Comparison of watershed-scale hydrological and hydraulics model for Chambery region in France using 2D and integrated 1D-2D PCSWMM models

The city of Chambéry and its surrounding cities, located north to the city of Grenoble in France, are well known for its flooding problems due to two main sources: the river overflows and the urban runoff. The group of cities decided to analyse the problem and find solutions through an innovative approach. Rather than working alone and looking at each problem independently, they choose to work on the entire watershed scale through a unique integrated hydrological and 2D hydraulic model. This watershed, classified as being a flood risk, is approximately 4700 ha and has more than 125,000 residents, which are primarily located in the northern region.

Based on that context, the objective of the first study was to compare different possible model development methods. Three methods for setting up 2D watershed-scale SWMM5 projects are explored using PCSWMM and watershed data from the mixed rural - urban settlement of Chambéry. 

The first method involved setting up an entirely 2D model using two boundary types: one representing the primary channels and the other representing the overland area.  In this case, channels were modeled using a directional mesh and the overland area was represented using an adaptive mesh, allowing for a higher resolution in areas with elevation fluctuations. The second method also modeled the area completely in 2D but used 4 boundary polygons; one representing the channels and the other three representing the overland area, where mesh resolutions transition from fine to coarser moving upstream within the watershed. In both previous scenarios full-2D scenarios, inflows were added directly to the mesh representing an equivalent, area-weighted, 100-year design storm rainfall.    

The third method used an integrated 1D-2D approach where the channelized flow was represented using 1D hydraulics and flooding from overtopped channels was modeled using a 2D overland mesh.  In this case, 1D-2D connections were established using side orifices. Model inflows were represented in that case using SWMM5 subcatchments generated using PCSWMM’s built-in watershed delineation tool. 

The results of the three methodologies are compared and the advantages and disadvantages of each method are considered in terms of time and effort needed to construct the model, the time needed to process the results and the convenience to optimise the model. The results give a unique and very interesting point of view about the various possible approaches that people can use for comparable projects before deciding the best approaches to choose.