Hydraulic Flume Research Articles

Mechanism and modeling of different plant root effects on soil detachment rate

Plant roots significantly affect soil erosion, although the mechanism of how roots affect the soil detachment rate (SDR) by overland flow on the Loess Plateau is not fully understood. This study was conducted to investigate the mechanism of how roots affect SDR using natural undisturbed soil samples taken from five sampling plots with the dominant species of Agropyron cristatum, Caragana korshinskii, Medicago sativa, Caragana korshinskii, and Robinia pseudoacacia. Bare land was used as the control plot. The samples were subjected to flow scouring through hydraulic flume experiments under six different shear stresses. The results showed that the roots could significantly reduce the SDR by >83.48% compared with the bare soil. The herbs with fibrous roots (A. cristatum) had a strong root-reducing effect on the SDR. Due to the weaker root-reducing effect of herbs with taproots, the comprehensive reducing effect of herb roots was lower than that of shrubs (C. korshinskii). Because of the higher soil organic matter, the tree roots (R. pseudoacacia) still had a higher reduction effect on the SDR even with a smaller root content, and its SDR was between that of herbs and shrubs. The SDR could be divided into 3 zones based on root mass density (RMD): zone Ⅰ (SDR is sensitive to root action), zone Ⅱ (the effect of soil properties on SDR is prominent), and zone III (SDR is relatively stable). The critical conditions between different zones and the main controlling factors affecting the SDR in different zones were analyzed. The prediction accuracy of the SDR model could be improved by constructing the model using the zoning method (NSE > 0.95). A prediction model of SDR suitable for root-soil complexes of different plant species was developed (NSE = 0.90).

К ВОПРОСУ РАЗРАБОТКИ ЛАБОРАТОРНОЙ УСТАНОВКИ ДЛЯ МОДЕЛЬНОГО ИССЛЕДОВАНИЯ ДЕГРАДАЦИИ ПОЧВ (НА ПРИМЕРЕ ВОДНОЙ ЭРОЗИИ)

Purpose: development of a laboratory facility for multicomponent soil analysis in model experiments on their sprinkling. Materials and methods of research. The works of Russian and foreign scientists, patents, laboratory facilities presented in specialized laboratories were used as a material for the study. Research methods included patent search, analysis and synthesis of the data obtained. Results and discussions. To conduct field experiments, an appropriate technical base which may require both the allocation of experimental plots for field studies in the surveyed areas, and significant costs, is needed, so laboratory experiments are the best alternative for model study of various processes and factors. In laboratory installations, the soil is placed in special containers called a flume, an erosive flume, or a hydraulic flume (hydroflume). Depending on the objectives of the study, various installations are used to ensure the water flow to the soil surface: a jet-drop tube, a sprinkler, a perforated plate with a thermostat. A laboratory facility “installation for studying the processes of water erosion of soils” was proposed by scientists of Russian Scientific Research Institute of Land Reclamation Problems. The laboratory installation includes all necessary equipment for conducting experiments with great variability. It can be used to model the terrain slope, rain intensity, the angle of its fall, the study of infiltration, etc. Conclusions. To obtain more accurate results, it is necessary to use multicomponent installations that allow simulating a combination of a large number of factors. The proposed laboratory facility has a wide range of possibilities for studying the erosion resistance of soils, the composition of surface runoff and infiltration, as well as the efficiency of a number of reclamation measures.

Modeling the effect of central impervious core and downstream filter geometry on seepage through earth dams

This investigation aims to identify the seepage discharge in the earth dam resting on an impervious foundation. Twenty-one models of a homogenous earth dam with and without horizontal filter and a central impervious core of different width were constructed in a hydraulic flume. In the present study, fluid and geometrical parameters of the dam, horizontal filter’s and the central impervious core were varied to find their effect on seepage. It was observed that with increasing the dam’s height, filter’s length, downstream slope, and longitudinal slope, there was an increase in seepage discharge and vice versa with upstream slope, clay core’s thickness, the viscosity of the fluid, and the earth dam’s length. Several studies have neglected the variation in fluid's viscosity for the seepage study in earth dam modeling, which is included in this study. Using Slope/w, the stability of the upstream and downstream faces was checked under full reservoir conditions.

