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Funding Information: Monika Barbara Kalinowska, Emilia Karamuz, and Michael Nones were supported within statutory activities no. 3841/E-41/S/2022 of the Ministry of Science and Higher Education of Poland. Adam Kiczko, Andrzej Brandyk, Adam Kozioł, and Marcin Krukowski were supported by the Polish National Centre for Research and Development (grant no. BIOSTRATEG3/347837/11/NCBR/2017). Kaisa Västilä was supported by Maa- ja vesitekniikan tuki ry (grant no. 33271) and the Academy of Finland (grant no. 330217). Funding Information: Monika Barbara Kalinowska, Emilia Karamuz, and Michael Nones were supported within statutory activities no. 3841/E-41/S/2022 of the Ministry of Science and Higher Education of Poland. Adam Kiczko, Andrzej Brandyk, Adam Kozioł, and Marcin Krukowski were supported by the Polish National Centre for Research and Development (grant no. BIOSTRATEG3/ 347837/11/NCBR/2017). Kaisa Västilä was supported by Maa- ja vesitekniikan tuki ry (grant no. 33271) and the Academy of Finland (grant no. 330217). Thanks to Łukasz Przyborowski from the Institute of Geophysics Polish Academy of Sciences, and Aesha Marsoumi and Shea Nee Chew from Warsaw University of Technology for helping with the field and laboratory measurements of rhodamine concentration. We appreciate comments from Steve Wallis, Paweł Rowi´nski, and an anonymous referee that contributed to the article’s improvement. Publisher Copyright: © 2023 Monika Barbara Kalinowska et al. In temperate climates, agricultural ditches are generally bounded by seasonal vegetation, which affects the hydrodynamics and mixing processes within the channel and acts as a buffer strip to reduce a load of pollutants coming from the surrounding cultivated fields. However, even if the control of such vegetation represents a key strategy to support sediment and nutrient management, the studies that investigated the effect of different vegetation maintenance scenarios or vegetation coverage on the flow and mixing dynamics at the reach scale are very limited. To overcome these limitations and provide additional insights into the involved processes, tracer tests were conducted in an agricultural ditch roughly 500gm long close to Warsaw in Poland, focusing on two different vegetation scenarios: highly vegetated and fully cut. Under the highly vegetated scenario, sub-reaches differing in surficial vegetation coverage are analysed separately to better understand the influence of the vegetation conditions on the flow and mixing parameters. Special attention has been paid to the longitudinal dispersion coefficient in complex natural conditions and its dependency on vegetation coverage (V). The vegetation maintenance decreased the travel and residence times of the solute by 3-5 times, moderately increasing the peak concentrations. We found that the dispersion coefficient decreased approximately linearly with the increase of vegetation coverage at V>68g%. Further research is needed at lower vegetation coverage values and different spatial plant distributions. The obtained longitudinal dispersion coefficient values complement dispersion value datasets previously published in the literature, which are barely available for small natural streams. The new process understanding supports the design of future investigations with more environmentally sound vegetation maintenance scenarios.

Vegetation notably influences transport and mixing processes and can thus be used for controlling the fate of substances in the hydro-environment. Whilst most work covers fully vegetated conditions, the novelty of this paper is to focus on flows with real-scale flexible willow patches. We aimed to investigate how longitudinal dispersion varies according to the spatial distribution, density and coverage of the patches and to evaluate the explanatory power of predictors that consider the hydraulics, vegetation and channel geometry. Salt tracer experiments were performed in a trapezoidal channel where we established 3-4 m long and 1-1.6 m wide patches of artificial foliated willows that reproduced the shapes and plant densities observed on woody-vegetated floodplains. We examined sparsely distributed patches with low areal/volumetric coverage of 6-11%, and non-vegetated conditions for reference. Flow depths and surface widths were 0.7-0.9 and 6-7 m, respectively, and the mean flow velocities ranged at 0.3-0.6 m/s. The emergent patches generated from a negligible to over a four-fold increase in the longitudinal dispersion when compared with non-vegetated conditions. The patches with a preferential location in low-velocity areas, such as near banks, or with a high plant density and a blockage of the cross-sectional flow area.0.4, led to the largest dispersion and residence times. Patches under such configurations enhanced the normalized differential velocity defined as the difference between the highest (90th percentile) and lowest (10th percentile) cross-sectional flow velocities divided by the mean velocity, thus increasing shear dispersion. As existing analytical predictors failed to estimate the effect of different patch configurations, we proposed the change in the normalized differential velocity between vegetated and corresponding non-vegetated conditions as a basic predictor of the reach-scale longitudinal dispersion coefficient under patchy vegetation. In contrast, we observed no clear relationship between flow resistance and dispersion. Thus, our findings indicated that bankside vegetation may allow for reduced peak concentrations and lengthened residence times, supporting pollutant management, while ensuring good flow conveyance. Such rare field-scale analyses improve the estimation of solute transport in real vegetated flows.

