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.
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.
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.