Variation in soil detachment capacity of structural and sedimentary crusts induced by simulated rainfall formed on ridge and furrow

The response of erosion to soil physical crusts (i.e., structural and sedimentary crusts) has been extensively investigated. Yet few studies have quantified the effects of physical crusts on their detachment capacity (Dc). Accordingly, we investigated the variation in Dc of physical crusts as induced by simulated rainfall. The runoff plot (2.0 m long × 1.0 m wide × 0.5 m high) was set to a slope of 5° and filled with Lou soil (Haplic Luvisol) using horizontal tillage with ridges (1.0 m long × 0.2 m wide × 0.1 m high) and cm furrows (1.0 m long × 0.3 m wide × 0.1 m deep). Simulated rainfall was applied at a single intensity (60 mm h−1) for seven durations (0, 5, 10, 15, 20, 25, and 30 min) to obtain structural (at ridges) and sedimentary (at furrows) crusts. Crust samples were collected to obtain Dc, by scouring them for 2 min at a constant inflow discharge (0.8 L min−1) on a hydraulic flume (15° slope). For both structural and sedimentary crusts, with increasing rainfall duration, the measured bulk density (BD), crust thickness (Ct), micro-aggregate content (MIA), and disintegration index (DI) increased, whereas the macro-aggregate content (MAA) decreased. Dc of 0 min and structural crust formed within 5 min of rainfall decreased with a longer scouring time. By contrast, Dc of the other crust types initially rose with an extended scouring time but then plateaued after ∼ 60 s. Dc of sedimentary crusts were significantly lower than that of structural crusts. Notably, crusts formed by rainfall at 30 min had greatest detachment reduction benefit. Dc was inversely related to BD, Ct, MIA and DI, whereas increased with increasing MAA. This research provides a timely reference for the effect of physical crust on concentrated flow in arid and semi-arid areas.

The shell phenotypic variability of the keyhole limpet Fissurella latimarginata: insights from an experimental approach using a water flow flume

ABSTRACT Hydrodynamics are a major environmental factor on intertidal rocky shores. Morphological responses to this factor are expected to strongly influence spatial distribution of species across environmental gradients. We here analysed the shell phenotypic variability of the limpet Fissurella latimarginata using geometric morphometric analysis. The limpets were obtained from a sheltered intertidal coastal area and a wave-exposed environment. To determine whether the shell shape variation of the intertidal molluscs is linked to their resistance to differential intertidal wave exposure, mesocosm studies were developed in a hydraulic flume to explore the effects of hydrodynamic forces on this limpet species. A unidirectional current was used to test the impacts of step-by-step increased current flow velocities for each limpet. The phenotypic variability observed in the populations of F. latimarginata was associated with habitats characterized by contrasting wave exposure. Limpets from exposed environments showed a flattened, round to laterally wider and posteriorly narrower shell shape, larger foot and higher full limpet height, and were dislodged at higher velocities. A more laterally compressed and peaked shape was found in limpets from sheltered areas and these showed a lower resistance to wave action by dislodging at lower velocities.

Towards resilient municipal wastewater sewerage systems in khartoum: A surcharged flow measurement device

