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
Universal Citation: NJ Admin Code 5:21-7.8
Current through Register Vol. 56, No. 18, September 16, 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.
2. Detention
components: emergency spillways, as follows:
i. Vegetated emergency spillways shall have
side slopes not exceeding three horizontal to one vertical.
ii. Maximum velocities in emergency spillways
shall be checked based on the velocity of the peak flow in the spillway
resulting from routing the spillway design storm hydrograph as defined in the
NJ DEP Dam Safety Rules ( N.J.A.C. 7:20) for all detention facilities
classified as dams and the 100-year storm hydrograph for all other facilities
(the routed Emergency Spillway Hydrograph). The design of the emergency
spillway will be based on the 100-year inflow to the basin except for Class IV
dams, which shall comply with the Dam Safety Standards, N.J.A.C. 7:20. The
design of the emergency spillway assumes the principal spillway is
malfunctioning and will not allow any discharge or flow. Where maximum
velocities exceed those contained in Table 7.5 suitable lining shall be
provided.
iii. Where maximum
velocities exceed the allowable velocities for soil stability as determined in
the Standards for Soil Erosion and Sediment Control in New Jersey, at N.J.A.C.
2:90, suitable lining should be provided. Design engineers also may check
maximum velocities in emergency spillways based on the velocity of the peak
flow in the spillway resulting from routing the spillway design storm
hydrograph as defined in the NJ DEP Dam Safety Rules (N.J.A.C. 7:20) for all
detention facilities classified as dams and the 100-year storm hydrograph for
all other facilities. Where maximum velocities exceed those contained in Table
7.5 below, suitable lining shall be provided. Linings shall meet specifications
required in Hydraulic Engineering Circular No. 15-Design of Roadside Channels
with Flexible Linings, published by the U.S. Department of Transportation,
Federal Highway Administration, or in the Standards for Soil Erosion and
Sediment Control in New Jersey as cited above.
TABLE 7.5
PERMISSIBLE VELOCITIES FOR EMERGENCY
SPILLWAYS WITH UNIFORM STANDS FOR VARIOUS
WELL-MAINTAINED GRASS COVERS
| Permissible Velocities |
| On: |
| Erosion- | Easily | ||
| Slope | resistant | eroded | |
| Ground Cover | Percent | soils (fps) | soils (fps) |
| Kentucky bluegrass | 5-10 | 6 | 4 |
| Lawn grass mixture | 0-5 | 5 | 4 |
| 5-10 | 4 | 3 | |
| Weeping lovegrass | |||
| Alfalfa | 0-5 | 3.5 | 2.5 |
| Crabgrass | |||
| Note: fps = feet per second |
Designs are not limited to the ground covers shown above. Design engineers may use reinforced grass technologies and other types of ground cover in accordance with appropriate authoritative standards.
Source: Soil Conservation Service, U.S. Department of Agriculture (Washington, DC: Government Printing Office, 1959). Cited in ULI-ASCE-NAHB, Residential Storm Water Management: Objectives, Principles, and Design Considerations (Washington, DC: Government Printing Office, 1975).
3. Detention
components: dams, as follows:
i. "Dam" refers
to any artificial dike, levee, or other barrier with appurtenant works that is
constructed to impound water on a permanent or temporary basis and raises the
water level five feet or more above the usual, mean, low-water height when
measured from the downstream toe-of-dam to the emergency spillway crest, or in
the absence of an emergency spillway, the top of the dam.
ii. Design engineers shall design all dams in
accordance with the Dam Safety Standards, N.J.A.C. 7:20.
4. Detention basin berms and embankment
ponds, as follows:
i. A detention basin berm
is a water impoundment made by either constructing an embankment (a facility
referred to as an embankment pond), or excavating a pit or dugout that does not
qualify as a dam. Detention basin berms constructed by the second method are
referred to as excavated ponds.
ii.
Site conditions shall be such that runoff from the design storm can safely pass
through: a natural or constructed emergency spillway designed to accept the
entire 100-year flow; a combination of a principal spillway and the emergency
spillway designed to ensure passage of the 100-year flow when either the
principal spillway and/or the emergency spillway flows are impeded by debris;
or a principal spillway designed so as to allow it to continue to function
reliably, passing the 100-year flow, when impeded by debris.
(1) The drainage area of the pond shall be
protected against erosion so that expected sediment does not shorten the
planned effectiveness of the structure.
(2) When necessary, embankment ponds shall
have foundation cutoff walls of relatively impervious material under the berm.
