Physical modeling of localized scour for the Yangtze Estuary Waterway Improvement Project, Phase I

In order to examine the effectiveness of engineering protection against loc alized scour in front of the south groin-group of the Yangtze Estuary Water way Improvement Project, Phase I, an undistorted physical model on a geomet ric scale of 1:250 is built in this study, covering two groins and their ad acent estuarine areas. By use of rinsing fix-bed model as well as localized mobile-bed model the experiment is undertaken under bi-directional steady flow. According to the experimental results, waterway dredging leads to the increase in steram velocity, the increase being larger during the ebb than during the flood. Construction of the upstream groin has some influence on the flow patterns near the downstream groin. Localized scour in front of t he groin-heads is controlled mainly by ebb flow. In the case of a riverbed composed entirely of silt, the depths of localized scour in front of the tw o groin-heads are 27 m and 29 m, respectively. In reality, the underneath s ediment of the prototype riverbed is clay whose threshold velocity is much higher than the stream velocity in the Yangtze Estuary; therefore, the dept hs of localized scour will not be much larger than the thickness of the sil t layer, i. e. 7.4 m and 4.7 m, respectively. The designed aprons covering the riverbed in fron of the groin-heads are very effective in scour control . Aprons of slightly smaller size can also fulfill the task of protection, but the area of localized scour increases significantly.