New Jersey Administrative Code › Title 5 - COMMUNITY AFFAIRS › Chapter 21 - RESIDENTIAL SITE IMPROVEMENT STANDARDS › Subchapter 7 - STORMWATER MANAGEMENT › Section 5:21-7.8 - Detention basins and other stormwater management facilities

Current through Register Vol. 56, No. 14, July 15, 2024
(a) When structural
				measures are used, they shall comply with the requirements of these rules and the
				Best Management Practices Manual, April 2004 edition.
(b) Design engineers shall locate detention
				facilities (either "wet" or "dry") so as to not interfere with or adversely affect
				existing surface waters on the site or adjacent to the site. Excavation for
				detention facilities shall be designed to be the maximum practical distance above
				seasonal high ground water elevation. In the case of "wet" detention facilities,
				storage may only be presumed to be available above the elevation of the seasonal
				high ground water. If the facility is designed as an infiltration basin, the bottom
				of the basin shall be a minimum of two feet above the elevation of the seasonal high
				water table. The determination of the seasonal high water table shall be made by the
				applicant's engineer.
(c) Design of
				outlets from detention basins and other stormwater management facilities shall
				account for tailwater effects up to the flood hazard design flood
				elevation.
(d) The following list of
				general structural criteria shall be used to design stormwater detention basins.
1. Detention components: principal basin control
				structure (quantity control), as follows:
i.
				Principal basin control structures will consist of orifice and/or weir control
				devices. Design engineers shall design orifices based upon the following equation:
Q = C A (2gH)< 0.5 >
where
Q = the flow rate in cubic feet per second
C = 0.6 (The orifice flow coefficient "C" may vary, depending on
				  entrance conditions. Design engineers may use other coefficients with appropriate
				  references.)
A = cross sectional area of flow in square feet
H = the vertical distance in feet between the center of the
				  orifice and the water surface
2g = 64.4 feet per second< 2 >.
To minimize the chance of clogging, orifices intended solely for
				  runoff quantity control will be at least two and one half inches in diameter (or its
				  equivalent). All joints are to be watertight. In addition, trash racks and/or
				  anti-vortex devices shall be required. When weirs are used alone or in conjunction
				  with orifices, design engineers shall use the following equation:
Q = C[w]L(h)< 3/2 >
where
Q = the flow rate in cubic feet per second
C[w] = 3.2 (design engineers may use other coefficients with
				  appropriate references)
L = length of the weir in feet
h = the vertical distance in feet between water surface elevation
				  and the crest of the weir.
All weirs shall be constructed as part of a reinforced concrete
				  structure with appropriate grates.
ii. Trash racks, and/or anti-vortex devices shall
				be installed at the intake to the outlet structure as appropriate, and shall have
				parallel bars with one-inch spacing between bars to the elevation of the water
				quality design storm. For elevations higher than the water quality design storm, the
				parallel bars at the outlet structure shall be spaced no greater than one-third the
				width of the diameter of the office or one-third the width of the weir, with a
				minimum spacing between bars of one inch and a maximum spacing between bars of six
				inches. The spacing shall be designed so as not to adversely affect the hydraulic
				performance of the outlet pipe or structure. In addition, the design of trash racks
				shall comply with the requirements of (d)6 below.
iii. Eight-inch thick, anti-seep collars are to be
				installed along outlet pipes when required by the municipal engineer. Reinforcement
				steel shall be No. 5 bars at 12 inches both ways, with two inches of cover on both
				faces (minimum).
iv. Where necessary for
				stability of the outlet pipe, a concrete cradle shall be provided.
v. All principal basin control structures shall be
				precast or reinforced concrete. All joints are to be watertight.
vi. Suitable lining shall be placed upstream and
				downstream of principal basin control structures, as necessary, to prevent scour and
				erosion. Such lining shall conform to Standards for Soil Erosion and Sediment
				Control in New Jersey, N.J.A.C. 2:90.