Incorporation of scour uncertainty to current AASHTO LRFD bridge design specifications

United Kingdom waters have morphologically dynamic bays and estuaries through which offshore windfarm export cables are laid. The export cables must cross existing cable and pipeline infrastructure before making landfall. Rock berms have traditionally been installed at such crossings and at locations of shallow cable burial for cable separation and protection. In some instances, the rock berms have caused scour and been affected by general seabed changes. Recent field observations of scour and seabed change at cable protection rock berms is presented. In one location two separate rock berms are exposed to 1.0 m to 1.5 m general seabed lowering. One berm is aligned oblique to the current and has experienced excessive scour, while the other berm is aligned more parallel with the current flow direction and has experienced only minor scour. A second location comprise a cable crossing with double rock berms aligned oblique to the current direction. The current speed was asymmetric being largest during flood and smaller during ebb tide. Excessive scour was observed in-between the two berms and on the in-shore side, while only limited scour was observed at the off-shore side of the berms. The asymmetry in scour is linked to the asymmetry in current speed indicating a strong scour dependence on the currents speed. In non-dimensional form, this dependence is linked to the so-called Shields parameter governing magnitude and mode of sediment transport.