(A) Computations for storm drainage from watersheds less than 200 acres may be based on the rational method using NWS TP-40 and NWS HYDRO 35 frequency curves for Grayson County. For larger watersheds, SCS or USACOE HEC methods shall be used.
(B) Underground drainage structures for residential areas, shopping centers and industrial developments shall be designed for a ten-year frequency rainfall, and central business districts for a 25-year frequency rainfall.
(C) An adequate storm sewer system consisting of inlets, pipes and other underground drainage structures with approved outlets shall be constructed where runoff of storm water and the prevention of erosion cannot be accomplished satisfactorily by surface drainage facilities. Storm sewers shall be reinforced concrete pipe.
(D) Open channel drainage structures shall be designed for the 100-year rainfall and shall provide for one foot of freeboard with sub-critical flow conditions.
(E) The drainage system shall be designed and constructed to handle rainfall runoff that originates in and crosses the subdivision.
(F) The drainage system shall be designed so that water shall not be greater than curb-deep and shall not flow farther than 1,000 feet before reaching an inlet in thoroughfare, collector, and local streets under design rainfall conditions. Curb inlets shall be installed at the upstream end of all valley gutters crossing thoroughfare and collector streets.
(G) Street crowns shall not be flattened or warped from one side of the street to the other side.
(H) Drainage systems, streets, terrain, building sites, building, public and private improvements shall be constructed to enable the runoff from a 100-year frequency rainfall to pass through the subdivision without inundating the ground floor of any home or business.
(I) Except when provided otherwise in the developer’s contract, the developer shall pay for all costs of the drainage system.
(Ord. 933, passed 8-9-05) Penalty, see § 153.999