Bedload Flux Research Articles

Passive Acoustic Measurement of Bedload Transport: Toward a Global Calibration Curve?

AbstractBedload transport is recognized as a key process in the development of river channel forms; however, most rivers suffer from an absence of data. Performing bedload measurements to document bedload transport rates is a challenge, as the deployment of traditional bedload samplers is time consuming and risky in floods. Consequently, bedload measurements are rarely executed. Alternative techniques are being developed to complement the use of traditional bedload measurements and to provide continuous monitoring. Passive acoustic measurements are made with hydrophones, measuring the underwater sounds naturally generated by bedload impacts in rivers. This paper proposes an innovative deployment of hydrophones to record bedload sounds at the scale of a cross section. The measured acoustic signals are interpreted with bedload samplings and with hydraulic and river bed parameters. Field experiments were done in 14 different sites, exploring a diversity of rivers. Bedload flux was observed to be the most consistent variable explaining the monitored acoustic power. Based on 25 experiments on 14 rivers, the cross‐section‐averaged acoustic power was related to the specific bedload flux and showed a good agreement (60% of bedload flux estimated within a factor of 2). The robustness of the obtained calibration curve remains to be tested. However, the potential of passive acoustic profiles to provide a continuous measurement of bedload sounds that could be used in the development of bedload gauging stations is shown.

Numerical modeling of bedload and suspended load contributions to morphological evolution of the Seine Estuary (France)

This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E0), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.

Seismic Monitoring of a Subarctic River: Seasonal Variations in Hydraulics, Sediment Transport, and Ice Dynamics

AbstractHigh‐latitude rivers are commonly covered by ice for up to one third of the year. Our understanding of the effects of ice on channel morphodynamics and bedload transport is hindered by the difficulties of sensing through the ice and dangers of field work on thin ice or during ice break‐up. To avoid this drawback, we used seismic signals to interpret processes and quantify water and sediment fluxes. Our objective was to determine seasonal differences in hydraulics and bedload sediment transport under ice‐covered versus open‐channel flow conditions using a small seismic network and to provide a first‐order estimation of sediment flux in a Fennoscandian river. Our study reach was on a straight, low‐gradient section of the Sävar River in northern Sweden. Interpretations of seismic signals, from a station 40 m away from the river, and inverted physical models of river stage and bedload flux indicate clear seasonal differences between ice‐covered and open‐channel flow conditions. Diurnal cycles in seismic signals reflecting turbulence and sediment transport are evident directly after ice break‐up. Analysis of seismic signals of ice‐cracking support our visual interpretation of ice break‐up timing and the main ice break‐up mechanism as thermal rather than mechanical. Assuming the bulk of sediment moves during ice break‐up and the snowmelt flood, we calculate a minimum annual sediment flux of 56.2 ± 0.7 t/km2, which drastically reduces the uncertainty from previous estimates (0–50 t/km2) that exclude ice‐covered or ice break‐up periods.

Importance of Wave Non-linearity for 3D Morphodynamic Modelling

Mengual, B.; Bertin, X., and Martins, K., 2020. Importance of wave non-linearity for 3D morphodynamic modelling. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1201-1205. Coconut Creek (Florida), ISSN 0749-0208.The effect of wave non-linearity on morphological changes of a sandbar is investigated through a realistic application at Duck Beach (North Carolina, USA) of a 3D state-of-the-art process-based morphodynamic model, which couples sediment transport, currents and waves (vortex force formalism). From simplified 1D/2DH models, previous studies highlighted that acceleration skewness of non-breaking waves over a sandbar could promote its progressive onshore migration. This process can counterbalance the offshore migration occurring under breaking waves through the development of strong offshore-directed “undertow” currents near the seabed. Based on the existing literature, an additional bedload flux associated to acceleration skewness of waves is implemented in the 3D model. Numerical experiments with and without this additional term clearly demonstrate the need to account for this supplementary wave-induced transport to reproduce onshore migration phases of the sandbar. Effectively, even a model integrating wave asymmetry effects on bedload flux estimates and 3D wave-current interactions fails to reproduce the observed onshore migration of the sandbar.

