The main purpose of a detention basin is to store runoff and reduce peak discharge by allowing flow to be discharged later at a controlled rate, and within a reasonable time. This controlled discharge rate is based on either limited downstream capacity (regional and local facilities) or on a limit on the increase in flows over predevelopment conditions (local facilities only). Detention has been shown to be very beneficial in controlling flood peaks in an urbanized area. Use of detention includes local detention such as in-channel or within a parking lot, and regional detention, such as a large recreational lake or reservoir and off-line detention facilities. Regional and local detention facilities are more fully discussed below. Policy regarding detention and extended detention (retention) is found in GJMC §
28.16.090 and
28.16.100.
It is important that any extended detention (retention) be supported
by valid water rights that allows for retention without impacting
any vested water rights on the stream system.
(Res. 40-08 (§ 1401), 3-19-08)
Stormwater storage reservoir types are numerous, but they essentially
fit into one of two categories: detention or retention. The words
“pond” and “basin” are used interchangeably
when used in connection with both detention and retention reservoirs.
A detention basin or pond detains water temporarily, releasing water
through a pipe or channel by means of a weir, orifice, or pump. Because
of its ability to release flow during inflow, the overall volume of
storage required for a given storm event is reduced. Another advantage
of the detention basin is the positive means of outflow, resulting
in fewer problems with long-term ponding. A retention basin or pond
retains water without any initial release during inflow. Once the
storm event is over, pond drainage may occur due to evaporation and
percolation into the soil. In some instances, retention basins may
also involve a gated pipe or pump which is closed or inoperative during
the storm event. However, if a gated pipe or pump is an available
or desirable option, it would normally be advantageous to release
water during stormwater inflow, which would change the basin from
a retention basin to a detention basin. The difference in detention
and retention basins is depicted in Figure 28.56.020.
The words “pond” and “basin” may be
used to refer to reservoirs that remain dry outside of storm events,
or store water for other purposes, e.g., irrigation, recreation, aesthetics,
etc., in addition to receiving stormwater during storm events. Words
“wet” and “dry” are used as prefixes to describe
the condition of ponds and basins. However, a wet pond and dry basin
each have a specific meaning.
Wet ponds may be desirable compared to dry basins in some circumstances.
It may be that ample storage volume exists to provide an aesthetic,
recreational, or irrigation pond below the required reservoir volume.
Use of irrigation storage facilities for stormwater detention purposes
must be reviewed by the appropriate jurisdiction(s) on a case-by-case
basis. It is required that the stormwater reservoir volume provided
must be in addition to the maximum expected base storage (irrigation
or wet pond) volume. This is depicted in Figure 28.56.020.
(Res. 40-08 (§ 1402), 3-19-08)
Local detention facilities are usually designed by and financed
by developers or local property owners. The facilities are intended
to allow development by protecting a site from existing flooding conditions
or to protect downstream property from increased runoff caused by
development. Two classes of local facilities are defined below.
(a) Local Minor Facilities.
Local minor detention facilities
are defined as serving a single development with a hydrologic basin
smaller than or equal to 20 acres and are designed to mitigate the
impact of increased runoff due to development. The outlet capacity
is generally based on predevelopment hydrology, and the detention
structures are generally small (0.01 to one acre-foot). Detention
storage volume may be provided as small landscaped or turfed basins,
parking lot storage, or a suitable combination of all.
(b) Local Major Facilities.
Local major detention facilities
are defined as serving more than a single development or serving hydrologic
basins greater than 20 acres in size. These facilities may serve dual
functions. They typically reduce existing flooding problems to allow
more development and/or control increased runoff caused by additional
development. These facilities may store significant flood volumes
and will generally be funded by the developer. They may handle both
off-site and on-site flows. Due to their larger size, these basins
are designed much the same as regional detention facilities.
