Revision of Flood Alert Thresholds for Forecasting and Management in Walloon Municipalities

Services for the development and application of a coherent and reproducible methodology for the revision or definition of alert thresholds for flood announcement, monitoring, and forecasting, as well as their definition for affected Walloon municipalities. This includes validating, setting, or revising flow thresholds causing first overflows, as well as those corresponding to pre-alert and alert phases, supplemented by the 100-year return period flow or the largest known flood as an extreme flood threshold. The Walloon territory will be divided into hydrologically homogeneous 'catchment areas / sections' for the dissemination of pre-alert and alert phases. A coherent, reproducible methodology will be developed, based on three components: hazard (physical characterization of floods, 1st overflows, most extreme historical flood), stakes (evaluation of potential effects on people, goods, activities), and context (exceptional consideration of conjunctural elements). A transition zone will be defined for each threshold value to account for uncertainties. The method is to be applied to 85 "main" measurement stations. Furthermore, four threshold values and their transition zones are to be defined at the communal level for approximately 190 Walloon communes (out of 261) with navigable and non-navigable 1st category watercourses, based on geomatic approaches, analysis of flood databases, and collection of testimonies. A simplified determination of the 4 threshold values (1st overflows, pre-alert, alert, extreme flood) for approximately 125 other active flow/height measurement stations on hydraulic channels and 1st category non-navigable watercourses will be done by interpolating/extrapolating the values from the 85 main stations. Key deliverables include 1 detailed methodological report (Word, PDF), for each of the 85 main stations 1 analysis grid and 1 visualization scale, a listing of the 4 interpolated/extrapolated thresholds for the 125 other stations, for each of the approximately 190 communes 1 analysis grid and 1 visualization scale per overflowing watercourse, a geomatic database of thresholds and exposed stakes, a consultation tool for these deliverables in a geomatic environment, and all useful intermediate results. The first phase (firm tranche, 8 months) covers methodology development and application to the Ourthe, Amblève, Vesdre, and Mehaigne catchments (22 main stations, 49 communes), including interpolated/extrapolated thresholds for other measurement stations in these catchments. The second phase (conditional tranche, 16 months) concerns the application of the method to the rest of the Walloon territory (63 main stations). The estimated value ranges from 400,000 € to 1,200,000 € excl. VAT.

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