In the preparation of a surface water management plan, the following
general principles shall be adhered to:
A. The rate and velocity of runoff from the site following
completion of the planned development shall not exceed that which would prevail
under total coverage in a meadow of good hydrologic conditions (permanent
meadow), as defined by Soil Conservation Service standards, or previous cover,
whichever produces the least amount of runoff.
B. Maximum use shall be made of presently existing surface
water runoff control devices, mechanisms or areas, such as existing berms,
terraces, grass waterways, favorable hydrologic soils, swamps, swales, watercourses,
woodlands, floodplains, as well as any proposed retention structures.
C. Evaluation shall be made of the nature of the subwatershed
of which the site is a part, the receiving stream channel capacities and point
of concentration structures as shown on the base maps showing roads, streams,
culverts and bridges and described in a report entitled "Stormwater Management
Study,'' June 1, 1973.
D. Surface water runoff shall not be transferred from one
watershed to another.
E. The plan shall coordinate with the soil erosion-sediment
control plan, and the Critical Area Ordinance and Tree Protection Ordinance
provisions shall be adhered to where applicable.
F. To the greatest possible extent, the plan shall avoid
the concentration of flow and shall provide for dissipation of velocities
at all concentrated discharge points.
G. Reestablishing vegetative cover shall be in accordance
with Standards and Specifications for Soil Erosion and Sediment Control in
New Jersey, adopted by the Hunterdon County Soil Conservation District, latest
edition.
H. Timing for the plan shall establish permanent surface
water management measures prior to construction or other land disturbance,
including seeding and establishing sod or grass waterways.
Design standards for engineering review by the Township Engineer shall
be as follows:
A. For calculating runoff controls, either of the following
methods may be used in computing runoff: Soil Conservation Service Method
under the United States Department of Agriculture or the Rational Method.
|
Criteria
|
SCS Method
|
Rational Method
|
---|
|
Surface conditions
|
Meadow
|
Average cultivation or light growth, r = 0.20 to 0.40
|
|
Collection system
|
15-year storm
|
15-year storm
|
|
Storage
|
100-year storm
|
100-year storm
|
|
Outlet discharge
|
10-year storm
|
10-year storm
|
|
Emergency spillway
|
100-year storm
|
100-year storm
|
|
Soil type
|
A, B, C, D as determined by map (Hydrologic Soils)
|
Loam
|
|
Maximum velocity at pipe outlets
|
4 feet per second
|
4 feet per second
|
|
Intensity
|
SCS Method
|
Water policy rainfall curves
|
B. All outfalls are to be designed in a manner to retard
velocities at the outfall and provide stream channel protection.
C. When a natural drainage pattern is necessarily intercepted,
as by a street, this shall be considered.
D. All structures and land treatment practices shall conform
to Standards and Specifications for Soil Erosion and Sediment Control in New
Jersey, latest edition, adopted by the Hunterdon County Soil Conservation
District.
E. All water-carrying structures and/or retention areas
shall be completed and stabilized prior to diversion of water to them.
F. As preliminary to developing the surface water management plan, there shall be an inventory of the site showing all existing natural and man-made drainage-related features as listed in §
316-12B (berms, terraces, grass waterways, favorable hydrologic soils, poorly drained soils, swamps, swales, watercourses, woodlands, floodplains). These shall be incorporated into the plan to the greatest possible extent in accordance with their functional capability.
G. Drainageways and watercourses which normally carry or
receive surface water runoff shall not be overloaded with increased runoff,
sediment or other pollution resulting from disturbance of soil and vegetation
or incident to development, construction or other activity.
H. Due consideration shall be given to the relationship
of the subject property to the natural or established drainage pattern of
the subwatersheds of which it is a part as shown on any applicable map of
the Watersheds Overlay, Franklin Township, Stormwater Management Study, June
1, 1973.
I. Surface water runoff controls shall be designed to assure
that the land in question uses no more than its proportionate watershed share
of the natural stream and culvert capacity as set forth in the tables, Appendix
C, Stormwater Management Study, June 1, 1973, or as ascertained by field measurements.
J. The curbing requirement may be relaxed by the Township
Engineer in the interest of acceptable drainage accommodation that will relieve
concentration of flow or discharge to a stormwater sewer system.
K. Innovative surface water runoff control and recharge
devices may be proposed, such as rooftop storage, dry wells, cisterns, roof
drain infiltration trenches, provided that they are accompanied by detailed
engineering plans and performance capabilities.