On-site, underground stormwater retention /detention accomplishes the capture and storage of stormwater collected from surrounding impervious areas. Riser pipes or curb cuts lead surface storm water to subsurface vaults or systems of large diameter interconnected storage pipes or chambers. Stored water is then released directly through an outlet pipe back into natural waters at rates designed to reduce peak water flows during storms to mimic pre-development conditions. In some cases stored water can be allowed to infiltrate to recharge groundwater (if soil types are suitable and the groundwater table is located sufficiently below the water storage units).

Underground stormwater storage provides minimal stormwater quality benefits, but can be a successful segment to a development’s overall stormwater management plan, when coupled in-line with other stormwater BMPs. The addition of pretreatment features at the system’s inlet can facilitate improvements to water quality by removing floatables, skimming of oils and grease and trap some level of sediments through deposition. Pretreatment is most important if stored water is to be allowed to infiltrate into the soil, otherwise rapid clogging of the system could occur. Pretreatment features can be designed and built into the system or there are commercially available, prefabricated units that can be incorporated within the system during initial planning and design.

Typical Underground Stormwater Storage System
Source: Montgomery County, MD

Subsurface storage relies on construction of water storage structures made of concrete (vaults) or large diameter, rigid pipes or arches with capped ends and made of plastic, steel or aluminum. A number of pre-built, modular systems are commercially available (view some examples). Storage structures, inlet and outlet pipes and maintenance access (man holes) are fitted and attached in a predetermined excavated area and then the entire area is back-filled to surrounding landscape surface height with gravel and subsequently surfaced. Because of on going maintenance requirements and the potential of needed repairs at some later date, underground storage facilities should not be built over and preferably should be located in areas where large sized maintenance vehicles can easily operate and excavation remains possible, if required.

Applications

Underground storage is most often used in developments where land availability, shape and land costs predicate against the development of surface stormwater Best Management Practices (BMPs).

On-site storage attenuates peak stormwater flows from surrounding impervious surfaces and provides storage for future controlled release into surface waters. These systems should not be expected to substantially improve water quality unless coupled in-line with additional stormwater BMPs.

On site stormwater storage is ideal for use under parking lots, roadways and paved areas associated with commercial, industrial and residential developments. Advantages of use include:

1. Reduction of stormwater runoff flow.
2. Extended storage and slow, measured release of collected stormwater runoff.
3. Being a good option for high density or urban areas with limited available space or unusual shapes or where land is expensive.
4. System instillation can be accomplished rapidly using prefabricated modular systems.
5. Durability and long life (50 years plus for most systems).
6. Increased level of public safety over open ponds and other surface stormwater BMPs.
7. Insulation from freezing
8. Aesthetically pleasing to public in that such systems are out-of-sight and thus out-of-mind

Examples

Lot G behind the UMD Library produces stormwater that flows to the west branch of Tischer Creek – a designated trout stream. The lot was refurbished in 2004 to include an underground “vault” to temporarily store storm parking lot runoff.

Corner of Haines Rd and Miller Trunk Highway
(No other information at present)

Materials and Installation

1. Site Selection and Planning:
   • Check with local authorities regarding their design requirements and necessary permits for construction of underground storage. There is great variability between localities in requirements and permissible construction materials.
   • The use of experienced engineers, suppliers (if using a proprietary system) and installers is strongly recommended.
   • Placement of underground stormwater storage is site specific. During early site inspections, special note should be made of site size, shape and physical characteristics of the landscape. These factors will help determine basic structure of the detention system and what materials are best used in construction. 
   • The suggested maximum area of stormwater drainage to be collected for one underground storage system is 25 acres.
   • If underground stormwater storage is one part of a series (or train) of a comprehensive stormwater plan, the storage system can be first or last in line and positioning in train is usually dictated by local authorities. Other components of a development’s stormwater plan should be expected to improve water quality and the storage system should be expected to slow the flow of cleaned water into native systems.
2. Material Selection:
   • Materials for underground storage of stormwater include
   • Site-specific conditions that can influence material selection:
     • Depth and area of excavation- deeper and larger excavated areas require more fill for maintaining the integrity of plastic or metal pipe.
     • Shape of space- continuous space allows the use of concrete, while angular spaces favor the use of pipe systems. However:
       • Pipes store less water than square concrete vaults per cross sectional area.
       • Pipes require more fill than concrete structures, thus using more excavated area.
       • Use the largest pipe diameter possible. Doubling pipe diameter quadruples capacity and only doubles cost.
     • Depth of water table- a high, close to system water table can cause plastic pipe to float upwards.
     • Construction costs- locality will influence costs of materials and labor
3. Design:
   • Capacity and discharge rate will depend on local government stormwater requirements. Typical design requirements include:
     • System to result in reduction of stormwater flow rates to a rate that mimics pre- development conditions – no net increase in runoff for a design storm event in a newly developed area.
     • System required  to handle a particular size storm event over a particular length of time
   • Pipes and floors of vaults should be designed with a maximum of two percent slopes to facilitate drainage of water.
   • An Emergency overflow system should be designed to convey flows larger than can be handled by the storage system or to divert water in case system becomes inoperable for any reason.
   • Sufficient personnel access points (man holes) should be incorporated in design to facilitate easy maintenance. Placement should, at a minimum, occur near the intake and another at the outlet end of the system. The number depends on maintenance methods used.
   • Rip-rap at the outflow is required to reduce erosion.
4. Maintenance:
   • Periodic inspections of the inlet and outlet areas to ascertain correct operation of system and to clean materials trapped on grates protecting catch basins and inlet area should be required monthly.
     • Routine sweeping and cleaning of impervious drainage areas will reduce floatables and sediment loading to underground stormwater storage.
   • The primary maintenance concerns are removal of floatables that become trapped and removal of accumulating sediments within the system; this should be done at least on an annual basis.  Proprietary traps and filters associated with stormwater storage units should be maintained as recommended by the manufacturer.
     • Confined space safety procedures must be followed by workers entering an underground stormwater storage facility.
     • Sediments are best removed mechanically rather than flushing. If flushing is the only option then great care must be taken not to flush sediments downstream into native waters.
   • Any structural repairs required to inlet and outlet areas should be addressed in a timely manner on an as needed basis.
   • Local authorities may require annual inspection or require that they carry out inspections and maintenance.
5. Cost:
   • Underground stormwater facilities are more expensive than surface stormwater treatment methods. However in areas where land is expensive or in short supply underground storage is cost effective.
   • Cost is highly variable depending on:
     • Materials used
     • Storage volume required
     • Construction and labor costs
     • Site location
     • Physical conditions of site location
   • Reported price ranges of three dollars per cubic foot of water storage to $10 per cubic foot of water storage in plastic and metal pipes reflect price variability based on scale of projects, size of pipes that could be used, amount of excavation, fill amounts required and  Construction/labor costs.

