Recirculating flow behind groynes for compound-channel geometries

摘要

Overbanks flows in a straight geometry are quite uncommon in the field. They usually cross transition reaches where the flow width can significantly varies. The extreme case is met when important obstacles, such as bridge-abutments, are present on the floodplain. They locally promote a rapid variation of the hydraulic parameters of the flow. Indeed, they first act like a groyne in a single-bed channel and so create downstream a slack-flow where silting can occur, while they promote local scour near their corner. Then, in the particular case of compound channels, they also partially block off the flow in the flood plain and promote a huge discharge transfer to the main channel. Available results on abrupt expansions in single-bed channel show that the flow in the vicinity of the obstacle obeys to two asymptotic regimes: deep or shallow water flows (Babarutsi et al., 1989) defined by a bed friction number. First, they have to be extended to the case of groynes and secondly to the case of compound channel geometries where the overspeed in the main channel affects the recirculation zone. Experiments are conducted both in single and compound channels. They confirm the validity of the expansion observations in the case of groynes, but under some corrections. They also put in evidence the crucial influence of another parameter in compound channels: the relative water depth in the floodplain.