|The customer is SIARV (Syndicat Intercommunal pour l'Assainissement de la Région de Villeneuve-Saint-Georges), an inter-commune association in charge of wastewater and stormwater management for 17 communes, as well as downstream water quality improvement for the river Yerres and downstream course control to ensure effective protection of the valley against flooding and to enrich the area's environmental inheritance.|
The software has been used to help prevent problems occurring in the river Yerres area, which is situated within French départements 91 and 94 (suburbs of the outer Paris region beyond the affluent left bank of the river Seine). The region is subject to frequent extreme flooding, notably the downstream course (the last 11 miles - 20km - of the river), which was subject to catastrophic flooding in 1978 following heavy rainfall. Because of this history of flooding, the Ministry of Environment and Sustainable Development added the task of reducing the river's propensity to flood to its list of proposed projects in 2003.
The area in question lies between an old water mill in the commune of Varennes-Jarcy and the Rochopt river dam in the commune of Boussy-saint-Antoine. This region is particularly badly affected, with some parts flooding at least once a year and sometimes more often.
SIARV appointed French engineering consultancy PROLOG Ingénierie to undertake a study to determine the optimum solution for the area's flooding problems. Complicating the task was the fact that the area under study is dotted with numerous tributaries - old water mill races and many secondary mill race diversions for periods of high flow, as well as many fixed and mobile structures both on and beside the river.
The studies were undertaken with the aim of devising a global management scheme for that stretch of the river, sizing the works to enable the river to be managed without flooding when its waters rise and to protect the most vulnerable zones at times of unusually high flows.
The chosen option was based on French government agricultural and environmental research body CEMAGREF's ‘ Inondabilité' method - a risk analysis method in which information on socio-economic vulnerability is cross-referenced with the flood risk for different return periods, allowing organizations to determine, for each return period, which areas are protected and which are not.
A preliminary hydrological analysis made it possible to determine the required hydrograms using the QDF method (flow-duration-frequency). A hydraulic model was then created using Wallingford Software's InfoWorks RS software solution, which was acquired from the company's French distributor GEOMOD.
The project began in January 2003 and was completed in March/April 2005.The model was based on a stretch of the river some 10km long, comprising 730 nodes and 960 hydraulic links. A digital model of the surrounding terrain was also created within the scope of the project.
The data sources were numerous, and included:
- cross sections from paper maps;
- topographic data imported from AutoCAD (1/200 and 1/500 scale);
- photogrammetry for other locations.
The DTM was elaborated with Covadis (an AutoCAD application) from the above data sources. This was converted into 3D ArcView shape files and integrated into InfoWorks RS. The model predictions were based on various return periods, from five to 100 years, to enable characterization of the flood risk, before and after employing a number of management options such as management of the many sluices.
Characterizing the risk and comparing it to the vulnerability analysis made it possible to determine the overall flood risk for the entire study area. The model was then used to work out management strategies for floods generated by high water levels, based on the concept of allowing floodwaters to overflow to less vulnerable areas.
GEOMOD Director Rene Dupas said: ‘InfoWorks RS with RTC [real time control] was chosen because it could dynamically integrate the results with DTM [digital terrain modeling] and GIS to become a decision-making tool. The project has produced excellent results that have enabled efficient and effective resolution of the area's serious flooding problems.'