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The Influence of Macrophyte Cutting on the Hydraulic Resistance of Lowland Rivers

J. Aquat. Plant Manage.
47: 2009. 65
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The Influence of Macrophyte Cutting on the
Hydraulic Resistance of Lowland Rivers
Macrophyte growth has an important effect on river flow
velocity patterns during summer when high vegetation biom-
ass (up to 1100 g.m
) is present (Champion and Tanner
2000). Within vegetation patches, velocity measurements
have shown a decline in stream velocity (Watson and Rose
1982, Sand-Jensen and Mebus 1996). Due to this decreased
velocity, flow is deflected around the vegetation patches
(Sand-Jensen and Pedersen 1999) resulting in highly vari-
able stream velocities within natural cross sections (Marshall
and Westlake 1990, Sukhodolova et al. 2004). In general, the
average flow is obstructed and channel resistance increases,
leading to greater water depths (Pitlo and Dawson 1990).
The increase in channel resistance can be an order of magni-
tude greater than the minimal channel resistance (Bakry et
al. 1992), but under exceptional conditions this parameter
can increase even further (Green 2003).
The natural variation of macrophytes in space and time is
highly variable (Barrat-Segretain 1996, Feijoó et al. 1996).
When a patch is formed, flow conditions change and coloni-
zation by other species is possible. This gradual appearance
and decline of species alters the dynamic characteristics of
the resistance associated with vegetation; therefore, a better
understanding of seasonal vegetation development is need-
ed.To reduce flow resistance and ensure drainage of sur-
rounding arable land and prevent flooding at high precipita-
tion events, macrophytes are mechanically cut and removed
from the system. River management objectives are to ensure
hydraulic efficiency, minimize the impact on river ecosys-
tems, and monitor the effect of vegetation regrowth on flow
resistance. For example, high vegetation cutting regimes re-
duce hydraulic resistance but are detrimental to the ecosys-
tem and increase maintenance costs (Dawson 1989). Also,
the effectiveness of this technique is dependent on the vege-
tation regrowth capacity. The regrowth capacity from cut
stems is high, and within three to five weeks (Rawls 1975,
Cooke et al. 1990, Crowell et al. 1994, Bal et al. 2006) biom-
ass can reach pre-harvested values, resulting in a second and
even third cutting regime. With this frequent vegetation re-
moval the travel time of water is reduced (Hamill 1983), re-
Department of Biology, Ecosystem Management Research Group, Uni-
versity of Antwerp, Universiteitsplein 1C, B-2610 Wilrijk, Belgium. Received
for publication March 4, 2008 and in revised form September 25, 2008.
J. Aquat. Plant Manage.
47: 65-68
J. Aquat. Plant Manage.
47: 2009.
sulting in increased flooding risks downstream of the
maintained river stretch (Trepel et al. 2003). Therefore, a
better understanding of the seasonal vegetation variation
and the impact of vegetation cutting on channel flow resis-
tance is necessary to increase the efficiency of vegetation re-
We investigated the impact of vegetation removal on the
hydraulic flow resistance of three lowland rivers to (1) in-
crease the knowledge on the seasonal variation of macro-
phyte abundance in combination with their effect on river
flow resistance, and (2) quantify the temporal impact of river
management on the hydraulic efficiency of lowland rivers.
We calculated resistance on three lowland river stretches
(Desselse Nete, Wamp, and Grote Caliebeek) in the Nete
catchment (Belgium) during the growing season of 2005.
These rivers were chosen because they have well-developed
aquatic vegetation populations that were cut once a year,
with the exception of the Wamp where the vegetation was
not removed. The macrophyte community in these river
stretches was dominated by a single species,
Potamogeton na-
L. (>75% of the macrophyte coverage). Other species,
such as
Sagittaria sagittifolia
Callitriche platycarpa
Stuckenia pectinatus
L., and
Ranunculus penicillatus
present but in densities reaching a maximum of 5%. Because
P. natans
concentrates its leaves on/or nearby the water sur-
face, and due to its relatively high stem diameter (up to 0.6
cm), a high impact on the resistance is expected.
