Automated Dynamic Floodway Evaluation in SWMM

As part of the City of Jacksonville Master Stormwater Management Plan (MSMP), a new tool has been developed using the Stormwater Management Model (SWMM) to perform floodway analysis as part of FEMA Flood Insurance Studies. The standard approach in performing floodway analysis is to use HEC-RAS and the imbedded automated floodway calculation tools. However, in many areas, the FEMA standard methods are not the most appropriate for the analysis of flood hazards due to the loss of floodplain storage and increases in flows and volumes. It is also common for many communities to have an existing SWMM, which can be used for mapping purposes without the need for a conversion to HEC-RAS.

The standard FEMA floodway analysis consists of modeling an encroachment of the floodplain until a 1.0 ft increase in peak stage is simulated. With HEC-RAS, each reach in the stormwater system is analyzed separately under steady state flow conditions. Without a dynamic model simulating the entire stormwater network, the increases in peak stage are often underestimated due to increases in flows, tailwater effects, and loss of floodplain storage, among other possibilities.

As part of the MSMP and the FEMA Map Modernization Program’s DFIRM revision for the City of Jacksonville, CDM was tasked to perform revised hydrology and hydraulic analyses for 60 watersheds within the countywide study area. In order to incorporate the floodway analysis into the flood map revision, CDM developed an automated tool in SWMM, similar to HEC-RAS Method 4, to perform the floodway analysis. The tool runs SWMM, compares peak stage results, revises transects for each reach in the study, and re-writes the input file in iterative fashion. The method considers the loss of storage and well as conveyance in the calculations for flows and stages, and the associated flow rate and velocity increases are also directly considered.

This presentation discusses the principles and benefits of the methodology as well as the algorithms used to develop the automated floodway tool in SWMM.