Modelling in applied hydraulics: More accurate in decision-making than in science?

Marked differences occur between modelling in scientific hydraulic research, in hydraulic engineering and in public decision-making. This study reviews differences in the required accuracy of model results and differences in the choice between physical and numerical modelling. Physical models are used for studying elementary processes and their interactions under controlled conditions in scientific research; for the planning and design of interventions in hydraulic engineering; and for explanation and demonstration in public decision-making. Numerical models are powerful tools in scientific research, but field applications cannot be verified or validated according to rigorous scientific standards. Hydraulic engineers use numerical models for various purposes, some requiring a high accuracy and some not. They are used to uncertainty and deal with this by means of sensitivity analyses or probabilistic approaches. Numerical models are also used for decision-making on interventions that affect stakeholders, sometimes even having the last word in corresponding protocols or legislation. The suggested or perceived accuracy of model results is in this context much higher than the real accuracy. This leads to the paradoxical situation that decision makers and stakeholders put higher demands on accuracy than scientists do.