Where part of a detention basin will contain a permanent pool of water, all the items required for detention storage shall apply except that the system of drains with a positive gravity outlet required to maintain a dry bottom basin will not be required. A controlled positive outlet will be required to maintain the design water level in the wet bottom basin and provide required detention storage above the design water level. However, the following additional conditions shall apply:
a. Basins designed with permanent pools or containing permanent ponds shall have a water area of at least five (5) acres. If fish are to be maintained in the pond, a minimum depth of approximately ten (10) feet shall be maintained over at least twenty-five percent (25%) of the pond area. The remaining pond area shall have no extensive shallow areas, except as required by subsection (c) below.
b. In excavated ponds, the underwater side slopes in the pond shall be stable. In the case of valley storage, natural slopes may be considered to be stable.
c. A safety ledge four (4) to six (6) feet in width is required and must be installed in all ponds approximately thirty (30) or thirty-six (36) inches below the permanent water level. In addition, a similar maintenance ledge twelve (12) to eighteen (18) inches above the permanent water line shall be provided. The slope between the two (2) ledges shall be stable and of a material such as stone or riprap which will prevent erosion due to wave action.
d. Retention ponds containing a permanent pool of water have all slopes between the riprap and high water line sodded and the remaining land area hydroseeded; are equipped with electrically driven aeration devices, if required to maintain proper aerobic conditions and sustain aquatic life; have a four (4) foot wide crushed limestone walkway at the high water line entirely around the body of water; provide suitable public access acceptable to the responsible governmental agency; and have the high water line not closer than seventy-five (75) feet to any property line;
e. Periodic maintenance is required in ponds to control weed and larval growth. The pond shall also be designed to provide for the easy removal of sediment which will accumulate during periods of pond operation. A means of maintaining the designed water level of the pond during prolonged periods of dry weather is also required.
f. For emergency use, basin cleaning or shoreline maintenance facilities shall be provided or plans prepared for auxiliary equipment to permit emptying and drainage.
g. Facilities to enhance and maintain pond water quality shall be provided, if required to meet applicable water quality standards. Design calculations to substantiate the effectiveness of these aeration facilities shall be submitted with final engineering plans. Agreements for the perpetual operation and maintenance of aeration facilities shall be prepared to the satisfaction of the Department of Planning and Building after review and recommendation of the Plan Commission.
(Ord. No. 1708, 6-13-07)