A computational framework for flood risk assessment in The Netherlands

The safety of dikes in the Netherlands, located in the delta of the rivers Rhine, Meuse and Scheldt, has been the subject of debate for more than ten years. The safety (or flood risk) of a particular area may depend on the safety of other areas. This is referred to as effects of river system behaviour on flood risk (quantified as the estimated number of casualties and economic damage). In the Netherlands, such effects are found to be important for the understanding and evaluation of flood risks. A computational framework was developed to assess these effects. One component in this system selects reasonable dike strength parameters (based on the measured strength parameters and their uncertainty) and flow conditions via a Monte Carlo simulation. A second computes maps of the water level and flow velocities in the study area as a consequence of a dike breach. The third and final component uses these maps and maps of the population density and economic activities to estimate the potential number of casualties and the economic damage. Flood risk is in and by itself a probabilistic problem: it depends on stochastic parameters such as the actual hydraulic load (the flood event) and the strength of dike sections, which is known only approximately. To handle this aspect, we used a Monte Carlo method to select 81 computations for which dike breaches would occur given the hydraulic and strength characteristics, out of a total of seven million. Preparing the input data, running the various computations and getting the final results was automated using Tcl scripts as much as possible. The hydraulic computations would last several days and were conducted in parallel on a Linux cluster to keep the time required within reasonable limits. Though this complex system was set up ad hoc, we can describe it more formally as a tuplespace, each computational step for each of the 81 computations thus is represented as a unique tuple, a combination of job ID and status. Within the Linux cluster each processor was assigned one such tuple, so that the next step could be carried. The tuplespace paradigm allows one to set up arbitrarily complex systems or to improve certain aspects of such systems. 1 The Deltares Institute was formed in 2008 from two independent institutes and parts from two others. One of these institutes was WL | Delft Hydraulics. 2 E-mail: [email protected]