Field Application and Validation of a Seismic Bedload Transport Model

AbstractBedload transport drives morphological changes in gravel‐bed streams and sediment transfer in catchments. The large impact forces associated with bedload motion and its highly dynamic spatiotemporal nature make it difficult to monitor bedload transport in the field. In this study, we revise a physically‐based model of bedload‐induced seismic ground motion proposed by Tsai et al. (2012) and apply it to invert bedload flux from seismic measurements alongside an Alpine stream. First, we constrain the seismic response of a braided river reach with a simple active experiment using a series of large‐rock impacts. This allows the characterization of surface wave propagation and attenuation with distance from the impact source. Second, we distinguish bedload‐generated ground vibrations from those caused by turbulent flow using frequency‐based scaling relationships between seismic power and discharge. Finally, absolute bedload transport rates are quantified from seismic measurements using inverse modeling based on a simplified formulation of bedload particle motion. The results are verified with a large data set of bedload samples (between 0.01 and 1 kg/m/s), demonstrating that seismic measurements can provide an indirect measure for bedload flux with an uncertainty of less than one order of magnitude. Larger deviations may be due to the contribution of turbulent flow effects and remaining uncertainties in particle impact mechanics (elasticity and velocity). When constraining these uncertainties, seismic monitoring may provide an accurate and continuous means to investigate bedload dynamics in gravel‐bed streams.

Joint Stochastic Bedload Transport and Bed Elevation Model: Variance Regulation and Power Law Rests

We describe the joint dynamics of bedload transport and bed elevation changes with a stochastic population model, and we analyze (1) the dependence of bedload flux statistics on local bed elevations and (2) resting time distributions for sediment undergoing burial in the fluctuating sedimentary bed. The model involves entrainment and deposition in a control volume characterized by elevation‐dependent rates, and it exhibits a statistical regulation effect, whereby bed aggradation suppresses the variance of the bedload flux while degradation enhances it. This variance regulation effect is contingent on collective entrainment, whereby moving grains destabilize stationary grains in a positive feedback. When collective entrainment is turned off,bedload transport fluctuations become independent of the bed elevation. Return times from above in the bed elevation time series provide heavy‐tailed power law distributions of resting times with tail behavior characterized by the mean erosion rate and the active layer depth. These results imply bedload statistics measurements on relatively short timescales can be strongly biased by bed elevation changes when collective entrainment occurs, and they support the growing consensus that sediment burial generates heavy‐tailed sediment resting times that ultimately generate anomalous bedload diffusion.

Estimating Bedload From Suspended Load and Water Discharge in Sand Bed Rivers

AbstractEstimates of fluvial sediment discharge from in situ instruments are an important component of large‐scale sediment budgets that track long‐term geomorphic change. Suspended sediment load can be reliably estimated using acoustic or physical sampling techniques; however, bedload is difficult to measure directly and can consequently be one of the largest sources of uncertainty in estimates of total load. We propose a physically informed predictive empirical model for bedload sand flux as a function of variables that are measured using existing acoustic or physical sampling techniques. This model depends on the assumption that concentration and grain size in suspension are in equilibrium with reach‐averaged boundary conditions. Bayesian inference is used to fit model parameters to data from eight sand‐bed rivers and to simulate bedload flux over the available gage record at one site on the Colorado River in Grand Canyon National Park. We find that the cumulative bedload flux during the 9 year period from 2008 to 2016 was 5% of the cumulative suspended sand load; however, instantaneous bedload flux ranged from as little as 1% of instantaneous suspended sand load to as much as 75% of instantaneous suspended sand load due to fluctuations in flow strength and sediment supply. Changes in bedload flux at a constant discharge are indicative of short‐term sediment supply enrichment and depletion. Long‐term average bedload flux cannot be expected to remain constant in the future as the river adjusts to changes in sediment runoff and the dam‐regulated discharge regime.

