Countermeasure Toe Protection at Spill-Through Abutments

An experimental study of scour countermeasures for spill-through abutments situated on the flood plain of a compound channel is reported. The purpose of the study was to determine the variations in the scour hole geometry under clear water conditions by varying the compound channel and abutment geometries, and to determine the extent and type of scour countermeasure toe protection provided. This approach avoids one of the inherent difficulties in conducting scour countermeasure experiments-that is, the subjectivity of determining whether the countermeasure used in the experiment is a success or a failure. Riprap and cable-tied block countermeasures are incorporated. The results show that for most cases, as the countermeasure apron width (i.e., the extent of toe protection) is increased, the scour hole is deflected further away from the abutment and reduces in size. However, for abutment and compound channel configurations where the scour hole forms close to the main channel bank, the scour hole increases in size as the apron width is increased. The results also show that cable-tied block mats allow the scour hole to form closer to the abutment than equivalent riprap aprons and result in deeper scour holes. A suggested design methodology for the extent of apron protection is presented. The method is an improvement on the current, rather-simplified practice of providing aprons of fixed width equal to twice the flow depth.