Despite the development of advanced process-based methods for estimating the discharge capacity of vegetated river channels, most of the practical one-dimensional modeling is based on a relatively simple divided channel method (DCM) with the Manning flow resistance formula. This study is motivated by the need to improve the reliability of modeling in practical applications while acknowledging the limitations on the availability of data on vegetation properties and related parameters required by the process-based methods. We investigate whether the advanced methods can be applied to modeling of vegetated compound channels by identifying the missing characteristics as parameters through the formulation of an inverse problem. Six models of channel discharge capacity are compared in respect of their uncertainty using a probabilistic approach. The model with the lowest estimated uncertainty in explaining differences between computed and observed values is considered the most favorable. Calculations were performed for flume and field settings varying in floodplain vegetation submergence, density, and flexibility, and in hydraulic conditions. The output uncertainty, estimated on the basis of a Bayes approach, was analyzed for a varying number of observation points, demonstrating the significance of the parameter equifinality. The results showed that very reliable predictions with low uncertainties can be obtained for process-based methods with a large number of parameters. The equifinality affects the parameter identification but not the uncertainty of a model. The best performance for sparse, emergent, rigid vegetation was obtained with the Mertens method and for dense, flexible vegetation with a simplified two-layer method, while a generalized two-layer model with a description of the plant flexibility was the most universally applicable to different vegetative conditions. In many cases, the Manning-based DCM performed satisfactorily but could not be reliably extrapolated to higher flows.

Funding Information: This research was supported by the RIED 2018 campaign project of KICT‐REC (Korea Institute of Civil Engineering and Building Technology – River Experiment Center), the International Matching Joint Research Project (Grant No. 20210485) of KICT, Aalto University, Maa‐ ja vesitekniikan tuki ry (Grant No. 33271), and Academy of Finland (Grant No 330217). The authors gratefully acknowledge Donggu Kim, Jongmin Kim, Myeonghui Ahn, Inhyeok Bae, Eun‐Kyung Jang, Mooyoung Na, and the technical staff of REC who participated in the extensive measurement campaign. Funding Information: This research was supported by the RIED 2018 campaign project of KICT-REC (Korea Institute of Civil Engineering and Building Technology ? River Experiment Center), the International Matching Joint Research Project (Grant No. 20210485) of KICT, Aalto University, Maa- ja vesitekniikan tuki ry (Grant No. 33271), and Academy of Finland (Grant No 330217). The authors gratefully acknowledge Donggu Kim, Jongmin Kim, Myeonghui Ahn, Inhyeok Bae, Eun-Kyung Jang, Mooyoung Na, and the technical staff of REC who participated in the extensive measurement campaign. Publisher Copyright: © 2022 The Authors. Hydrological Processes published by John Wiley & Sons Ltd. Riparian vegetation patches growing on river banks and floodplains influence in-channel and overbank hydromorphological processes. The current knowledge on patch-scale hydrodynamics is largely based on laboratory flume experiments with simplified vegetation. The aim of this study is to provide new understanding of the flow and wake characteristics for real riparian vegetation patches based on field-scale experiments with natural willows, in order to inform hydromorphological and ecological modelling. The focus was placed on the effects of foliage as the main driver of the seasonal changes in vegetation and on the influence of the flexibility-induced reconfiguration on the flow in the wake and around the patches. The patch drag, defined by its flow blockage factor, was increased by 3.0–4.4 times by the presence of foliage and decreased by up to 60% because of the streamlining and reconfiguration of foliage with increasing flow velocity. Such large changes in the patch drag altered the flow and wake characteristics, affecting the onset of a patch-scale vortex street. Seasonality and flexibility modified the patch sheltering effect, that is, the magnitude of velocity, turbulent kinetic energy, and bed shear stress reduction in the wake, relative to the background level. In the presence of foliage, mean flow velocity and bed shear stress in the wake were reduced on average by ~50% and ~70%, respectively. The sheltering effect was lower for the leafless conditions than for the foliated conditions. For the foliated cases, the spatial extents of the over-depth and the near-bed sheltered region were on average 1.5 and 1.8 times larger than in the corresponding leafless cases, respectively. Overall, seasonal changes in vegetation and flexibility-induced mechanisms were identified as key controls for the flow associated with patches of riparian vegetation, with major implications on developing models for predicting hydromorphological processes and the potential to preserve and create habitats.

