No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Historical geomorphic analysis (1932–2011) of a by-passed river reach in process-based restoration perspectives: The Old Rhine downstream of the Kembs diversion dam (France, Germany)
Abstract
The Old Rhine downstream of the Kembs diversion dam is one of the largest by-passed river reaches in the world (50 km). It offers a unique opportunity to study the morphological effects of by-passing and address physical and ecological restoration approaches in regulated rivers. We conduct a space-time analysis of channel adjustment over a period of 80 years (1932 to 2011). We examine planform changes (from aerial photographs), erosional and depositional patterns (from vertical profiles), sediment sizes within the active channel and the new established floodplain, and we date riparian vegetation encroachment. Results show that the Old Rhine exhibited rapid response to the completion of the by-passing scheme in the 1950s, with a 26% narrowing in median active channel width between 1956 and 2008, from vegetation encroachment on dewatered channel margins (mostly groyne fields). The narrowing was accompanied by overbank fine sediment deposition (similar to 1.5 cm y(-1) aggradation since 1950) as well as slight bed degradation (similar to 0.7 cm y(-1) since 1950). We found no downstream propagation of active channel narrowing over time, nor propagation of bed degradation. The channel was already significantly adjusted prior to the diversion scheme, following the nineteenth century river straightening and groyne construction. By-passing (dewatering) mainly provided new pioneer habitat for synchronous vegetation establishment and promoted channel stability by decreasing sediment transport owing to peak flow reduction. The morphological budget calculated over the past 20 years estimated a downstream output for coarse sediments at 16,000 m(3) y(-1), with 80% originating from bed degradation and 20% from bank erosion, without significant inputs from upstream. The present-day morphodynamics remain sensitive to changes because of dynamic bed armouring (
... Designed for hydroelectric power generation, irrigation and/or navigation, the dams on large European rivers such as the Rhine, the Danube, the Durance and the Rhône divert most of a river's water discharge into a bypass channel running parallel to the original river course, and the diverted water is returned to the main river course at some downstream point. In this way, the original channel is bypassed for several kilometres of flow length (Arnaud et al., 2015). At present, other mega-water diversion projects are being designed in China, India and South Africa. ...
... Thus, the set of benches represents a new morphological pattern in bypassed channels. Only the works of Arnaud et al. (2015) and Räpple (2018) attempted to assess the behaviour of bypassed channels in a more complex, geomorphic-sedimentary and ecological way; these authors also made recommendations for the management of bypassed channels. Räpple (2018) pointed to the vertical accretion from 0.2 cmÁyear -1 to 10.8 cmÁyear -1 after diversion of the main flow in the Rhône River. ...
... The analysis of the vertical accretion thickness for the 2013 flood event, the period from 1992 to 2012 and period 2014-2017 does not decrease with the heigher topography of benches, and the thickness of accretion on benches and their GUs varied, likely due to the variability of the vegetation cover: shrubs are considered to trap sediment the most effectively (e.g., Corenblit et al., 2009;Arnaud et al., 2015). Vegetation-induced landform accretion results in changes in the composition of the vegetation communities, while the reduced frequency and magnitude of flooding resulting from landform accretion, combined with successional changes in the vegetation, alters the vegetation's contribution to sedimentation (Bendix & Hupp, 2000;Corenblit et al., 2007). ...
Abstract
This article is focused on the investigation of the spatio-temporal variability of the Danube River reach’s vertical accretion thickness due to the response of the Danube River reach to bypassing. Five groyne-induced benches (GIBs) of the bypassed channel developed after water diversion in 1992 was studied by changes in topography for three-time spans (for the original gravel surface, for the surface before the 2013 flood and for the surface after the 2013 flood). The allostratigraphic approach was applied to 548 drilling probes at all GIBs and toptop, supra-platform, tail and backchannel geomorphic units have been identified at each GIB. The main to side-channel system connectivity increase sedimentation rates and the accretion was controlled by large flood events. The 100-year flood in 2013 contributed to the total volume by almost 26%. During study period 1992–2017, totally 1,146,589 m3 was accreted on five GIBs, of this 209,752 m3 during flood event in 2013 and 267,700 m3 in post flood period 2014–2017. The top geomorphic unit exhibits the highest median values of vertical accretion and for all GIBs accretion thickness are not related to the height above the mean channel water level. The thickness of accretion varied, likely because the variability of the vegetation cover caused variable hydraulic conditions and accretion rate span from 3.8 cm.year−1 to 5.3 cm.year−1. The investigation of the sediment thickness over long time spans 24 years and a large flood event) allowed us to conclude that the thickness is spatially variable for individual GIBs; however, its trend over time remains constants depending on the intake of sediments during large floodsd events. This article also provides a methodological template for the detailed investigation of river channel adjustment due to bypassing.
... The associated RMSEs vary between 4.1 ± 1.6 m (1938 series) and 2.2 ± 0.9 m (2002 series), indicating that the associated rectification has fair goodness-of-fit (cf. Gurnell, 1997;Arnaud et al., 2015). ...
... River and other highly engineered rivers 5.1.1. Methodological contribution GIS-based assessment of geomorphological evolution is often performed at a large scale as part of an overall planform study (Arnaud et al., 2015;Arnaud et al., 2019;Piégay et al., 2020;Tena et al., 2020). By developing a specific georeferencing method consisting of standardized georeferencing zones associated with recurrent "dike-focused" ground control points, the bias due to deformations was reduced and the precision at the dike field scale was increased (low RMSE). ...
... The 1902 and 1962 WL at low water shows a slight lowering of 1.1 m, which could be more or less pronounced according to the potential discharge difference between low water of 1962 (260 m 3 /s) and that of 1902 (exact discharge unknown). Although all these hydromorphological changes could indicate riverbed incision (Simon and Rinaldi, 2006;Zawiejska and Wyżga, 2010;Arnaud et al., 2015), the PDR reach remains relatively preserved (average elevation difference between 1897 and 1969 of − 0.9 m) in comparison with other sectors of the Rhône River (e.g., Pierre-Bénite reach: − 4,1 m, cf. Parrot, 2015), even if it can locally reach high values (max. of the reach: − 3.4 m at RK 59). ...
Hydraulic structures, such as groyne fields, were commonly used to channelize European and North American rivers, thus forming engineered margins on the edges of the active channel. On the Rhône River (France), which was corrected with dike fields (classical groyne fields and specific ones closed with a longitudinal submersible dike; i.e., closed fields) and equipped with numerous dams (mid-twentieth century), the engineered margins have mostly been filled with fine sediments and become terrestrial. On the 11 km bypassed studied reach (middle Rhône), 55.6 ha in 167 dike fields (i.e., 75% of the cumulated surface) have been subject to terrestrialization (i.e., the transformation of aquatic areas into terrestrial ones). Our study aimed to understand the trajectory of dike fields that serve as both drivers (e.g., inducing in-dike fine sediment trapping) and witnesses (e.g., informing the diversion impact) of hydrosedimentary changes. We combined geohistorical analyses (aerial photographs, riverbed elevations, and water levels) and GIS modeling, as well as topographic and ground-penetrating radar surveys (GPR), to emphasize the terrestrialization patterns and hierarchize the drivers and processes involved. We obtained a classification of diachronic patterns (five types) that highlight local recurrences and specificities (inherited forms) in the terrestrialization trajectory. Topographic and GPR surveys complemented our data, shedding light on the main characteristics of sediment deposits (volumes, thicknesses) and structural units. We also determined that the contribution to terrestrialization of phase 1 (channelized state; 1900s-1970s) is lower than that of phase 2 (channelized and bypassed state; 1970s-2000s), at 42% (23.4 ha) and 58% (32.2 ha), respectively. In phase 1, fine sediment deposition leads to deposit construction, triggered by the dike field setting, which shapes spaces with reduced shear stresses. The riverbed incision (induced by channelization) is shallow on this reach, so its contribution (by lowering the water level) is considered negligible. In phase 2, terrestrialization is mainly provoked by the diversion-induced drop in water level that has caused the retraction of the active channel and promoted its abandoned edges as new terrestrial margins. Understanding the evolution of these ecotones made it possible to adjust our recommendations in terms of management and restoration. It also highlights the relevance of strategic dike field reconnections to support the river by recreating gradients of hydrological connectivity (which are favorable to habitat diversity).
... The associated RMSEs vary between 4.1 ± 1.6 m (1938 series) and 2.2 ± 0.9 m (2002 series), indicating that the associated rectification has fair goodness-of-fit (cf. Gurnell, 1997;Arnaud et al., 2015). ...
... River and other highly engineered rivers 5.1.1. Methodological contribution GIS-based assessment of geomorphological evolution is often performed at a large scale as part of an overall planform study (Arnaud et al., 2015;Arnaud et al., 2019;Piégay et al., 2020;Tena et al., 2020). By developing a specific georeferencing method consisting of standardized georeferencing zones associated with recurrent "dike-focused" ground control points, the bias due to deformations was reduced and the precision at the dike field scale was increased (low RMSE). ...
... The 1902 and 1962 WL at low water shows a slight lowering of 1.1 m, which could be more or less pronounced according to the potential discharge difference between low water of 1962 (260 m 3 /s) and that of 1902 (exact discharge unknown). Although all these hydromorphological changes could indicate riverbed incision (Simon and Rinaldi, 2006;Zawiejska and Wyżga, 2010;Arnaud et al., 2015), the PDR reach remains relatively preserved (average elevation difference between 1897 and 1969 of − 0.9 m) in comparison with other sectors of the Rhône River (e.g., Pierre-Bénite reach: − 4,1 m, cf. Parrot, 2015), even if it can locally reach high values (max. of the reach: − 3.4 m at RK 59). ...
Many large European and American rivers have been channelized in the 19 th century and since then feature Dike Fields (DFs) forming engineered alluvial margins. Drivers and witnesses of contemporary geomorphological and ecosystem changes, these initially aquatic DFs have for the most been filled with fine sediments and become terrestrial. On the Rhône River (France) which has not only been corrected but also equipped with numerous dams (mid-20 th century), we studied the terrestrialization ( i.e., transformation of aquatic areas in terrestrial ones) in two types of DFs: open fields (groyne fields) and closed fields (groyne fields closed by a longitudinal dike). A classification of spatio-temporal terrestrialization patterns (5 types) has been obtained under GIS thanks to aerial photographs and completed by ground penetrating radar surveys to characterize the sediment structural organization of the deposits. It highlights local specificities (inherited forms) within a generalized trajectory of fluvial disconnection. Studying the evolution of the water lines and riverbed elevation allowed to emphasize the control factors and the associated forcings leading to terrestrialization. During phase 1 (reach only channelized – 1890s to 1970s), it is 47% of the closed fields areas which have been terrestrialized and 16% for open fields. Since the incision is not very pronounced on the reach, it appears to be mainly due to accretionary processes as a result of lower shear stresses within the DFs. The terrestrialization from phase 2 (channelized and bypassed reach – 1970s-2000s) corresponds to 32% of the areas of closed fields and 51% of open fields. A cross-validation between the planimetric approach and a lateral connectivity model shows that dewatering caused by the flow diversion has provoked the emersion of almost the half of the DF extent on the upper – and most impacted – part of the reach (75% of the total terrestrialized area). In terms of fluvial rehabilitation, to understand the DFs trajectories provides new insights to guide future restoration design in line with the societal stakes and the current hydrological conditions. Strategical DF reconnections (removing or lowering dikes) could support the river to gain space and recreate hydrological connectivity gradients favorable to habitat diversity that it is currently unable to create or maintain on its own.
... Caskey et al. (2014) also came to similar conclusions: in mountain streams, low gradient reaches respond more actively to flow diversion by channel width reduction than do steep channels. In the case of the River Rhine (Arnaud et al., 2015), previous navigation improvement works accentuated channel narrowing and bed degradation. Dépret et al. (2018) and Vazquez-Tarrio et al. (2019) showed that by-passing reduced the bedload transport capacity of the Rhône River because of a slight lowering of peak flow and a flood water slope decrease. ...
... Ibisate et al. (2013) also stated that evidence for unit-and cumulative-effects of RoR dams is difficult to disentangle from other factors causing change, as they seem to accentuate a long trend of river simplification initiated by changes in land cover and upstream damming. Working on the Old Rhine, Arnaud et al. (2015) also concluded that river adjustments related to by-passing were less intense than those during previous periods during which river training works resulted in a reduction of the active channel area by 45% between 1828 and 1872. ...
... Nevertheless, we can still state that the channel had adjusted to new conditions in 2011, despite the low potential of this reach to adjust (with less than 20% of the active channel occupied by gravel bars), thereby giving a relaxation time of around 25 yr. This observation is consistent with results obtained by Curtis et al. (2010) in Vermont, which indicated a 20-yr reaction time and a 30-yr relaxation time, as well as with those of Arnaud et al. (2015), who estimated a 20-yr relaxation time on the Old Rhine. ...
In this paper, we evaluate morphological changes related to the Plan d'Arem dam (1970), a run-of-river (RoR) dam located on the Upper Garonne (central Pyrenees), and disentangle its morphological effects from other drivers (post-Little Ice Age [LIA] climate change, changes in agricultural practice, catchment afforestation, upstream damming, and bypassing). The work is based on a before-after-control-impact approach, a space-time framework that allowed the stating of four hypotheses distinguishing the effects of the considered dam from other pressures. We first examined the potential reduction to the flow regime (QL) and bedload transport (QS) from these pressures, then assessed planimetric changes (1942–2019), vertical evolution (1922–2014), and sediment size within the channel. The results show the river completed adjustments related to post-LIA climate change and catchment afforestation at the beginning of the study period, with channel narrowing affecting the whole study reach and ranging from 0.6% to 1.2% yr⁻¹. Upstream dams and catchment afforestation reduced both the frequency and magnitude of peak flows and sediment supply, resulting in an increase in the channel narrowing rate on the upstream sub-reach (−1.2% yr⁻¹). However, downstream tributaries buffered these changes, and no downstream propagation was found. The effects of the Plan d'Arem started around 15 yr after its construction, with channel narrowing at a rate of 0.9% yr⁻¹ until the 2010s. The exceptional flood of June 2013 resulted in important channel widening followed by a new period of narrowing upstream of the Plan d'Arem dam, combined with channel stability downstream caused by a new dam management regime (flushing actions). We conclude that the before-after-control-impact approach is effective for isolating the effects of an RoR dam from those of other pressures, and that flushing actions mitigated the effects of the dam.
... Since the beginning of the 19 th century, 3 the upper Rhine was profoundly impacted by three successive engineering works: (i) 4 channelization (19 th century), (ii) regularization, which consisted of the construction of 5 alternate groyne fields (beginning and middle of the 20 th century) and (iii) damming, and in 6 some cases, flow diversion (middle of the 20 th century up to 1977) (Fig. 1b). These works 7 induced severe degradation of both geomorphological and ecological functionality, notably 8 along the Old Rhine, which is a 50 km reach between Basel and Breisach that is bypassed 9 by the Grand Canal d'Alsace (GCA) (Arnaud et al. 2015;Arnaud et al. 2019) . The 10 instream flow varied between 20 and 30 m 3 /s before December 2010. ...
... The sediment volume deposited on the two mid-27 bars (5,798 m 3 ; P1) was much higher than the sediment volume provided by bank re-28 erosion (1,456 m 3 ; P1). This shows that deposited sediment was also supplied from the 29 scouring of the two pools (9,366 m 3 ; P1) ( Fig. 3; Table 2) and probably also from residual 30 morphodynamics of the Old Rhine upstream of the restored bank (Arnaud et al. 2015) . The 31 bed grain-size diversification and fining that occurred along the whole restored length and 32 downstream along a total length of 600 m (Fig. 6) was induced by the abovementioned 33 sediment supply and the reduced hydraulic conditions and countercurrents downstream of 34 the groynes. ...
... This mainly resulted in the implementation of the two 39 transverse groynes. Interestingly, the CSD index values revealed a relatively weak but 40 significant morphological diversification from 1985/93 to 2009 (Fig. 5) due to two high flood 41 events during this period (Q 100 in 1999 and Q 50 in 2007; (Arnaud et al. 2015). Post-42 restoration geomorphological evolution induced positive ecological effects in the short 43 term, such as an increase in the diversity and richness of macroinvertebrate, fish and 44 macrophyte communities (Aelbrecht et al. 2014;Staentzel et al. 2018;) . ...