The cutoff walls shall extend up to abutments as required and be deep enough to
extend into a relatively impervious layer, or provide for a stable structure
when combined with seepage control. The cutoff trench shall have a bottom width
adequate to accommodate the equipment used for excavation, backfill, and
compaction operations. Cutoff wall side slopes shall not be steeper than one
horizontal to one vertical. The cutoff walls shall extend up to the normal
water line and the minimum depth shall be at least three feet.
(3) Design engineers shall include seepage
controls if any of the following conditions exist: pervious layers are not
intercepted by the cutoff wall; seepage creates swamping downstream; such
control is needed to insure a stable embankment; or special problems may
require drainage for a stable berm. Seepage may be controlled by foundation,
abutment, or embankment drains; reservoir blanketing; or a combination of these
measures.
(4) The minimum top width
for a berm shall be six feet. The minimum top width of dams should be 10
feet.
(5) All slopes must be
designed to be stable. If needed to protect the slopes of the berm, special
measures such as rock riprap, sand gravel, fabrics, geofabrics, geomembranes,
or special vegetation shall be provided, as specified by the standards in:
Guide for Design and Layout of Vegetative Wave Protection for Earth Dam
Embankments, TR 56, and Riprap for Slope Protection Against Wave Action, TR 69.
Both reports are published by the NRCS and are incorporated herein by
reference.
(6) The minimum
elevation of the top of the settled embankment shall be one foot above the
water surface in the detention basin, with the emergency spillway flowing at
the design depth. The minimum difference in elevation between the crest of the
emergency spillway and the settled top width of the structure shall be two feet
for all berms having more than a 20-acre drainage area or more than 20 feet in
effective height. Design engineers shall increase the design height of the
structure by the amount needed to insure that, after settlement, the height of
the berms equals or exceeds the design height. This increase shall not be less
than five percent, except where detailed soil testing and laboratory analysis
show that a lesser amount is adequate.
(7) Design engineers shall place a pipe
conduit with needed appurtenances under or through the berm except where rock,
concrete, or other types of mechanical spillways are used, or where the rate
and duration of flow can be safely handled by a vegetated or earth
spillway.
iii. The
design elevation of the top of all embankments and berms shall be one foot or
greater than the maximum water surface elevation in the basin, when stormwater
from the 100-year flood passes over the emergency spillway. The design height,
defined as the vertical distance from the top to the bottom of the deepest cut,
shall be constructed to insure that the top elevation will be maintained
following all settlement.
(1) When the design
discharge of the principal spillway is considered in calculating peak outflow
through the emergency spillway, the crest elevation of the inlet shall be such
that the full flow will be generated in the conduit before there is discharge
through the emergency spillway. The inlets and outlets of the principal
spillway shall be designed to function satisfactorily for the full range of
flow and hydraulic head anticipated. The capacity of the pipe conduit shall be
adequate to discharge long-duration, continuous, or frequent flows without flow
through the emergency spillways. The pipe diameter shall be no less than six
inches. If the pipe conduit diameter is larger than 10 inches, its design
discharge may be considered when calculating the peak outflow rate through the
emergency spillway.
(2) Pipe
conduits under or through the berm shall be capable of withstanding external
loading without yielding, buckling, or cracking. Flexible pipe strength shall
not be less than that necessary to support the design load with the maximum of
five percent deflection. The inlets and outlets shall be structurally sound and
made of materials compatible with those of pipe. All pipe joints shall be made
watertight by the use of couplings, gaskets, or caulking.
iv. In earthen berms and embankment ponds,
acceptable pipe materials are corrugated polyethylene, reinforced concrete,
polyvinyl chloride, and ductile iron. When necessary for stability, concrete
and ductile pipe shall be laid in a concrete bedding. Corrugated polyethylene
pipe exposed to direct sunlight shall be made of ultraviolet-resistant
materials and protected by coating or shielding, or provisions for replacement
should be made as necessary. Connections of corrugated polyethylene pipe to
less flexible pipe or structure must be designed to avoid stress concentrations
that could rupture the plastic. Design engineers shall follow specifications in
Table 7.6 for polyvinyl chloride (PVC) pipe. Design engineers shall provide for
seepage control if the conduit is of smooth pipe larger than eight inches in
diameter.
TABLE 7.6
ACCEPTABLE PVC PIPE FOR USE
IN EARTH BERMS +
| Normal pipe | Schedule for | Maximum depth of |
| size | standard dimension | fill over pipe |
| (inches) | ratio (SDR) | (feet) |
| schedule 40 | 15 | |
| 4 or smaller | schedule 80 | 20 |
| SDR 26 | 10 | |
| schedule 40 | 10 | |
| 6, 8, 10, 12 | schedule 80 | 15 |
| SDR 26 | 10 |
+ Polyvinyl chloride pipe, PVC 1120 or PVC 1220, conforming to ASTM D1785 or ASTM D2241.
v. Seepage along pipes extending through
embankments shall be controlled by use of a filter and drainage diaphragm,
unless it is determined that anti-seep collars will adequately serve the
purpose.
