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Abstract

Dam removal has been demonstrated to be one of the most frequent and effective fluvial restoration actions but at most dam removals, especially of small dams, there has been little geomorphological monitoring. The results of the geomorphological monitoring implemented in two dams in the rivers Urumea and Leitzaran (northern Spain) are presented. The one from the River Urumea, originally 3.5m high and impounding 500m of river course, was removed instantaneously whereas that in the River Leitzaran, 12.5m high, and impounding 1500m of river course, is in its second phase of a four-stage removal process. Changes in channel morphology, sediment size and mobility and river bed morphologies were assessed. The monitoring included several different techniques: topographical measurements of the channel, terrestrial laser scanner measurements of river bed and bars, sediment grain size and transport; all of them repeated in four (May, August, November 2011 and May 2012) and five (July and September 2013, April and August 2014 and June 2015) fieldwork campaigns in the River Urumea and River Leitzaran, respectively. Geomorphic responses of both dam removals are presented, and compared. Morphological channel adjustments occurred mainly shortly after dam removals, but with differences among the one removed instantaneously, that was immediate, whereas that conducted by stages took longer. Degradational processes were observed upstream of both dams (up to 1.2m and 4m in the River Urumea and River Leitzaran, respectively), but also aggradational processes (pool filling), upstream of Inturia Dam (2.85m at least). Less evident aggradational processes were observed downstream of the dams (up to 0.37m and 0.50m in the River Urumea and River Leitzaran, respectively). Flood events, especially a 100 year flood registered during the monitoring period of Mendaraz Dam removal, reactivated geomorphological processes as incision and bank erosion, whereas longitudinal profile recovery, grain-size sorting and upstream erosion took longer. Copyright © 2016 John Wiley & Sons, Ltd.

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... Several dozen operations have been studied and published in connection with one or more hydromorphological aspects (e.g. Egan and Pizutto, 2000;Stanley and Doyle, 2003;Major et al, 2012;Randle et al., 2015;Ibisate et al., 2016;Magilligan et al., 2016;Gilet et al., 2018). Based on numerous case studies, a series of reviews have already gathered and compared findings in the literature on the geomorphic effects of dam removal (Skalak and Pizutto, 2005; many hydro-physical conditions will interact with each other from one site to another (Bellmore et al., 2019). ...
... This allowed the rapid formation of a channel that enlarged due to the upstream displacement of a mobile knickpoint. These types of knickpoint have frequently been described in the literature (Pizutto, 2002;Doyle et al., 2003;Stewart, 2006;Major et al., 2012;Magilligan et al., 2016;Ibisate et al., 2016). A slower and more local incision and some local widenings continued until the second removal step in October 2017. ...
... These blocks are multi-decimetric to metric and, together with some bedrock outcrops, slowed down the vertical erosion. Indeed, other studies have shown that these hard areas (or other materials that are more cohesive and resistant than the previously eroded layer) can limit incision and regressive erosion (Pearson et al., 2011;Wilcox et al., 2014;Gartner et al., 2015;Warrick et al., 2015;Ibisate et al., 2016), and by fixing the bed, may isolate the surrounding sediments of the reservoir (Major et al., 2012). ...
Article
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The paper emphasizes the main lessons learned from hydromorphological monitoring following the removal of a medium-sized dam (7.29 m) located on a medium energy gravel bed river over a four year period (2015–2019). The Pierre Glissotte dam was previously located on the upper Yonne river (Morvan massif), where it was an obstacle to sediment continuity and was almost completely filled with sand and silts. The dam was removed in two steps, the first in July 2015 and the second in October 2017. Several methods were used (topographical surveys, SFM photogrammetry, RFID tracking, hydrological monitoring) to characterize river adjustments, i.e. the nature of the morpho-sedimentary dynamics, their rates, their temporal and spatial variations, and their control mechanisms. The results highlight the complex and nonlinear response of the Yonne river and the relevance of a regular prolonged monitoring. The changing patterns in space and over time, underline the vast range of uncertainties surrounding this type of restoration and the difficulty involved in predicting post-removal hydromorphology around the dam (return to pre-dam functioning, no changes, new equilibrium conditions). For instance, up to now, the study shows that intense morpho-sedimentary dynamics in the reservoir and effective restoration of bedload continuity do not necessarily lead to changes in the downstream conditions (bed mobility and morphological configuration) previously shaped under the influence of the dam, thus mitigating the success of the river restoration operation.
... Esta opción ya se ha llevado a cabo en más de 3800 presas en el mundo (Ding et al., 2019), aunque principalmente se realiza en azudes y pequeñas presas que han quedado obsoletas. En la Península destacan los ejemplos de Gotera y Retuerta (Primo y González, 2015) en la cuenca del Duero en los años 2011 y 2014 (respectivamente), Robledo de Chavela en la Cuenca del Tajo en el año 2014, e Inturia en la cuenca del Oria entre 2013 y 2016 (Ollero et al., 2014;Ibisate et al., 2016). ...
... ej. Palau, 1998aPalau, , 1998bEast et al., 2015;Ibisate et al., 2016;Espa et al., 2019;Itsukushima et al., 2019). Todos ellos concluyen que los efectos dependen de la cantidad de sedimento liberado y de las características de sedimento (p. ...
... Comiti (2012) summarizes research into the historical channel adjustments of rivers in the Italian Alps, presented in order to document the impacts of human pressure at different basin scales and for different river morphologies. Ibisate et al. (2016) analyzed the geomorphic responses of dam removals. Two dams in the north of Spain were considered. ...
Article
In the last few years, the suggestion of a new geological epoch has been subject of a progressively intense discussion within the Earth science community: the question as to whether or not we are living in an epoch of extensive anthropogenic influence, not only on the biotic environment, but also on the sedimentary and geomorphological processes that affect the shape of the Earth's surface. Human activities have left signatures (e.g. agricultural practices, Tarolli et al. 2014) on the Earth for millennia, and the magnitude of this fingerprint is currently growing, with clear impacts upon in morphology (Wilkinson, 2005; Wohl, 2013; Tarolli and Sofia, 2016), ecosystems (Ellis, 2011), sediments and climate (Waters et al., 2016). The recognition and the analysis of these changes represents a challenge for understanding the evolution of the Earth's landscape. The scientific community is now discussing a formal definition of the Anthropocene as a geologic epoch, stratigraphically distinct from the Holocene. The purpose of this special issue is to join such a debate, by offering a geomorphologic perspective on the effects of human activities on the Earth. It summarizes the work partly published in the last few years in the journal Earth Surface Processes and Landforms, and partly presented at the EGU General Assembly 2014, representing a synthesis of the state of the science of the role of humans as geomorphology agents. The presented papers are grouped in three different sections: (a) Anthropocene and landscape impact, (b) Anthropocene and Earth surface dynamics, (c) Anthropocene and its formalization as geologic epoch.
... In addition, the transport process of deposited sediment can be classified into two stages: An initial process in which a large amount of cohesive sediment discharges and a later process that includes sediment discharge due to relatively large floods that exceed the riverbank [13]. Deposited sediment on the riverbank was not also discharged during normal-level floods at other dam removal sites [50,51]. Our results indicate that no large amount of deposited sediment on the riverbank was discharged, which means that a large flood may be needed to transport these sediments. ...
Article
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Dam removal is typically intended for river restoration or as a countermeasure for aging dams. The influence of dam removal has mainly been studied in large rivers. This study is intended to investigate the influence of the sediment supplied after opening a check dam drain in a small steep stream to contribute to the establishment of sediment release technology form check dam by accumulating the basic knowledge about the influence of sediment release. Deposited sediment in the impoundment was rapidly discharged immediately after opening the drain outlet, and a moderate sediment discharge followed. The water course of the sediments deposited by repeated channel widening and riverbed degradation tended to stop longitudinal topographic changes from downstream. In addition, the turbidity during a flood was high in the first year and tended to decrease in the second year. As for the ecosystem response, changes in the benthic macroinvertebrate community were confirmed in downstream sites, and net-spinning species especially deceased immediately after the sediment supply began. Our monitoring results suggest that the increasing turbidity was suppressed during the flood because sediment release was conducted from the small-scale facility. As a result, a negative impact on the aquatic ecosystem seemed to be reduced.
... Las presas y los azudes se han construido con la finalidad de retener agua (y, por ende, sedimentos), aunque también son las que mayores consecuencias tienen. Las soluciones de restauración pasan por su demolición si están inutilizadas [137] o por restablecer el sedimento al río [150]. Un caso especial en las cuencas semiáridas son las presas de derivación para el riego de turbias mediante boqueras. ...
Technical Report
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Conama publica el informe “Soluciones ante los riesgos climáticos en ríos y costas”, una recopilación de propuestas para hacer frente al calentamiento global en las zonas costeras y fluviales más vulnerables.
... Dam removals are becoming more common, particularly in North America [34,35] but also in other parts of the world (e.g., Europe and Asia) [36][37][38][39]. Sediment released during and following dam removal, especially in cases with significant storage of reservoir sediment, is a primary driver of physical and ecological change in fluvial systems [40][41][42]. ...
Article
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The coastal marine ecosystem near the Elwha River was altered by a massive sediment influx—over 10 million tonnes—during the staged three-year removal of two hydropower dams. We used time series of bathymetry, substrate grain size, remotely sensed turbidity, scuba dive surveys, and towed video observations collected before and during dam removal to assess responses of the nearshore subtidal community (3 m to 17 m depth). Biological changes were primarily driven by sediment deposition and elevated suspended sediment concentrations. Macroalgae, predominantly kelp and foliose red algae, were abundant before dam removal with combined cover levels greater than 50%. Where persistent sediment deposits formed, macroalgae decreased greatly or were eliminated. In areas lacking deposition, macroalgae cover decreased inversely to suspended sediment concentration, suggesting impacts from light reduction or scour. Densities of most invertebrate and fish taxa decreased in areas with persistent sediment deposition; however, bivalve densities increased where mud deposited over sand, and flatfish and Pacific sand lance densities increased where sand deposited over gravel. In areas without sediment deposition, most invertebrate and fish taxa were unaffected by increased suspended sediment or the loss of algae cover associated with it; however, densities of tubeworms and flatfish, and primary cover of sessile invertebrates increased suggesting benefits of increased particulate matter or relaxed competition with macroalgae for space. As dam removal neared completion, we saw evidence of macroalgal recovery that likely owed to water column clearing, indicating that long-term recovery from dam removal effects may be starting. Our results are relevant to future dam removal projects in coastal areas and more generally to understanding effects of increased sedimentation on nearshore subtidal benthic communities.