River regulation alters hydrological and sediment regimes and consequently affects habitat complexity and dynamics, biodiversity and ecosystem services. Although channel bank
erosion is a key geomorphological process supplying alluvial channels with coarse sediments and diversifying aquatic and riparian habitats, banks have often been stabilized to limit erosion risk to human activities and facilities. The objective of this paper is to assess the effects, and their sustainability, of bank protection removal on a 300 m long reach of the Old Rhine (France/Germany) to promote sediment supply, channel diversification and a rehabilitation of fluvial morpho-sedimentary processes. This action was combined with the construction of two island groynes to locally increase bank erosion processes. Yearly detailed monitoring was implemented over 6 years, including classical
bathymetric surveys, airborne topo-bathymetric and terrestrial LiDAR, and bed grain-size and bedload tracking. Following a Q15 flood, the restoration induced a weak sediment supply. The restoration diversified habitats due to the implementation of the two island groynes, inducing bank scouring and the creation of new macroforms, as well as local bed grain-size diversification and fining. The cross-sectional diversity of the restored water channel was close to the regularization engineering phase. Channel bedform diversification persisted six years due to the persistence of the two island groynes. The action induced the rehabilitation of fluvial forms, in a static manner, rather than the rehabilitation of fluvial morpho-sedimentary processes, which raises questions about the sustainability of the benefits of such management actions in terms of fluvial functionality and naturality.
... Caskey et al. (2014) also came to similar conclusions: in mountain streams, low gradient reaches respond more actively to flow diversion by channel width reduction than do steep channels. In the case of the River Rhine (Arnaud et al., 2015), previous navigation improvement works accentuated channel narrowing and bed degradation. Dépret et al. (2018) and Vazquez-Tarrio et al. (2019) showed that by-passing reduced the bedload transport capacity of the Rhône River because of a slight lowering of peak flow and a flood water slope decrease. ...
... Ibisate et al. (2013) also stated that evidence for unit-and cumulative-effects of RoR dams is difficult to disentangle from other factors causing change, as they seem to accentuate a long trend of river simplification initiated by changes in land cover and upstream damming. Working on the Old Rhine, Arnaud et al. (2015) also concluded that river adjustments related to by-passing were less intense than those during previous periods during which river training works resulted in a reduction of the active channel area by 45% between 1828 and 1872. ...
... Nevertheless, we can still state that the channel had adjusted to new conditions in 2011, despite the low potential of this reach to adjust (with less than 20% of the active channel occupied by gravel bars), thereby giving a relaxation time of around 25 yr. This observation is consistent with results obtained by Curtis et al. (2010) in Vermont, which indicated a 20-yr reaction time and a 30-yr relaxation time, as well as with those of Arnaud et al. (2015), who estimated a 20-yr relaxation time on the Old Rhine. ...
In this paper, we evaluate morphological changes related to the Plan d’Arem dam (1970), a run-of-river (RoR) dam located on the Upper Garonne (central Pyrenees), and disentangle its morphological effects from other drivers (post-Little Ice Age [LIA] climate change, changes in agricultural practice, catchment afforestation, upstream damming, and bypassing). The work is based on a before-after-control-impact approach, a space-time framework that allowed the stating of four hypotheses distinguishing the effects of the considered dam from other pressures. We first examined the potential reduction to the flow regime (QL) and bedload transport (QS) from these pressures, then assessed planimetric changes (1942–2019), vertical evolution (1922–2014), and sediment size within the channel. The results show the river completed adjustments related to post-LIA climate change and catchment afforestation at the beginning of the study period, with channel narrowing affecting the whole study reach and ranging from 0.6% to 1.2% yr-1. Upstream dams and catchment afforestation reduced both the frequency and magnitude of peak flows and sediment supply, resulting in an increase in the channel narrowing rate on the upstream sub-reach (−1.2% yr-1). However, downstream tributaries buffered these changes, and no downstream propagation was found. The effects of the Plan d’Arem started around 15 yr after its construction, with channel narrowing at a rate of 0.9% yr-1 until the 2010s. The exceptional flood of June 2013 resulted in important channel widening followed by a new period of narrowing upstream of the Plan d’Arem dam, combined with channel stability downstream caused by a new dam management regime (flushing actions). We conclude that the before-after-control-impact approach is effective for isolating the effects of an RoR dam from those of other pressures, and that flushing actions mitigated the effects of the dam.
... Table 35 chapitre 6), celles-ci étant comparables à celles observées dans la littérature (cf. Arnaud et al., 2015 : les RMSE variaient entre 1,1 à 4,5 m sur 1946 séries photographiques). ...
... Ces valeurs sont issues d'analyses réalisées sur la fraction inférieure à 63µm, ce qui permet une comparaison directe du « signal chimique » avec notre jeu de données. Les carottes pédologiques de Dendievel et al (2020) sont décrites comme des dépôts lamineux stratifiés dont la constitution est caractéristique de l'érosion des roches calcaires, molasses et cristallines du Haut Rhône (Dendievel et al.,2020 ;Arnaud et al., 2015), qui par leur localisation sur le Rhône médian et les apports des différents affluents, sont aussi influencées par le fond géologique résultant de l'altération des roches cristallines et volcaniques du Massif Central. 2020) optent pour une normalisation par l'Aluminium, mais dans les deux cas, ils évoluent sur des sédiments provenant de fraction totale ou tamisés à 2 mm. ...
... Table 35) and comparable to the ones observed in the literature (cf. Arnaud et al.,2015: RMSE ranged from 1.1 to 4.5 m on 1946 photographic series). The photograph series at t+1 give the information of the deposit stability and prevent from interpretation errors due to the photograph quality variability. ...
Hydrosystème affecté par des « pressions multiples», le Rhône est un cours d’eau franco-suisse fortement anthropisé. En effet, ce fleuve alpin est à la fois chenalisé depuis la fin du 19ème siècle par des ouvrages de correction appelés « casiers Girardon » (sorte de champs d’épis fermés par une digue longitudinale submersible) et à partir du milieu du 20ème siècle, est équipé pour la production hydro-électrique par de nombreux barrages en dérivation. Les secteurs d’étude, situés sur le Rhône moyen et aval (Pierre-Bénite, Péage-de-Roussillon, Montélimar et Donzère-Mondragon) ont enregistré ces deux phases d’aménagement. Sujets à l’atterrissement et à la végétalisation, les « casiers Girardon » présentent dans ces secteurs des patrons sédimentaires qui témoignent de l’impact de chacune des phases d’aménagement. Grâce à un découpage géohistorique sous SIG (Système d’Information Géographique), nous obtenons des patrons planimétriques d’atterrissement très concordants avec l’organisation topographique et macrostructurale sédimentaire des dépôts (identifiée à partir de radargrammes et de transects topographiques). Ces données permettent de comprendre l’histoire de la dynamique et des processus sédimentaires au sein de ces milieux et les différences existant également entre eux. Les sédiments fins piégés sont généralement peu à modérément contaminés en ETM (e.g. Igéo plomb : 1,22 ± 0,48), mais portent tout de même les marques de l’histoire des usages du 20ème siècle : les signatures chimiques (zinc, plomb) obtenues par spectrométrie à fluorescence X (FP-XRF) corroborent une concordance spatio-temporelle entre les périodes d’atterrissement et un pic de contamination durant la période des années 1970 à 1980 (période connue de flux de polluants sur le Rhône). Les concentrations en ETM retrouvées dans les matrices végétales (foliaires et dendrologiques) reflètent la complexité des dépôts sédimentaires et révèlent notamment certaines de leurs spécificités (e.g. concentrations en nickel importantes, à l’image du fond géochimique rhodanien).
Ainsi, face à ces « Impacted ecosystems », pour la plupart atterris ou s’atterrissant encore, les casiers Girardon sont des milieux pour lesquels il existe des enjeux de gestion. Entre action de conservation et de démantèlement, une stratégie alternative consisterait à abaisser ou enlever stratégiquement certaines digues afin de recréer des conditions d’écotone et des gradients de connectivité hydrologique favorables à une mosaïque d’habitats semi-aquatiques.
... Although the results (mainly those for the active channel width) may be affected by some errors due to this fact, the magnitude of these would not interfere into the observed general trends. The spacing (i.e., 300 m) between cross sections was established based on: (i) the average length of the main morphological sequence (riffle-pool) in the study reach (i.e., 200-400 m); and (ii) the average active width of the study reach for the entire analysed period (i.e., 300 m; following Arnaud et al., 2015). A total of 40 cross sections segmented the 12 km length study reach. ...
... The impacts of lateral embankments on the reduction of active channel width and active geomorphic units were observed in several studies (e.g., Surian and Rinaldi 2003;Surian et al., 2009;Comiti et al., 2011;Downs et al., 2013;Ibisate et al., 2013;Latapie et al., 2014;Arnaud et al., 2015). Of particular interest is the study by Arnaud et al. (2015) in which the impact caused by the embankments and by-passing actions in the River Rhine (Germany) during the nineteenth century was demonstrated as a main factor for the decrease of the active channel area of around 45% between 1828 and 1872. ...
... The impacts of lateral embankments on the reduction of active channel width and active geomorphic units were observed in several studies (e.g., Surian and Rinaldi 2003;Surian et al., 2009;Comiti et al., 2011;Downs et al., 2013;Ibisate et al., 2013;Latapie et al., 2014;Arnaud et al., 2015). Of particular interest is the study by Arnaud et al. (2015) in which the impact caused by the embankments and by-passing actions in the River Rhine (Germany) during the nineteenth century was demonstrated as a main factor for the decrease of the active channel area of around 45% between 1828 and 1872. Similarly in the Santa Clara River in California (USA), Downs et al. (2013) found that the construction of lateral embankments together with other human disturbances or influences (e.g., reservoirs, gravel mining) reduced the mean channel width by almost 50% during the second half of the 20th century. ...
In this paper we analyse the geomorphic response of a mountain river (Upper Cinca River, Central Southern Pyrenees) in relation to multi-scale disturbances during almost a century (i.e., 1927–2015). The integration of multiple geomorphic descriptors (i.e., morphology and topography) and disturbances (i.e., floods, land use changes, dams, embankments and gravel mining activities) allows better understanding of the cause-effect relations and their significance. The paper presents a novel methodology integrating the application of SfM-MVS algorithms to historical imagery to extract multitemporal channel topography and morphology; and the use of statistical tools to infer on the existence of significant trends and breakpoints in the temporal evolution of the geomorphic descriptors and disturbances. Results show that afforestation was the main land use change exhibited between 1957 and 2015, although mainly concentrated during 1977–2015. Annual runoff presented a general negative trend with a wet (1959–1983) and dry period (1983–2015). Extensive gravel mining occurred after flood events, together with channel engineering works such as the construction of embankments. These disturbances, together with the construction of three small dams in the headwaters, had a direct impact on lateral connectivity by confining and constraining channels and concentrating flows, resulting inchannel incision and stability. The evolution of the disturbances, process changes and geomorphic descriptors indicated that, during the last century, the river channel is in a transient state, in which the magnitude of the changes oscillate. The river is adjusting to the disequilibrium imposed by multi-scale disturbances acting at different temporal and spatial scales by means of two contrasting channel states. First, Channel State 1 (1927–1984) was characterised by having high geomorphic activity and channel incision. Conversely, Channel State 2 (1984–2015) was characterised, by having a low geomorphic activity, when channel constraining was the dominant process and incision was more localised and marginal. The results of the Upper Cinca may be relevant to infer in future fluvial metamorphosis associated with the effects of global change on water and sediment fluxes in mountain catchments, and to assist objective-based rehabilitation measures of rivers subjected to multi-scale disturbances.
... In other words, the observed synchronicity between dams and geomorphic change does not directly translate into causation. It is very common, particularly in European fluvial settings, for dams constructed during the twentieth century to be on rivers previously subjected to a long history of anthropogenic influences and human management related to catchment-scale land-use changes, river navigation, irrigation and/or flood control going back to at least the Industrial Revolution (Frings et al., 2009(Frings et al., , 2014aArnaud et al., 2015;Vázquez-Tarrío et al., 2019). Additionally, in many cases multiple dams were constructed along the same river (e.g. ...
... Similar effects are seen in cases of river embankments and flood protection works that decouple channels and banks reducing sediment supply from bank erosion and secondary channels. Embankments are often associated with channel straightening and narrowing or widening, altering sediment transport capacities and leading to bed textural changes (e.g., Frings et al., 2009;Bravard, 2010;Arnaud et al., 2015). This constitutes a nice example of equifinality: the same degree of surface coarsening arising from different controls: sediment starvation following sediment-supply reductions resulting from dam construction or shear stress increase caused by embankments and flood protection works. ...
Present-day river forms and processes are in many cases conditioned by the consequences of anthropogenic modifications such as dams, embankments and gravel mining. Fluvial geomorphologists have typically investigated the effects of these human impacts using a so-called expert-based approach, whereby observed association or synchronicity between geomorphological changes and a given, preidentified impact, are interpreted as evidence of causation. This approach has important limitations when the effects of multiple human interventions interact along the same river corridor or overlap with the legacy of natural changes affecting the sediment - water balance. In such situations, the establishment of causal links between channel morphology and single impacts is not as straightforward as commonly assumed and the conclusions are susceptible to ‘confirmation biases’. In this paper we highlight this risk through an assessment of human impacts on the Rhône River within a multi-driver context. The French Rhône is an excellent example of an Anthropocene river impacted by two main development phases during the twentieth century: embankments (1890s–1930s) followed by a series of multiple dams (1950s–1990s). We began by laying out several geomorphologically consistent hypotheses for the geomorphological trajectory of the Rhône over the twentieth century. Next, we tested these hypotheses against grain size data collected in the field in a structured and hypothesis-oriented way. Using this hypothesis-driven and deductive attribution analysis we identified the relative impacts of the different development phases on the present-day grain size distribution and in particular on armouring in the Rhône River, and proposed a hierarchy of dominant drivers of geomorphological change along the Rhone over the last century and a half. Our results led us to conclude that in the case of the Rhône, the effect of dams on armouring was negligible compared to a legacy of natural heritages and embankments.
... The mean annual discharge is 1059 m 3 /s at Basel (Uehlinger and Wantzen, 2009). After two engineering phases of correcting and regulating the flow (Arnaud et al., 2015;Schmitt et al., 2018;Arnaud et al., 2019), the Rhine River became bypassed by the French Grand Canal d'Alsace (GCA), along 50 km from Kembs to Breisach where four power plants were built. The flow in the by-passed section, which is named Old Rhine, is controlled upstream by the Kembs derivation dam. ...
... Spillovers occur into the Old Rhine River when the Rhine discharge exceeds 1400 m 3 /s at Basel, which is the maximum flow capacity discharge of the GCA. The channel bottom of the study reach is composed mainly of gravels and cobbles (Arnaud et al., 2015). The mean slope and mean width are equal to 0.09% and 100 m, respectively ( Figure 1b). ...
Most grain size monitoring is still being conducted by manual sampling in the field, which is time consuming and has low spatial representation. Due to new remote sensing methods, some limitations have been partly overcome, but methodological progress is still needed for large rivers as well as in underwater conditions. In this paper, we tested the reliability of two methods along the Old Rhine River (France/Germany) to estimate the grain size distribution (GSD) in above-water conditions: (i) a low-cost terrestrial photosieving method based on an automatic procedure using Digital Grain Size (DGS) software and (ii) an airborne LiDAR topo-bathymetric survey. We also tested the ability of terrestrial photosieving to estimate the GSD in underwater conditions. Field pebble counts were performed to compare and calibrate both methods. The results showed that the automatic procedure of terrestrial photosieving is a reliable method to estimate the GSD of sediment patches in both above-water and underwater conditions with clean substrates. Sensitivity analyses showed that environmental conditions, including solar lighting conditions and petrographic variability, significantly influence the GSD from the automatic procedure in above-water conditions. The presence of biofilm in underwater conditions significantly altered the GSD estimation using the automatic procedure, but the proposed manual procedure overcame this problem. The airborne LiDAR topographic survey is an accurate method to estimate the GSD of above-water bedforms and is able to generate grain size maps. The combination of terrestrial photosieving and airborne topographic LiDAR methods is adapted to assess the GSD over several kilometers long reaches of large rivers.
