Giving Room to the River: A Nature-Based Solution for Flash Flood Hazards? The Brague River Case Study (France)

Abstract The Brague River basin (68 km 2 ) is located on the French Mediterranean coast. It experiences a high risk of flash floods. The potential efficacy and efficiency of flood protection strategies based on green (Nature Based Solutions: NBS) or grey (civil-engineering) measures, as well as their co-benefits, are studied in this chapter. Two NBS flood alleviation strategies combine both small natural water retention areas, along with a widening of the river corridor. Two more classical grey scenarios were based on large wood-trapping racks, and another based on retention dams. This chapter synthesizes (i) the flood risk assessment; (ii) the estimation of total costs; (iii) how benefits related to each strategy were estimated. Benefits were evaluated with both a top-down method (avoided damages and transfer of valuations made elsewhere for co-benefits) and with a bottom-up approach surveying citizen willingness to pay. The cost-benefit analysis demonstrates that costs are higher than the avoided damage, and that co-benefits are much higher than avoided damage for most strategies and for both approaches. Depending on the number of household considered in the co-benefits valuation, the balance may reach higher benefits than costs for NBS strategies, though not for the grey solution based on large dams.