Optimum design of large flood relief culverts under the A89 motorway in the Dordogne?Isle confluence plain

Abstract

The planned A89 motorway will go through the Dordogne — Isle confluence plain, which is regularly flooded under the effect of both river discharges and ocean tides. Hydraulic transparency of the motorway embankment was one of the prerequisites imposed by the French government. In order to optimize the cost/efficiency ratio of large culverts located under the motorway, their position and size had to be determined with great accuracy. This task has been fulfilled thanks to complementary sophisticated physical scale models and 2-D numerical models. The schematization principles adopted in the 2-D numerical model concerning bridge pier grouping were validated using the physical scale model. The finite element TELEMAC-2D code was used to draw up the optimum design of this river engineering scheme. TELEMAC-2D solves the 2-D Shallow Water equations on non-structured grids in the presence of alternately dry and wet beds. The 2-D numerical modelling based on high quality topographic data allows considerable improvements to be made in the computation of flood flows in a flat valley, as compared with traditional one-dimensional techniques. Near flood relief culverts in particular, the various physical effects contributing to the overall head loss can be distinguished: bottom roughness, medium size bed variations, strong curvature of the streamlines and vortices behind abutments. The hydraulic impact of the optimum solution has been studied under different aspects (rise of maximum flood levels, increase in maximum velocity, changes in the water flow patterns, submersion time of flooded land, modification of flood routing characteristics) for a wide range of flood hydrological events. Copyright © 2000 John Wiley & Sons, Ltd.