(Res. 40-08 (§ 1403.1), 3-19-08)
Regional detention facilities are those identified in the current
Grand Valley Stormwater Master Management plan or as designated by
local jurisdiction. Generally, these facilities control flow on major
drainageways, are large in size, and are owned and maintained by public
agencies. The purpose of these facilities is to significantly reduce
downstream flows in order to maximize the capacity of existing systems
and maintain flows at or below historic rates.
(Res. 40-08 (§ 1403.2), 3-19-08)
The developer may be given the option of paying a drainage fee in lieu of providing adequate detention facilities. Such an option may be considered only if the developer completes an engineering analysis to show that all downstream facilities have adequate capacity to handle the undetained flows from the proposed development. If such option is allowed, this does not waive the requirements for drainage planning submittal requirements as outlined in Chapter
28.12 GJMC.
(Res. 40-08 (§ 1404), 3-19-08)
(a) For stormwater management purposes, charges assessed to existing
and new development to offset cost of providing drainage facilities
and services are typically categorized as an impact fee or a utility
fee.
(b) An impact fee is based on the cost of upgrading drainage services
within a watershed, shared among all new developments within the watershed.
The basis for sharing costs can be as simple as the size of the water
tap and as complex as calculating imperviousness of the property then
making adjustments for various best management practices, such as
detention. An impact fee is a one-time charge against new development.
(c) A utility fee is based on the cost of upgrading and maintaining drainage
services within a watershed to meet current standards or level-of-service.
A utility fee is often assessed to all properties, developed or not,
and often is limited to providing maintenance of the system and minor
capital improvements. Utility fees are normally charged on a monthly
basis.
(d) Since the local jurisdiction charge is a fee in lieu of detention
for new development, it is considered a limited impact fee. Reasonableness
tests applied to impact fees include:
(1) The system impact fee shall be related to the amount of drainage
improvements needed to meet local standards.
(2) The impact fee shall only cover capital and related (i.e., engineering,
construction administration, etc.) costs for improvements.
(Res. 40-08 (§ 1404.1), 3-19-08)
(a) The formula used by local jurisdictions to calculate drainage fee
is of the form:
Drainage Fee ($) = B(CD –
CH)A0.7
|
(28.56-1)
|
Where:
B
|
=
|
Fee constant (established annually by City Council of local
jurisdiction)
|
CD
|
=
|
100-year runoff coefficient (expressed as a decimal) based on
developed land use conditions
|
CH
|
=
|
100-year runoff coefficient (expressed as a decimal) based on
pre-developed land use conditions
|
A
|
=
|
Area of development (acres)
|
Assigning B = $14,000, A = 1 acre, CD = 0.60 and CH = 0.51, based on SCS Type C
soils, and an imperviousness of two percent pre-developed and 50 percent
developed, the fee in lieu of detention would be $1,260.
|
(b) The equation used in Mesa County and the other local jurisdictions
takes into account key factors for a fee in lieu of detention, such
as:
(1) The fee is based on area.
(2) The fee takes into account imperviousness by using the rational method
C values.
(3) The fee takes into account the difference between pre- and post-development
runoff rates, which is the main function of on-site detention.
(c) The constant B needs to closely reflect the actual cost of providing
detention, and must be changed with time as the need arises. Local
jurisdictions will review this figure annually and make appropriate
recommendations for its adjustment.
(Res. 40-08 (§ 1404.2), 3-19-08)
In addition to the primary purpose of detention and retention
facilities to reduce peak flows, they can be adapted to enhance stormwater
quality.
The outlet for a detention basin can be designed to allow for
a slower rate of release and thus provide time for particulate pollutants
to settle out of the stormwater. Such basins are called extended detention
basins (EDB). It is recommended that an EDB outlet be designed to
completely drain a full basin in 48 hours, allowing for the removal
of a significant portion of insoluble pollutants with proper operation
and maintenance.
A retention pond (RP) is a sedimentation facility designed to
have a permanent water pool. This pool of water is mixed with stormwater
during storm runoff events from frequently occurring storms and allows
for sedimentation and thus water quality enhancement to occur.
