Hydraulic and design aspects of constructed rock riffles in gravel-cobble bed rivers

摘要

Riffle-pool reconstruction has become an important river rehabilitation technique in disturbed channels. However, design has often lacked sufficient hydraulic analysis to investigate the impact of restoration on flood levels and sediment transport capacities. This paper describes a component of a recent hydraulic study of four gravel-cobble bed rivers where it was found that energy losses across both natural and constructed riffles varied between 50 and 100% of total mini-reach losses over a range of discharges up to and including bankfull. This implies that riffle restoration may increase flood levels, and that the additional flow resistance should be an important consideration in design. Notably, the results indicate a significant relationship between the riffle energy loss and the structure amplitude, suggesting that restoration flood level impacts can be investigated without the need to specify appropriate roughness coefficients for the riffle-pool sequence. It is believed that this approach would represent a useful tool in the design of riffle-pool restoration projects.