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Detention Pond Design Considering Varying Design

StStorms

Robert PittDepartment of Civil, Construction and

Environmental EngineeringUniversity of Alabama

Tuscaloosa, AL

Land Development Results in Increased Peak Flow Rates and Runoff Volumes

Developed area

Similar undeveloped area

Large Rain Small Rain

Receiving Water Effects of Water Pollutant Discharges

• Sediment (amount and quality)• Habitat destruction (mostly through high flows and• Habitat destruction (mostly through high flows and

sedimentation)• Eutrophication (nutrient enrichment)• Low dissolved oxygen (from organic materials)• Pathogens (mostly from municipal wastewater and

agricultural runoff)agricultural runoff)• Toxicants (heavy metals and organic toxicants)• Temperature• Debris and unsafe conditions• etc.

Wet Detention Ponds

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Extended Detention Ponds

Caltrans, San Diego and Los Angeles, California

Dry Ponds with Pilot Channels

Unusual Dry Detention Pond Located on Hillside to Meet “100 year” Peak Flow Rate Criterion

Large Corrugated Pipes used for Underground Detention Below Parking Area

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Examples of Proprietary Underground Detention Systems

Contech Construction Products, Inc. Invisible Structures, Inc.

StormTech StormTrap

Basinwide Hydraulic Analyses• Basinwide analyses are needed to identify the most Basinwide analyses are needed to identify the most

suitable locations and sizes for flood control detention ponds

• If just follow “pre” and “post” development peak flow rate criterion (the peak flow rate after development must be no larger than the peak flow rate before development for a specific design storm), worse conditions are likely to occur at downstream areasat downstream areas

• WinTR-55 is the easiest and cheapest tool available to perform a basinwide hydraulic analysis to ensure that hydrographic interferences will not occur.

Developing subwatershed requiring detention pond

Predevelopment hydrographs from upstream area and from developing subarea

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Final hydrographs from subareas and total area with detention pond to meet predevelopment peak flow criterion

Probability distribution of typical Alabama rains (by count) and runoff (by depth).

<0.5”: 65% of rains(10% of runoff)

0.5 to 3”: 30% of rains(75% of runoff)

3 to 8”: 4% of rains(13% of runoff)

>8”: <0.1% or rains(2% or runoff) EPA report on wet weather flows, Pitt, et al. 1999

Same general pattern in other parts of the country, just shifted.

Pitt, et al. (1999)

Design Issues (<0.5 inches)• Most of the events (numbers of rain storms)

Little of ann al r noff ol me• Little of annual runoff volume• Little of annual pollutant mass discharges• Probably few receiving water effects• Problem:

ll i lik l d– pollutant concentrations likely exceed regulatory limits (especially for bacteria and total recoverable heavy metals) for each event

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Design Issues (0.5 to 3 inches)

• Majority of annual runoff volume and• Majority of annual runoff volume and pollutant discharges

• Occur approximately once a week• Problems:

• Produce moderate to high flows• Produce moderate to high flows• Produce frequent high pollutant loadings

Design Issues (3 to 8 inches)• This range of rains can include drainage-design storms

(depending on rain intensity and site time of concentration). Most of these storms last for one to two days. Drainage design storms of these depths would last only for a few hoursonly for a few hours.

• Establishes energy gradient of streams• Occur approximately every few months (two to five

times a year). Drainage design storms having high peak intensities occur every several years to several decades)

• Problems:– Unstable streambanks– Habitat destruction from damaging flows– Sanitary sewer overflows– Nuisance flooding and drainage problems/traffic

hazards

Design Issues (> 8 inches)• Occur rarely (once every several years to once

every several decades, or less frequently). Three rains were recorded that were >8 inches in the 37 years between 1952 and 1989 in Huntsville, AL.