For frequently occurring storms, both EDBs and RPs capture the total runoff as a surcharge. However, in the case of RPs, the stormwater is allowed to mix with the permanent pool water as it rises above the permanent pool level. All surcharged water above the permanent pool level is to be released over 48 hours. Details related to the design of EDBs and RPs can be found in Chapter 28.6 GJMC, Construction Site Stormwater Runoff Control, and Chapter
28.64 GJMC, Post-Construction Stormwater Management.
For the 100-year detention, the water quality volume is considered
to be a part of the volume required for detention purposes.
(Res. 40-08 (§ 1405), 3-19-08)
Peak runoff from a site may not be increased in the 10- and 100-year storms due to development. The site runoff may be a composite of detention/retention basin release/overflow and direct runoff, both of which must be considered. If direct runoff is allowed from the site, the sum of the direct runoff plus the release from the detention basin must not exceed the historic rate. This is depicted in Figure 28.56.090 and discussed further in GJMC §
28.56.240.
(Res. 40-08 (§ 1406.1), 3-19-08)
For proper function and safety considerations, geometric requirements
shall be as shown on Figure 28.56.100.
(Res. 40-08 (§ 1406.2), 3-19-08)
Most drainage conveyance systems are designed to divert even
minor nuisance flows to stormwater storage facilities. For dry basins,
this can present an aesthetic and maintenance problem. Conveyance
facilities to a dry basin shall be capable of transporting flow to
the outlet facility rather than causing a soggy bog condition that
cannot be properly maintained. Facilities conveying trickle or nuisance
flows, such as from irrigation sprinklers, shall be adequate to convey
at least 0.5 cfs. Reference is made to Figures 28.56.110(a) and 28.56.110(b).
The outlet facility for a retention basin would be a dry well
or riprap filled dissipation pit. For a detention basin, the nuisance
flows shall be conveyed to the basin outlet.
(Res. 40-08 (§ 1406.3), 3-19-08)
All reservoirs or ponds which serve more than a single lot or site must be provided with a detention/retention tract dedicated for maintenance access. Maintenance of required volume and inflow and outflow works is necessary for the facility to function as required. Dedicated rights-of-way and/or easements must be provided as required in GJMC §
28.16.020(b).
(Res. 40-08 (§ 1406.4), 3-19-08)
Storage volume shall be calculated by the methods shown prescribed
in Figure 28.56.130.
(Res. 40-08 (§ 1406.5), 3-19-08)
After final grading, the slopes and bottom of each detention
and retention basin shall be protected from erosion by seeding and
mulching, sodding or other approved groundcover and shall be in accordance
with jurisdictional specifications.
The planting of trees and shrubs on the slopes of stormwater
basins is also encouraged. Temporary and/or permanent irrigation systems
shall be provided as required for the type of groundcover and landscape
installed and approved.
(Res. 40-08 (§ 1406.6), 3-19-08)
The minimum required freeboard for open space detention/retention
facilities is 1.0 foot above the computed 100-year water surface elevation.
(Res. 40-08 (§ 1406.7), 3-19-08)
Any dam constructed for the purpose of storing water, with a
surface area, volume, or dam height as specified in §
37-87-105, C.R.S., as amended, shall require the approval of the plans by
the State Engineer’s Office. All detention and retention storage
areas shall be designed and constructed in accordance with these criteria.
Those facilities subject to State statutes shall be designed and constructed
in accordance with the criteria of the State.
(Res. 40-08 (§ 1406.8), 3-19-08)
Whenever a detention pond uses an embankment to contain water,
the embankment shall be protected from catastrophic failure due to
overtopping. Overtopping can occur when the pond outlets become obstructed
or when a larger than 100-year storm occurs. Failure protection for
the embankment may be provided in the form of a buried heavy riprap
layer on the entire downstream face of the embankment or a separate
emergency spillway having a minimum capacity of twice the maximum
release rate for the 100-year storm. Structures shall not be permitted
in the path of the emergency spillway or overflow. The invert of the
emergency spillway shall be set equal to or above the 100-year water
surface elevation.
