Comparison of three methodologies for quasi-2D river flooding modeling with EPA SWMM

Fast quasi-2D mesh construction in association with a computational engine such as the US Environmental Protection Agency’s Storm Water Management Model (SWMM) is making two dimensional hydraulic modeling even more economically feasible. As in any hydraulic modeling, care must be taken to sufficiently represent the reality with the available modeling objects.

Three methods for quasi-2D model setup are explored with the PCSWMM 5.6 (Computational Hydraulics International) software using real project data from a hydraulic study of the Birse River in the town of Court in Switzerland. Two of the three methods use an integrated 1D river channel – quasi-2D floodplain approach, while the third method uses a complete quasi-2D approach for the entire mesh. The first method is the standard method for constructing a 1D-2D model with PCSWMM (quasi-2D mesh with 1D bottom orifice connections towards the 1D conveyance network). Care needs to be taken in the model construction to avoid double accounting of the channel conveyance. The methodology describes the steps taken to avoid double accounting. The second method uses the technique of representing the river channel with a 1D approach and making orifice side connections with the quasi-2D floodplain mesh. Side connections can be tedious to construct, so a routine using FME software was developed to automatically create the orifice side connections. The third method used a complete quasi-2D model to represent the river and floodplain conveyance.  In this method, care needs to be taken in representing the bridges. In the three methods, Geomensura Mensura Genius and ESRI ArcGIS are used extensively to improve the topographic definition with a TIN terrain model.

A 100 year return period flood is simulated for each PCSWMM model. The results of the three methodologies are compared. The advantages and disadvantages of each method are considered in terms of the time needed to construct the model and the time needed to process results. In the end, the EPA-SWMM quasi-2D river flood modeler will choose the modeling methodology according to the data and software available, and his/her proficiency with the available software. Regardless which method is used, quasi-2D modeling provides a fast and efficient method for investigating two dimensional flood problems.