Swift Creek Watershed Master Plan | Town of Cary

Gradient’s capability in managing and executing large-scale urban watershed planning projects, utilizing advanced watershed and stormwater modeling techniques, integrating comprehensive data collection, and supporting community engagement can be leveraged to address Raleigh’s flooding and water quality issues effectively.

Overview

The Swift Creek watershed model and plan is being developed by Gradient for the 20-square- mile urban watershed with funds from a USEPA community grant.

The Swift Creek watershed includes downtown Cary and areas which experienced flooding during significant rainfall events. Swift Creek is also an impaired stream with a total maximum daily load (TMDL).

Due to the large watershed size, Cary decided to develop the watershed model in different phases. Given the potential for increased development in this watershed, Phase I of this project focused on modeling the upper reaches of the watershed, consisting of approximately 1.15 square miles.

The study analyzed the existing conditions of the watershed to form a baseline for future development.

The watershed model geometry and topography was built primarily from the Quality Level 2 LIDAR data as well as stormwater structure GIS data. The 2-, 10-, 25-, 50- and 100- year, 24-hour storm discharges were estimated for the watershed area using PCSWMM software (running the EPA SWMM engine).

Rainfall distributions based on NOAA Atlas 14 precipitation-frequency data were used to develop runoff hydrographs for the 24-hour duration rainfall events for the area. A flood study analysis was performed using the PCSWMM software, including a combination of 1D and 2D modeling.

Gradient Solution

As part of Phase II and II, Gradient is expanding the model to the rest of Town jurisdictional area and also performing model validation with actual loT (internet- of-things) hydrologic sensors data that were deployed in the watershed in 2022.

Gradient also investigated watershed’s sediment load estimation in PCSWMM.

This comprehensive model (with floodplain extents) is anticipated to be completed in Fall 2024 and used by Cary staff to map the regulatory floodplain, understand flooding risk, and guide land development projects in the watershed.