Sewerage and sanitation-related disasters are becoming more frequent, especially in developing countries. The aftermath of natural disasters is no less devastating than the immediate destruction they cause. In this way, one of the most pressing needs in rapidly urbanizing cities, especially African capitals, is the efficient sanitary facilities in which the flow measurement is an essential element to evaluate the sewerage systems' performance. The traditional devices of measurements are not suitable due to the enormous discharge of sewage network and many solid particles in wastewater might cause a blockage in sewers. Consequently, the accuracy of measurement declines rapidly. This research aims to develop a flow-measuring device for full pipe flow, also known as sanitary sewer overflow. This particular case is an existing phenomenon that faces sewerage authorities worldwide and impacts the whole society. Flow measurement in such a situation is a formidable challenge and needs cutting-edge research to find fitting flowmeters. The device is a modified fiberglass-built venturi that was manufactured locally without the diverging part (cutthroat venturi). The manufacturing was both simple and achieved at a relatively low cost. Two sensors were used to measure the surcharged flow, one in the inlet section and the other in the throat. The modified venturi connects to an Arduino board that is programmed to output the differential pressure. To calibrate the device, a closed pipe submerged into a hydraulic flume channel is adopted to simulate the overflow sewers. To calculate the actual flow, a stopwatch was used together with the known volume of the channel to compute the actual flow, which was compared with the discharge measured by the device. The results showed an acceptable discharge coefficient “Cd” of about 0.9, which meets the standard discharge coefficient of the venturi meter.

Performance analysis of Helical Savonius Hydrokinetic turbines arranged in array

The Helical Savonius hydrokinetic turbines (HSHKT) are regarded as the beneficiary equipment to increase the power generation in low head rivers. This study investigates the performance of HSHKT arranged in arrays. Firstly, performance analyses of two HSHKT rotors are experimentally performed, arranged in inline formation along the fluid flow direction, for different spacing between them (L). These spacing are considered 3D (D = rotor diameter), 3.5D, 4D, 4.5D, 5D, and 5.5D. Experiments are performed in a hydraulic flume of 6 m length at 0.5 m/s velocities for different loading conditions. The optimum performance (i.e., coefficient of power and coefficient of torque) of both HSHKTs together is obtained at a distance of 4D. Consequently, the study is simulated through the ANSYS-CFX for 0.5 m/s upstream velocity and optimum performance is validated at L = 4D. Further, the study is extended in ANSYS-CFX for three HSHKTs arranged in arrays of different patterns, namely triangular and staggered formation. The analysis is performed for different velocities and varying distances between them. It is observed that at a 4D distance, the optimum performance of the three HSHKTs is achieved. Moreover, it is also found that HSHKT performance is better in staggered formation compared to the triangular formation.

Bio-composites treatment for mitigation of current-induced riverbank soil erosion

Mitigation of erosion along the riverbanks is a global challenge. Stabilisers such as cement can control erosion, but it risks the river ecology. This paper presents the erosion characteristics of riverbank soil treated with two biological stabilisers that alleviate the ecological cost. The riverbank soil of one of the largest river systems, Brahmaputra, is treated by bio-polymeric and bio-cement binders and their composite. Moreover, a novel selective bio-stimulation technique has been employed to achieve bio-mineralisation. The soil stabilisation is assessed by needle penetration tests and CaCO3 contents. The specimens were tested in a flow-controlled hydraulic flume subjected to a critical current profile ranging from 0.06 to 0.62 m/s. Soil samples treated up to four cycles of biocementation have been tested at three different slopes (30°, 45° and 53°). The eroded depth and erosion rate are evaluated with image analysis. Up to four-fold reduction in the erosion rate was observed with biocementation treatment. However, cementation beyond a threshold led to the formation of brittle chunks. A bio-composite was devised through a pre-treatment of low-viscosity biopolymer along with biocementation. The bio-composite was found to effectively mitigate the current-induced erosion with 36% lower ammonia production than the equally erosion resistant biocemented counterpart. The dual characteristics of the bio-composite were confirmed with the microstructural analysis. This study unravels the potential of biopolymer-biocement composite as a sustainable erosion mitigation strategy.

