Page - 379 - in Book of Full Papers - Symposium Hydro Engineering

The SSC decreased with each successive release, despite the increased nominal flow rate (10, 15, 20 m3/s). This decline in SSC production has been observed in other studies and is the result of the progressive scouring of fine sediment from the riverbed. These initial results confirm that regular releases at relatively low flow rates (10 m3/s) and lasting 5 hours could prevent clogging by silts and clays in the Selves River. 4.3. EFFECT ON BEDLOAD TRANSPORT Fluorescent sand tracers were collected after each release. Their proportion in the sediment samples and the transport distance from the injection point were recorded. The results show a progressive change in mean position, distance travelled by a given fraction of the tracers and maximum distance travelled as a function of flow. The median, average and maximum transport distances have more than doubled between 10 and 15 m3/s, which is not the case between 15 and 20 m3/s. These findings clearly show that release efficiency increases with flow and that there is a relationship between the flow and the distance travelled by the tracers. The 10 m3/s release mobilized only a few gravels. The second release at 15 m3/s mobilized the same grain size classes with a slightly larger amount of particules entrained. However, these latter were mobilized over short distances. During the last release, fewer plots were set up, but the results show a more damaging transport of coarse particles. 4.4. EFFECT ON HABITAT The linear survey made it possible to characterize the initial state (t0) of sand and silt deposits along the Selves and to identify the tributaries contributing the most in fine sediments. It was then possible to measure the evolution between the initial state and the state after the three successive releases. The results are spatially heterogeneous. Both declines and increases in which both variables were observed over the entire length of the bypassed section. In particular, heavily clogged areas near the tributaries have tended to become unclogged and water depth has increased. Changes in the fine sediment thickness are closely linked to the different geomorphic units. Fine sediment accumulation was non-existent in lotic reaches and in areas where pre-release clogging was already negligible. The most significant changes occurred in glides for which the median values and distribution were considerably reduced and concentrated. Surveys carried out over the length of the bypassed reach showed that fine sediment thickness has been nearly divided by 3 on average. The proportion of sands and fine gravel remained essentially the same as the 3 releases were not 379