Vegetation cutting times differed between the beginning
of May for Grote Caliebeek and end of June for Desselse
Nete. Biomass was collected with a lightweight sampler ac-
cording to the design of Marshall and Lee (1994). This 360º
rotating device cut the stems just above the sediment surface
on an area of approximately 0.22 m
. For each sampling site,
10 biomass samples were collected randomly along a length
of approximately 100 m. Dry mass of the samples was deter-
mined by drying them for 48 hr at a temperature of 75 °C.
The most widely used equation to calculate the hydraulic
resistance, especially in vegetated streams in which flow resis-
tance is mainly generated due to skin drag (Lee and Fergu-
son 2002), is the Manning equation:
= k R
where R = hydraulic radius (m); S = energy gradient (dimen-
sionless); k = factor to keep the equation dimensionally cor-
rect (m
); n = Manning coefficient (dimensionless); Q =
discharge (m
); and A = cross-sectional area (m
The downside of using this equation is that the resistance
factor (n) in the equation assumes that the objects, in this
case macrophytes, are rigid. However, due to their flexibility
and their marginal positive buoyancy, macrophytes tend to
lay over with increasing velocity, reducing the Manning coef-
ficient in this manner.
The energy grade line (S) of the different lowland rivers
was measured by determining elevation differences between
up- and downstream points. The hydraulic radius of a river
channel is defined as the ratio of its cross sectional area (A)
versus the wetted perimeter (part of the river bank in contact
Figure 1. Seasonal variation of the Manning’s n (a) and macrophyte biomass
(b) on three slow flowing lowland rivers during 2005. Error bars (n = 10) are
Figure 2. The effect of biomass (a) and discharge (b) on the Manning’s n
J. Aquat. Plant Manage.
47: 2009. 67
with the water). By measuring the bottom profile and the wa-
ter level on a fixed location, the wetted perimeter and cross
sectional area of the three river stretches could be deter-
mined and thus the hydraulic radius calculated.
No river bends were present in the study stretches. Veloci-
ty measurements were taken at fixed cross sections on a
bridge on stretches not disrupted by aquatic vegetation, re-
sulting in more accurate discharge calculations. Five or six
velocity measurement points were performed with an elec-
tromagnetic flow meter (Valeport, type 801) for 30 sec each
along the width of each cross section (Table 1). At each
point, a vertical velocity profile was measured with intervals
of 20 cm. When water depth was below 40 cm, water veloci-
ties were measured each 10 cm. By integrating the velocities,
cross section discharges were calculated. Velocity and water
depth measurements started March 2005, when new vegeta-
tion appeared, and continued until December 2005.
The hydraulic resistance on all three lowland rivers, as in-
dicated by the computed Manning’s n, increased in spring
until June, when maximum values were reached of 0.34,
0.66, and 0.71 for the Desselse Nete, Grote Caliebeek, and
Wamp, respectively (Figure 1a). On other river systems, resis-
tance values ranged between 0.25 and 2.25 (Dawson 1978,
Watson 1987, Hearne and Armitage 1993, Green et al. 2006),
which is within the range of our experiments. Resistance in-
creased for Wamp and Grote Caliebeek by a factor of 7.6 and
2.2, respectively, in 1 mo, comparable with the three- to five-
fold increase found in other river systems (Bakry et al. 1992,
Champion and Tanner 2000, Sellin and van Beesten 2004).
This sharp increase in resistance until June was not seen on
Desselse Nete because biomass, consisting entirely of leafless
Potamogeton natans
L. stems, was still high in winter (260 g m
; data not shown), resulting in increased resistance. These
stems were not divided evenly along the river, but followed
the cutting pattern used in 2004. Points with lower stem den-
sity were those where vegetation was mechanically cut. Next
winter, after a total removal of the vegetation during sum-
mer, much lower and more evenly spread
P. natans
tions were observed. Further research is needed to explain
this variation.
