A.

Computation of the rate of flow at any given location shall be based on the rational formula: Q = C.I.A., in which:

Q = volume in cubic feet per second

C = runoff factor

I = intensity of rainfall in inches per hour

A = watershed area in acres

In setting the value of the runoff coefficient "C", consideration will be given to the physical features of the drainage basin and the best available data on the future density of development of the drainage basin. In no case shall it be less than 0.25.

B.

The intensity of the storm shall be based on the following:

1.

As a minimum, a ten (10) year storm shall be used at low points with a relief swale, or twenty-five (25) year storm where carried in a pipe.

2.

The Rainfall Intensity Duration Frequency Curve for Philadelphia, presented in Technical Paper No. 25, prepared by U.S. Department of Commerce Weather Bureau, shall be used.

C.

Standard headwalls shall be installed on all pipes and additionally, trash bars shall be installed on all pipes equal to or greater than twenty-four (24) inches in diameter.

(Prior code § 13-27.1)

A.

Design Methods. Storm sewer pipelines shall be designed by either of the following two methods. Both shall be based on the Manning equation and shall utilize the following friction factors:

n = 0.013 Concrete Pipe

n = 0.021 Corrugated Metal Pipe

The minimum allowable pipe size is fifteen (15) inches.

1.

Submerged Pipeline.

A.

This method is based on the assumption that when the storm sewer system is under maximum load the hydraulic gradient will be at or above the crown of the pipe and that flow in the lines will be controlled by head differentials between structures or other locations where the system is open to the ground surface, such as inlets or outlets, manholes and stream inlets or outlets. The head of water above the crown of any pipe can range from zero feet to a point which will not cause surface flooding.

B.

The slope of the hydraulic gradient in any section of storm sewer between opening to the surface shall be calculated on the assumption that the pipe is flowing full at a constant velocity and at the required capacity.

C.

The elevation of the hydraulic gradient at any point in the pipe shall be no lower than the crown of the pipe and no higher than the surface of the ground.

D.

The elevation of the hydraulic gradient at any point where the system opens to the surface, such as an inlet or manhole, shall be three feet below the surface of the ground.

E.

At all structures, such as manholes, inlets, etc., where the pipe size does not change, the elevation of the hydraulic gradient shall be dropped two-tenths feet to allow for losses therein. Where the inlet and outlet pipe sizes are not the same, the elevation of the hydraulic gradient shall be dropped an amount based on the following formula:

H = 0.2 feet + 0.8 (I-O) feet

I = diameter of inlet pipe

O = diameter of outlet pipe

If O is larger than I, head loss = 0.2 feet

F.

The minimum slope of any pipe shall be such that a minimum velocity of two and one-half feet per second shall be maintained when the pipe is flowing one-quarter full.

2.

Pipeline Flowing Full.

a.

This method is based on the assumption that the hydraulic gradient will match the inside top of the pipe when the system is under maximum hydraulic load.

b.

For this method, head losses through manholes, inlets, etc., shall be ignored.

c.

The minimum slope of any pipe shall be such that a minimum velocity of two and five tenths feet per second shall be maintained when the pipe is flowing one-quarter full.

d.

When the pipe sizes change, the inside tops of the pipes shall be matched.

B.

Pipe Profiles. Continuous profiles for each reach of pipe shall be plotted, along with the location of the hydraulic gradient and the hydraulic information pertinent to each reach within the system. This information shall include the pipe size and type, the "n" factor, the slope of the hydraulic gradient, the slope of the pipe, the design capacity and the velocity at the design capacity.

C.

Inlet Design.

1.

Stormwater inlets shall be equal to New Jersey State Highway Department inlet Type "B". The maximum collecting capacities of the inlets shall be considered to be:

a.

When installed on streets where the grade is 1.00 percent—five cubic feet per second.

b.

When installed on streets where the grade is 2.00 percent—4.8 cubic feet per second.

c.

When installed on streets where the grade is 3.00 percent—4.6 cubic feet per second.

d.

When installed on streets where the grade is 4.00 percent—4.4 cubic feet per second.

e.

When installed on streets where the grade is 5.00 percent—4.2 cubic feet per second.

f.

When installed on streets where the grade is 6.00 percent—4.0 cubic feet per second.

2.

Sufficient inlets shall be located and constructed so that the length of surface runoff will not contribute a runoff to the inlet exceeding the preceding designated collecting capacities.