... Specifically, researchers have used field expeditions, remote sensing, physical experimentation, and numerical modeling to explore the effect of sediment supply changes due to meander cutoffs (Zinger et al., 2011), landslides/debris flows (Brummer and Montgomery, 2006;Hoffman and Gabet, 2007;Nelson and Dubé, 2016;Sutherland et al., 2002), flood events (Lisle, 1982;Madej , 1999), sediment augmentation (Humphries et al., 2012;Juez et al., 2016;Nelson et al., 2015;Sklar et al., 2009), mining operations (Ferguson et al., 2015;Pickup et al., 1983), reservoir sediment releases Wohl , 2003, 2001;Wohl and Cenderelli, 2000), and general supply increases (Jackson and Beschta, 1984;Lisle et al., 1997Lisle et al., , 2000Madej , 2001;Maturana et al., 2014;Podolak and Wilcock , 2013). Additionally, with the growing frequency of dam removal projects, studies related to subsequent geomorphic effects have been plentiful Cui et al., 2014;Ibisate et al., 2016;Pace et al., 2017b;Pearson et al., 2011;Pizzuto, 2002;Zunka et al., 2015). ...
Thesis
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Channel geometry, water discharge, and sediment supply work together to influence gravel-bed morphodynamics. How these forcings change and interact affects instream mesoscale geomorphic units, such as riffles and pools, which are often important habitat areas for aquatic organisms. Riffles and pools, defined as vertical undulations in the longitudinal bed profile, are often co-located with variations in channel width and their maintenance in natural systems is often attributed to unsteady flow effects. However, little work has been done to investigate the interaction between unsteady flow and the periodic width variations that often accompany riffle-pool morphology. Surficial sediment sorting, which is largely dependent on sediment supply, is also invoked as an important factor for riffle-pool maintenance. However, there is a lack of studies exploring how riffles and pools respond, or are maintained, in the case of increased sediment supply, such as might be experienced due to dam removal. In general, little is known about how constriction-forced riffles and pools interact with unsteady flow and changes to sediment supply. This dissertation investigates the interplay between channel geometry, discharge, and sediment supply using numerical methods, laboratory experiments, and field exploration. Chapter 2 presents a one-dimensional morphodynamic model which was used to investigate the controls on sediment pulse evolution in coarse-bed rivers. The model uses the standard step backwater method to compute hydrodynamics, calculates bedload, and simulates elevation changes. A stratigraphy submodel retains data related to vertical grain size sorting in the channel subsurface. The results suggest that sediment pulses move downstream with a greater degree of translation with smaller pulse sizes, longer pulse feed times, finer pulse grain sizes, and prolonged higher discharges. In Chapter 3, a two-dimensional morphodynamic model was used to systematically investigate the influence of width variations, unsteady flow, and changing sediment supply rates on equilibrium morphodynamics. Multiple channels with various amplitudes and wavelengths of sinusoidal width variations were modeled under conditions of steady and unsteady discharge and different sediment supply rates. Results suggest that the amplitude of width variations exerts a primary control on riffle-pool relief and that under cycled hydrographs a reversal in the location of maximum shear stress occurs providing a riffle-pool maintenance mechanism. Complementary flume experiments are presented in Chapter 4, where two geometries (constant- and variable-width) were subjected to the same sequential phases of steady flow and constant sediment supply, unsteady flow and constant sediment supply, and unsteady flow and increased sediment supply. Results show that the variable-width channel adjusts to an increased sediment supply by reducing the elevation relief between adjacent riffles and pools and decreased cross-sectional elevation variability, effectively reducing the form drag, rather than increasing the overall bed slope. Finally, Chapter 5 presents a field investigation of the Elwha River downstream of the former Glines Canyon Dam site, using the dam removal as a natural experiment. Three annual topographic surveys were conducted along with hydrodynamic modeling to investigate the impact of increased sediment supply to a natural channel with riffle-pool morphology. Results show aggradation and channel widening have resulted in shallower, slower flows. Field surveys were complemented with historical aerial image analysis which suggests that channel widening and lateral migration rates have increased substantially since dam removal.
... It is also an index that can give us a better picture about erosivity or aggressiveness of precipitation and its effects on soil erosion than the usual indices based on monthly precipitation (Bessaklia et al. 2018), such as the monthly precipitation concentration index developed by Oliver (1980) and the modified Fournier Index developed by Arnoldus (1980). This is an important subject in many countries because erosion reduces soil fertility, modifies the conditions for crops, alters agricultural practices and causes the rapid clogging of reservoirs (Scholz et al. 2008;Ibisate et al. 2016). There are two country groups more affected by soil erosion: very arid countries with poor vegetation cover and countries with high precipitation. ...
Article
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Abstract Empirical frequency distribution of daily precipitation amounts can be fitted by a negative exponential distribution, because anywhere there are many small daily totals and few large ones. Therefore, the cumulative percentages of days with precipita-tion, sorted in increasing order according to their amounts, against the cumulative percentage of the rainfall amounts that they contribute are fitted by positive exponential curves Y = aXebx, a and b constants. Based on these curves, the Concentration Index (CI) evaluates the contribution of the rainiest days to the total amount. In this study the CI has been calculated for 15 meteorological stations in Da Nang city and Quang Nam province in Central Coast Vietnam, for the 1979–2016 period. The results show high values of CI, ranging from 0.62 to 0.72. Conversely, the linear correlation between altitude and CI is negative (R = -0.60, p < 0.01). There are no correlations between the latitude nor the annual mean number of precipitation days and the CI. CI change for the sub-periods of 1979–1997 and 1998–2016 is also analyzed. Keywords : Concentration Index, Daily precipitation, Correlation, Monsoon climate, Vietnam
... It is also an index that can give us a better picture about erosivity or aggressiveness of precipitation and its effects on soil erosion than the usual indices based on monthly precipitation (Bessaklia et al, 2018), such as the monthly precipitation concentration index developed by Oliver (1980) and the modi ed Fournier Index developed by Arnoldus (1980). This is an important subject in many countries because erosion reduces soil fertility, modi es the conditions for crops, alters agricultural practices and causes the rapid clogging of reservoirs (Scholz et al, 2008;Ibisate et al, 2016). ...
Preprint
Full-text available
Empirical frequency distribution of daily precipitation amounts can be fitted by a negative exponential distribution, because anywhere there are many small daily totals and few large ones. Therefore, the cumulative percentages of days with precipitation, sorted in increasing order according to their amounts, against the cumulative percentage of the rainfall amounts that they contribute are fitted by positive exponential curves Y = aX, a and b constants. Based on these curves, the Concentration Index (CI) evaluates the contribution of the rainiest days to the total amount. In this study the CI has been calculated for 15 meteorological stations in Da Nang city and Quang Nam province in Central Coast Vietnam, for the 1979–2016 period. The results show high values of CI, ranging from 0.62 to 0.72. Conversely, the linear correlation between altitude and CI is negative (R=-0.60, p < 0.01). There are no correlations between the latitude nor the annual mean number of precipitation days and the CI. CI change for the sub-periods of 1979–1997 and 1998–2016 is also analyzed.
... Besides new strategies for sediment management at catchment scale, there is a need for increasing connectivity along European rivers which are probably the more heavily modified at worldwide scale (Belletti et al., 2020). For instance dam removal, one of the more effective strategy to re-establish water and sediment connectivity (Foley et al., 2017), is still not so common in Europe and only in few countries (e.g., France, Spain, United Kingdom, Sweden, and Finland) a significant number of barriers or dams was removed over the last few years (Ibisate et al., 2016). The EU 2030 Biodiversity Strategy (BDS 2030; European Commission, 2020), which aims to reverse the loss of biodiversity, could represent the appropriate framework to improve the geomorphic functioning of European rivers in the next decades. ...
Chapter
Fluvial systems have changed notably during the Anthropocene in many regions of the Earth. This article gives an overall picture of fluvial changes in Europe since the mid-20th century by addressing key aspects such as type and magnitude of changes, causes (e.g., catchment and reach human activities, climate changes), and practical implications of geomorphic changes. Although European rivers have a long history of human impact and significant changes occurred in the 19th century or even before (e.g., channelization of large rivers), remarkable acceleration of fluvial changes occurred since the mid-20th century. Channel narrowing and incision, that in several cases led also to changes in channel pattern, were widespread. Such channel changes were due to decreases in sediment supply and to alteration of flow regime (decreases in floods and formative discharges). A very strong link does exist between human activities and fluvial changes. Some human activities had direct and clear effects on rivers and their catchments (e.g., in-channel sediment mining, dams, land-use changes). Notwithstanding the role of climate was not so evident during the second half of the 20th century, it is likely that it will be a significant driving factor during the 21st century, at least in some European regions (e.g., arctic and subarctic regions, Alps). Fluvial changes had several negative effects (e.g., decreased environmental quality, increased flood hazard) and there is a need for improving river functioning and morphology. An “Anthropocene” perspective in which rivers are viewed critically as socio-biophysical systems co-evolving with human activity should be used in restoration and management of European rivers.
... If the flow rate during removal was less than the mean annual flood flow rate, we considered the removal to have occurred during a low flow. The majority of the applicable studies came from dams removed from gravel and sand rivers with less than 10% silt Ibisate et al., 2016;Pearson et al., 2011;Podolak & Pittman, 2011;Tullos, Finn, & Walter, 2014;Wang & Kuo, 2016). Fewer studies have documented dam removals or failures on systems with silt as part of the sediment mixture Latrubesse et al., 2020;Riggsbee et al., 2012;Sanders & Sauer, 1979;Wilcox et al., 2014). ...
