Non-Proprietary BMPs

An Extended Detention (ED) Pond relies on 12 to 24 hour detention of stormwater runoff after each rain event. An under-sized outlet structure restricts stormwater flow so it backs up and is stored within a pond or wetland. The temporary ponding enables particulate pollutants to settle out and reduces the maximum peak discharge to the downstream channel thereby reducing the effective shear stress on downstream banks. ED differs from stormwater detention since it is designed to achieve a minimum drawdown time, rather than a maximum peak rate of flow (which is commonly used to design for peak discharge or flood control purposes and often detains flows for just a few minutes or hours). ED ponds rely on gravitational settling as their primary pollutant removal mechanism. Consequently, they generally provide fair to good removal for particulate pollutants but low or negligible removal for soluble pollutants, such as nitrate and soluble phosphorus. The use of ED alone generally has the lowest overall pollutant removal rate of any stormwater treatment option. As a result, ED is normally combined with wet ponds or constructed wetlands to maximize pollutant removal rates (see Table 1).

Designers should note that ED ponds are the final element in the roof to stream runoff reduction sequence, and should only be considered if there is water quality or channel protection volume to manage after all other upland runoff reduction practices have been considered and properly credited. Designers may need to submit documentation to the local plan review authority showing that all runoff reduction efforts were explored and were found to be insufficient.

The major design goal for the Chesapeake Bay is to maximize nutrient removal and runoff reduction. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced Level 2 design that maximizes nutrient and runoff reduction. To qualify for the higher nutrient reduction rates for Level 2 design, ED ponds must be designed with a treatment volume equal to 1.25(Rv)(A). Table 2 lists the design criteria for Level 1 and 2 designs. See Section 5 for more detailed design guidelines.

Table 2. Extended Detention (ED) Pond Criteria
Level 1 Design (RR:0; TP:15; TN:10)	Level 2 Design (RR:15; TP:15; TN:10)
Tv = [(1.0) (Rv)(A)/12] – volume reduced by upstream BMP	Tv = [1.25(Rv) (A)/12] – volume reduced by upstream BMP
At least 15% of Tv in permanent pool	More than 40% of Tv in deep pool or wetlands
Length/Width ratio OR Flow path = 2:1 or more	Length/Width ratio OR Flow path = 3:1 or more
Length of shortest flow path/overall length = 0.4 or more	Length of shortest flow path/overall length = 0.7 or more
Average ED time of 24 hours or less	Average ED time of 36 hours
Vertical ED fluctuation exceeds 4 feet	Maximum vertical ED limit of 4 feet
Turf cover on floor	Trees and wetlands in the planting plan
Forebay and micropool	Additional cells or treatment methods (e.g., sand filter or biotretention on pond floor)
CDA less than ten acres	CDA greater than ten acres

The following feasibility issues need to be evaluated when ED ponds are considered as the final practice in a treatment train:

Contributing Drainage Area: A minimum contributing drainage area of 10 acres is recommended for ED ponds in order to sustain a permanent micropool that prevents clogging. ED may still work on drainage areas less than 10 acres, but designers should be aware that these “pocket” ponds will typically have very small orifices that will be prone to clogging, experience fluctuating water levels, and generate future maintenance problems.

Available Head: The depth of an ED pond is usually determined by the amount of hydraulic head available at the site. The bottom elevation is normally set by the existing downstream conveyance system to which the ED pond discharges. Typically, a minimum of 6 to 10 feet of head is needed to construct an ED pond.

Soils: The permeability of soils is seldom a design constraint for micropool ED ponds. Soil infiltration tests need to be conducted at proposed pond sites to estimate infiltration rates which can be significant in Hydrologic Soil Group (HSG) “A” soils and some group “B” soils. Infiltration through the bottom of the pond is encouraged unless it will impair the integrity of the embankment. Geotechnical tests should be conducted to determine the infiltration rates and other subsurface properties of the soils underlying the proposed ED pond. If these tests indicate the site has active karst features, an alternative practice or combination of practices should be employed at the site. See Technical Bulletin No.1 for guidance on stormwater design in karst terrain (CSN, 2008).

