Detention storage located in underground tanks or vaults ... · underground vault or tank being the component that is downstream in the treatment train. This will prevent the underground

c

Chapter 6.17. Underground DetentionCITY OF BIRMINGHAM POST CONSTRUCTIONSTORMWATER DESIGN MANUAL (ver. Oct. 2018)

Description:Detention storage located in underground tanks or vaults is designed

to provide water quantity control through detention and/or extended

detention of storm water. Variations:Detention vaults Detention tanks Detention chambers Detention vaults with pre-treatment

Key Advantages:

Ideal for highly urbanized areas where land is limited Can be used for storm water quantity control downstream of other runoff reducing or water quality treating GIPs Some designs require minimal drop between inlet and outlet Key Limitations:Not designed to provide storm water quality benefits unless designed in series with other GIPs and pre-treatment is incorporated Underground installation may make these systems difficult to maintain Performance dependent on design and frequency of inspection and cleanout of unit Some designs may require a confined space entry for maintenance and repairs

Suitable for this practiceProvides partial benefits

PERFORMANCE STANDARD COMPLIANCEWater QualitySt reamCh annel P rot ecti on O verb ank Fl oo

d P rot ecti on E xt remeFl ood P rot ecti onTreatmentVolume 80% TSS R emovalLevel 1 Level 2NotApproved NotApproved

Construction CostOperation andMaintenance CostGroundµLevelEncroachmentBuilding FootprintEnhancementTriple BottomµLineBenefits

Figure 6.17.1. Underground DetentionConstruction


6.17.1 General ApplicationDetention vaults are box-shaped underground storm water storage facilities typically constructed with

reinforced concrete. In contrast, detention tanks are underground storage facilities typically constructed with a

large diameter metal or plastic pipe. Both serve as an alternative to surface detention for storm water quantity

control, particularly for space-limited areas where there is not adequate land for a dry detention basin or

multi-purpose detention area.

Both underground vaults and tanks can provide channel protection through extended detention of the channel

protection volume (CPv) and overbank flood Qp25 (and in some cases protection for the extreme flood Qf).

Basic storage design and routing methods are the same for detention basins except that a bypass for high flows

is typically included.

Underground detention vaults and tanks are not intended for water quality treatment. Therefore, they shall be

used in a treatment train approach with other BMPs and GIPs that provide treatment of the WQv with the

underground vault or tank being the component that is downstream in the treatment train. This will prevent the

underground vault or tank from becoming clogged with trash or sediment and significantly reduces the

maintenance requirements for an underground detention system. It should be noted that some underground

detention vaults and tanks can be designed with integrated pre-treatment.

Prefabricated concrete vaults are available for commercial vendors. In addition, several pipe manufacturers have

developed packaged detention systems. Figure 6.17.2 and Figure 6.17.3 show example design schematics for

underground detention systems. Figure 6.17.2. Example Underground Detention Tank System (Source: WDE, 2000)


Figure 6.17.3. Example Underground Detention Tank System (Source: WDE, 2000)

The remainder of this page intentionally left blank.


6.17.2 Planning and Physical FeasibilityThe following feasibility issues need to be evaluated when underground detention is considered as the final

practice in a treatment train. The following criteria provided in Table 6.17.1 shall be considered when

evaluating the suitability of underground detention for a development site. Table 6.17.1. Underground Detention ConstraintsContributing DrainageArea Hotspot Land Uses Site Slope Site Layout25 acres max. No constraints, well#suited for hot spotrunoff. Underground detention may beinstalled on sites with slopes upto 15%. Adequate maintenanceaccess to each chamber shall beprovided for inspection andcleanout of undergrounddetention units.Not allowed in areas where operationmay create a risk for basement flooding,interfere with subsurface sewagedisposal systems, or affect otherunderground structures. Overflow shalldrain away from buildings to preventdamage to foundations.Soils Requirement Space Required Trout StreamsInfiltration andgeotechnical testsrequired. Structuralload bearing capacityof subsurface soilsshall be adequate tosupport the detentiondevice and stormwater runoff.

Undergrounddetention is installedunderground;therefore, minimalsurface area isrequired for thefacility.Not effective for protecting troutstreams due to potential forthermal impacts, suspendedsolids, or soluble pollutants.

The data listed below is necessary for the design of underground detention and shall be included with the

development plan. See Appendix B for more information on required elements for the development plan.

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

Impervious and pervious areas. Other means may be used to determine the land use data.

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

Design data from nearby storm sewer structures.

Infiltration testing of native soils at proposed elevation of underground detention.

Geotechnical and structural load bearing capacity of subsurface soils. 6.17.3 Design RequirementsThe designer shall calculate CPv, Qp25, and Qf, in accordance with the guidance presented in Chapter 4.

Then, the designer shall determine pretreatment volume. A separate sediment sump or vault chamber sized to

0.1 inches times the impervious acres of contributing drainage area shall be provided at the inlet for

underground detention systems that are in a treatment train with off-line water quality treatment GIPs.

Then, the designer shall design the emergency overflow system. Set up a stage-storage-discharge relationship

for the control structure for the 1-year, 24-hour rainfall event orifice. Size and determine the invert elevation of

the CPv orifice to ensure that the channel protection volume is stored for at least 24 hours within the

underground detention facility. Then, the designer shall set up a stage-storage-discharge relationship for the

control structure for the 25- and 100-year, 24-hour storm orifices to design embankment(s) and spillway(s).


