5 county-level rules, plus city-specific rules for 1 city in Williamson County, Tennessee.
Verified from official government sources
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.
Williamson County Zoning Ordinance, Art. VII, Sec. 7220.A
Each development shall provide for the on-site or off-site detention of excess stormwater runoff resulting from that development.
Land disturbance in Williamson County triggers a soil erosion and sedimentation control plan once excavation or fill tops 500 cubic yards or clearing exceeds 5,000 square feet. Stream work always requires a plan.
Williamson County Zoning Ordinance, Art. VII, Sec. 7230.A
excavation, fill, or any combination thereof will exceed five hundred (500) cubic yards
Williamson County is landlocked in Middle Tennessee, so coastal rules do not apply. Development near its streams and drainageways is instead governed by open-space buffers, floodplain limits, and erosion controls protecting the Harpeth River system.
Williamson County Zoning Ordinance, Art. VII, Sec. 7113.A
Fifty (50) percent of the drainageway shall be maintained as open space. Drainageways shall be protected in such a way that the entire length can continue to function and be used for drainage purposes.
Williamson County enforces floodplain standards through its zoning ordinance to stay in the National Flood Insurance Program. New buildings in a Special Flood Hazard Area must sit at least one foot above base flood elevation.
Williamson County Zoning Ordinance, Art. VII, Sec. 7111.3.a
New construction or substantial improvement of any residential building (or manufactured home) shall have the lowest floor, including basement elevated no lower than one (1) foot above the base flood elevation.
Williamson County requires erosion plans for significant earthwork and forbids development from sending more stormwater onto downstream neighbors. Post-development flows cannot exceed the peak rate, volume, or velocity of the land in its natural state.
Williamson County Zoning Ordinance, Art. VII, Sec. 7220.B
No development shall cause downstream property owners, water courses, channels, or conduits to receive stormwater runoff from proposed developments at a higher peak flow rate, at higher volumes, or at higher velocities than would have resulted from the same storm event occurring over the site of the proposed development with the land in its natural, undeveloped condition.
1 cities in Williamson County have their own environmental rules rules. Each link goes to that city's dedicated page with code citations.
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