6.2.16 dry extended detention basin - augustaga.gov

6.2.16 Dry Extended Detention Basin

Description: A dry extended detention (ED) basin is a surface storage basin or facility designed to provide water quality treatment and water quantity control through extended detention of stormwater. Dry ED basins differ from dry detention basins in that they provide 24-hour detention of the channel protection volume (CPv).

LID/GI Consideration: Similar to dry detention, dry ED basins contribute to a site’s overall perviousness and aesthetics. The open-grassed area of a dry ED basin can also be used for multiple purposes, such as landscaped or recreational areas. This is considered to be a WQv IMP.

Augusta Stormwater Management Manual

Version: July 20206.2.16- 1


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KEY CONSIDERATIONS STORMWATER MANAGEMENT SUITABILITY

DESIGNCRITERIA: ` Embankments shall be less than 20 feet in height and have side slopes no steeper than 2:1 (horizontal to vertical), internally and 3:1 externally

` The depth of the basin shall not exceed 10 feet ` Sediment forebay is required ` Geotechnical slope stability analysis is recommended for embankments greater than 10 feet in height

` All embankments shall be designed to State of Georgia guidelines for dam safety

` Storage volumes greater than 100 acre-feet are subject to the requirements of the Georgia Safe Dams Act (Georgia Annotated Code 12-5-370) unless the facility is excavated to this depth

` The bottom area of storage facilities shall have a 2% minimum grade toward the outlet to prevent standing water conditions.

` The outfall of dry ED basins shall always be stabilized to prevent scour

` An emergency spillway shall be provided in addition to and separate from the primary outlet structure to safely convey large flood events

` A one 10 foot bench shall be provided at 2% for every 20 feet in vertical drop on the exterior of the pond.

ADVANTAGES/BENEFITS: ` Moderate removal rate of urban pollutants ` High community acceptance ` Useful for water quality treatment, channel protection, and flood control

` Dry ED basins can serve multiple purposes on a development site

` Settling pools within the basin mitigate potential thermal impacts

DISADVANTAGES/LIMITATIONS: ` Dam height restrictions for high relief areas ` Drainage from the dry ED basin can be problematic for low relief terrain

ROUTINEMAINTENANCEREQUIREMENTS: ` Remove debris from inlet and outlet structures ` Maintain side slopes and outlet structure ` Remove invasive vegetation

` Monitor sediment accumulation and remove periodically

Residential Subdivision Use: Yes High Density/Ultra-Urban: Not Recommended Roadway Projects: Yes

RUNOFF REDUCTION CREDIT

` 0% of the runoff reduction volume provided

POLLUTANTREMOVAL

60% Total Suspended Solids

10%/30%Nutrient removal (Total Phosphorous/Total Nitrogen)

50%Metals - Cadmium, Copper, Lead, and Zinc removal

NA% Pathogens – Fecal Coliform

Runoff Reduction

u TSS Removal

Channel Protection

Overbank Flood Protection

Extreme Flood Protection Retrofit

suitable for this practice

u may provide partial benefits

Medium Land Requirement

Low Capital Cost

Low Maintenance Burden

IMPLEMENTATION CONSIDERATIONS




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6.2.16.1 GeneralDescriptionandStormwaterManagementSuitabilityDry ED basins are surface facilities intended to provide for the temporary storage and treatment of stormwater runoff to reduce downstream water quality and water quantity impacts. These facilities temporarily detain stormwater runoff, releasing the flow over a period of time. They are designed to completely drain within 24 to 72 hours after a storm event and are normally dry between rain events. A typical schematic for a basin is shown in Figure 6.2.16-1.

Dry ED basins provide downstream channel protection through extended detention of the channel protection volume (CPv). They can also provide overbank flood protection (peak flow reduction of the 2-, 10-, 25-, and 50-year, 24-hour storms) and can be designed to control the extreme flood (100-year, 24-hour, Qp100) storm event.

Figure 6.2.16-1. Schematic of Dry ED Basin

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Dry ED basins provide limited pollutant removal benefits and are not intended for runoff reduction. These IMPs must be used in a treatment train approach with other IMPs that provide runoff reduction, additional water quality treatment, and channel protection.

6.2.16.2 PlanningThe following criteria must be considered when evaluating the suitability of dry ED basins for a development site:

Physical Feasibility Requirements ` Drainage Area – In general, dry ED basins shall be used on sites with a minimum drainage area of 10 acres.

` Space Required – Approximately 2-3% of the contributing drainage area is required.

` Site Slope – Dry ED basins can be used on sites with slopes up to about 15%.

` Embankments – Vegetated and riprap embankments shall be less than 20 feet in height and have side slopes no steeper than 2:1 (horizontal to vertical), although 3:1 is preferred.