Biocementation mediated by native microbes from Brahmaputra riverbank for mitigation of soil erodibility

Riverbank erosion is a global problem with significant socio-economic impacts. Microbially induced calcite precipitation (MICP) has recently emerged as a promising technology for improving the mechanical properties of soils. The present study investigates the potential of selectively enriched native calcifying bacterial community and its supplementation into the riverbank soil of the Brahmaputra river for reducing the erodibility of the soil. The ureolytic and calcium carbonate cementation abilities of the enriched cultures were investigated with reference to the standard calcifying culture of Sporosarcina pasteurii (ATCC 11859). 16S rRNA analysis revealed Firmicutes to be the most predominant calcifying class with Sporosarcina pasteurii and Pseudogracilibacillus auburnensis as the prevalent strains. The morphological and mineralogical characterization of carbonate crystals confirmed the calcite precipitation potential of these communities. The erodibility of soil treated with native calcifying communities was examined via needle penetration and lab-scale hydraulic flume test. We found a substantial reduction in soil erosion in the biocemented sample with a calcite content of 7.3% and needle penetration index of 16 N/mm. We report the cementation potential of biostimulated ureolytic cultures for minimum intervention to riparian biodiversity for an environmentally conscious alternative to current erosion mitigation practices.

Root Characteristics and Water Erosion-Reducing Ability of Alpine Silver Grass and Yushan Cane for Alpine Grassland Soil Conservation

In Taiwan, intensive forest fires frequently cause serious forest degradation, soil erosion and impacts on alpine vegetation. Post-fire succession often induces the substitution of forest by alpine grassland. Alpine silver grass (Miscanthus transmorrisonensis Hay.) and Yushan cane (Yushania niitakayamensis (Hay.) Keng f.) are two main endemic species emerging on post-fire alpine grassland. These species play a major role in the recovery of alpine vegetation and soil conservation of alpine grassland. However, their root traits, root mechanical properties and water erosion-reducing ability have still not been well studied. In the present study, root characteristics were examined using a complete excavation method. Root mechanical characteristics were estimated by utilizing the uprooting test and root tensile test, and hydraulic flume experiments were performed to investigate the water erosion-reducing ability using 8-month-old plants. The results show that the root architecture system of Alpine silver grass belongs to fibrous root system, while the Yushan cane has sympodial-tufted rhizomes with a fibrous root system. Root characteristics reveal that relative to Alpine silver grass, Yushan cane has remarkably larger root collar diameter, higher root biomass, larger root volume, higher root density, and a higher root tissue density. Furthermore, uprooting resistance of Yushan cane is notably higher than that of Alpine silver grass. However, the root tensile strength of Alpine silver grass is significantly higher than that of Yushan cane. Additionally, hydraulic flume experiments reveal that Yushan cane has significantly lower soil detachment rates than that of Alpine silver grass. Collectively, these findings clearly show that Yushan cane has superior root characteristics and water erosion-reducing ability than Alpine silver grass and is thus more suitable for the conservation of alpine grassland.

Heat and water flux modeling in an earth dam.

This study aims to identify the water flux in an earth dam using heat flux due to convection. Sixteen earth dam models were constructed in a hydraulic flume by varying geometrical and flow input parameters to identify heat and water flux. Homogeneous as well as earth dams with clay cores were built in a hydraulic flume. Temperature measurements were done to calculate heat flux in the experimental model. A finite element model of the earth dam using Seep/w was developed to obtain water flux, while temp/w was used to obtain heat flux. These results were used as input in Temp/w and Seep/w in Geostudio 2020. Significant reduction of the heat and water flux was seen while comparing the homogeneous models with central impervious core models. An increase in the heat and water flux was observed on increasing the downstream filter's length, longitudinal slope, and vice versa with the upstream slope and the thickness of the clay core. Comparing fluxes in a homogeneous dam model (model 1) with the clay core model (model 9) with top width 2.4 m and bottom width 18 m in model 9, both water flux and heat flux were reduced by 78.46%. While comparing it with model 10, with bottom core width of 18 m and top core width of 1.9 m, both water flux and heat flux reduced by 77.72%. Heat flux measurements were found to be a valuable alternative to detecting water flux and seepage in an earth dam at a reduced cost.