The increase in resistance followed almost the same pat-
tern as the biomass growth (Figure 1b). When resistance was
plotted against biomass, significant linear relations were
found for Desselse Nete (R
= 0.70, p < 0.01, F = 13.9) and
Wamp (R
= 0.56, p = 0.05, F = 6.45; Figure 2a). This relation
was not significant for Grote Caliebeek (R
= 0.06, p = 0.61, F
= 0.3) due to high resistance values in June and November,
1. S
Grote Caliebeek Desselse Nete Wamp
May Discharge (m
) 0.36 0.34 0.63
Depth (m) 0.84 1.24 0.52
S (m m
) 0.0014 0.0012 0.0005
R (m) 5.18 5.48 8.84
June Discharge (m
) 0.02 0.22 0.23
Depth (m) 0.28 1.08 0.83
S (m m
) 0.0007 0.0010 0.0011
R (m) 4.06 5.16 9.46
July Discharge (m
) 0.30 0.54 0.42
Depth (m) 0.50 0.81 1.06
S (m m
) 0.0005 0.0010 0.0003
R (m) 4.50 4.62 10.02
August Discharge (m
) 0.08 0.29 0.37
Depth (m) 0.51 1.04 1.06
S (m m
) 0.0005 0.0010 0.0002
R (m) 4.52 5.08 9.92
September Discharge (m
) 0.05 0.33 0.34
Depth (m) 0.41 1.11 0.77
S (m m
) 0.0007 0.0012 0.0001
R (m) 4.32 5.22 9.34
November Discharge (m
) 0.07 0.38 0.26
Depth (m) 0.49 0.79 0.40
S (m m
) 0.0028 0.0009 0.0002
R (m) 4.48 4.58 8.60
December Discharge (m
) 0.29 0.44 0.68
Depth (m) 0.38 0.77 0.36
S (m m
) 0.0028 0.0008 0.0010
R (m) 4.26 4.54 8.52
J. Aquat. Plant Manage.
47: 2009.
despite the low biomass, probably a result of cutting activities
and natural senescence. In June, these high resistance values
were the result of very low discharges, around 0.015 m
(Figure 2b). The high resistance values in November were
the result of the high-energy gradient (0.0028 m m
) and low
discharges (0.07 m
; Table 1).
Together with the growth of aquatic vegetation, the water
depth increased on all three lowland rivers, as shown by Daw-
son and Robinson (1984). This linear relation between water
depth and biomass was significant (all p values <0.03) for all
three rivers.
On both managed river stretches (Desselse Nete and
Grote Caliebeek) the resistance was strongly reduced after
vegetation removal to values around 0.09 and 0.07
but in-
creased again due to fast regrowth (Rawls 1975, Cooke et al.
1990, Crowell et al. 1994, Bal et al. 2006), resulting in a sec-
ond resistance peak later in the season. This regrowth indi-
cates that vegetation cutting did not happen when biomass
reached a peak, or belowground reserves would be depleted
(Linde et al. 1976) resulting in low regrowth. In the unman-
aged stretch of Wamp, only one resistance peak was seen ear-
ly in the vegetation season (June). This peak did not occur
when biomass was highest (July), but one month earlier due
to low discharges and a high energy gradient (Table 1).
From this moment on, a decrease in resistance occurred to-
gether with a decline in biomass.
Vegetation cutting in combination with harvesting is a
useful tool to increase drainage when flooding threatens
human activities, but only for a short period. Within one
month, biomass will increase again resulting in increased
water levels. Over time an integrated approach is necessary,
including the reduction of nutrients to prevent unnaturally
high macrophyte biomass. With strategic timing and fre-
quency of cutting practices, river management can result in
less but more efficient cutting activities causing fewer hy-
draulic problems.
Special thanks goes towards IWT (Institute for Promotion
of Innovation through Science and Technology, Flanders)
and FWO (Fund for Scientific Research, Flanders) for fund-
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... Other studies focused on the hydrodynamics of aquatic macrophites such as Potamogeton spp., Stuckenia spp., Ranunculus spp. Myriophyllum spp, etc., which show an herbaceous habitus, not presenting rigid stems or straight emergent leaves (Green, 2005;Nikora et al., 2008;Bal and Meire, 2009;Old et al., 2014;Verschoren et al., 2017). Besides hydraulic resistance, these studies focused on the effects of plant growth on turbulence, on nutrients retention, as well as on the impacts and effectiveness of different management strategies (Baatrupp-Pedersen, 2018). ...