• Produce relatively small fraction of the annual pollutant mass dischargesP d t l l fl d th l t• Produce extremely large flows and the largest events exceed drainage system capacity (depending on rain intensity and time of concentration of drainage area)

• Smallest storms should be captured on-site for reuse or infiltrated

Combinations of Controls Needed to Meet Many Stormwater Management Objectives

reuse, or infiltrated • Design controls to treat

runoff that cannot be infiltrated on site

• Provide controls to reduce energy of large

h ldevents that would otherwise affect habitat

• Provide conventional flooding and drainage controls

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Figure and Table from Center for Watershed Protection

Impervious Cover Model

Urban Steam Classification

Sensitive0 – 10%

Imperviousness

Impacted11– 25%

Imperviousness

Damaged26–100%

ImperviousnessImperviousness Imperviousness ImperviousnessChannel Stability Stable Unstable Highly Unstable

Aquatic Life Biodiversity Good/Excellent Fair/Good Poor

Recurrence Interval (yrs)

Existing Flowrate

Exceedence for Predevelopment

Exceedence for Existing

Exceedence for Ultimate

Hours of Exceedence of Developed Conditions with Zero Runoff Increase Controls Compared to Predevelopment Conditions (MacRae (1997)

Interval (yrs) Flowrate(m3/s)

Predevelopment Conditions (hrs per 5 yrs)

Existing Development Conditions, with ZRI Controls (hrs per 5 yrs)

Ultimate Development Conditions, with ZRI Controls (hrs per 5 yrs)

1.01 (critical mid-bankfull

1.24 90 380 900

conditions)

1.5 (bankfull conditions)

2.1 30 34 120

Can calculate the hours of exceedence of various flow targets for different development scenarios

Rainfall Frequency• Rainfall frequency is commonly expressed as the

average return period of the event.g p• The value should be expressed as the probability of

that event occurring in any one year.• As an example, a 100-yr storm, has a 1% chance of

occurring in any one year, while a 5-yr storm has a 20% chance of occurring in any one year. 20% chance of occurring in any one year.

• Multiple rare events may occur in any one year, but that is not very likely.

Example Intensity - Duration - Frequency (IDF) Curve

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Developed by S. Rocky Durrans

SCS (NRCS) Rainfall Distributions Zones of Different Rainfall Distributions

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Rainfall Distributions in the Southeastern U.S. Probability of design storm (design return period) not being exceeded during the project life (design p j ( gperiod).

As an example, if a project life was 5 years, and a storm

t t bwas not to be exceeded with a 90% probability, a 50 year design return period storm must be used.

Estimating Storage Requirements of the Detention Pond

• The detention basin is the most widely used measureThe detention basin is the most widely used measure for controlling peak discharge.

• It is generally the least expensive and most reliable of the measures that have been considered.

• It can be designed to fit a wide variety of sites and can accommodate multiple-outlet spillways to meetcan accommodate multiple outlet spillways to meet requirements for multi-frequency control of outflow.

Estimating the Effects of Storage (Based on Chapter 6 of TR-55)

• Hydrologic routing procedures can be used to y g g pestimate the effect on hydrographs. – Both the TR-20 (SCS 1983) and DAMS2 (SCS 1982)

computer programs provide accurate analysis methods. WinTR55 also has improved routing.

• This chapter in TR-55 contains a manual method for quick estimates if the effects of temporary detention q p yon peak discharges. – The method is based on average storage and routing effects

for many structures.

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Estimating the Effects of Storage (Based on Chapter 6 of TR-55) Estimating the Effects of Storage: Input

Requirements• The figure is used to estimate storage volume

(Vs) required or peak outflow discharge (qo).• The most frequent application is to estimate

Vs, for which the required inputs are runoff volume (Vr), qo, and peak inflow discharge (qi).

• To estimate qo, the required inputs are Vr, Vs, and qi.

Estimating the Effects of Storage: Estimating Vs

• Use worksheet 6a to estimate Vs, storage volume required, by the following procedurethe following procedure.

1. Determine qo. Many factors may dictate the selection of peak outflow discharge. The most common is to limit downstream discharges to a desired level, such as predevelopment discharge. Another factor may be that the outflow device has already been selected.y

2. Estimate qi by either the graphical peak discharge or tabular hydrograph methods. Do not use peak discharges developed by another procedure.