... Generally, studies of long-term channel incision are primarily focused on the analyses of historical geodetic profiles from topographic or hydrometric surveys (Table 1). Channel bed elevation lowering is detected by contrasting historical longitudinal and crosssectional profiles with present channel conditions (Arnaud et al., 2015;Dufour & Piégay, 2010;Ferrer-Boix et al., 2023;James, 1991;Kondolf et al., 2002;Škarpich et al., 2013Surian et al., 2009;Wyżga, 2001). As indirect approaches, are used informations from lowering the annual minimum water level, following condition that the channel width remains constant over time (Chiriloaei et al., 2012;Hajdukiewicz et al., 2017;R adoane et al., 2013;Wyżga, 1991Wyżga, , 1993Wyżga, , 2001 or historical photographs of undercutting bridges are compared (Galay, 1983;Kondolf & Swanson, 1993;Ferrer-Boix et al., 2023). ...
Channel incision is an evident trend for river evolution in many European rivers and notably the Western Carpathians, whose former braided and multichannel wandering river system is transforming into a single‐thread channel, but it is often difficult to separate drivers and determine if incision is finished or is still ongoing. To overpass these research gaps, this paper presents an innovative approach to assess the multidecadal incision of the Belá River in the Western Carpathians since 1949 by LiDAR‐based analyses of floodplain surfaces above the river channel dated from historical aerial images. Detailed analyses of ongoing incision were also calculated based on DEM of differences (DoD) using Structure‐from‐Motion (SfM) photogrammetry‐derived topo‐bathymetric models. The study applied the BACI (Before‐After‐Control‐Impact) approach that compared pre‐state (Before), post‐state (After) and reach ( Control ) that is not affected by potential external effects with degraded (impacted) reach to be able to distinguish the driver effects. Floodplain channel surface analyses indicate the maximum incision up to 4 m and incision rate of 5.7 cm/year that occurred in the most degraded reach. Moreover, cross‐section profiles point to accelerated incision of 24.5 cm/year in the last 10 years (2011–2021) by the propagation of incision upstream. Overall, the net changes from the UAV survey pointed to 22 759 m ³ of gravel sediments, constituting outwash from the 1.6 km long channel system (2015–2022) by incision, whereas analyses of historical channel surfaces estimated erosion of 573 303 m ³ from impacted reaches between 1949 and 2020. Incision evidence is only observed in the downstream part below the control section due to local drivers (channel regulation, comprising embankment and gravel mining that activated a backward erosion of the system with knickpoint migration upstream). This analysis shows the benefits of combining different sources of data to separate long‐term and ongoing channel responses and the BACI‐approach to better target cause–effect relationships in space and time.
... Instead, homogenous areas of either water or sediment were manually mapped from the polygon of the active channel. Following the methodology given in Arnaud et al. (2015), the January 2015 satellite image was utilised to establish cross sections at regular intervals of 100 m, to provide a comprehensive depiction of the geometry of the active channel along S -S1. Utilising this image, the upper cross section was constructed perpendicular to the wetted channel, which served as a representation of the upstream boundary of this subsegment, and the downstream boundary was defined using the uppermost cross section of S-S2 (see Iroumé et al., 2020). ...
Data collected between 2015 and 2022 was utilised to assess longitudinal connectivity and explain the temporal and spatial fluctuation of large wood (LW) along the Blanco River, which was severely affected by the 2008 eruption of the Chaitén Volcano in southern Chile. The study was performed along a ~ 10.6-km long segment, which was divided into two sub-segments (upper and lower). LW source areas were identified using information from previous research, complemented by the analysis of satellite images to cover the study period. LW fluctuation was studied for the period 2017-2022, using field data and images from an unmanned aerial vehicle along seven 80 m-long reaches located within the lower sub-segment of the river. Using the generated geore-ferenced orthomosaics and digital elevation models, LW and wood jams (WJs) locations, abundance, dimensions, volumes, and WJs stability were assessed, as well as wood deposition patterns. Streambank erosions and landslides were found to be the main LW sources. Considering wood sourcing, and the variations in the number of LW and WJs, in the dimensions of individual LW and WJs volume and deposition patterns as indicators, it was possible to infer the longitudinal connectivity conditions for the studied segment through time. The results showed that the upper and lower subsegments were poorly connected. However, changes in LW abundance over time, its dimensions and volumes, and the very low stability of WJs, and LW deposition patterns indicative of fluvial transport confirmed that the lower subsegment was longitudinally connected. The results also verified that the presence of LW did not affect water or sediment longitudinal connectivity along the fluvial system. The volume of individual LW and WJs showed a decreasing trend with time in all study reaches. As the bank erosions able to source LW to the channel are also reducing with time, pieces of wood deposited in the active channel may become the only way to sustain LW availability for the next few years. Things could change with the occurrence of an extreme and infrequent flood able to connect the upper and lower sub-segments and force the river to initiate a new phase of planform adjustments.
... Also, bank stabilisation, by limiting sediment remobilization and promoting channel incision, induced a decrease in species richness and a change in composition in favor of drought-tolerant species (Baattrup-Pedersen et al., 2005;Helfield et al., 2007;Dufour et al., 2007). However, and despite the fact that many large rivers have been both damned and channelized (e.g., Alexander, Wilson & Green, 2012;Arnaud et al., 2015;Luan et al., 2018), very few studies have investigated the cumulative impact of altered water flow and sediment transport on riparian plant communities. ...
In riverine ecosystems, human‐induced stressors related to flow regulation and bank stabilisation have accumulated over time. The restoration of these highly anthropised ecosystems has become a major issue over the last few decades, with ambitious stated objectives. However, while the individual impact of flow regulation and channelisation on river functioning has been extensively studied, the response of ecological communities to multiple co‐occurring human‐induced stressors remains largely unexplored.
Using a sampling design based on five river reaches in the Rhône catchment, ranging from unregulated in flow and bedload transport to intensively regulated reaches, we sought to understand how the accumulation of anthropogenic stressors influenced the functional response of riparian plants communities on gravel bars. By using 12 ecological and morphological traits, we performed a classification analysis to construct six riparian guilds and investigated whether their representativeness, as well as the mean value of individual traits, varied with increasing levels of anthropogenic stressors.
Species cover and redundancy in several guilds increased or decreased significantly with increasing pressures. Thus, the guild of small taproot herbs with low nutrient and soil moisture requirements (xero‐oligotro‐taproot species guild) and the guild of taproot herbs adapted to very bright and dry conditions (mesoxero‐mesotro‐taproot species guild) dominated the unregulated reaches with active bedload transport. Conversely, regulated reaches with stabilised baseflow and inactive transport were dominated by the guild of flood‐tolerant trees (hygro‐perennial tall species guild) and the guild of vegetatively reproducing and flood‐tolerant perennial herbs (hygro‐perennial clonal species guild).
Analysis of individual traits revealed a shift in environmental conditions, from full light to shade tolerance and from dry to humid, with increasing anthropogenic stressors. In response to this decrease in drought levels, plants traits shifted from annual to perennial species, from sexual to vegetative strategies and from taproots to a fibrous root system.
Our results highlight the accumulated effects that anthropogenic stressors can have on riparian communities, inducing a progressive shift in certain traits related to life history, reproductive strategies, and drought adaptations. This effect on a set of shared traits reveals the strong influence that human infrastructures can have on the ecological niche of species and the morphological adaptations of riparian vegetation.
From an applied point of view, and for highly anthropised rivers, our results suggest that restoration actions targeting a single stressor will not be sufficient to reorient riparian plant communities towards an ecological state closer to reference systems. Since human‐induced stressors have often deeply altered the flow and sediment regimes of rivers, a more integrated approach based on the restoration of erosion and flooding processes is essential to allow the expression of a wider diversity of riparian plant communities and habitats.
... River planform changes, including channel widening, may occur at various temporal and spatial scales. Feng et al. (2022) provided one of the first multi-decadal river extent changes at the global scale; however, most studies are regional or river specific (e.g., Surian et al., 2009;Arnaud et al., 2015;Scorpio et al., 2015;Hajdukiewicz et al., 2019). These longer-term studies focused on identifying trends and phases of channel narrowing and widening over several decades and attributed them to different drivers, such as flow regulation, in-channel gravel mining, or afforestation. ...
River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures.
... Engineers demonstrated considerable technical prowess in their efforts to train rivers into stable, hydraulically efficient conduits that are conceived to be more predictable and manageable (Knight, 2016). Short-term successes in this regard, however, led to long-term unsustainable outcomes in biophysical, socioeconomic, and cultural terms (Table 1; Arnaud et al., 2015;Bravard, 2010;Frings et al., 2019;Knox et al., 2022;Tena et al., 2020). Impacted systems have been referred to as zombie rivers (Mitchell & Williams, 2021)-rivers that are increasingly devoid of life and diversity. ...
Contemporary management practices have artificially confined (strangled) river systems in Aotearoa New Zealand to support intensified land use in riparian areas. These practices work against nature, diminishing the functionality and biodiversity values of living rivers, and associated socio‐cultural relations with rivers. River confinement can accentuate flood risk by promoting development in vulnerable locations and limiting the flexibility to adapt to changing climate, prospectively accentuating future disasters. To date, uptake of space‐to‐move management interventions that seek to address such shortcomings is yet to happen in Aotearoa New Zealand. This is despite the fact that such practices directly align with Māori (indigenous) conceptualizations of rivers as indivisible, living entities. Treaty of Waitangi obligations that assert Māori rights alongside colonial rights of a settler society provide an additional driver for uptake of space‐to‐move initiatives. This article outlines a biophysical prioritization framework to support the development and roll out of space‐to‐move interventions in ways that work with the character, behavior, condition, and evolutionary trajectory (recovery potential) of each river system in Aotearoa.
This article is categorized under: Water and Life > Conservation, Management, and Awareness
Science of Water > Water and Environmental Change
... 51 (1938,1949,1974), l'impact direct de la mise en place de la dérivation 75 (1979), les changements rapides post-dérivation (1982 et 1986) et les ajustements plusieurs 76 décennies après la dérivation (1991,2002,2009 cohérence avec ce qui est habituellement observé (cf. Arnaud et al., 2015). Notre typologie des 81 patrons planimétriques d'atterrissement est basée sur une approche empirique reposant sur la 82 récurrence des motifs de remplissage sédimentaire ; s'inspirant notamment des patrons de dépôts 83 inter-épis de Sukhodolov et al. (2002). ...
Sur les grands cours d’eau européens et nord-américains, des champs d’épis ont couramment été implémentés au 19ème siècle afin de corriger et stabiliser leur géométrie. Au 20ème siècle, beaucoup se voient équipés de barrages pour la production hydro-électrique. Travaillant sur le Rhône qui cumule ces deux types d’aménagement, nous observons un phénomène d’atterrissement de ses marges alluviales « construites ». L’étude s‘appuie sur la planimétrie diachronique sous SIG (9 dates de 1938 à 2009) de plus de 150 structures (champs d’épis et « casiers » fermés par une digue longitudinale submersible) du tronçon court-circuité de Péage-de-Roussillon (Rhône moyen). Elle est complétée par des relevés au radar géologique (GPR) informant de l’organisation des dépôts sédimentaires. Nous référant aux patrons sédimentaires inter-épis de Sukhodolov et al. (2002), nous décrivons des motifs supplémentaires qui retranscrivent l’histoire des aménagements et les spécificités géomorphologiques du Rhône. En deux phases, l’atterrissement est tout d’abord initié par les structures de correction qui induisent du piégeage sédimentaire (intra-digue) puis la dérivation entraîne une rétraction du chenal en eau et l’émersion des abords du lit mineur, favorisant le stockage des sédiments fins. Ces deux phénomènes contribuent à la déconnection du fleuve de ses marges alluviales et à la réduction des annexes fluviatiles. Comprendre l’évolution de ces écotones permet notamment de mieux évaluer leur fonctionnalité actuelle et de formuler des recommandations en termes de gestion et de restauration.
... Over the past three decades, enormous efforts have been made to examine channel planform behavior locally and globally (Bora and Goswami 2021;Hasanuzzaman et al. 2021). Many geospatial technology-based studies have been conducted across the world, including in the USA on the four rivers of Olympic National park (East et al. 2017), Taiwan on the Zhuoshui River and the Gaoping River (Kuo et al. 2017), Italy on the Scrivia river (Mandarino et al. 2020), Germany on the Old Rhine downstream (Arnaud et al. 2015), China on the lower yellow river (Kong et al. 2020;Guo et al. 2021) and the lower jingjiang reach (Yang et al. 2015), India on the Koshi river (Sinha et al. 2014), the Sharda river (Midha and Mathur 2014), the Dwarkeswar river (Ghosh and Mukhopadhyay 2021), the middle lower part of Ganga (Pal and Pani 2019), and the Ramganga river basin (Agnihotri et al. 2020), and Bangladesh on the lower Padma river (Rashid 2020;Nawfee et al. 2018;Halder and Chowdhury 2021), the rivers in southern estuarine Region (Islam et al. 2018), the Lower Meghna river (Mahmud et al. 2020), Madhumati river (Biswas et al. 2021), the lower Teesta river (Akhter et al. 2019), and the Brahmaputra river (Rashid et al. 2021). Lower sections of rivers regularly change their course because of hydrological variability and sedimentological readjustments (Agnihotri et al. 2020;Pal 2017, 2018;Saha and Pal 2019;Nawfee et al. 2018;Dewan et al. 2017). ...
The confluence of the Ganges, Jamuna, and Padma rivers is one of the most dynamic in the
world, an internationally important research area because of the confluence of two of the
world’s major rivers, the Ganges and the Brahmaputra. Morphological changes in this area
have resulted in severe erosion along the banks. Riverbank erosion is one of Bangladesh’s
most critical issues, requiring a substantial solution. Riverbank erosion affects millions of
people in Bangladesh each year because of erosion in this confluence zone. Consequently,
understanding the morphological shifting pattern of the confluence is crucial. The research
aims to quantify actual bank shifting near the confluence of the Ganges, Jamuna, and
Padma rivers in terms of shifting rate and area during a 25-year period (1990–2015). The
acquired satellite images were geo-referenced, and bank lines were digitized to carry out
this research. The bank line is the linear construction that separates the outside boundary
of the river channel from the floodplains. To assess channel width fluctuation, the distance
between the extreme borders of the left and right banks, including mid-channel sandbars,
was measured. This period is split into five stages, each lasting 5 years, to measure the
maturity of change. Furthermore, the long-term trend from 1972 to 2015 is qualitatively
discernible. Landsat satellite pictures were used to investigate this morphological change.
The study provides current and reliable information on the planform dynamics of the
Ganga–Jamuna confluence. This study will help design and implement drainage development
plans and erosion control techniques in this crucial confluence zone.
... Several works analysed the morphological responses of fluvial systems to reductions in sediment supply (e.g., Church, 1995;Graf, 2006;Surian & Rinaldi, 2003) and identified three main effects: bed-level incision, channel narrowing and changes in planform morphological configuration. Fewer studies have focused on anthropic interventioninduced alterations in sediment fluxes, likely because of the aforementioned limitations in sediment transport estimation (Arnaud et al., 2015;Preciso et al., 2012;Surian & Cisotto, 2007;Vázquez-Tarrío et al., 2019;. Even less quantification has been directed to these changes over expansive time and spatial scales (e.g., the last century and a large river network, respectively). ...
Knowledge about historical changes in sediment fluxes in most coarse‐bedded rivers worldwide is extremely limited. In consideration of this deficiency, we developed a width‐based approach to estimating multi‐decade changes in coarse sediment fluxes occurring at reaches of the Po River and 21 of its tributaries in northern Italy. The estimation was based on temporal variations in the reach‐averaged width of the river’s active channel, and such width was expressed through a dimensionless index of coarse bed material load (Iq). The index was determined in two periods: 1954 to 1998 and 1998 to 2020. Statistically significant relationships were found between temporal variations in Iq occurring in reaches of the Po River and at key locations of each specific reach (i.e. upstream reaches and tributaries). Such evidence of coherent changes in sediment transfer through space and time led us to conclude that Iq variations can be regarded as a reliable proxy for historical changes in sediment transport in a river reach. The application of the approach to the investigation of the Po River catchment provided new insights into the historical changes characterising coarse sediment fluxes along the river and its major tributaries. From 1954 to 1998, an average decrease in coarse sediment fluxes of about –20% and –30% occurred along the river and the terminal sectors of its tributaries, respectively. The estimations showed that coarse sediment fluxes exhibited a slightly lower decrease in the last two decades, with sediment flux recovery occurring only in some tributaries. The results suggest that a profound change in sediment dynamics and fluxes has occurred, and is likely still ongoing, in the Po River system, despite the decrease in human disturbances (e.g. in‐channel sediment mining) in more recent times.
