Author(s): B. Gjunsburgs; G. Jaudzems; J. Parilkova

Linked Author(s): Boriss Gjunsburgs

Keywords: Multiple floods; Local scour; Hydraulic contraction; River bed layering; Computer modelling

Abstract: Based on laboratory tests, method elaborated, computer modelling results were presented data how multiple floods, contraction rate of the flow or river bed layering impact on scour near structures in flow. Transport system infrastructures, such as roads, bridges, dams, and water intakes in rivers, are under permanent impacts of multiple floods. To estimate their safety and stability during scour development at hydraulic structure foundations, a multidisciplinary approach, involving the principles of hydraulics, hydrology, morphology, geology, and so on, is required. The scour hole parameters (depth, width, and volume) during floods under clear-water conditions in the floodplain are summed up and increase from flood to flood. Therefore, it is difficult to predict how multiple floods will impact on local scour near the structure, whether it will be destroyed after a current or forthcoming event, whether the scour depth will or will not exceed the designed equilibrium depth, if the floods are lower probability than the calculated ones, and how long the structure will stay undamaged and safe enough after unexpected multiple flash floods. Using the differential equation of equilibrium of the bed sediment movement development in time at engineering structures during floods has been elaborated. Matching experimental and calculated results allow us to use this method for computer modelling of the scour process in nature during floods with different probability, duration, frequency, and sequence. This method enables us to compute the scour depth at any stage of the flood during the maintenance period or at the stage of designing the bridge crossings. 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 increases the possibility of damage. The stability of intakes, piers, abutments, guide banks and spur dikes in floods depends on the depth and dimensions of the scour hole at foundations. Local scour is depending on multiple floods, contraction rate of the flow, river bed and structures parameters and correct understanding of the impact of each parameter can reduce failure possibility of the structures. It was found that the scour hole parameters increase during multiple floods with decreasing probability and with increasing duration and frequency of the floods. The sequence of floods can increase or reduce the scour depth development in time, depending on their probability. The successive floods of the same probability considerably increase the value of scour depth. The impact of multiple floods of different probability, duration, frequency and sequence on depth of scour is presented in figures. The contraction of the river floodplain by embankment and bridge structures, namely abutments, piers, guide banks, and spur dikes, when some part of floodplain discharge is blocked, leads to streamline concentration, increase in velocity, vortex structure, turbulence, general scour, and local scour. The paper explores impact of hydraulic contraction, the discharge redistribution between channel and floodplain during the flood, local flow modification and river bed layering on depth, width and volume of scour hole near low-land rivers. Experiments in a flume, method for scour calculation and computer modelling results confirm a considerable impact of the contraction rate of the flow, the discharge redistribution between channel and floodplain, the local velocity, backwater and river bed layering on the depth, width, and volume of scour hole in steady and unsteady flow, under clear water condition.

DOI: https://doi.org/10.3850/978-981-11-2731-1_013-cd

Year: 2018