Oregon Sea Grant - Drywells

by Maria Cahill, Green Girl Land Development Solutions; Derek C. Godwin, Oregon Sea Grant Extension; and Marissa Sowles, Oregon Sea Grant Extension
February 2011

ORESU-G-11-004
© Oregon State University
This publication may be photocopied or reprinted in its entirety for noncommercial purposes.

Drywells are vertically installed, underground infiltration facilities that receive runoff from impervious surfaces via buried pipes. Drywells reduce runoff flow rates, volumes, and temperature by infiltrating runoff into the surrounding soils (BES 2006). Drywells have limited storage capacity, but do store some runoff during a rainfall event. These systems provide fewer benefits than other low-impact development measures. They also tend to be one of the more expensive types of facilities to construct and are complex to permit. Because of these factors, they are generally chosen as a last resort when surface-infiltration facilities (such as rain gardens, planters, and porous pavements) either cannot be used or cannot be placed on the site in such a way that they reduce storm water volumes. By their design, drywells are underground injection control (UIC) devices and will trigger state UIC requirements. (See Permits section below.)

In Oregon, there are generally two kinds of drywells that are commercially available, both of which are perforated manhole rings. In other areas of the country, the information available suggests designs that don’t use a prefabricated chamber at all, but instead fill the entire vertical excavation with storage aggregate and line it with geotextile fabric on all sides. While this isn’t commonly seen in Oregon, it is allowed.

For runoff from large areas, a drywell is usually made from perforated concrete manhole rings installed vertically, 10 to 30 feet deep. Rings with a diameter of 4 feet are the most widely available size. A foot of clean, washed, storage drain rock is usually placed around the outside of the drywell as backfill. For runoff from small areas, a cost-effective variation of the concrete drywell is a mini drywell designed with polyolefin (a very hard plastic). These facilities measure about 2 feet in diameter and 2 feet in depth, and their small size and lightweight material makes them quite versatile. For both types, the drywell should be wrapped in nonwoven geotextile to prevent fine sediments from the surrounding soils (that is, silts and clays) from migrating into the system and clogging it.

Design

Throughout Oregon, drywells must be designed to infiltrate varying volumes of runoff. In Portland, where combined sewer overflows are the driving factor for infiltration requirements, the 10-year, 24-hour design storm is used, while in Eugene, the planners have determined that infiltrating the 5-year, 24-hour design storm will meet the city’s stormwater-management goals. Infiltration facilities are typically designed to capture and treat the stormwater runoff from surfaces draining to them during 80 to 90% of annual storm events, on average. This will often be a 1-inch, 24-hour design storm, but this volume probably will vary on the Oregon coast. For jurisdictions with no standards, the minimum infiltration volume should be chosen to protect scouring of the downstream natural waterways, which in Oregon has often been found to be the 1- to 2-year, 24-hour design storm (FWS 2006). In some cases, cities may require rain gardens to infiltrate larger storm events, especially where local soils drain well or where the drywell has no overflow. Check with your local planning department for specific design requirements for your area.

Plantings at the surface are acceptable, but they must allow access for maintenance (BES 2006) and do not provide any water-quality benefit, since runoff will be conveyed via a buried pipe underneath the plantings. Generally, uniformly graded drain rock with a minimum 30% void ratio is placed between the walls of the drywell and the filter fabric.

Sizing

Sizing of these facilities is based on the runoff volume they receive from both pervious and impervious surfaces. The amount of runoff routed to the drywell will depend on local rainfall patterns, the drainage area, and how much of the water runs off these surfaces. Because of western Oregon’s pattern of small, frequent storms, drywells are designed to drain within 24 to 36 hours. The volume of runoff that a drywell is capable of infiltrating depends on the depth and diameter of the perforated rings and the infiltration rates of soils surrounding the facility.

For the small, plastic drywells, installing 1 unit per 500 square feet of impervious area is recommended, but this suggestion may vary with the infiltration rate of the soil.

