The drainage plan includes detailed plans of
all stormwater drainage facilities, street profiles and other utilities
which may be required by the Planning Board. These plans are considered
to be an integral part of the final plat submission and are, therefore,
the basis for the performance bond and inspection fee.
A.
It is intended that all drainage problems within the
limits of the development or subdivision area will be completely resolved
by the design as approved by the Planning Board before any work is
started. However, in the event that an error on the part of the designing
engineer, either of design or judgment, becomes apparent during the
progress of the work or after completion of the work, the Planning
Board reserves the right to stop the work and/or order the necessary
changes to correct the situation. The Planning Board shall be held
blameless for any change so made.
B.
In order to achieve better comprehensive drainage
design, subdivision storm drainage systems shall, upon recommendation
of the Planning Board, be partially or completely integrated with
the area storm drainage system or designed in such a manner as to
be partially or completely integrated with the area storm drainage
system. In the following cases, developers may be required to pay
to the Town a drainage contribution towards the cost of such area
storm drainage system:
(1)
Subdivisions lying within drainage areas for which
the Town has designed storm drainage collection and disposal systems.
The drainage contribution toward the cost of participating in the
Town drainage system shall be borne by the developer on the basis
of his proportional share of the drainage contributed to the total
system and shall be computed by the Engineer for the Town.
(2)
Subdivisions which, because of the small area being
drained, would result in a disproportionate high cost of maintenance
for the system. In such case, the developer shall participate in the
cost of the future drainage systems on the basis of the alternate
cost to him of a drainage system if constructed within the subdivision.
The drainage plans must be prepared by a licensed
professional engineer in conformance with the design criteria and
standards contained herein. These plans shall be submitted in final
form, together with the final plat, at least 15 days prior to advertising
for the public hearing. The final plat shall not be signed until the
drainage plan is approved by the Engineer.
A.
General. The drainage plans shall be prepared in the
following forms:
(1)
Sheet size: 24 inches by 36 inches.
(2)
Scales:
(a)
Plan: one inch equals 100 feet. Note: Where required because of area, in addition to a one-hundred-foot-scale plan, a plan with a scale of one inch equals 200 feet will be required on Sheet 1, showing all applicable data under Subsection B.
(c)
Alternate scales may not be used unless approved
by the engineer.
(3)
Title box: standard.
(4)
Match lines: Where required, clearly defined match
lines shall be properly drawn and designated.
(5)
Materials: ink on linen.
B.
The plan of the complete stormwater drainage system
for the entire subdivision, with appropriate development stages for
each final plat section, shall include the following:
(4)
Surface drainage structures (e.g., valley gutters
and warped intersections):
(a)
Valley gutters shall be designated by the symbol
"V.G." A separate detail may be required if grades are in excess of
3%.
(5)
Concrete headwalls: All but standard concrete headwalls
shall be detailed.
(6)
Special structures; e.g., timber bulkhead, concrete
sluices, overflow structures, culverts, etc.
(7)
Detailed drawings required: stormwater, recharge basin
(scale: one inch equals 40 feet), easements, streams, waterways, canals,
etc.
C.
Profiles. Profiles for all proposed streets in the
subdivision and for all existing streets related to the development
of the subdivision shall include the following:
(1)
United States Coast and Geodetic Survey datum.
(2)
All existing natural grades and proposed grades.
(3)
Intersecting streets and stations.
(4)
Vertical curve data. (Vertical curves are required
where the algebraic difference of slopes is 1% or greater.)
(5)
System of survey stations.
(6)
Drainage pipes and structures.
(7)
Sanitary sewers.
D.
Design computations. The storm sewer system design
computations shall be submitted with the final map submission on acceptable
design sheets, signed by the engineer and dated.
A.
Collection system. The stormwater collection system
shall be designed in accordance with the rational method:
|
Q
|
=
|
A i R
| ||
|
Where
| ||||
|
Q
|
=
|
Discharge in cubic feet per second
| ||
|
A
|
=
|
Tributary drainage area in acres
| ||
|
i
|
=
|
Coefficient of runoff of the drainage area.
