FIRM 2D: A New Methodology for Using HEC-RAS 2D Models in Flood Insurance Maps
Historically, the United States Army Corps of Engineers (USACE) employed the HEC-RAS software for one-dimensional modeling (1D), and therefore, all the flood insurance maps were developed using this technique. The launch of Flow Two-Dimensional (2D) Modeling in the 2016 USACE HEC-RAS Release 5.0 lacked the capability to perform floodways analysis, which is a crucial component in the Flood Insurance Rate Map (FIRM). However, the latest release of USACE HEC-RAS, version 6.4.1 in June 2023, has incorporated an encroachment tool that enables the delineation of floodways in 2D modeling, so now is a good time to make the transition from 1D to 2D flow modeling.
This article presents a case study of a watershed (HUC-12) as a representative sample. The study aims to demonstrate the process of 2D flow modeling and the methodology involved in creating the 2D flow model. The process begins with obtaining digital elevation models (DEM) and conducting hydrology and hydraulic analysis using USACE HEC-HMS and HEC-RAS software. The results are visualized and analyzed using ArcGIS.
The study also examines the appearance of the FIRM 2D and concludes by determining the Base Flood Elevation (BFE) for a specific building as a sample within the study area, which is required for the elevation certificate (EC).
Case Study
The Redhouse Creek-Back River watershed, designated as Hydrologic Unit Code (HUC) 12, has been selected as the central focus for this study’s proposal. This watershed is situated within the jurisdictions of Baltimore City/County, Maryland. The area of this location is estimated to be roughly 20,016 acres, as illustrated in Figure-1.
Figure-1: Redhouse Creek-Back River Watershed (HUC 12), Baltimore City/ County, MD
Methodology
a. Digital Elevation Model (DEM)
Elevation data for the study area was obtained from the USGS (United States Geological Survey) by using RAS Mapper (refer to Figure-2)
Figure-2: Digital Elevation Model (DEM) For the Site
b. Hydrology
The HEC-HMS version 4.11 computer software was employed to conduct this hydrological study, which analyzed a 100-year storm scenario (refer to Figure-3).
Figure-3: 100 Year Storm Hydrograph for the Watershed
c. Hydraulic
The HEC-RAS 2D model employed for the site integrates the HEC-HMS watershed model as a boundary condition for the simulation of unsteady flow. A computational mesh with dimensions of 10 feet by 10 feet was used for the HEC-RAS 2D region. The HEC-RAS software’s “Manning’s n by Land Cover” capability was applied to establish a corresponding “n” coefficient for each land use classification.
It is important to highlight that hydraulic structures are excluded from consideration in this model due to the lack of a site survey at this study.
Results
The water surface elevation (WSE) associated with the 100 Year Flood Elevation (BFE) was obtained using HEC-RAS Mapper and exported at a resolution of 0.1 ft intervals, as illustrated in Figure- 4.
Figure-4: BFE Contour Lines
According to the data presented in Figure- 4, the maximum Base Flood Elevation (BFE) observed for the large white structure situated in the lower left corner of the map is 31 feet.
Conclusions
The advantages of 2D flow modeling over 1D modeling are manifold. First, in 2D modeling, it is possible to simulate all streams within any size of watershed in batches, resulting in significant savings in both time and financial resources. Additionally, 2D modeling allows for the simulation of ponding areas for shallow flows outside of the stream channels (AO and AH Zones), utilizing Rain-on-Grid as a boundary condition.
Although it is possible to utilize the outcomes of the 2D flow model by adhering to the current FIRM format, which involves stream cross-sections and profiles in the Flood Insurance Study (FIS), it is no longer necessary to include these stream cross-sections and profiles. This is due to the capability of displaying the results of the 2D model for the Base Flood Elevation (BFE) at any point within the watershed as contour lines in the FIRM 2D. This applies regardless of whether the location is within or outside of the floodplain of the stream. Consequently, it will be more convenient to obtain the elevation certificate for any house within the watershed without the need to consult the floodway data table or the profiles.
Nazir Hejazi, MSc, CFM, is a water resources engineer at Tetra Tech. He can be reached at nazir.hejazi@tetratech.com
References
FEMA, National Flood Insurance Program (NFIP), Floodplain Management Requirement, A Study Guide and Desk Reference for Local Officials, February 2005.
Hydrologic Engineering Center, 2023. HEC-HMS User’s Manual, U.S. Army Corps of Engineers, Davis CA., October 2023.
Hydrologic Engineering Center, 2023. HEC-RAS 2D Modeling User’s Manual, U.S. Army Corps of Engineers, Davis CA., October 2023.