Estimating the Effects of Storage: Estimating Vs

3. Compute qo/qi and determine Vs/Vr from figure 6-1.

4. Q (watershed runoff in inches) is determined from the CN plot using the 24-hr rain depth associated with the design storm. It must be converted to the units in which Vs is to be expressed—most likely, acre-feet or cubic feet. The most common conversion of Q to Vr is expressed in acre-feet:

Vr = 53.33Q(Am)Vr 53.33Q(Am)

Where Vr = runoff volume (acre-ft)Q = runoff (in)Am = drainage area (mi2), and53.33 = conversion factor from in-mi2 to acre-ft.

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Solution of the SCS Runoff Equation(from TR-55, Urban Hydrology for Small Watersheds, Soil Conservation Service,

U.S. Department of Agriculture): Estimating the Effects of Storage: Estimating Vs5. Use the results of steps 3 to 4 to compute Vs:

Where Vs = storage volume required (acre-ft).

6 Th t i th d t ti b i di t V t b

r

srs V

VVV

6. The stage in the detention basin corresponding to Vs must be equal to the stage used to generate qo. 1. In most situations a minor modification of the outflow device can be

made. If the device has been preselected, repeat the calculations with a modified qo value.

Detention Pond Size Example for Single “Design Storm• A development is being planned in a 75-acre (0.117 mi2)

watershed that outlets into an existing concrete-lined channel designed for present conditions. If the channel capacity is exceeded, damages will be substantial. The watershed is in the type II storm distribution region.

• The present channel capacity, 180 cfs, was established by computing discharge for the 25-year frequency storm by the Graphical Peak Discharge method.

• The developed-condition peak discharge (qi) is 360 cfs, and runoff (Q) is 3.4 inches. Since outflow must be held to 180 cfs, a d i b i h i h i fl di h ( ) illdetention basin having that maximum outflow discharge (qo) will be built at the watershed outlet.

• How much storage (Vs) will be required to meet the maximum outflow discharge (qo) of 180 cfs, and what will be the approximate dimensions of a rectangular weir outflow structure?

Detention Pond Example Sizing, Single Stage (single “design storm” objective) Example 6-2, TR-55

• How much storage (Vs) will be required to meet the maximum outflow discharge (qo) of 180 cfs (calculated to be associated with the “25-yr design storm,” and what will be the approximate dimensions of a rectangular weir outflow structure? The peak inflowdimensions of a rectangular weir outflow structure? The peak inflow had been determined to be 360 cfs for this event.

• Figure 6-2 shows how worksheet 6a is used to estimate required storage (Vs = 5.9 acre-ft) and maximum stage (Emax = 105.7 ft). The rectangular weir was chosen for its simplicity; however, several types of outlets can meet the outflow device proportion requirement.

• An outlet device should be proportioned to meet specific objectives. A single-stage device was specified in this example because only one storm was considered. A weir is suitable here because of the low head. The weir crest elevation is 100.00 ft. Using Vs = 5.9 acre-ft (figure 6-2, step 9) and the elevation-storage curve, the maximum stage (Emax) is 105.7 ft.

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Detention Pond Size

Estimation: Example forExample for

single design storm

General Weir Equation2/3CLHQ

Where C = weir coefficient (see table)L = weir length

Detention Pond Size Estimation: Example for single design storm

• The rectangular weir equation is:

• Qo=3.2LwHw1.5

Where qo = peak outflow discharge (cfs)Lw = weir crest length (ft)Hw = head over weir crest (ft)

• Hw and qo are computed as follows:w qo pHw = Emax – weir crest elevation = 105.7 - 100.0 = 5.7 ft.

Detention Pond Size Estimation: Example

• Since qo is known to be 180 cfs, solving for Lw yields

ftft

ftL

HqL

w

ww

1.4)7.5(2.3

sec/180

2.3

5.1

3

5.10

• In summary, the outlet structure is a rectangular weir with crest length of 4.1 ft, Hw = 5.7 ft, and qo = 180 cfs corresponding to a Vs = 5.9 acre-ft.

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Example for Multi-Stage Structure (multi-year discharge objectives) (example 6-2, ch 6, TR-55)

• In addition to the 25-yr design storm objective shown in the above example, an additional limit of 50 cfs (associated with the “2 d i t ”) t l b t Th k i fl f thi“2-yr design storm”) must also be met. The peak inflow for this event had been determined to be 91 cfs.