Funding Information: The shares of financing of the investigated TSCs are reported in Table S9. The differences in financing illustrate the absence of a systematic nationwide approach for financing TSCs. The farmers’ share of the costs was rather low in the six pilot studies, averaging less than 10%. Most financing has been arranged through individual, externally funded research and development projects. Currently, in Finland, a TSC project may obtain state financial support in the form of either a drainage subsidy from Agrifood Research and Development Fund (MAKERA) managed by the ELY Centers (Regional State Authority) or from CAP-AES’s non-productive investments as wetland investment and maintenance allowance. However, overlapping subsidies cannot be received for the same drainage area. Support can cover no more than 40% of the costs, but environmentally friendly (i.e., nature-based) solutions, and particularly expensive structures, may increase the support percentage. In recent years, MAKERA has granted approximately 2.5 million €/year of state support for about 50 dredging projects. Funding Information: Funding: This research was funded by the Joint Research Centre of European Commission (contract no. 939642), Academy of Finland (grant no. 330217), the Finnish Cultural Foundation (grant no. 00201229) and by the Valumavesi project. Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Conventional dredging of ditches and streams to ensure agricultural drainage and flood mitigation can have severe environmental impacts. The aim of this paper is to investigate the potential benefits of an alternative, nature-based two-stage channel (TSC) design with floodplains excavated along the main channel. Through a literature survey, investigations at Finnish field sites and expert interviews, we assessed the performance, costs, and monetary environmental benefits of TSCs in comparison to conventional dredging, as well as the bottlenecks in their financing and governance. We found evidence supporting the expected longer-term functioning of drainage as well as larger plant and fish biodiversity in TSCs compared to conventional dredging. The TSC design likely improves water quality since the floodplains retain suspended sediment and phosphorus and remove nitrogen. In the investigated case, the additional value of phosphorus retention and conservation of protected species through the TSC design was 2.4 times higher than the total costs. We demonstrate how TSCs can be made eligible for the obligatory vegetated riparian buffer of the European Union agri-environmental subsidy scheme (CAP-AES) by optimising their spatial application with respect to other buffer measures, and recommend to publicly finance their additional costs compared to conventional dredging at priority sites. Further studies on biodiversity impacts and long-term performance of two-stage channels are required.

Funding Information: The authors thank the technician Antti Louhio for his help in arranging the experiments at the Environmental Hydraulics Lab of Aalto University. The authors acknowledge the support from the technicians Antonio Fusco and Domenico Palmiero of the Laboratory of Hydraulics of University of Naples Federico II. The authors acknowledge funding from Maa‐ ja vesitekniikan tuki ry (Grant No. 33271), Maj and Tor Nessling Foundation (Grant No. 201800045), Academy of Finland (Grant No. 330217), and from statutory activities No. 3841/E‐41/S/2020 of the Ministry of Science and Higher Education of Poland. Publisher Copyright: © 2021. The Authors. Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Natural riparian vegetation generally presents a complex hydrodynamic behavior governed by plant morphology and flexibility. By contrast, hydrodynamic processes in partly vegetated channels are conventionally simulated by using simplified model vegetation, such as arrays of rigid cylinders. The aim of this study is to investigate the impacts of embedding natural plant features in the experimental simulation of flow in partly vegetated channels. Unique comparative experiments were carried out with both reconfiguring vegetation made of natural-like shrubs and grasses, and with rigid cylinders. While the lateral distributions of flow properties presented a high similarity governed by the shear layer differential velocity ratio, the bulk vegetative drag, and the presence of large-scale vortices, the flexibility-induced mechanisms of natural-like vegetation markedly affected the flow at the interface. Differences in plant morphology and spacing, and the dynamic motion of flexible foliated plants induced deeper vortex penetration into the vegetation. The normalized shear penetration was 6–10 times greater than observed for rigid cylinders, resulting in wider zones significantly exchanging momentum with the adjacent open water. The efficiency of lateral momentum transport for flexible foliated vegetation was up to 40% greater than the corresponding rigid cylinder case. Overall, the results indicated that improving the representativeness of model vegetation is a critical step toward the accurate simulation of hydrodynamic and transport processes in natural settings.

