RIVER ECOSYSTEM MODELING - APPLICATION OF THE PROSE MODEL TO THE SEINE RIVER (FRANCE)

The Seine river crosses the most densely populated and industrialized area of France, Paris (16 million inhabitants), surrounded by fertile land with intensive agriculture. In the framework of a CNRS (Scientifi c Research National Center) research project, computer programs have b een designed to tackle problems related to eutrophication, non-point p ollution and the impact of sewage during dry or wet periods (urban run of and sewage network overflow). The PROSE software has been specially designed to simulate the behaviour of the most disturbed stretches of the Seine ecosystem on the last 300 kilometers of the river, upstream of the estuarine area. The 1-D hydraulic sub-model of PROSE is based on a finite difference solution of Saint-Venant equations solved with the Preissman scheme. It simulates steady state situations as well as highly transient situations such as fast changes in river discharge du ring rainy periods or dam motions. The biological sub-model is based o n the RIVE model, describing the major processes in a river ecosystem: primary production, heterotrophic bacterial activity and organic matt er decomposition, major nutrients species (nitrogen, phosphorus), nitr ifying activity and oxygen balance. Water column and sediment variable s are simulated. Most of the parameters have been estimated during lab oratory experiments or field studies. Different situations observed be tween 1989 and 1991 allowed a detailed validation of the model. The mo del was then used to explore the reaction of the ecosystem (particular ly its oxygen status) to changes in physical constrains (discharge, re oxygenation at darns) or in biological processes (release of microorga nisms accompanying waste water discharge).