Swales, Drains, and Site Grading for Stormwater Control

Scope Images
Swales reduce and filter stormwater runoff.
Swales reduce and filter stormwater runoff.
Scope

Install swales and drains and grade the site to control stormwater runoff on site and to prevent soil saturation around the foundation.

  • Slope surface to grade away from the house.
  • If site grade is level or directed toward the house and if space is available on the site, install swales to capture, control, and filter stormwater runoff, allowing it to percolate into the soil on site. Swales should have a 3-to-1 width-to-height ratio, a slope along the longitudinal channel of 2% to 4%, and be large enough to accommodate the volume of runoff from a 6-month, 24-hour storm event.
  • If site grade is level or directed toward the house and space on site is limited, install drains to collect and direct water via underground French drains to drywells or off site to the sewer or to community swales or retention ponds.

See the Compliance tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS. 

Description

A few inches of rain falling on a typical suburban lot can produce 9,750 gallons of water runoff (HUD 2019, EPA 2018). This runoff must be channeled away from the building foundation to keep the basement or crawlspace dry and to prevent water from seeping into the building interior where it may create moisture problems. Water management is an important part of the design and construction of the home. Keeping water from entering the house is critical to ensuring the long-term structural integrity of the building. Water saturation of foundations and siding can lead to mold problems, as well as poor indoor air quality.

The ground around the house should be graded to slope away from the house, as described in the Building America Solution Center guides Final Grade Slopes Away from Foundation and Patio Slabs, Porch Slabs, Walks, and Driveways Slope Away from House. Other solutions are available for sites that are level or that have unavoidable slope toward the house. Or, if there are municipal requirements in place to retain stormwater runoff on site. Swales can be constructed to provide a means to drain stormwater from the site’s surface. Drains can also be installed to handle higher volumes of surface water. More information about onsite water drainage can also be found in the Solution Center guide Gutters and Downspouts. Information about minimizing erosion on site can be found in the guide “Erosion Control for Slopes, Stream Banks, and Dunes.”

Swales

A swale is a trapezoidal channel that is dug to receive storm water overflow, allowing it a path to flow away from the home (Figure 1). Swales can provide a means to slow water runoff and allow natural percolation into the soil on site. They are typically located along property boundaries or at the base of a natural grade. They will also draw in water that would otherwise sit in a flat yard and they can divert water flowing toward the house from an upward sloping yard. Swales are typically planted with specific types of vegetation to reduce erosion, to help remove any pollutants found in the storm water runoff, and to improve the aesthetics of the design.

Swales are troughs that collect site stormwater and filter it with vegetation, soil, and gravel layers allowing it to percolate slowly into the soil
Figure 1. Swales are troughs that collect site stormwater and filter it with vegetation, soil, and gravel layers allowing it to percolate slowly into the soil. (Source: City of Binghamton NY 2016).

The installed vegetation should adequately remove and retain heavy metals. Examples of vegetation appropriate for use in swales include reed canary grass, grass-legume mixtures, and red fescue. The swale should be large enough to comfortably divert the volume of precipitation (rain fall or snowmelt runoff) expected for a 6-month-frequency, 24-hour storm event. The sides of the swale should have a slope no steeper than 3 to 1, and the first 8 inches of the soil should drain well (Figure 2) (Enterprise Communities 2019).

Swales can be installed in combination with berms across slopes to slow the descent of stormwater and reduce erosion (Figure 3). For faster drainage where large water volumes are expected, a French drain can be installed at the base of the swale to direct water to a drywell or a community retention pond, or to a lower part of the site (Figure 4). While each site is unique, the following steps provide general guidance for constructing a swale to divert water away from the house (EPA 1999).

Swales are trapezoidal channels dug to receive storm-water overflow, with specific vegetation planted to improve aesthetics, filter stormwater runoff, and prevent erosion
Figure 2. Swales are trapezoidal channels dug to receive storm-water overflow, with specific vegetation planted to improve aesthetics, filter stormwater runoff, and prevent erosion. (Source: PNNL).

