APPENDIX
A
DRY
DETENTION BASIN POLICY
Section I Description
A detention basin (also known as a
detention pond) is the most common method to satisfy both stormwater detention
and stormwater quality requirements. It
is applicable to small and large developments, can be easily designed and
constructed, and is long-lasting and durable while reducing peak flows (with
adequate inspection and maintenance).
This practice will also provide a significant reduction in sediment, as
well as a partial reduction in nutrients, toxic materials, heavy metals,
floatable materials, oxygen demanding substances, and oil and grease.
A dry detention basin is intended to drain dry between storm
events, but sometimes may not have a chance to drain completely between closely
occurring storm events. The detention
basin begins to fill as stormwater runoff enters the facility. The first flush volume is captured in order
to ensure water quality. One or more
outlet structures then release the stormwater runoff slowly to reduce peak
discharge rates and to provide time for sediments to settle. Litter and debris are prevented from leaving
the detention basin, and soluble pollutants are captured by a combination of vegetation
and soils.
Section II - Selection
Criteria
·
The
primary objective is to reduce the incoming peak flow discharge and slow the
stormwater runoff response from a particular property or development, thus
reducing flooding downstream.
·
The
secondary objective is to remove suspended sediments, trash and debris, oil,
grease and other pollutants to protect the water quality of
·
Dry
detention basins can also supply multiple benefits for passive recreation
during dry periods (recreational trails, ball fields, picnicking). Portions of a dry detention basin that are
not wetted frequently can be attractively landscaped or used for other
purposes.
Section III Design
and sizing considerations
·
A
permanent detention basin design must be stamped by a professional engineer
licensed in the state of
·
As
the primary objective, dry detention basins must be designed to have adequate
detention storage and outlet structures.
Multi-stage detention is required for the 2-year, 5-year and
10-year, rainfall design storm events in all watersheds. Additional stages (i.e. 25-year,
50-year and 100-year) may be required for special watersheds. The
engineering design professional shall use the methodology that is best suited
for conditions to calculate runoff.
·
As
the secondary objective, water quality is obtained through the use of the first
flush treatment volume. The first 0.5
inches of stormwater runoff, over the entire contributing drainage area of the
development, is defined as the first flush volume (with a minimum value of 4500
cubic feet). The initial wave of stormwater runoff is more likely to contain
aerially-deposited sediments, particulates from vehicles (such as incomplete
combustion, dust from brake linings, tire particles), leaves, trash, cigarette
butts, etc. The first flush volume must
be captured and then slowly released over a minimum 24-hour period (and maximum
of 72 hours). The overall goal for
stormwater treatment is based on 75% removal of total suspended sediments for
first flush volume.
·
Additional
measures may be required to improve stormwater quality, depending upon the
nature of the land use and expected pollutants.
Pretreatment of stormwater runoff with a media filtration inlet or
oil/water separator may be necessary. A
trash rack for capturing floating debris is generally considered to be standard
equipment for a stormwater treatment BMP.
·
Stormwater
runoff that falls onto pavement and rooftops should be detained and treated in
a manner that will reduce thermal impacts to streams. This may include locating a detention basin
away from sunlight by using trees or buildings as shade.
Section IV Location
and layout
Basic elements of a dry detention
basin are illustrated in Figure 1. The illustrated
design includes the use of a sediment forebay to reduce sediment loading,
particularly if the post-construction detention basin is a modification from a
temporary sediment basin during the construction phase. The use of an upper stage (for storage of
infrequent storms) is optional; there are both benefits and drawbacks. A shallow detention basin with a large
surface area will usually perform better than a deeper detention basin with the
same volume. However, shallow storage
areas increase the overall surface area needed for detention.
Design flow paths to minimize potential short-circuiting by
locating the inlets as far away from the outlet structure as possible. The length-to-width ratio of a basin should
be at least 3:1. Baffles or backslope
drains may be used to prevent short-circuiting.