` Minimum Depth to Water Table – Except for the case of hotspot runoff, the only consideration regarding groundwater is that the base of the dry ED basin shall not intersect the groundwater table.

` Soils – Dry ED basins can be used in almost all soils and geology, with minor design adjustments for regions of karst (i.e., limestone) topography or in rapidly percolating soils, such as sand. In these areas, dry ED basins shall be designed with an impermeable liner to prevent groundwater contamination or sinkhole formation.

` Hotspots – Dry ED basins can accept runoff from stormwater hotspots, but need significant separation from groundwater when used for this purpose.

` Trout Stream – Dry ED basins shall not be used where receiving water temperature is a concern. In addition, careful consideration shall be given to the potential for perched or raised groundwater levels.

The data listed below are necessary for the design of a dry ED basin and shall be included with the SWMP:

` Existing and proposed site, topographic and location maps, and field reviews

` Impervious and pervious areas

` Roadway and drainage profiles, cross-sections, utility plans, and soil report for the site

` Design data from nearby storm sewer structures

` Water surface elevation of nearby water systems as well as the depth to seasonally high groundwater

` Setback distances from property lines, building foundations, drinking water wells, open water, etc




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See Appendix B for more information on required elements for the SWMP.

DesignThe following criteria shall be considered minimum standards for the design of a dry ED basin:

` Dry ED basins are sized to provide storage and control for multiple rain events, including:

` Temporary storage of the water quality volume (WQv)

` Temporary storage of the channel protection volume (CPv)

` Temporary storage of the volume of runoff required to provide overbank flood (Qp2, Qp10, Qp25, and Qp50) protection (i.e., reduce the post-development peak flows of the 2- and 50-year storm events to the pre-development rates)

` Control of the 100-year storm

` Routing calculations shall be used to demonstrate that the storage volume is adequate. See Section 5.3 (Storage Design) for procedures on the design of detention storage.

` Storage volumes greater than 100 acre-feet are subject to the requirements of the Georgia Safe Dams Act (Georgia Annotated Code 12-5-370) unless the facility is excavated to this depth.

Specifications ` Vegetated embankments shall be less than 20 feet in height and shall have side slopes no steeper than 2:1 (horizontal to vertical) internally and 3:1 exernally. Riprap-protected embankments shall be no steeper than 2:1. Geotechnical slope stability analysis is recommended for embankments greater than 10 feet in height and is mandatory for embankment slopes steeper than those given above. All embankments shall be designed to State of Georgia guidelines for dam safety.

` A one 10 foot bench shall be provided at 2% for every 20 feet in vertical drop on the exterior of the pond.

` The depth of the basin shall not exceed 10 feet.

` Areas above the normal high water elevations of the detention facility shall be sloped toward the basin to allow drainage. Careful finish grading is required to avoid creation of upland surface depressions that may retain runoff. The bottom area of storage facilities shall be graded toward the outlet to prevent standing water conditions.

` Designing dry ED basins with a high length to width ratio (i.e., at least 1.5:1) and incorporating other design features to maximize the flow path effectively increases detention time by eliminating the potential of flow to short circuit the basin. Designing basins with relatively flat side slopes can also help to lengthen the effective flow path.

` A minimum 2% slope is required from the basin inlet to the basin outlet to ensure that flow can move through the basin to the outlet.

` A low-flow or pilot channel across the facility bottom from the inlet to the outlet is recommended to convey low flows and prevent standing water conditions. To prevent stream warming, designers shall place landscaping to provide shade around the pilot channel and the basin




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outlet. Designing the pilot channel as a grass channel also reduces thermal impacts. A minimum slope of 2% is required for grass swales and 0.5% for armored pilot channels.

` Maintenance access having a minimum width of 20 feet shall be provided for all dry ED basins.

` The pretreatment/inlet system shall meet the following specifications:

` Inflow channels are to be stabilized with flared aprons, or the equivalent.

` Pretreatment for a dry ED basin is usually provided by a sediment forebay. The sediment forebay shall be sized to 0.1 inch of runoff per impervious acre of contributing drainage area for basins

` Pretreatment may also be provided by using a grass filter strip, pea gravel diaphragm, or grass channel. Where filter strips are used, 100% of the contributing runoff shall flow across the filter strip. Refer to Section 6.2.4 for design criteria for a grass channel and Section 6.2.15 for vegetated filter strips.

` The outlet structures shall meet the following specifications:

` For a dry ED basin, the outlet structure is sized for control of all required storms (based upon hydrologic routing calculations) and can consist of a weir, orifice, outlet pipe, combination outlet, or other acceptable control structure. Small outlets that will be prone to clogging or difficult to maintain are not acceptable.