... Zhao et al. (2017) investigated the hydraulic resistance of three emergent species naturally colonizing drainage ditches and observed Manning's n larger than 0.43. Bal and Meire (2009) examined lowland rivers colonized by various flexible aquatic macrophytes and observed Manning's n in the range 0.34-0.71 at the maximum growth stage. ...
Drainage channels are a widespread component of agricultural and urbanized lowland landscapes. Management of instream and riparian vegetation along drainage channels must be planned by reconciling the need to ensure channel hydraulic efficiency with the need to preserve the riparian habitat. The present paper reports the experimental results of a study conducted on a drainage channel colonized by Phragmites australis in undisturbed natural conditions. The impacts of common reed on flow resistance, flow velocity distribution and turbulence parameters were examined with different discharges under three different scenarios of channel vegetation, which were obtained by means of machineries traditionally used in land reclamation areas. Removing either totally or partially the channel vegetation had great effects on streamwise velocity distribution and turbulence patterns, with small differences in global flow resistance. The experimental results suggest that clearing the channel vegetation just in the center of the cross section can improve the channel conveyance to values close to those obtained with the total removal of the vegetation, while maintaining relatively high levels of turbulent intensities.
... weed cutting) (Dawson, 1978;Fox and Murphy, 1990;Caffrey, 1993;Old et al., 2014). Weed cutting seeks to optimize drainage of water from agricultural lands by reducing the water retention capacity of the stream (Bal and Meire, 2009). ...
... Despite the profound effects of weed cutting on the physical instream environment, only a few studies have quantified the physical degradations (Baattrup- Pedersen andRiis, 1999, 2004;Old et al., 2014), whereas most studies focus on hydrological effects such as current velocity, discharge capacity, hydrological resistance, and water level in relation to flooding risk (e.g. Bal and Meire, 2009;Bal et al., 2011;Curran and Hession, 2013) and on nutrient transport (e.g. Verschoren et al., 2017). ...
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Climate change has increased the frequency and intensity of stream flooding events. In response, managing authorities may increase frequency and intensity of aquatic plant removal (weed cutting) to lower the water level in rivers possibly impairing physical and hydromorphological stream conditions. We studied 32 Danish lowland streams subjected to three different weed cutting practices, representing a gradient in weed cutting intensity, and uncut controls to compare physical and hydromorphological habitat quality parameters among stream groups. Moreover, we measured short-term changes in dissolved oxygen (DO) concentrations and suspended sediment (SS) transport in two streams before, during, and just after weed cutting for the least and most pervasive weed cutting method, respectively. Our results indicated a lower habitat quality affiliated with increasing intensity of weed cutting practice, notably an association with silt cover at the expense of hard substrate. DO concentrations were relatively unaltered but an abrupt increase in SS transport comparable to storm events was observed during cutting with the most pervasive method. Our results indicate that ecological and hydromorphological effects of high intensity weed cutting should be carefully studied and considered before large scale implementation.
... As stated by Baczyk et al. [41] the analysis of 203 cases of maintenance work on river ecosystems demonstrated that in 96 % cases the observed effects of silt removal, plant removal or other works performed in river beds, adversely impacted species composition of fish, macrophytes, macrozoobenthos and water quality were adverse. Mowing and removal of vegetation, as a maintenance measure, directly affects the functioning of buffer, while indirectly affecting hydrochemical parameters of water (increased eutrophication) and habitats of invertebrates and fish in the water course [42,43]. ...
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Abiotic and biotic bounty of riparian waters may be affected by inadequate maintenance works. Improper planning and execution of maintenance works cause changes to hydrological and hydrochemical condition of water in small rivers, affecting biocenose of riverbeds by modifying the taxonomic composition of organisms inhabiting the regulated river section. Five (5) rivers were subject to studies - Plonia, Mysla, Tywa, Rurzyca, and Wardynka (Odra river basin), which were monitored before and after maintenance works consisting in desilting, mowing and removal of aquatic plants. This study examined hydrological (mean depth and width of small rivers, speed and flow), physical and chemical parameter of water (temperature, pH, O2, N-NO3, N-NH4, P-PO4) before and after dredging of selected rivers. Obtained results and resulting statistical analysis demonstrated increase in hydrological indices - depth, width, speed and flow. Among other physical and chemical properties that significantly increased following completion of maintenance works, were O2 and NH4. NO3 concentration and temperature dropped, but not statistically significant. Changes in hydrological and hydrochemical properties of waters caused by maintenance works may affect biodiversity of the regulated river sections, including changes in composition of ichthyofauna species.