How to Construct a Swale

  1. Do not construct vegetated swales in gravelly and coarse sandy soils that cannot easily support dense vegetation.  If available, use alkaline soils and sub-soils to promote the removal and retention of metals. Use non-settling compact soils, as determined by a certified hydrologist, soil scientist, or engineer. Know that the following factors will decrease the effectiveness of a swale: compacted soils, large storm events, frozen ground, short grass heights, steep slopes, high runoff velocities, and high discharge rates (EPA 1999).
  2. Do not compact the soil in the swale trench. Soil infiltration rates should be greater than 0.2 millimeters per second (one-half inch per hour); therefore, care must be taken to avoid compacting the soil during construction (EPA 1999).
  3. Choose plants that will aide in filtration. A fine, close-growing, water-resistant grass should be selected for use in vegetated swales because increasing the surface area of the vegetation exposed to the runoff improves the effectiveness of the swale system. Pollutant removal efficiencies vary greatly depending on the specific plants involved so the vegetation should be selected with pollution control objectives in mind. In addition, care should be taken to choose plants that will be able to thrive at the site. Examples of vegetation appropriate for swales include reed canary grass, grass-legume mixtures, and red fescue. The best plants to use in a swale will depend on the region and location. Local cooperative extension programs based at land grant universities often have a list of plants that will work well in a swale (EPA 1999).
  4. Construct the swale in a parabolic or trapezoidal cross-section with side slopes no steeper than a 3:1 width-to-height ratio. The overall slope of the swale (the longitudinal channel or from water-entry point to water exit point) should slope consistently between a 2% and 4% grade. Slopes less than 2% can lead to pooling; slopes greater than 4% can lead to excessive water velocity and contribute to soil erosion (West Virginia 2012).
  5. Build the swale large enough (deep and wide) to comfortably divert the volume of water occurrence (rain fall or snowmelt runoff) expected for a 6-month-frequency, 24-hour storm event. To determine the rain volume for this type of storm event in your location, contact the U.S. Geological Survey. Swales are generally not used where the maximum flow rate exceeds 140 liters/second (5 cubic feet per second) (EPA 1999).
  6. Schedule a site visit to fill in soil and conduct final grading after settling has occurred (e.g., after the first rainy season) (EPA 1999).
. A swale and berm can be installed together across a slope to slow the downhill flow of water.

Figure 3. A swale and berm can be installed together across a slope to slow the downhill flow of water. (Source: Enterprise Community Partners 2019).

An engineered bioswale uses perforated pipe laid in rock and landscape fabric at the bottom of a vegetated trench to direct water away from a site.

Figure 4. An engineered bioswale uses perforated pipe laid in rock and landscape fabric at the bottom of a vegetated trench to direct water away from a site. (Source: PNNL).

Drains

Drains can be installed to collect and direct surface water away from the foundation. Metal grated drains can be installed at the base of downward-sloped driveways or walkways. Drain pipe may be installed under pavers, stone, turf, or mulch at low points in the yard where drainage is needed to reduce surface ponding. This type of underground drainage is often referred to as a French drain (Figure 6 and Figure 7). It consists of a perforated pipe that is wrapped in rock and landscape fabric. The perforated pipe carries the collected stormwater to a lower part of the site or to a drywell for below-ground dispersal or off site to the stormwater sewer system or to a community retention pond. If the drain empties to daylight, it should terminate onto a sloped grade that directs water away from the building. A splash block or stones placed beneath the pipe opening or a sprinkler head to disperse outflow will help control erosion at this location (EPA 2012). Many communities have ordinances in place directing that stormwater be handled on site.

The French drain should be located 18 or more inches below ground and have a positive slope toward the drain outlet, which could be underground to a drywell, or a storm sewer, or to a downslope location on the site. Smooth PVC pipe is preferable to corrugated pipe, which is more likely to collect sediment and clog. See the following steps for constructing a French drain.

A French drain contains a perforated drain pipe wrapped in rock and landscape fabric. (
Figure 5. A French drain contains a perforated drain pipe wrapped in rock and landscape fabric. (Source: FEMA 2013).
This double French drain provides drainage for a significant volume of storm water.
Figure 6. This double French drain provides drainage for a significant volume of storm water. (Source: Enterprise Community Partners 2019).

How to Install French Drains

  • Dig trench at least 22 inches deep at minimum and 14 inches wide. Slope away from house toward termination point. Trenchers do not typically dig a wide enough trench. If you use a trencher, widen the trench with a shovel.
  • Lay 8-oz landscape filter fabric in bottom of trench. Use fabric wide enough to extend past sides of trench on each side.
  • Lay 4 inches of clean rock (¾ to 2-inch) in the base of the trench.
  • Lay4-inch perforated PVC pipe, perforations down, in trench. Noncorrugated pipe is preferred. Or, install perforated pipe that comes prewrapped in filter fabric, or prewrapped in a filter fabric “sock” filled with Styrofoam packing peanut “rocks.”
  • Fill in around and above the pipe with more clean rock to a depth of 4 to 6 inches over the pipe.
  • Pull the sides of the fabric over the pipe, lapping one side over the other.
  • Top with 9 inches or more of more rock or stone or topsoil or landscaping mulch.
  • Avoid planting shrubs or trees over or near the drain pipe.
A drywell, shown here used for downspout catchment, can also be used to receive water from a French drain.
Figure 7: A drywell, shown here used for downspout catchment, can also be used to receive water from a French drain. (Source: EPA WaterSense).

