Williamson County, TN Environmental Rules: Stormwater Management (2026)
Key Facts
- Program
- Phase II MS4 (TDEC)
- Authority
- County Engineering Dept.
- Detention
- Match natural runoff rate
- Design Storm
- 1 to 100-year
- Watershed
- Harpeth River
Summary
Williamson County runs a Phase II MS4 stormwater program under a TDEC permit, protecting the Harpeth River watershed. Every development must detain excess runoff on-site so post-development flows do not exceed the natural rate.
Each development shall provide for the on-site or off-site detention of excess stormwater runoff resulting from that development.
Full Breakdown
The county Engineering Department administers stormwater under Tennessee's Phase II MS4 general permit through TDEC. Zoning Ordinance Section 7220 requires each development to detain the excess stormwater generated by new impervious surface, forest removal, or altered drainageways, with storage sized for the 1- through 100-year storm. Sites disturbing one acre or more also need coverage under the state construction general permit (TNCGP, TNR100000). The county samples the Harpeth, Little Harpeth, Owl Creek, and Rutherford Creek to track water quality.
Violations & Fines
Building without an approved stormwater plan draws a stop-work order. Illicit discharges to storm drains and unmaintained detention facilities bring correction notices and escalating fines until the site complies.
Frequently Asked Questions
Do I need a stormwater plan?
Why is the Harpeth River a focus?
Sources & Official References
Other rules in Williamson County
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