` A dry ED basin has a channel protection orifice with a minimum diameter of 3 inches and shall be adequately protected from clogging by an acceptable external trash rack. The orifice diameter may be reduced to 1 inch if internal orifice protection is used (e.g., an overperforated vertical stand pipe with 0.5-inch orifices or slots that are protected by wirecloth and a stone filtering jacket). Adjustable gate valves can also be used to achieve this equivalent diameter.

` Seepage control or anti-seep collars shall be provided for all outlet pipes.

` Riprap, plunge pools or pads, or other energy dissipators are to be placed at the end of the outlet to prevent scouring and erosion. If the basin discharges to a channel with dry weather flow, care shall be taken to minimize tree clearing along the downstream channel, and to reestablish a forested riparian zone in the shortest possible distance from the ED basin.

` The following safety features are required:

` An emergency spillway shall be included in the dry ED basin design to safely pass the extreme flood flow. The spillway prevents basin water levels from overtopping the embankment and causing structural damage. The emergency spillway shall be designed to State of Georgia guidelines for dam safety and located so that downstream structures will not be impacted by spillway discharges.

` A minimum of 1 foot of freeboard shall be provided, measured from the top of the water surface elevation for the extreme flood, to the lowest point of the dam embankment, not counting the emergency spillway. A safety bench shall be provided for embankments greater than 10 feet in height and having a side slope steeper than 3:1. For large basins,




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the safety bench shall extend no less than 15 feet outward from the normal water edge to the toe of the basin side slope. The slope of the safety bench shall not exceed 6%.

` Stormwater shall be conveyed to and from dry ED basins safely and to minimize erosion potential.

` GI-IMPs shall not be designed and constructed within the basin.

Landscaping ` Designers shall maintain a vegetated buffer around the dry ED basin and select plants within the detention zone (i.e., the portion of the basin up to the elevation where stormwater is detained) that can withstand both wet and dry periods. The side slopes of dry ED basins shall be relatively flat to reduce safety risks.

` Bare soil is not permitted in the ED area. Non-vegetated areas shall be stabilized using mulch or other appropriate ground cover. See the mulch specifications in Section 6.2.1.3.

` Plantings shall be designed not to conflict with the current drainage of the basin.

` All trees shall be kept away from any drainage structures to allow for maintenance access and repairs as needed.

Design ProceduresStep 1. Determine if the development site and conditions are appropriate for the use of the dry ED basin.Consider the application and site feasibility criteria in this Chapter. In addition, determine if site conditions are suitable for a dry ED basin. Create a rough layout of the basin dimensions, taking into consideration existing trees, utility lines, and other obstructions.

Minimum setback requirements for dry ED basins are as follows: ` 10 feet from a property line

` 10 feet from a building foundation

` 100 feet from a private well

` 200 feet from a public water supply reservoir

` 100 feet from open water

` 1,200 feet from public water supply wells

Step 2. Determine the goals and primary function of the ED basin.

Consider whether the dry ED basin is intended to: ` Meet a water quality (treatment) target. For information on the sizing of the IMP utilizing the water quality treatment approach, see Step 3.

` Be “oversized” to include partial credit for storage capacity for other stormwater requirements (channel protection volume [CPv ])

` Provide a possible solution to a drainage problem

` Enhance landscape and provide aesthetic qualities




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Identify any restrictions or other requirements that may apply or may affect the design.

Complete Step 3 for a water quality treatment approach. Refer to Chapter 5 for detailed information on compliance calculations

Step 3. Calculate WQv.

Calculate the WQv using the following formula:WQv=(1.2)(Rv)(A)/12

whereWQv = water quality volume (ft3)1.2 = target rainfall amount to be treated (inches)Rv = volumetric runoff coefficient, which can be found by Rv = 0.05+0.009(I)

and where I = new impervious area of the contributing drainage area (%)

A = area draining to this practice (ft2)12 = unit conversion factor (in/ft)

Step4.Sizeflowdiversionstructure,ifneeded.

A flow regulator (or flow splitter diversion structure) shall be supplied to divert the WQv (or RRv) to dry ED basin.

Size low flow orifice, weir, or other device to pass Qwq.

Step 5. Determine pretreatment volume.

A sediment forebay is provided at each inlet, unless the inlet provides less than 10% of the total design storm inflow to the basin. The forebay shall be sized to contain 0.1 inch per impervious acre of contributing drainage.

Step6.Designtheemergencyoverflowsystem.

Calculate the CPv (2-year, 24-hour storm), Qp2, Qp10, Qp25, Qp50, and Qp100flow rates and volumes.

Step7.Designembankment(s)andspillway(s).