Calculations of "i" shall be shown on the design sheet.
| ||
|
R
|
=
|
Rainfall intensity in inches per hour as determined
by:
| ||
 |
|
Where
| ||||
|
t
|
=
|
Time of concentration in minutes
| ||
(1)
Pipes, conduits or ditches shall be designed by the
use of Mannings formula:
 |
|
Where
| ||||
|
V
|
=
|
Velocity in feet per second
| ||
|
R
|
=
|
Hydraulic radius in feet
| ||
|
n
|
=
|
Coefficient of roughness where:
| ||
|
n
|
=
|
0.13 for reinforced concrete pipes 24 inches
or smaller in diameter
| ||
|
n
|
=
|
0.15 for reinforced concrete pipes 18 inches
or smaller in diameter
| ||
|
n
|
=
|
To be determined and approved for special conduits
and ditches. Maximum velocity shall be 10.0 feet per second.
| ||
|
S
|
=
|
Slope of the pipe in feet/feet to be used as
hydraulic gradient of pipe flow except where the system will be surcharged
when the recharge basin is full. Design gradient in this case may
be assumed as the lowest inlet invert to the water surface at 80%
storage volume where no overflow is provided. In case of outfall in
tidal water, use the maximum tide elevation.
| ||
(2)
Manholes shall be a maximum distance apart of 350
feet.
(3)
Pipe alignment and location. Storm sewer pipe shall
be in a straight line between manholes and shall be placed between
the center line of the road and the curbline.
(4)
Valley gutters. Valley gutters will be permitted at
T-intersections where they will be parallel to the center line of
the through road and shall be installed only after design approval
by the Town Engineer. In general, the crown of the intersecting road
shall be gradually eliminated starting from a point about 30 feet
back from the flow line of the through street. At other than T-intersections,
valley gutters will be permitted only when warranted by limited traffic
use as approved by the Engineer.
(5)
Gutter flow. Surface drainage in gutters shall be
limited to that flow which will not exceed five inches in depth at
the curb based on the individual conditions of road grade, runoff
factor, rainfall intensity and tributary area and inlet capacity.
(6)
Catch basins. Not more than two catch basins shall
be interconnected before being connected to a manhole.
(7)
Brooks, ditches and stream beds. Where existing brooks,
water-bearing ditches and dry stream beds giving evidence of seasonal
runoff use are encountered, such area shall be maintained for drainage
purposes or other adequate means of providing for such drainage and
shall be installed at the developer's expense, after approval by the
Engineer.
(a)
A right-of-way or drainage easement of sufficient
minimum width to include a twelve-foot access strip in addition to
the width of the ditch, brook or stream bed as measured from bank
top to bank top shall be offered to the Town for drainage purposes.
Such right-of-way or drainage easement shall be shown on the drainage
plan and on the final plan with property bearings and distances indicated.
(8)
Drainage structures on state or county land. Drainage
structures which are located on state highway rights-of-way shall
be approved by the appropriate highway engineer's office, and a letter
of approval from that office shall be filed with the Town Planning
Board. Where county highways are involved, the Planning Board will
obtain the approval from the county pursuant to § 239-k
of the General Municipal Law.[1]
[1]
Editor's Note: General Municipal Law § 239-k
was repealed L.1998, c.451, § 2, effective 7-1-1998. See
now General Municipal Law § 239-f.
B.
Recharge basins. The stormwater recharge basins shall
be designed in accordance with the following;
(1)
Storage capacity: a five-inch rainfall with coefficient
of runoff based on runoff characteristics (minimum: C equals 0.30).
Overflow is to be provided unless special criteria is approved by
the Engineer.
(2)
A test hole in the proposed site must indicate sand
to at least five feet below the designed depth.
(3)
The maximum storage depth is to be 10 feet.
(4)
Side slopes, berm, planting fence, curb and gate shall
be in accordance with construction standards.