Trout Streams: The use of ED ponds in watersheds containing trout streams is restricted to situations where upland runoff reduction practices cannot meet the full channel protection volume. In these instances, a micrpool ED pond must be designed with (1) a maximum 12 hour detention time, (2) have the minimum pool volume needed to prevent clogging, (3) be planted with trees so it becomes fully shaded and (4) be located outside of any required stream buffers.

ED is normally combined with other stormwater treatment options such as wet ponds, sand filters and constructed wetlands to enhance its performance and appearance. The most common design variations for ED include:

Figure 1 illustrates several ED pond design variations. While ED ponds can provide for channel and flood protection, they will rarely provide adequate runoff reduction and pollutant removal to serve as a stand-alone compliance strategy. Therefore, designers should always maximize the use of upland runoff reduction practices, such as rooftop disconnections, small scale infiltration, rain tanks, bioretention, grass channels and dry swales that reduce runoff at its source (rather than treating the runoff at the terminus of the storm drain system). Upland runoff reduction practices can be used to satisfy most or all of the runoff reduction requirements at most sites, but an ED pond may still be needed to provide any remaining channel protection requirements. Upland runoff reduction practices will greatly reduce the size, footprint and cost of the downstream ED pond.

Designers can use a site-adjusted Rv or CN to reflect the use of upland runoff reduction practices to compute the remaining volume that must be treated by the ED pond using the accepted local or state runoff reduction method. ED ponds are then designed to capture and treat the remaining runoff volume for the water quality storm and the channel protection storm (if needed). Runoff treatment credit may be taken for the entire water volume below the normal pool (including micropools, forebays and shallow marsh areas), as well as any temporary extended detention above the normal pool. To be eligible for the higher Level 2 design removal rates for water quality, the ED pond must be sized with 1.25 of the remaining water quality (but not any additional Channel Protection volume).

The kerplunk approach can be used to estimate the required volume for channel protection, using the NRCS methods presented in Appendix A of this Specification.

Soil borings should be taken below the proposed embankment, in the vicinity of the proposed outlet area, and in at least two locations within the ED pond treatment area. Soil boring data is needed to (1) ascertain the physical characteristics of excavated material, (2) determine its adequacy for use as structural fill or spoil, (3) provide data for structural designs for outlet works (e.g., bearing capacity and buoyancy), (4) determine compaction/composition needs for the embankment, (5) fix the depth to groundwater and bedrock and (6) evaluate potential infiltration losses (and the consequent need for a liner).

Sediment forebays are considered an integral design feature to maintain the longevity of ED ponds. A forebay must be located at all major inlets to trap sediment and preserve the capacity of the main treatment cell.

The forebay shall consist of a separate cell, formed by an acceptable barrier. Typical examples include earthen berms, concrete weirs, and gabion baskets.
A major inlet is defined as an individual storm drain inlet pipe or open channel serving at least 10% of the ED pond’s contributing drainage area.
The forebay should be at least 4 feet deep and shall be equipped with a variable width aquatic bench for safety purposes. The aquatic benches should be 4 to 6 feet wide and placed 18 inches below the water surface. The total volume of all forebays should be at least 15% of the total WQv (inclusive).
The forebay should be designed in such a manner that it acts as a level spreader to distribute runoff evenly across the entire bottom surface area of the main treatment cell.
The bottom of the forebay may be hardened (i.e., concrete, asphalt, or grouted riprap) in order to make sediment removal easier. A fixed vertical sediment depth marker should be installed in the forebay to measure sediment deposition over time.

Non-clogging Low Flow Orifice: Unless the drainage area to an ED pond is unusually large, the diameter of the ED orifice will be less than 6 inches. Small diameter pipes are prone to chronic clogging by organic debris and sediment. Designers should always look at upstream conditions to assess the potential for higher sediment and woody debris loads. The risk of clogging in such small openings can be reduced by:

Providing a micropool at the outlet structure:

Utilize a reverse-sloped pipe that extends to a mid-depth of the permanent pool or micropool
Install a downturned elbow or half-round CMP over a riser orifice (circular, rectangular, V-notch, etc.) to pull water from below the micropool surface
The depth of the micropool should be at least 4 feet deep, which will not draw down by more than 2 feet during a 30 day summer drought (for water balance method, see Section 6.2 of Stormwater Design Specification No 13-constructed wetlands)..