Despite the development of advanced process-based methods for estimating the discharge capacity of vegetated river channels, most of the practical one-dimensional modeling is based on a relatively simple divided channel method (DCM) with the Manning's flow resistance formula. This study is motivated by the need to improve the reliability of modeling in practical applications while acknowledging the limitations on the availability of data on vegetation distributions and densities required by the process-based methods. We investigate whether the advanced methods can be applied to modeling vegetated compound channels by identifying the missing characteristics as parameters through the formulation of an inverse problem. We developed a new probabilistic approach for comparing six models of channel discharge capacity in respect of their uncertainty, with the model with the lowest uncertainty considered the most favorable. Calculations were performed for flume and field settings varying in floodplain vegetation submergence, density, and flexibility, and in hydraulic conditions. The output uncertainty, estimated on the basis of a quasi-Bayes approach, was analyzed for a varying number of observation points, demonstrating the significance of the parameter equifinality. The results showed that very reliable predictions with low uncertainties can be obtained for process-based methods with a large number of parameters. The equifinality affects the parameter identification but not the uncertainty of a model. The best performance for sparse, unsubmerged, rigid vegetation was obtained with the Mertens method and for dense, flexible vegetation with the generalized two-layer method combined with a description of the flexibility-induced reconfiguration. We found that the process-based methods are superior when applied for vegetative conditions they were developed for while the Manning based DCM seems to be the most flexible technique.

The presence of vegetation has a significant impact on the flow conditions in streams and rivers by changing the roughness, which has an important effect on flow velocity distribution. The article focuses on modelling and analysing the depth-averaged velocity distribution in a rectangular channel partially covered by submerged grassy vegetation. The possibility of using two selected models (CCHE2D flow model and Shiono and Knight model) has been evaluated using the measurement data from a laboratory experiment. The measurements and modelling have been carried out for different flow conditions. Applied models calibrated for low velocities were found to be extendable to the same vegetative condition during high velocities.

Funding Information: We gratefully acknowledge the help of laboratory technician Antti Louhio in the flume setup and trainee Marco Maio for the help in the data collection. The research was funded by Maa- ja vesitekniikan tuki ry (No 36537 and 33271), Maj and Tor Nessling Foundation (No 201800045) and Academy of Finland (No 330217). Publisher Copyright: © 2021 The Author(s) River flows are greatly influenced by floodplain vegetation with implications on hydrological and hydraulic conditions from cross-sectional to river reach scales. Flow models need to reliably reflect changes in the riverine environment, such as vegetation growth associated with altered flow regimes, increased sediment loads and eutrophication. Leaf area index (LAI) based approaches are increasingly used as tools to predict the flow resistance caused by natural vegetation. However, current LAI-based modelling involves uncertainty at low and high vegetation densities and flow velocities due to a lack of research and validation at the outer ranges. The aim of this paper is to investigate the flow resistance for a mixture of flexible floodplain vegetation consisting of woody plants and understory grasses of low to high densities (LAI = 1–5) over a wide range of mean flow velocities (0.05–1.2 m/s) at low relative submergences of 1–2. A novel flume setup was designed by using high-accuracy pressure sensors to measure flow resistance and force sensors to measure plant drag forces. Flow resistance decreased by 35–90% when the submergence H/hv increased from 1 to 2, which is a highly relevant range for floodplain flows. The results provided new evidence that LAI-based modelling of vegetative friction factors can be reliably extended from low to high LAI values for non-submerged vegetation. However, adjustments to existing LAI-based approaches are required for water stages higher than the vegetation height. Furthermore, the friction by the understory grasses cannot be neglected as often assumed in literature especially for cases of low plant densities and low relative submergences.