(1) The drain is to consist of sand
meeting fine concrete aggregate requirements (at least 15 percent passing
through the No. 40 sieve, but no more than 10 percent passing through the No.
100 sieve). If unusual soil conditions exist, design engineers shall make a
special design analysis. The drain shall be a minimum of two feet thick, and
extend vertically upward and horizontally at least three times the pipe
diameter, and vertically downward at least 18 inches beneath the conduit
invert. The drain diaphragm shall be located approximately parallel to the
centerline of the embankment. The drain shall be outletted at the embankment
downstream toe, preferably using a drain backfill envelope continuously along
the pipe where it exits in the embankment. Protecting drain fill from the
surface erosion will be necessary.
(2) When anti-seep collars are used in lieu
of a drainage diaphragm, they shall have a watertight connection to the pipe.
Maximum spacing shall be approximately 14 times the minimum projection of the
collar measured perpendicular to the pipe. Collar material shall be compatible
with the pipe materials. The anti-seep collar(s) shall increase by 15 percent
the seepage path along the pipe. When anti-seep collars are used in lieu of a
drainage diaphragm, the design engineers shall use the following criteria to
determine the size and number of anti-seep collars.
Let V = vertical projection and minimum horizontal projection of the anti-seep collar in feet.
Let L = length in feet of the conduit within the zone of saturation, measured from the downstream side of the riser to the toe drain or point where the phreatic line intercepts the conduit, whichever is shorter.
Let n = number of anti-seep collars.
The ratio (L+2nV)/L shall be at least 1.15. Anti-seep collars should be equally spaced along part of the barrel within the saturated zone at distances of not more than 25 feet.
vi. Closed-circuit spillways designed for
pressure flow must have adequate anti-vortex devices. To prevent clogging of
the conduit, an appropriate trash guard shall be installed at the inlet or
riser.
vii. Emergency spillways
convey the design flow safely past earth embankments when the principal or
auxiliary spillway is disabled. Design engineers shall provide for an emergency
spillway for each basin.
(1) Emergency
spillways shall provide for passage of the design flow at a safe velocity to a
point downstream where the berm will not be endangered. The maximum permissible
velocity in the exit channel shall be four feet per second, where only sparse
vegetative cover can be expected; where excellent vegetative cover and a
vigorous sod can be expected and maintained, the maximum permissible velocity
is six feet per second.
(2) If
chutes or drops are used for the principal or emergency spillways, they shall
be designed according to standards in NRCS's Part 650 (Engineering Field
Handbook) and National Engineering Handbook, Part 630 (Hydrology), Section 5,
"Hydraulics"; Section 11, "Drop Spillways"; and Section 14, "Chute Spillways,"
incorporated herein by reference. The minimum capacity of a structural spillway
shall be that required to pass the peak flow expected from the design
storm.
viii. For
excavated basins, provisions shall be made where needed for a principal
spillway, emergency spillway, and embankment in accordance with the embankment
and berm criteria described in this section.
(1) Where soil conditions and safe
maintenance practices allow, side slopes of the excavated basin shall be stable
and no steeper than three horizontal to one vertical.
ix. The material placed in the fill shall be
free of detrimental amounts of sod, roots, frozen soil, stones more than six
inches in diameter (except rock fills), and other objectionable material.
(1) Drain fill shall be kept from being
contaminated by adjacent soil materials during placement by either placing it
in a cleanly excavated trench, or by keeping the drain at least one foot above
the adjacent earth fill.
(2)
Selected drain fill and backfill material shall be placed around structures,
pipe conduits, and anti-seep collars at about the same rate on all sides to
prevent damage from unequal loading. Fill material shall be placed and spread
beginning at the lowest point in the foundation, and then bringing it up in
continuous horizontal layers thick enough that the required compaction can be
obtained. The fill shall be constructed in continuous horizontal layers. If
openings or sectionalized fills are required, the slope of the bonding surfaces
between the embankment in place and the embankment to be placed shall not be
steeper than the ratio of three horizontal to one vertical. The bonding surface
shall be treated the same as that specified for the foundation to insure a good
bond with the new fill.
(3) The
distribution and gradation of materials shall be such that no lenses, pockets,
streaks, or layers of material shall differ substantially in texture or
gradation from the surrounding material. If it is necessary to use materials of
varying texture and gradation, the more impervious material shall be placed in
the center and upstream parts of the fill. If zoned fills of substantially
differing materials are specified, the zones shall be placed according to lines
and grades shown on the drawings. The complete work shall conform to the lines,
grades, and elevations shown in the drawings or as staked in the
field.
