Dynamic morphology in a bridge-contracted compound channel during extreme floods: Effects of abutments, bed-forms and scour countermeasures

Abstract

Fluvial morphology in compound river channels usually undergoes drastic variation during extreme floods due to scour and erosion processes. Those processes can be highly dynamic consisting of multiple elements (e.g. bed-form migration, bank erosion, riprap break-down, etc) that are inter-dependent. The existence of bridge abutments and embankments that lead to flow contraction on the floodplain brings more complexity and uncertainty. The present study investigated flood-related processes around bridge sites in a compound channel with different abutment/embankment configurations. Results show that a longer abutment/embankment pushes the high-shearing zone on the floodplain (around abutment toe) further towards the bank slope and causes a higher risk of undercutting at the toe area, while vertical flow pressurization had a more significant effect on enhancing general bed shearing. The main channel bank tends to retain a constant slope (2:1) as scour progresses gradually during floods. The channel bed scour is closely related to the unit discharge contraction ratio determined by both lateral and vertical flow contractions. Regarding the interaction between dynamic processes, migrating bed-forms enhanced sediment erosion by cyclic “embedment–winnowing” processes when they met destabilized loose riprap rocks. Thus, rocks sliding into the main channel may cause a higher risk of bed subsidence and then embankment failure. In total, four bed-form migration types (i.e. interaction types) are observed for different abutment/embankment lengths and flow intensities. In general, the observed processes provide a clear picture of how channel morphology is altered dynamically by multiple factors during extreme floods. Additionally, the findings in this study are integrated under a framework to evaluate the potential scour and erosion hazards in a real river section. It is found that those hazards may go beyond individual sites, affecting each other and then the morphological evolution at a catchment scale. A more comprehensive study in the future will enable this framework to performed more quantitative analysis.