• The smallest event is used to design the lower portions of the pond and is calculated first, the larger storm pond requirements are then added to this, but the volume and outfall discharge for the smaller event are “subtracted” from the larger event

i trequirements. • The qo/qi ratio for the small event is therefore 0.55 (50/91).

This flow ratio corresponds to a Vs/Vr ratio of 0.26 TR-66 Fig 6-1 chart for the type II event.

The prior runoff volume was determined to be 1.5 watershed-inches, or 9.4 acre-ft, and the t l (V ) i th fstorage volume (Vs) is therefore

2.4 acre-ft.

The storage-stage diagram on the left shows that this storage volume is met at an elevation of 103.6 ft. With an datum elevation of 100 ft (the bottom of the pond and the ( pbottom of the lowest weir), the maximum height for the weir crest elevation is therefore 3.6 ft.

Using the rectangular weir equation, this corresponds to a weir length of 2.3 ft.

• The top elevation of the weir (for the larger event) was previously determined to be at 5.7 ft. in order to provide the needed Vs. The first stage weir that is 2.3 ft long with a maximum elevation of 5.7 ft will discharge 100 CFS. Therefore, another 80 CFS must be discharged through a second stage (or another discharge) between the 3 6 and 5 7 ft (a total depth ofanother discharge) between the 3.6 and 5.7 ft (a total depth of 2.1 ft).

• Assuming the second discharge structure is completely separate from the lower one, the weir length can then be calculated with a depth of 2.1 ft and an 80 CFS discharge rate. For a rectangular weir, this corresponds to a weir length of 8.2 ft.I t it i d th t th i “ t k d ” b t• In most cases, it is assumed that these weirs are “stacked,” but that likely introduces errors (if stacked, the actual discharge will be less than assumed).

• If the lower discharge is an orifice (or culvert) and the upper discharge is rectangular, then they will be separate and this method can also be used.

• In all cases, it is a good idea to confirm these initial sizing and discharge device calculations using a routing model.

• The following example shows how WinTR-55 can be used to evaluate pre- and post-development flows and to evaluate a pond. Unfortunately, WinTR-55 has some limitations by only allowing

i l l d i d l h f l d ia single stage outlet device and only has a few outlet devices available. The inflow calculations and associated flow routing also assume open channel flow and not pipe flow, but that is probably OK for a small site having minimal piping.

• Other models can be used. One inexpensive commercial model that is very flexible is HydroCad, which is based on TR-20

ti t h l d h dl i t t d drouting technology and can handle many interconnected ponds, different outlets, and pipe and open channel routing. TR-20 can also be used (USDA source) and is free, but is a DOS program still (WinTR-55 incorporates the TR-20 more accurate routing, but has the limitations mentioned above. SWMM5 (EPA) is free and very flexible and can also be used, with few limitations.

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Example of Sizing of Underground Detention Facility Considering Several Design Storms

• Design an underground detention facility considering the following site conditions andconsidering the following site conditions and objectives:– Area of 5 acres in Tuscaloosa, AL– Assume pre-development CN of 58 (good woods, B soil)

and Tc of 45 minutes– Assume post-development CN of 98 (pavement and roofs)

and Tc of 6 minutes– Post-development peak flows must be equal to, or less than,

pre-development peak flows for the following series of design storms: 1, 2, 5, 10, 25, 50, and 100 year events.

Pre-development peak flows for all 1 to 100 year storm:

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Post-development site runoff volumes (Vr), in watershed-inches (select “WinTR-20 Reports/printed page

1 yr: 3.361 in2-yr: 3.961 in5 5 160 i

file” and scroll down to the various rains), and calculated ft3, using the 5 acre area:

5-yr: 5.160 in10-yr: 6.060 in25-yr: 6.859 in50-yr: 7.559 in100-yr: 8.359 in

Storm event

Predev. peak discharge (qo), CFS

Postdev. Peak discharge (qi), CFS

Postdev. Runoff (Vr), inches

Postdev.Runoff (Vr), ft3

qo/qi Needed Vs/Vr (Type III rain distribution)

