§ 801-27. Drainage.

A. 

General provisions.

(1) 

The subdivision shall be designed so that all drainage shall be contained and disposed of within the subdivision; no drainage shall be discharged on to adjoining property or on to the public right-of-way at a rate greater than existed prior to the construction of the subdivision. The applicant shall submit information as to pre- and post-development runoff volumes and peak flows during the ten-year and one-hundred-year storm. No drainage shall be discharged directly into any wetland or surface water body, or into any drain, ditch, culvert or retention pond that leads into wetlands or surface water bodies.

(2) 

All drainage systems within the subdivision shall be designed in accordance with the soil conservation service method or an alternative system acceptable to the Board's engineer. Calculations shall be made from the source of drainage runoff using topographic maps for the entire drainage area, including those areas outside the subdivision. Copies of all drainage calculations shall be submitted with the definitive plan. Percolation tests may be required at the discretion of the Board.

(3) 

Drainage systems shall be designed and installed so as to prevent stormwater runoff from becoming a hazard or a nuisance to the subdivision residents or the public at large. The applicant is ultimately responsible for the adequacy of the drainage system in reaching this end. Should the system not adequately achieve the goal, the applicant shall make whatever modifications are needed to ensure that the drainage system performs adequately in the opinion of the Planning Board. All modifications shall be acceptable to the Board's engineer.

B. 

Subsurface drains or subdrains.

(1) 

In areas where the finished grade of the roadway is less than four feet above the water table or in areas where less than four feet of fill is placed above water in swampy places or any standing water, or in other areas, where in the opinion of the Board the subgrade must be drained, a system of subdrains shall be designed for such areas. The subdrain shall consist of a minimum of one longitudinal drain for each forty-foot width of roadway or fraction thereof.

(2) 

In addition, laterals shall be required as directed by the Board in areas in which an undue amount of water could accumulate in the subgrade. The system of subdrains shall be discharged into the storm drainage system or otherwise disposed of in a manner satisfactory to the Board.

(3) 

Subdrains shall also be required where test borings show an impervious layer of soil above a permeable layer of soil which is located at or above one foot below the proposed basement floor elevation.

C. 

Storm drains.

(1) 

A complete storm drain system shall be designed for each street of the subdivision and, to the satisfaction of the Board, shall be so laid out to provide adequate drainage of all portions of the street system so that water does not accumulate thereon, to intercept stormwater runoff from the adjacent lots of the subdivision, and to eliminate undesirable or unnatural accumulation of water on any portion of the subdivision or surrounding property. Those conditions which result from a ten-, twenty-five- or fifty-year storm as required shall be assumed as a basis for design of the street drains. The storm drain system shall include gutters, catch basins, manholes, culverts, drain lines, headwalls, vegetated swales, detention ponds and such other items as may be required to complete the system to the satisfaction of the Board. Information regarding a procedure which may be utilized in drainage designs is available from the Town Engineering office.

(2) 

The Soil Conservation Service drainage calculation methodology or an alternative systematic method acceptable to the Board's engineer shall be used in the design of the drainage systems.

(3) 

Best management practices (BMPs) shall be utilized to treat the first one inch of rainfall, or 1/2 inch of runoff, whichever is greater. BMPs shall consist of grass-lined swales, grass-lined retention basins, or other treatment facilities acceptable to the Board's engineer.

(4) 

Side slopes on BMPs shall not be steeper than four to one and generally should be much flatter. BMPs shall be distributed throughout a subdivision and not concentrated in any one location to better dilute the effects of any pollutants left untreated. For this reason, the use of swales in areas with relatively level terrain is desirable. The bottom of the BMPs should be four feet above the probable high groundwater. All BMPs shall be located within drainage easements or the road right-of-way.

(5) 

Catch basins shall be located in pairs, one on each side of the roadway, at all low points or sag curves in the roadway, at intervals of not more than 300 feet on continuous grades of the roadway, and at or near the corners of the roadway at intersecting streets.

(6) 

Manholes shall be located at all changes in direction, either horizontally or vertically, of a drain line or at the intersection of two or more drain lines, or so located that no drain line greater than 300 feet in length would exist without either a catch basin or manhole.

(7) 

Culverts shall be designed on the assumption that the entire drainage area is built up to that density and in the manner which the applicable section of the Zoning Ordinance allows. The calculations (or a copy thereof) necessary to determine the size of any culvert which carries a brook, stream, river or other natural waterway shall be submitted to the Board for review. All culverts shall have a headwall at each end, and any culvert over 36 inches in diameter shall include at the upstream end additional protection, as approved by the Board, for the roadway side slopes.

Editor's Note: See Ch. 240, Zoning, of the Code of the Town of Barnstable.

(8) 

All drains shall be a minimum of 12 inches in diameter and shall be laid on a slope of not less than 1/2 of 1%. The minimum design velocity shall be three feet per second and maximum design velocity shall be 10 feet per second. If the system is designed as a self-cleaning system, the Board may accept a lesser minimum slope for the drain lines. Pipe shall have a capacity 25% greater than required by the calculations. In such cases as it is deemed necessary and acceptable by the Board and its engineer, surface water may be disposed of by a leaching system of the proper size and design. Calculations for the design of such system shall be submitted with other drainage calculations for the subdivision and under the same provisions. Provision shall be made for the disposal of surface water intercepted or collected by the system in such a manner that no flow is conducted over Town ways, or over the land of others unless a drainage easement is obtained or unless such flow, in essentially the same quantity, previously existed in the same location. Where adjacent property is not subdivided, provision shall be made for extensions of the system by continuing appropriate drains to the boundary of the subdivision at such size and grade as will allow their proper projection.

(9) 

All recharge systems shall be designed with a fail-safe feature which will provide a safe, legal off-the-road overflow area for the runoff in the event that the catch basins overflow. The overflow areas shall be sized to accommodate a one-hundred-year storm. The overflow areas shall preferably be located in naturally vegetated shallow kettle holes or other depressions; however, if there are not any naturally occurring areas, then the shallow areas with gently sloping sides shall be excavated for stormwater storage. Overflow areas shall be located within drainage easements. Separate drainage lots are discouraged and will not be permitted without the permission of the Planning Board.

(10) 

A headwall shall be provided at the outfall end of all drains where required.

(11) 

All recharge infiltration type drainage systems shall be designed by the applicant's engineer to prevent stormwater from breaking out of any slopes. Breakout calculations shall be provided by the applicant's engineer.