On the fine sediment deposition patterns in a gravel bed open-channel flow

One of the most important problems in rivers restoration and management is the problem of fine sediment deposition in the bottom of gravel beds. In fact, such phenomena can affect fauna and flora in various area. In Present study, a series of video camera measurements in an open channel with gravel bed was carried out in order to investigate the process of fine particles entry in the matrix of a gravel bed. Specifically, the present study focuses on a spatial pattern of fine sediment deposition and entrapment. The results show that deposition and entrapment of fine particles caused by the intrusion of large gravel particles and thus fine sediment deposition pattern are mostly in agreement with bed topography. Indeed, fine particles generally deposited on the downstream side of the gravel particles, while they rarely settled down in the upstream side of the gravel particles. Moreover, the results highlighted the formation of quite long longitudinal sand bars which are repeated in whole cross-section. These observations are in agreement with near bed common flow characteristics such as sweep and ejection events and strong secondary currents formation. The combined effects of sand ribbons and bed topography lead to the complex spatial pattern of deposition which questions the applicability of common transport thresholds which were developed based on the bulk properties of the flow like shear velocity.