Size the emergency spillway, calculate the 100-year water surface elevation, set the top of the embankment elevation, and analyze safe passage of the Qp100. Set the invert elevation of the emergency spillway 0.1 foot above the 100-year water surface elevation.

Step8.Investigatepotentialbasinhazardclassification.

The design and construction of the dry ED basin may be required to meet the Georgia Dam Safety standards.

Step 9. Prepare vegetation plan.

A vegetation scheme for the dry ED basin shall be prepared to indicate how the basin bottom, side slopes, and embankment will be stabilized and established with vegetation. The use of native vegetation is highly recommended for these facilities.




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6.2.16.3 Inspection,ProtectionandMaintenanceRequirementsAll IMPs require proper construction, protection, and long-term maintenance or they will not function as designed, and may cease to function altogether. The design of all IMPs includes considerations for maintenance and maintenance access. A legally binding IMP Maintenance Agreement shall be completed. For Augusta policies, additional guidance and forms pertaining to IMP protection, inspection, and maintenance requirements, see Chapter 8 of this Manual.

Requirements DURING Construction ` A dense and vigorous vegetative cover or other effective soil stabilization practice shall be established over the contributing pervious drainage areas before stormwater can be accepted into the dry ED basin. This will prevent sediment from accumulating in the basin.

Protection Requirements ` Providing signage for the IMP will:

` Allow for easy identification and location of the IMP

` Serve as a general education tool, making those responsible for property, landscape or IMP maintenance and the general public aware of the water quality features of the IMP and to avoid encroachment

` Design the layout of the dry ED basin such that maintenance access can be achieved without the need for vehicles or equipment in the stormwater treatment area.

Inspection RequirementsWhile AED requires an annual inspection, more frequent inspections help identify problems earlier and keep maintenance costs lower. The following are recommended frequent inspection items:

Dry ED Basin Routine Maintenance Activities and ScheduleActivity Schedule

` Remove trash, sediment, and debris from forebay and inlet and outlet structures.

` Mow the embankment and maintenance access. Periodically mow along maintenance rights-of-ways and the embankment. Remove grass clippings.

Monthly or as needed

` Repair and revegetate eroded areas.

` Remove and dispose of vegetation that may hinder the operation of the pond.

Seasonally

` Perform structural repairs to pond, outlet structures, embankments, control gates, valves, or other mechanical devices.

As needed (roughly every 20-50 years, but will vary based on the characteristics

of the drainage area and amount of sediment entering the practice)

For dry ED basin, inspections of the following elements are important for proper function of the IMP, and inspection frequency and diligence are critical.




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` Inspect the areas where stormwater flows into or out of the basin for clogging or sediment build-up.

` Inspect trees, shrubs and other vegetation to ensure they meet landscaping and vegetation specifications. Replace if necessary.

` Inspect the property that drains to the basin for erosion, exposed soil or stockpiles of other potential pollutants.

Maintenance RequirementsA dry ED basin provides temporary storage of stormwater runoff to control the peak rate of runoff by allowing the stored water to release slowly over a period of time. This practice is mostly used to control water quantity, although some water quality benefits can be obtained by the settling of floatables and sediment. This basin is designed to maximize water quality benefits.

There are some common problems to be aware of when maintaining a dry ED basin. They include, but are not limited to, the following:

` Sediment build-up

` Trash, litter, and debris accumulation

` Clogging in the inlet and outlet structures

` Erosion

` Structural repairs to inlets and outlets

` Clogging in the emergency spillway

` Mosquitoes breeding in the practice

Routine maintenance should be performed on a dry ED basin to ensure that the structure is properly functioning. Note: During the first year the dry detention basin is built, maintenance may be required at a higher frequency to ensure the proper establishment of vegetation in the practice. In the event of snow, check to make sure that the materials used to de-ice the surrounding areas stay out of the practice to avoid clogging and further pollution.

Inspect the dry ED basin after a large rainstorm. Keep drainage paths (both to and from the IMP) clean so that the water can properly flow into the basin. If the basin is not draining properly, check for clogging of the inflow and outflow structures.

If the forebay or basin has received a significant amount of sediment over a period of time, then the sediment at the bottom of the forebay or basin may need to be removed. Accumulated sediment in the practice decreases the available storage volume and affects the basin’s ability to function as it was designed.

Specifically, the following maintenance actions are of critical importance to the long-term performance of the IMP:

` Perform weeding, pruning, mowing, and trash removal as needed to maintain appearance and prevent obstacles to the intended drainage and maintenance of the ED basin

` Inspect vegetation to evaluate health and replace if necessary

` Keep inlets and outlets clear of debris to prevent clogging and clear if necessary


Version: July 20206.2.16- 10


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6.2.16 dry extended detention basin - augustaga.gov

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