B. Drainage

B. Drainage

  1. Adequate disposal of surface water shall be provided. Provisions must be made for maintaining natural watercourses and, unless a drainage easement is provided, limiting the peak runoff from new or improved roads to the existing conditions during both the 2-year and 25-year rainfall events. Road cross-culverts and water flow in the curb line shall be designed for at least the 25-yr event and driveway culverts for at least the 10-year event. Water flow along the curb line shall not extend more than 4 feet into the travel lane. Drainage calculations must be submitted to the Planning Board for their approval. For comparison of existing conditions and proposed conditions, or where hydrograph routing is necessary, the SCS TR-20 methods shall be utilized. For stormdrainage system sizing where the drainage area is less than 200 acres the Rational Method or SCS methods may be utilized. For storm-drainage system sizing where the drainage areas are greater than 200 acres, the Potters Small Watershed Analysis, SCS methods, or other methods as approved by the Planning Board or their representative may be used.
  2. All road culverts shall be at least fifteen (15) inches in diameter and all driveway culverts shall be at least twelve (12) inches in diameter. Culverts shall be corrugated metal pipe (complying with AASHTO M36, Type III ) or reinforced concrete pipe (Class III or higher complying with NHDOT Section 603), or HDPE plastic pipe (by Hancor or ADS complying with AASHTO M294, Type S). No aluminum shall be allowed. See Detail 6A.
  3. When pipe systems are included in a street design, plan/profile sheets shall be required for the systems showing proper sizing, slopes, inverts, etc.
  4. When roadside drainage ditches are used, they shall be in accordance with Details 1A and 1B. Riprap-lined roadside ditches are allowed only behind curbed roads. In such cases the riprap shall extend no closer than four feet from the back of the curb. Crossculverts and/or closed storm-drain systems shall be used to limit the flow in ditches to 10 cfs and velocity to 3 fps in grass-lined ditches. In no case shall the ditch length exceed 600 feet. Refer to paragraph 131-67.C.2 for other conditions that require the use of curbing, catch basins and closed storm drains. Permanent turf reinforcement mats (TRMs) are not allowed in roadside ditches.
  5. The street crown, center to shoulder, shall be 1/4 inch per foot. Superelevation shall be required where the ADT is greater than 400. When designing a superelevated street, the superelevation rate shall not exceed 3/4 inches per foot, and must be accompanied by engineering calculations.
  6. Headwalls with properly designed cut-offs shall be required at the inlet end of all culverts. See Detail 2. For driveway culverts, prefabricated end-sections with toe plates may be used in lieu of headwalls.
  7. Underdrains shall be used under ditches where the seasonal high water table is within three feet of the subgrade elevation. Where the road runs nearly perpendicular to the existing contours, additional underdrains shall be installed laterally across the road and spaced no greater than 100 feet apart. Where the road runs nearly diagonal to the contours, underdrains shall be installed laterally across the road and spaced no greater than 200 feet apart. Underdrain shall be installed in accordance with Detail 3. Stormdrain pipes may be perforated in lieu of providing separate underdrain pipes. Perforations in storm-drain pipes shall be positioned at 10 and 2 o’clock, and the depth of flow in the pipe shall be kept below the perforations. 
  8. Infiltration devices shall require: pre-treatment of the storm water before infiltration; test pits demonstrating that the bottom of the devices are at least 3 feet above the seasonal high groundwater table; the bottom of the entire infiltration surface to be at least three feet below finished grade; and the infiltration area to be located outside the travel way and shoulder. In areas of SCS Adams soils, confirmed by test pitting, the exfiltration velocity may be assumed to be 0.0022 cfs/sf. In all other areas, the exfiltration velocity shall be determined by field permeability testing or established correlations to grain-size distributions, with a 1.5 factor of safety applied.
  9. Drainage easements shall be provided where the peak rate of runoff will increase and/or where the runoff flow types changes to concentrated flow. Drainage easements shall extend from point of increased flow and/or the point the flow becomes concentrated to the point the runoff reaches an NHDES jurisdictional wetland /channel. Drainage easements are not required within the road right-of-way.
  10. For subdivisions, the assumed impervious area per building lot shall not be less than 5,000 square feet and the assumed cleared area per building lot shall not be less than 10,000 square feet. The assumed impervious and cleared areas shall be stated on the subdivision plan with a note specifying that if the actual impervious and/or cleared areas exceed the assumptions, the lot owner shall revise the drainage analysis accordingly and provide measures to limit the flow to those assumed in the drainage design. Note, impervious area includes both paved and compacted gravel surfaces. travel way.