§ 26-198. Requirements for developments that drain to sinkholes.

(a)

Copies of the appropriate permits from TDEC shall be required prior to approval of a stormwater management plan for developments and redevelopments on property that includes a sinkhole. After review of these permits, the director may require additional information related to structural integrity and flood protection. If the proposed development does not require TDEC approval, a letter from TDEC stating that a TDEC permit is not required shall be submitted prior to the approval of a stormwater management plan.

(b)

For site development or redevelopment projects that must satisfy the flood protection criteria provisions in subsection 26-175(d) that have sinkholes located entirely within the development boundaries, calculations shall be provided showing that 100-year frequency, 24-hour design storm will not flood any structures assuming plugged conditions (0 cfs outflow) for the sinkhole. If the contributing watershed is less than 50 acres and there is documented evidence that resurgence is not a contributing factor to flood elevations, calculations shall be provided showing that there will not be a rise in the sinkhole floodplain elevation or a decrease in the sinkhole floodplain storage volume between the pre-and post-developed conditions for the 100-year frequency, four-day duration storm (7.8 inches, over a four-day period), assuming plugged conditions (0 cfs outflow) for the sinkhole. These calculations must include the entire contributing watershed for the sinkhole. An easement is required around the sinkhole to include an area that is a minimum of five feet horizontally outside the highest closed contour.

(c)

For site development or redevelopment projects that must satisfy the flood protection criteria provisions in subsection 26-175(d) that have sinkholes located partially on site, calculations shall be provided showing that there will not be a rise in the sinkhole floodplain elevation or a decrease in the sinkhole floodplain storage volume between the pre- and post-developed conditions for the 100-year frequency, 24-hour duration storm, assuming plugged conditions (0 cfs outflow) for the sinkhole. If the contributing watershed is less than 50 acres and there is documented evidence that resurgence is not a contributing factor to flood elevations, calculations shall be provided showing that there will not be a rise in the sinkhole floodplain elevation or a decrease in the sinkhole floodplain storage volume between the pre-and post-developed conditions for the 100-year frequency, four-day duration storm (7.8 inches, over a four-day period), assuming, plugged conditions (0 cfs outflow) for the sinkhole. These calculations must include the entire contributing watershed for the sinkhole. An easement is required at a minimum of five feet horizontally outside the highest closed contour on the section of the sinkhole located on the developed property. A rise in the 100-year water surface elevation is allowable when no structures will be flooded and all parties with ownership of the sinkhole agree in writing to allow the rise. In this case, an easement is required around the sinkhole to include an area that is a minimum of five feet horizontally outside the highest closed contour.

(d)

Retention of stormwater runoff or satisfaction of the provisions stated in subsection (c) is required for developments and redevelopments that require approval of a stormwater management plan and are located in one of the following watersheds:

(1)

Ten Mile Creek;

(2)

Sinking Creek;

(3)

Harrell Hills watershed (near Cranberry Dr., Clairmont Dr., and Gaines Rd.);

(4)

The Dead Horse Lake/Dutchtown Road sinkhole area;

(5)

Any watershed area which will drains exclusively to a sinkhole;

(6)

Any area of known flooding where deemed necessary by the director.

(e)

Retention facilities shall be designed so that the overflow in the one-year, two-year, five-year, ten-year, 25-year and 100-year design storms meet the pre-developed discharges in addition to retaining the difference in the pre-developed and post-developed 100-year design storm. In basins or sub-basins where there is a documented historical draw down time for the sinkhole or region being drained to, it may be acceptable for a detention pond to be used instead of retention. For detention to be approvable, the draw down time of the detention pond shall be a minimum of one and a half times the draw down time for the region.

(f)

The director has authority to condition the approval of a permit upon the compliance with additional requirements, including but not limited to measures to avoid and/or protect the sinkhole throat, detention, conveyance facilities, or other stormwater management solutions required to reduce the adverse impact of the proposed development on other properties or on the subject development.

(Ord. No. O-07-12-101, § 2(§ 5.8), 1-28-08)