How to Install a Drywell

  1. Dig a 4 ft by 4 ft wide by 4 to 6 ft deep hole.
  2. Line the hole with landscape filter fabric.
  3. Fill with clean ¾” to 1.5” rock (no fines). Tamp down the rock every few inches.
  4. When the hole is half full, lay perforated pipe to enter the well area at half the height or higher.
  5. Fill the remainder of the well with rock.
  6. overlap side pieces of landscape fabric to cover the top of the rock. Add additional landscape fabric to cover the entire top with 6 inches of overlap. Cover with at least 8 inches of dirt flush with grade.
  7. If using a premade plastic perforated drywell, dig the hole at least 16 inches wider than the plastic drywell, line with fabric, fill with 6 inches of rock, install drywell, insert pipe, continue adding rock to surround drywell, install lid on drywell.
Ensuring Success

Follow local ordinances regarding stormwater runoff.

Climate

The following are some climate- and region-specific factors to consider:

  • In areas with potentially high winds and heavy rains, increase gutter and rain leader capacity to accommodate heavy rain and large roof areas. Use metal rather than vinyl gutters and downspouts in areas susceptible to forest fire (Baechler et al. 2010).18
  • State Runoff Limitations: Many states regulate offsite runoff draining through a site. Retention systems such as swales are an approved way to return runoff to its original watershed (FDOT 2017).
Annual mean precipitation across the U.S.
Figure 1. Annual mean precipitation across the U.S. (Source: NOAA 2017).
Regions susceptible to hurricanes are subject to heavy rains and an increased reliance on stormwater mitigation systems
Figure 2: Regions susceptible to hurricanes are subject to heavy rains and an increased reliance on stormwater mitigation systems. (Source: FEMA 2019).

 

Videos
Publication Date
Author(s)
American Concrete Institute
Organization(s)
ACI
Description
Video from American Concrete Institute explaining the differences between foundation construction strategies and heights to mitigate damage from flooding.
Publication Date
Author(s)
Milwaukee Metropolitan Sewerage District
Organization(s)
MMSD
Description
Video from Milwaukee Metropolitan Sewerage District on properly grading the site surrounding a home to control erosion.
CAD

Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

U.S. Environmental Protection Agency (EPA) WaterSense® New Home Specification

The EPA WaterSense New Home Specification states that:

Slopes in excess of 4 feet of horizontal run per 1-foot vertical rise (4:1) shall be vegetated (EPA 2014).

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

Water Management System Builder Requirements

1. Water-Managed Site and Foundation.
1.2 Back-fill has been tamped and final grade sloped ≥ 0.5 in. per ft. away from home for ≥ 10 ft. Alternatives in Footnote.2

Footnote 2) Swales or drains designed to carry water from the foundation are permitted to be provided as an alternative to the slope requirements for any home, and shall be provided for a home where setbacks limit space to less than 10 ft. Also, tamping of back-fill is not required if either: proper drainage can be achieved using non-settling compact soils, as determined by a certified hydrologist, soil scientist, or engineer; OR, the builder has scheduled a site visit to provide in-fill and final grading after settling has occurred (e.g., after the first rainy season).

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.

EPA Indoor airPLUS (Revision 04)

Certified under ENERGY STAR Qualified Homes Version 3 or 3.1. 
No additional Indoor airPLUS requirements. 

 

2009, 2012, 20152018, and 2021 IRC

Section R401.3 Drainage. This requires that surface water should drain to a storm sewer conveyance or another approved collection point that doesn’t create a hazard. Lots must be graded to drain water away from the foundation walls and the grade should fall at least 6 inches within the first 10 feet. If physical barriers such as walls, slopes, or the lot lines limit the ability to achieve 6 inches of slope within 10 feet of the foundation, then drainage can be achieved by installing drains or swales. 

P3302.1 Subsoil Drains: Subsoil drains shall be open-jointed, horizontally split or perforated pipe conforming to one of the standards indicated in Table P3302.1. Such drains shall not be less than 4 inches (102 mm) in diameter. Where the building is subject to backwater, the subsoil drainage shall be protected by an accessibly located backwater valve (2018 IRC).

