[1] To calculate bed load, engineers often use flow resistance equations that provide estimates of bed shear stress. In these equations, on the basis of the estimate of the appropriate hydraulic radius associated with the bed only, the bed roughness k s is commonly set as a constant, whatever the bed load intensity. However, several studies have confirmed the existence of feedback mechanisms between flow resistance and bed load, suggesting that a flow-dependent bed roughness should be used. Therefore, using a data set composed of 2282 flume and field experimental values, this study investigated the importance of these feedback effects. New flow resistance equations were proposed for three flow domains: domain 1 corresponds to no bed load and a constant bed roughness k s = D (where D is a representative grain diameter), whereas domain 3 corresponds to a high bed load transport rate over a flat bed with a constant bed roughness k s = 2.6D. Between these two domains, a transitional domain 2 was identified, for which the bed roughness evolved from D to 2.6D with increasing flow conditions. In this domain, the Darcy-Weisbach resistance coefficient f can be approximated using a constant for a given slope. The results using this new flow resistance equation proved to be more accurate than those using equations obtained from simple fittings of logarithmic laws to mean values. The data set indicates that distinguishing domains 2 and 3 is still relevant for bed load. In particular, the data indicate a slope dependence in domain 2 but not in domain 3. A bed load model, based on the tractive force concept, is proposed. Finally, flow resistance and bed load equations were used together to calculate both shear stress and bed load from the flow discharge, the slope, and the grain diameter for each run of the data set. Efficiency tests indicate that new equations (implicitly taking a feedback mechanism into account) can reduce the error by a factor of 2 when compared to other equations currently in use, showing that feedback between flow resistance and bed load can improve field bed load modeling.Citation: Recking, A., P. Frey, A. Paquier, P. Belleudy, and J. Y. Champagne (2008), Feedback between bed load transport and flow resistance in gravel and cobble bed rivers, Water Resour. Res., 44, W05412,

Effects of animals on the transport of solids (e.g., sediments) can modify earth-surface processes and landforms (role as geomorphic agents) and resources for other organisms (role as ecosystem engineers). Therefore, we studied the impact of crayfish (Orconectes limosus) behavior on sand-gravel erosion and bottom habitat over riffle-pool sequences in experimental streams. We manipulated the availability of crayfish food and refugia. Refugia availability had clear effects on overall crayfish activity. The establishment of dominance hierarchies among the crayfish produced patterns in crayfish activity that differed between riffles and pools. Crayfish activity significantly affected sand and gravel erosion. High local sediment erosion averaged ഠ2.8 (riffles) and ഠ1.4 (pools) kg dry weight mϪ1 when crayfish hierarchies were established and refugia were available. Removing the refugia increased these rates to ഠ4.0 (riffles) and ഠ3.2 (pools) kg dry weight m Ϫ2 d Ϫ1. This direct erosion caused by crayfish should be lower than that caused by floods. Crayfish distinctly (1) increased bedform roughness (riffles), (2) decreased sand dune height (pools), (3) shifted the transition between gravel (riffles) and sand (pools) downstream, (4) decreased sand in gravel interstices (riffles), (5) decreased filamentous algae growth on gravel (riffles), and (6) decreased biofilm cover on sand dunes (pools). These sediment changes due to crayfish activity at baseflow should have opposite effects on sediment erosion during subsequent floods. Crayfish impact on physical habitat at baseflow could largely affect population and community structure of the benthos, as well as egg survival of gravel-breeding fish.

Abstract. There is growing evidence for impacts of small stream organisms on the transport of fine sediments. However, such impacts on the transport of coarser sediments remained unconsidered. Therefore we studied whether a silk-producing stream insect (Hydropsyche siltalai) consolidated gravel through silk fixed among gravel pieces. We exposed gravel for 2 months in a stream along a gradient of Hydropsyche abundance and measured the critical shear stress for this gravel in a laboratory flume. The critical gravel shear stress was very significantly (p -7.7 x 10-•) related to a variable (overall biomass) describing gravel consolidation by Hydropsyche silk. We showed that Hydropsyche in a range of abundance typical of many streams increases critical shear stress for gravel by as much as a factor of 2. Surface gravel arrangement (i.e., the spatial distribution and orientation of different gravel sizes and forms) had a less significant impact on critical shear stress, while other sediment variables so far considered in erosion had no significant impact. Future studies on the transport of solids and on habitat conditions should consider the high potential of stream invertebrates to consolidate gravel.

International audienceExperiments were conducted over uniform gravel bed materials to obtain 143 friction factor values under bed-load equilibrium flow conditions in an attempt to add to the scarce data available on slopes between 1 and 9% for Shields numbers between 0.08 and 0.29. Analyses showed that when only flows over flat beds are considered, a distinction must be made between flows with and without bed load. More particularly, fitting flow resistance equations indicated that the roughness parameter increases by a factor of 2.5 from clear water flow to intense bed-load transport. Between these two states, the flow resistance can be approximated by a constant for a given slope