Optimum design of large hood relief culverts under the A89 motorway in theDordogne-Isle confluence plain

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 t he 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 b een fulfilled thanks to complementary sophisticated physical scale models a nd 2-D numerical models. The schematization principles adopted in the 2-D n umerical model concerning bridge pier grouping were validated using the phy sical scale model. The finite element TELEMAC-2D code was used to draw up the optimum design o f this river engineering scheme. TELEMAC-2D solves the 2-D Shallow Water eq uations on non-structured grids in the presence of alternately dry and wet beds. The 2-D numerical modelling based on high quality topographic data al lows considerable improvements to be made in the computation of flood flows in a flat valley, as compared with traditional one-dimensional techniques. Near hood relief culverts in particular, the various physical effects cont ributing to the overall head loss can be distinguished: bottom roughness, m edium size bed variations, strong curvature of the streamlines and vortices behind abutments. The hydraulic impact of the optimum solution has been studied under differe nt aspects (rise of maximum flood levels, increase in maximum velocity, cha nges in the water flow patterns, submersion time of flooded land, modificat ion of flood routing characteristics) for a wide range of flood hydrologica l events. Copyright (C) 2000 John Wiley & Sons, Ltd.