Verification of Delft FLS

Abstract

Nowadays flooding because of a dike breach can be simulated with high accuracy by use of computer models. Delft FLS, which stands for DELFT Flooding System, developed by WL - Delft Hydraulics, is such a model. The hydrodynamic processes occurring during a flooding are complex, for example moving hydraulic jumps and propagating bores. The implementation of the hydrodynamic laws in the computer model must be done correctly to be able to calculate these processes accurately. To verify the accuracy of the model an experiment was developed and executed in which these hydrodynamic processes occurred. The experimental results were compared with the numerical results to allow statements about the accuracy of the model. The experiment concerned the flooding of a basin of 8 m by 23 m. When a gate was lifted a reservoir emptied into the basin, which was thereby flooded. Three experiments were executed, two with different initial water layers in the basin and one with an initially dry basin. A video camera above the basin recorded the development of the flooding in the horizontal plane. The water depths were registered with wave gauges. The first numerical results seemed to agree reasonably well with the experimental data. However the calculated lateral flow velocity was too high. The dike-break is modelled in the computer program as a linear in time decreasing height of a dam. A numerical momentum conservative method is normally used in Delft FLS to calculate the flow through the breach. In the experiment a lifted gate represented the dike-break. The flow could not be calculated with the momentum conservative method because of a loss of momentum due to a force against the gate. An energy conservative numerical method is needed to calculate the flow. After applying the energy conservative numerical method the calculated results agreed very well with the experimental results. Verification of the computer model Delft FLS showed that the numerical methods used in the computation core of the program are very suitable to calculate the complex hydrodynamic processes of flooding. The methods are mass, energy and momentum conservative, which results in very accurate computations of two-dimensional dynamic flow with discontinuities in it.