(Res. 40-08 (§ 1406.9), 3-19-08)
The outlet from the detention basin shall consist of a short
(maximum 25 feet) length(s) of CAP or RCP with an 18-inch minimum
diameter. Multiple pipe outlets may be required to control different
design frequencies. The invert of the lowest outlet pipe shall be
set at the lowest point in the detention pond or at the top of the
minimum pool, if present. The outlet pipe(s) shall discharge into
a standard manhole or into a drainageway with proper erosion protection
for the proposed structure. If an orifice plate is required to control
the release rates, the plate(s) shall be hinged to open into the detention
pipes to facilitate back flushing of the outlet pipe(s).
Inlet to the detention pipes can be by way of surface inlets
and/or by a local private storm sewer system.
(Res. 40-08 (§ 1406.10), 3-19-08)
It is rarely the case that a new development comprises an entire
drainage basin. Instead, developments are most often located within
a larger drainage basin. This means that unless the new development
is located at the very upstream edge of a drainage basin, it will
most likely receive sheet flow, if not concentrated flow, from off-site
areas. Developers must look not only at the new development but also
at the surrounding topography to determine just how much off-site
area drains to the new development site.
For drainage design of new developments, there are two considerations
for accommodating off-site storm runoff. These are peak runoff rates
from off-site areas and routing of off-site storm runoff relative
to the new development.
(a) Peak Storm Runoff Rates.
(1) Undeveloped Off-Site Areas.
Runoff rates from off-site
areas that are undeveloped but have a comprehensive development plan
(comp plan) in place will be calculated using imperviousness values
in Table 28.28.020 for the land use or surface characteristic specified
by the comp plan. Runoff rates from off-site areas that are undeveloped
and do not have a comp plan in place will be calculated using an imperviousness
of 45 percent as specified in Table 28.28.020 (or as specified by
the local jurisdiction).
(2) Developed Off-Site Areas.
Runoff rates from off-site
areas that are developed should be determined by calculating the actual
imperviousness of the area using approved drainage reports, aerial
photography, and field investigations. If this information is not
available, imperviousness shall be determined based on general land
use or surface characteristic as shown in Table 28.28.020.
The reduction of peak runoff rates from detention shall only
be assumed for developments that are confirmed to have detention facilities
and shall be subject to approval by the local jurisdiction. The discharge
rates from these detention facilities shall be assumed to be those
rates approved in the final drainage report for each off-site upstream
area having detention facilities.
(b) Routing of Off-Site Storm Runoff.
There are two methods
by which off-site flows can be routed relative to the new development.
These are routing around the new development and routing through the
new development.
(1) Routing Around the New Development.
Ideally the new development will be graded so off-site storm runoff is routed around the development based on peak rates determined in subsection
(a) of this section. If this method is chosen, off-site flows must be routed to their historic path immediately downstream of the new development. Concentrated flows draining to the development should be maintained as concentrated flows downstream of the development and sheet flows should be maintained as sheet flow.
(2) Routing Through the New Development.
If the site cannot
be graded to accommodate rerouting off-site flows, the development’s
detention pond must be sized to account for any off-site flow that
comes into the new development and drains to the new detention pond.
It is possible that routing off-site runoff through the new detention
pond will significantly alter the volume and outlet requirements of
the new detention pond.
The effects of routing storm runoff from off-site areas through
the new detention pond shall be determined by hydrograph routing methods.
A hydrograph of the off-site area, assuming ultimate development conditions
without detention, shall be generated to achieve a total volume of
runoff. The developed condition hydrograph shall then be truncated
at the allowable release rate and extended in time such that the total
runoff volume is the same. This modified hydrograph is a reasonable
representation of the ultimate development runoff that is routed through
a detention pond within the off-site development.