Article
The process of dam removal establishes the channel morphology that is later adjusted by high flow events. Generalities about process responses have been hypothesized, but broad applicability and details remain a research need. We completed laboratory experiments focused on understanding how processes occurring immediately after a sediment release upon dam removal or failure affect the downstream channel bed. Flume experiments tested three sediment mixtures at high and low flow rates. We measured changes in impounded sediment volume, downstream bed surface, and rates of deposition and erosion as the downstream bed adjusted. Results quantified the process responses and connected changes in downstream channel morphology to sediment composition, temporal variability in impounded sediment erosion, and spatial and temporal rates of bedload transport. Within gravel and sand sediments, the process response depended on sediment mobility. Dam removals at low flows created partial mobility with sands transporting as ripples over the gravel bed. 37% of the reservoir eroded, and half the eroded sediment remained in the downstream reach. High flows generated full bed mobility, eroding sands and gravels into and through the downstream reach as 38% of the reservoir eroded. Although some sediment deposited, there was net erosion from the reach as a new, narrower channel eroded through the deposit. When silt was part of the sediment, the process response depended on how the flow rate influenced reservoir erosion rates. At low flows, reservoir erosion rates were initially low and the sediment partially exposed. The reduced sediment supply led to downstream bed erosion. Once reservoir erosion rates increased, sediment deposited downstream and a new channel eroded into the deposits. At high flows, eroded sediment temporarily deposited evenly over the downstream channel before eroding both the deposits and channel bed. At low flows, reservoir erosion was 17‐18% while at the high flow it was 31‐41%.
... This was the third-tallest dam removed intentionally thus far globally (Major et al., 2017), and provided a case study that is unusual in three respects. First, as the largest dam removal in a Mediterranean hydroclimatic setting (Ibisate et al., 2016), the potential for transporting sediment and restructuring channel morphology differed from previous large-dam-removal situations. As is typical for Mediterranean-type rivers, the Carmel River flow varies over more than three orders of magnitude seasonally; all of its sediment-transport capacity occurs during a winter rainy season, with no secondary contribution from spring snowmelt high flows. ...
Article
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Dam removal provides a valuable opportunity to measure the fluvial response to changes in both sediment supply and the processes that shape channel morphology. We present the first study of river response to the removal of a large (32‐m‐high) dam in a Mediterranean hydroclimatic setting, on the Carmel River, coastal California, USA. This before‐after/control‐impact study measured changes in channel topography, grain size, and salmonid spawning habitat throughout dam removal and subsequent major floods. During dam removal, the river course was rerouted in order to leave most of the impounded sediment sequestered in the former reservoir and thus prevent major channel and floodplain aggradation downstream. However, a substantial sediment pulse occurred in response to base‐level fall, knickpoint migration, and channel avulsion through sediment in the former reservoir above the newly rerouted channel. The sediment pulse advanced ~3.5 km in the first wet season after dam removal, resulting in decreased riverbed grain size downstream of the dam site. In the second wet season after dam removal, high flows (including a 30‐year flood and two 10‐year floods) transported sediment >30 km downstream, filling pools and reducing cross‐channel relief. Deposition of gravel in the second wet season after dam removal enhanced salmonid spawning habitat downstream of the dam site. We infer that in dam removals where most reservoir sediment remains impounded and where high flows follow soon after dam removal, flow sequencing becomes a more important driver of geomorphic and fish‐habitat change than the dam removal alone.
... The turn of the century (1900) may be spotted as the fastest increasing supply cut in the case of a 2 km/yr disturbance (or the incipient cut for a 0.5 km/yr disturbance) in order to explain the shift from progradation to retreat in the delta. Obviously, the recovery of free span in height in the middle river by removing small dams would be effective to increase the sediment delivery to the delta, in the long term (Ibisate et al., 2016). ...
Article
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The human pressure upon an alluvial river in the Mediterranean region has changed its riverine and deltaic landscapes. The river has been channelized in the last 70 years while the delta has been retreating for more than a century (a set of data unknown, so far). The paper concentrates on the fluvial component, trying to connect it to the delta evolution. Is the channelization responsible for the delta retreat? We develop a method to compute the actual bed load transport with real information of the past river morphology. The paper compares the computation with very limited measurements, among which are bulk volumes of trapped material at a modern, deep river mouth. The decrease in sediment availability in the last 30 km of the channelized river is deemed responsible for the decrease in the sediment yield to the delta. Moreover, power development and flood frequency should be responsible for a baseline delta retreat during the 20th century. The sediment trapping efficiency of dams is less important than the flow regulation by dams, in the annual sediment yield. Therefore, it is more effective to dismantle channelization than to pass sediment at dams, to provide sand to the beaches.
Article
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Debris flows are one of the most serious natural disasters on Earth and cause great losses in human life and property each year. Check dams have been widely utilized in debris flow hazard mitigation, and over time, some control projects have exceeded their designed service lifetime. However, the processes of stored sediment erosion with multiple surges of debris flows after check dam removal are unknown. Here, the stored sediment erosion processes associated with different reductions in the dam height (i.e., the removed height) were revealed by physical experiments. The results showed that erosion was concentrated on the stored sediment scarp formed by dam removal and that the erosion process mainly included undercutting and widening. The erosion efficiency of the debris flow gradually decreased with the development of erosion and was positively correlated with removed height, negatively correlated with the accumulated debris flow volume, and proportional to the remaining potential volume of sediment to be eroded. The erosion volume development process was well described by an exponential function model, and the physical meanings of the parameters in the model and their empirical values were clarified. Moreover, the distribution of the erosion volume along the flume was shown to satisfy the Weibull distribution, and the distribution parameters were related to the removed height, erosion volume and sediment volume fraction of the debris flow. We anticipate that our results will serve as a starting point for the demolition and reconstruction of debris flow hazard mitigation engineering in mountainous areas.
Article
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Abstract The removal of dams and reservoirs may seem to be an unforeseen and sometimes controversial step in water management. The removal of barriers may be different for each country or region, as each differs greatly in terms of politics, economy and social and cultural awareness. This paper addresses the complex problem of removing dams on rivers and their connected reservoirs. We demonstrate the scales of the changes, including their major ecological, economic, and social impacts. Arguments and approaches to this problem vary across states and regions, depending on the political system, economy and culture, as confirmed by the qualitative and quantitative intensities of the dam removal process and its global geographical variation. The results indicate that the removal of dams on rivers and their connected reservoirs applies predominantly to smaller structures (
Chapter
Rivers of the Iberian Peninsula have been influenced for a long time by intensive human use. This, together with a largely unpredictable climate and scarce water resources, resulted in a large number of hydraulic infrastructures, with more than 1000 large reservoirs spread throughout Iberian watercourses. Although climatic variation is high, most of the Iberian Peninsula is semi-arid in terms of rainfall, with a northeast to south gradient in aridity, and many streams and rivers in the Mediterranean basin are intermittent. The main Iberian rivers cross the Peninsula either east to west (Duero, Tagus, Guadiana) into the Atlantic, or west to east (Ebro) into the Mediterranean. The Iberian rivers are subject to strong hydromorphological alterations as well as widespread point-source and diffuse pollution. In addition to eutrophication in a large number of waterbodies, organic contaminants such as pesticides, and pharmaceutical and industrial products cause multiple stress and threaten a rich biodiversity. Predictions for climate change indicate that the flow regime of most Mediterranean watercourses will be affected ecologically, especially under future scenarios of increasing human land occupation.
Article
Is geomorphology at the forefront of river management? The aims of this article are to explore potential answers to this question in terms of role, barriers, motivation and prospects for river management in the Anthropocene Era. We justify and execute our analysis, first through the growing interest in applied geomorphology and its role to improve river ecology and river policy design; second, by interviewing 24 specialists (researchers (i.e., biologists, ecologists, geomorphologists), engineers, river managers, planners) from different countries. We detected three barriers (academic, management and social) that prevent geomorphology from being more involved in river policy. We then propose three principles for living with rivers, considering geomorphology one of the key factors: (i) working across disciplinary frontiers, (ii) promoting integrated approaches, and (iii) improving fluvial education. Our conclusions look to rivers as natural and dynamic systems where geomorphological knowledge can improve the skills of engineers, ecologists and embrace a transdisciplinary approach. The new riverscape that we propose for the Anthropocene Era must be conceived using negotiation and discussion between an interconnected network of actors, regulators, scientists (sometimes), and natural and cultural values, where management objectives are raised and designed.
Article
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The October 2007 breaching of a temporary cofferdam constructed during removal of the 15-meter (m)-tall Marmot Dam on the Sandy River, Oregon, triggered a rapid sequence of fluvial responses as ∼730,000 cubic meters (m3) of sand and gravel filling the former reservoir became available to a high-gradient river. Using direct measurements of sediment transport, photogrammetry, airborne light detection and ranging (lidar) surveys, and, between transport events, repeat ground surveys of the reservoir reach and channel downstream, we monitored the erosion, transport, and deposition of this sediment in the hours, days, and months following breaching of the cofferdam.
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El cambio global puede manifestarse en los sistemas fluviales bien como cambio moderado pero duradero de caudales líquidos y sólidos o bien en forma de procesos extremos más frecuentes o violentos. En la práctica y en el ámbito de la cuenca del Ebro, está siendo mucho más evidente la pri- mera manifestación. Las principales perturbaciones antrópicas se deben a los embalses y actúan sobre los sistemas fluviales en la misma línea que el cam- bio global, siendo por tanto muy difícil identificar efectos causados por unos u otros. En el artículo se exponen estas dificultades y se propone una me- todología de evaluación basada en protocolo de campo y en la aplicación de indicadores hidromorfológicos.
