A drainage system shall be provided for by means of culverts under roadways and other drainage structures or outlet ditches that are necessary to provide adequate drainage of storm water for all streets in the subdivisions, and for adjoining property where necessary. All such drainage systems shall be designed in accordance with sizes and specifications established in the North Carolina Department of Transportation Standard Specifications and the Roadway Standard Drawings.
(A) Storm water systems shall be designed by a professional engineer and approved by the Director of Public Works or his or her designee. Installation of same shall be in accordance with city specifications and standards.
(B) Adequate storm drainage shall be provided throughout by means of pipes or graded channels.
(1) Storm pipe shall be placed at all low points in the street grade to transmit storm water transversely across the street, with catch basins being constructed on both sides of the street at the low points.
(2) In no case, shall storm water be transmitted more than 500 feet in the gutter line.
(3) No open ditches will be permitted within the limits of the street rights-of-way, except for preexisting stream channels, which may be approved by the Director of Public Works or his or her designee.
(C) Drainage pipe.
(1) The minimum size pipe shall be 15 inches, regardless of drainage area.
(2) The minimum cover for all pipe shall be two feet.
(3) Sub-drainage shall be provided where the ground water table is within two feet of the subgrade.
(4) Six-inch, corrugated, black plastic pipe or corrugated metal pipe, with open joints or perforations laid on six inches of clean sand or washed stone, covered with six inches of clean sand or washed stone, shall be used to lower the water table.
(5) Ditches shall be a minimum of 36 inches deep and two feet wide.
(6) All surface drainage pipe shall be concrete, conforming to State Highway Commission Standard Specification.
(7) For special conditions, pipe recommended by the manufacturer for the type of installation involved, and approved by the Director of Public Works, or his or her designee, will be considered.
(8) Any concrete pipe laid between the concrete curbs shall be reinforced.
(9) All pipe shall be laid with the bell or groove upgrade and joint entirely interlocking.
(D) Catch basins shall be built as shown on the standard drawings.
(1) Improvised grates will not be acceptable.
(2) Catch basins walls shall be built straight, with inside joints struck smooth.
(3) Precast catch basins may be acceptable with the approval of the Director of Public Works or his or her designee.
(E) Catch basins, with frame, grates and hoods installed in curb-and-gutter sections less than two feet, six inches wide, shall offset the frame, grate and hood to the back of the structure, to maintain a consistent width of roadway.
(F) Frames, grates and hoods shall not be offset from the catch basin more than four inches, front to back.
(G) All pipe shall be installed using NCDOT Specifications outlined in the NCDOT Standard Specifications for Roads and Structures, Section 300, unless otherwise noted in this chapter.
(H) Pipe velocities for the design storm event shall not exceed 20 feet per second.
(I) The minimum pipe slope is 0.5% for all pipes, excluding special structures used for stream crossings, which should match the existing stream bed grade.
(Ord. passed 8-8-2019)