Joint Sensing of Bedload Flux and Water Depth by Seismic Data Inversion

AbstractRivers are the fluvial conveyor belts routing sediment across the landscape. While there are proper techniques for continuous estimates of the flux of suspended solids, constraining bedload flux is much more challenging, typically involving extensive measurement infrastructure or labor‐intensive manual measurements. Seismometers are potentially valuable alternatives to in‐stream devices, delivering continuous data with high temporal resolution on the average behavior of a reach. Two models exist to predict the seismic spectra generated by river turbulence and bedload flux. However, these models require estimating a large number of parameters and the spectra usually overlap significantly, which hinders straightforward inversion. We provide three functions contained in the R package “eseis” that allow generic modeling of hydraulic and bedload transport dynamics from seismic data using these models. The underlying Monte Carlo approach creates lookup tables of potential spectra, which are compared against the empirical spectra to identify the best fitting solutions. The method is validated against synthetic data sets and independently measured metrics from the Nahal Eshtemoa, Israel, a flash flood‐dominated ephemeral gravel bed river. Our approach reproduces the synthetic time series with average absolute deviations of 0.01–0.04 m (water depth, ranging between 0 and 1 m) and 0.00–0.04 kg/sm (bedload flux, ranging between 0 and 4 kg/sm). The example flash flood water depths and bedload fluxes are reproduced with respective average deviations of 0.10 m and 0.02 kg/sm. Our approach thus provides generic, testable, and reproducible routines for a quantitative description of key metrics, hard to collect by other techniques in a continuous and representative manner.

Mechanics of bed particle saltation in turbulent wall-shear flow

In this paper, we explore the mechanics of bed particle saltation in turbulent wall-shear flow, analysing the forces on a particle to perform saltation. The hydrodynamic drag encompasses the form drag and turbulent drag. The hydrodynamic lift comprises the Saffman lift, Magnus lift and turbulent lift. The subtle role of the Basset force in governing the particle trajectory is accounted for in the analysis. The bedload flux, emanating from the mathematical analysis of bed particle saltation, is determined. The results reveal that for the particle parameter range 20–100, the transport stage function equalling unity corroborates the threshold of bed particle saltation, where the saltation height and length are 1.3 and 9 times the particle size. For a given transport stage function, the relative saltation height and length decrease with an increase in particle parameter. For the particle parameter range 20–100, the relative saltation height and length increase with an increase in transport stage function, reaching their peaks, and then, they decrease. For a given particle parameter, the peak and mean particle densimetric Froude numbers increase as the transport stage function increases. The bedload flux curves for particle parameters 26 and 63 produce the upper and lower bound curves, respectively.

The impact of inter‐flood duration on non‐cohesive sediment bed stability

AbstractLimited field and flume data suggests that both uniform and graded beds appear to progressively stabilize when subjected to inter‐flood flows as characterized by the absence of active bedload transport. Previous work has shown that the degree of bed stabilization scales with duration of inter‐flood flow, however, the sensitivity of this response to bed surface grain size distribution has not been explored. This article presents the first detailed comparison of the dependence of graded bed stability on inter‐flood flow duration. Sixty discrete experiments, including repetitions, were undertaken using three grain size distributions of identical D50 (4.8 mm); near‐uniform (σg = 1.13), unimodal (σg = 1.63) and bimodal (σg = 2.08). Each bed was conditioned for between 0 (benchmark) and 960 minutes by an antecedent shear stress below the entrainment threshold of the bed (τ*c50). The degree of bed stabilization was determined by measuring changes to critical entrainment thresholds and bedload flux characteristics.Results show that (i) increasing inter‐flood duration from 0 to 960 minutes increases the average threshold shear stress of the D50 by up to 18%; (ii) bedload transport rates were reduced by up to 90% as inter‐flood duration increased from 0 to 960 minutes; (iii) the rate of response to changes in inter‐flood duration in both critical shear stress and bedload transport rate is non‐linear and is inversely proportional to antecedent duration; (iv) there is a grade dependent response to changes in critical shear stress where the magnitude of response in uniform beds is up to twice that of the graded beds; and (v) there is a grade dependent response to changes in bedload transport rate where the bimodal bed is most responsive in terms of the magnitude of change. These advances underpin the development of more accurate predictions of both entrainment thresholds and bedload flux timing and magnitude, as well as having implications for the management of environmental flow design. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