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Dam removal is becoming an increasingly important component of river restoration, with > 1100 dams having been removed nationwide over the past three decades. Despite this recent progression of removals, the lack of pre- to post-removal monitoring and assessment limits our understanding of the magnitude, rate, and sequence of geomorphic and/or ecological recovery to dam removal. Taking advantage of the November 2012 removal of an old (~ 190 year-old) 6-m high, run-of-river industrial dam on Amethyst Brook (26 km2) in central Massachusetts, we identify the immediate eco-geomorphic responses to removal. To capture the geomorphic responses to dam removal, we collected baseline data at multiple scales, both upstream (~ 300 m) and downstream (> 750 m) of the dam, including monumented cross sections, detailed channel-bed longitudinal profiles, embeddedness surveys, and channel-bed grain size measurements, which were repeated during the summer of 2013. These geomorphic assessments were combined with detailed quantitative electrofishing surveys of stream fish richness and abundance above and below the dam site and throughout the watershed and visual surveys of native anadromous sea lamprey (Petromyzon marinus) nest sites. Post-removal assessments were complicated by two events: (1) upstream knickpoint migration exhumed an older (ca. late eighteenth century) intact wooden crib dam ~ 120 m upstream of the former stone dam, and (2) the occurrence of a 10–20 year RI flood 6 months after removal that caused further upstream incision and downstream aggradation. Now that the downstream reach has been reconnected to upstream sediment supply, the predominant geomorphic response was bed aggradation and associated fining (30–60% reduction). At dam proximal locations, aggradation ranged from 0.3 to > 1 m where a large woody debris jam enhanced aggradation. Although less pronounced, distal locations still showed aggradation with a mean depth of deposition of ~ 0.20 m over the 750-m downstream reach. Post-removal, but pre-flood, bed surveys indicate ~ 2 m of incision had migrated 25 m upstream of the former reservoir before encountering the exhumed dam, which now acts as the new grade control, limiting progressive headcutting. Approximately 1000 m3 of sediment was evacuated in the first year, with ~ 67% of the volume occurring by pre-flood, process-driven (e.g., changes in base level) controls. The combination of changes in channel-bed sedimentology, the occurrence of a large magnitude flood, and the emergence of the new crib dam that is a likely barrier to fish movement was associated with major reductions in abundance and richness in sites downstream and immediately upstream adjacent to the former dam in post-removal sampling. At the same time, we documented the presence of four species of fish, including sea lamprey, which were not present above the dam prior to removal, indicating that upstream passage has been achieved; and we also documented lamprey spawning activity at sites immediately below the dam, which had previously been unsuitable owing to an excessively coarse and armored riverbed. Our results point to the importance of interactions between dam removal and flood disturbance effects, with important implications for short- and long-term monitoring and assessment of dam impacts to river systems.
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The Elwha River restoration project, in Washington State, includes the largest dam-removal project in United States history to date. Starting September 2011, two nearly century-old dams that collectively contained 21 ± 3 million m3 of sediment were removed over the course of three years with a top-down deconstruction strategy designed to meter the release of a portion of the dam-trapped sediment. Gauging with sediment-surrogate technologies during the first two years downstream from the project measured 8,200,000 ± 3,400,000 tonnes of transported sediment, with 1,100,000 and 7,100,000 t moving in years 1 and 2, respectively, representing 3 and 20 times the Elwha River annual sediment load of 340,000 ± 80,000 t/y. During the study period, the discharge in the Elwha River was greater than normal (107% in year 1 and 108% in year 2); however, the magnitudes of the peak-flow events during the study period were relatively benign with the largest discharge of 292 m3/s (73% of the 2-year annual peak-flow event) early in the project when both extant reservoirs still retained sediment. Despite the muted peak flows, sediment transport was large, with measured suspended-sediment concentrations during the study period ranging from 44 to 16,300 mg/L and gauged bedload transport as large as 24,700 t/d. Five distinct sediment-release periods were identified when sediment loads were notably increased (when lateral erosion in the former reservoirs was active) or reduced (when reservoir retention or seasonal low flows and cessation of lateral erosion reduced sediment transport). Total suspended-sediment load was 930,000 t in year 1 and 5,400,000 t in year 2. Of the total 6,300,000 ± 3,200,000 t of suspended-sediment load, 3,400,000 t consisted of silt and clay and 2,900,000 t was sand. Gauged bedload on the lower Elwha River in year 2 of the project was 450,000 ± 360,000 t. Bedload was not quantified in year 1, but qualitative observations using bedload-surrogate instruments indicated detectable bedload starting just after full removal of the downstream dam. Using comparative studies from other sediment-laden rivers, the total ungauged fraction of < 2-mm bedload was estimated to be on the order of 1.5 Mt.
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This paper presents the results of a Gilbert-type delta progradation experiment within an impoundment created by a dam. The delta was composed of a poorly-sorted sand-gravel mixture in a bedload-dominated environment. The main goal of the paper is to analyze the sorting process of material within the deposit as the delta progrades towards the dam. Bed profile evolution has been documented and the entire delta has been extensively sampled in order to study sorting processes. Longitudinal and vertical sorting mechanisms are illustrated. What is novel in this investigation is the complete record, within an entire deltaic deposit, of the vertical distribution of streamwise sorting in the absence of suspended load. The data presented herein provide a detailed description of sorting processes in a Gilbert-type delta. The experimental set-up, the water flow and the sediment feed rate chosen determine the evolution of the delta: it initially progrades with little topset aggradation and degrades afterwards. Experimental results fit well with a previously-presented empirical sorting model, in spite of the fact that the experimental conditions used here were well outside the range of those used to derive that model. The relative coarsening of the upper layers of the delta is found to be related to the slow speed at which the delta progrades, the formation of a mobile armour layer and the erosion of the topset towards the end to the run. Furthermore, a strong correlation between the coarsening of the bottom layer of the delta and its front height has been documented and explained: as the delta gets higher, since there is more space to sort sediment, it is more likely that coarse particles failing near the top of the foreset reach the bottom of the foreset. These findings provide new and useful data documenting sediment sorting in granular, bedload-dominated deltas.This article is protected by copyright. All rights reserved.
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The Mekong River, largely undeveloped prior to 1990, is undergoing rapid dam construction. Seven dams are under construction on the mainstem in China and 133 proposed for the Lower Mekong River and tributaries. We delineated nine distinct geomorphic regions, for which we estimated sediment yields based on geomorphic characteristics, tectonic history, and the limited sediment transport data available. We then applied the 3W model [Minear and Kondolf, 2009] to calculate cumulative sediment trapping by these dams, accounting for changing trap efficiency over time and multiple dams on a single river system. Under a 'definite future' scenario of 38 dams (built or under construction), cumulative sediment reduction to the Delta would be 51%. Under full build-out of all planned dams, cumulative sediment trapping will be 96%. That is, once in-channel stored sediment is exhausted, only 4% of the pre-dam sediment load would be expected to reach the Delta. This scenario would have profound consequences on productivity of the river and persistence of the Delta landform itself, and suggests that strategies to pass sediment through/around dams should be explored to prevent the consequences of downstream sediment starvation.
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A methodology for dam removal monitoring and the results of two case studies is presented: Mendaraz dam (Urumea River) and Inturia dam (Leitzaran River), both located in Gipuzkoa. This monitoring is conducted by river survey cross-sections, measurements of processes and granulometrical analysis. Fast sediment erosion and sedimentation together with geomorphological adjustments were detected after dam removal. In Mendaraz an extraordinary flood favored these processes. The recovery of fluvial dynamics shows benefits not only from the ecological point of view but also for the restoration of natural river dynamics. Geomorphological monitoring is a key tool to quantify and assess river evolution and dynamics after dam removal.
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Condit Dam on the White Salmon River, Washington, a 38-m-high dam impounding a large volume (1.8 million m3) of fine-grained sediment (60% sand, 35% silt and clay, 5% gravel), was rapidly breached in October 2011. This unique dam decommissioning produced dramatic upstream and downstream geomorphic responses in the hours and weeks following breaching. Blasting a 5-m-wide hole into the base of the dam resulted in rapid reservoir drawdown, abruptly releasing ~1.6 million m3 of reservoir water, exposing reservoir sediment to erosion, and triggering mass failures of the thickly accumulated reservoir sediment. Within 90 minutes of breaching, the reservoir's water and ~10% of its sediment had evacuated. At a gaging station 2.3 km downstream, flow increased briefly by 400 m3 s-1 during passage of the initial pulse of released reservoir water, followed by a highly concentrated flow phase —up to 32% sediment by volume—as landslide-generated slurries from the reservoir moved downstream. This hyperconcentrated flow, analogous to those following volcanic eruptions or large landslides, draped the downstream river with predominantly fine sand. During the ensuing weeks, suspended-sediment concentration declined and sand and gravel bedload derived from continued reservoir erosion aggraded the channel by > 1 m at the gaging station, after which the river incised back to near its initial elevation at this site. Within 15 weeks after breaching over 1 million m3 of suspended load is estimated to have passed the gaging station, consistent with estimates that > 60% of the reservoir's sediment had eroded. This dam removal highlights the influence of interactions among reservoir erosion processes, sediment composition, and style of decommissioning on rate of reservoir erosion and consequent downstream behavior of released sediment.
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The rates and styles of channel adjustments following an abrupt and voluminous sediment pulse are investigated in the context of site and valley characteristics and time‐varying sediment transport regimes. Approximately 10.5 x 106 m3 of stored gravel and sand was exposed when Barlin Dam failed during Typhoon WeiPa in 2007. The dam was located on the Dahan River, Taiwan, a system characterized by steep river gradients, typhoon‐ and monsoon‐driven hydrology, high, episodic sediment supply, and highly variable hydraulic conditions. Topography, bulk sediment samples, aerial photos, and simulated hydraulic conditions are analyzed to investigate temporal and spatial patterns in morphology and likely sediment transport regimes. Results document the rapid response of the reservoir and downstream channel, which occurred primarily through incision and adjustment of channel gradient. Hydraulic simulations illustrate how the dominant sediment transport regime likely varies between study periods with sediment yield and caliber and with the frequency and duration of high flows. Collectively, results indicate that information on variability in sediment transport regime, valley configuration, and distance from the dam is needed to explain the rate and pattern of morphological changes across study periods. Copyright © 2013 John Wiley & Sons, Ltd.