... Finally, interactions between mobile sediment and an immobile armour layer caused concerns in the Old Rhine (Rest-Rhein, Vieux Rhin) at the border between France and Germany. Nourishment of sediment for ecological reasons (Die Moran et al., 2013;Arnaud et al., 2015Arnaud et al., , 2017Staentzel et al., 2018) was feared to trigger further river incision by breaking up the armour layer (Figs. 1 and 2), rendering flood retention polders along the river (e.g. Peters et al., 2001) ineffective. ...
Predicting the formation and break-up of immobile layers is of crucial importance for river management, as these processes greatly affect the morphodynamic evolution of the river bed. Two models are currently available for studying these processes: Struiksma's and Hirano's model. In this paper, we show that both models present limitations. This is done by numerical modelling of a laboratory experiment and two thought experiments. Struiksma's model does not predict break-up and Hirano's model yields unrealistic results when part of the sediment is immobile. We propose two alternatives that overcome these limitations: the ILSE and HANNEKE models. They differ in the interpretation of the top part of the bed interacting with the flow. Moreover, only the HANNEKE model explicitly predicts the formation of coarse layers, at the expenses of a more limited application range.
... In this chapter, we look specifically at river restoration and what constitutes a restored river (or landscape and waterscapes) in the context of the Waipā River. Over the last three decades, an ever-expanding and diverse body of scholarship on river restoration has emerged; including research from the fields of historical ecology (Beller et al. 2016;Bhatt et al. 2016;Kurashima et al. 2017;Stein et al. 2010), geomorphology (Abernethy andRutherfurd 1998;Arnaud et al. 2015;Jacobson et al. 2011), engineering (Palmer et al. 2014), environmental management (Bhatt et al. 2016;Morandi et al. 2014;Waltham et al. 2014). The emphasis remains placed on the need to address the degradation of places, ecosystems, or keystone species through targetted restoration efforts. ...
This book was inadvertently published with incorrect information with reference to The Waikato River Authority on pages 306, 307, 369 and 370. This text has now been revised and updated.
... There is a flourishing number of studies in the literature using such resources, with examples of characterisation of river corridors (Wiederkehr et al., 2010), assessment of catchment-scale alterations (Llena et al., 2020), and river rehabilitation planning (e.g. Arnaud et al., 2015;Large & Petts, 1996;Rollet et al., 2014). This source of information can also be used beyond the scope of planning. ...
Gravel augmentation has become common practice to mitigate the effects of decline in upstream sediment supply in gravel-bed rivers. However, the functional aspects of river systems are often left out of rehabilitation monitoring programmes. Despite temperature being a fundamental parameter determining the general health of rivers, a limited number of studies have tested whether gravel augmentation can aid restoring thermal functions. Using airborne thermal infrared (TIR) imagery, this paper explores potential positive outcomes through the monitoring of gravel augmentation actions on 3 rivers in France using hydro-morphological indicators within a trajectory-based Before-After Control-Impact (BACI) framework. This design, combining long-term geomorphic evolution with TIR-based CI strategy, indicated that restoring forms was not sufficient to restore thermal functions in their spatial dimension. Despite forms showing various degrees of recovery, the thermal regime of rehabilitated reaches (i.e. temperature gradient, cold-water patches density, etc.) failed to recover. Nonetheless, hydro-morphological indices can be used to estimate long-term evolution of groundwater-surface water interactions. We emphasise the benefits of trajectory-based BACI assessment to identify current conditions, understand the past evolution (trajectory) of the system to define the framework within which rehabilitation can be objectively assessed, especially when assessing hydrological (here thermal) processes. From a hydrological perspective, the gap between restored forms and restored thermal processes stems from the alteration of connectivity pathways, which potentially require more time (or efforts) to be recovered than forms do. With an increasing number of rehabilitation schemes and increasing pressure of global changes on rivers, we suggest that monitoring of water temperature becomes a routine part of rehabilitation projects.
This article is protected by copyright. All rights reserved.
... Therefore, increasing efforts are made to improve the hydromorphological structure of European rivers in response to the Water Framework Directive (WFD) (Arnaud et al., 2015;Campana et al., 2014;Wohl et al., 2015), and here floodplain restoration along braided Alpine rivers has high priority. Most restoration projects along Alpine rivers aim to improve the flow regime, the channel structures, or aquatic habitat conditions, while improving sediment fluxes is difficult (Habersack & Piégay, 2007;Kurth & Schirmer, 2014). ...
Braided reaches were common along near-natural Alpine rivers, and the associated habitat dynamics supported plant and animal species specialized on early-successional stages. The extensive riparian zones could mitigate climate change by absorbing floods and by retaining water during droughts. Human impacts largely reduced active river corridors through altered discharge and construction of dykes, while recent restoration projects aim at increasing river dynamics. The causes and consequences of Alpine river degradation are well understood, but there are only few quantitative studies on flood- plain degradation and restoration. Thus, we have reconstructed historical changes of gravel bars along five Alpine rivers (Iller, Inn, Isar, Lech, and Wertach) in Southern Germany in the period 1808–2009, based on historical maps and aerial images. We found losses of >90% in gravel bar area along these rivers since the mid-19th century. The decline was caused by a reduction of the active river corridor and by ongoing suc- cession of the remaining open habitats. Within the past 30 years, at the Isar River, res- toration measures were realized with the aim to widen the active river corridor and to recreate gravel bars. In four restored reaches, we found that 5% of the historical gravel bar area recovered, and that the proportion of restored gravel bar area was highest after intermediate flooding. We conclude that the active river corridors of German Alpine rivers are almost completely lost, and that more extensive restoration needs to be done to preserve the habitat dynamics and biodiversity of these systems, and to adapt Alpine rivers to climate change.
... Ce projet de recherche a permis de démontrer pour la première fois l'existence de sédiments hérités liés à l'aménagement d'un fleuve européen. Si ce caractère innovant constitue un des points forts de ce travail, la contrepartie est que, malgré la dimension spatiale ambitieuse de l'étude -avec ses trois sites d'études répartis le long du Rhône-, on peut légitimement se demander si ces sédiments hérités ne constituent pas un phénomène propre au Rhône plutôt qu'un processus géomorphologique à large échelle comme le sont les sédiments hérités « originaux » observés aux Arnaud et al., 2015). Il serait ainsi plus probable de retrouver des sédiments hérités caractérisés par des modifications typologiques analogues. ...
Si des interactions complexes entre les hommes et les rivières existent depuis plusieurs millénaires, les pressions anthropiques exercées sur les hydrosystèmes ont fortement augmenté au cours des deux derniers siècles. Il est donc essentiel de mieux appréhender l’évolution récente des corridors fluviaux en lien avec les activités humaines qui s’y sont développées, et l’investigation du compartiment sédimentaire est particulièrement appropriée pour ce type d’approche rétrospective. Cette thèse s’appuie sur l’étude du Rhône, un fleuve européen majeur soumis à de fortes pressions anthropiques, et s’intéresse particulièrement à deux de ces pressions : la contamination et l’aménagement du cours d’eau. Plus précisément, elle vise à 1) prouver l’existence de sédiments hérités des aménagements mis en place sur le Rhône, et les caractériser ; 2) développer une méthodologie pour l’interprétation d’archives sédimentaires issues d’un milieu fluvial fortement anthropisé, dans le cadre des contaminations historiques et émergentes ; 3) démontrer l’intérêt du concept de sédiments hérités dans un contexte européen et en lien avec la mise en place d’infrastructures fluviales. La méthodologie développée dans ce travail combine une analyse diachronique des sites d’étude, leur investigation à l’aide de sondages géophysiques et le prélèvement de carottes sédimentaires permettant la quantification de nombreux paramètres sédimentaires (e.g., granulométrie, carbone organique total, teneur en contaminants et radionucléides). Son implémentation a permis de démontrer l’existence de deux types de sédiments hérités sur le Rhône, en lien avec les deux phases d’aménagement auxquelles le fleuve a été soumis (correction du tracé et aménagements hydroélectriques). Dans les deux cas, les sédiments hérités résultent d’un processus de déconnexion entre le chenal principal et les environnements de dépôt concernés, et la granulométrie est le paramètre principal permettant de les distinguer des dépôts sous-jacents. Une évolution de la définition de sédiments hérités est donc proposée afin d’y inclure ces nouvelles caractéristiques. Il est également prouvé que la capacité d’enregistrement des sédiments fluviaux est fortement influencée par l’environnement de dépôt étudié et par les modifications de leurs conditions hydrodynamiques induites par les différents aménagements. La méthodologie proposée permet néanmoins d’interpréter de façon robuste ces archives et de reconstruire la trajectoire récente du Rhône et les tendances temporelles de contamination de polluants historiques (éléments métalliques, polychlorobiphényles) ou émergents (retardateurs de flammes bromés) à partir de carottes issues de divers environnements. Une telle approche rétrospective est essentielle afin de mieux appréhender l’ensemble des impacts anthropiques en lien avec les enjeux actuels de gestion des cours d’eau, et de proposer des politiques publiques et des mesures de suivi et/ou de restauration cohérentes avec l’évolution à long terme du fleuve.
... Spills occur on the Old Rhine River when the Rhine River discharge exceeds 1,400 m 3 /s in Basel, which is the maximum discharge capacity of the Grand Canal d'Alsace (GCA). The channel bottom of the study reach is composed mainly of gravel and cobble (Arnaud et al., 2015). The mean slope and mean width are equal to 0.09% and 100 m, respectively ( Figure 1b). ...
The quantification of the bed grain size distribution (GSD) of river surfaces is primarily conducted through manual approaches in the field. These methods are time consuming and not able to accurately represent the spatial diversity of the grain size distribution of rivers. Recently, several software programmes and procedures have been developed using semi-automatic and automatic methods to estimate bed GSD from digital imagery. The purpose of this study is to compare softwares accuracy between reference GSDs and estimated GSDs using geometric approches (Basegrain software and a procedure developed on ImageJ), statistical approaches (Digital Grain Size (DGS) and PebbleCounts softwares) and a machine learning framework (SediNet). This study evaluates ten digital images recorded along the Rhine River downstream of the city of Basel. The results showed that all software programmes considerably underestimated the manually measured GSDs. Nevertheless, it is possible to significantly improve the estimation of bed GSD by applying calibration laws. Both DGS and Basegrain softwares are reliable to estimate the GSD,while the three others softwares are accurate for percentiles equal and higher than the D50. After linear regression correction, the mean NRMSE of percentile errors did not exceed 13% for DGS and Basegrain software, while the others did not exceed 22% for percentiles coarser than the D50.
... Tremendous endeavors have been undertaken to investigate channel planform behaviour locally and worldwide over the last three decades (Bora and Goswami, 2021; Hasanuzzaman et al. 2021). Numerous geospatial technology-based studies have been conducted across the world, including in the United States on the four rivers of Olympic National park (East et al., 2017) , Taiwan on the Zhuoshui River and the Gaoping River (Kuo et al., 2017), Italy on the Scrivia river (Mandarino et al., 2020), Germany on the Old Rhine downstream (Arnaud et al., 2015), China on the lower yellow river (Kong et al., 2020;Guo et al., 2021) and the lower jingjiang reach (Yang et al., 2013), India on the Koshi river (Sinha et al., 2014), the Sharda river (Midha and Mathur, 2014), the Dwarkeswar river (Ghosh and Mukhopadhyay, 2021), the middle lower part of ganga , and the Ramganga river basin (Agnihotri et al., 2020), and Bangladesh on the lower padma river (Rashid, 2020; Nawfee et al., 2018;Halder et al., 2021), the rivers in southern estuarine Region , the Lower Meghna river (Mahmud et al., 2020), Madhumati river (Biswas et al., 2021), the lower Teesta river (Akhter et al., 2019), and the Brahmaputra river (Rashid et al., 2021). Lower sections of rivers regularly change course as a result of tectonic tilting, hydrological variability, and sedimentological readjustments (Agnihotri et . ...
The Ganges-Jamuna-Padma confluence is one of the world's most active confluences. The confluence of two of the world's greatest rivers, the Ganges and the Brahmaputra, makes this a globally significant site. Severe erosion along the banks has been caused by morphological changes in this region. Riverbank erosion is one of Bangladesh's most serious problems, as it necessitates costly intervention. Riverbank erosion in Bangladesh affects millions of people each year as a result of erosion in this confluence zone. As a result, it's critical to comprehend the confluence's morphological changing pattern. This study aims to quantify actual bank shifting around the confluence of the Ganges, Jamuna, and Padma in terms of shifting rate and area during a twenty-five-year period (1990-2015). To conduct this study the collected satellite image were geo-referenced and digitize bank lines from using ArcGIS program. The bank line is the linear structure that divides the river channel's outer border from the flood plains. The distance between the extreme margins of the left and right banks, including mid-channel sandbars, was measured to determine channel width variation. To assess the maturity of change, this time frame is subdivided into five phases, each lasting five years. In addition, the long-term shift from 1972 to 2015 is qualitatively noticeable. This morphological alteration was studied using LANDSAT satellite images. The research gives current and trustworthy information on the Ganga-Jamuna confluence's planform dynamics. This research will be useful in the planning and execution of drainage development plans and erosion control strategies in this critical confluence zone.
... Operation of reservoirs created by dams directly affects downstream reaches of the interrupted rivers by altering flow and the flood regime and reducing the amount and size of the flushed sediment (Depret et al., 2019;Jin et al., 2019;Kondolf et al., 2014;Ren et al., 2020). This effect migrates progressively downstream of the impoundment, leading to spatially (how far it might reach) and temporally (how long it may take) variable responses of channel morphology (Arnaud et al., 2015;Ayles & Church, 2014;Downs & Pi egay, 2019;Nelson et al., 2013). In the two upstream reaches (i.e., A and B) of the Lhasa River, the magnitude and variability of the natural flow regime after the closure of the first (Zhikong) dam were obviously reduced in terms of the data at the Tangjia station ( Figs. 1b and 6). ...
Among braided rivers developed on the Qinghai-Tibet Plateau of China at very high elevations (>3500 m), the middle and lower reaches of the Lhasa River have been affected by comprehensive human activities mainly involving dam construction, urbanization, farming, afforestation, and mining. In the current study, the impacts of these human activities on hydrology and morphology of the four braided reaches downstream of a cascaded of two dams are investigated. The study period was divided into 1985-2006 (P1), 2006-2013 (P2), and 2013-2019 (P3), representing the natural and changed flow regimes by dams. Using available daily discharge data at two stations within the four braided reaches, dam-induced hydrological alteration was analyzed based on the indicators of hydrologic alteration and range of variability approach and key discharge proxies were calculated. Remotely sensed images also were selected in the three periods and morphological metrics extracted from them were compared for the four reaches among these periods. Attenuated hydrological regimes were found for only two reaches. The total channel width (Wc) and braiding intensity (BIt) followed different temporal trends among the four reaches. Annual average shift rates of the main channel in the four reaches were higher in the short (P2-P3) than in the long (P1-P2 and P1-P3) periods. The longitudinal changes of Wc and the number of channels did not have any identifiable trend among the four reaches. By linking the morphological changes to quantified spatial and temporal patterns of various human activities, it was found that (1) the two dams had insignificant impact on channel morphology, suggesting that the studied braided river might have a short relaxation time and (2) the evolutional trajectories of morphological changes in most of the four reaches were similar, suggesting that temporal trends of morphological changes due to complex human activities are not affected by the different physiographic settings of the reaches. Continuous exploitation of the valley area requires comprehensive river management strategies for coordinating various human activities.
