Section 12
12.0 The following design criteria shall apply to proposed drainage systems:
12.1 Off Site Drainage Considerations
In the course of designing the on-site drainage systems, the subdivider shall ensure that there is no net increase in runoff to off-site areas. If increased runoff is not preventable, the subdivider shall obtain an easement in the impacted areas, and make drainage improvements there to contain the increased runoff. The easement document shall contain a section holding the Town of Newbury harmless for any claims for damage in the easement area and in any down-slope areas potentially impacted.
12.2 Storm Frequency Design Requirements
12.2.1 Cross culvert pipes - 25 year storm frequency; check impacts for 50 year storm;
12.2.2 Closed drain system - 10 year storm frequency; check impacts at sag points for a 25 year storm;
12.2.3 Storm water detention facilities - 25 year storm frequency; check impacts for 50 year storm;
12.2.4 Storm water retention facilities - 25 year storm frequency; check impacts for 50 year storm.
12.2.5 Box culverts and bridges - 50 year storm frequency; check impacts for 100 year storm.
Note: The Board reserves the right to require that storm drain systems be designed for less frequent, more intense rainfalls where conditions warrant.
12.3 Culvert Design
12.3.1 All computations for culvert pipes shall be documented and included in the drainage report. These shall be performed under the supervision of an engineer. The engineer shall stamp and sign the drainage report.
12.3.2 All available records concerning rainfall and floods shall be used in the design of culverts and storm sewers.
12.3.3 Pipe culverts will be designed as open flow channels. They will either be under inlet or outlet control. The exact control can be found by following the procedure outlined in "Hydraulic Charts for the Selection of Highway Culverts", published by Bureau of Public Roads as H.E.C. No. 5. This material can also be found in the NH DOT, DRAINAGE MANUAL.
12.3.4 Minimum pipe culvert sizes are as follows:
Roadways - 15"
Drives - 12"
12.3.5 Pipe culverts with a span of 10 feet or more will be considered as bridges.
12.3.6 It is preferred that culverts be located to fit natural channels in lines and grade.
12.3.7 The minimum grade of culverts shall be 0.4% or able to maintain a velocity of 2 fps while flowing one-third full.
12.3.8 Collars will be designed for culverts whose grade exceeds 20%; erosion control should be considered on all culverts.
12.3.9 When the computed outlet velocity is in the range beyond normal design of 10 fps additional outlet protection shall be considered.
12.3.10 The maximum headwater depth of flow immediately upstream from a pipe culvert shall be controlled by the following:
Damage to adjacent property;
Damage to culvert and the roadway;
Traffic interruption;
Hazard to human life; and,
Damage to stream & flood plain environment.
As a guide, the following table may be used under "normal conditions":
PIPE SIZE MAXIMUM ALLOWABLE HEADWATER
12" - 30" 2 times pipe diameter
36" - 48" 1 ½ times pipe diameter
54" - up 1 times pipe diameter
12.3.11 Minimum cover for culverts, measured between the pipe crown and finished grade, shall be as follows:
Paved and Unpaved Roads - 3' for all type of pipe material
Under Drives - 1' for all type of pipe material
Under Grassed - 2' for all type of pipe material
Acceptable pipe material for road culverts is reinforced concrete. Acceptable pipe material under drives include reinforced concrete; corrugated, galvanized steel; or corrugated aluminum.
12.3.13 All culverts shall be constructed with end sections, headers, or stone slope paving as specified below. End sections shall be permitted on all pipes less than 48" diameter, except 24" diameter where there is an active stream. Stone slope paving or riprap shall be permitted at culvert ends for pipes up to 24" in diameter.
12.4 Closed Drainage System Design
12.4.1 A closed drainage system design may be required at the Planning Board's discretion in commercial, densely developed residential areas, or in areas with excessively steep grades.
12.4.2 All computations for closed drainage system designs, shall be documented and included in the drainage report. These shall be performed under the supervision of an engineer.
12.4.3 Storm drains shall be designed on the assumption that each inlet intercepts all runoff that contributes to it, providing the inlet capacity is equal to or greater than the design runoff.
12.4.4 The compatibility of grate capacity, pipe capacity and design flow must be considered in closed system design.
12.4.5 The minimum grade of closed system pipes will be 0.4% or able to maintain a velocity of 2 fps while flowing one-third full.
12.4.6 Manholes shall be placed wherever a change in grade of alignment of a storm drain occurs but, in any case, a storm drain shall normally not have a manhole, catch basin or drop inlet more than 300 feet apart.
12.4.7 In general use catch basins rather than drop inlets. Call for drop inlets only where no pipe inlet occurs and where soils and other debris are not liable to wash in, on slope drainage, in embankments, and in culverts. Refer to the Typical Section showing the Catch Basin and Drop Inlet Design detailed in Exhibit I.