If topography or aesthetics require the pond to have
an irregular shape, increase pond area and volume to compensate for dead
spaces. It is important to reduce
the velocity of incoming stormwater using riprap or other energy dissipaters.
Although dry detention basins are
generally less expensive to construct and maintain than wet detention basins,
they provide lower water quality benefits.
The primary disadvantage of a dry detention basin is the amount of
surface area required, which can be reduced somewhat by using concrete
retaining walls on one or more sides. In
general, concrete retaining walls should not face southward in order to reduce
the potential for heating on hot summer days.
Bedrock and topography must be considered during preliminary
design. Karst topography may indicate
fractured bedrock, dissolved limestone passages, or sinkholes, for which a
detention basin would be highly detrimental.
The additional water volume that is introduced to the underground
limestone passages, or even the additional weight of ponded water, could
intensify karst activity and eventually collapse the bed of the detention
pond.
Interaction with site utilities must be considered during
preliminary design. Typical utilities
include electrical, telephone, cable TV, water, sewer, natural gas, petroleum,
etc. These utilities may or may not be
in a dedicated utility easement, so it is always necessary to conduct a careful
site survey. Detention basins (including
embankments) should not be allowed over utility lines. Conversely, utility trenches should not be
constructed on existing detention basin structures.
Detention basin easements and access must be considered
during preliminary design, in order to allow for the construction easement and
maintenance. Detention basins that are
not frequently inspected and maintained often become more of a nuisance than a
beneficial part of a stormwater management program. In particular, provide access for inspection
and maintenance to the sediment forebay and to the outlet control structure. It may also be desirable to encourage or
discourage public access to the detention basin (by using site grading, signs,
fences or gates). Additional safety
elements include trash racks, grating over pipes and culverts, gentle side
slopes whenever possible, increased visibility and/or lighting in residential
areas, etc.
Small detention basins serving individual properties do not
offer as much recreational benefits as community or regional detention basins
would. Regional facilities can often be
landscaped to offer recreational and aesthetic benefits. Jogging and walking trails, picnic areas, and
ball fields are some of the typical uses.
For example, portions of the facility for flood control of major design
storms can be used for exercise areas, soccer fields, or football fields. Wildlife benefits can also be provided in the
form of islands, buffer areas, or preservation zones. It is important to maintain such areas,
however as their primary purpose is for stormwater management. Under no circumstances should debris be
allowed to accumulate near the outlet.
Section V Volume and
size
The volume of a
dry detention basin consists of two elements: the live pool (the upper portion
of the basin representing detention capability) and the first flush volume (the
lower portion of the basin representing stormwater quality treatment). Since the post-development peak runoff may
not exceed the pre-development peak flow rate, the upper sections volume
should be greater than or equal to this difference in volume.
Detention basins shall be sized to collect the 0.5 inches of
stormwater runoff from the entire contributing area, or the first 4500 cubic
feet of stormwater runoff, whichever is greater. The first flush volume must be
released at a controlled rate over a minimum 24-hour period (and a maximum
72-hours period).
As a warning to those who design detention basins, it should
be realized that future stormwater regulations are likely to be more stringent
than the current regulations. This is
mostly driven by national and state laws and regulations, which will require
municipalities and county governments to accomplish additional pollution
reduction with a proportional effort for water quality monitoring and
enforcement.
Section VI Grading
Side slopes of detention basins and embankment dams shall be
3: 1 (H: V) or flatter, except where approved by the engineering
department. This encourages a strong
growth of vegetation on the side slopes, helps to prevent soil erosion, and
allows for safer mowing. Steep slopes,
particularly on embankments or other fill soils, will contribute to soil
erosion if not properly vegetated or stabilized, and thereby reduce or negate
the effectiveness of a dry detention basin with respect to water quality. Vegetate the side
slopes and basin bottom to the maximum extent practical. If significant side erosion is expected,
consider the use of soil stabilization or armoring techniques. Detention basins should not be located
immediately above or below a steep slope or grade, because impounded water may
create slope stability problems.