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ABSTRACT Fluvial geomorphology can be used as a diagnostic tool to assess the state of a river system and also for the spatial and temporal monitoring of channel restoration or rehabilitation actions. For these objectives a wide range of geomorphological control variables, processes, forms, attributes, indexes and ratios can be used. Within all of these variables ten geomorphological indicators have been selected: i) channel morphometry, ii) activity of erosion and sedimentation processes, iii) mobility of sediments, iv) vertical dynamics, v) slope and longitudinal structure, vi) cross section and transversal structure, vii) dominant discharge, viii) specific stream power, ix) sediment size and shape, x) in-channel vegetation. The methodology of application and usefulness of each indicator is explained. RESUMEN La geomorfología fluvial puede emplearse como herramienta de diagnóstico sobre el estado del sistema fluvial, así como para el seguimiento en el espacio y en el tiempo de actuaciones de restauración o rehabilitación en cauces. Para estos objetivos puede manejarse un amplio espectro de variables geomorfológicas de control, de procesos, formas, atributos, índices y ratios. De entre todas estas variables se han seleccionado diez indicadores geomorfológicos: i) morfometría del cauce, ii) actividad de procesos de erosión y sedimentación, iii) movilidad de sedimentos, iv) dinámica vertical, v) pendiente y estructura longitudinal, vi) sección y estructura transversal, vii) caudal geomórfico, viii) potencia específica, ix) tamaño y forma de sedimentos, x) vegetación en el cauce. Se explica la metodología de aplicación y la utilidad de cada uno de ellos.
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Dam removal projects are playing an increasingly important role in stream restoration, and offer unparalleled opportunities to study sediment dynamics following disturbance. We used the removal of the ˜4-m high Merrimack Village Dam (MVD) on the Souhegan River in southern New Hampshire to measure processes and rates of channel evolution in a sand-filled impoundment. From 2007 to 2010, we repeatedly surveyed 11 cross sections and the longitudinal profile, and collected sediment samples to measure changes in channel morphology and bed texture. The dam removal in August 2008 resulted in a nearly instantaneous base level drop of 3.9 m and caused a two-phased channel response. The initial, process-driven phase (2 months) was characterized by rapid incision and removal of the impounded sand (up to 1013 t d-1), followed by channel widening. Once incised to base level, the rate of sediment removal slowed (30.7 t d-1) and adjustments became event-driven, and the former impoundment segmented into a nonalluvial section and an alluvial section with erosion and deposition influenced by vegetation on the channel banks. Two years after the dam removal and two high-magnitude floods, the river has excavated 79% of the original sediment. Continued response will be substantially influenced by the establishment of bank vegetation within the former impoundment and the magnitude and frequency of high discharge events. Initial channel development and sediment erosion occurs rapidly (weeks to months) in sand-filled impoundments, but excavation of the remaining sediment occurs more slowly depending on vegetation feedbacks and flood events.
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This paper is devoted to a morphodynamic model of incision into a reservoir deposit driven by partial or total removal of the associated dam. The model considers the erosional processes upstream of the position of the former dam, rather then the deposition that occurs downstream. A theory is developed to predict the evolution of both the width and depth of the incisional channel that develops as erosion progresses. The theory is implemented in a numerical model, which is tested against and verified with flume experiments on sudden, complete removal of a dam. In these experiments a channel of a given initial width is allowed to freely incise into a noncohesive reservoir deposit after sudden dam removal. These experiments show a phenomenon that we refer to as “erosional narrowing.” That is, as a channel of a given initial width rapidly incises into the deposit, it can become narrower. As the rate of incision slows, this short period of rapid narrowing is followed by a longer period of widening. In the model the incisional channel is abstracted to a trapezoidal channel with well-defined bed and bank regions, both of which are allowed to erode. Balance between bed and bank erosion plays a key role in the morphodynamics of the channel. More specifically, rapid erosion of the bed can cause the channel to narrow even as bank erosion progresses. As the rate of bed erosion slows, bank erosion causes channel widening. This observed pattern is explained in the context of a theoretical model tested against the experiments.
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The present paper reports on a laboratory investigation of the erosion of a deltaic front induced by the removal of a dam. We built a laboratory model of a dam, and observed both the sedimentation in the reservoir due to the downstream propagation of a delta front and the erosion of the delta front during dam removal, including measurement of channel morphology and flow field. The experiments provide a detailed view of a phenomenon that has not been described in detail to date: erosional narrowing. After the sudden removal of a dam, the flow incises into the reservoir deposit, first rapidly and then more slowly. During the initial period of rapid incision the width of the channel can in some circumstances undergo rapid and substantial narrowing, all the while incorporating sediment from its sidewalls. As the rate of incision slows, the channel first stops narrowing and then enters a phase of slow widening. This pattern of narrowing followed by widening tends to propagate upstream. The minimum channel width attained at a cross section, however, increases with upstream distance from the dam. While the period of erosional narrowing is very short, the incision is so intense that large amounts of sediment are delivered downstream in a short period of time. The process thus has practical implications in regard to the strategy of dam removal. Undistorted Froude similitude is used to scale the results up to field dimensions.
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This study documents changes in channel geometry, bed level profile, and bed grain size distribution and their relations with the sediment transport at the reach scale, following the removal of a low-head dam. After the removal, net sediment deposition occurred downstream of the dam, and net erosion occurred in the reservoir, but approximately less than 1% of the sediment stored in the reservoir was transported downstream. No bank erosion was evident either upstream or downstream of the dam. Bed deposition and scouring in the reservoir accounted for a decrease in the bed slope of 30%. The stations downstream of the dam had surface bed material sizes at least 40% finer than preremoval conditions. However, the sediment transport rates downstream of the dam were not significantly different from predam to postdam removal or from an upstream control. Overall, the removal of the dam had only minor effects on the channel adjustment downstream of the dam. A simple analysis linking transport to channel geometry explains this effect.
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The availability of suitably sized spawning gravels limits salmonid (salmon and trout) populations in many streams. We compiled published and original size distribution data to determine distinguishing characteristics of spawning gravels and how gravel size varies with size of the spawning fish. Median diameters of 135 size distributions ranged from 5.4 to 78 mm, with 50% falling between 14.5 and 35 mm. All but three spawning gravel size distributions were negatively skewed (on a log-transformed basis), with 50% of the skewness coefficients falling between -0.24 and -0.39. Fewer than 20% of the distributions were bimodal. Although tending to be coarser, spawning gravels had sorting and skewness values similar to other fluvial gravels reported in the literature. The range of gravel sizes used by fish of a given species or length is great, but the relation between fish size and size of gravel can be described by an envelope curve. In general, fish can spawn in gravels with a median diameter up to about 10% of their body length.
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Depositional deltas form in the headwaters of most reservoirs. The deltas are characterized by a developing foreset slope that eventually approaches the submerged angle of repose. Deltas extend both upstream and downstream; downstream growth seriously depletes reservoir storage, while upstream evolution raises local ground-water levels and increases flood frequency. Flow separates as it passes downstream over the delta lip into the deeper part of the reservoir. Because of this flow separation and attendant recirculation, traditional finite difference modeling approaches are invalid near the steep foreset slope and cannot model delta growth accurately. A method of numerically fitting a vertical shock face to the evolving delta is developed and illustrated. Conditions upstream of the shock are described with the traditional St. Venant equations; downstream conditions are constant. A one-dimensional mobile-bed computer model is developed and compared to a simulated reservoir in a laboratory flume. The simulated delta closely matches the growth and propagation rate of the observed delta.
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Removal of two dams > 30 m from the Elwha River, on Washington State's Olympic Peninsula, can provide an unprecedented opportunity to study the geomorphic and biologic consequences of this activity. Resulting information can inform management decisions regarding Elwha resources, as well as future dam removal projects. Research and monitoring priorities for each river section (above, between, and below the dams) and nearshore depend on the location-specific effects of the dams, planned active restoration efforts, and conceptions of Elwha ecosystem dynamics. Several river section-or discipline-specific workshops were held 2001 to 2005 to describe impacts to the Elwha River, potential responses to dam removal and priorities for research and monitoring. We present conceptual models based on summaries of these workshops to provide a framework to integrate and relate studies that are currently planned or are underway. We identify the need for an organizational framework – including conceptual models, study designs, data management and integrated sample designs – for research and monitoring that will increase under-standing of ecosystem response, and engender additional financial support.
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Sediment management is frequently the most challenging concern in dam removal but there is as yet little guidance available to resource managers. For those rivers with beds composed primarily of non-cohesive sediments, we document recent numerical and physical modelling of two processes critical to evaluating the effects of dam removal: the morphologic response to a sediment pulse, and the infiltration of fine sediment into coarser bed material. We demonstrate that (1) one-dimensional numerical modelling of sediment pulses can simulate reach-averaged transport and deposition over tens of kilometres, with sufficient certainty for managers to make informed decisions; (2) physical modelling of a coarse sediment pulse moving through an armoured pool-bar complex shows deposition in pool tails and along bar margins while maintaining channel complexity and pool depth similar to pre-pulse conditions; (3) physical modelling and theoretical analysis show that fine sediment will infiltrate into an immobile coarse channel bed to only a few median bed material particle diameters. We develop a generic approach to sediment management during dam removal using our experimental understanding to guide baseline data requirements, likely environmental constraints, and alternative removal strategies. In uncontaminated, non-cohesive reservoir sediments we conclude that the management impacts of rapid sediment release may be of limited magnitude in many situations, and so the choice of dam removal strategy merits site-specific evaluation of the environmental impacts associated with a full range of alternatives.