The modified off-site hydrograph is then added to the developed
conditions hydrograph for the new development (assuming no on-site
detention). This composite hydrograph shall then be routed through
the new detention pond to verify that the release rates from the new
detention pond meet allowable release rates. In this instance, the
allowable release rate is the sum of the allowable release rates from
both the off-site area and the new development. The design of the
detention pond may require iteration on the detention volume and outlet
configuration to achieve the required results.
(Res. 40-08 (§ 1406.11), 3-19-08)
Retention will only be considered on a case-by-case basis with
the following minimum requirements:
(a) Retention basins must drain within 48 hours of all storm events up to the 100-year storm event, beginning either at the start of the storm or when runoff first reaches the basin. Total runoff volume shall be determined by calculating imperviousness then converting this to a curve number (CN) following the procedures in Chapter
28.28 GJMC. Using the CN, excess precipitation for the design storm is determined using procedures described in GJMC §
28.28.070. The design storm shall be the 24-hour precipitation amount of 2.01 inches multiplied by a factor of 1.5.
(b) The design of the retention basin must be supported by vertical hydraulic
conductivity data for subsurface soil and/or rock obtained via an
appropriate engineering test such as a tri-axial cell test, a permeameter
test, or a vertical conductivity aquifer test conducted by a qualified
geotechnical engineer familiar with the Mesa County soils and geology.
Test data shall then be used in an appropriate calculation to determine
the required size of the retention basin and the time required for
complete infiltration of the 100-year storm event. One of the following
models shall be used to demonstrate the design meets all criteria:
(1) Hydrus-2D (U.S. Dept. of Agriculture, Agricultural Research Service).
(2) SEEP/W (GeoSlope International, Inc.).
(3) Equivalent model (upon approval of local jurisdiction).
(c) When using vertical hydraulic conductivity data in these calculations,
a saturated K-value shall only be used when it has been demonstrated
that the infiltration media over the entire length of the infiltration
path are saturated. If the infiltration path is characterized by unsaturated
conditions, a partially saturated K-value shall be used for infiltration
calculations.
(d) Measures that minimize sediment from entering and clogging the pond
surface shall be implemented. These include pre-sediment basins and/or
frequent sediment removal. Depth of storage shall be minimized and
surface area shall be maximized for infiltration purposes. The wetted
sides of the pond shall not be included in the area used to calculate
infiltration releases.
(e) An emergency overflow from the retention area shall be provided with
a minimum capacity of the 100-year peak inflow rate. The overflow
path shall be carefully selected and analyzed for capacity and downstream
impact. The overflow shall be located at the maximum retention volume
water surface. A minimum of 1.0 foot of freeboard shall be provided
above the maximum retention volume water surface.
(f) An operation maintenance plan shall be developed and be part of the
development agreement that includes monitoring and reporting requirements
and penalties for noncompliance. This plan may include pumping to
meet the drain time requirements and should consider that there may
be power outages during a major storm.
(Res. 40-08 (§ 1406.12), 3-19-08)
The hydrologic design of detention facilities is based on the type of facility (regional vs. local) and the method used to estimate the runoff (HEC-1 vs. the rational method). If HEC-1 is used, a full hydrograph is available for traditional storage routing. If the rational method is used, only a peak flow rate is available. GJMC § 28.52.220 through 28.56.250 discuss the procedures for these two methods. Note that the procedures used to calculate storm runoff are described in Chapter
28.28 GJMC.
(Res. 40-08 (§ 1407), 3-19-08)
The HEC-1 method may be used to develop inflow hydrographs for
hydrologic basins of any size, and may be used for both local and
regional facilities. The inflow hydrographs shall be based on ultimate
development conditions.
This program can calculate a hydrograph for any location in
the hydrologic basin. The data input file must be structured so that
the proposed detention basin site is a hydrograph-routing or hydrograph-combining
point.
(a) Detention Basin Outflow Limitations.