Typical drywell depths range from 2 to 20 feet. There are a few excavators who can install drywells as deep as 30 feet, but 20 feet tends to be the maximum constructible depth in most areas. If adequate disposal cannot be achieved by building the maximum depth of drywell constructible in your area, drywells can be linked together in series by piping. Consult a civil engineer to model the stormwater runoff for your area and provide recommendations for drywell diameter, depth, number, and spacing.

Soils

Because stormwater runoff is concentrated from a large area to a facility with a relatively small storage capacity, soils must drain well, and an infiltration test or bore log test should be submitted and approved by the local jurisdiction (BES 2008). Soils may vary with depth and must be tested in the location of the facility and in the different soil horizons, to ensure that the facility will function as designed. See the fact sheet “Infiltration Testing” for more information.

Routing

Different pretreatment mechanisms are required, depending on the runoff source, prior to entering the drywell. In most drywells, silt traps are suggested to prolong the life of the facility (BES 2008). For drywells collecting roof runoff, a silt basin is highly recommended, in addition to gutter screens to filter debris (NRCS 2008). For drywells capturing runoff from impervious areas other than residential roofs, employing proprietary treatment facilities with a filtering medium is a common pretreatment strategy (BES 2006). A filtration rain garden or flow-through planter could also serve this pretreatment need.

A bypass system is used for occasions when the drywell reaches saturation. In this case, an overflow pipe can deposit runoff onto a splash block (NRCS 2008). Alternatively, it can deliver the excess runoff to a secondary facility (SEMCOG 2008). Excess runoff should never be directed to neighboring private property without a utility easement, nor should it be allowed to back up the system. Figure 1 shows Portland’s standard residential installation, in which runoff from a large storm could back up and cause flooding. This flooding would occur at the point where the downspout connects to the drain pipe at finish grade, unless the rim of the catch basin, set lower than that elevation, is used as a large-storm relief valve.

Due to pipe cover requirements, the overflow pipe will often be too deep to daylight with these options and is connected to another pipe. An overflow pipe at the top of the facility or some other conveyance system should be used to convey water to an approved disposal point. An approved disposal point is usually a public or private storm pipe, or an outfall location that won’t impact structures and other property.

Setbacks

Setbacks can vary by jurisdiction. The City of Portland (BES 2008) requires a 5-foot setback from property lines and a 10-foot setback from building foundations. Additionally, drywells should be located 20 feet from cesspools and 500 feet from drinking wells (BES 2006, NRCS 2008, BES 2008). However, in eastern Oregon, where fractured bedrock can be shallow and many feet deep, drywells are often drilled into bedrock as long as the water table is still sufficiently deep to allow positive drainage out of the drywell.

Since the small, plastic drywells are not as deep as the concrete drywells, the City of Portland requires only an 8-foot setback from a building foundation rather than the standard 10 feet (BES 2008), and it’s possible that an even smaller setback is required. A common guideline to determine an appropriate setback distance for infiltration facilities in general is to assume that water moves equally horizontally as it does vertically (in other words, that it moves in a 1H:1V ratio). An appropriate setback can be calculated by ensuring that saturated zone of soil is outside the structural components of the building (see figure on previous page). Consult the state building codes and a geotechnical engineer to confirm how close your infiltration facilities can be located to critical infrastructure.

Physical Setting

Impervious surfaces of varying types are acceptable. Areas of caution are those with minimally infiltrating soils, wellhead protection areas, and wastewater drainage areas (BES 2006). Flat slopes are preferred, and slopes 20% or greater require geotechnical evidence prior to installation (NRCS 2008, PSMM 2008).

Since these are vertical systems with depths of up to 30 feet, they cannot be used in areas with high water tables. For roof runoff, the Oregon DEQ requires 5 feet between the bottom of a drywell and the top of the seasonal high groundwater table, and 10 feet for all other runoff.

