Southampton Township
City Zoning Code
City Zoning Code
12-5.3 Drainage
floodwater protection and stormwater management.
[Amended by Ord. No. 2003-13]
a.
Drainage.
1.
A stormwater collection system shall be provided for all land development. Inlets and pipes shall be provided where the same are necessary for proper drainage. The system shall be adequate to dispose of or store the stormwater and natural drainage water which originates within the development boundaries and that which originates beyond the development boundaries and passes through the development, calculated on the basis of maximum potential development as permitted under the zoning provisions of this chapter. In general, no stormwater runoff or natural drainage water shall be so diverted as to overload existing drainage systems or to create flooding or the need for additional drainage structures on other private properties or public lands without proper, approved provisions being made.
2.
Blocks and lots shall be graded to secure proper drainage away from all buildings, to prevent the ponding of stormwater on lots and to provide for reasonable use of the land without detrimental effect to adjoining land.
b.
Flood plain protection, delineation and encroachment.
1.
Flood plain protection, delineation and encroachment shall be as described and in conformance with all applicable rules and regulations of the New Jersey Department of Environmental Protection (NJDEP), Division of Water Resources, Bureau of Flood Plain Management, as amended and revised to date. All streams or watercourses shown as a blue line on the New Jersey State Atlas or United States Coast and Geodetic Survey maps or with an upstream drainage area of 50 acres or more, which affect a development, shall be delineated by the developer and submitted and approved by the NJDEP if previous delineation has not been made. Such delineation shall be by backwater calculations and based upon the HEC-2 analysis or the standard step method based upon the Bernoulli and Manning equations.
2.
Land subject to periodic or occasional flooding shall not be platted for residential occupancy, nor shall such lands be platted for any other purpose, which may endanger life or property or aggravate the flood hazard. Such land shall be considered for open space, yards or similar uses in accordance with flood plain regulations.
3.
Areas within the flood plains and other critical areas shall be delineated on the preliminary and final plats and shown in such a manner that their boundaries can be accurately determined.
4.
In the case of any lot containing a floodway and on which regrading and/or construction is proposed, the regrading or construction shall not be permitted unless a stream encroachment permit has been issued by the NJDEP, where required by state regulations, or any permit required by law.
c.
Stormwater management.
1.
Purpose.
(a)
To reduce artificially induced flood damage to public health, life and property.
(b)
To minimize increased stormwater runoff from any new land development.
(c)
To maintain the adequacy of existing and proposed culverts and bridges, dams and other structures.
(d)
To induce water recharge where natural storage and geologically favorable conditions exist.
(e)
To prevent, to the greatest extent feasible, an increase in nonpoint source pollution.
(f)
To maintain the integrity of stream channels for their biological functions, as well as for drainage and other purposes by minimizing disturbance of native vegetation.
(g)
To reduce the impact of development upon stream erosion.
(h)
To reduce erosion from any development or construction project by minimizing clearing and grading to the maximum extent possible.
(i)
To minimize the increase in runoff pollution due to land development, which otherwise would degrade the quality of water and may render it both unfit for human consumption and detrimental to biological life.
(j)
To preserve and protect water supply facilities and water resources by means of controlling increased flood discharges, stream erosion and runoff pollution, and minimizing impervious surfaces to the maximum extent possible.
(k)
To minimize impacts to the wetlands with State and Federally listed endangered species.
(l)
To maintain 100% of average annual ground water recharge volume.
2.
General provisions. Stormwater management and stormwater quality for all subdivision and site plans shall be in accordance with the New Jersey Stormwater Management Act, P.L. 1981, c.32, and, where required due to location, the New Jersey Pinelands Comprehensive Management Plan, both as amended and revised to date.
d.
Design criteria.
1.
Runoff computations.
(a)
Computation of the rate of flow at any given location shall be based on the rational formula, as follows:
Q = CIA |
Where: | ||||
Q | = | Volume in cubic feet per second | ||
C | = | Runoff factor | ||
I | = | Intensity of rainfall in inches per hour | ||
A | = | Watershed area in acres | ||
(b)
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 0.40.
2.
The intensity of the storm shall be based on the following:
(a)
As a minimum, a five-year storm shall be used where excess flow can continue downhill in the street without flooding adjoining properties.
(b)
As a minimum, a ten-year storm shall be used at low points with a relief swale or a twenty-five-year storm where carried in a pipe.
(c)
As a minimum, all culvert designs shall be based on a twenty-five-year storm. The Rainfall-Intensity-Duration-Frequency Curve for Philadelphia, presented in Technical Paper No. 25, prepared by United States Department of Commerce Weather Bureau, shall be used.
(d)
Standard headwalls shall be installed on all pipes, and, additionally, trash bars shall be installed on all pipes equal to or greater than 24 inches in diameter.
3.
Pipeline design. 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 factors:
n = 0.015, concrete pipe | ||
|---|---|---|
n = 0.021, corrugated metal pipe | ||
The minimum allowable pipe size is 15 inches. | ||
(a)
Pipeline submerged.