... This has negative consequences for salmonid eggs (Argent & Flebbe, 1999;Greig et al., 2005) and macroinvertebrates (Jowett, 2003;Wood et al., 2005;Wood & Armitage, 1999). Therefore, removal of some aquatic vegetation may be necessary for practical engineering reasons (i.e., flow conveyance, flood mitigation and land drainage) in addition to river ecosystem health (Bal & Meire, 2009;Dawson, 1989;Dunderdale & Morris, 1996). Potential aquatic vegetation removal techniques include flushing flows (Tena et al., 2017), chemical control (Murphy & Barrett, 1990), biological control with herbivorous fish (Pipalova, 2006;Van der Zweerde, 1990), and mechanical removal (Wade, 1990). ...
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This paper contributes a field study of suspended sediment transport through aquatic vegetation. The study was run over a 3 month period which was selected to coincide with scheduled weed cutting activities. This provided the opportunity to obtain data points with no vegetation cover, as well as to investigate the effects of weed cutting on Suspended Sediment Concentrations (SSC), particle size distributions and river hydraulics. Aquatic vegetation cover was quantified through remote sensing with Unmanned Aerial Vehicles (UAVs) and biomass estimated from ground truth sampling. SSC was highly dependent on aquatic vegetation abundance, and the distance upstream that had been cleared of aquatic vegetation. The data indicates that fine sediment was being trapped and stored by aquatic vegetation, then likely remobilised after vegetation removal. Investigation of suspended sediment spatial dynamics illustrated changes in particle size distribution due to preferential settling of coarse particles within aquatic vegetation, for example D50 decreased from 36.08 μm to 15.64 μm after suspended sediment travelled 304.2 m downstream and passed ~3700 kg of aquatic vegetation biomass. Hydraulic resistance in the study reach (parameterized by Manning's n) dropped by over 70% following vegetation cutting. Prior to cutting hydraulic resistance was discharge dependent (likely due to vegetation pronating at higher flows), while post cutting hydraulic resistance was approximately invariant of discharge. Aerial surveying captured interesting changes in aquatic vegetation cover prior to vegetation cutting, where some very dense regions of aquatic vegetation were naturally removed (without any high flow events) leaving behind unvegetated riverbed and fine sediment. The weed cutting boat had a lower impact on SSC than was originally expected, which indicates that it may offer a less damaging solution to aquatic vegetation removal in rivers than some other approaches such as mechanical excavation. This paper contributes valuable field data (which are generally scarce) on the research topic of flow-vegetation-sediment interactions, to supplement laboratory and numerical studies. This article is protected by copyright. All rights reserved.
... Macrophytes provide important protection to riverbanks and the lake littoral zone and stabilise the sediment by reducing flow velocities (Kaenel et al., 1998;Verschoren et al., 2017;Wilcock et al., 1999). In rivers, macrophyte removal generally enhanced discharge capacity, where flow velocities increased by 30-40% (Old et al., 2014;Wilcock et al., 1999), water level was lowered by up to 50% (Kaenel et al., 2000) and the Manning roughness coefficient was reduced by 25-73% (Bal and Meire, 2009;Old et al., 2014;Vereecken et al., 2006;Verschoren et al., 2017). The most profound effects on hydraulics were found when macrophytes were removed from larger areas . ...
