Suction Removal of Sediment from between Armor Blocks. II: Waves

Abstract

When a stone/armor layer on a sand bed is exposed to flow, the sand underneath will be agitated by the flow turbulence. When the flow velocity reaches a critical value, the sand will be sucked (winnowed out) from between the armor blocks. In a previous investigation, we studied suction removal of sediment in steady currents. The present study is an extension of our previous investigation to waves. The critical condition for the onset of suction is determined. It is found that the onset of suction is governed by three parameters: (1) the sediment mobility number (based on the sediment size); (2) the ratio of sediment size to stone size, d∕D ; and (3) the Keulegan-Carpenter (KC) number, based on the armor block/stone size. The variation of the critical mobility number for suction as a function of d∕D and KC is determined for the ranges of the parameters 0.001<d∕D⩽1 and 1<KC<50 . The case of steady current is included as a reference case. The effect of waves superimposed on current, the effect of a multilayer stone cover, and the effect of regularly placed armor blocks are investigated. Suction of the base-bottom sediment will cause sinking of the armor layer and, therefore, general lowering of the bed. The time scale of the latter process and the downward displacement of armor blocks/stones are also investigated.