Pollutant Removal

According to some sources, drywells provide nominal treatment, but infiltration of runoff can also mean groundwater contamination from pollutant runoffs (NRCS 2008). If the runoff is from vehicular areas, it is necessary to pair this facility with a secondary water-quality facility. Based on published research, the Center for Watershed Protection estimated that the total amount of phosphorus to be removed is 60% to 93%, and nitrogen to be removed is 57% to 92%. However, this and other water-quality benefits are not recognized by Oregon DEQ’s UIC Program. See Permits section below for more detailed discussion.

Construction

To prevent clogging, provide erosion-control measures such as a compost sock and diversion methods. You may also consider staging construction of the drywell in such a way that the surrounding land has been stabilized to prevent clogging (NJDEP 2004).

Construction of the drywell should disturb as little of the surrounding soil as possible. To avoid clogging the drywell during construction, at no time should runoff from other areas be directed to the drywell until those areas have been stabilized or fully constructed. Steps to install the drywell itself are as follows:

Inlet and outlet pipe installation can be done by the same means used to connect pipes to standard manholes. Surface backfill construction should follow project specifications.

Maintenance

The City of Gresham’s drywells have been cared for properly and are over 80 years old. It is unlikely that a facility can be repaired when it becomes clogged, so maintenance is very important. Maintenance activities include removing excess debris, controlling erosion and trash, and maintaining inlets and outlets. Vegetation that may clog inlets or outlets should be removed (BES 2006). When the facility clogs, it must be replaced (BES 2004). Inspections should occur frequently, declining in frequency with larger facilities (NRCS 2008). The design we recommend has a 3-foot-deep solid sump at the bottom to make suctioning excess sediment easier.

Permits

Drywells are considered a Class V Injection Well and must be approved by the Oregon Department of Environmental Quality (ODEQ) though the Underground Injection Control (UIC) permitting process. According to the U.S. Environmental Protection Agency, a Class V UIC well is, by definition, any bored, drilled, or driven shaft, or dug hole that is deeper than its widest surface dimension. These systems are regulated throughout the state because they don’t remove pollutants like rain gardens and swales do, and because they are much more likely to pollute groundwater because of their vertical nature. Special care must be taken to protect the groundwater by treating runoff first. For roofs, the ODEQ doesn’t require any treatment, but a silt basin that reduces sediment to protect the system against long-term clogging is recommended. In sensitive basins, additional treatment may be required since roofs may have a number of soluble pollutants, such as biological contaminants and hydrocarbons from asphalt shingles. Runoff from vehicular traffic requires more robust treatment.

In addition to the permitting paperwork, a long-term stormwater management plan must be developed, including a description of the best management practices for the entire site, spill prevention and response, a maintenance plan and schedule, and an employee training record. The plan must be revisited every 5 years, or immediately after a spill, and the drywell itself must be reevaluated. The ODEQ has very specific guidelines for pretreatment and other permitting requirements that you can find on their Web site (ODEQ 2010a).

It’s worth noting that “State rules prohibit the use of injection systems where better treatment or protection is available (for example, when a stormwater or municipal sewer is available), and must also meet the requirements of the state Groundwater Act (Div. 40). By policy, these services are considered available if the system is not at capacity or is within 300 feet of the site. ODEQ will confirm availability with the jurisdiction before allowing the use of injection systems” (ODEQ 2010b).

Most UICs are under rule authorization, not permits. Individual residential roof drywells are not required to register and get ODEQ approval as UICs, unless the city requests the review because of high groundwater concerns. Fourplexes, apartment houses, commercial, and industrial roof drains do need ODEQ approval though rule authorization. Usually no pretreatment is needed, as long as the discharge is only roof runoff. If the drywell serves a parking area, driveway, garbage bin, or loading dock, it needs to be registered and approved by ODEQ. Please note that cities and counties cannot approve UICs; only ODEQ can do this as the agency designated by EPA.