Needed Vs,ft3

1-yr 0.94 14.97 3.361 61,000 0.063 0.6* 36,600

Data Summary Table and Sizing Calculations

2-yr 1.65 17.50 3.961 71,890 0.094 0.56* 40,260

5-yr 3.48 22.57 5.160 93,650 0.15 0.50 46,830

10-yr 5.07 26.36 6.060 110,000 0.19 0.46 50,590

25-yr 6.61 29.73 6.859 124,500 0.22 0.44 54,780

50-yr 8.02 32.68 7.559 137,200 0.25 0.41 56,250

100-yr 9.71 36.04 8.359 151,700 0.27 0.40 60,690

• The smallest qo/qi on the NRCS graph is 0.1, which corresponds to a Vs/Vr ratio of 0.55 for a type III rain distribution. These values are therefore slightly extrapolated.

• If using 5 ft tall pre-fabricated StormTrap rectangular modular units, 12,140 ft2(0.29 acres) of area will be needed for the largest storage volume requirement, corresponding to about 5.7% of the 5 acre site area.

With a 5 ft elevation for the maximum stage for the 100 yr event, the maximum stages providing the necessary storage for the other events can be calculated. In this simple case with vertical walls (a rectangular box), the elevation for each event is easily calculated. With circular pipes (or other irregular shape), a storage – stage plot will need to be prepared, like in the TR-55 examples shown previously.

Design Stage Elevation Storage Max. allowable Difference betweenstorm (ft) of orifice

(ft)volume (ft3)

discharge (CFS)

each stage (ft)

100 – yr 5.00 ft 4.63 60,690 9.71 0.3750 – yr 4.63 4.51 56,250 8.02 0.1225 – yr 4.51 4.17 54,780 6.61 0.3410 – yr 4.17 3.86 50,590 5.07 0.315 – yr 3.86 3.31 46,830 3.48 0.555 yr 3.86 3.31 46,830 3.48 0.552 – yr 3.31 3.02 40,260 1.65 0.291 - yr 3.02 0 36,600 0.94 3.02 ft from bottom

Next is the selection and sizing of the discharge devices to provide these maximum allowable discharges at the stages in the storage unit. In this example, I used orifices and located them at the bottom of the storage increment associated with each event, as shown in the above table.

The accumulative discharges of the lower orifices at the higher stages need to be subtracted from the maximum allowable discharge for each stage. As the water depth increases for the larger events, the stage increase causes an increased discharge from the orifices Therefore the sizing of the orifices starts at the bottom andorifices. Therefore, the sizing of the orifices starts at the bottom and works up, with there corrections.

As an example, the 10-yr event control is located at 4.17 ft from the bottom of the tank in order provide the necessary 50,590 ft3 of storage. At this elevation, the maximum discharge is 5.07 CFS. However, the 3 lower orifices also contribute flows, as shown on , ,the following table. The actual increased flow that needs to be accommodated at this elevation is therefore only 1.03 CFS, requiring a much smaller orifice (only about 3 inches in diameter) than if no discharges were located below this stage.

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Peak flow at stage allowed

2-yr (3.31 ft)

5-yr (3.86 ft)

10-yr (4.17 ft)

25-yr (4.51 ft)

50-yr (4.63 ft)

100-yr (5.0 ft)

Corrected flow after subtraction

Orifice size (inches)

1-yr 0.94 CFS at 3.02 ft

0.98 1.06 1.10 1.15 1.16 1.21 0.94 1.6

2-yr 1 65 CFS n/a 1 13 1 33 1 50 1 57 1 74 1 65 – 0 98 = 0 67 2 4

Flow contributions at higher stages for larger events

2-yr 1.65 CFS at 3.31 ft

n/a 1.13 1.33 1.50 1.57 1.74 1.65 – 0.98 = 0.67 2.4

5-yr 3.48 CFS at 3.86 ft

n/a n/a 1.61 1.90 1.99 2.25 3.48 – 1.13 – 1.06 = 1.29

2.8

10-yr 5.07 CFS at 4.17 ft

n/a n/a n/a 1.50 1.63 1.98 5.07 – 1.61 – 1.33 –1.10 = 1.03

2.9

25-yr 6.61 CFS at 4.63 ft

n/a n/a n/a n/a 0.65 0.87 6.61 – 1.50 – 1.90 –1.50 – 1.15 = 0.56

2.1

50 yr 8 02 CFS n/a n/a n/a n/a n/a 2 06 8 02 0 65 1 63 3 750-yr 8.02 CFS at 4.63 ft