The controlled outlet capacity has direct influence on the required size of the basin. The outflow limitation can be based on either the existing undeveloped peak flow from the hydrologic basin or on limitations in the capacity of the downstream conveyance system (based on a hydrologic analysis of local conditions). The outflow limitation for local facilities is stated in GJMC §
28.16.090. The design maximum outlet capacity of a regional facility must be coordinated with the local jurisdiction.
(b) Hydrologic Calculation Method.
After the inflow hydrograph
has been calculated and the outflow limits have been determined, the
required storage volume can be estimated. Separate methods for calculating
required storage are used depending on the method used to estimate
the inflow hydrograph.
In order to calculate the required storage volume at a particular
detention basin site, the following information must be available
or prepared:
(2) Outlet capacity limitation;
(3) Proposed outlet discharge versus elevation data for the proposed
basin site;
(4) Proposed storage vs. elevation data for the proposed basin site;
(5) Proposed drain time for the proposed basin site.
(c) The HEC-1 computer program can be used to determine the required
storage volume and outflow limitation based on a reservoir routing
procedure. Initial estimates of outlet size are made and the program
is run. The output is reviewed and changes are made to the outlet
configuration as needed until the desired degree of flood peak attenuation
and acceptable drain time is achieved.
(Res. 40-08 (§ 1407.1), 3-19-08)
The minimum required volume shall be determined using the HEC-1
method or the following equations. These empirical equations were
developed as part of the UDFCD hydrology research program and modified
to reflect rainfall conditions in Mesa County. The equations are based
on a computer modeling study and represent average conditions.
One of the most difficult aspects of storm drainage is obtaining consistent results between various methods for estimating detention requirements. These equations provide consistent and more effective approaches to the sizing of on-site detention ponds. The equations presented in this section may be used for hydrologic basins with a total area of less than 160 acres, per the rational method restrictions set forth in GJMC §
28.28.100(c). The use of these equations in the design of regional detention facilities must be approved by Mesa County or the applicable local jurisdiction.
(a) Minimum Detention Volumes.
For the 100-year,
K100 = (1.78P - 0.002P2 - 3.56)(X100/900)
|
(28.56-4)
|
For the 10-year,
K10 = (0.95P - 1.90)(X10/1,000)
|
(28.56-6)
|
Where:
V
|
=
|
required volume for the 100- or 10-year storm (acre-feet)
|
P
|
=
|
Developed basin imperviousness (percent)
|
A
|
=
|
Tributary area (acres)
|
X
|
=
|
Mesa County and the other local jurisdictions adjustment factor
per Table 28.56.230(a)
|
Table 28.56.230(a): Detention – Volume Adjustment
Factor
|
---|
Ultimate Development Percent Imperviousness
|
X100
|
X10
|
---|
< 50%
|
0.42
|
0.26
|
≥ 50%
|
0.48
|
0.38
|
(b) Allowable Release Rates.
The maximum release rates at
the ponding depths corresponding to the 10- and 100-year volumes are
as follows:
Table 28.56.230(b): Allowable Release Rates for Detention Ponds
(cfs/acre)
|
---|
Control Frequency
|
Soil Group
|
---|
A
|
B
|
C
|
---|
10-year
|
0.05
|
0.09
|
0.12
|
100-year
|
0.25
|
0.43
|
0.5
|
The predominant soil group for the total basin area
tributary to the detention pond shall be used for determining the
allowable release rate. Information on the soils in Mesa County can
be found in published SCS soil surveys.
(Res. 40-08 (§ 1407.2), 3-19-08)
(a) If any storm runoff will be discharged from the property without
first being routed through a detention pond, on-site detention facilities
are to be designed using the compensating detention procedure. The
total of all undetained area shall not exceed five percent or 5,000
square feet, whichever is less.