Purpose: Riparian vegetation imposes a critical control on the transport and deposition of suspended sediment with important implications for water quality and channel maintenance. This paper contributes (1) to hydraulic and morphological modeling by examining the parameterization of natural riparian vegetation (trees, bushes, and grasses) and (2) to the design and management of environmental channels by determining how the properties of natural floodplain plant stands affect the erosion and deposition of suspended sediment. Materials and methods: Laboratory and field data were employed for enhancing the physical description of flow–plant–sediment interactions with a consideration of practical applicability. A drag force parameterization that takes into account the flexibility-induced reconfiguration, and the complex structure of foliated plants was validated for small natural trees under laboratory conditions, while the data from a small vegetated compound channel demonstrated the approaches at the field scale. Based on the field data, we identified three key vegetative factors influencing the net deposition and erosion on the floodplain. The significance of these factors was evaluated for vegetative conditions ranging from almost bare soil to sparse willows and dense grasses. Overall, the investigated conditions covered flexible and rigid vegetation with seasonal differences represented by foliated and leafless states. Results and discussion: The drag and reconfiguration of woody plants were reliably predicted under leafless and foliated conditions. Subsequently, we present a new easy-to-use methodology for predicting vegetative drag and flow resistance. The methodology is based on a physically solid parameterization for five widely used coefficients or terms (Eqs. (2)–(6)), with the necessary parameter values presented for common riparian species. The methodology was coupled with existing approaches at the field scale, revealing that increasing vegetation density and the associated decreasing flow velocity within vegetation significantly increased net deposition. Further, deposition increased with increasing cross-sectional vegetative blockage and decreasing distance from the suspended sediment replenishment point. Thus, longitudinal advection was the most important mechanism supplying fine sediment to the floodplain, but long continuous plant stands limited deposition. Conclusions: The proposed parameterization (Eqs. (2)–(6)) can be readily implemented into existing hydraulic and morphological models to improve the description of natural vegetation compared to the conventional rigid cylinder representation. The approach is advantageous for evaluating, for example, the effects of both natural succession and management interventions on floodplains. Finally, guidance is provided on how floodplain vegetation can be maintained to manage the erosion and deposition of suspended sediment in environmental channel designs.

Quantifying vegetative effects on the flow, suspended sediment concentrations, and sediment transport is complicated by uncertainties associated with the correct conceptualization of flow-vegetation-sediment interactions. The aim of this paper is to investigate the vegetative effects on the flow, the spatially varying sediment transport mechanisms, and sediment fluxes for both the unvegetated and vegetated areas of the channel. Experiments were conducted in a laboratory flume at medium to high bulk flow velocities. Care was taken to reproduce vegetated conditions typical of floodplain flows, where the unvegetated main channel and vegetated floodplain are clearly separated, inducing a strong shear flow. Vegetation was represented by a combination of artificial understory grasses and flexible woody plants. Instantaneous flow velocities were measured by acoustic Doppler velocimetry and suspended sediment concentrations by optical turbidity sensors. The suspended sediment concentration increased for positions closer to the bed in the unvegetated part of the channel. However, in and adjacent to the vegetative areas, the vertical profiles of concentration showed more complex distributions. Based on the paired measurements of the flow and concentration the streamwise sediment fluxes were estimated. In the investigated partly vegetated channel the unit sediment discharge was two to four times higher in the unvegetated part compared to the vegetated part of the channel. Data and findings in the present study provide insight on the vertical and lateral variability of suspended sediment fluxes and are useful for predicting sediment transport in partly vegetated channels.

Flow disturbances generated by individual patches of submerged, flexible aquatic vegetation were investigated for two naturally growing macrophyte species, Potamogeton crispus L. and Myriophyllum spicatum L., in a sandy lowland river. Through acoustic Doppler velocimetry, 24 vertical profiles of the 3D velocity field were recorded downstream of each of the patches. The morphological features and biomechanical properties of the plants were also evaluated. The experiments showed the relationship between biomechanical characteristics and turbulence statistics. M. spicatum, which was stiffer and therefore less prone to dynamic reconfiguration, showed a greater effect on velocity damping, causing an increase in Reynold stresses, turbulence intensities, and turbulent kinetic energy downstream of the patch. These effects were present in regions both above and below plant height. In contrast for P. crispus, these effects were present only below plant height. The stiffer plant produced a mixing layer in its wake similar to that of dense plant canopies. The patch of less stiff and more streamlined P. crispus with longer leaves presented a much weaker effect on the flow. In contrast to previous studies conducted with rigid plant surrogates, we concluded that reconfiguration of the living flexible plants allows the plants to minimize drag forces, and therefore, their influence on the flow field was weaker than the effects reported for rigid surrogates.