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Macrophytes are generally considered a nuisance when they interfere with human activities. To combat perceived nuisance, macrophytes are removed, and considerable resources are spent every year worldwide on this practice. Macrophyte removal can, however, have severe negative impacts on ecosystem structure and functioning and interfere with management goals of healthy freshwater ecosystems. Here, we reviewed the existing literature on mechanical macrophyte removal and summarised current information from 98 studies on short- and long-term consequences for ecosystem structure and functioning. In general, the majority of studies were conducted in rivers and streams and evaluated short-term effects of removal on single ecosystem properties. Moreover, most studies did not address the interrelationships between ecosystem properties and the underlying mechanisms. Contrasting effects of removal on ecosystem structure and function were found and these discrepancies were highly dependent on the context of each study, making meaningful quantitative comparisons across studies very difficult. We illustrated how a Bayesian network (BN) approach can be used to assess the implications of macrophyte removal on interrelated ecosystem properties across a wide range of environmental conditions. The BN approach could also help engage a conversation with stakeholders on the management of freshwater ecosystems.
... These high vegetation densities can lead to river management problems through enhanced sedimentation and flood risk [1]. To mitigate these problems, nuisance macrophytes such as Ranunculus penicillatus may be mechanically cut and removed from rivers [2]; however, this may have negative ecological consequences, resulting in decreased abundance of invertebrates and fish [3]. It is desirable to develop river management strategies that balance engineering and ecological considerations [4]. ...
Conference Paper
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Aquatic plants such as macrophytes and bryophytes inhabit many of the river systems around the world. They are primary producers that provide a 3D substrate matrix for invertebrates and small fish, but at high levels of instream biomass they can have a significant impact on flow conveyance and sediment transport. Investigation of flow interactions with aquatic vegetation is challenging however, due to the wide range of scales that these interactions occur over. For example, flow interactions with large patches are not a summation of flow interactions with stems and leaves (due to clumping and reconfiguration), which makes extrapolation of small scale laboratory studies to real vegetation in natural channels problematic. This issue is further enhanced by the lack of data on the size distribution of natural vegetation patches. To address these knowledge gaps, two field studies were undertaken: one of patch characteristics and the other of fluid velocities around patches. In the former, over 1,000 Ranunculus penicillatus patches were surveyed in the River Urie (North East Scotland) using an unmanned aerial vehicle. This survey revealed that natural R. penicillatus patches at the end of summer have mean planform area of 1.32 m2 , mean length of 2.95 m, mean aspect ratio of 5.63, over 1,000 m of total stem length and over 15,000 leaves. These average natural patches are 2-3 orders of magnitude larger than laboratory replicas by biomass. In the second field study, flow interactions with a natural patch of the macrophyte R. penicillatus were investigated using high resolution stereoscopic field Particle Image Velocimetry (PIV). The macrophyte patch caused substantial changes to velocity distributions and turbulence in its wake. Flow fields were highly three dimensional and log profiles did not occur in the wake of the macrophyte patch. 1 INTRODUCTION Aquatic macrophytes are found at high densities in nutrient rich lowland rivers, such as those along the North East coast of Scotland. These high vegetation densities can lead to river management problems through enhanced sedimentation and flood risk [1]. To mitigate these problems, nuisance macrophytes such as Ranunculus penicillatus may be mechanically cut and removed from rivers [2]; however, this may have negative ecological consequences, resulting in decreased abundance of invertebrates and fish [3]. It is desirable to develop river management strategies that balance engineering and ecological considerations [4]. These management strategies need to be based on reliable field data that covers the size of and flow around natural macrophyte patches, since laboratory studies frequently use surrogate vegetation that may not accurately represent natural conditions [5; 6]. To efficiently quantify the geometry of macrophyte patches in the field, new techniques are needed. The most promising technique for this is surveying from Unmanned Aerial Vehicles (UAVs) [7]. This enables much larger numbers of macrophyte patches to be surveyed than would be possible using instream manual measurements. To quantify the flow fields around aquatic vegetation in high resolution, techniques that result in planes of measurements rather than point measurements are required. The most promising technique is the use of stereoscopic PIV [8]. This extended abstract reports the sizes of over 1,000 R. penicillatus patches following an aerial surveying campaign. It also reports the turbulent wake behind an individual R. penicillatus patch as investigated with stereoscopic PIV.
... In contrast, organisms may also mediate kinetic energy by obstructing the water flow. For example, aquatic vegetation causes flow impedance, leading to a reduction of potential washout of aquatic organisms downstream [15]. Another form of energy transport is found in the transport of thermal energy. ...
