Specific Requirements for the permanent stormwater management portion of the Stormwater Management and Sediment Control Plan approval process will include, but are not limited to the following:
(A) Stormwater management must be addressed on a watershed basis to provide a cost effective water quantity and water quality solution to the specific watershed problems.
(B) All storm drainage systems will be classified based on the size of the contributing drainage basin or sub-watershed as follows:
(1) Minor drainage systems: Zero to < 40 acres.
(a) All street drainage, curb and gutters, pipe systems, culverts, ditches and channels which drain less than 40 acres will be designed to carry flows resulting from a 10-year frequency storm. Minimum allowable pipe diameter shall be 15 inches.
(b) The plan shall identify all drainageways.
(2) Collector systems: Forty to < l00 acres.
(a) All drainage systems draining at least 40 acres but less than 100 acres shall be designed to carry flows resulting from a 25-year frequency storm.
(b) The plan shall identify all drainageways.
(3) Major drainage channels: One hundred to < 300 acres. For all drainage systems draining at least 100 acres but less than 300 acres, encroachment upon major drainage channels and the adjacent overflow land shall be avoided as much as possible. All unavoidable improvements such as channel improvements, culverts or bridges along these channels shall be designed to carry a flow resulting from a 50-year frequency storm. In a subdivision, an easement sufficient to accommodate the flow from a 50-year frequency storm shall be dedicated to Newberry County for scheduled maintenance of the channel, and any other maintenance required may be permitted to be performed by a third party, upon written request to the county. In all other development activities, the plan shall identify all drainageways.
(4) County or FEMA flood plains: Three hundred and more acres.
(a) For all natural channels, creeks or rivers draining more than 300 acres, encroachment upon these channels and the adjacent overflow land shall be avoided as much as possible. All unavoidable improvements such as culverts or bridges along these channels shall be designed to carry a flow resulting from a 100-year frequency storm.
(b) If there are no detailed flood studies and base flood elevations available in these areas, the applicant shall provide a detailed flood study delineating the 100-year flood plain and the floodway. One-hundred year flood limits along with base flood elevation and the floodway will be shown on the Stormwater Management and Sediment Control Plans and also on the final plat if applicable. In a subdivision, the property line shall stop at the floodway line and the floodway shall be dedicated to Newberry County. In all other development activities, the plan shall identify all drainageways.
(5) Drainage systems to be analyzed and revised. All drainage systems regardless of its size or classification shall be analyzed and revised as necessary to ensure that overflow of the system would not result in a likelihood of dwelling flooding, property damage or public access and/or utility interruption be greater than 1% in any given year. All hydrological computations shall be based on the contributing watershed being fully developed.
(C) All hydrologic computations shall be accomplished using a volume based hydrograph method acceptable to the Planning Engineer. The storm duration shall be the 24-hour rainfall event, applicable SCS distribution with a 5-minute burst duration time increment. The Rational or Modified Rational Method is acceptable for sizing individual culverts or storm drains that are not part of a pipe network or system and do not have a contributing drainage area greater than 20 acres. The storm duration for this method shall be equal to the time of concentration of the contributing drainage area or a minimum of 5 minutes, which ever is greater.
(D) Stormwater management requirements for a specific project shall be based on the entire area to be developed, or if phased, the initial submittal shall control that area proposed in the initial phase and establish a procedure and obligation for the total site control.
(E) (1) Stormwater quantity control is an integral part of overall stormwater management. Detention ponds to control the peak run-off and volume may be required in area where there are known drainage problems or where the downstream natural or manmade channels are inadequate to accommodate projected run-off. The requirements for a detention pond and/or downstream capacity improvements will be determined by the Planning Engineer, in accordance with applicable state or federal regulations. Newberry County strongly encourages developers and engineers to make use of existing natural channels in their natural state, not to encroach into the natural flood prone areas and use channel improvements and/or detention ponds only as a last resort.
(2) The following design criteria for flow control shall be used where required by the Environmental Management Administrator:
(a) Post-development peak discharge rates shall not exceed pre-development discharge rates for 2-, 10- and 25-year frequency, 24-hour duration storm events unless the subject development is in a known problem watershed such that additional flow will negatively impact the watershed or there is insufficient flow capacity downstream in which case the peak discharge rate after the development may be restricted up to 1/2 the pre-development rates or whatever the system capacity downstream.
(b) Where ponds are the proposed method of control the person responsible for the land disturbance shall submit to Newberry County, when required, an analysis of the impact of stormwater flows downstream in the watershed for the 2-, 10-, 25-, 50- and 100-year storm events. The analysis shall include hydrologic and hydraulic calculations necessary to determine the impact of hydrograph timing modifications of the proposed land disturbance, with and without the pond. The results of the analysis will determine the need to modify the pond or to eliminate the pond requirements. Lacking a clearly defined downstream point of construction, the downstream impacts shall be established, with the concurrence of Newberry County.
(F) Discharge velocities shall be reduced to provide a non-erosive velocity flow from a structure, channel, or other control measure or the velocity of the 10-year, 24-hour storm runoff in the receiving waterway prior to the land disturbance, whichever is greater.
(G) All ponds, other than a retention or detention pond engineered as part of a stormwater system, that impede, encroach or alter a major drainage channel or flood plain must be capable of accommodating stormwater from a 100-year storm event based on built-out conditions for the watershed.
(H) Where existing wetlands are intended as a component of an overall stormwater management system, the approved Stormwater Management and Sediment Control Plan shall not be implemented until all necessary federal and state permits have been obtained.
(I) All stormwater management and sediment control practices shall be designed, constructed and maintained with consideration for the proper control of mosquitoes and other vectors.
(Am. Ord. 06-21-05, passed 6-15-2006)