Erosion-reducing effects of plant roots during concentrated flow under contrasting textured soils

The effects of plant roots on water erosion are likely to vary according to the range of root traits and soil textures. However, little is known about the erosion-reducing potential of roots under different soil textures due to the lack of sufficient texture variations which come from a comparable system. In this study, we systematically investigate how root functional traits respond to soil sand contents and their combined effects on rill erosion. Seeding of Cynodon dactylon (L.) Pers. were grown in a greenhouse under varied proportions of sand mass (ranging from 14% to 83%) by mixing silty clay soil (Ultisols derived from quaternary red clay) and engineering sand. After six months growing, the root-soil composites were subjected to concentrated flow in a hydraulic flume to obtain soil detachment capacity (Dc), then roots were separated to measure their morphology and architecture traits. The results showed that sand content significantly affected root morphology traits (e.g., RD and RLD) (p < 0.01). In contrast, root architecture traits (e.g., root fractal dimension, topological index and radius frequency distribution function) showed no significant changes with soil texture (p > 0.05). Generally, root strongly promoted soil cohesion and they mitigated rill erodibility in a coordinated way. Rill erodibility was mostly determined by soil texture even in soils with dense roots, and the erosion-reducing effects of roots were less efficient in heavy-textured soils. Moreover, no significant correlations were found between critical flow shear stress (τc) and root traits, and soil with a loam texture displayed the lowest τc. Those relationships are likely to exist elsewhere, but its strength and direction could be changed according to the root trait variability and the varied root-soil feedbacks. The results of this study would be quite relevant for soil erosion modeling and imply the bare sandy soils are more urgent to be recovered during engineering constructions. Moreover, the inclusion of plant with dense fibrous roots could be the optimal species mixtures to stable sandy slopes.

Plant structural diversity alters sediment retention on and underneath herbaceous vegetation in a flume experiment.

Sediment retention is a key ecosystem function provided by floodplains to filter sediments and nutrients from the river water during floods. Floodplain vegetation is an important driver of fine sediment retention. We aim to understand which structural properties of the vegetation are most important for capturing sediments. In a hydraulic flume experiment, we investigated this by disentangling sedimentation on and underneath 96 vegetation patches (40 cm x 60 cm). We planted two grass and two herb species in each patch and conducted a full-factorial manipulation of 1) vegetation density, 2) vegetation height, 3) structural diversity (small-tall vs tall-tall species combinations) and 4) leaf pubescence (based on trait information). We inundated the vegetation patches for 21 h in a flume with silt- and clay-rich water and subsequently measured the amount of accumulated sediment on the vegetation and on a fleece as ground underneath it. We quantified the sediment by washing it off the biomass and off the fleece, drying the sediment and weighting it. Our results showed that all manipulated vegetation properties combined (vegetation density and height, and the interaction of structural diversity and leaf pubescence) explained sedimentation on the vegetation (total R2 = 0.34). The sedimentation underneath the vegetation was explained by the structural diversity and the leaf pubescence (total R2 = 0.11). We further found that vegetation biomass positively affected the sedimentation on and underneath the vegetation. These findings are crucial for floodplain management strategies with the aim to increase sediment retention. Based on our findings, we can identify management strategies and target plant communities that are able to maximize a floodplain's ability to capture sediments.

The effectiveness of selected vegetation communities in regulating runoff and soil loss from regraded gully banks in the Mollisol region of Northeast China