If a drywell is to be decommissioned, there is a separate rule-authorization process that includes testing, reporting, and additional paperwork.

Check with your local planning, engineering, or development services department for specific design requirements for your area.

Cost

Cost varies with the size of the facility, but in general, both construction and maintenance of small residential facilities costs between $1,200 and $1,500.1 The construction cost of larger concrete facilities is similar to the cost associated with installing a standard, deep manhole—around $10,000 to $15,000.

If runoff is from a surface other than a rooftop, a water-quality facility will be required for pretreatment. When drywells are used in locations where other surface infiltration (non-UIC) facilities cannot fit, proprietary treatment structures with filter cartridges must be used for water quality. These proprietary pretreatment systems range in cost from $6,000 to $25,000 installed, depending on the system and the amount of impervious area being treated. In addition, an annual maintenance contract to replace the filter cartridges, costing about $120 per year per cartridge, is also required.

Permitting costs for drywells vary from $100 to $300, and the cost of preparing the required permitting documentation can be as high as $2,000. Purchasing spill-response materials to be kept on site, and training in spill response measures for employees or maintenance staff, will also add to the cost.

New state regulations went into effect in September 2001, requiring numerous existing drywells around Oregon to meet the current standard (ODEQ 2010a) This regulation indicates that there will be no grandfathering of these systems and, if new rules are believed necessary to protect Oregon’s groundwater resources, there could be an unpredictable future cost associated with retrofitting.

References and Resources

Arnold, J. A. (ed.), D. E. Line, S. W. Coffey, and J. Spooner. 1993. Stormwater Management Guidance Manual. Raleigh: North Carolina Cooperative Extension Service and North Carolina Division of Environmental Management.

Oregon Association of Clean Water Agencies (ACWA). 2003. Underground Injection Wells for Stormwater, Best Management Practices. Retrieved from http://www.oracwa.org/Pages/UIC.pdf

Center for Watershed Protection (CWP) and Chesapeake Stormwater Network (CSN). 2008. Technical Memorandum: The Runoff Reduction Method. Ellicott City, MD.

Hicks, R. G., P. Curren, and J. R. Lundy. 1998, rev. 2003. Asphalt Paving Design Guide. Asphalt Pavement Association of Oregon.

New Jersey Department of Environmental Protection (NJDEP). 2004. New Jersey Stormwater Management Best Practices Manual. Retrieved from Web site: www.state.nj.us/dep/stormwater

Oregon Department of Environmental Quality (ODEQ). 2010a. Regulations. Water Quality, Underground Injection Control (UIC) Program. Retrieved from http://www.deq.state.or.us/wq/uic/regs.htm

Oregon Department of Environmental Quality (ODEQ). 2010b. Program Information. Water Quality, Underground Injection Control (UIC) Program. Retrieved from http://www.deq.state.or.us/wq/uic/uic.htm

Editing and layout by Rick Cooper; template design by Patricia Andersson.

© 2011 by Oregon State University. This publication may be photocopied or reprinted in its entirety for noncommercial purposes. To order additional copies of this publication, call 541-737-4849. This publication is available in an accessible format on our Web site at http://seagrant.oregonstate.edu/sgpubs/onlinepubs.html. For a complete list of Oregon Sea Grant publications, visit http://seagrant.oregonstate.edu/sgpubs

This report was prepared by Oregon Sea Grant under award number NA06OAR4170010 (project number A/ESG-07) from the National Oceanic and Atmospheric Administration’s National Sea Grant College Program, U.S. Department of Commerce, and by appropriations made by the Oregon State Legislature. The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of these funders.

Note: This is an accessible version of a document originally produced for the Web in .pdf format. While it contains all significant content of the original print document, it omits layout and graphic elements which contribute to the look and feel of the original, and make the .pdf version more suitable for printing.

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Drywells