For facility sizing criteria, the basis for hydrologic and hydraulic evaluation of development sites are as follows:
(A) Impervious cover is measured from the site plan and includes any material or structure on or above the ground that prevents water from infiltrating through the underlying soil. IMPERVIOUS SURFACE is defined to include, without limitation: paved parking lots, sidewalks, roof tops, driveways, patios, and paved, compacted gravel and compacted dirt surfaced roads.
(B) Off-site areas shall be assessed based on their “pre-developed condition” for computing the water quality volume (i.e, treatment of only on-site areas is required). However, if an offsite area drains to a proposed BMP, flow from that area must be accounted for in the sizing of a specific practice.
(C) Off-site areas draining to a proposed facility should be modeled as “present condition” for peak-flow attenuation requirements.
(D) The length of sheet flow used in time of concentration calculations is limited to no more than 50 feet for predevelopment conditions and 50 feet for post development conditions.
(E) Detention time for the one-year storm is defined as the center of mass of the inflow hydrograph and the center of mass of the outflow hydrograph.
(F) The models TR-55 and TR-20 (or approved equivalent) will be used for determining peak discharge rates.
(G) The standard for characterizing pre-development land use for on-site areas shall be woods.
(H) For purposes of computing runoff, all pervious lands in the site shall be assumed prior to development to be in good condition regardless of conditions existing at the time of computation.
(I) If an off-site area drains to a facility, off-site areas should be modeled, assuming an “ultimate buildout condition” upstream.
(J) Determination of flooding and channel erosion impacts to receiving streams due to land development projects shall be measured at each point of discharge from the development project and such determination shall include any runoff from the balance of the watershed which also contributes to that point of discharge.
(K) The specified design storms shall be defined as a 24-hour storm using the latest published rainfall distribution for Pickens County by National Oceanic and Atmosphere Administration (NOAA) on their website; http://hdsc.nws.noaa.gov/hdsc/pfds/orb/sc_pfds.html.
(L) Proposed residential, commercial, or industrial subdivisions shall apply these stormwater management criteria to the land development as a whole. Individual lots in new subdivisions shall not be considered separate land development projects, but rather the entire subdivision shall be considered a single land development project. Hydrologic parameters shall reflect the ultimate land development and shall be used in all engineering calculations.
(M) Detention must be addressed whenever there is an increase in stormwater runoff resulting from the proposed development. This can be achieved by appropriate detention facilities including wet or dry bottom reservoirs, flat or green roofs, parking lots, underground storage, and appropriate LID techniques such as rain gardens. The following shall govern the design of detention facilities:
(1) Storm volume. The volume of storage potential provided in detention facilities shall be sufficient to control the excess stormwater runoff, as determined to be the difference between the stormwater quantity from the site in its developed state for a ten-year, 24-hour frequency rainfall less the ten-year runoff quantity from the site prior to development.
(2) Release velocity. Detention facilities shall release stormwater at a nonerosive velocity. Protected channels receiving detention discharge shall incorporate features to reduce velocity to nonerosive levels at the point where such discharge enters the unprotected channel. If release is into a subsurface conduit the energy gradient in the receiving facility shall not be increased beyond the slope of the conduit.
(3) Spillway. Emergency spillways shall be provided to permit the safe passage of runoff generated from a 100-year, 24-hour storm.
(4) Freeboard. Detention facilities shall have adequate capacity to contain the storage volume of tributary stormwater runoff with at least one foot of freeboard above the water surface of flow in the emergency spillway in a 100-year storm event.
(5) Fencing. Ground reservoir detention facilities with a depth greater than 24 inches shall be enclosed within a permanent six-foot high fence. Wet ponds do not have to be fenced when they are also used for an aesthetic reason. All detention facilities shall be landscaped on the outside of the fence or reservoir by a screen of the owner’s choosing, including berms, trees, shrubbery, and the like.
(Ord. 2008-06, passed 4-14-08) Penalty, see § 52.999