AbstractThe steep slopes of actively developed gullies are extremely sensitive to erosion in the Mollisol region of Northeast China; however, decision making on the optimal configuration for gully bank protection is under discussion. Here, five vegetation communities were established on regraded gully banks with bare slopes as the control to identify their controlling effects on flow hydraulics and soil erosion through in situ scouring experiments. A hydraulic flume experiment was conducted to clarify the ability of roots to reduce erosion. For vegetated slopes, flow velocity, stream power, and unit stream power decreased by 42–67%, 0–18%, and 44–68%, respectively, while flow depth, shear stress, the Darcy–Weisbach friction coefficient, and Manning coefficient increased by 55–150%, 69–156%, 5–22‐times and 2–5‐times, respectively. Changes in the seven flow hydraulic parameters were significantly influenced by vegetation coverage, aboveground biomass, root mass density and litter biomass (p &lt; .05). Consequently, the five vegetation communities experienced decreased runoff and soil loss by 19–30% and 78–97%, respectively. Vegetation with tap‐roots and rich roots of 1.0–2.0 mm in diameter was more effective in controlling gully bank erosion. Thus, the vegetation community consisting of Dactylis glomerata, Festuca arundinacea, Uraria crinita and Medicago sativa (25:25:25:25) is perfectly acceptable for gully bank protection in the Mollisol region of Northeast China. The results provide a scientific reference for selecting optimal vegetation configuration for gully bank rehabilitation.

A METHOD FOR EVALUATING TSUNAMI LOADING ON SEAWALLS DURING OVERFLOW

Seawall constructions are one of the main ways of protecting coasts against tsunamis. The estimation of tsunami loading on a structure is important for evaluating the fragility of seawalls. In this study, hydraulic flume tests were conducted to investigate the characteristics of tsunami loading when tsunamis flow over seawalls. Correlations between the loading on seawalls and the specific energy of the flow are investigated. A method for evaluating the maximum tsunami loading on the seawall is proposed. The approach can evaluate the maximum tsunami loading, even when tsunamis flow over seawalls.

Model Study to Determine the Manning’s Coefficient for Gabion Lining against Canal Erosion

Canal erosion often happens in unlined canals due to high velocity water flows. Because of this natural phenomenon, numerous problems occur in canal systems and finally change the topography of the canals in the long-term. Canal lining is the method adopted worldwide, to prevent canal erosion using different lining materials available such as concrete, rock masonry, brick or clay, with the modern practice being gabion nets filled with rubble. Considering several factors such as, economy, stability, material availability and manpower requirement, gabion lining is a suitable canal lining method compared to other prevention methods used at present. To make the gabion lining method very effective, economical and feasible in canal lining applications, the surface roughness or the Manning’s coefficient (n) value of gabion linings needs to be experimentally determined first, as this value is unknown at present and will be useful for the scientists and engineers in design stages later on. A 300 mm x 300 mm rectangular field channel was selected as the prototype to model it dimensionally in a laboratory hydraulic flume. The Manning’s formula, which applies to uniform flow conditions in open channel flow, was used to determine “n” experimentally. Present experimental observations indicate a value range of 0.05044 m-1/3s to 0.0552 m-1/3s as Manning’s coefficient for the test model study and that of 0.0597 m-1/3s to 0.0654 m-1/3s for the prototype.

Hydrodynamic characteristics and evolution law of roll waves in overland flow

The frequent occurrence of roll waves in overland flow can increase the water depth and accelerate sheet erosion. To investigate the variation laws and hydrodynamic characteristics of roll waves, indoor artificial experiments with five roughness types, seventeen flow discharges, and five slope gradients were conducted in a hydraulic flume. The results showed that the evolution process of roll waves is divided into three phases: monochromatic, quasi-sine, and mature wave regions. In addition, the parameter roll-wave velocity increased in terms of its power function with the unit discharges, while the slopes showed that a positive line was related to this increase. An empirical formula of the wave velocity was obtained by conducting a regression analysis after synthesizing the influencing factors. The wavelength varied at a single peak with increasing unit discharges, showing a negative linear correlation with the test slope. Additionally, an empirical formula of the wavelength was derived using regression methods. The correction coefficients in the laminar flow instability zone varied from 0.27 to 0.39, while the Reynolds number was approximately 100. The critical average Freud number Fr and the Onsager value Z were 0.6 and 3.33 × 10−3, respectively. In addition, the correction coefficients in the turbulent flow instability zone varied within 0.412–0.730, while the Reynolds number fluctuated between 900 and 3300. The range of the critical Fr value was 1.59–2.20, and the critical average Z was 1.88 × 10−3. These results are vital for understanding the hydrodynamic characteristics and evolution laws of roll waves in overland flow.