Providing an over-sized forebay to trap sediment, trash and debris before it reaches the ED low-flow orifice.
Installing a trash rack to screen the low-flow orifice;

Utilizing a perforated pipe under a gravel blanket with an orifice control in the riser structure.

Emergency Spillway: ED ponds shall be constructed with overflow capacity to pass the 100-year design storm event through either the Primary Spillway or a vegetated or armored Emergency Spillway. Refer to Introduction Appendix C Emergency Spillways.

Adequate Outfall Protection: The ED pond shall have a stable outfall for the 10-year design storm event. The channel immediately below the pond outfall shall be modified to prevent erosion and conform to natural dimensions in the shortest possible distance. This is typically done by installing appropriately sized riprap, placed over filter fabric, that can reduce flow velocities from the principal spillway to non-erosive levels (3.5 to 5.0 fps). Flared pipe sections that discharge at or near the stream invert or into a step pool arrangement should be used at the spillway outlet.

Long Flow Path: ED pond designs should have an irregular shape and a long flow path from inlet to outlet to increase water residence time, treatment pathways, and pond performance. In terms of flow path geometry, there are two design objectives: (1) the overall flow path through the wetland, and (2) the length of the shortest flow path (Hirschman et al., 2009):
The overall flow path can be represented as the length to width ratio OR the flow path ratio (see the Specs Introduction chapter for diagrams and equation). These ratios shall be at least 2:1 for Level 1 designs and 3:1 for Level 2. Internal berms, baffles, or topography can be used to extend flow paths and/or create multiple pond cells.
The shortest flow path represents the distance from the closest inlet to the outlet (see Specs Introduction chapter). The ratio of the shortest flow to the overall length shall be at least 0.4 for Level 1 and 0.7 for Level 2. In some cases -- due to site geometry, storm sewer infrastructure, or other factors -- some inlets may not be able to meet these ratios; however, the drainage area served by these “closer” inlets should constitute no more than 20% of the total contributing drainage area.

Required Storage: The total water quality storage volume may be provided by a combination of permanent pool, shallow marsh and/or extended detention storage. The storage needed for channel protection must be provided by temporary ED only.

Vertical Extended Detention Limits: The maximum ED water surface elevation shall not extend more than 5 feet above the basin floor or normal pool. The maximum vertical elevation for ED over shallow wetlands is 1 foot. The bounce effect is not as critical for larger flood control storms (e.g., the 10-year design storm) and these events can exceed the 5 foot vertical limit if they are managed by a multi-stage outlet structure.
Safety Features:
The principal spillway opening shall not permit access by small children.
End walls above pipe outfalls greater than 48 inches in diameter shall be fenced to prevent a hazard.
An emergency spillway and associated freeboard shall be provided in accordance with applicable local or state dam safety requirements. The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges.
Both the safety bench and the aquatic bench may be landscaped to prevent access to the pool.

5.7. Landscaping and Planting Plan
A landscaping plan shall be provided that indicates the methods used to establish and maintain vegetative coverage within the ED pond and its buffer. Minimum elements of a plan include: delineation of pondscaping zones, selection of corresponding plant species, planting plan, sequence for preparing wetland bed (including soil amendments, if needed) and sources of native plant material.

The landscaping plan should provide elements that promote greater wildlife and waterfowl use within the stormwater wetland and buffers. The planting plan should allow the pond to mature into a native forest in the right places yet keeps mowable turf along the embankment and all access areas. The wooded wetland concept proposed by Cappiella et al., (2005) may be a good option for many ED ponds.
Woody vegetation may not be planted or allowed to grow within 15 feet of the toe of the embankment and 25 feet from the principal spillway structure.
A buffer should be provided that extends
The landscaping plan should 25 feet outward from the maximum water surface elevation of the ED pond. Permanent structures (e.g., buildings) should not be constructed within the buffer. Existing trees should be preserved in the buffer area during construction.

For more guidance on planting trees and shrubs in ED pond buffers, consult Cappiella et al (2006) and Appendix E of the Introduction to the New Virginia Stormwater Design Specifications , as posted on the Virginia Stormwater BMP Clearinghouse web site).

Several ED pond potential maintenance problems can be addressed during design. Good maintenance access is needed so crews can remove sediments from the forebay, alleviate clogging and make riser repairs.