... In the light of the relationships existing between bed material transport and morphologies of alluvial rivers (Church, 2006), a sound understanding of river adjustment to anthropogenic disturbances can be addressed only through a space-time analysis of the whole "river morphodynamics," intended as the interaction between the processes that mobilize the streambed sediments, the bed characteristics, and the river morphologies (Church and Ferguson, 2015). Therefore, it is crucial to consider both the channel morphological adjustments, and the transport regime (i.e., sediment mobility and budgeting) (see for instance Surian and Cisotto, 2007;Arnaud et al., 2015;and Vázquez-Tarrío et al., 2019). Furthermore, the timing and rate of recovery of more natural conditions after the cessation of the impacts causing sediment starvation is still poorly understood. ...
The alteration of sediment transport regime represents one of the key factors controlling the response of fluvial systems to human impacts. Additional research efforts are required to define sound cause-effect relationships between the human pressures and the morphodynamic responses of sediment-starved gravel-bed rivers. This work deals with the evolution of the Parma River, a large Italian gravel-bed river affected over the last 160 yr by multiple anthropogenic disturbances (i.e., land-use changes, in-channel mining and, more recently, the construction of a detention basin), which strongly limited the sediment availability to the high-plain portion of its course. The data collected allowed us to reconstruct the history of the impacts, the river morphological evolutionary trajectory, the sedimentological characteristics of the bed, and to estimate, leveraging a morphological approach, the variation in coarse sediment regime over the most recent period. After the cessation of intense in-channel mining in 1990s, the study sector located upstream from the detention basin shifted from a degrading (i.e., narrowing and incising) to a recovery phase, characterized by a mean coarse transport of 15,000–31,000 m³yr⁻¹. These results demonstrate that the in-channel mining was the most impactful historical pressure acting on the river, and that a channel preserving some natural characteristics and passing from a condition of sediment-starvation to one of increasing sediment availability can recover an active morphodynamics within a short time period. The downstream sector was greatly affected by the reduction of sediment transfer that occurred in the 2000s caused by the construction of a detention basin. Its degradation continued over the most recent period and the coarse transport is currently very low (up to about 3000 m³yr⁻¹ 4 km downstream from the basin). This sector is characterized at the present by a morphodynamics defined as “slack”, a condition induced by the complete evacuation of the mobile channel sediments, leading to extremely low bed material mobility and transport, and, consequently, to morphological degradation up to the achievement of a new quasi-equilibrium. We hypothesized that such an extreme state of alteration is relatively limited in space, affecting a gravel-bed river only at the reach scale in response to multiple overlapped impacts or because of an extremely severe perturbation (i.e., reduction) of the sediment availability and dynamics.
... Series of maps, aerial images, topographic, and bathymetric surveys are generally selected at regular time steps, if available, or before and after major events (floods, river engineering phases) to assess changes in the river hydromorphological conditions. This set of archival materials is used to extract channel features such as active channel width, bar-island areas (Hohensinner et al., 2004(Hohensinner et al., , 2021Michalková et al., 2011;Scorpio et al., 2015;Lestel et al., 2018), bed degradation and floodplain aggradation rates (Steiger et al., 2001;Downs et al., 2013;Arnaud et al., 2015), riparian vegetation patterns (Cadol et al., 2011;Belletti et al., 2013;Kui et al., 2017;Safran et al., 2017), and landscape unit characteristics (Dufour et al., 2015;Solins et al., 2018;Piégay et al., 2020). Following Roux et al. (1989) and Habersack and Piégay (2008), comparative analyses of river changes, notably in Europe, have become a critical issue for addressing a series of questions of interest to geographers, historians, engineers, ecologists, geologists, and geochemists. ...
Space and time analyses of channel changes, especially within large rivers
subject to high levels of human impact, are critical to address multiple
questions about rivers in the Anthropocene era. The reconstruction of
long-term (> 150 year) evolutionary trajectories permits an
understanding of how natural and anthropogenic factors impact
hydromorphological and ecological processes in rivers, helps with the design
of sustainable management and restoration options, and may also help in the
assessment of future changes. However, the reconstruction of channel changes
can be challenging: historical data are often scattered across many
archives, and the quantity and accuracy of information generally decreases
as one goes back in time. This data article provides a historical database
of 350 cartographic and topo-bathymetric resources on the French Rhône
River (530 km in length) compiled from the 17th to mid-20th
century, with a temporal focus prior to extensive river training (1860s).
The data were collected in 14 national, regional, and departmental archive
services. A table describes the properties of each archived data item and
its associated iconographic files. Some of the historical maps are available
in a georeferenced format. A GIS layer enables one-click identification of
all archive data available for a given reach of the French Rhône River.
This database provides substantial new material for deeper analyses of
channel changes over a longer time period and at a finer time step compared
with previously available data. The database has several potential
applications in geomorphology, retrospective hydraulic modelling, historical
ecology, and river restoration, as well as permitting comparisons with other
multi-impacted rivers worldwide. The dataset is available at https://doi.org/10.1594/PANGAEA.922437 (Arnaud et al., 2020a). Iconographic extracts of the
350 archived items are available at http://photo.driihm.fr/index.php/category/52 (last access: 2 May 2021).
... -1 at Basel gauging station, which corresponds to the maximal capacity of the GCA power plants (Figure 1). This discharge was exceeded an average of 69 days per year between 1932 and 2011 (Arnaud et al., 2015a). This Rhine section exhibits poor ecological functionalities due to severe geomorphological alterations induced by successive engineering works: damming and bypassing, as well as channelization and construction of groin fields since the 19 th century (Piégay et al., 2011). ...
Over the last two centuries, rivers were strongly regulated, inducing notable sedimentstarvation and flow reduction. This regulation altered aquatic habitats, biocenosis andecosystem services. Gravel augmentation (GA) is increasingly promoted to restorebedload transport, rejuvenate bed sediments and diversify aquatic and riverinehabitats. However, practical questions remain in terms of efficiency and sustainability.The objective of this study is to provide feedback from three GA experimentsconducted along the Rhine River downstream of the Kembs dam (France/Germany).The restored sites were monitored over a time period from 1 to 7 years according tothe sites so that the stockpile deposits (SDs) and channel responses were comparedto highlight the strengths and limitations of such experiments. The monitoring wasbased on topo-bathymetry, bedload tracking and grain size surveys. Two-dimensionalhydraulic modeling was also performed to link the SD erosional patterns to critical flowconditions. We show that (i) SD erosion is dependent on the grain size of theintroduced sediments and the local flow velocities, (ii) sediment dispersion occurredmostly along the thalweg and increased over time, with a progressive decrease in thevirtual velocities, (iii) bathymetric simplification and bed grain-size fining were observedwhen the downstream sediment diffusion began, but (iv) at mid-term (~ 5 years) localchannel diversifications occurred due to the fragmentation of the sediment waves withdeposition occurring preferentially on riffles, and (v) new sediment starvation conditionsappeared when the sediment waves travelled downstream in the absence of anupstream sediment supply. The results allow us to propose operationalrecommendations and management scenario to improve both the efficiency and thesustainability of GAs conducted in large regulated rivers.
... The river naturally controls the build-up of residue in it. But sometimes some human activities, such as building bridges, extracting sand from the stream in an unscientific way, upsets the nature of the natural storage of river sediments (Arnaud et al. 2015). If the accumulation of river sediment is artificially treated, it affects the river flow pattern, channel adjustment, and riverbank stability. ...
Sediment deposition is a significant feature of a dynamic floodplain river. As the river's carrying capacity decreases, it is forced to deposit sediment on the river bed. However, due to the construction of some hydraulic structures over the river bed at different times (bridges, dams), the velocity of the river in the floodplain area decreases with accelerating sedimentation process, which hinders water transport. The sub-catchment basin (Nabadwip-Kalyani stretch) of the Bhagirathi–Hooghly river in West Bengal (Ghosh et al. 2020), India is frequently used as a means of water transport by local stakeholders. Because of sedimentation, the thalweg depth of the river is constantly decreasing in Nabadwip–Kalyani stretch and the water transport is being disrupted. Therefore, it is very important to know the function of hydraulic structures over the river bathymetry and resultant sediment flow, transportation, and sediment deposition in this region. Therefore, this article mainly assesses to develop a 1D hydro-dynamic sediment transport model on river morphological change through seasonal quasi-unsteady flow accumulation and sediment budget in a given year (2018). The hydro-dynamic model is calibrated by 1D HEC-RAS v 5.0.7 software (Gibson et al. 2017) based on seasonal quasi-unsteady flow using various empirical equations and Manning's roughness coefficient and validated with R2, NSE, and RSR. The model also examines the role of hydrodynamic structures in scouring and sedimentation due to the location of these structures above the river. This model output will help local stakeholders to understand the amount of sediment in the river and keep river transport viable in all seasons.
... However, when suitably interpreted, sedimentary sequences may allow to reconstruct the evolution of a river corridor in correlation with the anthropogenic pressures it was subject to throughout time. Such an understanding is essential to implement relevant restoration measures and mitigate further anthropogenic impact on the river system (Wohl et al., 2005;Arnaud et al., 2015;Słowik, 2015;Eschbach et al., 2018). ...
In this study, we explore the variability of sedimentation conditions (e.g., grain-size, accumulation rate, contamination) according to fluvial depositional environments. Indeed, sediment cores are commonly used as archives of natural and anthropogenic activities in hydrosystems, but their interpretation is often complex, especially in a fluvial context where many factors may affect the quality, continuity, and resolution of the record. It is therefore critical to thoroughly understand the nature and dynamics of an environment in which a sediment core is sampled to be able to interpret it. To that end, four depositional environments from a bypassed reach of the Rhône River were comparatively investigated through geophysics in order to assess the range of sedimentation conditions: a floodplain, a semi-active secondary channel, an active secondary channel, and a dam reservoir. Sediment cores were retrieved from each environment and thoroughly characterised (e.g., grain-size, Total Organic Carbon, organic contaminants). Robust age-depth models were elaborated for each core based on 137Cs, 210Pbex, and Persistent Organic Pollutants (POPs) trends. The results show that each depositional environment recorded a different time-period, and therefore different contamination levels and trends. In particular, a shift from polychlorinated biphenyls (PCBs) to polybrominated diphenyl ethers (PBDEs) as the predominant POP in the sediments can be observed, the tipping point being set in the 1970s. Two types of infrastructure-induced legacy sediments related to two periods of river engineering in the reach were also identified using grain-size analysis. The combination of geophysical methods (Ground Penetrating Radar) and sediment cores is therefore confirmed as a relevant methodology that should be promoted in fluvial contexts in order to reconstruct the sedimentary evolution of fluvial corridors. The study also highlights the challenges of dating recent fluvial sediments and proposes a multi-proxy dating methodology using POPs contamination trends.
... https://doi.org/10.1111/j.1365-2427.2009.02281.x Arle, J., Claussen, U., Müller, P., 2011 Arnaud, F., Piégay, H., Schmitt, L., Rollet, A.J., Ferrier, V., Béal, D., 2015. Historical geomorphic analysis (1932 Aubry, P., Piegay, H., 2001. ...
Les données issues de la surveillance des rivières sont volumineuses, avec des relations complexes. Des méthodes de fouille de données non supervisées peuvent s’y appliquer et donner des résultats pertinents pour leur gestion, sous réserve d’une collaboration étroite entre hydroécologues et informaticiens. L’extraction de motifs partiellement ordonnés à partir de séquences temporelles de pressions physico-chimiques précédant un état biologique a été réalisée. Ces motifs temporels permettent d’identifier une partie des pressions en cause dans un état écologique dégradé ou non, de préciser l’importance de la durée des séquences avant l’évaluation de l’état biologique, d’identifier les altérations caractéristiques à l’échelle d’une hydro-écorégion. A terme nous envisageons d’élargir ces motifs aux pressions hydromorphologiques.
The combined analysis of past evolutionary trajectories of channel morphology and temporal patterns of driving factors is fundamental to understanding present river conditions, supporting river management and evaluating future changes. Rivers in Europe underwent important channel changes during the Anthropocene in response to changing natural drivers and anthropogenic pressures. A considerable number of papers have been published on this topic, in the last decades. In this study, a comprehensive meta‐analysis on channel changes during the last 200 years in Europe was performed, aiming to provide quantitative information on the intensity of changes, to highlight regional scale similarities and dissimilarities in evolutionary morphological trajectories and to discuss the main causes of such changes. Based on a review, 102 papers were selected, addressing 145 channel reaches flowing through five main mountain ranges (Iberians, Alps, Apennines, Balkans and Carpathians) in the southern and eastern parts of Europe. The results show that active channel narrowing (between 26% and 36% on average) and incision (between 1 and 2 m) prevailed in most rivers between the 1800s and the 1950s, although widening was documented in some rivers of the Alps and the Apennines. Most multi‐thread reaches maintained their pattern until the mid‐20th century. Active channel changes accelerated during the 1950s–1990s (or 2000s) period, with channel narrowing up to 60% and channel incision up to 14 m. Multi‐thread patterns strongly decreased in frequency, with anabranching channels disappearing and single‐thread patterns becoming predominant. The cumulative effect of multiple and concomitant human pressures (gravel mining, channelisation and damming) was identified as the main driving factor for these accelerated changes. These findings must feed the public debate about preventing alterations of river ecosystems—exerted by anthropic disturbances—in a context of rapid economic development, especially in river systems still poorly altered and thus preserving wide, active and heterogeneous fluvial corridors.
Methods for microplastic analysis in marine matrices, microplastic exposureexperiments and microplastic degradation experiments, as developed, optimized and/or applied in the JPI Oceans Project Andromeda.
Background
Climate change and increasing anthropogenic pressure are two of the major drivers of increasing extreme events like droughts and floods. To deal with the increasing number of flooding events hitting Europe in the last few decades, around twenty years ago the European Commission started to develop ad-hoc legislation to reduce flood risk by mapping flood hazard and risk areas, such as the Directive 2007/60/EC on the Assessment and Management of Flood Risk. This Directive looks to identify regions where flood management strategies should be prioritized. Despite this holistic approach, flaws connected to the consideration of sediment transport and morphological changes in rivers exist, leading to potential underestimations of the impact of floods affecting active watercourses or areas subjected to frequent morphological changes.
Results
By discussing six examples related to European lowland and mountain watercourses affected by significant floods in the last 20 years, the present mini-review aims to provide additional evidence on the need for a rethinking of flood risk mapping, moving from a “clear water” perspective to a more integrated approach, where the interactions between all the fluvial components (water, sediment, biota, and humans) are adequately considered.
Conclusions
The examples reported here show the importance of considering sediment and wood in flood risk management, suggesting the need for integrating flood-related studies with other disciplines like geomorphology and ecohydrology.
The peninsular bound rivers in the state of Bihar join the median principal drainage, River Ganga along its south bank flowing from south to north. These emanate mostly from the rocky uplands of the Chotanagpur Gneissic Complex, a few from the Vindhyans, Bihar Mica Belt, Gondwanas and other geological formations. Amongst them, River Falgu has its emergence from the Chotanagpur Gneissic Complex and is a promising repository for sand used for construction purposes. Understandably, there is a composite licence awarded by the state government to harness the sand potential of the river along its fertile stretches which is sometimes unchecked from the point of view of mining complying with the requisites of existing guidelines. The region which experiences hot and humid climate conditions has suffered irreversible vagaries in the past few decades with a conspicuous decrease in the overall precipitation, thus influencing the groundwater conditions. The recent past is replete with climatic surprises from unexpected rainfall in a couple of seasons to intimidating drought-like conditions in others. Undoubtedly, these events cannot be segregated from short-term climatic changes which are now affecting different parts of the country with a certain amount of regularity. This has influenced the sedimentation pattern and carrying capacity of the river. The aspect of sedimentation in a stretch of one of the peninsular bound rivers viz. Phalgu, originating from Chotanagpur Gneissic Complex (CGC) and debouching into the River Ganga near Patna district, forms the soul of the present study in light of fluctuation in sedimentation pattern, anthropogenic interference (particularly aberrations in sand mining), the effect of cross drainage structures (viz. barrage, bridge, weir) and diversion of the streamflow for various irrigational purposes which together or in isolation have greatly influenced the ecological health of the river. The study has been carried out on 11 no Sand Mining ‘Ghats’ officially identified as lease areas for minor minerals in Jehanabad district, Bihar along the course of River Phalgu. Studies reveal that anthropogenic activities are significantly affecting the river bed morphology by influencing sediment supply and transportation in the river basin. Transportation of sediment is the predominant work of the river through various hydrological processes such as a solution, suspension, saltation and traction under highly fluctuating hydrodynamic conditions, but decreased precipitation has affected the carrying capacity of the river. This chapter deals with the tight rope walking in arriving at the best possible balance resulting from environmental safety guideline procedures involved in mining, safeguarding the ecology of the river amidst challenges and ways to minimize the unprecedented effects of short-term climatic change. It also highlights aspects like drivers responsible for the reduced flow, erratic modes of sand deposition vis-a-vis undisciplined extraction and recommendations towards sustainable exploitation in tune with the expectation and guidelines served by the Hon’ble National Green Tribunal in this case along Phalgu River, in Jehanabad district, Bihar.