(1)
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.
(2)
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.
(3)
The elevation of the hydraulic gradient at any point in the pipe shall be no lower than the surface of the ground.
(4)
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.
(5)
At all structures, such as manholes, inlets, etc., where the pipe size does not change, the elevation of the hydraulic gradient shall be dropped 0.2 foot 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 foot + 0.8 (D2 - D1) feet | |
|---|---|---|---|
D1 | = | Diameter of inlet pipe | |
D2 | = | Diameter of outlet pipe | |
If D1 is larger than D2, head loss equals 0.2 foot. | |||
(6)
The minimum slope of any pipe shall be such that a minimum velocity of 2.5 feet per second shall be maintained when the pipe is flowing 1/4 full.
(b)
Pipeline flowing full.
(1)
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.
(2)
For this method, head losses through manholes, inlets, etc., shall be ignored.
(3)
The minimum slope of any pipe shall be such that a minimum velocity of 2.5 feet per second shall be maintained when the pipe is flowing 1/4 full.
(4)
When the pipe sizes change, the inside tops of the pipes shall be matched.
(c)
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, slope of the pipe, the design capacity and the velocity at the design capacity.
4.
Inlet design.
(a)
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:
(1)
When installed on streets where the grade is 1.00%: five cubic feet per second.
(2)
When installed on streets where the grade is 2.00%: 4.8 cubic feet per second.
(3)
When installed on streets where the grade is 3.00%: 4.6 cubic feet per second.
(4)
When installed on streets where the grade is 4.00%: 4.4 cubic feet per second.
(5)
When installed on streets where the grade is 5.00%: 4.2 cubic feet per second.
(6)
When installed on streets where the grade is 6.00%: four cubic feet per second.
(b)
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.
(c)
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 drop-off or dip at the inlet will not be created. At such locations where drainage is entirely dependent on inlets, the collecting capacities of the inlets shall be designed for 1/2 the preceding considered capacities.
(d)
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.
(e)
Access manholes shall be spaced at five hundred-foot intervals (maximum) through rights-of-way and at sewer junctions where there are no catch basins.
5.
Open channel design. Open ditches or channels will not be permitted when the design capacity requires a fifteen-inch pipe or larger unless approved by the Township Engineer. Where permitted, open-channel design should be based on the following hydraulic considerations:
(a)
Manning's equation.
(1)
Factors:
n = | 0.015, best concrete-lined ditch | |
n = | 0.025, best unlined ditch | |
n = | 0.03 to 0.15, fair-to-poor natural streams and watercourses |
(2)
Velocity.
Excavation Material | Velocity (feet per second) | |
|---|---|---|
Fine sand and firm load | 2.50 to 3.5 | |
Stiff clay and hardpan | 3.75 to 6.0 | |
Concrete-lined ditch | 15.0 |
(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.
6.
Culverts. All culverts shall be limited to a single opening. Multiple pipes will not be permitted. The design of culverts shall be such as to minimize the probability of debris accumulation.
7.
Retention basin design.
(a)
Retention ponds will be required in all major development unless deemed unnecessary by the Township Engineer.
(b)
Retention ponds shall be designed to limit the stormwater runoff after development to a controlled rate of flow equal to or less than the stormwater runoff prior to development. The required storage in the basin should be for twenty-five-year storm, with the outflow from the basin limited to a ten-year storm. Complete calculations for the basin should be supplied at the time the preliminary plan is submitted. These calculations should include runoff prior to development, runoff after development and complete calculations for the sizing of the basin, including the outfall pipe.
(c)
Detention facilities shall be designed to accommodate runoff from the development of the site for the two-, ten- and one-hundred-year storm events so that the post project construction peak runoff rates are reduced by 50%, 75% and 80% respectively.
(d)
Additionally, the following graphs should be included:
(1)
Depth in pond versus storage in pond.
(2)
Inflow to pond versus time and allowable outflow from pond versus time (on same graph).
(e)
The design calculations should be based on time intervals of five to 10 minutes and indicate inflow, average inflow by time interval, outflow, average outflow by time interval, incremental change in storage and height of water in pond.
(f)
In most instances, retention ponds will be designed to completely empty after a rainstorm occurs and will only have standing water for a short period of time during a storm. In those instances where existing or proposed permanent ponds will be used as retention ponds, they must have a minimum of four feet in depth and provide adequate freeboard to function as a normal retention pond.
(g)
If a stormwater management plan for the region or watershed exists, approved and adopted pursuant to the NJDEP Rules at N.J.A.C. 7:8, the stormwater management system shall conform to that plan.
8.
The water quality design storm shall be defined as the one year frequency S.C.S. Type III, twenty-four-hour storm or 1.25 inches of rainfall falling uniformly in two hours. Where detention basins, wet basins, ponds, constructed wetlands, infiltration structures, dry wells or other devices are used to control the quantity or rate of development runoff, they shall reduce suspended solid loads by 80% and reduce nutrient loads to the maximum extent possible.