Riparian shrubs and trees present a complex, seasonally variable morphology, with flexible stems and leaves efficiently adapting to the flow forcing (reconfiguration). The aim of this paper is to investigate how foliage and reconfiguration affect the flow and mixing in a partly vegetated channel. Specific attention was placed on the velocity statistics, onset and coherence of turbulent structures, and lateral momentum transport at the horizontal interface between vegetation and open water. The experimental flume arrangement was novel in that it allowed investigating the lateral shear layer induced by flexible riparian plants. The natural-like vegetation consisted of emergent woody plants and a grassy understory, with density, morphology and reconfiguration behavior comparable to those found in real riparian areas. Investigations were conducted under foliated and leafless conditions to determine the seasonality effects. The mean and turbulent flow structure was determined with acoustic Doppler velocimetry, and dynamic plant motions were investigated from video footage. The presence of foliage enhanced the drag discontinuity at the interface, resulting in more pronounced velocity gradients between the vegetated and open areas compared to the leafless conditions. Foliation induced stronger shear layer-scale mixing, whereas, under leafless conditions, the local mixing induced by stems was more important. The reconfiguration decreased the coherence of the two-dimensional large-scale vortices at the interface while their characteristic frequency was consistent with the canonical mixing layer theory. Our results indicated that shear layer dynamics in partly vegetated channels was influenced strongly by morphology and reconfiguration of complex plants, with more efficient lateral momentum transport at the interface in the foliated conditions than previously reported for shear layers induced by simpler vegetation.

Hydrodynamic processes in partly vegetated channels have been often investigated by simulating vegetation with arrays of rigid cylinders. By contrast, natural riparian vegetation is generally flexible and presents a complex morphology that influences the dynamic and reconfiguration behavior, deeply affecting the flow structure. The aim of this study is to investigate the impacts of embedding natural plant features in the experimental simulation of flow in partly vegetated channels, in comparison with the rigid cylinder representation. Experiments were carried out with both reconfiguring vegetation made of bushes and grasses, and with rigid cylinders in two facilities. The results on flow structure gained in the two setups are compared. The lateral distributions of normalized mean velocity and lateral Reynolds stress of the considered hydraulically similar vegetated shear layers were observed to depend mainly on the differential velocity ratio and the overall vegetative bulk drag, observing analogous distributions for the two vegetation models. Nevertheless, different shear penetration within the vegetation was observed for flexible and rigid vegetation, with a systematically higher penetration found for natural-like vegetation. The flexibility-induced mechanisms of natural vegetation were found to significantly affect the turbulent flow structure, markedly modifying the lateral exchanges across the interface.

New sustainable, cost-effective solutions are urgently needed for river management since conventional practices have posed serious ecological threats on streams, rivers and the surrounding riparian areas. Besides addressing the societal needs e.g. for flood management, river management should increasingly address the ecosystem requirements for improved water quality and biodiversity. We argue that it is not feasible to solve existing and future river management challenges with intensive restoration projects. Instead, we believe that less resource-intensive solutions using natural channel processes and features, including vegetation, should be investigated. Besides directly supporting biota, aquatic and riparian vegetation traps, takes up and helps to process nutrients and harmful substances, and thus this paper emphasizes vegetation as a tool for nature-based solutions (NBS) in river management. We synthesize findings from key literature, showing that the fate of substances in channel systems is largely controlled by abiotic and biotic processes facilitated and modified by vegetation, including flow hydrodynamics, channel morphology, and sediment transport. Subsequently, we demonstrate how vegetation can be incorporated into channel designs, focusing on a two-stage (compound) design to improve resilience to flooding, control the transport of substances, and enhance the ecological status. As a conclusion, clever use and maintenance of vegetation present an unused potential to obtain large-scale positive environmental impacts in rivers and streams experiencing anthropogenic pressures.

Value of the Data • These data were obtained with reference to two vegetation representations in partly vegetated channels: a novel experimental arrangement in which vegetation was simulated by using artificial flexible foliated plant stands and a second configuration with rigid cylinders. The value of these data relies in the hydraulic similarity achieved between the two sets of experiments and in the use of natural-like vegetation of complex morphology. • River engineers, environmental scientists and researchers interested in flow-vegetation interaction and hydrodynamic processes in partly vegetated channels can benefit from these data. • The described data are optimal for investigating the flow-vegetation interaction in partly vegetated channels, with reference to the mean and turbulent flow structure. These data can be used for developing and validating models for velocity prediction for vegetated flows. Vegetation, generally present along river margins and floodplains, governs key hydrodynamic processes in riverine systems. Despite the flow-influencing mechanisms exhibited by natural vegetation and driven by its complex morphology and flexibility, vegetation has been conventionally simulated by using rigid cylinders. This article presents a dataset obtained from hydraulic experiments performed for investigating the flow-vegetation interaction in partly vegetated channels. Vegetation was simulated by using both natural-like and rigid model plants. Specifically, two sets of experiments are described: in the first, vegetation was simulated with natural-like flexible foliated plants standing on a grassy bed; in the second, rigid cylinders were used. Experiments with rigid cylinders were designed to be compared against tests with natural-like plants, as to explore the effects of vegetation representation. The following experimental data were produced: 3D instantaneous velocity measured by acoustic Doppler velocimetry, vegetation motion video recordings, and auxiliary data including detailed vegetation characterization. These experiments are unique both for the use of natural-like flexible woody vegetation in hydraulic experiments and for the similarity achieved between the resulting observed vegetated shear layers. These data are expected to be useful in vegetated flows model development and validation, and represent a unique benchmark for the interpretation of the flow-vegetation interaction in partly vegetated channels.