Suggested References – Guidebooks, websites and pamphlets:

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1. The Urban Small Sites Best Management Practices (BMP) Manual (2003) by the Metropolitan Council of Minnesota’s Twin Cities [ 570 KB]
   Detailed information on 40 BMPs for stormwater pollution management with comments on their application in a cold-climate setting.
2. Stormwater Technology Fact Sheet-On-Site Underground Retention/Detention [ 4.3 MB] (September 2001) by USEPA. A strong overview on use of underground stormwater detention systems. Particularly good sections on material selection, variables impacting costs and operation/maintenance. Also provides a number of case studies.
3. Water Detention/Recharge Systems-Innovative Site Solutions & Stormwater Management (2002) [ 457 KB] by CONTECH CONSTRUCTION PRODUCTS INC. A good overview provided by a commercial supplier of steel pipe systems for stormwater detention and groundwater recharge. Contains link to useful software that offers help on underground stormwater design and a very useful chart on capacity of different sized corrugated steel pipes, spacing and depth of fill required above pipe. Also briefly introduces a proprietary stormwater treatment that can be used in conjunction with underground detention systems.
4. Subsurface Detention-A variety of devices and designs to temporarily hold runoff (July/August 2005) by Carol Brzozowski in Stormwater-The Journal for Surface Water Quality Professionals. A review of significant underground stormwater projects around the nation.
5. Maintenance Goes Underground: Owners and vendors talk about how to keep stormwater systems working by Carol Brzozowski in Stormwater-The Journal for Surface Water Quality Professionals (July/August 2004). A review of maintenance and its challenges of stormwater systems built underground.
6. Hospital Board Goes with Precast Concrete Boxes to Manage Stormwater (Winter 2005) [ 500 KB]
   by James Loaney in Concrete Pipe News-The Magazine of the American Concrete Pipe Association. A review-case study of underground stormwater storage using precast concrete boxes. Interesting because installed in a cold climate under winter conditions. (Page 6 and 7 of 16)

Tips and Wisdom

1. Selection of experienced engineers, suppliers (if using a proprietary system) and installers is highly recommended.
2. Acceptable for use in cold climates. Pipes and vaults can be insulated from freezing temperatures. Close attention to maintenance is extremely important in cold climate use, specifically:
   • Drainage area needs to be kept swept clean of sediments like sand. Attention to spring maintenance is especially critical.
   • Judicious use of sand to prevent slippery surfaces is required. Sand will wash into and accumulate in a holding system, the result being an increased number of costly maintenance visits to remove sediment build-up.
   • Use of deicer also requires judicious use. Water collected from a drainage area where deicer is in heavy use will leave detention system with high and unacceptable levels of chloride.  In any scenario where deicer is used, a stormwater BMP that addresses treatment of chloride should be installed to remove pollutant prior to entering native waters.
   For more information: Chapter 9 [ 1.4 MB] of the MN Stormwater Manual covers Cold Climate Impact on Runoff Management
3. Presence of non-porous clay soils dictates against the use of arch systems that allow stored stormwater to infiltrate into soil and eventually recharge groundwater. For more information: Chapter 13 [ 1.7 MB] of the Minnesota Stormwater Manual.
4. If infiltration into soil is possible, then consideration must be given to:
   • The depth of soil between the stored water and groundwater must be sufficient to treat and remove pollutants associated with the drainage area and that will be washed into the stormwater storage system.
   • The addition of water quality improvement BMPs to pretreat and remove pollutants from storm water prior to entering storage system.
5. Some soils have corrosive properties. Check/test local conditions to ensure the selection of metal pipe is compatible with local environment and offer the desired longevity of the system.
6. Remember that pipe systems can be organized to fit into unusual landscapes.
7. For ease of maintenance adequate access is important. Pipes should be a minimum of 72 inches in diameter

Limitations

1. Underground detention systems provide little or no water quality improvement. To achieve water quality improvement additional stormwater BMPs must be incorporated in-line with storage system.
2. Often require extensive, costly excavation.
3. High materials cost compared to surface storage methods
4. Allowing direct recharge of groundwater from underground stormwater storage units is not usually recommended. Exceptions can be made if:
   •  Pretreatment removes pollutants prior to entering a storage unit.
   • Soils are suitable for infiltration.
   • There is sufficient depth of soil between the storage tank and groundwater.
5. Maintenance costs are more expensive than those of surface located stormwater management practices.
   • Must be located in accessible areas where maintenance is possible
   • Special equipment (and access) might be required to perform routine maintenance
   • Noxious gasses may form in system requiring confined space protocols during inspections and maintenance.