Minimum width for top of embankment is 6 feet. The embankment height should allow for up to
10% settlement of embankment, unless the embankment is thoroughly compacted
with vibratory equipment or sheepsfoot rollers. The top of embankment (after expected
settlement) shall generally be at least 2 feet above the top of outlet
structure and at least 1 foot above the peak 100-year water surface elevation. Compaction in the immediate area of the
emergency spillway can be difficult, but is necessary.
The use of a backslope drain can be very beneficial in
preventing erosion at detention basins.
See Figure 5 for a typical detail.
The backslope drain is also useful for increasing lengths of flow paths
to prevent short-circuiting of the detention basin. Intercepted stormwater can be routed around
the detention basin to enter at the most hydraulically distant point from the
outlet structure.
Section VII Outlet
Structure
Detention
basin outlet structures should be constructed of durable materials, such as
concrete or masonry block. Corrugated
metal pipe (CMP) and plastic (HDPE) risers and drain pipes are popular in
engineering design, but are susceptible to crushing and flotation in detention
basins and shall not be approved.
A concrete outlet structure is generally preferable to a masonry block
structure because it is sturdier and more durable. Provisions should be made for sufficient
reinforcement and anchoring.
The
specific flow-controlling elements of an outlet structure may include one or
more of the following: a circular
orifice, a noncircular orifice, a rectangular weir, a trapezoidal weir, a
triangular weir, a V-notch weir, culvert entrance control or a riser overflow
opening.
Figures
2 and 3 illustrate possible designs for the outlet structure. These details are only two possible ways to
accomplish stormwater detention and stormwater quality control. The first flush volume is typically drained
during a minimum time of 24 hours by using an orifice with a designed
size. Maximum drain time should be less
than 72 hours to allow for sufficient volume recovery prior to the next period
of rainfall. The first flush volume can
be filtered through sand by using an underdrain system (shown in Figure 2) or
by an aboveground filter box with sand or aggregate (shown in Figure 3). Figure 4 shows an alternative outlet
structure with a water quality manhole.
Provide an emergency spillway in order to route large storms through the
facility without overtopping.
Section VIII - Emergency
Spillway
An
emergency spillway should be included in addition to the primary outlet
structure on a detention pond. The
purpose of this spillway is to pass storm events that exceed the design
capacity of the pond, in order to prevent overtopping the embankment. The emergency spillway should be located over
an undisturbed abutment area and not over the embankment fill for
stability reasons, except where approved by the engineering department. The emergency spillway capacity should be
designed to prevent overtopping the embankment structure or dam during a storm
event commensurate with the impoundment volume, dam size, and downstream flood
hazard potential in event of dam failure.
The minimum spillway capacity should be capable of handling a 100-year
storm event. If the emergency spillway
is incorporated into the outlet structure, a secondary emergency spillway must
be provided. The designer is referred to the requirements set forth in the
Tennessee Safe Dams Act and Regulations at:
www.state.tn.us/environment/permits/safedam.htm.
Section IX Other Design
Elements
·
Sediment
forebay (if used) to facilitate the cleanout
of sediment, trash, debris, leaves, etc.
The sediment forebay typically contains 5% to 10% of the total
volume. It should be located at a point
where velocities have dissipated, to allow large sediments and debris to settle
out. A forebay can be separated from the
remainder of a detention basin by several means: a lateral sill with rooted wetland
vegetation, rock-filled gabion, rock retaining wall, or rock check dam placed
laterally across the basin. The sediment
forebay should be easily accessible so that it can be inspected and maintained.
·
Public
safety should be considered, particularly in residential areas. Operating detention basins often attract
neighborhood children. Avoid steep slopes and drop-offs; consider routes for
escaping the detention basin if a person accidentally falls in. Avoid depths over 4 feet when possible;
provide fencing and signs in areas where children may potentially play, and
where steep slopes are used in the detention area.