Sizing of Underground Detention PracticesStormWater QualityAnother BMP or GIP shall be used in a treatment train with underground detention to provide storm water

quality as they are not designed to remove pollutants. In addition, some underground detention vaults and

tanks can be designed with integrated pre-treatment and manufacturer’s specifications shall be consulted.Required Geotechnical TestingSoil borings shall be taken in at least two locations within the proposed underground detention area. Soil boring

data is needed to (1) determine the physical characteristics of the excavated material, (2) determine its adequacy

for use as structural fill or spoil, (3) provide data for structural designs of the outlet works (e.g., bearing capacity

and buoyancy), and (4) determine compaction/composition needs for the embankment. Pre�TreatmentThe pretreatment/inlet system shall meet the following specifications:

A separate sediment sump or vault chamber sized to 0.1 inches times the impervious acres of contributing

drainage shall be provided at the inlet for underground detention systems that are in a treatment train with

off-line water quality treatment GIPs; and

For CPv control, a low-flow orifice capable of releasing the channel protection volume over 24 hours shall

be provided. The channel protection orifice shall have a minimum diameter of 3 inches and 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 (i.e., an over-perforated vertical stand pipe with 0.5 inches orifices or

slots that are protected by wire cloth and a stone filtering jacket). Adjustable gate valves can also be used to

achieve an equivalent diameter. Conveyance and OverflowThe outlet structures shall meet the following specifications:

For overbank flood protection, an additional outlet is sized for Qp25 control (based upon hydrologic routing

calculations) and can consist of a weir, orifice, outlet pipe, combination outlet, or other acceptable control

structure.

Riprap, plunge pools or pads, or other energy dissipators shall be placed at the end of the outlet to prevent

scouring and erosion.

A high-flow bypass shall be included in the underground detention system design to safely pass the extreme

flood flow (Qf). Internal Design FeaturesThe following criteria shall be considered minimum standards for the design of an underground detention

system:

Routing calculations shall be used to demonstrate that the storage volume is adequate.

Underground detention vaults and tanks shall meet structural requirements for overburden support and

traffic loading, if appropriate.


Safety features shall include the following:

Maintenance activities for an underground detention device may require a confined space entry.

Vaults that are greater than 4 feet deep shall be equipped with a safety ladder.

A minimum 20-foot wide maintenance right-of-way or drainage easement shall be provided for the

underground detention. Maintenance Reduction FeaturesGood maintenance access is needed so crews can remove sediments, alleviate clogging, and make repairs. The

following underground detention issues can be addressed during design in order to make on-going maintenance

easier:

Adequate maintenance access is required and shall have sufficient area to allow vehicles to turn around.

Access roads shall (1) be constructed of load-bearing materials or be built to withstand the expected

frequency of use, (2) have a minimum width of 12 feet, and (3) have a profile grade that does not exceed

15%. Steeper grades are allowable if appropriate stabilization techniques are used such as a gravel road.

A maintenance right-of-way or easement shall extend to the underground detention from a public or

private road.

Adequate maintenance access shall be provided for all underground detention systems. Access shall be

provided over the inlet pipe and outflow structure. Access openings shall consist of a standard frame, grate,

and solid cover or a removable panel.

Vaults with widths of 10 feet or less shall have removable lids.

Underground Detention Material Specifications

Minimum 3,000 psi structural reinforced concrete shall be used for underground detention vaults. All

construction joints shall be provided with water stops. Cast-in-place wall sections shall be designed as

retaining walls.

The minimum pipe diameter for underground detention tanks is 36 inches.

The maximum depth from finished grade to the vault invert shall be 20 feet. 6.17.4 Construction, Protection, and Maintenance RequirementsAll BMPs 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 BMPs includes considerations for maintenance

and maintenance access. A legally binding Inspection, Protection, and Maintenance agreement shall be

completed. For City policies, additional guidance and forms pertaining to BMP protection, inspection, and

maintenance requirements (see Appendix B of this manual). Requirements During ConstructionSimple erosion and sediment control measures, such as temporary stabilization, silt fencing, and inlet

protection, shall be used in areas of the site that drain to the underground detention.

A dense and vigorous vegetative cover, or other effective soil stabilization practice, shall be established over

the contributing pervious drainage areas before storm water can be accepted into the underground

detention. This will prevent sediment from accumulating in the underground detention.


Newly installed underground detention shall be inspected prior to being placed in service. Sediment and

debris that may have collected in the system during delivery and installation shall be removed. Protection RequirementsProvide signage for the BMP. o Allows for easy identification and location of the BMP. o Serves as a general education tool, making those responsible for property, landscape or BMP

maintenance, and the general public aware of the water quality features of the BMP and to avoid

encroachment.

Design the layout of the underground detention such that maintenance access can be achieved without the

need for vehicles or equipment in the storm water treatment area. Inspection RequirementsInspect the areas where storm water flows into or out of the underground detention for clogging or

sediment buildup.

Inspect the property that drains to the underground detention for erosion, exposed soil, or stockpiles of

other potential pollutants. Maintenance RequirementsRemove any trash, debris, and sediment buildup in the underground vaults or tanks.

Perform structural repairs to inlet and outlets, as needed.

Follow manufacturer's instructions for additional maintenance requirements.

Documents

Detention storage located in underground tanks or vaults ... · underground vault or tank being the component that is downstream in the treatment train. This will prevent the underground