Memphis/Shelby County Stormwater Management Manual (SWMM)

Memphis/Shelby County Stormwater Management Manual 3rd Edition (FINAL VERSION 11-1-2024)

The City of Memphis and Shelby County (with the assistance of Christian Brothers University’s Surface Water Institute) have completed the process of developing a revised stormwater management manual.    The Third Edition SWMM sets forth requirements for the modeling and design of the stormwater systems for new development projects.

The Memphis & Shelby County Stormwater Management Manual 3.0 came into effect FEBRUARY 1, 2025.   All new applications for developments will need to follow the new regulations. Requests to submit engineering reviews using the New SWWM prior to January 31, 2024 will be approved on a case by case bases by the Engineering Division

Summary of Significant Technical Changes

1. Puts more emphasis on the active and passive drainage systems.
2. Puts greater stress on the requirements appropriate for specific components of a drainage system.
3. Eliminates the terms “minor” and “major” for a drainage system or any component of a drainage system.
4. Eliminates drainage area as a criterion for determining an active system return period.
5. Sets the 25-year event as the lowest return period for the active system.
6. Sets specific active event return periods for types of system components.
7. Prohibits the use of the modified rational method.
8. Substitutes an IDF table in one-minute steps for IDF curves.
9. Eliminates the requirement described in the Second Edition SWMM to control detention outflow volume between hour-11 and hour-15 of the 24-hour for 2-year and 5-year events.
10. Requires no water quality treatment other than that required by TDEC.