Complete Dynamic 2D Modeling for Cost Benefit Analysis of Flood Mitigation: Case of a Densely-Populated Coastal City in France

Located on the north coast of France, the city of Morlaix is subject to frequent floods, particularly the dense downtown area at the confluence of two rivers, where flood levels are aggravated by sea tides. Following many recent floods, the municipality sought global and sustainable solutions, and a study was conducted 10 years ago to propose appropriate hydraulic works for both watersheds that would reduce peak flows upstream. Reduced peak flows would have reduced damages; however, no specific study quantified the impact and financial benefits of the proposed works, and so the process stopped.

Appropriate cost benefit analysis is the main objective of the present study. Detailed PCSWMM 2D modeling covers the entire hydraulic system, which comprises: two converging rivers, underground sewer networks and flood flows through the downtown area. To better understand the dynamic behavior of the flows, scenarios based on various flood and tide conditions were analyzed. Derived maximum flow depths and velocities helped quantify existing flood cost damages and costs (i.e. without the proposed hydraulic works).

Investments in the hydraulic works were found to pay off in a 12 year period, and the economic analysis reinforced public interest in targeting the proposed works within 50 years. Sensitivity analyses on all parameters validated the strength of the approach. PCSWMM 2D was found to be an effective and user-friendly method for cost analysis for flood mitigation in Morlaix.