Town of East Hampton, NY Stormwater Collection Systems

All stormwater collection systems shall be designed in accordance with the following minimum specifications:

The collection and storage system required for residential and commercial properties shall be designed using the following equations:

(1)

Storage runoff to seepage pools:

		Sc = A x R c
	Where:
		Sc = storage capacity in cubic feet.
		A = tributary drainage area in square feet.
		R = rainfall equals 0.167 feet two inches.
		c = coefficient of runoff of the drainage area.
	The following minimum values of c and a combination thereof shall be used (combination of c):

	Hilly Terrain	Flat Terrain
Concrete and asphalt	0.9	0.9
Bare earth	.5	.2
Grass meadow	.35	.1
Forest area	.3	.1
Cultivated area	.4	.1

(2)

Positive outfalls or recharge basins.

(a)

When positive outfalls or recharge basins are used, the collection system shall be designed using the rational method of design.

(b)

The equations for the rational method of design are as follows:

		Q = Aci
	Where:
		Q = discharge in cubic feet per second.
		A = tributary drainage area in acres.
		c = coefficient of runoff of the drainage area.
		i = intensity of rainfall in inches per hours, and shall be determined by the following formula:

i = 120

t x 20

	Where:
		t = time of concentration in minutes.

(c)

In no case shall positive outfall into streams or bays be allowed without special permission of the Planning Board.

Installation.

(1)

Manholes. The maximum distance between manholes shall be 350 feet. Man holes shall be provided wherever branches are connected, pipe sizes are changed or there are changes in pipe alignment or pipe gradients.

(2)

Catch basins. No more than two basins shall be interconnected before being connected to a manhole. In residential areas no catch basin inlet will be permitted to accept more than three acres of runoff from a residential subdivision and no more than 1 1/2 acres in business and commercial areas. In no case shall water be allowed to flow in the gutter in excess of 1,000 feet.

(3)

Piping. Pipelines shall be laid on accurate grade and in a straight line between manholes. Pipelines shall be placed as nearly as possible along the edge of the pavement within the right-of-way.

(4)

Capacity of gutters. All runoff control gutters shall be designed to contain the flow within the boundaries set by the property lines and the edge of the roadway shoulder line.

(5)

Treatment of gutters and roadside ditches. All roadside drainage shall be treated as follows:

(a)

Slopes, 2% or less: seed.

(b)

Slopes, 2% to 4%: sod.

(c)

Slopes, greater than 4%: asphalt or concrete gutters.

Stormwater recharge basins.

(1)

Storage requirements. Stormwater recharge basins shall provide storage capacity for five inches of rainfall on the tributary area. Where a positive method of overflow is not provided, the recharge basin capacity shall be increased by 50%.

(2)

Maximum depth. The maximum storage depth shall be 10 feet, measured two feet below the top of the slope.

Seepage pools.

(1)

Where stormwater drainage is to be provided by using seepage (leaching) pools, the design volume shall be computed in accordance with § 220-3.03A(1). Additional storage capacity can be required if test holes indicate poor leaching material. Inlets shall be catch basins, designed in accordance with § 220-3.03B(2), and the system shall be designed to tolerate overflow in an acceptable manner.