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Rivers transport sediment from eroding up- lands to depositional areas near sea level. If the continuity of sediment transport is interrupted by dams or removal of sediment from the channel by gravel mining, the flow may become sediment-starved (hungry water) and prone to erode the channel bed and banks, producing channel inci- sion (downcutting), coarsening of bed material, and loss of spawning gravels for salmon and trout (as smaller gravels are transported without replacement from upstream). Gravel is artificially added to the River Rhine to prevent further inci-
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This paper demonstrates the application of Terrestrial Laser Scanning (TLS) to determine the full population of grain roughness in gravel-bed rivers. The technique has the potential to completely replace the need for complex, time-consuming manual sampling methods. Using TLS, a total of 3.8 million data points (mean spacing 0.01 m) were retrieved from a gravel bar surface at Lambley on the River South Tyne, UK. Grain roughness was extracted through determination of twice the local standard deviation (2σz) of all the elevations in a 0.15 m radius moving window over the data cloud. 2σz values were then designated to each node on a 5 cm regular grid, allowing fine resolution DEMs to be produced, where the elevation is equivalent to the grain roughness height. Comparisons are made between TLS-derived grain roughness and grid-by-number sampling for eight 2 m2 patches on the bar surface. Strong relationships exist between percentiles from the population of 2σz heights with measured a-, b-, and c-axes, with the closest matches appearing for the c-axis. Although strong relationships exist between TLS-derived grain roughness (2σz), variations in the degree of burial, packing and imbrication, results in very different slope and intercept exponents. This highlights that conventional roughness measurement using gravel axis length should be used with caution as measured axes do not necessarily represent the actual extent to which the grain protrudes into the flow. The sampling error inherent in conventional sampling is also highlighted through undertaking Monte Carlo simulation on a population of 2000 clasts measured using the grid-by-number method and comparing this with the TLS-derived population of grain roughness heights. Underestimates of up to − 23% and overestimates of up to + 50% were found to occur when considering the D84, and − 20% and overestimates of up to + 36% were found to occur when considering the D50.
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Like many rivers in the western U.S., the Elwha River, Washington, has changed substantially over the past century in response to natural and human forcing. The lower river is affected by two upstream dams that are slated for removal as part of a major river restoration effort. In preparation for studying the effects of dam removal, we present a comprehensive field and aerial photographic analysis of dam influence on an anabranching, gravel-bed river.Over the past century with the dams in place, loss of the upstream sediment supply has caused spatial variations in the sedimentary and geomorphic character of the lower Elwha River channel. Bed sediment is armored and better sorted than on the naturally evolving bed upstream of the dams. On time scales of flood seasons, the channel immediately below the lower dam is fairly stable, but progresses toward greater mobility downstream such that the lowermost portion of the river responded to a recent 40-year flood with bank erosion and bed-elevation changes on a scale approaching that of the natural channel above the dams. In general, channel mobility in the lowest 4 km of the Elwha River has not decreased substantially with time. Enough fine sediment remains in the floodplain that – given sufficient flood forcing – the channel position, sinuosity, and braiding index change substantially. The processes by which this river accesses new fine sediment below the dams (rapid migration into noncohesive banks and avulsion of new channels) allow it to compensate for loss of upstream sediment supply more readily than would a dammed river with cohesive banks or a more limited supply of alluvium. The planned dam removal will provide a valuable opportunity to evaluate channel response to the future restoration of natural upstream sediment supply.
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Dam removal continues to garner attention as a potential river restoration tool. The increasing possibility of dam removal through the FERC relicensing process, as well as through federal and state agency actions, makes a critical examination of the ecological benefits and costs essential. This paper reviews the possible ecological impacts of dam removal using various case studies. Restoration of an unregulated flow regime has resulted in increased biotic diversity through the enhancement of preferred spawning grounds or other habitat. By returning riverine conditions and sediment transport to formerly impounded areas, riffle/pool sequences, gravel, and cobble have reappeared, along with increases in biotic diversity. Fish passage has been another benefit of dam removal. However, the disappearance of the reservoir may also affect certain publicly desirable fisheries. Short-term ecological impacts of dam removal include an increased sediment load that may cause suffocation and abrasion to various biota and habitats. However, several recorded dam removals have suggested that the increased sediment load caused by removal should be a short-term effect. Pre-removal studies for contaminated sediment may be effective at controlling toxic release problems. Although monitoring and dam removal studies are limited, a continued examination of the possible ecological impacts is important for quantifying the resistance and resilience of aquatic ecosystems. Dam removal, although controversial, is an important alternative for river restoration.
Conference Paper
Terrestrial laser scanning is a modern measurement technology. Laser scanning technology has changed the way of phenomena of surrounding reality analysis, including the determination of river bank. Quick creation of the 3D terrain models and the spatial visualization of the extent of the river are the new possibilities. The results of the Biebrza river valley measurements taken with a ScanStation 10 Leica scanner was used in the research. After point cloud filtration, DTM was developed. Based on comparison of the obtained and collected materials: orthophotomaps and topographic maps, usefulness of terrestrial scanner in the determination of the riverbank was evaluated.
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Using a dam removal on the Ashuelot River in southern New Hampshire, we test how a sudden, spatially non-uniform increase in river slope alters sediment transport dynamics and riparian sediment connectivity. Site conditions were characterized by detailed pre- and post-removal field surveys and high-resolution aerial lidar data, and locations of erosion and deposition were predicted through one-dimensional hydrodynamic modeling. The Homestead Dam was a ~ 200 year old, 4 m high, 50 m wide crib dam that created a 9.5 km long, relatively narrow reservoir. Following removal, an exhumed resistant bed feature of glaciofluvial boulders located 400 m upstream and ~ 2.5 m lower than the crest of the dam imposed a new boundary condition in the drained reservoir, acting as a grade control that maintained a backwater effect upstream. During the 15 months following removal, non-uniform erosion in the former reservoir totaled ~ 60,000 m3 (equivalent to ~ 9.3 cm when averaged across the reservoir). Net deposition of ~ 10,700 m3 was measured downstream of the dam, indicating most sediment from the reservoir was carried more than 8 km downstream beyond the study area. The most pronounced bed erosion occurred where modeled sediment transport increased in the downstream direction, and deposition occurred both within and downstream of the former reservoir where modeled sediment transport decreased in the downstream direction. We thus demonstrate that spatial gradients in sediment transport can be used to predict locations of erosion and deposition on the stream bed. We further observed that bed incision was not a necessary condition for bank erosion in the former reservoir. In this characteristically narrow and shallow reservoir lacking abundant dam-induced sedimentation, the variable resistance of the bed and banks acted as geomorphic constraints. Overall, the response deviated from the common conceptual model of knickpoint erosion and channel widening due to dam removal. With thousands of dams likely to be considered for removal or repair in the coming decades, this study helps to advance predictions of the geomorphic response to dam removal and contributes to a broader understanding of the variability in both style and timing of fluvial responses to disturbances.
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Base-level lowering of reservoirs impounding upstream sediment supply triggers a series of channel evolution steps such as degradation, lateral erosion, and redeposition that can dramatically alter the reservoir landscape and decouple the relationship between stream power and sediment supply. Many case studies exist for small dam removals with a few years of sediment storage or dam breaches triggering instantaneous large sediment releases. However, quantitative information for a controlled drawdown initiating erosion of a large sediment deposit is rare. We investigate reservoir sediment response to the phased and concurrent drawdown of two reservoirs on the Elwha River, Washington, USA, during the largest dam removal in history by measuring changes in reservoir topography and channel morphology as a function of base-level lowering, river discharge, and cohesion.
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Dam removal is commonly used for river restoration. However, there are still some uncertainties associated with dam removal, mainly related to the sediment transport rates released downstream from the deposit that had previously filled the impoundment. This research studies the physical response to dam removal in the antecedent deposit by answering the following questions: a) how does an initial channel excavated into the deposit evolve, and b) what is the time distribution of the material released during the early stages of the process. These goals are achieved by an experimental campaign using a poorly sorted mixture of sediment in the antecedent deposit. The research shows that for the given conditions of our experiments, the rate at which the sediment is released depends on the height of the removed dam, the water discharge and the maximum potential volume of sediment to be eroded. This investigation provides new insights of the width evolution when the sediment is composed of a poorly-sorted mixture. This evolution is linked to the bed degradation rates: channel narrows during a rapid incisional phase, and subsequently widens when bed degradation rates decrease. Channel width changes propagate upstream as a convection-like perturbation associated with a kinematic wave starting at the location of the antecedent dam. These features are modeled through a new numerical model accounting for mixtures. More specifically, a set of equations has been derived for the variation of bed elevation, channel bottom width and bed grain size distribution, that when solved numerically, describe the observed channel processes.
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Pulses of fine sediment in gravel-bedded rivers can cause extensive fine sediment infiltration (FSI) into void spaces in coarse bed material, potentially altering river morphodynamics and aquatic ecosystems. Previous work suggests a conceptual model of FSI whereby FSI occurs to a limited depth as a function of the relative grain size of bed sediment compared with infiltrating sediment and is influenced by fine sediment supply and local flow dynamics. Our study applies this conceptual model to a complex reach of a wandering, medium-sized, gravel-bed river to investigate the spatial and temporal controls on FSI. To constrain the timing of FSI, we use the release of contaminated sediment from an upstream dam removal and complementary field methods (bulk sampling, freeze cores and infiltration bags) to capture sediment across varied depositional settings. Our results indicate that, even in a morphologically complex reach, fine-sediment content in the bed does not vary significantly among deposition settings or vertically below the bed surface. We also found that the most contaminated fine sediments released into our study river by a dam removal are not present within the bed material and that substrate has likely been reworked over the period between the release of contaminated sediment and sampling. Our observations also suggest that seals of fine sediment causing void pore space at depth, which have previously been associated with FSI, are not evident in our field area. This suggests that in natural systems, high sediment supply and mobile beds may limit seal formation and persistence. Copyright © 2013 John Wiley & Sons, Ltd.
Article
Removal of two dams 32 m and 64 m high on the Elwha River, Washington, USA, provided the first opportunity to examine river response to a dam removal and controlled sediment influx on such a large scale. Although many recent river-restoration efforts have included dam removal, large dam removals have been rare enough that their physical and ecological effects remain poorly understood. New sedimentary deposits that formed during this multi-stage dam removal result from a unique, artificially created imbalance between fluvial sediment supply and transport capacity. River flows during dam removal were essentially natural and included no large floods in the first two years, while draining of the two reservoirs greatly increased the sediment supply available for fluvial transport. The resulting sedimentary deposits exhibited substantial spatial heterogeneity in thickness, stratal-formation patterns, grain size and organic content. Initial mud deposition in the first year of dam removal filled pore spaces in the pre-dam-removal cobble bed, potentially causing ecological disturbance but not aggrading the bed substantially at first. During the second winter of dam removal, thicker and in some cases coarser deposits replaced the early mud deposits. By 18 months into dam removal, channel-margin and floodplain deposits were commonly >0.5 m thick and, contrary to pre-dam-removal predictions that silt and clay would bypass the river system, included average mud content around 20%. Large wood and lenses of smaller organic particles were common in the new deposits, presumably contributing additional carbon and nutrients to the ecosystem downstream of the dam sites. Understanding initial sedimentary response to the Elwha River dam removals will inform subsequent analyses of longer-term sedimentary, geomorphic and ecosystem changes in this fluvial and coastal system, and will provide important lessons for other river-restoration efforts where large dam removal is planned or proposed. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.