A bottom mounted wavemaker in water wave flumes

Reflections of the surface waves in hydraulic wave flumes could affect the performance of the common piston, hinged and plunger type wavemakers. This paper proposes a new wavemaker to produce surface waves where the possible reflections are barely able to affect its performance. The proposed wavemaker includes an oscillating paddle at the bottom of the flume where the produced upward flow induces travelling surface waves. An analytical study is carried out on the wave generation of the system. It is shown that the paddle length could significantly affect the height of the generated waves, which could not happen in conventional wavemakers. Effects of the water depth and the excitation frequency on the induced surface waves are also investigated. It is demonstrated that for a specific paddle length, water depth and forcing period the best performance of the system could be achieved.

Variations in Soil Erosion Resistance of Gully Head Along a 25-Year Revegetation Age on the Loess Plateau

The effects of vegetation restoration on soil erosion resistance of gully head, along a revegetation age gradient, remain poorly understood. Hence, we collected undisturbed soil samples from a slope farmland and four grasslands with different revegetation ages (3, 10, 18, 25 years) along gully heads. Then, these samples were used to obtain soil detachment rate of gully heads by the hydraulic flume experiment under five unit width flow discharges (2–6 m3 h). The results revealed that soil properties were significantly ameliorated and root density obviously increased in response to restoration age. Compared with farmland, soil detachment rate of revegetated gully heads decreased 35.5% to 66.5%, and the sensitivity of soil erosion of the gully heads to concentrated flow decreased with revegetation age. The soil detachment rate of gully heads was significantly related to the soil bulk density, soil disintegration rate, capillary porosity, saturated soil hydraulic conductivity, organic matter content and water stable aggregate. The roots of 0–0.5 and 0.5–1.0 mm had the highest benefit in reducing soil loss of gully head. After revegetation, soil erodibility of gully heads decreased 31.0% to 78.6%, and critical shear stress was improved by 1.2 to 4.0 times. The soil erodibility and critical shear stress would reach a stable state after an 18-years revegetation age. These results allow us to better evaluate soil vulnerability of gully heads to concentrated flow erosion and the efficiency of revegetation.

Microplastic retention by marine vegetated canopies: Simulations with seagrass meadows in a hydraulic flume.

Marine canopies formed by seagrass and other coastal vegetated ecosystems could act as sinks of microplastics for being efficient particle traps. Here we investigated for the first time the occurrence of microplastic retention by marine canopies in a hydraulic flume under unidirectional flow velocities from 2 to 30cms-1. We used as model canopy-forming species the seagrass Zostera marina with four canopy shoot density (0, 50, 100, 200 shoots m-2), and we used as microplastic particles industrial pristine pellets with specific densities from 0.90 to 1.34gcm-3 (polypropylene PP; polystyrene PS; polyamide 6PA; and polyethylene terephthalate PET). Overall, microplastics particles transported with the flow were retained in the seagrass canopies but not in bare sand. While seagrass canopies retained floating microplastics (PP) only at low velocities (<12cms-1) due to a barrier created by the canopy touching the water surface, the retention of sinking particles (PS, PA, PET) occurred across a wider range of flow velocities. Our simulations revealed that less dense sinking particles (PS) might escape from the canopy at high velocities, while denser sinking particles can be trapped in scouring areas created by erosive processes around the eelgrass shoots. Our results show that marine canopies might act as potential barriers or sinks for microplastics at certain bio-physical conditions, with the probability of retention generally increasing with the seagrass shoot density and polymer specific density and decreasing with the flow velocity. We conclude that seagrass meadows, and other aquatic canopy-forming ecosystems, should be prioritized habitats in assessment of microplastic exposure and impact on coastal areas since they may accumulate high concentration of microplastic particles that could affect associated fauna.