ED ponds generally use materials on site except for plantings, inflow and outflow devices such as piping and riser materials, possibly stone for inlet and outlet stabilization, and filter fabric for lining banks or berms.

The basic material specifications for earthen embankments, principal spillways, vegetated emergency spillways and sediment forebays shall be as specified in Appendices A-D of the Introduction to the New Virginia Stormwater Design Specifications , as posted on the Virginia Stormwater BMP Clearinghouse web site, at the following URL:

When reinforced concrete pipe is used for the principal spillway to increase its longevity, ‘O” ring gaskets (ASTM C-361) should be used to create watertight joints, and they should be inspected during installation.

Active karst regions are found in much of the Ridge and Valley province of the Bay watershed, and complicate both land development in general and stormwater design in particular. Designers should always conduct geotechnical investigations in karst terrain to assess this risk in the planning stage. Because of the risk of sinkhole formation and groundwater contamination in regions with active karst, ED ponds are highly restricted. If these studies indicate that less than three feet of vertical separation exist between the bottom of the ED pond and the underlying active karst layer, ED ponds should not be used. If ED ponds are used, they must have an acceptable liner per Stormwater Design Specification No. 13 (Constructed Wetlands).

The lack of sufficient hydraulic head and the presence of a high water table of many coastal plain sites significantly constrain the application of ED ponds. Excavating ponds below the water table creates what are known as dugout ponds where the water quality volume is displaced by groundwater, reducing pond efficiency and mixing and creating nuisance conditions. In general, shallow constructed wetlands are a superior alternative to ED ponds for the coastal plain environment.

Winter conditions can cause freezing problems within inlets, flow splitters, and ED outlet pipes due to ice formation. The following design adjustments are recommended for ED ponds installed in higher elevations of the Virginia:

The following is a typical construction sequence to properly install a dry ED pond. The steps may be modified to reflect different dry ED pond designs, site conditions, and the size, complexity and configuration of the proposed facility.

Step 1: Use of ED pond as an E&S Control. An ED pond may serve as a sediment basin during project construction. If this is done, the volume should be based on the more stringent sizing rule (erosion and sediment control or water quality). Installation of the permanent riser should be initiated during the construction phase, and design elevations should be set with final cleanout and conversion in mind. The bottom elevation of the ED pond should be lower than the bottom elevation of the temporary sediment basin. Appropriate procedures should be implemented to prevent discharge of turbid waters when the basin is being converted into an ED pond.

Step 2: Stabilize Drainage Area. ED ponds should only be constructed after the contributing drainage area to the pond is completely stabilized. If the proposed pond site will be used as a sediment trap or basin during the construction phase, the construction notes should clearly indicate that the facility will be dewatered dredged and re-graded to design dimensions after construction is complete.

Step 3: Assemble Construction Materials on-site, make sure they meet design specifications and prepare any staging areas.

Step: 5 Install Project E&S Controls, including temporary dewatering devices, erosion and sediment controls, and stormwater diversion practices prior to construction. All areas surrounding the pond that are graded or denuded during construction are to be planted with turf grass, native planting, or other approved methods of soil stabilization.

Step 7: Install the Riser or Outflow Structure and ensure the top invert of the overflow weir is constructed level at the design elevation.

Step 8: Construct the Embankment and any Internal Berms in 8 to 12-inch lifts and compacted with appropriate equipment.

Step 9: Excavate/Grade until the appropriate elevation for the bottom of the ED pond is reached and desired contours and acceptable side slopes are achieved.

Step 10: Construct Emergency Spillway in cut or structurally stabilized soils.

Step 12: Stabilize Exposed Soils with temporary seed mixtures appropriate for the pond buffer. All areas above the normal pool should be permanently stabilized by hydroseeding or seeding over straw.

Step 13: Plant the Pond Buffer Area, following the pondscaping plan (see Section 7.5).

Multiple construction inspections are critical to ensure that stormwater ponds are properly constructed. Inspections are recommended during the following stages of construction:

A construction inspection form for ED ponds can be accessed at the CWP website at www.cwp.org/postconstruction (see Tool #6 of Managing Stormwater in Your Community: A Guide for Building Effective Post-Construction Programs). For larger ED ponds, the expanded construction inspection form provided in Appendix B of CWP (2004) should be used.