Article
Acquiring high resolution topographic data of natural gravel surfaces is technically demanding in locations where the bed is not exposed at low water stages. Often the most geomorphologically active surfaces are permanently submerged. Gravel beds are spatially variable and measurement of their detailed structure and particle sizes is essential for understanding the interaction of bed roughness with near-bed flow hydraulics, sediment entrainment, transport and deposition processes, as well as providing insights into the ecological responses to these processes. This paper presents patch-scale laboratory and field experiments to demonstrate that through-water terrestrial laser scanning (TLS) has the potential to provide high resolution digital elevation models of submerged gravel beds with enough detail to depict individual grains and small-scale forms. The resulting point cloud data requires correction for refraction before registration. Preliminary validation shows that patch-scale TLS through 200 mm of water introduces a mean error of less than 5 mm under ideal conditions. Point precision is not adversely affected by the water column. The resulting DEMs can be embedded seamlessly within larger sub-aerial reach-scale surveys and can be acquired alongside flow measurements to examine the effects of three-dimensional surface geometry on turbulent flow fields and their interaction with instream ecology dynamics. Copyright © 2011 John Wiley & Sons, Ltd.
Article
The ability to predict the effects of dam removal in highly sediment-filled systems is increasingly important as the number of such dam removal cases continues to grow. The cost and potential impacts of dam removal are site-specific and can vary substantially depending on local conditions. Of specific concern in sediment-impacted removals is the volume and rate of reservoir deposit erosion. The complexity and potential accuracy of modeling methods used to forecast the effects of such dam removals vary substantially. Current methods range from predictions based on simple analysis of pre-dam channel geometry to sophisticated data-intensive, three-dimensional numerical models. In the work presented here, we utilize data collected from past dam removals to develop an additional tool for predicting the rate and volume of sediment deposit erosion. Through the analysis of sediment, discharge, deposit, removal timeline, channel, and watershed data, in conjunction with post-removal monitoring data from a wide range of dam removal projects, some significant trends in the evolution of reservoir deposits following dam removal can be seen. Results indicate that parameters such as median grain size, level of cohesion, spatial variability of the deposit, and removal timeline are among the most influential factors in determining the rate and volume of sediment erosion. By comparing local conditions of dams and reservoirs slated for removal with those of past removals, we hope that predictions of the rate and volume of sediment deposit erosion can be usefully constrained.
Article
Bed material waves are temporary zones of sediment accumulation created by large sediment inputs. Recent theoretical, experimental and field studies examine factors influencing dispersion and translation of bed material waves in quasi-uniform, gravel-bed channels. Exchanges of sediment between a channel and its floodplain are neglected. Within these constraints, two factors influence relative rates of dispersion and translation: (1) interactions between wave topography, flow and bed load transport; and (2) particle-size differences between wave material and original bed material. Our results indicate that dispersion dominates the evolution of bed material waves in gravel-bed channels. Significant translation requires a low Froude number, which is uncharacteristic of gravel-bed channels, and low wave amplitude which, for a large wave, can be achieved only after substantial dispersion. Wave material of small particle size can promote translation, but it primarily increases bed load transport rate and thereby accelerates wave evolution. Copyright © 2001 John Wiley & Sons, Ltd.
Article
The practice of dam removal has received increasing attention as a consequence of maintenance and liability concerns related to the advanced age of many of these structures. Most dams that have been removed thus far are small run-of-river structures. As the number of removals of run-of-river dams increases, it is crucial to understand the effects that these structures have on river geomorphology and sedimentology while in place and how rivers respond to removals so that possible responses to future removals can be anticipated and predicted. This paper reviews current knowledge related to the influence of run-of-river dams on the hydraulics and geomorphology of rivers and suggests types of studies that need to be undertaken to address gaps in current knowledge. Compared to studies of large impoundment dams, field investigations of channel morphology and sedimentology upstream and downstream of run-of-river dams are few and limited in geographic scope. Available studies indicate that the response of rivers to the long-term existence of run-of-river dams is variable both in terms of upstream sediment storage and downstream channel erosion. Future research should focus on how geomorphological responses of rivers to run-of-river dams vary with geographical context and on integration of process-based field studies, numerical modeling and experimental investigations to determine the influence of these dams on flow structure, sediment transport, and patterns of channel erosion and deposition.
Article
A reservoir sediment release on the North Fork Poudre River supplied ~7000 m3 of silt- to pebble-sized sediment to an originally boulder bed channel. Deposition along the 12 km of channel downstream from the reservoir occurred primarily in pools. During the subsequent snowmelt hydrograph, sediment was progressively scoured from the upstream and then the downstream pools. Initial sediment reworking in the pools created a deep, narrow thalweg scoured to the original pool bed, with additional sediment deposition in lateral eddies. Continued reworking reduced but did not completely remove these eddy deposits. The channel became supply-limited with respect to finer grain-sized fractions (clay to medium sand) first at upstream and then at downstream sites and eventually became supply-limited with respect to coarser grain-sized fractions (coarse sand to pebbles). Bedload transport rates at a site were strongly linked to the depletion of sediment stored in upstream pools. Magnitude, duration, and sequence of flows were all important controls on bedload transport and return of the channel to its prerelease state.
Article
River bed degradation can proceed downstream as well as upstream depending upon the cause of degradation. The causes of downstream progressing degradation are primarily related to changes in independent river channel variables, such as increase in water discharge, decrease in size of bed material, and decrease in bed material discharge. The causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man-made changes. Study of various case histories indicates that river slopes are increased by lowering a base level, by decreasing the length of a river, or by removal of a control point. Case histories also show that downstream and upstream progressing degradation can act in combination along a river system: downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary.
Article
Although .70 dams have been decommissioned in Wisconsin over the past 30 y, little is known about the physical and ecological effects of dam removal on riverine ecosystems. The purpose of our study was to document changes in channel form and macroinvertebrate assemblages following the removal of a low-head, run-of-river dam from the Baraboo River, Wisconsin, in January 2000. We surveyed cross sections and collected benthic macroinvertebrate samples in 6 reaches (an upstream reference reach, reaches immediately above and below the dam that was to be removed, and sequential unimpounded and impounded reaches further downstream) in a multiple-dam sys- tem. Surveys were conducted in December 1999, before dam removal, in March 2000 immediately after dam removal, and in August 2000 following a flood. Benthic sediments were collected from selected sites in March and August to measure particle size shifts associated with the dam breach. Before dam removal, impounded reaches were characterized by relatively deep, wide channels, ex- tensive deposits of loose sediments, and macroinvertebrate taxa characteristic of lentic or depositional habitats. Removal of the dam significantly decreased the cross-sectional area of the active channel in the former impoundment from 59 m 2 to 11 m2, but did not alter channel form in downstream reaches. However, we found an increase in loose sediments and in the relative abundance of the sand fraction (0.5-2.0 mm) below the dam immediately after it had been removed. A flood in June increased cross- sectional area in the former impoundment by widening the channel. Sediments that had settled in the reaches below the dam in March were transported ;3.5 km downstream, where they became trapped in another impoundment. The flood had little or no detectable effect on the other 5 study reaches. Within 1 y of dam removal, macroinvertebrate assemblages in formerly impounded reaches did not significantly differ from those in either the upstream reference site or in other unimpounded reaches below the dam site. All unimpounded sites were characterized by lotic taxa such as net- spinning caddisflies and heptageniid mayflies regardless of their impoundment history. Thus, dam removal caused relatively small and transient geomorphic and ecological changes in downstream reaches, and apparently rapid channel development to an equilibrium form within the impoundment, associated with both dam removal and the subsequent June flood. These muted changes and rapid recovery likely result from the relatively large channel size and the small volume of stored sediment available for transport following dam removal.
Article
We examined channel response following the removal of low-head dams on two low-gradient, fine- to coarse-grained rivers in southern Wisconsin. Following removal, channels eroded large quantities of fine sediment, resulting in deposition 3–5 km downstream. At one site (Baraboo River), upstream changes were rapid and included bed degradation, minimal bank erosion, and sediment deposition on channel margins and new floodplain. Sand was transported through the former impoundment and temporarily deposited downstream. At the second site (Koshkonong River), head-cut migration governed channel adjustments. A deep, narrow channel formed downstream of the head-cut, with negligible changes upstream of the head-cut. Fluvial changes were summarized in a conceptual channel evolution model that highlighted (1) similarities between adjustments associated with dam removal and other events that lower channel base-level, and (2) the role of reservoir sediment characteristics (particle size, cohesion) in controlling the rates and mechanisms of sediment movement and channel adjustment.
Article
There is a pressing need for tools to predict the rates, magnitudes, and mechanisms by which sediment is removed from a reservoir following dam removal, as well as for tools to predict where this sediment will be deposited downstream and how it will impact downstream channel morphology. In the absence of adequate empirical data, a good initial approach is to examine the impacts of dam removal within the context of the geomorphic analogies of channel evolution models and sediment waves. Channel changes at two dam breaching sites in Wisconsin involved a succession of channel forms and processes consistent with an existing channel evolution model. Sediment transported downstream after removal of other dams suggests that reservoir sediment may be translated downstream either as a distinct wave or gradually eroded away. More extensive data collection on existing dam removals is warranted before undertaking the removal of a large number of dams. However, if removal is to proceed based on current knowledge, then geomorphic analogies can be used as the foundation for sediment management and stabilization schemes.
Article
Onsite surveys and 75 measurements of discharge were made on 21 high-gradient streams (slopes greater than 0.002) for the purpose of computing the Manning roughness coefficient, n, and to provide data on the hydraulics of these streams. These data show that: (1) n varies inversely with depth; (2) n varies directly with slope; and (3) streams thought to be in the supercritical flow range were actually in the subcritical range. A simple and objective method was employed to develop an equation for predicting the n of high-gradient streams by using multiple-regression techniques and measurements of the slope and hydraulic radius. The average standard error of estimate of this prediction equation was 28% when tested with Colorado data. The equation was verified using other data available for high-gradient streams. Regimeflow equations for velocity and discharge also were developed.