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Food production for a growing world population relies on application of fertilisers and pesticides on agricultural lands. However, these substances threaten surface water quality and thereby endanger valued ecosystem services such as drinking water supply, food production and recreational water use. Such deleterious effects do not merely arise on the local scale, but also on the regional scale through transport of substances as well as energy and biota across the catchment. Here we argue that aquatic ecosystem models can provide a process-based understanding of how these transports by water and organisms as vectors affect – and are affected by – ecosystem state and functioning in networks of connected lakes. Such a catchment scale approach is key to setting critical limits for the release of substances by agricultural practices and other human pressures on aquatic ecosystems. Thereby, water and food production and the trade-offs between them may be managed more sustainably.
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Riverine floods cause increasingly severe damages to human settlements and infrastructure. Ecosystems have a natural capacity to decrease both severity and frequency of floods. Natural flood regulation processes along freshwaters can be attributed to two different mechanisms: flood prevention that takes place in the whole catchment and flood mitigation once the water has accumulated in the stream. These flood regulating mechanisms are not consistently recognized in major ecosystem service (ES) classifications. For a balanced landscape management, it is important to assess the ES flood regulation so that it can account for the different processes at the relevant sites. We reviewed literature, classified them according to these mechanisms, and analysed the influencing ecosystem characteristics. For prevention, vegetation biomass and forest extent were predominant, while for mitigation, the available space for water was decisive. We add some aspects on assessing flood regulation as ES, and suggest also to include flood hazard into calculations.
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Aquatic vegetation, hydraulics and sediment transport have complex interactions that are not yet well understood. These interactions are important for sediment conveyance, sediment sequestration, phasing of sediment delivery from runoff events, and management of ecosystem health in lowland streams. To address this knowledge gap detailed field measurements of sediment transport through natural flexible aquatic vegetation are required to supplement and validate laboratory results. This paper contributes a field study of suspended sediment transport through aquatic vegetation and includes mechanical removal of aquatic vegetation with a weed cutting boat. It also provides methods to quantify vegetation cover through remote sensing with Unmanned Aerial Vehicles (UAVs) and estimate biomass from ground truth sampling. Suspended sediment concentrations were highly dependent on aquatic vegetation abundance, and the distance upstream that had been cleared of aquatic vegetation. When the study reach was fully vegetated (i.e. cover >80%), the maximum recorded SSC was 14.6 g/m (during a fresh with discharge of 2.47 m/s), during weed cutting operations SSC was 76.8 g/m at 0.84 m/s (weedcutting boat 0.5-1 km upstream from study reach), however following weed cutting operations (4.6 km cleared upstream), SSC was 139.0 g/m at a discharge of 1.52 m/s. The data indicates that fine sediment was being sequestered by aquatic vegetation and likely remobilised after vegetation removal. Investigation of suspended sediment spatial dynamics illustrated changes in particle size distribution due to preferential settling of coarse particles within aquatic vegetation. Hydraulic resistance in the study reach (parameterized by Manning’s n) dropped by over 70% following vegetation cutting. Prior to cutting hydraulic resistance was discharge dependent, while post cutting hydraulic resistance was approximately invariant of discharge. Aerial surveying captured interesting changes in aquatic vegetation cover, where some very dense regions of aquatic vegetation were naturally removed leaving behind unvegetated riverbed and fine sediment.
Technical Report
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Expertise on good practices of river maintenance works and hydrotechnic projects. Environmental impact was presented. Mitigation measures in a planning and implementation phase were proposed. Ekspertyza w zakresie minimalizacji oddziaływania na środowisko typowych prac utrzymaniowych oraz inwestycji hydrotechnicznych na rzekach. Zawiera m. in. omówienie typowych oddziaływań na środowisko, rekomendacje co do etapu planowania prac (m. in. schemat prostej analizy kosztów:korzyści), rekomendacje co do sposobów wykonywania, propozycje dotyczące nowoczesnego zarządzania ciekami. Opracowana na zlecenie Ministerstwa Środowiska, 2018 r.