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program. The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel. Access to ED ponds should be covered by a drainage easement to allow inspection and maintenance.

It is also recommended that the maintenance agreement include a list of qualified contractors that can perform inspection or maintenance services, as well as contact information for owners to get local or state assistance to solve common nuisance problems, such as mosquito control, geese, invasive plants, vegetative management and beaver removal. CWP (2004) provides some excellent templates on how to respond to these problems.

Maintenance of ED ponds is driven by annual inspections that evaluate the condition and performance of the facility (see Table 3). Based on inspection results, specific maintenance tasks will be triggered. An annual maintenance inspection form for ED ponds can be accessed at CWP website at www.cwp.org/postconstruction (see Tool #6 of Managing Stormwater in Your Community: A Guide for Building Effective Post-Construction Programs). A more detailed maintenance inspection form is also available from Appendix B of CWP (2004).

ED ponds are prone to a high clogging risk at the ED low flow orifice. These aspects of pond plumbing should be inspected at least twice a year after initial construction. The constantly changing water levels in ED ponds make it difficult to mow or manage vegetative growth. The bottom of ED ponds often become soggy, and water-loving trees such as willows may take over. The maintenance plan should clearly outline how vegetation in the pond and its buffer will be managed or harvested in the future. Periodic mowing of the stormwater buffer is only required along maintenance rights-of-way and the embankment. The remaining buffer can be managed as a meadow (mowing every other year) or forest.

The maintenance plan should schedule a shoreline cleanup at least once a year to remove trash and floatables that tend to accumulate in the forebay, micropool and on the bottom of ED ponds.

Frequent sediment removal from the forebay is essential to maintain the function and performance of an ED pond. Maintenance plans should schedule cleanouts every 5-7 years, or when inspections indicate that 50% of forebay capacity has been lost. Designers should also check to see whether removed sediments can be spoiled on-site or must be hauled away. Sediments excavated from ED ponds are not usually considered toxic or hazardous, and can be safely disposed by either land application or land filling.

Table 3. Suggested Annual Maintenance Inspection Points for ED ponds

Activity

Measure sediment accumulation levels in forebay.

Monitor the growth of wetlands, trees and shrubs planted. Record species and approximate coverage, and note presence of any invasive plant species.

Inspect the condition of stormwater inlets to the pond for material damage, erosion or undercutting

Inspect upstream and downstream banks for evidence of sloughing, animal burrows, boggy areas, woody growth or gully erosion that may undermine embankment integrity

Inspect pond outfall channel for erosion, undercutting, rip-rap displacement, woody growth, etc.

Inspect condition of principal spillway and riser for evidence of spalling, joint failure, leakage, corrosion, etc.

Inspect condition of all trash racks, reverse sloped pipes or flashboard risers for evidence of clogging, leakage, debris accumulation, etc.

Inspect maintenance access to ensure it is free of woody vegetation and check to see whether valves, manholes or locks can be opened and operated.

Inspect internal and external pond side slopes for evidence of sparse vegetative cover, erosion or slumping, and repaired immediately

Note: For a more detailed maintenance inspection checklist, see Appendix B in CWP(2004) Stormwater Pond and Wetland Maintenance Guidebook

SECTION 9: COMMUNITY AND ENVIRONMENTAL CONCERNS

There are several community and environmental concerns about ED ponds that can be anticipated during design, as follows:

Aesthetics: ED ponds tend to accumulate sediment and trash, which residents are likely to perceive as unsightly and creating nuisance conditions. Fluctuating water levels in ED ponds also create a tough landscaping environment. In general, designers should avoid designs that rely solely on dry ED.

Existing Wetlands: ED ponds should not be constructed within existing natural wetlands nor should they inundate or otherwise change the hydroperiod of existing wetlands.

Existing Forests: Designers can expect a great deal of neighborhood opposition if they do not make a concerted effort to save mature trees during design and pond layout. Designers should also be aware that even modest changes in inundation frequency can kill upstream trees (Wright et al., 2007).

Stream Warming Risk: ED ponds have less risk of stream warming than other pond options, but they can warm streams if they are un-shaded or contain significant surface area in shallow pools. If an ED pond discharges to temperature-sensitive waters, it should be forested, contain the minimum pools to prevent clogging and have a maximum detention time of 12 hours or less.

