(A) Storm drainage design. Storm drainage for roads shall be planned and designed in accordance with the following standards.
(1) Pipe and ditches. Sufficient pipe shall be installed to carry existing watercourses in the street right-of-way and to drain both the proposed street or streets or extensions thereof or other streets which, based on topography will be served by the same drainage systems. No open ditches or channels shall be provided in the street right-of-way except with the express written permission of the Board of Selectmen, and adequate provision is made to maintain the standard cross-section, and for protective guide posts or rails as may be required to insure public safety. Road drainage systems shall be coordinated with, general drainage requirements for the use and development of the adjoining land.
(2) Design storm. Flows for roadways and roadway storm drains shall be designed using a minimum 25-year design storm frequency. Flows developed for drainage areas with no established watercourse shall be based on a 50-year design storm frequency. Drainage basins with designated watercourses shall use a design frequency of 50 years for average annual daily stream flows of less than five c.f.s. and 100 years for average annual daily stream flows greater than five c.f.s. It shall be the applicant’s responsibility to adequately demonstrate whether a stream flow is less than or greater than five c.f.s. using stream gauging stations, published government documents and other acceptable statistical rainfall data as determined by the Town Engineer.
(3) Design methods. The Rational Method may be used for drainage areas of 200 acres or less. For areas greater than 200 acres, the SCS method, FHWA method or USGS method shall be used as approved by the Board of Selectmen and/or the Town Engineer.
(4) Pipe design. The minimum size of a stormwater pipe shall be 15 inches in diameter. Coefficients used in design for reinforced concrete pipe shall be N=0.015 and for asphalt-coated corrugated pipe shall be N=0.021. The minim vim slopes of pipe shall be 0.5%.
(5) Pipe size and slope. Pipe size and slope shall be such that the head on the invert will not exceed two point zero (2.0) diameter at design storm and this head shall be contained without damage to any adjacent property. Pipe, except for under drains, shall be laid on straight alignments, both horizontally and vertically, with manholes and catch basins providing access at all deflection points or at the junction of two or more lines. The open end of any pipe shall be provided with an end well, culvert end or other appropriate device. The minimum cover over the top of the pipe shall be three feet. Culverts under streets shall extend to the edge of the right-of-way.
(6) Spacing of catch basins. Catch basins shall be connected to a stormwater pipe and shall be provided in sufficient number and suitable locations to properly drain the surface. Surface water shall not travel on any street a distance of more than 300 feet unless otherwise approved by the Board of Selectmen. No surface water shall travel across any intersections. Spacing and number of basins must be supported by complete gutter flow analysis.
(7) Discharge. The discharge of all stormwater shall be into suitable streams or rivers or into town or state drains or ditches or other town drainage facilities with adequate capacity to carry the additional water. Where the discharge shall be into private property adjoining the street, proper easements and discharge rights shall be secured by the applicant for the town before approval of the drainage plan. Discharge shall be done in a manner that protects watercourses and wetlands from pollution and shall not increase flood hazard due to increase in runoff. Plunge pools and other devices shall be employed to reduce velocities, as directed by the Town Engineer.
(B) Drainage construction. Storm drainage shall be constructed in accordance with the following standards.
(1) Pipe. Stormwater pipe shall conform to one of the following standards.
(a) In general, asphalt-coated corrugated metal pipe shall be used where culvert gradients are in excess of 10% or unsuitable base is encountered, such as organic soils areas. Said pipe shall conform to Form 814, shall be minimum 16 gauge and have paved invert.
(b) Reinforced concrete pipe shall be Class IV and conform to Form 814.
(2) Methods. Prior to laying pipe, the trench shall be excavated to the required depth, the bottom of which shall be graded to afford a uniformly firm bearing for the pipe throughout its length. Where rock is encountered, it shall be excavated to not less than 12 inches below the bottom of the trench and this depth shall be refilled with crushed stone approved by the Board of Selectmen. All pipe shall be carefully laid, true to the lines and grades given, hubs upgrade and with the spigot ends fully entered into adjacent hubs. Preferably, joints in concrete pipes shall have rubber gaskets; otherwise joints shall be thoroughly wetted and filled with a portland cement mortar and caulked. Asphalt-coated corrugated metal pipe shall be carefully joined and firmly clamped together by approved connecting bands, which shall be properly bolted in place before any backfill is placed. Backfill will consist of clean well graded soil free from large stones, frozen lumps, wood or other debris. Backfill shall be placed in layers of not more than six inches and adequately compacted. Any suitable material removed in excavating for the trench may be used as backfill. Fill approximately the same elevation at the same time. Rock fill or stones larger than two inches shall not be placed closer than two feet from the pipe on the sides as well as the top.
(3) Appurtenances. Catch basins, manholes, drop inlets, end wells and other appurtenances to the storm drainage system shall be constructed in accordance with Form 814 and the Connecticut State Highway Department Standard Sheets.
(4) Cleaning. All pipe and drainage structures shall be thoroughly cleaned prior to acceptance of the road and drainage system by the town.
(Ord. passed 1-11-1984)