(4) The moisture content of
the fill material shall be adequate for obtaining the required compaction.
Material that is too wet shall be dried to meet this requirement, and material
that is too dry shall be wetted and mixed until the requirement is met.
Construction equipment shall be operated over each layer of fill to insure that
the required compaction is obtained. Special equipment shall be used if needed
to obtain the required compaction. If a minimum required density is specified,
each layer of fill shall be compacted as necessary to obtain that
density.
(5) Fill adjacent to
structures, pipe conduits, and drain fill or anti-seep collars shall be
compacted to a density equivalent to that of the surrounding fill by hand
tamping, or by using manually directed power tampers or plate vibrators. Fill
adjacent to concrete structures shall not be compacted until the concrete has
had time to gain enough strength to support the load.
x. All permanent and temporary stabilization
should be applied pursuant to the Standards for Soil Erosion and Sediment
Control in New Jersey, at N.J.A.C. 2:90.
xi. In a principal spillway, pipe materials
shall conform to the appropriate specifications. Anti-seep collars shall be
made of materials compatible with that of the pipe and shall be installed
according to the manufacturer's instructions. It may be firmly and uniformly
bedded throughout its length, and shall be installed to the line and grade
shown on the drawings.
xii. The
mix, design, and testing of concrete shall be consistent with the size
requirements of the job. Mix requirements or necessary strength shall be
specified. The type of cement, air entrainment, slump, aggregate, or other
properties shall be specified as necessary. All concrete is to consist of a
workable mix that can be placed and finished in an acceptable manner. Necessary
curing shall be specified. Reinforcing steel shall be placed as indicated on
the plans and shall be held securely in place during concrete placement.
Subgrades and forms shall be installed to line and grade, and the forms shall
be mortar tight and unyielding as the concrete is placed.
xiii. Foundation and embankment drains, if
required, shall be placed to the line and grade shown on the drawings. Detailed
requirements for drain material and any required pipe shall be shown in the
drawing and specifications for the job.
xiv. Concerning excavated basins, the
compacted excavation shall conform to the lines, grades, and elevations shown
on the drawings or as staked in the field.
xv. Concerning embankment and excavated
berms, construction operations shall be carried out so that erosion and air and
water pollution are minimized, and held within legal limits. All work shall be
conducted in a skillful manner. The completed job shall present a workmanlike
appearance.
(1) Measures and construction
methods that enhance fish and wildlife values shall be incorporated as needed
and practical. Ground cover to control erosion shall be established as needed
and practical. Fencing shall be provided as needed.
5. Detention facilities in flood
hazard areas, as follows:
i. Detention
development must comply with all applicable regulations under the Flood Hazard
Area Control Act,
58:16A-50
et seq.
6. The following
safety provisions shall apply to stormwater management basins and parts
thereof.
i. Trash racks shall be installed at
the intake to the outlet from the stormwater management basin to ensure proper
functioning of the basin outlets.
ii. Bar spacing for trash racks shall be in
accordance with (d)1ii above.
iii.
The average velocity of flow through a clean trash rack is not to exceed 2.5
feet per second under the full range of stage and discharge. Velocity is to be
computed on the basis of the net area of opening through the rack.
iv. Any outlet structure with an overflow
grate must have the grate secured but removable for emergencies and
maintenance. Grate spacing shall be no greater than two inches across the
smallest dimension.
v. Trash racks
and overflow grates shall be constructed and installed to be rigid, durable,
and corrosion resistant, and shall be designed to withstand a perpendicular
live loading of 300 pounds per square foot.
vi. Every outlet structure of a basin shall
have escape provisions in or on the structure. Escape provisions include the
installation of permanent ladders, steps, rungs, or other features that provide
easily accessible means of egress from the stormwater management basin.
Free-standing outlet structures may be excluded at the discretion of the
approving authority.
vii. Safety
ledges shall be constructed on the slopes of all new retention basins, with a
permanent pool of water deeper than 2 1/2 feet. Ledges shall be comprised of
two steps, each four to six feet in width, one located approximately 2 1/2 feet
below the permanent water surface, and the second located one to 1 1/2 feet
above the permanent water surface.
viii. In new stormwater management basins,
maximum interior slopes for earthen dams, embankments, or berms shall not
exceed three horizontal to one vertical.
(e) Guidelines for the following stormwater management practices are found in the Best Management Practices Manual, April 2004 edition.
1. Bioretention
systems;
2. Constructed stormwater
wetlands;
3. Dry wells;
4. Extended detention basins;
5. Infiltration structures;
6. Manufactured treatment devices;
7. Pervious pavement;
8. Sand filters;
9. Vegetative filters; and
10. Wet ponds.
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