Northside Road Flood Elevation Analysis
City of Perry
DATE OF SERVICES: October 2020 – January 2021
PROJECT STATUS: Complete
ROLE OF FIRM:
- Flood Elevation Analysis
- Burke Murph – Project Manager
- Matt Taylor – QC Manager
- Barrett Neal – Assistant Project Manager
- Michael Griffore – Senior Designer
CLIENT: City of Perry
The City of Perry (City) contracted with GWES to determine the potential cause and/or source(s) of the flooding that occurred at 1913 Northside Road in Perry, GA during the Hurricane Sally rain event on September 16th and 17th, 2020. The study included analysis of recent development in the vicinity of this property, improvements to the downstream retention pond, and existing stormwater infrastructure. According to Section 2.2.2 of the City of Perry Stormwater Local Design Manual, stormwater conveyance infrastructure for a neighborhood roadway shall be sized to handle the 25-year storm event.
The former Cherokee Pines retention pond’s (pond) contributing watershed is comprised of approximately 352-acres of residential and commercial properties that conveys stormwater through a 20’ x 4’ box culvert beneath Northside Road. Stormwater is then conveyed through a 30-foot-wide channel approximately 700 feet before it reaches an existing retention pond. Recent pond modifications made to the outlet control structure along with the installation of a retaining wall has increased the pond’s water elevation by approximately 1.07 feet.
There are a considerable number of unknowns within the watershed that required GWES to make a variety of assumptions. Through hydrologic and hydraulic calculations, it was determined that the existing stormwater conveyance infrastructure beneath Northside Road is adequate to properly convey the 25-year storm event and is in compliance with the City’s stormwater requirements for a neighborhood roadway.
As far as the flooding that occurred at 1913 Northside Road is concerned, it is our opinion that the culvert under Northside Road is adequate to handle the rain event that occurred on September 16th and 17th, 2020 without the current tailwater condition. It is our opinion that the tailwater condition was increased by modifications made to the outlet control structure at the pond’s dam.