Bedload transport measurements in the Gilgel Abay River, Lake Tana Basin, Ethiopia

Little is known about the contribution of bedload flux to the total sediment load exported from tropical sand-bed rivers. Yet, predicting bedload transport rates contributes to better river management, more appropriate river engineering works design, accurate knowledge of sediment delivery rates, which greatly benefits sediment-related water resources management. Therefore, bedload, suspended load and runoff discharge were measured at the lower Gilgel Abay River in the Blue Nile catchment of Ethiopia. Catchment area is 3887 km2, average stream discharge in the rainy season 315 (±164) m3 s−1, and the stream surface gradient at the sampling station 0.0007 m m−1. A standard Helley-Smith (U.S. BL-84) cable-suspended sampler was used to take 315 samples to measure bedload fluxes for the 2017 rainy season. The bedload predominantly comprises well to moderately well sorted, medium-sized sand (250–500 µm) with a symmetric and leptokurtic distribution. The mean daily observed bedload discharge varies from 0.7 to 427.5 Mg d−1 with an average rate of 175.1 Mg d−1. Field observations also show that the bedload dynamics become larger near the river banks and decrease towards the middle of the river section due to the presence of a central bridge pier. The average suspended sediment load ranges from 247 to 220,028 Mg d−1 and dominates the total sediment yield (>94.6%). The bedload accounts for only 0.1 to 5.4% of the total sediment load. The area-specific bedload and suspended load are also 0.6 and 22 Mg ha−1 yr−1 respectively. The elongated planform growth of the Gilgel Abay River delta in Lake Tana is also in line with the predominance of very fine, suspended load in the sediment transport of this river, in addition to limited alongshore drifting given the low waves of Tana lake. Among the tested bedload prediction criteria, the stream-power based equations of Martin and modified Bagnold performed better. Yet, the bedload prediction performance of simple power regression equations based on runoff discharge and excess unit stream power parameters provided the best agreement with the observed bedload data (Adj. R2 0.82–0.87). Finally, this study provides plausible estimates of bedload fluxes that are within predicted ranges reported for sand-bed rivers in Africa and other tropical regions.

Impact of Vegetation on Bed Load Transport Rate and Bedform Characteristics

AbstractThe impacts of aquatic vegetation on bed load transport rate and bedform characteristics were quantified using flume measurements with model emergent vegetation. First, a model for predicting the turbulent kinetic energy, kt, in vegetated channels from channel average velocity U and vegetation volume fraction ϕ was validated for mobile sediment beds. Second, using data from several studies, the predicted kt was shown to be a good predictor of bed load transport rate, Qs, allowing Qs to be predicted from U and ϕ for vegetated channels. The control of Qs by kt was explained by statistics of individual grain motion recorded by a camera, which showed that the number of sediment grains in motion per bed area was correlated with kt. Third, ripples were observed and characterized in channels with and without model vegetation. For low vegetation solid volume fraction (ϕ ≤ 0.012), the ripple wavelength was constrained by stem spacing. However, at higher vegetation solid volume fraction (ϕ=0.025), distinct ripples were not observed, suggesting a transition to sheet flow, which is sediment transport over a plane bed without the formation of bedforms. The fraction of the bed load flux carried by migrating ripples decreased with increasing ϕ, again suggesting that vegetation facilitated the formation of sheet flow.

Bed particle saltation in turbulent wall-shear flow: areview.

Bed particle saltation in turbulent wall-shear flow remains an intriguing phenomenon in applied hydro-dynamics. In this review, we report the current state of the art of bed particle saltation in turbulent wall-shear flow, highlighting the physical characteristics of bed particle saltation and its mathematical modelling. A critical appraisal of the mechanics of bed particle saltation is presented thorough ample experimental evidence. The salient features of bed particle saltation, encompassing the saltation height, saltation length, particle velocity, saltation duration, particle collision with the bed, particle rotation, particle resting time and particle re-entrainment, are thoroughly discussed. Both the deterministic and computational fluid dynamics approaches in modelling bed particle saltation are summarized, and the subtle role of the hydrodynamic forces is elaborated. The estimation of bedload flux in a fluvial environment, emanating from the mathematical modelling of bed particle saltation, is delineated using different modelling approaches. Finally, the challenges in modelling bed particle saltation are highlighted, and a new look at bed particle saltation is furnished.