[1] Both the foliage and stem essentially influence the flow resistance of woody plants, but their different biomechanical properties complicate the parameterization of foliated vegetation for modeling. This paper investigates whether modeling of flow resistance caused by natural woody vegetation can be improved using explicit description of both the foliage and stem. For this purpose, we directly measured the drag forces of Alnus glutinosa, Betula pendula, Salix viminalis, and Salix x rubens twigs in a laboratory flume at four foliation levels, parameterized with the leaf-area-to-stem-area ratio AL/AS. The species differed in the foliage drag but had approximately equal stem drag. For the foliated twigs, increasing AL/AS was found to increase the reconfiguration and the share of the foliage drag to the total drag. The experiments provided new insight into the factors governing the flow resistance of natural woody vegetation and allowed us to develop a model for estimating the vegetative friction factor using the linear superposition of the foliage and stem drag. The model is novel in that the foliage and stem are separately described with physically based parameters: drag coefficients, reconfiguration parameters, and leaf area and frontal-projected stem area per ground area. The model could satisfactorily predict the flow resistance of twig to sapling-sized specimens of the investigated species at velocities of 0.05–1 m/s. As a further benefit, the model allows exploring the variability in drag and reconfiguration associated with differing abundance of the foliage in relation to the stem.

Summary Reliable estimation of vegetative flow resistance calls for physically sound and readily measurable plant properties. Laboratory flume investigations were conducted to examine four reference area properties in relation to the drag, reconfiguration, and flow resistance of foliated Black Poplar twigs. The experiments were novel in that three characteristic reference areas (leaf area A L , frontal projected area under flow A P , and still-air frontal projected area A 0 ) as well as the foliage–stem reference area ratio ( A L / A S ) were evaluated. The drag forces were simultaneously measured for up to eight specimens in a plant stand at both partly and just submerged conditions. Due to the high A L / A S of the twigs, leaves contributed 74–98% of the total drag at mean velocities of 0.1–0.9 m/s. Both the partly and just submerged poplars had similar A P and drag per characteristic reference area. Thus, the derived parameter values could be used to estimate the friction factors of the poplar stands at low to just submerged conditions, with each of the three characteristic reference areas providing satisfactory estimates. The flow resistance estimation with A L may be further improved by using A L / A S as a secondary area parameter to take into account the share of the stem to the total drag. Comparison to literature data on other deciduous species suggested that the foliage–stem reference area ratio was an essential property for explaining the between-species variation in A P and flow resistance per A L .

The purpose of this study was to quantify how vegetation influences the flow and sediment processes relevant to the design and management of environmental compound channels. Therefore, a two-year field investigation was conducted in a cohesive two-stage channel, focusing on the flow resistance and net deposition in five subreaches with different floodplain vegetation conditions. In the grassy subreaches, the cross-sectional blockage factor was the key vegetation property governing the flow resistance, with a process-based model providing reliable estimates under widely variable hydraulic and vegetative conditions. The net deposition of cohesive sediment was best explained by vegetation height while high stand length and density created supply-limited conditions on the inner floodplain. These results showed that the two-stage approach offers potential for controlling the sediment processes through appropriate vegetation maintenance. The novelty of this research is that straightforward analyses acco...

The authors regret that the article contains a typing mistake in Eq. (5), with an u C 2 term accidentally appearing at the right hand side of the squared brackets.