·
A
low-flow channel (or concrete trickle ditch) can assist in completely draining
detention basins with flat slopes. It
also assists with the observation and removal of accumulated sediment. A typical design may be a triangular ditch,
maybe 4 wide and 3 deep with a slope of 0.5 to 1.0 percent.
·
Depending
on the embankment soil, height of dam, and amount of compaction for the
embankment, an anti-seep collar or a cutoff layer of compacted clay is required
around the outlet pipe to prevent internal piping and erosion. An anti-seep collar should extend at least
one pipe diameter from the culvert in all directions, with compacted clay
backfill using small mechanical tampers.
·
To
prevent the outlet riser from clogging, include trash racks or other debris
barriers with a maximum opening size of 6 inches on all outlet structures,
except for any emergency spillway structures that are designed for a 25-year
storm or greater return period. Trash
racks that are placed at an angle to the direction of flow tend to force debris
up and away from the outlet opening and are somewhat less vulnerable to clogging. These racks should be regularly cleaned and
maintained.
·
Provide
a permanent means for vehicle access to the detention basin. Detention basins must be located in a
maintenance easement so that authorities have the right to inspect the
facility. This easement should be free
of large trees and excessive vehicle grades.
·
Include
a skimmer, oil/water separator or other type of stormwater runoff pretreatment
for drainage areas with greater than 50 percent impervious surface or where
there may be a potential source of oil and grease contamination. In addition to most large parking lots, oil
and grease contamination is also likely for vehicle fueling and maintenance
facilities.
·
An
anti-vortex device for the outlet structure may be potentially needed for very
large detention basins in areas where public access is not controlled. The anti-vortex device may be a combination
of vanes above the outlet structure or guide walls around the outlet structure,
that increases the inlet flow efficiency and might lessen the chance of humans
drowning or reduce the potential for erosion and structural undercutting.
Section X
Construction /Inspection Considerations
Inadequate storage is the most frequent problem that occurs
in the design review before construction, and also for the as-built review
after construction. This can occur for
several reasons:
·
The
design engineer did not allow enough room to construct the detention basin
(most often due to insufficient design detail such as slope transitions,
setbacks, parking lot widths, inaccurate contours, utilities not shown).
·
The
engineer who performs the stormwater computations is not the same person as the
design engineer who does site layout and grading. The required detention storage volume and
outlet structure details need to be communicated clearly to the design engineer
for inclusion on the plans and for construction layout.
·
The
construction contractor does not correctly follow the design plans, and
consequently, does not excavate deep enough or build berms of sufficient height
to hold the required detention volume.
This may occur due to rock formations encountered or to groundwater. It
is important that the elevation-volume configuration shown on the plans be
preserved during construction so that the detention basin functions according
to intended design.
·
The
construction contractor changes the basin configuration during the construction
without being aware of the required volume.
Approval from the engineer was not obtained for a design change.
It
is highly recommended that the design engineer is involved in the construction
and inspection of the detention basin.
Special attention should be given to the detention basin volume,
elevations of each outlet, embankment crest and emergency spillway crest; side
slopes, size and shape of various weirs or orifices, and installation of cutoff
collars in embankments.
Proper
hydraulic design of the outlet is critical to achieving good performance for
both stormwater detention and stormwater quality of the dry detention
basin. The two most common problems for
detention basin outlets are:
·
The
discharge capacity of the outlet system is too great at the detention design
depth. This causes excessive basin
outflows and results in fast drawdown times and inadequate filling of the
detention basin volume. Both stormwater
detention and stormwater quality will suffer.
·
The
outlet structure clogs because it is not adequately protected against trash and
debris. The use of innovative trash
racks is recommended. Effective trash
racks are often created using welded rebar with 6-inch openings. Sloped trash racks are preferable to vertical
ones for forcing floating debris upward and away from the opening, rather than
being forced against the trash rack, and causing clogging. This is sufficient to stop most beverage
cans, fast food containers, tree limbs, etc.