3.

The gutter grate of all inlets shall be set not less than two inches, nor more than four inches below the gutter grade. The surface of the paving adjacent to the inlets shall be constructed to blend into the lowered gutter grade at the inlet in such a manner that a sudden dropoff or dip at the inlet will not be created. At such locations where drainage is entirely dependent upon inlets, the collecting capacities of the inlets shall be designed for one-half the preceding considered capacities.

4.

Where surface water is collected from two directions at one street corner, inlets shall be placed at or near the tangent points of both ends of the radius. The use of one inlet in the radius shall not be allowed.

5.

Access manholes shall be spaced at five hundred (500) foot intervals, maximum, through rights-of-way and at sewer junctions where there are no catchbasins.

(Prior code § 13-27.2)

Open channel design should be based on the following hydraulic considerations:

A.

Manning's Equation:

n = .015—best concrete lined ditch

n = .025—best unlined ditch

n = .03 to .15—fair to poor natural streams and watercourses

2.

Velocity.

Excavation Material

Velocity

Fine sand and firm loam

2.50 to 3.5 fps

Stiff clay and hardpan

3.75 to 6.0 fps

Concrete lined ditch

15 fps

B.

Ample freeboard should be provided on all channels.

C.

The channel should be designed to conform, wherever possible, to the adjacent ground conditions. This means that it should not be projecting excessively above the surrounding ground or placed excessively below the surrounding ground.

D.

Continuous profiles for each reach of open channel shall be plotted, along with the adjacent average ground and the hydraulic information pertinent to each reach within the system. This information shall include the type of channel lining, the "n" factor, the width of the channel bottom, the side slopes, the water depth, the design capacity and the velocity at the design capacity.

E.

Open channels shall have a maximum side slope of three to one and shall have adequate slope protection as required by the soil erosion and sediment control ordinance.

(Prior code § 13-27.3)

All culverts shall be limited to a single opening. Multiple pipes shall not be permitted. The design of culverts shall be such as to minimize the probability of debris accumulation.

(Prior code § 13-27.4)

Detention basins shall be required on all major subdivisions, unless deemed unnecessary by the township engineer, and shall meet the following design and construction requirements:

A.

Determine project area runoff by:

1.

Using existing ground coefficient,

2.

Using ultimate developed ground coefficient;

B.

Require storage for one hundred (100) year storm event;

C.

Indicate flooded area if additional two inches of rain fall during twenty-four (24) hours following design storm;

D.

Allowance for percolation and evaporation out of basins during and after the storm will not be considered in the calculations;

E.

Basin storage capacities shall be calculated as follows: Storage volume in cubic feet = project: area in square feet × developed land surface coefficient × (total rainfall)/12;

F.

Outlet structure shall be capable of discharging water equal to or less than the contribution calculated for the drainage basin area in its natural state before development for a ten (10) year storm period;

G.

If required, profiles and sections indicating downstream capacities and proposals or easements for reconstruction or clearing of all streams shall be submitted prior to consideration of the first final section;

H.

The maximum side slope of a detention basin shall be five to one, and shall have adequate slope protection as required by Section 16.18.130. Where a basin is proposed on an individual lot in and interstate business district B-1 or B-2, or the industrial zone, I, the side slope may be reduced to three and one-half to one, provided that:

1.

The basin is owned by the owner of the lot,

2.

There is a guarantee that the basin will be maintained,

3.

The basin is integrally designed as part of the site as amenity,

4.

The planning board determines that the basin is not a detriment to the site plan or the surrounding area;

I.

The minimum depth from the bottom of any basin to the seasonal high water table, SHWT, shall not be less than two feet;

J.

The maximum overall depth of any detention basin shall not exceed six feet from the top of the berm to the bottom of the basin.

(Prior code § 13-27.5)

A.

Detention basin maintenance shall be funded by the developer for all basins proposed for township dedication. The township engineer or staff shall provide the township committee with an estimate of the maintenance costs.

B.

If a homeowners' association is formed, the developer shall fund the maintenance of the basin with a capital contribution to a trust fund, the interest of which shall be available for the maintenance of the basin. In the event the homeowners' association ceases to exist or defaults on the maintenance obligation, the trust fund shall accrue to the township, which shall use the interest of such funds to maintain the detention basin.

(Prior code § 13-27.6)