Adaptation to climate change has become one of the focal points of current development discussion. This article summarises the findings from a multidisciplinary research project looking at climate change impacts and adaptation in the Mekong River Basin in Southeast Asia. The research highlights the central role that the hydrological cycle has in mediating climate change impacts on ecosystems and societies. The findings indicate that climate change should not be studied in isolation, as there are several other factors that are affecting the hydrological cycle. In the Mekong, the most important such factor is the on-going hydropower development that is likely to induce changes at least as radical as climate change, but with shorter timescales. The article concludes that climate change adaptation should broaden its view to consider environmental changes likely to occur due to different factors at various spatial and temporal scales. It is also important to recognise that climate change adaptation is a dynamic, development-orientated process that should consider also broader socio-political context. To enable this, we propose that an area-based adaptation approach should be used more actively to complement the dominant sector-based approaches.

Riparian plants exert flow resistance and largely influence the flow structure, which affects erosion, deposition and transport processes of fine sediments. Predicting these vegetative effects is important for flood, sediment and nutrient management. However, predictions on the fate of sediments are complicated by uncertainties associated with the suitable parameterization of natural plants and the associated effects on the turbulent flow field and on the variables in the transport equations. The aim of this study is to quantify deposition and transport of fine sandy sediment in a partly vegetated channel under laboratory conditions. Care was taken to reproduce conditions typical of vegetated floodplain flows including dense flexible grassy understory as a starting point. The experiments were conducted in a flume that is specifically designed to recirculate fine sediment. We measured suspended sediment concentrations with optical turbidity sensors and determined patterns of net deposition over the vegetated parts of the cross section. The flow field was determined with acoustic Doppler velocimetry. Our investigations are intended to improve future predictions of fine sediment storage and transport in natural or constructed vegetated channels, and the first results reported herein were useful in designing further, on-going experiments with complex combinations of vegetation and channel geometry. Key words: sediment transport, suspended sediment, deposition, riparian vegetation, flow field.

The flood pulse is a key element characterizing the hydrology of the Mekong River and driving the high ecosystem productivity in the Lower Mekong floodplains, both in the Cambodian lowlands and the Mekong Delta in Vietnam. This paper assesses the impacts of climate change, both in terms of changed basin water balance and sea level rise, on the Lower Mekong flood pulse. The impacts were simulated by a three-dimensional hydrodynamic model using the projected changes in sea level and the Mekong mainstream discharge under the influence of climate change as boundary conditions. The model simulations projected that average and maximum water levels and flood duration increase in 2010–2049. The most consistent and notable changes occurred in the average and dry hydrological years. Sea level rise had the greatest effects in the Mekong Delta, whereas the impacts of changed basin water balance were more notable in the upper areas of the Mekong floodplains. The projected impacts were mostly opposite to those resulting from regional water infrastructure development. Higher and longer flooding could cause damage to crops, infrastructure and floodplain vegetation, and decrease the fertile land area. On the other hand, it might boost ecosystem productivity and enhance dry season water availability.

Vegetation affects flow hydrodynamics and transport processes across various spatial scales and is increasingly put forward as a powerful tool for nature-based solutions (NBS) in river management. Complex mixtures of different plant types at a wide range of densities are common on real floodplains, but such conditions are rarely properly reproduced in experimental investigations. This paper reports on the first results of an ongoing study, with a view on flow resistance associated with low relative submergences (H/hv = 1-2) and low vegetation densities (LAI = 0.12-0.58). Such experimental investigations are challenging due to the necessity of a high-accuracy sensor system suited for the bulk scale. A mixture of artificial elements was selected to mimic the behaviour of natural floodplain shrubs and herbaceous vegetation in a laboratory flume. Results suggested that also at very low LAI values the vegetative resistance increased linearly with LAI. An increase of H/hv from 1 to 2 caused a 25% to 50% reduction in the friction factor for very low to intermediate LAI, respectively.

Design of environmentally preferable agricultural drainage channels calls for an improved understanding of cohesive sediment processes. Flow and cohesive sediments were investigated in a new demonstration and test channel where a floodplain was excavated on one side of the existing channel to improve flood conveyance. In this approach, the existing, naturally recovered channel was mostly left intact to reduce environmental impacts. Continuous monitoring of discharge and suspended sediment concentration (SSC) during 1 year revealed that the construction work of the two-stage channel caused 2% of the annual suspended sediment (SS) load. Agricultural areas covering 13% of the catchment were estimated to contribute over half of the annual SS, predominantly during the rising stages. Seasonal positive hysteresis was found in SSC which was explained by different drainage efficiencies of two distinct sediment sources. The temporally varying shares of the two sources caused scatter in the rating curves between dis...