Properly designed and installed trash racks also provide a measure of
safety to children who may otherwise be pulled toward and held against the
opening.
Section XI Maintenance[u1]
Effective and safe operation of a detention basin depends on
continuous maintenance of all system components. Detention basin easements and access must be
considered during the planning stage in order to allow for proper inspection
and maintenance.
·
Inspect
the dry detention basin every 4 months and particularly after heavy
rainfall events. Record all observations
with pictures and document any measurements taken. Perform any maintenance and repair erosion
promptly. Remove debris and trash after
storm events. Check outlet structures
regularly for clogging.
·
Remove
sediment when accumulation becomes noticeable (1 to 2 over a wide area) or if
resuspension is observed or probable.
Sediment may be permitted to accumulate if the detention basin volume
has been overdesigned with adequate controls to prevent further sediment
movement. If a sand underdrain is used,
look for reduced infiltration or ponded water; sand layer replacement may be
needed.
·
Maintain
a thick and healthy stand of vegetation (usually grass). Mow or trim at regular intervals to encourage
thick growth. Remove leaves, grass
clippings, or sticks from detention basin regularly to prevent stormwater pollution. Remove trees or nuisance vegetation as
necessary to ensure structural integrity of the basin. This is especially true in embankments. Signs should be posted at detention ponds to
discourage local homeowners from depositing yard trimmings, waste, and fill
materials inside the basin. Appropriate
signs and barriers such as fences should also be considered at detention basins
where children have easy access to the site.
Section XII Sediment Removal
A primary
function of stormwater treatment BMPs is to collect and remove sediments. The sediment accumulation rate is dependent
on a number of factors including watershed size, facility sizing, construction
upstream, and nearby industrial or commercial activities, etc. Sediments should be identified before
sediment removal and disposal is performed.
Special attention or sampling should be given to sediments accumulated
from industrial or manufacturing facilities, heavy commercial sites, fueling
centers or automotive maintenance areas, parking areas, or other areas where
pollutants are suspected. Sediment
should be treated as potentially hazardous until proven otherwise.
Some sediment may contain contaminants for which TDEC
requires special disposal procedures.
Consult TDEC Division of Water Pollution Control if there is any
uncertainty about what the sediment contains or if it is known to contain
contaminants. Clean sediment may be used
as fill material or land spreading. It
is important that this material not be placed in a way that will promote or
allow resuspension in stormwater runoff.
Some demolition or sanitary landfill operators will allow the sediment
to be disposed at their facility for use as cover. This generally requires that the sediment be
tested to ensure that it is innocuous.
Section XIII limitations
·
A
dry detention basin will require frequent inspection and maintenance. Trash, debris, leaves and other large items
should be removed from the detention basin following each rainfall event. If upstream erosion is not properly controlled,
dry detention basins can be maintenance-intensive with respect to sediment
removal, nuisance odors, insects and mosquitoes, etc.
·
A
dry detention basin may not have sufficient vegetation on the slopes and bottom
to prevent erosion. Vegetation must be
maintained and cut at adequate intervals.
Remove grass clippings from detention basin immediately after cutting,
using rakes or other hand equipment.
·
A
dry detention basin that impounds more than 30 acre-feet of volume (and minimum
6 feet high) or which is higher than 20 feet (and minimum 15 acre-feet of
volume) is subject to the Tennessee Safe Dams Act of 1973 and as amended by
law. The Safe Dams Act is administered
by the TDEC Division of Water Supply; further information on design standards,
regulations and permit applications is available at the TDEC website:
http://www.state.tn.us/environment/permits/safedam.htm
·
Dry detention basins require
a relatively large surface area (typically 1% to 3% of the contributing drainage
area) in order to provide sufficient pond volume for detention and water quality.
Dry detention basins require a differential elevation between inlets
and outlets, for which extremely flat areas may not be suitable.
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