Significance of Time Dependence in The Effect of Wing Dams on Water Levels

Mainly in the 20th century, many wing dams (spur dikes, groins) were built worldwide, primarily to improve the navigability of rivers. As the wing dams narrow the riverbed, the water depth increases. However, the accelerated flow increases bed erosion, which in turn causes the bed level to decrease. It is not clear how the two effects affect water levels, especially at high water, when the water flows over the wing dams. The study is further complicated by considering the added resistance and thus the slope change caused by such interventions, which also affects the sediment transport and water levels. The impact of wing dams on water levels has been studied mainly by trend analysis of measured time series. Trends have shown that the water levels of flood discharges are increasing in rivers with wing dams. However, it not clear when and with what properties the new equilibrium will be reached. In the present study, we used a schematic model for calculating bed change and flow in a 1D manner to analyze the Middle Mississippi River. We found that the installation causes a sudden substantial local erosion, causing a significant sediment surplus downstream. This phenomenon has a local effect on the bed level, which also can stimulate the backwater effect. The passage of the sediment wave can take centuries for Mississippi-scale rivers. Our model results show that until the new state is reached, the water level initially increases (following the trend analysis results), but later decreases.