Reduction of Bend Scour by an Outer Bank Footing: Footing Design and Bed Topography

Protection of banks against erosion is an important but very expensive task in river management. The outer banks in river bends are most vulnerable to erosion and require an enhanced protection. This paper investigates, in an experimental flume, the efficiency of scour reduction and bank protection near the outer banks in open-channel bends by means of a horizontal foundation, called footing, protruding into the flow. First it is experimentally verified that bed mobility has a minor influence on the bed topography, which is mainly shaped by bend effects. Subsequently, the influence of the footing width and vertical elevation on the bed topography is investigated under clear water scour conditions. A maximum scour reduction of more than 40% was obtained with a footing placed at one-third of the maximum scour depth without bank protection and a footing width of about two-thirds of this maximum scour value. But a footing that is too narrow and/or not deep enough is vulnerable to underscour and subsequent bank failure. The experiments convincingly demonstrate the efficiency of this bank protection technique. The optimal footing parameters in the presented experiments should merely be seen as indicative, however, as they are expected to be case dependent.