This study presents a method to generate historical orthomosaics using Structure-from-Motion (SfM ) photogrammetry, historical aerial photographs, and lidar data, and then analyzes the horizontal accuracy and factors that can affect the quality of historical orthoimagery products made with these approaches. Two sets of historical aerial photographs (1934 and 1951) were analyzed, focused on the town of Woodstock in Connecticut, U.S.A. Ground control points (GCPs) for georeferencing were obtained by overlaying multiple data sets, including lidar elevation data and derivative hillshades, and recent orthoimagery. Root-Mean-Square Error values of check points (CPs ) for 1934 and 1951 orthomosaics without extreme outliers are 0.83 m and 1.37 m, respectively. Results indicate that orthomosaics can be used for standard mapping and geographic information systems (GIS ) work according to the ASPRS 1990 accuracy standard. In addition, results emphasize that three main factors can affect the horizontal accuracy of orthomosaics: (1) types of CPs, (2) the number of tied photos, and (3) terrain.
Reconstructing evolutionary trajectories of river systems gives valuable insights into the main drivers (i.e. hydro-climatic versus anthropogenic) of their historical morphodynamics. Understanding past and modern adjustments of hydrosystems is therefore a key for appropriate sustainable management schemes. In this respect, the historical evolution (~150 years) of a 6 km dynamic reach of an Upper Rhine sub-tributary, the Bruche River, is thoroughly studied using a wide array of planimetric and topographic data (ancient maps and longitudinal profile, orthophotos, LiDAR). The primary aim is to precisely quantify the lateral and vertical mobility, taking uncertainties into account. Lateral mobility is studied using a specifically designed indicator, the Aggregated Migration rate Index (AMI), which enables precise assessment of planform changes in m.yr-1 on elementary sub-reaches (~100 m). A twofold decrease in mean lateral mobility in the mid-20 th century, most probably related to functional channel modifications associated with the installation of an underground pipeline, represents the most striking result. Vertical mobility is analysed through the comparison of diachronic LiDAR data (decadal timescale) and historical archive (secular timescale). From the mid-20 th century onwards, an unexpected general aggradation of the riverbed (~0.6 m) has occurred, for which several explanations are discussed (i.e. bank erosion, deforestation, sediment release following weir lowering), though remaining speculative. Overall, our results show that historical geomorphological adjustments are primarily controlled by various anthropogenic factors. As the Bruche hosts one of the highest concentrations of spawning beds of emblematic fish species in the French Upper Rhine, this study lays the foundation for future restoration strategies.
Pressure on large fluvial lowlands has increased tremendously during the past twenty years because of flood control, urbanization, and increased dependence upon floodplains and deltas for food production. This book examines human impacts on lowland rivers, and discusses how these changes affect different types of riverine environments and flood processes. Surveying a global range of large rivers, it provides a primary focus on the lower Rhine River in the Netherlands and the Lower Mississippi River in Louisiana. A particular focus of the book is on geo-engineering, which is described in a straight-forward writing style that is accessible to a broad audience of advanced students, researchers, and practitioners in global environmental change, fluvial geomorphology and sedimentology, and flood and water management.
The construction of the Karkheh Dam is an important factor contributing to the occurrence of dust storms in the Khuzestan Province of Iran. It has reduced the annual mean flow discharge in the Karkheh River from 120 to 50 m3/s and dried land around the river. The area of dried land is 90.17 km2 around the river and 333.45 km2 in the Hawr-al-Azim wetland. The Rosgen method, Fluvial-12 software, and Shulits equation demonstrate the instability of the plan, cross sections and longitudinal slope of the river, respectively, around Pay-e-pol hydrometric station (upstream of the river). After dam construction, extreme erosion occurred in this part of the river. The type of sediment is clay and silt with D50 = 8 μm. The eroded sediment settles downstream (around Hamidiyeh hydrometric station) and the Hawr-al-Azim wetland. The wind can easily lift these particles especially from May to July. Because of the size of these particles, the haze concentration increased from 25% to 45% in dust storms. After construction of the dam, the dust storm days increased to 90 days in 2008. By increasing the stability of the river, the dust storms reduced from 2011. The annual volume of generated haze by geomorphological characteristic changes is almost 3107 m3. HIGHLIGHTS
Identifies a relationship between haze generation and changes in river morphological characteristics.;
Identifies the difference between haze and dust storms.;
Identifies the effects of dam construction on river morphological characteristics.;
Identifies the effects of dam construction on haze generation.;
Identifies the effects of river topographic instability on haze generation.;
Remote data have frequently been used in fluvial geomorphology to observe landforms and derive geomorphological data such as water depth, grain size, and digital elevation models. This article reviews the types and historical development of remote data used in fluvial geomorphology and introduces examples of recent applications. The focus is on raw remote data and their derivatives, and both classic and recent techniques. Further consideration is given to current problems with the use of remote data in fluvial geomorphology and related future perspectives.
We argue that it is important to acknowledge that river restoration (both in theory and practice) still remains largely located within the realm of the hegemonic Western knowledge systems. In this chapter we challenge the Eurocentrism of dominant ecological restoration projects by documenting the different framing and approaches to restoration being employed by Māori (the Indigenous of Aotearoa New Zealand). We focus our attention on the collective efforts of one tribal group (Ngāti Maniapoto) who are working to decolonise how their ancestral river is managed and restored through the use of Indigenous Knowledge, augmented by Western scientific techniques. A key focus is on restoration that is underpinned by the principle of kaitiakitanga (environmental guardianship) and devoted to healing fractured relationships between humans and more-than-humans.
Many terrestrial ecosystems have undergone profound transformation under the pressure of multiple human stressors. This may have oriented altered ecosystems toward transient or new states. Understanding how these cumulative impacts influence ecosystem functions, services and ecological trajectories is therefore essential to defining effective restoration strategies. This is particularly the case in riverine ecosystems, where the profound alteration of natural disturbance regimes can make the effectiveness of restoration operations questionable. Using the case study of legacy dike fields, i.e., area delimited by longitudinal and lateral dikes, along the regulated Rhône River, we studied the impacts of long-term channelization and flow regulation on environmental conditions and riparian forests attributes along a 200km climatic gradient. We characterized the imprint of human stressors on these forests by comparing the dike field stands to more natural stands in both young and mature vegetation stages. Across four reaches of the river between Lyon and the Mediterranean Sea, we found that channelization consistently promoted high rate of overbank sedimentation and rapid disconnection of dike field surfaces from the channel. The rapid terrestrialisation of dike field surfaces, i.e., the process by which former aquatic areas transition to a terrestrial ecosystem as a result of dewatering or sedimentation, fostered a pulse of riparian forest regeneration in these resource-rich environments that differs from more natural sites in structure and composition. Within the dike fields, older pre-dam stands are dominated by post-pioneer and exotic species, and post-dam stands support large, aging pioneer trees with a largely exotic understory regeneration layer. These patterns were associated with differences in the relative surface elevation among dike fields, whereas species shifts generally followed the river's longitudinal climate gradient. To enhance the functionality of these human-made ecosystems, restoration strategies should target the reconnection of dike fields to the river by dismantling part of the dikes to promote lateral erosion, forest initiation and community succession, as well as increasing minimum flows in channels to improve connection with groundwater. However, since a river-wide return to a pre-disturbance state is very unlikely, a pragmatic approach should be favoured, focusing on local actions that can improve abiotic and biotic function, and ultimately enhancing ecosystem services such biodiversity, habitat, and recreation opportunities.
Share Link:
https://authors.elsevier.com/a/1c4y314Z6tekgX
Riječni su sustavi diljem Europe od kraja 19. stoljeća vidljivo izmijenjeni zbog antropogenih utjecaja i klimatskih promjena. Predmet su ovoga istraživanja geomorfološke promjene korita rijeke Cetine tijekom posljednjih 150 godina. Na temelju povijesnih karata, arhivskih aerofotogrametrijskih snimaka i suvremenih ortofoto-karata provedena je detaljna digitalizacija korita, sprudova i riječnih otoka. Rijeka je podijeljena na 15 segmenata u kojima je analizirana promjena širine aktivnog korita te pojavnost sprudova i otoka. Posebno su analizirane promjene složenih korita u izvorišnom području, obilježja doline koja je danas potopljena ispod Perućkog jezera i promjene ušća. U razdoblju do sredine 20. stoljeća zabilježeno je djelomično sužavanje korita uz isušivanje močvarnih zona u izvorišnom području, vjerojatno zbog smanjene vlažnosti nakon završetka malog ledenog doba, te povećanje površine sprudova i progradacija delte zbog pojačanog donosa sedimenta uslijed poljoprivredne intenzifikacije. U posljednjih pedeset godina na rijeci je izgrađeno pet hidroelektrana, čiji je rad dodatno uzrokovao sužavanje korita za oko 50% na trećini duljine rijeke. Smanjenje površine sprudova od 85%, osim uz izgradnju brana, povezano je i s antropogeno uzrokovanim promjenama u vegetacijskom pokrovu. Rezultati ovog rada dakle pokazuju utjecaj ljudskog djelovanja na degradaciju riječnog korita, ali predstavljaju i polazišnu točku u potencijalnom planu obnove rijeke.
The present study explores the evolutionary trajectory of the glacier-fed Mareit River (South Tyrol, Italian Alps), where
a large restoration programme was implemented in 2008–2009. River corridor changes before and after the restoration works were
assessed using historical maps, recent field observations, topographic surveys and topographic differencing. Trends of anthropic (forest cover, channel works, gravel mining) and natural (glacial cover, precipitation, flow regime) factors controlling channel morphology– at both catchment and reach scales – were reconstructed. From the mid-19th century, the evolutionary trajectory of the Mareit River followed a degradational trend, characterized by channel narrowing, bed incision and planform simplification. Direct,
in-channel human alterations – mainly in the form of bank protections (in the late 19th century), gravel mining (mostly in the 1970s) and grade-control works (since the 1980s) – dominated the historical adjustments before the restoration. In 2008–2009, a segment
of the Mareit was restored by widening the channel, partly removing the check-dams and shaping a braided pattern within a
laterally constrained corridor. Post-work monitoring shows that the restoration improved both the morphological quality and the geomorphic diversity. At present, the channel is subject to narrowing and slight bed level incision, with islands and floodplains progressively expanding at the expenses of the active channel. This trend is likely to continue in the next decades based on the expected
future flow regime, and indeed the Mareit River seems to be attaining a ‘miniaturized’ version of the anabranching pattern of the mid-19th century. Overall, this restoration approach and the associated evolutionary trajectory is considered positive, because it leads to a complex mosaic of geomorphic units, dynamically self-adjusting to the time-varying driving variables. The formation of a morphodynamically active corridor, while keeping artificially non-erodible boundaries, represents an optimal strategy to integrate
ecological improvements with flood risk mitigation in the densely populated Alpine valleys.
The Seine River basin (65,000 km²) is extremely rich in cartographic documents generated over the past two centuries: general maps describing the territory, fiscal land registries, navigation charts (e.g. bathymetric profiles and maps), etc. After 1830 river engineers (Ponts et Chaussées) started to develop a huge network of waterways, which were charted with precision and accuracy. These documents, retrieved from various archives, have been checked, selected, geo-referenced and digitalised within an open-access database (ArchiSeine). It has allowed researchers to fully quantify the state of rivers, often in their lateral, longitudinal and vertical dimensions, their long-term and slow natural dynamics (e.g. meander movement) and their abrupt modifications by man-made river works due to various and evolving river use (water supply, wood rafting, navigation, hydropower, sand extraction, flood protection), all closely connected to Paris growing demands, and the adjustments of the fluvial system to these changes. From headwaters to the estuary, the physical attributes of the Seine River system have been substantially modified. Examples of such environmental trajectories are provided for the Versailles plateau headwaters, the Bassée alluvial plain and the Lower Seine sector.
The Upper Rhine River has undergone several anthropogenic modifications since the last two centuries for flood protection, navigation and hydropower generation. Channel rectification, groyne fields and lateral "Grand Canal of Alsace" construction heavily altered the hydro-sedimentary functioning of the 50 km-long "Old Rhine" by-passed between Kembs and Breisach: channel pattern simplification, bed incision, armouring and ecological habitats alterations. Two complementary projects have been initiated to restore the sediment transport and the reach alluvial dynamics. This thesis investigated space-time and experimental geomorphic approaches within the morpho-ecological restoration of a modified fluvial system. The archive and field data-based work aimed at understanding the Old Rhine historical trajectory. The geomorphological monitoring of a gravel input experiment also consisted in evaluating environmental benefits and potential risks for society and proposing physical assessment indicators. Results will help river stakeholders to define strategies to restore morphological processes and associated ecological functions in the Old Rhine.
The Upper Rhine between France and Germany has been heavily impacted by channelization for navigation
and then by dams works during the last two centuries. Most of the flows are now conveyed in a canalized
section, a minimum discharge been reserved in the old section, so called the “Old Rhine”. Between Huningue and Neuf-Breisach (45 km), the longest disconnected section, engineering works induced simplification and stabilization of the channel pattern from a formerly braiding reach to a single incised channel (incision of 7m between 184 and 1920 before Kembs dam construction), hydrological modifications, channel bottom armouring due to bedload decrease, and thus ecological alterations.
Within the framework of current research works on river restoration, a new international and interdisciplinary
project on the Upper Rhine entitled “INTERREG IV - Redynamisation of the Old Rhine” began in early
2009 running for four years. The purpose of this project is to evaluate the feasibility of an important hydromorphological and ecological restoration plan on the 45 km reach, expanding and gathering some independent national projects, by a closed partnership between French and German scientists, water management organisations and practitioners. The finality of the project is to provide possible prospective scenarios, in order to partially restore a braiding channel pattern on the Old Rhine channel, by promoting left bank erosion, channel enlargement and/or artificial sediment inputs. The Alsace Region will conduct the project administratively, and the scientific group will be coordinated by the University of Lyon (UMR 5600 CNRS). The study will involve historical, hydro-morphological, ecological and sociological parts. On one hand a historical approach, led by UMR 5600 and Electricité de France (EDF), will be necessary to understand long term channel changes since 1800 and thus to orientate restoration strategies. This task will be based on old maps, aerial photographs and topographical data analysis using a GIS. On the other hand, a hydro-morphological approach will be performed to determine the equilibrium between the sediment transport capacity of the channel and the sediment volume to be introduced in order to restore the morphological dynamics without increasing overflows. It will consist (i) in in situ experiments (controlled bank erosion conducted by EDF and artificial sediment introduction, and field monitoring by UMR 5600) and (ii) in hydro-dynamic modelling (LWI laboratory and the Cemagref). Lastly, an ecological approach (University of Strasbourg and environmental organisations – Conservatoire des Sites Alsaciens and Association de la Petite Camargue Alsacienne) will evaluate gravel introduction benefits on the different components of the ecosystem. The compromise of conciliation between social uses and ecological issues will also be evaluated using a sociological approach (University of Strasbourg). Results of these complementary approaches will permit to propose large scale and long term innovative, international and inter-disciplinary restoration scenarios, on the large scale of one of the largest rivers in Europe, over a twenty year period.
