ABSTRACT
Elliptical and straight guide banks usually are constructed to direct flow and sediments to the bridge opening, when part of the floodplain is blocked by approach embankment(s). Contraction of the flow leads to the flow modification; at the heads of the guide banks take place streamline concentration, sharp drop in water level, local increase in velocity, vortex structure, increased turbulence, local scour and as well general scour up and down of the bridge. The differential equation for the bed sediment movement under clear-water conditions is used, and a method for computing the scour development during multiple floods is presented. The method is confirmed by experimental data. Experiments in the flume with steady and unsteady flow, with different rate of contraction, derived method of calculation of scour development in time (Gjunsburgs et al. 2006, 2008) analysis, computer modelling of the scour with different contraction rate of the flow Q/Qb, different discharge distribution between channel and the floodplain confirmed considerable impact contraction rate of the flow on depth, width and volume of the scour hole at the guide banks. Contraction rate of the flow is not constant during the floods, contrary to the geometrical contraction of the flow. Local velocities, depth, width and volume of the scour hole are depending on the contraction rate: with increase of the contraction rate of the flow local velocity and depth of scour are increasing, as well with reduction of the critical velocity. River bed stratification leads to the increase depth of scour when fine sand layer is under the coarse sand layer or reduce it when coarse sand layer is under the fine sand layer. All results are confirmed in tests, theoretically and also presented in figures.