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RealiStiC estimates of hydraulic resistance are essential to the proper design and analysis of irrigation canal networks. Extensive field studies in Egypt provide data on the magnitude and variability of hydraulic resistance in earthen irrigation canals infested with aquatic weeds. Values of Manning resistance coefficient, n, were calculated from 280 measurements at selected cross sections in 23 stable canals with emergent ditch-bank vegetation. The temporal sample mean of monthly, values of n estimated within a year ranged from 0.017-0.062 for cross sections where five or more measurements were made. The temporal sample coefficient of variation ranged from 0.03-0.42. Dependence of n on flow regime (hydraulic depth and product of average velocity and hydraulic radius) was explored. Studies also were conducted on nine canals containing submerged vegetation. These studies included 312 sonar measurements of vegetation density as well as 156 measurements to estimate n at selected cross sections. Regression analysis revealed that variability in flow regime and in vegetation density contribute significantly to variability in n. Temporal mean of monthly values of vegetation density within a year ranged from 0.06-0.25, with temporal coefficient of variation between 0.17 and 0.92. Vegetation density at measured cross sections showed a clear seasonal pattern. The spatial mean and coefficient of variation in vegetation density along a canal were found to range monthly from 0.06-0.36 and from 0.18-0.76, respectively. Temporal mean values of n in canals with submerged vegetation ranged from 0.028-0.074, with temporal coefficient of variation between 0.13 and 0.57.
Aquatic macrophytes can severely retard flow rates in the river channels that they occupy. Consequently, there is a need to improve our ability to model vegetation resistance, to aid flood prediction and allow for better-informed channel management. An empirical model is developed to calculate flow resistance (Manning’s resistance coefficient) of channels containing the submergent macrophyte Ranunculus (water-crowfoot). Blockage factors (the proportion of a cross-section blocked by vegetation) were determined for up to nine cross-sections at each of 35 river sites. These were used to create blockage-factor percentiles, which were regressed against vegetation resistance. An exponential best-fit relation involving the 69th blockage-factor percentile gave the best results. A parameter relating the length of the vegetated/solid boundary in contact with the open channel to the length of the conventionally-defined wetted perimeter improved the model fit by acting as a pseudo-measure of the turbulent-energy losses generated within the unvegetated stream by the macrophytes. The model was tested on three additional sites containing different macrophyte species and much higher vegetation blockages, and was found to work well.
The water pumped from mineworkings during periods of cheap “off-peak” electricity formed a series of relatively uniform solitary waves in the River Skerne. The passage of the waves was recorded at three river gauging stations during 1973 and 1974. Analysis of the hydrographic records showed that the time of travel of the waves varied significantly according to season as a result of the growth of aquatic weeds in the river channel. This could have important implications with respect to predicting the time of travel of pollutants in rivers.
Rooted macrophyteisn temperatelo wlands treamsa re often distributedin monospecific patches which control flow, carbon fluxes, and the abundance of invertebrates and fish. Small high-resolutionh ot-wirep robes providedd etailedm easureso f flow velocitiesw ithina nd aroundm acrophytep atcheso f four plant specieso f contrasting morphologiesin Danishs treams.F low velocityd eclinedr apidlya t the surfaceo f the plant patches and species with large leaf area on bushy shoots (e.g. Callitriche cophocarpaa nd Elodeac anadensisr) educedt he flow more than speciesw ith streamlined, strap-formedle aves (e.g. Sparganiume mersum).V ariablef low-resistancer esulted in flow velocities at 2 cm above the sediment which were 11 -fold lower inside C. cophocarpap atchest han upstreamo f the patches,w hereasn o significantd ifferences in near-bed velocities were found inside and outside the more open patches of S. emersumT. he reducedv elocity within flow-resistanpt atches remainss ufficiently fast (i.e. >1 cm s -) to prevent carbon depletion and oxygen accumulation and should be optimal to photosynthesis and plant growth. The deflected flow is accelerateda roundt he patchesa nd contributest o form a mosaic of highlyv ariable plant cover, flow and substrate conditions. These relations have important implications for flow resistance, areal expansion of patches and spatial variability of sedimenta nd invertebratceo mpositioni n streams.