The key features of the equilibrium scour depth dse, width Wse, length Lse and the volume Vse with large abutment
aspect ratios (i.e., abutment width Lc divided by its length L) in floodplain are different compared to narrow
abutments. Seven models with Lc/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 Lc/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.