Extreme sediment fluxes in a dryland flash flood

A flash flood on 28th September, 2012, rose to a peak discharge of 2357 m3 s−1 from zero within one hour in the ephemeral Nogalte channel in SE Spain. Channel morphology and sediment sizes were measured at existing monitored sites before and after the flood and peak flow hydraulics calculated from surveyed floodmarks and cross-sections. Maximum peak sediment fluxes were calculated as ~600 kg s−1 m−1, exceeding maximum published, measured dryland channel values by 10 times and common perennial stream fluxes by 100 times. These high fluxes fit the established simple bedload flux - shear stress relations for dryland channels very well, but now extended over a much wider data range. The high sediment fluxes are corroborated by deposits at >1 m height in a channel-side tank, with 90 mm diameter sediment carried in suspension, by transport of large blocks and by massive net aggradation as extensive, structureless channel bars. Very high sediment supply and rapid hydrograph rise and recession produced the conditions for these exceptional sediment dynamics. The results demonstrate the extreme sediment loads that may occur in dryland flash floods and have major implications for catchment and channel management.

Determining the scales of collective entrainment in collision-driven bed load

Abstract. Fluvial bed-load transport is notoriously unpredictable, especially near the threshold of motion where stochastic fluctuations in sediment flux are large. Laboratory and field observations suggest that particles are entrained collectively, but this behavior is not well resolved. Collective entrainment introduces new length scales and timescales of correlation into probabilistic formulations of bed-load flux. We perform a series of experiments to directly quantify spatially clustered movement of particles (i.e., collective motion), using a steep-slope 2-D flume in which centimeter-scale marbles are fed at varying rates into a shallow and turbulent water flow. We observe that entrainment results exclusively from particle collisions and is generally collective, while particles deposit independently of each other. The size distribution of collective motion events is roughly exponential and constant across sediment feed rates. The primary effect of changing feed rate is simply to change the entrainment frequency, although the relation between these two diverges from the expected linear form in the slowly driven limit. The total displacement of all particles entrained in a collision event is proportional to the kinetic energy deposited in the bed by the impactor. The first-order picture that emerges is similar to generic avalanching dynamics in sandpiles: “avalanches” (collective entrainment events) of a characteristic size relax with a characteristic timescale regardless of feed rate, but the frequency of avalanches increases in proportion to the feed rate. The transition from intermittent to continuous bed-load transport then results from the progressive merger of entrainment avalanches with increasing transport rate. As most bed-load transport occurs in the intermittent regime, the length scale of collective entrainment should be considered a fundamental addition to any probabilistic bed-load framework.

The trouble with shear stress

Predicted bedload fluxes often have large errors partly because calculations usually do not include spatial and temporal flow variability. Sediment motion is driven by a temporal distribution of applied fluid forces, which are often simplified into time-averaged flow velocities or shear stresses in sediment transport equations. Spatial variations in these flow parameters are also often neglected. Given the nonlinearities in the relation between flow and sediment flux, the reach-averaged sediment flux calculated with an average velocity or shear stress will not necessarily equal the measured spatially averaged bedload flux. Such spatial and temporal flow variations are ignored with the commonly used reach-averaged shear stress (τds) that is a function of the flow depth and channel slope. To demonstrate the uncertainties with using reach-averaged shear stresses, and even local shear stresses, to predict bedload transport rates, we have three goals. First, we outline the derivations and assumptions involved with commonly used estimates of shear stress. Second, we summarize the literature on the uncertainties involved with using shear stress, and other space- and time-averaged flow parameters, to predict sediment flux. Third, we use a new data set from experiments to add further support to a growing body of literature that suggests τds may not be linked to the mechanics of bedload transport. We show considerable uncertainties exist in estimating a reach-averaged shear stress, by comparing τds and near-bed shear stresses calculated from velocity profiles, Reynolds stresses, and local turbulence. For example, τds was systematically greater than these near-bed stresses even for experiments without large bed roughness. We also find that correlations between local time-averaged sediment fluxes and local Reynolds stresses or turbulence intensities are relatively poor. The near-bed, time-averaged local flow velocity has the strongest relation with the time-averaged local bedload flux because it is closely related to the forces that actually drive sediment transport. However, sediment transport rates in individual experiments could still vary by orders of magnitude for a given value of this flow velocity, suggesting that additional work is needed to identity a parameter, or distributions of parameters, that will best predict sediment fluxes.