12.4.8 The minimum pipe size for closed systems under roadways is 15" diameter.
12.4.9 Minimum cover for closed drainage systems, measured between the pipe crown and finished grade, shall be as follows:
Paved and Unpaved Roads - 3' for all type of pipe material
Under Drives - 1' for all type of pipe material
Under Grassed - 2' for all type of pipe material
Acceptable pipe materials for closed drainage systems include: reinforced concrete; corrugated, galvanized steel; corrugated aluminum; and smooth lined, corrugated PVC pipe.
12.4.10 The type of grate chosen will be based on the following usage restrictions:
NH DOT GRATE STD. WHERE UTILIZED
A In roadways, ditches, medians where bicycle traffic is NOT anticipated.
B & B Alt. In roadways, ditches, medians, where bicycle and pedestrian traffic is anticipated.
C In ditches & sumps off the roadway where vehicles cannot make contact with the structure.
E & E Alt. Where high grate capacity is required and bicycle or pedestrian traffic is NOT anticipated.
All grates and frames shall be cast iron.
12.4.11 Placement of catch basins in curbed roadway sags shall conform to the following:
At least one catch basin with a double grate shall be located at the bottom of a sag. Depending on roadway classification and design con-sidera-tions, an addi-tional catch basin on either side could be necessary. The spacing between the three catch basins shall be such as to prevent ponding of ½ of the traveled way.
12.4.12 No surface flow shall be allowed across streets.
12.5 Ditches
12.5.1 A maximum length of 400 feet for a ditch to a catch basin or drop inlet is required. Local conditions may require variations. Any variations must be approved by the Board or the Board's Agent.
12.5.2 In order to keep the ditch self-cleaning, a minimum grade of 0.5% shall be required, except for "Site Specific Swales" as required by RSA 485-A:17.
12.5.3 All ditches shall be checked for possible erosion and subsequent siltation of streams. Acceptable methods of treatment include matting for erosion control, stone for erosion control, stone fill and riprap.
12.5.4 All ditches steeper than 5% shall be adequately protected against soil erosion. Matting for erosion control or stone linings shall be provided, as determined by an engineering evaluation.
12.5.5 Ditches shall be used at the top of back slopes only when excessive off-site runoff could damage slopes and/or overtax on-site systems.
12.6 Subsurface Drainage (Under drains)
Subsurface drainage systems (under drain pipe) shall be provided where the seasonal high ground water table is within five feet (5') of the finished roadway grade. Test pits or borings in roadway cut sections shall be taken, as required or ordered by the Board's Agent, to locate the Seasonal High Water (SHWT) Table and determine the need for under drain pipe. This pipe shall be perforated PVC with a minimum diameter of 6". Alternative under drain pipe material shall require specific approval by the Board or its Agent.
12.7 Stormwater Detention and Retention Pond/Basin
12.7.1 Consideration shall be given to use of stormwater detention and retention basins to reduce the peak rainfall run-off rate from the subdivision. For the purpose of this regulation the following definitions shall apply:
Detention Pond or Basin - A stormwater storage facility which acts as a temporary reservoir, allowing rainfall runoff to be released at slow, pre=determined rates.
Retention Pond or Basin - A stormwater storage facility which acts as a temporary reservoir which does not allow any surface release of rainfall runoff until after a storm, if at all. Most often the retained stormwater is discharged into the ground by infiltration, or into the atmosphere by evaporation and transpiration by plants.
12.7.2 All computations for Detention or Retention Ponds/Basins shall be performed under the supervision of an engineer.
12.7.3 Considerations shall be given to providing adequately for emergency or flooding conditions.
12.7.4 Detention Pond/Basin side slopes shall not exceed 4 to 1 (4:1) horizontal to vertical dimension ratio unless the area is enclosed with suitable fencing.
12.7.5 The Planning Board, at its sole discretion, may require suitable fencing of Retention Ponds/Basins.
12.7.6 Adequate drainage easements and access ways (drives) must be provided to assure access to the proposed Detention or Retention Pond/Basin for maintenance purposes.
12.8 Construction Details
The subdivider shall include the following in the proposed construction plans.
12.8.1 Pipe profiles for all culverts and closed drain system pipes. These may be shown on roadway cross sections, roadway profile or on a completely separate profile, including existing and proposed finished grades, as required. Plans shall specify pipe type, size, length, slope and invert elevations.
12.8.2 All catch basins, drop inlets and man-holes shall be shown in profile, the type of grate with rim elevations shall be specified.
12.8.3 Any ditch section proposed, which is different from the typical roadside ditch, shall be shown in detail giving the bottom width, side slopes and minimum depth of ditch.
12.8.4 Permanent erosion control measures shall be specified and detailed on the plans for all roadside ditches (where required) and the drainage pipe outlets. Sufficient detail shall be provided to properly construct the intended item (i.e., stone size, thickness of stone layer, subgrade preparation or protection, depth of stone lining in ditch, etc).
12.8.5 Sufficient plans and construction details for Detention and Retention Ponds shall be provided.