The European Water Framework Directive (2000/60/CE) requires to achieve the good ecological status of
bodies of surface water by 2015. Growing efforts have thus been made in last decades for restoring physical processes and associated ecological habitats of altered river reaches. Retrospective and experimental fluvial geomorphology should be used to help to design restoration practices acting on the physical component of rivers. Such approaches have been investigated recently on a reach of the Upper Rhine (French‑German border). During the last two centuries, channel rectification, groyne fields, damming and lateral canal construction heavily impacted the hydro‑sedimentary functioning of the 50 km‑long “Old Rhine” by‑passed between Kembs and Breisach, causing channel pattern simplification, bed incision, armouring and riverine and aquatic habitats alteration. This paper reports a geomorphic analysis realized within two complementary and interdisciplinary programs to evaluate the feasibility for restoring sediment transport and alluvial morphodynamics: 1) the European program “INTERREG Redynamization of the Old Rhine” (2009‑2012); 2) the program “sediment reintroduction from controlled bank erosion” initiated by Electricity de France (EDF) within the Kembs hydropower plant relicensing process since 2003. On one hand, a space‑time analysis compiling archive and field data over the period 1828‑2012 was aimed at better understanding the Old Rhine evolutionary trajectory and estimating the sensitivity to channel change. On the other hand, an experimental sediment reintroduction from an artificial input operation into the channel was monitored in situ with bathymetric and topographic surveys, bedload tracing, aerial imagery and grain size sampling, in order to assess first morpho‑ecological effects, potential risks and test evaluation indicators. Recommendations for defining large‑scale and sustainable restoration plans were discussed following diachronic and experimental approaches.
We evaluate three metrics representing the drivers of channel change downstream from dams. A balance between changes in sediment supply and transport capacity identifies conditions of sediment deficit or surplus. A Shields number represents the competence of postdam flows and the potential for incision under conditions of sediment deficit. A ratio of postdam to predam flood discharge provides a metric for the scale and rate of channel change, especially width. The metrics are calculated for more than 4000 km of some of the major rivers in the western United States. More than 60% of these rivers are in sediment deficit, and only a few reaches are in sediment surplus. The sediment balance can be used to assess the relative effort involved in reversing undesired conditions of deficit or surplus.
Les amenagements de Gabcikovo-Nagymaros, sur le secteur hongrois et slovaque du Danube, comprennent de facon classique : un reservoir (Hrusov), une derivation dotee d'un equipement pour la navigation et la production d'energie (Gabcikovo), un ouvrage de compensation aval (Nagymaros). Ces amenagements en cours de realisation ont ete precedes par une phase ď endiguement (XVe-XIXe siecle) et de correction fluviale du type Girardon (XIXe-XXe siecle) qui ont permis de stabiliser le cours du fleuve (correspondant a un cone de dejection bâti sur une rupture de pente a la sortie du secteur alpin) et d'ameliorer les conditions de la navigation. Ces anciens amenagements ne permettent pourtant pas le poussage, de sorte qu'a defaut des amenagements actuels, le secteur interesse introduirait une cesure dans un systeme etendu a l'Europe rhenane, danubienne et peut-etre polonaise. Face a ces interets d'ordre international, l'opinion hongroise reste sensible aux problemes de frontiere et a la qualite du milieu fluvial en amont de la capitale. Celui-ci connaitra une degradation sensible par modification des conditions de l'ecoulement fluvial, disparition d'une foret alluviale de grande valeur ecologique, reduction de la biomasse fluviale et degradation de la qualite des eaux.
The 346km of the middle Ebro River between Logroño and La Zaida is a free meandering channel in a wide floodplain. This reach contains a discontinuous riparian corridor, including valuable riparian forests and oxbow lakes. The Ebro has witnessed substantial changes in channel morphology, gravel bars, riparian vegetation and floodplain uses over the last 80years. The growth in sinuosity, migrations and meander cut-offs have been frequent before 1981. Afterwards, bank protections and dykes have stabilized the channel. There has been a progressive and significant decrease of both the area covered by water and the gravel bars without plant colonization. As a result the width of the riparian corridor has been dramatically reduced for human use. The deceleration and near elimination of the free meander dynamics of the Ebro channel represent an important loss of natural heritage. Dams, land-use changes throughout the basin, and construction of flood defences that restrict the main channel have changed the river system behaviour, which urgently needs a management plan combining both improvement and risk reduction. The solution proposed is the creation of a “Fluvial Territory”.
Small hydropower plants (SHP) affect river flow and sediment transport and thus impact river morphology. Eight hydropower schemes were studied along the meandering middle and lower reaches of Aragón River (Spain) to assess their effects on channel morphology and sediment dynamics from 1927 to 2010. GIS tools were used to measure changes in fluvial surfaces, channel planform and lateral and vertical dynamics. Three periods (early, middle and late twentieth century) were analysed to discern the effects of the main pressures, such as changes in land use, large reservoirs upstream and SHPs. Results were combined with field and topographical measurements and hydrological analysis. Active channel width and channel migration suffered a clear reduction in the whole period. They started as a consequence of land cover changes in the drainage basin, but their speed increased after a large reservoir was built upstream. More recent changes occurred since most of the SHPs were put into operation in the 1990s, especially in their short-circuited reaches and in the four more downstream ones. These changes are interpreted as a consequence of reduced discharge, transitory sediment trapping and reactivation of sediment transport after weirs became filled as well as by the impact of flood hydrology.
Floodplain development is associated with lateral accretion along stable channel geometry. Along shifting rivers, the floodplain sedimentation is more complex because of changes in channel position but also cutoff channel presence, which exhibit specific overflow patterns. In this contribution, the spatial and temporal variability of sedimentation rates in cutoff channel infill deposits is related to channel changes of a shifting gravel bed river (Ain River, France). The sedimentation rates estimated from dendrogeomorphic analysis are compared between and within 14 cutoff channel infills. Detailed analyses along a single channel infill are performed to assess changes in the sedimentation rates through time by analyzing activity profiles of the fallout radionuclides 137Cs and unsupported 210Pb. Sedimentation rates are also compared within the channel infills with rates in other plots located in the adjacent floodplain. Sedimentation rates range between 0.65 and 2.4 cm a-1 over a period of 10 to 40 years. The data provide additional information on the role of distance from the bank, overbank flow frequency, and channel geometry in controlling the sedimentation rate. Channel infills, lower than adjacent floodplains, exhibit higher sedimentation rates and convey overbank sediment farther away within the floodplain. Additionally, channel degradation, aggradation, and bank erosion, which reduce or increase the distance between the main channel and the cutoff channel aquatic zone, affect local overbank flow magnitude and frequency and therefore sedimentation rates, thereby creating a complex mosaic of sedimentation zones within the floodplain and along the cutoff channel infills. Last, the dendrogeomorphic and 137Cs approaches are cross validated for estimating the sedimentation rate within a channel infill.
The 350-km-long Austrian Danube river section has been changed dramatically by channelisation in the 19th century and by hydropower plant construction since the 1950s. These have drastically reduced the hydrological connectivity and flood retention capacity of the Danube river landscape. Restoration measures currently under discussion for the Machland floodplain (river-km 2094-2084) aim to re-establish lateral connectivity in floodplain areas that are hydrologically separated from the main channel. This would also help increase the flood retention capacity. In this discussion process, the analysis of historical records provides useful baseline data for estimating natural reference conditions prior to channelisation and improves the evaluation of the effectiveness of selected restoration measures. Here, surveys from 1812 were used to generate a digital terrain model of the former undisturbed river landscape. The present case study of the Danube River proves (1) that comprehensive historical records can quantitatively document the physical environment of an entire river landscape based on data analysis and modelling and (2) that historical analyses can yield valuable reference data for assessing the restoration potential. This approach also provides essential baseline data to evaluate the abiotic and biotic reference conditions according to the specifications of the EU Water Framework Directive.
Résumé Les questions de genre constituent certainement l’un des deux principaux fronts, avec la bioéthique, sur lesquels se mobilisent les autorités catholiques. Cette étude s’attache à montrer comment ces questions travaillent également de l’intérieur le catholicisme, notamment les pratiques liées à la place des filles et des femmes dans l’institution ecclésiale.
Fluvial geomorphology concerns the transport of weathered rock debris and its accumulation and organization into land-forms that evolve continuously. Sediment budgets define the most fundamental aspect of landform evolution: mass conservation as it is achieved by morphogenetic processes acting within the boundary conditions imposed by natural or anthropogenic controls. This chapter discusses the nature of sediment budgets, provides examples of how they have been used, and describes an approach for designing and constructing useful budgets. Sediment budgets provide a framework for organizing both qualitative information about process interactions and quantitative information about process rates. Sediment budgeting can also aid the assessment of environmental impacts from planned projects. A sediment system can be examined from many points of view, depending on the intended application. The chapter summarizes concepts that are particularly relevant to sediment budgeting, and describes assessment methods applicable to components of the sediment system.
Sediment deficit management is currently a recurring problem in many western gravel-bed rivers. The aim of this contribution is to show the operational benefits of establishing a sediment budget illustrated by the case of the lower Ain River; it was undertaken after establishing a diagnosis of river changes demonstrating the existence of a sediment deficit on this waterway. It is first a matter of providing a methodological framework for developing a sediment budget, based on a large set of data collected in the field (% of fines, bank heights, thicknesses of overbank deposits) or extracted from existing documents (topographic profiles, aerial photographs, etc.) in the particularly complex cases of laterally and vertically mobile rivers. In these cases, it is particularly difficult to provide an interpretation of plan-form changes in terms of sediment delivery. This sediment budgeting improves our understanding of the recent (characterisation, location of sediment input and storage) and future sedimentary behaviour of the lower Ain River and determines the potential resistance of this reach to the propagation of its sediment deficit. These results will finally provide concrete evidence to support decision-making about defining which measures are most relevant to managing the deficit, more particularly by re-exploring the concept of erodible corridor.
This reconnaissance investigation was undertaken to determine whether the channels of the North Platte and Platte Rivers in western and central Nebraska have been changing in character since the latter part of the 19th century and, if so, the general nature and extent of such changes. The 480-kilometer study reach extended from Minatare on the North Platte River to Grand Island on the Platte River. The channels have indeed changed considerably. Changes in the 365-km reach from Minatare to Overton differ in magnitude and sometimes in character from the downstream stretch from Overton to Grand Island (115 km). The decreases in channel width are related to decreases in water discharge. Such flow reductions have resulted primarily from the regulating effects of major upstream dams and the greater use of river water by man. Much of the former river channel is now overgrown with vegetation. Refs.
River Channel Management is the first book to deal comprehensively with recent revolutions in river channel management. It explores the multi-disciplinary nature of river channel management in relation to modern management techniques that bear the background of the entire drainage basin in mind, use channel restoration where appropriate, and are designed to be sustainable.
For thousands of years rivers have been altered by many forms of human activity. Deliberate changes of fluvial hydrosystems have resulted from the power of technology to satisfy the many demands upon water resources. River regulation by dams for domestic and industrial water supply and for irrigation, which today often include intercatchment transfers, and for flood control has markedly altered the flows, sediment loads and water quality characteristics of many rivers (Petts, 1984). Channelization for navigation and land reclamation has further altered the ecological character of fluvial hydrosystems by isolating the channel from its floodplain (Brookes, 1988). These direct impacts have been compounded by indirect impacts associated with the range of land-use changes that have altered the catchment areas over the period of human occupation — several thousand years in some cases.
Among the ways dams alter downstream physical habitat conditions in gravel-bed streams is through the elimination of the gravel supply from the upstream catchment. Physical rehabilitation in these cases may include increasing gravel mobility and gravel supplies through gravel augmentation. Although some gravel augmentations are limited to the construction of artificial spawning riffles, the practice is increasingly being used to increase bed mobility and support the geomorphic processes that maintain physical habitat integrity. However, the quantity and size distribution of gravel needed to support specific geomorphic and habitat objectives in a given river are difficult to predict, as are details regarding how and when the added gravel will propagate downstream. Computational methods for addressing these issues are subject to large uncertainties, whereas empirical data quantifying long-term gravel fluxes in reference streams are scarce. Some approaches used to guide geomorphic gravel augmentations on the Trinity River in California are summarized.
An examination of primary forest succession within two migrating bends along the Beatton River in northeast British Columbia provides information on the interrelationships between the geomorphic and biotic components of a meandering-river floodplain. Overbank sedimentation rates are estimated from changes in the wood structure of partially buried balsam poplars, and from distinct changes in floodplain slope. Particular emphasis is placed upon the changing age-structure of dominant tree species to describe successional changes within the floodplain forest. Rapid sedimentation on the youngest ridges favours the establishment of a dense, non-reproducing balsam poplar stand of uniform age, and prevents the establishment of white spruce seedlings. Following an abrupt decline in overbank sedimentation on surfaces approximately 50 years old, white spruce rapidly colonize the bare mineral soil beneath the poplar canopy and form a dense, relatively even-aged stand. With the death of mature poplars on surfaces 100-150 years in age, the spruce seedlings are released and develop into a mature, non-reproducing stand. This stand persists until its senescence on ridges 350-400 years old. Here, a second, similar, although less even-aged spruce stand establishes, maturing in a like manner to the first, to eventually decline on ridges 500-550 years in age. On this older surface there is the suggestion of yet a third generation of spruce seedlings. These oscillations are reflected in changes of the vascular understory-species during succession, and compare to the cyclic regeneration and climax instability described by Watt (1947) and Whittaker (1953). Tree densities and overbank sedimentation achieve minimum values on flood-plain surfaces greater than approximately 200 years in age. A reasonable explanation to account for negligible sediment deposition on these older surfaces is that the more open trunk density there permits relatively rapid overland flow, preventing the deposition of suspended sediment load. Sedimentation and vegetation appear to be strongly interrelated components within the total floodplain system.
Relations between topography, sedimentary nature of deposits and phytocenoses in the natural flood plain under forest of the Rhine in the upper Rhine graben. Sommerley Forest (Erstein, France). — In the reconstruction of the sedimentary arrangement of a former flood plain of the Rhine, which is now canalized, much difficulties arise. The classical geomorphological structure on the sides of a former channel is generally disrupted. Geomorphological taxons at différent scales overlap in a confuse manner, topography is rarely indicative.
To settle the problem, a great help to geomorphologists is plotting of phyto associations the indicative character of which is particularly useful in this context. Some examples are given.
The Old Rhine is a 45 km-long reach, by-passed by a hydropower scheme achieved in 1959. Since the 1950's, incision, constriction and armouring of the river bed were observed, along with a significant decrease in sediment transport. GIS analysis enabled a description of the reach evolution based on aerial photographs from 1949 till 2008 every 10 years. The vegetation expansion rate and the river planform dynamics were quantified by comparing the different dates (see the joined communication of Arnaud et al.). In order to better understand the effects of morphological changes on hydrodynamics and the armour layer establishment 1D numerical simulations were performed. Channel sensitivity to changes is estimated by distinguishing relative importance of causal factors. Two river geometries of 1950 and 1990 were used, allowing hydro-sedimentary and morphological comparisons. Bed roughness was estimated and distributed based on the type of vegetation observed thanks to the GIS study. The evaluation of the simulated bed shear stress over critical shear stress ratio for different grain sizes revealed the mean sediment mobility along the reach, according to the magnitude and duration of floods observed during the studied time periods. On specific cross-sections, the impacts of typical floods in terms of bed shear stress distribution and bed evolution were estimated to assess the GIS analysis. Both GIS study and 1D modelling are complementary revealing the morphological and vegetation evolution in a plan view and an overview of the bed dynamics, respectively. Both methods underlined the significant impact of vegetation development on the bed dynamics over decades. Numerical modelling reveals the influence of the roughness height distribution over a cross-section on the sediment dynamics. The simulation of grain sorting effects for the considered flood time-series reproduced the conditions for the armour layer establishment, as the coarsest fractions were less mobile than the finer ones.
Sediment deficit management is currently a recurring problem in many western gravel-bed rivers. The aim of this contribution is to show the operational benefits of establishing a sediment budget illustrated by the case of the lower Ain River; it was undertaken after establishing a diagnosis of river changes demonstrating the existence of a sediment deficit on this waterway. It is first a matter of providing a methodological framework for developing a sediment budget, based on a large set of data collected in the field (% of fines, bank heights, thicknesses of overbank deposits) or extracted from existing documents (topographic profiles, aerial photographs, etc.) in the particularly complex cases of laterally and vertically mobile rivers. In these cases, it is particularly difficult to provide an interpretation of plan-form changes in terms of sediment delivery. This sediment budgeting improves our understanding of the recent (characterisation, location of sediment input and storage) and future sedimentary behaviour of the lower Ain River and determines the potential resistance of this reach to the propagation of its sediment deficit. These results will finally provide concrete evidence to support decision-making about defining which measures are most relevant to managing the deficit, more particularly by re-exploring the concept of erodible corridor.