Morpho-textural implications to bedload flux and texture in the sand-gravel ephemeral Poveda Gully

We report on channel morpho-texture and bedload transport in a natural, steep, sand-gravel ephemeral channel draining the small Poveda Gully watershed in the mining area of the Alto Tajo Natural Park, Spain. First-ever continuous bedload flux and texture monitoring in a transitional sand-gravel environment was undertaken by two independent Reid-type slot samplers. Morphological changes in the feeder reach have been quantified by TLS (terrestrial laser scanning) and SfM (structure from motion) technologies. We identified a pattern in channel-bed morphology and texture (morpho-texture): when the channel is incised its texture is coarser, otherwise sand-filling occurs. These changes determine bedload flux and texture: sand fill brings rise to high fluxes and fine-grained bedload, whereas incision caused by evacuation of sand leaves a sandy-gravel surface with lower bedload fluxes and coarser texture. A video camera recording during events allowed identification of dramatic changes in bedload flux and texture owing to the appearance and erosion of bars, supplying field evidence to explain the difficulty in the prediction of bedload flux at short time intervals.

Effect of Flood Hydrograph Duration, Magnitude, and Shape on Bed Load Transport Dynamics

AbstractBed load sediment transport is an inherently challenging process to measure within a river, which is further complicated by the typically transient nature of the hydrograph. Here we use laboratory experiments to explore how sediment flux under transient—unsteady and intermittent—flow differ from those under steady flow. For a narrow unimodal sediment distribution, we calculated fluid stress and measured sediment flux for a range of hydrograph durations, magnitudes, shapes, and sequences. Within a hydrograph, we find considerable variability in sediment flux for a given stress above the threshold for motion. However, cumulative bed load flux resulting from a flood scales linearly with the integrated excess transport capacity (flow impulse). This scaling indicates that, to first order, flow magnitude, duration, shape, and sequence are only relevant to bed load flux in terms of their contribution to the total flow impulse, in agreement with prior field results. The flood impulse represents a quantitative parameter through which the effects of transient flow on coarse sediment transport may be parsed.

Indirect estimation of bedload flux from modern sand-bed rivers and ancient fluvial strata

Indirect estimation of bedload flux from modern sand-bed rivers and ancient fluvial strata

Incorporating macroinvertebrate biological structures into gravel-bedded stream fluid dynamics using 3D CFD modelling

In gravel-bedded streams and rivers, silk nets of hydropsychid caddisfly larvae (Trichoptera:Hydropsychidae) can increase the critical shear stress, τ∗c, required for sediment particle motion, reduce bedload flux, and provide a low flow refuge for other organisms. However, there is still a limited understanding of the mechanisms that regulate how biological silk structures might influence near-bed fluid dynamics. Part of the challenge in identifying both the per capita and population level influences of caddisfly silk nets lies in the difficulty of modelling such fine-scale and detailed architecture of the structures. Here, we use a 3D computational fluid dynamics (CFD) model to evaluate the impact of silk nets on shear stress and pore space velocity by varying silk net aerial density within a modelled gravel bedded stream where the silk nets are simulated as small rigid bodies of realistic distribution, geometry, and orientation. The model revealed that silk net densities of 330 and 735 nets/m2 reduce the average shear stresses by an average 6.0% and 11.6%, respectively, over a range of inflow velocities. Silk nets also result in near-bed flows close to 0 m/s by reducing velocity by up to 60%. Our research sheds light on the important influence of small but numerous aquatic animals on water flow paths and highlights the significance of adding biological complexity into water flow models in lotic ecosystems to improve erosion models that may be used in river channel and riparian habitat design.