Section R405.1. Footing drains of drain tile, gravel, crushed stone, perforated pipe, or some other approved systems must be installed at the footing perimeter of any building with concrete or masonry foundations that has a basement. The drainage pipe must sit on a bed of gravel 2 or more inches thick that extends at least a foot from the edge of the footing. The drain pipe should be covered with an approved filter fabric then be topped with 6 or more inches of washed crushed gravel or crushed rock. This drain pipe must discharge by gravity or mechanical means into an approved drainage system. A drainage system is not required when the foundation is installed on well-drained ground or sand-gravel soils designated as Group 1 soils by the United Soil Classification System.

Section R408.6. Finished Grade, permits the finished grade of the crawlspace floor to be level with the bottom of the foundation wall footings. However, if there is evidence that the groundwater table can rise to within 6 inches of the finished floor of the building perimeter or if there is evidence that surface water is not readily draining from the building site, then level of the crawlspace dirt floor must be as high as the outside finished grade, unless an approved drainage system is provided.

Retrofit: 

2009, 2012, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

Existing Homes

Once a home has been constructed and concrete driveways and patios are poured, it is a difficult and expensive process to regrade the site should water issues arise due to negative slope on one or more sides of the home. Other options for dealing with water that wants to flow toward the house are to install footing drains if they don’t currently exist and to install damp proofing and/or dimpled plastic moisture barrier on the exterior surface of the below-grade walls. Swales or other drainage systems can be constructed as described in the Description tab to carry water away from the home and to a drywell, stormwater sewer, or other drainage location downstream of the home. Drains can be installed at the base of driveways and patios that slope toward the house. Solid-surface driveways, patios, and walkways can be replaced with pervious surfaces such as pavers, gravel, or pebbles, to allow water to drain into the ground rather than flowing toward the house. Sometimes a combination of measures is needed to keep the basement or crawlspace dry.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

References and Resources*
Author(s)
U.S. Environmental Protection Agency
Organization(s)
EPA
Publication Date
Description
Document describing correct construction of swales, designed to trap particulate pollutants (suspended solids and trace metals), promote infiltration, and reduce the flow velocity of storm water runoff.
Author(s)
Coulbourne,
Jones,
Durham,
Kapur,
Koumoudis,
Line,
Low,
Overcash,
Passman,
Reeder,
Seitz,
Smith,
Tezak
Organization(s)
FEMA
Publication Date
Description
Volume 1 of a two-volume report providing a comprehensive approach to planning, siting, and risk management for homes constructed in coastal environments.
Author(s)
Enterprise Community Partners
Organization(s)
Enterprise Community Partners
Publication Date
Description
A guide providing tools to island and hurricane-prone communities on redevelopment and rehabilitation of homes to prepare for future natural disasters.
Author(s)
University of Wisconsin Sea Grant Institute,
U.S. Army Corps of Engineers
Organization(s)
University of Wisconsin Sea Grant Institute,
U.S. Army Corps of Engineers
Publication Date
Description
A publication describing how to protect coastal investments by adapting to natural processes and protecting coastal environments and shorelines.
Author(s)
Florida Department of Transportation
Organization(s)
FDOT
Publication Date
Description
Handbook describing the permitting process regulating the transfer of stormwater from private property owners to the State of Florida Department of Transportation stormwater drainage systems.
Author(s)
NOAA Office for Coastal Management
Organization(s)
NOAA
Publication Date
Description
Case study describing Alachua County, Florida’s, green infrastructure strategy for land use development that incorporates rain gardens, ponds, swales, and native vegetation to manage stormwater.
Author(s)
City of Binghampton
Organization(s)
City of Binghampton
Publication Date
Description
Brochure defines bioswales and various design considerations regarding the creation of a bioswale, such as slope, width, location, and types of native plants.
Author(s)
Mid-America Regional Council
Organization(s)
EPA
Publication Date
Description
A guide on the use of rain barrels for greater water efficiency and use in gardens and lawns.
Author(s)
Environmental Protection Agency
Organization(s)
EPA
Publication Date
Description
Indoor airPLUS is a voluntary partnership and labeling program that helps new home builders improve the quality of indoor air by requiring construction practices and product specifications that minimize exposure to airborne pollutants and contaminants.
Author(s)
U.S. Environmental Protection Agency
Organization(s)
EPA
Publication Date
Description
Website providing the technical specifications and related documents for home builders, subcontractors, architects, and other housing professionals interested in certifying a home to the EPA's Indoor airPLUS program requirements.
Author(s)
Jack Spirko
Organization(s)
Permaculture Research Institute
Publication Date
Description
Article describing the difference between swales and hugels (berms used to create soil from woody debris).
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Pacific Northwest National Laboratory

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