Localized scour around setback abutment in compound channel
Ahmed Abouelfetouh Abdelaziz Mohamed Younes
Date of Issue2019-01-31
School of Civil and Environmental Engineering
This study focused on localized scour formed around 45° wing-wall (WW) setback abutments in a compound channel. Fifty-five long duration scour experiments were carried out to investigate the effects of abutment length, width, degree of flow contraction on the floodplain, flow depth ratio and flow intensity (u/uc) on the time development of the geometry of scour hole and its characteristics, up to the equilibrium scour conditions. Four different water depths were tested, and different discharges were used for each water depth to achieve various flow intensities under clear water scour conditions. Seven 45° wing-wall models were used in these experiments. The experimental results showed that a secondary scour hole was formed immediately downstream the rear face of abutment models in some experiments. The longest abutment produces a secondary scour hole earlier compared to the shorter one. The narrowest model does not have a secondary hole. On the other hand, the widest model forms a secondary hole much later. The final shape of scour hole consists of a primary scour hole and a smaller secondary scour hole. The abutment width, floodplain flow depth, abutment length and flow intensity have profound effect on the formation of the secondary scour hole. The results revealed that the location of maximum scour depth generally occurs slightly downstream of the wing-wall abutment nose. The maximum scour location, Rx is affected by the flow intensity, flow depth, and abutment aspect ratio, (i.e. its length and width). This location is shifted laterally further from the abutment nose as flow intensity decreases for the same abutment aspect ratio. On the other hand. The maximum scour location Ry is only affected by flow depth, and abutment aspect ratio. However, previous studies suggested that this location is affected only by the relative abutment length. Generally, it was observed that with an increase in relative abutment length, all dimensions of scour hole and its area and volume increase. Therefore, a shorter abutment has smaller scour dimensions than the longer one. In addition, for high flow intensity, the values of these characteristics are greater than that of low flow intensity at the same value of abutment aspect ratio. The abutment width has a significant effect on the characteristics of scour hole, contrary to the findings of previous studies. Generally, with an increase in abutment width, the scour depth and width decrease. Thus, the narrower abutment has larger scour depth and width than the wider one. Conversely, the scour length, area and volume initially increase with an increase in the relative abutment width and then these characteristics reduce significantly with much wider abutment. The effect of increasing the abutment length on the scour depth and width was found to be greater than on the length of the scour hole. Moreover, the influence of decreasing the abutment width on the scour length was found to be less than on the depth and width of the scour hole. It was found that the scour width is approximately equal to 3.5 times the floodplain water depth, and this is much wider than the recommended riprap apron extension of two times the floodplain water depth suggested by previous guidelines. Equations are proposed to predict the dimension of scour hole and its area and volume. Finally, the exponential function was found to be suitable to represent the development of scour hole characteristics with time, and the proposed scour-time equations will be useful to calculate these characteristics when the flow does not last for sufficiently long time to reach its equilibrium scour conditions.