Equilibrium scour hole size at setback abutments with varied aspect ratios in floodplains

The key features of the equilibrium scour depth $d_{\mathrm{se}}$, width $W_{\mathrm{se}}$, length $L_{\mathrm{se}}$ and the volume $V_{\mathrm{se}}$ with large abutment aspect ratios (i.e., abutment width $L_c$ divided by its length $L$) in floodplain are different compared to narrow abutments. Seven models with $L_c/L$ ranging from 0.125 (narrow abutment) to 4 (very wide abutment) were tested. The results show that the combined effect in terms of the abutment aspect ratio $L_c/L$ is a key parameter with wide abutments. Furthermore, the average equilibrium scour width observed was much larger than previous studies and extends up to 3.5 times the floodplain water depth. This implies the current guideline by FHWA (Federal Highway Administration), (2009) to provide a riprap countermeasure apron width for 2 times the floodplain water depth may be insufficient. The much wider scour formation is caused by the migration of the maximum scour location around wide abutments. The results show generally the scour hole dimensions for setback abutments in compound channel are less than that abutments in rectangular channel under the same flow conditions. To this end, empirical equations, which agree well with the data from the present and previous studies are proposed to predict these characteristics at the equilibrium state.