Safety Risk: ED ponds are generally considered to be safer than other pond options since they have few deep pools. Steep side-slopes and unfenced headwalls, however, can still create some safety risks.

Mosquito Risk: The fluctuating water levels within ED ponds have potential to create conditions that lead to mosquito breeding. Mosquitoes tend to be more prevalent in irregularly flooded ponds than in ponds with a permanent pool (Santana et al., 1994). Designers can minimize the risk by combining ED with a wet pond or wetland.

SECTION 10: DESIGN REFERENCES

Cappiella, K., T. Schueler and T. Wright. 2006. Urban Watershed Forestry Manual: Part 2: Conserving and Planting Trees at Development Sites. USDA Forest Service. Center for Watershed Protection. Ellicott City, MD

Cappiella, K. et al . 2007. Urban Watershed Forestry Manual: Part 3: Urban Tree Planting Guide. USDA Forest Service. Center for Watershed Protection. Ellicott City, MD

Cappiella, K., L. Fraley-McNeal, M. Novotney and T. Schueler. 2008. The next generation of stormwater wetlands. Wetlands and Watersheds Article No. 5. Center for Watershed Protection. Ellicott City, MD

Center for Watershed Protection. 2004. Pond and Wetland Maintenance Guidebook. Ellicott City, MD.

Hirschman, D., L. Woodworth and S. Drescher. 2009. Technical Report: Stormwater BMPs in Virginia’s James River Basin: An Assessment of Field Conditions & Programs. Center for Watershed Protection. Ellicott City, MD.

Maryland Department of Environment (MDE). 2000. Maryland Stormwater Design Manual. Baltimore, MD

Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD

Virginia Department of Conservation and Recreation (VA DCR). 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA

This appendix provides a general discussion of the channel protection method for ED ponds. The Virginia Stormwater Management Handbook (Latest Edition) should be consulted for allowable computation methods for channel protection.

Step 1: Compute the runoff volume produced from the post-development 1-year, 24 hour design storm event, using TR-55, and adjusted CNs to account for upland runoff reduction.

Step 2: Use the Kerplunk method which assumes the pond volume (above the permanent pool or basin floor) instantaneously fills up. Determine the storage volume and Cpv maximum invert elevation using the short-cut method described below.

Step 3: Set the Cpv orifice invert above the permanent pool elevation and size the initial orifice diameter to drain the entire Cpv volume in 24 hours.

Step 4: Compute the average peak discharge rate for the Cpv event (i.e., Cpv/24).

Step 5: Using TR-20 or other accepted storage indication routing tool,, route the runoff through the pond and check to make sure that the peak discharge for Cpv does not exceed twice the average discharge, and that it meets the minimum 12 hour detention time.

Note: No permanent pool is involved in the design (although a micropool is recommended to keep orifice from clogging).

In most cases, an extended detention pond is utilized after upland runoff reduction practices provide some storage for channel protection and flood control design storms (in some cases, the runff reduction practices can be incorporated within the extended detention pond if adequate pretreatment is provided (e.g., sand filter or bioretention in the pond floor).

This section presents a modified version of the TR-55 (NRCS, 1986) short cut sizing approach. The method was modified by Harrington (1987) for applications where the peak discharge is very small compared with the uncontrolled discharge. This often occurs in sizing detention for the 1-year, 24-hour storm event.

Using TR-55 guidance, the unit peak discharge (qu) can be determined based on the Curve Number and Time of Concentration (Figure 15-A.1). Knowing qu and T (extended detention time), qO/qi (peak outflow discharge / peak inflow discharge) can be estimated using Figure 15-A.2.

Then using qO/qi, Figure 15-A.3 can be used to estimate VS/Vr. For a Type II or Type III rainfall distribution, VS/Vr can also be calculated using the following equation:

                        Where:             VS = required storage volume (acre-feet)
                                                Vr = runoff volume (acre-feet)
                                                qO = peak outflow discharge (cfs)
                                                qI = peak inflow discharge (cfs)

                        Where:             VS and Vr are defined above
                                                Qd = the developed runoff for the design storm (inches)
                                                A = total drainage area (acres)

EXTENDED DETENTION (ED) POND