Garner Road Townhomes Flood Study

The proposed Garner Road Townhomes development triggered a City of Raleigh requirement for a local flood study because the contributing drainage area exceeded 100 acres. No FEMA-designated SFHAs existed on the site, but that did not eliminate the regulatory obligation. The city requires developers to establish future-conditions floodplains, floodways, and RFPEs regardless of whether FEMA mapping already covers the area, meaning the work had to be done from the ground up.

The project also required design and sizing of a new stream crossing to satisfy City of Raleigh stormwater conveyance criteria. Getting that crossing right mattered not just for permitting but for the design team’s schedule: defining the floodplain limits and crossing parameters early enough to inform site layout was the only way to avoid costly redesigns or permitting delays down the line.

RiskHydro built a comprehensive modeling framework to characterize future-conditions hydrology and floodplain hydraulics for the site. The work ran on two tracks simultaneously: developing the flood study to meet City of Raleigh and UDO submittal requirements, and sizing the stream crossing to satisfy Stormwater Design Manual performance criteria. Key steps included:

• Simulating 100-year peak flows using HEC-HMS under future land use conditions to establish the design hydrology driving floodplain extent and crossing size
• Applying HEC-RAS 2D unsteady modeling with LiDAR terrain, survey data, and proposed grading plans to map flood extents and water surface elevations across the site
• Conducting a floodway encroachment analysis to define conveyance corridors that meet City of Raleigh allowable surcharge criteria
• Sizing the proposed stream crossing to comply with City of Raleigh Stormwater Design Manual requirements, including allowable headwater elevations (HW/D), structural embedment, and confirmation of no adverse increase outside the drainage easement
• Preparing documentation meeting all City of Raleigh Stormwater Design Manual and UDO standards for floodway and RFPE submittals

The study gave the design team a clear regulatory framework to work within and provided the City of Raleigh with the data it needed for a thorough, efficient review. By integrating floodplain compliance early in the planning process, the project avoided the redesign risk that often comes when flood limits are not defined until late in design. The five outcomes below reflect what that early investment delivered.

• Future-conditions SFHAs and RFPEs established, defining the regulatory limits that govern site development and giving the design team firm boundaries to work within
• No-rise conditions verified in upstream and downstream reaches, confirming the project does not increase flood risk to adjacent or downstream properties
• Stream crossing configuration established that satisfies City of Raleigh conveyance and stormwater performance criteria
• Clear, defensible data provided to support City of Raleigh review and reduce the likelihood of requests for additional information
• Floodplain compliance integrated early in planning, reducing permitting time and long-term risk exposure for the development team