Article
The boat-based, mobile mapping system (BoMMS) with a laser scanner allows the derivation of detailed riverine topographical data for fluvial applications. Combined with data acquisition from static terrestrial LiDAR (light detection and range) or mobile terrestrial LiDAR on the ground, boat-based laser scanning enables a totally new field mapping approach for fluvial studies. The BoMMS approach is an extremely rapid methodology for surveying riverine topography, taking only 85 min to survey a reach approximately 6 km in length. The BoMMS approach also allowed an effective survey angle for deep river banks, which is difficult to achieve with aerial or static terrestrial LiDAR. Further, this paper demonstrates the three-dimensional mapping of a point-bar and its detailed morphology. Compared with the BoMMS surface, approximately, 80% and 96% of the terrestrial LiDAR points showed a height deviation of less than 2 cm and 5 cm, respectively, with an overall standard deviation of ± 2·7 cm. This level of accuracy and rapidity of data capture enables the mapping of post-flood deposition directly after a flood event without an extensive time lag. Additionally, the improved object characterisation may allow for better 3D mapping of the point bar and other riverrine features. However, the shadow effect of the BoMMS survey in point bar mapping should be removed by additional LiDAR data to acquire entire riverine topography. The approach demonstrated allowed a large reach to be surveyed compared with static terrestrial LiDAR and increased the spatial limit of survey towards aerial LiDAR, but it maintains the same or even better temporal resolution as static terrestrial LiDAR. Copyright © 2009 John Wiley & Sons, Ltd.
Article
Despite some highly visible projects that have resulted in environmental benefits, recent efforts to quantify the number and distribution of river restoration projects revealed a paucity of written records documenting restoration outcomes. Improving restoration designs and setting watershed priorities rely on collecting and making accessible this critical information. Information within the unpublished notes of restoration project managers is useful but rarely documents ecological improvements. This special section of Restoration Ecology is devoted to the current state of knowledge on river restoration. We provide an overview of the section’s articles, reflecting on lessons learned, which have implications for the implementation, legal, and financing frameworks for restoration. Our reflections are informed by two databases developed under the auspices of the National River Restoration Science Synthesis project and by extensive interactions with those who fund, implement, and permit restoration. Requiring measurable ecological success criteria, comprehensive watershed plans, and tracking of when and where restoration projects are implemented are critical to improving the health of U.S. waters. Documenting that a project was put in the ground and stayed intact cannot be equated with ecological improvements. However, because significant ecological improvements can come with well-designed and -implemented stream and river restorations, a small investment in documenting the factors contributing to success will lead to very large returns in the health of our nation’s waterways. Even projects that may appear to be failures initially can be turned into success stories by applying the knowledge gained from monitoring the project in an adaptive restoration approach.
Article
Although dam removal has been increasingly used as an option in dam management, and as a river restoration tool, few studies provide detailed quantitative assessment of the geomorphological response of rivers to dam removal. In this study, we document the response of the Pine River, Michigan, to the gradual removal of Stronach Dam. In 1996, prior to the initiation of removal, 31 permanent cross-sectional transects were established in the 10-km study area. These transects were surveyed annually during the course of the removal (1996–2003) and for the three years following removal (2004–2006). Dam removal resulted in progressive headcutting of sediments in the former impoundment, extending upstream 3.89 km of the dam. Over the course of the 10 years since dam removal was initiated, a net total of 92 000 m3 of sediment erosion occurred. The majority of sediments stored in the former reservoir remained in place, with only 12% of the estimated reservoir sediment fill being eroded. Approximately 14% of the net erosion was deposited within the stream channel 1 km downstream of the dam location, with the remainder being transported further downstream or deposited in the floodplain. Sediment fill incision resulted in a narrower and deeper channel upstream, with higher mean water velocity and somewhat coarser substrates. Downstream deposition resulted in a wider and shallower channel, with little change in substrate size composition. Counter-intuitively, water velocity also increased downstream because of the increased slope that developed. Prior to removal, bedforms in the former impoundment were dominated by runs but are showing signs of restoration toward reference conditions. Continuing changes in river geomorphology are evident even three years following removal and are likely to occur for years to come.
Article
Dams have major impacts on river hydrology, primarily through changes in the timing, magnitude, and frequency of low and high flows, ultimately producing a hydrologic regime differing significantly from the pre-impoundment natural flow regime. This paper presents the analysis of pre- and post-dam hydrologic changes from dams that cover the spectrum of hydrologic and climatic regimes across the United States. Our overall goals are to document the type, magnitude, and direction of hydrologic shifts because of impoundment. Using the entire database for the National Inventory of Dams (NID) for dams possessing longstanding U.S. Geological Survey (USGS) gages downstream, we identified 21 gage stations that met length-of-record criteria encompassing an array of types of dams and spanning four orders of magnitude in contributing watershed area. To assess hydrologic changes associated with dams, we applied a hydrologic model, the Indicators of Hydrologic Alteration (IHA), supplemented with orientation statistics for certain hydrograph parameters. Dams had significant impacts on the entire range of hydrologic characteristics measured by IHA. For many characteristics, the direction and significance of effects were highly consistent across the 21 sites. The most significant changes across these sites occurred in minimum and maximum flows over different durations. For low flows, the 1-day through 90-day minimum flows increased significantly following impoundment. The 1-day through 7-day maximum flows decreased significantly across the sites. At monthly scales, mean flows in April and May tend to decline while mean flows in August and September increase. Other significant adjustments included changes in annual hydrograph conditions, primarily in the number of hydrograph reversals that has generally increased for almost all sites following impoundment. The number of high pulses has increased following impoundment but the average length declines. The mean rate of hydrograph rise and fall has declined significantly. These results indicate that the major pulse of dam construction during the previous century has modified hydrologic regimes on a nationwide scale, for large and small rivers.
Article
The effects of dams on downstream geomorphology are reviewed and a typology is devised, consisting of nine cases. The classification can be seen as a further development of Lane's balance between water discharge, sediment load, grain size, and river slope. Depending on changes in released water flow and changes in released sediment load, relative to the transport capacity of the flow, it is possible to estimate resulting cross-sectional geomorphology. The longitudinal extent of changes and their variability with time, and the tributary response to altered mainstream cross-section changes, are also discussed.
Article
In this paper, we synthesize a series of small dam removal studies to examine how changes in channel form can affect riparian vegetation, fish, macroinvertebrates, mussels, and nutrient dynamics. Each of the ecosystem attributes responded to the disturbance of dam removal in different ways and recovered at very different rates, ranging from months to decades. Riparian vegetation appeared to require the greatest time for recovery, while macroinvertebrates had the least. Mussel communities were the most adversely affected group of species and showed no signs of recovery during the time period of the study. Based on these and other studies, we suggest that ecosystems may follow two trajectories of recovery following dam removal. First, ecosystems may fully recover to pre-dam conditions, although this may be unlikely in many cases. Even if full recovery occurs, the timescales over which different attributes recover will vary greatly and may be perceived by the public or management agencies as not recovering at all. Second, ecosystems may only partially recover to pre-dam conditions as the legacy of environmental damage of long-term dam presence may not be reversible or because other watershed changes inhibit full recovery. The potential for full or partial recovery is likely driven by the sensitivity of particular organisms, the characteristics of the dam removed, and the local geomorphic conditions of the watershed. Scientists and management agencies should assess the potential for full or partial recovery prior to dam removal and, in particular, should identify those species or groups of species that are likely to not recover to pre-dam conditions. Such information is critical in the decision of whether, or how, to remove a dam.
Article
The hydrology and geomorphology of large rivers in America reflect the pervasive influence of an extensive water control infrastructure including more than 75,000 dams. One hundred thirty-seven of the very large dams, each storing 1.2 km3 (106 acre feet) of water or more, alter the flows of every large river in the country. The hydrologic effects of these very large dams emerge from an analysis of the stream gage records of 72 river reaches organized into 36 pairs. One member of each pair is an unregulated reach above a dam, whereas the other is a regulated reach downstream from the same structure. Comparison of the regulated and unregulated reaches shows that very large dams, on average, reduce annual peak discharges 67% (in some individual cases up to 90%), decrease the ratio of annual maximum/mean flow 60%, decrease the range of daily discharges 64%, increase the number of reversals in discharge by 34%, and reduce the daily rates of ramping as much as 60%. Dams alter the timing of high and low flows and change the timing of the yearly maximum and minimum flows, in some cases by as much as half a year. Regional variation in rivers, dams, and responses are substantial: rivers in the Great Plains and Ozark/Ouachita regions have annual maximum/mean flow ratios that are 7 times greater than ratios for rivers in the Pacific Northwest. At the same time, the ratio of storage capacity/mean annual water yield for dams is greatest for Interior Western, Ozark/Ouachita and Great Plains rivers and least for Pacific Northwest streams. Thus, in many cases those rivers with the highest annual variability have the greatest potential impact from dams because structures can exert substantial control over downstream hydrology. The hydrologic changes by dams have fostered dramatic geomorphic differences between regulated and unregulated reaches. When compared to similar unregulated reaches, regulated reaches have 32% larger low flow channels, 50% smaller high flow channels, 79% less active flood plain area, and 3.6 times more inactive flood plain area. Dams also affect the area of active areas, the functional surfaces that are functionally connected to the present regime of the river. Regulated reaches have active areas that are 72 smaller than the active areas of similar unregulated reaches. The geomorphic complexity (number of separate functional surfaces per unit of channel length) is 37% less in regulated reaches. Reductions in the size of hydrologically active functional surfaces are greatest in rivers in the Great Plains and least in Eastern streams. The largest differences in geomorphic complexity are in interior western rivers. The shrunken, simplified geomorphology of regulated large rivers has had direct effects on riparian ecology, producing spatially smaller, less diverse riparian ecosystems compared to the larger, more complex ecosystems along unregulated reaches of rivers.