(A) Stormwater channel location. Generally acceptable locations of stormwater channels in the design of a subdivision may include, but are not limited to, the following:
(1) Adjacent to roadways;
(2) In a depressed median of a double roadway, street or parkway provided the median is wide enough to permit slopes of one foot drop in six feet horizontal or flatter;
(3) Centered on lot lines or entirely within the real yards of a single row of lots or parcels;
(4) In each of the foregoing cases, a drainage easement with sufficient width (typically 20') to facilitate maintenance and design flow shall be provided and shown on the plat;
(5) If it is deemed to be appropriate by the design engineer to discharge concentrated surface water on an adjoining property at a location other than a natural or human-made drainage way, a written agreement between the parties or a general release, or an easement should be obtained for allowing the condition to exist; and
(6) The owner or developer may discharge storm water which has not been concentrated into a lower lying property, if the post-development peak rate does not exceed the pre-development peak rate and if the increase in volume caused by the development will not have an adverse impact on the lower lying property or if the landowner(s) of the lower lying property is made aware that the peak rate and/or volume for storm water has increased and agrees to accept the additional storm water and any and all imposed conditions. In the latter situation, the landowner(s) receiving the increase peak rate and/or volume increase and the developer and owner shall provide an agreement to the city stating said agreement, in a writing approved by the city, which writing shall include, in addition to the lower lying property owner’s consent, language which releases the City of Franklin and the Franklin-Simpson Planning and Zoning Office/Commission from all liabilities occurring from said stormwater and/or the provisions of said agreement and a provision indemnifying the City of Franklin and the Franklin-Simpson Planning and Zoning Office/Commission from any losses or damages as a result of the stormwater flow.
(B) Storm sewer outfall. The storm sewer outfall shall be designed to provide adequate protection against downstream erosion and scouring.
(C) Detention basin.
(1) Some major detention structures may also be required to comply with additional commonwealth design criteria.
(2) The facility shall be protected form soil erosion during all probable flow conditions imposed by the design storms.
(3) Unlined spillways shall be placed on either undisturbed ground or on a stabilized foundation and not on fill material.
(4) Side slopes for grassed areas shall not be steeper than two to one (horizontal: vertical).
(5) The earthen side slopes of retention basins (permanent ponds) steeper than three to one shall be protected from erosion using a rip rap layer one foot thick or other suitable material extending from a level three feet below the permanent pool design elevation to the maximum water surface elevation determined during routing of the ten-year, one-hour design storm runoff.
(6) Where possible, the shape of the basin shall conform to the natural topography.
(7) Where detention/retention basins are planned to be within 300 feet of a residence or recreational area, additional design criteria shall be considered so as not to create a potential or actual health or safety hazard. The following are possible requirements:
(a) Landscaping and/or fences;
(b) Trash racks and barriers to prevent entrance into the inlet and outlet structures;
(c) Grading to prevent steep drop-offs; and
(d) Signs to alert persons that the basin will flood during heavy rains.
(8) Control devices subject to theft or vandalism shall be adequately protected (bolted, heavy duty chain and lock and the like).
(9) Railing of fences shall be placed around the tope edge of inlet and outlet structures where the drop is equal to or greater than three and one-half feet.
(10) Grass bottoms in detention basins shall be designed with minimum slopes of 2% to promote adequate surface drainage.
(11) Where multiple-purpose use of the detention/retention area is planned or poor draining soils are found, provisions for adequate low flow drainage may be required.
(12) Detention/retention basins having permanent pool may be provided with a drain pipe and a suitable gate or valve located at the upstream end of the drain pipe. The drain pipe and valve shall be so configured to permit complete draining of the basin.
(13) To provide a maintenance access, the following requirements shall be satisfied:
(a) Unobstructed access ways shall be designed and shown on plans and constructed with the facility;
(b) The length of the access way from public right-of-way shall be minimized;
(c) Multiple accesses may be required;
(d) Grading of access ways to facilities should leave slopes which are not too steep (five to one maximum) to accommodate maintenance vehicles; and
(e) Detention/retention facilities requiring review by state agencies will have at least one all weather access roadway to include a minimum ten feet wide surface to the satisfaction of the City Engineer.