(b) Compensating detention is based on the following assumptions:
(1) The 10- and 100-year peak discharge from the property from detained
and undetained area when added together will be no greater than allowable
discharge. Therefore, the more undetained release of storm runoff
from the site, the less the detention pond is permitted to release,
which requires proportionally larger detention volume.
(2) Regardless of the method used, the volume of the detention pond must
be adjusted to result in reduced discharge rates. For the HEC-1 method,
the detention volume is determined using actual runoff hydrographs
and storage routing based on the outlet configuration. For the rational
method, the increase in volume is accomplished by simply computing
the volume based on the entire property area, not just the area tributary
to the detention pond. This is a reasonable assumption given the five
percent area or 5,000 square feet size limitation on undetained area.
(c) The compensating detention procedure is given in six steps below.
The following procedure applies to both the HEC-1 and rational method
methods for determining detention volumes, except as specifically
noted otherwise.
(1) Step 1.
If the undetained area is less than five percent
of the total project area or 5,000 square feet, whichever is less,
continue. If not, then the undetained area must either be reduced
in size or the site layout revised to be in compliance with the SWMM.
(2) Step 2.
Determine the allowable release rate for the
10- and 100-year flood events based on the pre-project site conditions
using the entire site area. See Table 28.56.230(b).
(3) Step 3.
Determine the post-project runoff rates for
the 10- and 100-year floods for the undetained area only.
(4) Step 4.
Determine the adjusted allowable release rates
by subtracting the runoff rates from the post-project, undetained
area from the allowable release rates in Step 2.
(5) Step 5.
Determine minimum required 100-year and 10-year storage volumes for the area tributary to the detention pond. If using the rational method, the storage volume is determined using the equations in GJMC §
28.56.230 based on the entire project area, not just the area tributary to the detention pond.
(6) Step 6.
Determine the final outlet configuration that
results in the adjusted allowable release rates at the computed detention
volumes for the entire site.
(Res. 40-08 (§ 1407.3), 3-19-08)
Over-detention is defined as detaining developed conditions
peak flows to the point that release rates are lower than pre-developed
conditions. Over-detention is sometimes required to meet capacity
limitations of downstream drainage facilities. Specific over-detention
requirements are identified in basin-wide or watershed master plans
or by the local jurisdiction within which the detention pond is proposed.
Over-detention requirements are unique for each basin or watershed
and are often based on a detailed hydrologic investigation using hydrograph
methods to generate peak flows and route the flows through the drainage
system. Simplified detention volume and release rate methods are not
appropriate, and over-detention volume and release rate requirements
shall be determined using a hydrograph analysis in accordance with
the SWMM requirements.
(Res. 40-08 (§ 1407.4), 3-19-08)
Hydraulic design data for sizing of detention facilities outlet
works is as follows:
(a) Weir Flow.
The general form of the equation for horizontal
crested weirs is:
Where:
Q
|
=
|
discharge (cfs)
|
C
|
=
|
weir coefficient (see Table 28.56.260)
|
L
|
=
|
horizontal length (feet)
|
H
|
=
|
total energy head (feet)
|
Another common weir is the v-notch, whose equation is
as follows:
Where:
θ
|
=
|
angle of the notch at the apex (degrees)
|
When designing or evaluating weir flow, the effects
of submergence must be considered. A single check on submergence can
be made by comparing the tailwater to the headwater depth. The example
calculation for a weir design on Figure 28.56.100 illustrates the
submergence check.
(b) Orifice Flow.
The equation governing the orifice opening
and plate is the orifice flow equation:
Where:
Q
|
=
|
Flow (cfs)
|
Cd
|
=
|
Orifice coefficient
|
A
|
=
|
Area (ft.2)
|
g
|
=
|
Gravitational constant = 32.2 ft./sec2
|
h
|
=
|
Head on orifice measured from center of the opening (ft.)
|
An orifice coefficient (Cd) value
of 0.65 shall be used for sizing of squared edged orifice openings
and plates.
(Res. 40-08 (§ 1408), 3-19-08)