n/a n/a n/a n/a n/a 2.06 8.02 – 0.65 – 1.63 –1.99 – 1.57 – 1.16 = 1.02

3.7

100-yr 9.71 CFS at 5.0 ft

n/a n/a n/a n/a n/a n/a 9.71 – 2.06 – 0.87 –1.98 – 2.25 – 1.74 –1.21 = -0.4

Slight excess flow for 100 yr event

This was only an example calculation and is not intended to show the “best” design. The flow rate reductions for this site are extreme (from good woods to complete pavement) and cover a wide range of design storms. The resulting necessary storage

l f ll f h h f i l d hvolumes for all of the events are therefore quite large, and the maximum discharge rates are quite low. The resulting rather small orifices calculated in this example could be expected to cause operational problems. It may be better to use a shallower unit, but that would result in a larger area for the detention facility, likely significantly increasing the cost. However, the decreased heads on the orifices would result in somewhat largerdecreased heads on the orifices would result in somewhat larger diameters, and fewer expected problems.

WinTR-55 Schematic Example

Sub-area 2(Reach Routing)

Sub-area 3

Legend

Outlet

Storage AreaSub-Area Inflow Points

Legend

WinTR-55 Structure Data Window

• Entry, editing and/or viewing of a pond’s surface area, structure outlet type and dimensions, and rating can be found on this window.

• Up to three outlet sizes (trials) may be defined for each structure.

• If the temporary structure storage or hydraulics require a complex rating, use another method like TR-20 or Sites.

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Pre-development flow calculations

Predevelopment hydrographs

Post development flow calculations

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Post development hydrographs with no pond

Pond sizing calculations

The area is 2 acres at the depth where the discharge begins, and is 3.5 acres in area 6.5 feet above this spillway elevation. If the upper area was not entered (it is an optional value), the pond is assumed to then have vertical side slopes (not a good idea). The “Discharge Description” is based on the spillway type selectedDischarge Description is based on the spillway type selected, either a pipe (using the pipe approach previously described), or a weir. If a weir is selected, it can be a broad-crested weir and the weir length entered. If a 0 value is entered for the weir length, the model will assume a 90o V-notch weir. If a pipe spillway is selected (as in this example), the pipe diameter (in inches) is given, ranging from 6 to 60 inches. When a pipe is selected, the g , g g p p ,height from the invert of the discharge end of the pipe to the spillway elevation is also needed for the simplified equation. This height must be at least twice the diameter of the pipe.

Up to three pipe diameters (or weir lengths) can be entered. The model will evaluate all three options, making the selection of the choice easier. As the dimensions are entered, the rating curves (flow vs. height) and storage below the elevations are displayed. This is a good indication of the correct spillway size, as the g p y ,maximum discharge close to the desired pond depth can be observed. In this case, the 40 inch pipe has the desired discharge of 139 cfs at a stage slightly above 4 feet, and well under 10 feet. The 36 inch pipe option would need about 10 feet of stage (greater than planned), while the 24 inch pipe would require even more (more than 20 ft). Therefore, it is expected that the 3rd pipe option, the 40 inch pipe would work best.

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The maximum total flow rate is about 450 plus 120 = 570 cfs, compared to a maximum of about 520 cfs without detention.

Much more information on detention ponds at:The Design and Use of Detention Facilities for Stormwater M t U i DETPONDManagement Using DETPOND:http://unix.eng.ua.edu/~rpitt/SLAMMDETPOND/WinDetpond/WinDETPOND%20user%20guide%20and%20documentation.pdf

Detention Pond Design and Analysis:http://unix eng ua edu/~rpitt/Class/Water%20Resources%2http://unix.eng.ua.edu/~rpitt/Class/Water%20Resources%20Engineering/WREMainPage.htm

Urban Hydrology for Small Watersheds (TR-55):http://unix.eng.ua.edu/~rpitt/Class/Erosioncontrol/Module4/tr55.pdf