(14) Underground box chambers for temporary storage of storm water shall be provided with more than one access point for ventilation and cleaning. Rectangular chambers shall have a minimum height of 36 inches to facilitate maintenance. Underground circular chambers shall have minimum height of 36 inches.
(15) Water-tight pipe shall be provided on all outlet where the 25-year design storm headwater exceeds twice the diameter of the pipe. The use of anti-seep collars shall be evaluated when the headwater exceeds six feet.
(D) Sinkhole drainage areas.
(1) For the purpose of this section, the following definitions shall apply unless the context clearly indicates or requires a different meaning.
IMMEDIATE SINKHOLE DRAINAGE AREA. Any area that contributes surface water directly to the sinkhole(s); this does not include areas which contribute surface water indirectly to a sinkhole (via streams).
SINKHOLE. Any closed depression formed by removal (typically underground of water, surficial soil, rock or other material). The existence of a sinkhole shall be as indicated by the closed depression contour lines on topographic maps or other documents. Its actual limits may, however, be determined by field measurements with concurrence of the city’s Drainage Coordinator. SINKHOLES may be either circular in plan or irregular depending upon structural control.
SINKHOLE CLUSTER AREA. Any area that contributes surface water other than by way of a stream to a sinkhole which is located in a group of two or more sinkholes clustering together.
(2) (a) Plan requirements. A sinkhole, the immediate sinkhole drainage area, a sinkhole cluster area or portions of such items shall be shown on any development or preliminary subdivision plan for land where they exist. Sinkhole related non-buildable areas and restricted fill area shall be shown on final subdivision plans and development plans.
(b) Sinkhole related non-buildable areas. Based upon the topography, geology, soils and known history of the sinkhole (such as past filling) and the developer’s engineer’s storm water analysis and plan, the city’s Drainage Coordinator shall establish sinkhole related non-buildable areas. No buildings, parking areas or other structures shall be permitted, within the sinkhole related non-buildable area. This non-buildable area shall follow the limits of the sinkhole in most cases; however, the non-buildable area may be expanded or contracted by action of the city where warranted due to the nature of the specific sinkhole, the underlying geology, soils, drainage and any related information such as depth to bedrock. In sinkhole cluster areas, the city may require the developer to provide recommendations from a consulting engineer and a consulting hydro-geologist based upon substantial and state-of-the-art field studies and evaluation of the specific sinkhole system. Such studies will be reviewed by the city’s Drainage Coordinator.
(c) Development in sinkhole drainage areas. Development may occur in the immediate sinkhole drainage area if the developer provides alternative surface drainage away from the sinkhole, while keeping the water in the same surface drainage basin, and provided further that the water shall not go into another sinkhole drainage area off the petitioner property, nor into another stream of known flooding problems. The immediate sinkhole drainage area (or portion thereof) which cannot be provided with an alternative drainage system can be deleted from the development area and be used to meet the normal open space requirements. For portions of the immediate sinkhole drainage area where alternative surface drainage methods cannot be provided, as determined by the city’s Drainage Coordinator, the developer may choose one of the following options:
(d) Sinkhole surface drainage analyses. The sinkhole can be used for surface runoff drainage of a proposed development if the conditions of either of the following alternatives are met:
1. Alternative 1: A sinkhole can be used for surface runoff of a proposed development with or without retention or detention facilities as recommended by a consulting engineer and a consulting hydro-geologist; provided that any increase in the quantity of surface runoff due to development of the entire sinkhole drainage area in question will not aggravate flooding on the proposed development adjacent existing development, or connected/adjacent sinkhole subsurface systems. Such engineering and geological report must be substantive and based on state-of-the-art field studies and evaluation of the specific sinkhole system. The city shall not approve development proposals subject to Alternative 1 provisions unless the study findings meet the requirements of this division (D)(2)(d)(1).
