Purvis, Rebecca

Rebecca Purvis is a Water Resources Engineer with Atkins, based in Willow Spring, NC.

Titles from this speaker

Bringing Fairfax County, VA Flood Maps To The 21st Century

Abstract

Learning Objectives:
-Application of geoprocessing automation in hydrologic analysis
-Best practices for applying a phased approach to large-scale hydraulic modeling production

Introduction
Current effective FEMA Flood Insurance Rate Maps (FIRMs) were initially developed for Fairfax County, VA in the 1970s. FEMA is currently updating the County's Flood Insurance Rate Maps (FIRMs). FEMA typically maps floodplains of drainage areas one square mile or greater. Fairfax County, however, intends to regulate the floodplains of all stream channels within the county that have drainage areas 70 acres and greater. Atkins is currently working with Fairfax County to develop a separate regulatory floodplain layer (map) for all 70-acre streams by leveraging the terrain and cross sections from the most recent FEMA study and developing flows based on the Anderson Method, which is the Fairfax County's preferred hydrologic method. The final regulatory floodplain layer would be assembled from the more conservative (higher BFE) results between the County and FEMA. Fairfax County plans to fully remap the entire county, which includes 27 watersheds, consisting of about 800 stream miles and 1200 stream crossings.

Methodology
To comply with Fairfax County's Public Facilities Manual (PFM), floodplain studies on areas less than 200 acres are to use the Rational Method or NRCS methodology. An automated ArcGIS-based geoprocessing tool was created to calculate all necessary watershed parameters needed for hydrology calculations, including curve number, drainage area, and longest flow path. The tool requires a shapefile of desired delineation locations as well as several watershed rasters. The output will provide an updated pour point shapefile in which the attributes report drainage area, curve number, slope, longest flow path, and longest flow path elevations (Figure 1). Additionally, longest flow path and pour point watershed shapefiles are generated. Parameters are input into an automated spreadsheet to calculate the Anderson Method discharge and Rational Method discharge. Values are also input into HEC-HMS to calculate the NRCS Method discharge. Updated flows for each watershed are then imported into a HEC-RAS model to simulate the updated flooding extent. The HEC-RAS models are being developed based on Fairfax County's 2018 Lidar data and have cross-sections spaced approximately 300 feet apart. This is a large-scale project with many moving parts, including hydrology development, hydraulic model development, survey collection and implementation, and final flooding extent results for 27 watersheds. Thus, a key component is to plan in phases to handle all data and tasks. Fairfax County watersheds have been split into Phase 1 and Phase 2 and three stages to manage the time requirements for each step (Figure 2). Each phase (group of watersheds) of the work will go through the three stages: preliminary HEC-RAS model development, field surveys of hydraulic structures, and final model development and mapping. Both the hydrologic and hydraulic analyses are being performed in parallel. Production is being done by watershed basis with the hydraulics for a given basin lagging the hydraulics for that basin.

Project Status
The project team has completed the geoprocessing, hydrology calculations, and hydraulic modeling for three watersheds. Figure 3 shows the floodplains generated for one of the watersheds. Development of the preliminary HEC-RAS models for the remaining watersheds within Phase 1 is ongoing. Subsequent stages of the work will include field surveys of hydraulic structures and final HEC-RAS model development and regulatory floodplain mapping.

Conclusions
Current flood maps for Fairfax County are up to 50-years old. Increases in development density within the watershed and climate change are not incorporated in these maps. Updating these maps will allow for reliable data in future developments and improved mitigation and response options. The use of a geoprocessing tool greatly optimizes the development of updated flow data thereby aiding in the completion of this massive undertaking within budget and schedule.

This paper was presented at the WEF Stormwater Summit, June 27-29, 2023.

SpeakerPurvis, Rebecca

Presentation time

15:45:00

16:15:00

Session time

13:30:00

16:45:00

SessionSession 13: Advancements in Modeling

Session number13

Session locationKansas City Convention Center

TopicWatershed-Based/Integrated Planning, Design and Modeling