Multiple floods impact on engineering structures in river flow

Abstract

According to the tests and the method suggested, the scour starts when the floodplain is flooded and usually stops at the flood peak. The scour development depends on the flow hydraulics, the river-bed parameters, the multiple floods probability, frequency, sequence and duration. At the flood peak, a scour hole is usually formed. Although the scour process can be continued further, it stops, because the flood is time-restricted. The scour time is always less than the flood duration. At the next flood of the same probability, the scour process does not start when the floodplain is flooded, but at a later time step, closer to the flood peak. This happens because of the scour hole developed in the previous flood, which reduces local flow velocity and flow capacity to remove sediments. The duration of the scour process at the second and forthcoming floods is less than at the previous floods. The scour hole depth, width, and volume increase from flood to flood.Method is confirmed by test resulrs. Proposed methods allows to compute the development of scour depth during expected usual or extreme flood events varying with flood probability, frequency, sequence, and duration at the stage of design or in maintanence period of the river engineering structures. Thus the most dangerous sequence of expected floods for engineering structures can be found in advance, and to take neccesary protection measures. Water flow in rivers during floods strongly impact transport system infrastructure - roads, bridges, dams, and etc. Frequency and intensity of flood events with high water levels and considerable discharges becomes more frequent and increases the loads on engineering structures in rivers and at the same time the possibility to be damaged. The stability of piers, abutments, guide banks and spur dikes in floods depends on the depth and dimensions of the scour hole at foundations. By preliminary computing scour development in time in multiple floods of different probability, frequency, sequence, and duration, we can estimate possible scour hole depth,width and volume values induced by expected floods. According to calculated scour we can take protection measures if necessary, and reduce the economical and environmental losses. During multiple floods the scour hole parameters near structures under clear-water conditions are summed up and increases from flood to flood, and each next flood can lead to failure. Contraction of the flow by bridge structures leads to considerable changes in flow pattern, local increase in velocities, and origin of turbulence, eddy and vortex structures. All those changes are the reason of local scour at the abutments, piers, guide banks, spur dikes. The differential equation of equilibrium for bed sediment movement in clear water was used, and a calculation method for the scour development at the abutment and guide banks during multiple floods was elaborated. According to the method proposed, the hydrograph was divided in time steps, and each step was divided into time intervals. Calculations were performed for each time step of the hydrograph, so that to estimate the influence of the flow unsteadiness during the flood, but in each time step, the flow was assumed to be steady.