2. Alternative 2: A sinkhole can be used for surface drainage of a proposed development if all of the following conditions and provisions are met:
a. That the runoff from the development area is either: completely retained in a retention basin; or retained in a detention basin. The flow rate out of the above basins shall be regulated so that it is no greater than the flow rate into the sinkhole of the development area prior to development for each of the following storms: ten-year/one-hour; 25- year/24-hour storm; or a 100-year/one-hour storm. The outflow rate shall not aggravate flooding on downstream properties for any of these storms;
b. As previously noted, the developer may elect to divert enough of the sinkhole drainage area so that the development of the remaining area does not increase the total quantity of runoff into the sinkhole. where additional runoff is anticipated, a consulting engineer and a hydro-geologist shall evaluate and show the effect of any additional quantity of runoff to the sinkhole and sinkhole system. For approval, the study must show the development will not aggravate flooding on the proposed development, adjacent lands or connected/adjacent sinkhole systems; and
c. Where the sinkhole outlet is offsite, either the runoff leaving the subject property must be shown to be no greater in flow or in quantity than that which existed before development or written approvals must be submitted from owners of the property where any increase in flow or quantity of water must go to reach the sinkhole outlet. Easement areas shall be approved by the city, based upon the developer’s engineer calculations of proposed ponding elevation.
(e) Filling in sinkhole drainage areas. Development may involve some filling of the sinkhole drainage area or sinkhole upon approval by the city’s Drainage Coordinator; however, no principal or accessory buildings with soil bearing foundations shall be permitted to be constructed on fill within the limits of any sinkhole.
(f) Required Plan notes. For any land which includes a sinkhole related non-buildable area, or restricted fill area, the developer shall place the following note on the final subdivision plan or development plan: based upon the evidence presented, the city has identified sinkhole related non-buildable areas on this plan; however, approval of this plan is not be interpreted as any guarantee that future sinkhole problems will not occur due to either natural or human activities.
(3) The following notes may be required in whole or part by the city depending upon the nature of the sinkhole and the method of treatment (if any) constructed by the developer:
(a) Any sinkhole related non-buildable area identified here has been determined to be unsuitable for any construction activity, and no buildings, parking areas or other structures shall be permitted within this area.
(b) Any sinkhole or restricted fill area identified here has been determined to be unsuitable for soil bearing foundations, and the entire structure of any building (including the floor system) constructed therein must be founded on solid rock.
(c) No basement or first floor elevations shall be lower than an elevation, USGS datum, to be determined on a case-by-case basis, said elevation being at least one foot above the 100-year 24-hour storm assuming no outflow from the sinkhole.
(4) Based upon the facts of each case, additional notes may be required or the above language modified as deemed appropriate by the city.
(E) Lot lines. Whenever the plans call for the passage and/or storage of stormwater runoff along lot lines, the grading of all such lots shall be prescribed and established for the passage and/or storage of waters, and no structure or vegetation which would obstruct the flow of stormwater shall be allowed, nor shall any change be made to the prescribed grades and contours of the specified stormwater channels.
(F) Manholes. All utility sewer manholes constructed in an area designed for the storage or passage of stormwater, shall be provided with either a water-tight manhole cover or be constructed with a rim elevation of a minimum of one foot above the high water elevation of the design storm.
(G) Easement. Permanent easements for the detention and conveyance of stormwater, including easements of access to structures and facilities shall be maintained by the developer, owner, Homeowners’ Association, or other person or entity approved by the city unless other acceptable maintenance plans are approved by the city.
(H) Obstruction of drainage. The keeping of disposal of grass clippings, trash, debris, obstructions or unwanted materials into the storm sewers or within or along stormwater channels or in adjacent flood plain areas which may wash into sewers and channels is prohibited.
(I) Permits required for construction within floodplains. Construction within floodplains will require authorization by the city, Commonwealth Natural Resources and Environmental Protection Cabinet (including FEMA) and the U.S. Army Crops of Engineers, where applicable. Each agency should be consulted well in advance of construction for assistance in completing the required permits.
(J) Maintenance. Provisions acceptable to the city for perpetual maintenance of detention facilities, outlet works and appurtenances shall be made, as provided in § 150.24.
(Ord. 980.2-9-94, passed 8-24-1994; Ord. 2023-022, passed 6-26-2023)