Laboratory flume studies on monochromatic wave-fine sandy bed interactions Part 2. Sediment suspensions

As reported in preceding paper (Part 1. Soil Fluidization), the observed phenomena of sediment suspensions above a fluidized sandy bed of Sand II (d(50)=0.092 mm) under monochromatic wave actions are quantitatively investigated. The suspended sediment concentration (SSC) at a single point within 5 cm above the bed was synchronously measured with water waves and bed soil's pore pressures with an intrusive optical sediment-concentration probe. The measurements show that SSC initiates several wave cycles after initiation of bed soil's fluidized response and grows to a peak value mainly in the post-fluidization phase. Under similar wave loadings in the same test series, SSC is usually higher over a resonantly fluidized (RF) bed than over a nonresonantly fluidized (NRF) bed. On the contrary, only relatively low SCC can be identified above an unfluidized bed. The analyses illustrate that to certain extent, peak values of SSC are directly proportional to the thickness of fluidized soil layer d(f). Values of df usually decrease with repeated fluidized response, longer consolidation periods, and in deeper water depths. Once the fluidized responses initiate, pore pressures are generally much significantly amplified in both shallow fluidized soil layers and near below the fluidized layer, especially during the resonance event. The resulting depth gradients of dynamic pore pressure amplitudes in shallow layers are likely to have caused higher initial rises of SSC in a RF bed than in the subsequent NRF bed. Those in deeper layer should have contributed to sustain the fluidization state for further SSC increments. Immediately after termination of wave loading, re-deposited suspended sediments always result in a typical flat bed form. For a pre-fluidized bed, wave-induced drastic sediment suspensions are still obtainable very near above the bed with even a rather thin fluidized surface soil layer. (C) 2008 Elsevier B.V. All rights reserved.