Aberdeen Township City Zoning Code
§ 25-5.6
Drainage.
a.
All streets shall be provided with manholes, catch basins and pipes where the same may be necessary for proper surface drainage. This requirement shall not be satisfied by the construction of dry wells.
b.
Total stormwater runoff from a site after development shall be limited to not more than the runoff from the site in its undeveloped state. Detention and retention facilities shall be utilized to limit such runoff. The Board may waive this requirement if the nature of the site, the character of adjacent previously developed areas or other factors make the utilization of runoff limiting devices unnecessary, inadvisable or impractical.
c.
The system shall be adequate to carry off or store the stormwater and natural drainage water which originates not only within the lot or tract boundaries, but also that which originates beyond the lot or tract boundaries. No stormwater run-off or natural drainage water shall be so diverted as to overload existing drainage systems or create flooding or the need for additional drainage structures on other private properties or public lands without proper and approved provisions being made for taking care of these conditions. Drains for the purpose of disposing of sump pump and/or roof leader runoff must outlet into an adequate water course or drainage system as approved by the Township Engineer.
d.
The existing system of natural drainage within each site shall be preserved to the maximum extent possible. To this end, the Board may require the preservation of natural drainage swales, recharge areas, wet weather ponds and similar features and may require suitable drainage and conservation easements. In its review of the proposed system, the Board shall consider the following criteria:
1.
The utilization of the natural drainage system to the fullest extent possible.
2.
The maintenance of the natural drainage system as much as possible in its unimproved state.
3.
When drainage channels are required, wide shallow swales with natural vegetation will be preferred to other sections.
4.
The construction of flow retarding devices, detention areas and recharge berms to minimize runoff value increases.
5.
Maintenance of the base flow in streams, reservoirs and ponds.
6.
The reinforcement, improvement and/or extension of the natural drainage system to such an extent as is necessary to eliminate flooding and excess maintenance requirements.
e.
The computation of stormwater runoff shall be made using the rational formula unless, in unusual cases, another method is approved by the Township Engineer. Runoff from upstream areas shall be calculated based on full development potential according to current zoning or current use, whichever produces the greatest runoff. Runoff coefficients used should generally fall in the following ranges:
Classification Fully Developed | Range of Coefficient |
|---|---|
Public parks, open space and land conservation | 0.15—0.30 |
Low density residential | 0.35—0.45 |
Medium density residential | 0. 40—0. 60 |
High density residential | 0.55—0.80 |
Commercial and Industrial | 0.75—0.95 |
Pavements, roadways, shoulders | 1 |
f.
Storm sewers, open channels, bridges and culverts, unless otherwise directed by the Township Engineer, shall be designed for minimum flow capacities as follows:
1.
For closed systems, ten (10) years; or if the above results in a pipe size at least equivalent to a twenty-one (21) inch reinforced concrete pipe, then twenty-five (25) years, or if the above results in a pipe size at least equivalent to a thirty (30) inch reinforced concrete pipe, then fifty (50) years; or if the above results in a pipe size at least equivalent to a fifty-four (54) inch reinforced concrete pipe, then one hundred (100) years.
2.
For open channels, ten (10) years; or if the tributary area exceeds fifty (50) acres, then twenty-five (25) years; or if the tributary area exceeds two hundred fifty (250) acres, then fifty (50) years. The flooding limits for storms with a return period of twice the design storm shall be determined for all open channels. Such limits shall be the drainage or conservation easements delineated on the plat or plan.
3.
For detention facilities, a twenty-four (24) hour flood with a return period not less than fifty (50) years or, if the tributary area exceeds fifty (50) acres, then one hundred (100) years.
g.
Velocities shall be restricted as follows:
1.
Velocities in closed systems at design flow shall be at least two and one-half (2.5) feet per second, but not more than that velocity which will cause erosion damage to the pipe.
2.
Velocities in open channels at design flow shall not be less than one-half (1/2) foot per second and not greater than that velocity which will begin to cause erosion or scouring of the channel.
3.
For unlined earth channels, the maximum velocity allowed will be two (2) feet per second.
4.
At transitions between closed systems and open channels or different types of open channels suitable provisions must be made to accommodate the velocity transitions. These provisions may include broken stone streambed protection, rip-rap, gabions, lining, aprons, chutes and checks, or others, all suitably detailed and approved as part of the plan submission.
h.
The design location and type of inlets and the size and location of manholes shall be approved by the engineer.
i.
The use of siltation and oil separation basins with controlled outflows shall be required to prevent pollution of waterways when discharge is into a standing body of water.
j.
Where, in the opinion of the Township Engineer, underdrainage is required, Type F underdrains shall be installed in accordance with the New Jersey State Highway Specifications of 1961 as amended and supplemented, and shall be of the size specified by the Township Engineer. All underdrain systems shall be provided with a clean-out.
k.
If the property on which a proposed development is to be located is proposed to be traversed by a drainage facility of any kind, including a pipe, channel, stream or swale, the Board may require that a stormwater and drainage easement or right-of-way along said facility be provided by the developer.
l.
The minimum easement for open channel sections shall be the maximum design top width of the channel section segment plus twenty-five (25) feet rounded to the next highest five (5) foot increment. The excess easement area shall be provided offset to that side of the channel most convenient for use by maintenance crews and equipment. The minimum distance between the channel top edge and any easement line shall be five (5) feet.
m.
Land subject to periodic or occasional flooding (flood plain areas) shall not be used for any purpose which may endanger life or property or aggravate the flood hazard. Site design, grading and drainage shall be so arranged to prevent damage to existing or proposed structures or adjacent properties under major storm conditions of a one hundred (100) year return frequency.
n.
Lots shall be graded to secure proper drainage away from buildings and into drainage structures, where possible. Additionally, drainage shall be provided in a manner which will prevent the collection of storm water in pools or other unauthorized concentrations of flow. To the extent possible, water shall not flow across adjacent property lines.