Flow diversions are widespread and numerous throughout the semi‐arid mountains of the western United States. Diversions vary greatly in their structure and ability to divert water, but can alter the magnitude and duration of base and peak flows, depending upon their size and management. Channel geometry and riparian plant communities have adapted to unique hydrologic and geomorphic conditions existing in the areas subject to fluvial processes. We use geomorphic and vegetation data from low‐gradient (≤3%) streams in the Rocky Mountains of north‐central Colorado to assess potential effects of diversion. Data were collected at 37 reaches, including 16 paired upstream and downstream reaches and five unpaired reaches. Channel geometry data were derived from surveys of bankfull channel dimensions and substrate. Vegetation was sampled using a line‐point intercept method along transects oriented perpendicular to the channel, with a total of 100 sampling points per reach. Elevation above and distance from the channel were measured at each vegetation sampling point to analyze differences in lateral and vertical zonation of plant communities between upstream and downstream reaches.
Since the 1990s, French operational managers and scientists have been involved in the environmental restoration of rivers. The European Water Framework Directive (2000) highlights the need for feedback from restoration projects and for evidence-based evaluation of success. Based on 44 French pilot projects that included such an evaluation, the present study includes: 1) an introduction to restoration projects based on their general characteristics 2) a description of evaluation strategies and authorities in charge of their implementation, and 3) a focus on the evaluation of results and the links between these results and evaluation strategies. The results show that: 1) the quality of an evaluation strategy often remains too poor to understand well the link between a restoration project and ecological changes; 2) in many cases, the conclusions drawn are contradictory, making it difficult to determine the success or failure of a restoration project; and 3) the projects with the poorest evaluation strategies generally have the most positive conclusions about the effects of restoration. Recommendations are that evaluation strategies should be designed early in the project planning process and be based on clearly-defined objectives.
As regards river restoration, it is fundamental to better link human pressures and environmental responses and to take into consideration not only target species or habitat but diverse ecological elements. This permits to assess sustainable restoration plan, especially concerning sediment augmentation below dams. The use of a hierarchical multicriteria approach on the Ain River permits us to assess a diagnosis of sediment deficit impact integrating several morphological (channel shifting, river bed degradation and river bed coarsening) and ecological components (Riparian and floodplain lake and fish communities). Our diagnosis also integrates a temporal and spatial approach better to link human pressures and environmental responses and to identify the dam effects amongst other drivers (e.g. grazing decline and channel regulation). The results confirm causality links between sediment deficit and slight channel bed degradation (0.01 m.year−1) or channel bed paving and thus highlight the impact of the dam on the drying of the riparian forest and on former channel community. However, the relationship between incision and reduction in active channel lateral mobility is more difficult to establish. The role of sediment deficit in the current variability of the riparian regeneration capacity and, thereby, landscape diversity along the lower valley remains unclear. This study also confirms the relevance of using different ecological indicators, notably because all components present different adjustment time scales, whereas some of them are more sensitive to other impacts. Copyright © 2013 John Wiley & Sons, Ltd.
The difficulties in obtaining reliable sediment transfer data from direct field measurement or from sediment transport formulae are widely recognized by geomorphologists and river engineers. Quantifying morphological changes (erosion and deposition) in rivers by the analysis of archive data or by field survey, however, can overcome many of these difficulties and provide a mechanism by which sediment budgets can be calculated over a variety of spatial and temporal scales. This paper applies three sediment budgeting methods based on morphological changes in a hypothetical braided reach. These methods range from a simple two-dimensional planform budget to more sophisticated three-dimensional cross-profile and morphological budgets. The development of each budget technique is described and the limitations and applicability of each identified. The three methods are then used to calculated sediment transfer rates in a multi-thread reach on the River Severn in mid-Wales, UK. Results show that across four budget periods spanning 2.5 years the reach was a net exporter of sediment. Application of the planform budget to eight time periods since 1836 shows a similar pattern of net sediment export in the nineteenth century, but during the majority of the twentieth century the reach was a net sediment sink. Finally, the applicability of applying budgeting techniques to extended centennial and millennial timescales is discussed and an assessment made of the role they might play in advancing our understanding of Holocene river dynamics and informing sustainable river management practices.
This paper focuses on one aspect of riparian zone change associated with channel incision. It presents information on sedimentation within the riparian zone of the River Garonne, France, derived from both historical and contemporary sources. The riparian zone investigated is morphologically complex, containing a remnant island and cut-off channel. Information drawn from historical and contemporary sources permits the reconstruction of channel planform and cross-profile change, as well as an assessment of changes in riparian zone sedimentation rates and patterns. Very significant changes are identified over the last 50 years: a change from a multiple thread to single thread channel planform; high rates of riparian zone sedimentation of the order of 0·5 to 2·5 cm a−1; clear spatial patterns in over-bank sedimentation, reflecting topographic and vegetational variations and flood event magnitude; and a reduction in morphological and vegetational diversity through the period, which is expected to continue as the zone is transformed into an increasingly homogeneous sediment sink. Copyright © 2001 John Wiley & Sons, Ltd.
The damming of a river changes the flow of water, sediment, nutrients, energy, and biota, interrupting and altering most of a river`s ecological processes. This article discusses the importance of geomorphological analysis in river conservation and management. To illustrate how subtle geomorphological adjustments may profoundly influence the ecological relationships downstream from dames, three case studies are presented. Then a geomorphically based approach for assessing and possibly mitigating some of the environmental effects of dams by tailoring dam designed and operation is outlined. The cases are as follows: channel simplification and salmon decline on the McKenzie River in Oregon; Channel incision and reduced floodplain inundation on the Oconee river in Georgia; Increased stability of a braided river in New Zealand`s south island. 41 refs., 10 figs., 1 tab.
Rivers of the coastal plain of the south-eastern USA are characteristically low-gradient meandering systems that develop broad floodplains subjected to frequent and prolonged flooding. These floodplains support a relatively unique forested wetland (bottomland hardwoods), which have received considerable ecological study, but distinctly less hydrogeomorphological study. The hydroperiod, or annual period of inundation, largely controls the development of characteristic fluvial landforms, sediment deposition and vegetation distribution patterns. Order-of-magnitude differences in wetted perimeter, width/depth, suspended sediment load and hydraulic roughness may exist between dry in-channel seasons and the hydroperiod. Substantial sediment (and adsorbed contaminants) retention and storage through lateral and vertical accretion is common (where not heavily impacted by flow regulation) along these Coastal Plain rivers. The present chapter summarizes our current understanding of the hydrology, fluvial geomorphology, general and local sedimentation patterns, and related plant ecological patterns of these Coastal Pplain bottomlands.
Alpine rivers have undergone significant changes over the two last centuries. Human activities have modified their geometry through engineering measures to gain land for agricultural purposes and settlements, as well as through active mining to exploit gravel resources. Their sediment and water transfers have also been altered by hydropower-plant construction, control works on high-gradient streams, and catchment land-use changes. The resulting river morphological changes have led to abiotic (e.g., river-bed degradation and narrowing) and biotic (e.g., longitudinal and lateral disconnection) disruption. The current critical management situation (channel instability problems, flood effects, biodiversity decrease) has made river restoration a major issue in the Alps and their surroundings. Such an approach is reinforced by the European Water Framework Directive, which aims to ensure that rivers attain a good ecological status by 2015.In the Alps, space is not always easily available and boundary conditions have changed over the long term. A major challenge in river restoration in the Alpine environment is therefore to identify the processes and key parameters for improving both geomorphological and ecological conditions under often-restricted boundary conditions. Early attempts at river restoration mainly focused on small-scale measures. Today, successful restoration projects in high-energy and bedload-transport-dominated conditions must include the full spectrum of scales, striving to initiate self-forming morphodynamics.In this context, we appraise restoration experiences from the Alps, focusing on channel widening and dike enlargement, former channel reconstruction and reconnection, promotion of bedload supply input from floodplains, tributaries, and hillslopes, as well as on bank erosion measures and restoration activities. We discuss the basic arguments behind such actions, their limitations, and research challenges.
Numerous measures are used in the literature to describe the grain-size distribution of sediments. Consideration of these measures indicates that parameters computed from quartiles may not be as significant as those based on more rigorous statistical concepts. In addition, the lack of standardization of descriptive measures has resulted in limited application of the findings from one locality to another. The use of five parameters that serve as approximate graphic analogies to the moment measures commonly employed in statistics is recommended. The parameters are computed from five percentile diameters obtained from the cumulative size-frequency curve of a sediment. They include the mean (or median) diameter, standard deviation, kurtosis, and two measures of skewness, the second measure being sensitive to skew properties of the "tails" of the sediment distribution. If the five descriptive measures are listed for a sediment, it is possible to compute the five percentile diameters on which they are based (phi 5 , phi 16 , phi 50 , phi 84 , and phi 95 ), and hence five significant points on the cumulative carve of the sediment. This increases the value of the data listed for a sediment in a report, and in many cases eliminates the necessity of including the complete mechanical analysis of the sediment. The degree of correlation of the graphic parameters to the corresponding moment measures decreases as the distribution becomes more skew. However, for a fairly wide range of distributions, the first three moment measures can be ascertained from the graphic parameters with about the same degree of accuracy as is obtained by computing rough moment measures.
In response to various types of human disturbance, most Italian rivers have experienced considerable channel adjustment during the last centuries and in particular in the last decades. This paper reviews all existing published studies and available data, and aims to reconstruct a general outline of the main channel adjustments that have occurred in Italian rivers during the past 100 years.Two main types of channel adjustment have been recognized: (a) incision, which is commonly on the order of 3–4 m, but in some cases is even more than 10 m; (b) narrowing, with channel width reduction up to 50% or more. In some reaches, these adjustments have led to changes in channel pattern in particular from braided to wandering.Such channel adjustments are due to several types of human intervention, particularly sediment extraction, dams and channelization. A strong temporal relationship (specifically, short reaction times) between human disturbance and channel adjustment can be inferred, but trends of adjustment are available for only a few rivers (e.g. the Po, the Arno and the Piave Rivers). These trends show that incision and/or narrowing are more intense immediately after the disturbance and then slow and become asymptotic; the same trends also suggest that larger rivers could have longer relaxation times.The results of this study are synthesised in a general classification scheme that summarises the main styles of adjustment observed in Italian rivers. According to the scheme, braided rivers adjust through prevalent narrowing with varying rates of incision, whereas single-thread rivers adjust mainly through a more pronounced incision accompanied by various amounts of narrowing. The scheme, representing initial and final (present) morphologies and not including intermediate stages of channel adjustment, will need to be tested on the basis of more detailed data to have a wider application both to the Italian context and to fluvial systems elsewhere, affected by similar types of human disturbance causing a reduction of sediment supply.
The Miribel canal is a former arm of the Rhône embanked between 1848 and 1857 over a length of 18 km to improve navigation at low discharges. The impact of this was a hydraulic tilting of the long profile characterised by 4 m of degradation upstream and 4–6 m of aggradation of bedload downstream. This phenomenon increased downstream flooding. Since 1937 a diversion dam has controlled upstream water input, thus reducing the transit of the pebble bedload. However, excessive harvesting of sands and gravels occurred between 1970 and 1980, resulting in a general lowering of the river bed and the accompanying water table with ecological consequences in the alluvial plain and for water supply. This development made it all the more necessary to obtain knowledge about the bedload discharges passing through the Miribel canal, and more broadly about the hydraulic conditions as functions of the varying discharge. Calculation of shear stresses and grain size measurements on the lateral bars after several floods in 1989-90 show that movement of bed-material is initiated at a discharge of 440 m3 · s−1 (equalled or exceeded 40 days · year−1), and becomes general at 550 m3 · s−1 (equalled or exceeded 30 days · year−1). Transport discharge is thus relatively frequent and involves distal fluvio-glacial deposits composed of fine-grained materials. Potential transport calculated by the Meyer-Peter formula is around 60,000 t · year−1 for the range of discharges between 440 and 850 m3 · s−1. For these discharge values, the canal experiences a loss of materials without replacement from upstream; for higher rates of discharge, the floodgates let through an unknown quantity of materials which partially make up the loss. Gravel harvesting ceased in 1991 but the diversion dam will have to be operated in a different way in order to increase the input of bedload into the canal.
The Platte River in central Nebraska responded to water development by rapid channel narrowing and expansion of native riparian woodland. Woodland expanded most rapidly in the 1930s and 1950s; open channel and woodland area stabilized in the 1960s and have remained stable for most reaches into the mid-1990s, despite relatively low flows and infrequent peak flows in the past decade. Open channel area may have been maintained or increased under recent lower flows because of increased erodibility of the floodplain as it has aggraded, developed vertical banks and as its woodland vegetation has become older, sparser and less protective of banks. One section of the Platte River, near Grand Island, has disequilibrated in the past decade by undergoing a 10% loss of channel area. The reach occurs below an area where vegetation has been removed to increase open channel area for migrating whooping and sandhill cranes and other water birds. Vegetation clearing may have liberated excess sediment, locally aggraded the channel and stimulated tree and shrub recruitment. This management practice needs to be examined before it is used more widely in the Platte River. © 1997 John Wiley & Sons, Ltd.
Recently, vegetation is a key of river management where environmental aspects should be taken care of as well as the safety against flood and the water resources utilization. Then, the management of fluvial processes related to vegetation is inevitably important, and fluvial hydraulics and hydraulics of flow with vegetation must support it. These two fields have been recently developed obviously, and coupling them must provide us answers to various emerging problems in new river engineering.In this paper, after an explanation how to treat the flow with vegetation, 2D analysis of fluvial process related to flow with vegetation are introduced with typical processes. Particular interests are paid on the topics of fluvial process related to flow with vegetation which concerns with changes of river landscape.
This determination of the size of material on the bed of a stream is based upon an analysis of the relative area covered by particles of given sizes. The method is applicable to those rivers which flow on coarse material and may be waded during periods of low water. Sampling consists of measuring the intermediate axis of 100 pebbles picked from the bed of the channel on the basis of a grid system. From this sample a frequency distribution is drawn from which the desired size parameters are read. The advantages of the areal sampling procedure over bulk sampling are (1) that it is applicable to very coarse materials, and (2) that it provides a more representative sample of an entire reach of a stream.
Rivers of the Coastal Plain of the southeastern United States are characteristically low-gradient meandering systems that develop broad floodplains subjected to frequent and prolonged flooding. These floodplains support a relatively unique forested wetland (Bottomland Hardwoods), which have received considerable ecological study, but distinctly less hydrogeomorphic study. The hydroperiod, or annual period of inundation, largely controls the development of characteristi c fluvial landforms, sediment deposition, and vegetation distribution patterns. Order of magnitude differences in wetted perimeter, width/depth, suspended sediment load, and hydraulic roughness may exist between "dry" in-channel seasons and the hydyoperiod. Substantial sediment (and adsorbed contaminants) retention and storage through lateral and bertical accretion is common (where not heavily impacted by flow regulation) along these Coastal PI&n rivers. The present chapter summarizes our current understanding of the hydrology, fluvial geomorphology, general and local sedimentation patterns, and related plant ecological patterns of these Coastal Plain bottomlands.
Data on the morphological effects of 11 Canadian interbasin river diversions are presented and some of the more fully documented cases are discussed and illustrated. A classification of diversion routes is suggested and procedures for evaluating the morphological effects in each classification are proposed.
This study describes changes in mean channel-bed elevation, channel width, bed-material sizes, vegetation, water discharges, and sediment loads downstream from 21 dams constructed on alluvial rivers. Most of the studied channels are in the semiarid western US. Flood peaks generally were decreased by the dams, but in other respects the post-dam water-discharge characteristics varied from river to river. Sediment concentrations and suspended loads were decreased markedly for hundreds of kilometers downstream from dams; post-dam annual sediment loads on some rivers did not equal pre-dam loads anywhere downstream from a dam. Bed degradation varied from negligible to about 7.5 meters in the 287 cross sections studied. In general, most degradation occurred during the first decade or two after dam closure. Bed material initially coarsened as degradation proceeded, but this pattern may change during later years. Channel width can increase, decrease, or remain constant in the reach downstream from a dam. Despite major variation, changes at a cross section in stream bed elevation and in channel width with time often can be described by simple hyperbolic equations. Equation coefficients need to be determined empirically. Riparian vegetation commonly increased in the reach downstream from the dams, probably because of the decrease in peak flows. 120 references, 49 figures, 14 tables.