Mitigating Flood Damage: Developing a Comprehensive Drainage Plan using SWMM, HEC-RAS, and ArcGIS

Scott M. Rybarczyk

ABSTRACT

Basement flooding and water overtopping the roadways is a common occurrence in one suburb of Buffalo, NY. Due to the flat topography and overdevelopment in this dense suburb, it has been difficult to mitigate these flooding problems. Only by developing a comprehensive drainage plan could the Town expect to find solutions to their flooding problems.

A plan was developed for the Town starting with a system of integrated models to assess the complex system of ditches on the eastern half of the Town. Approximately 4,700 acres of the Town were modeled using a watershed approach combining SWMM and HEC-RAS to adequately represent the hydrology and hydraulics of the system. ArcGIS was used in concert with both models to first develop inputs to each model and then for visualization of the model results.

This presentation will focus on the development of both the SWMM and HEC-RAS models and show how the models interact with each other to provide the results needed for the comprehensive drainage plan. SWMM was used to model the hydrology of individual subdivisions using subareas, catch basins, and pipes. SWMM was also used to provide an initial estimate of flows and flood timing in each ditch. The final ditch models were developed in HEC-RAS using inputs from SWMM. In some locations, additional iterations were needed between SWMM and HEC-RAS to balance the model results. In both SWMM and HEC-RAS, model inputs were constructed using GIS tools including HEC-GeoRAS. The final model results were plotted using HEC-GeoRAS to provide visualizations that could be understood by the general public.

The models developed were confirmed using existing field data and then applied through the use of various scenarios defined by the Engineer and the Town. These scenarios specified improvements to the system that included both traditional mitigation measures (ponds, increased culvert sizes) and green mitigation measures (rain gardens, artificial wetlands). A final scenario was developed and used to create the comprehensive drainage plan. This plan is now being implemented and the first series of projects have been financed and constructed.


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