A. 
General. Proposed drainage facilities shall be designed to accommodate the runoff from the entire upstream drainage area under conditions of maximum potential land development that could occur under the most current Zoning Regulations.
B. 
Potential overload. Where the proposed roadway and drainage facility construction and land development associated therewith are likely to cause an increase in the volume or rate of stormwater runoff such as to hydraulically overload existing downstream drainage structures or facilities, cause damage to said existing downstream structures or facilities, and/or cause flooding and/or physical damage of land and improvements adjacent thereto, adequate measures shall be taken by the person or firm constructing the road and drainage facilities to prevent or alleviate such harmful effects. Such measures may include runoff retardance by vegetative plantings, stormwater retention basins, replacement of existing inadequate downstream drainage structures and facilities, erosion protection and other measures deemed effective by the Commission.
C. 
Discharge. The discharge of all stormwater shall be into established streams, brooks, rivers, or Town or state highway drains having adequate capacity to accommodate such discharges. Where the discharge of stormwater shall be onto or through private property, perpetual drainage easements and discharge rights, running with the land and in favor of the owner of the road, shall be secured by the applicant.
D. 
Diversion. The diversion of stormwater runoff from one watershed or watercourse to another shall be avoided whenever possible. Where it is absolutely necessary to create such a diversion, special provisions shall be made to minimize the potential damages which may occur as a result of such diversion.
E. 
Existing watercourses. All work on established watercourses shall be accomplished in such a way as to minimize the effects which would be adverse to the regimen of such watercourse. Adequate provision shall be made to prevent or minimize scour or erosion in the adjacent upstream and downstream reaches of the watercourse.
F. 
Capacity within roadway. Storm drainage systems within the roadway, exclusive of culverts and bridges carrying flows under the road, shall be designed to safely accommodate flows resulting from storms of the maximum intensity which can be expected to occur on an average of once in 10 years (ten-year storm) without being surcharged.
G. 
Capacity under roadways. Culverts crossing under roadways shall be designed to accommodate the following flows:
(1) 
Flows resulting from storms of the maximum intensity which can be expected to occur on an average of once in 25 years (twenty-five-year storm) without causing the depth of the headwater pool to exceed 1.2 times the height of the culvert barrel or flooding of the roadway or adjacent buildings, sewage disposal systems and water supply systems; and
(2) 
Flows resulting from storms of the maximum intensity which can be expected to occur on an average of once in 100 years (one-hundred-year storm) without causing damage to or flooding of buildings, sewage disposal systems and water supply systems or damage to the culvert or roadway.
H. 
Bridges. Bridges over waterways shall be designed in accordance with Subsection G(2) and shall have a minimum under clearance (water surface to closest bottom projection of bridge) of one foot at the design flow.
A. 
General. Stormwater flows may be computed by use of the Rational Method or by use of the methods described in the most current edition of the United States Soil Conservation Service Technical Release No. 55. Other methods of computing stormwater flows may be utilized provided that they conform to sound engineering practice. In general, the use of the Rational Method is discouraged for use in computing flows from drainage areas in excess of 200 acres or for computing flows from one-hundred-year storms.
B. 
Computations. Where the Rational Method formula is used, rainfall intensities shall be computed as follows, where "t" is the time of concentration, in minutes.
Storm Return Frequency
(years)
Rainfall Intensity
(inches per hour)
10
170
(t+23)
25
230
(t+30)
C. 
Runoff coefficients. Where the Rational Method formula is used, the following runoff coefficients (C) shall be utilized for each type of surface and a composite "C" value computed for each tributary drainage area. In any case, a composite "C" value of less than 0.25 shall not be used.
Type of Surface
Runoff Coefficient "C" (1)
(10-year storm)
Pavement, roofs
0.90
Embankment slopes (cuts and fills)
0.60
Lawns
Flat slope (2)
0.17
Average slope (2)
0.22
Steep slope (2)
0.35
Cultivated fields and pasture
Flat slope (2)
0.20
Average slope (2)
0.30
Steep slope (2)
0.40
Meadows (moist, level grassland)
0.10
Forested areas
Flat slope (2)
0.10
Average slope (2)
0.15
Steep slope (2)
0.30
Notes:
(1)
For twenty-five-year storm, increase runoff coefficients by 50% (except for pavements, roofs and embankment slopes).
(2)
Flat slope equals 2% or less; average slope equals 2% to 7%; steep slope equals over 7%.
A. 
Culverts under roadways. Culverts under roadways shall have a minimum diameter of 24 inches.
B. 
Other. All other piping carrying surface drainage or a combination of surface drainage and groundwater shall have a minimum diameter of 15 inches.
A. 
General. Catch basins, or drop inlets where permitted, shall be provided in order that surface water will not travel along the roadway curbline without interception more than 350 feet on roads with grades up to and including 5% and not more 250 feet on roads with grades over 5%. Catch basins, or drop inlets where permitted, shall also be installed at all low points and at all corners of intersections.
B. 
Debris and/or sediment. Drop inlets (catch basins without sumps) shall not be used where the potential exists for debris and/or sediment entering the drop inlet to clog the storm drainage piping.
C. 
Depressed or set back. On steep grades or in special drainage situations the Commission may require catch basins or drop inlets to be depressed and/or set back from the street curbing to facilitate the collection of water and may require the furnishing and installation of double inlets.
A. 
General. In general, a manhole is less preferable to a catch basin or drop inlet and should only be provided where the use of a catch basin or drop inlet is not feasible.
B. 
Places. Manholes shall be provided at each change of drainage pipe slope or alignment, at all pipe junctions and otherwise at intervals of approximately 350 feet on long lengths of pipe where catch basins or drop inlets are not used.
The inlets and outlets of all exposed drainage conduits shall be protected by concrete or mortared stone endwalls. Endwalls shall be constructed with wingwalls where necessary to contain and protect the adjacent earthen slopes and/or direct the flow of water entering or leaving the conduit. Prefabricated pipe end sections will not be permitted.
A. 
General. Where open channel flow is required, the channel shall be properly designed to safely carry the design flow. Open channels shall be in the form of a trapezoid having a bottom width of at least two feet and side slopes of not less than two feet horizontal to one foot vertical. The channel shall be seeded, sodded, riprapped or otherwise stabilized as the flow quantities and velocities require.
B. 
Stabilization of open channels. Special attention shall be given to the stabilization of open channels in the immediate vicinity of pipe inlets and outlets, bridges, at bends and curves and at other critical locations as required to prevent scouring, erosion and/or siltation of watercourses and culverts and undermining of drainage structures.
C. 
Criteria. Hydraulic design of open channels and design of bed and bank stabilization shall be done in accordance with the applicable criteria of the most current edition of the Federal Highway Administration publication titled "Design of Roadside Drainage Channels."
The installation of underdrains will be required beneath the edge of pavement of a proposed street wherever the groundwater is known to be less than three feet below the proposed finished grade of the street. The Commission may require underdrains to be installed where localized seeps or springs are observed within the proposed street lines during construction.