Rain Gardens, Bioretention Cells, Bioswales, and Hedgerows
Effective landscape techniques that beautifully put plants to work for you



LID before and after.jpg

Example of a yard before and after installation of a raingarden. 

What is a rain garden? 

A rain garden is a landscaped depression in the land, with specially design soil that collects rainwater from a roof, driveway or other impervious surfaces and allows it to soak into the ground. Planted with grasses and flowering perennials, shrubs and trees, rain gardens can be a cost effective and beautiful way to keep water on your site where it is useful. 

Bioretention Cells, Bioswales, and some Hedgerows are sometimes referred to as "Rain Gardens", because they are basically doing the same thing in different situations. For example, a "Bioswale" is basically a kind of rain garden where water is slowed and filtered, but much of the water is directed to another location.

Bioretention is a more complex rain garden with drainage systems and amended soils.  

Hedgerows can be used as an edge to a Bioswale or as check dam to slow surface water as it heads down slope. 

rain-garden graphic.jpg



A vegetated or grassed swale is an area with dense vegetation that retains and filters the first flush of runoff from impervious surfaces. It is constructed downstream of a runoff source. After the soil-plant mixture below the channel becomes saturated, the swale acts as a conveyance structure to a bioretention cell, wetland, or infiltration area.

There is a range of designs for these systems. Some swales are designed to filter pollutants and promote infiltration and others are designed with a geo-textile layer that stores the runoff for slow release into depressed open areas or an infiltration zone.

Alternative Devices: Filter strip or vegetated buffer.

Typical Uses: Edges of paved areas (roads or parking lots), parking lot islands, intermediary common spaces, open space, or adjacent to buildings.

Land Use: Commercial, industrial, residential (urban, suburban, ultra-urban); transportation projects (highway medians and rail projects); new construction and retrofit projects.

Approximate Cost: $0.25 per square foot for construction only; $0.50 per square foot for design and construction.

Maintenance: Routine maintenance is required. Maintenance of a dense, healthy vegetated cover; periodic mowing; weed control; reseeding of bare areas; and clearing of debris and accumulated sediment.

Additional Benefits: Easily customized to various projects (size, shape, and depth) and land uses; enhances aesthetic value of site; uses small parcels of land, easements, right-of-ways; easily retrofitted into existing buildings/open space.

Design Specs and Supplementary Information:


Disconnecting impervious areas is a fundamental component of the LID approach. Roofs, sidewalks, and paved surfaces are disconnected from each other to allow for more uniform distribution of runoff into pervious areas. Conveying runoff into vegetated areas keeps the water from directly entering the storm drain network, reduces runoff volume, and promotes distributed infiltration.

Since paved surfaces make up a large portion of the urban (or developed) landscape, the use of permeable pavement is very effective at stabilizing the hydrologic condition of a site. Permeable surfaces can be used in conjunction with subsurface infiltration galleries (subsurface retention facilities) as seen in Section 6.

A secondary benefit of permeable paving is its performance in snowy conditions. Cahill Associates reports an increase in demand for the installation of permeable asphalt in the Northeast as a result of reduced maintenance costs (snow shoveling and desalting) due to rapid snowmelt on permeable surfaces.

Types of permeable pavement include permeable asphalt, permeable concrete, grid block pavers, plastic grids, vegetated grids, Belgium block (in photo), turf block, gravel, cobbles, brick, natural stone, etc.

Figure 20: Belgium block pavers in parking bays
Courtesy The Low Impact Development Center

Figure 21: Permeable parking bays
Courtesy Cahill Associates, Inc.

Typical Uses: Parking bays, parking lanes, sidewalks, roads. Blocks and porous pavement are generally used in high traffic parking and roadway applications; respectively grid systems are more commonly used in auxiliary parking areas and roadways.

Land Use: Ideal for commercial, industrial, and residential (urban, suburban, ultra-urban); suitable for new construction and retrofit projects.

Approximate Cost: Varies according to product. Typically, the cost is higher than conventional paving systems; however, they help reduce the overall storm water infrastructure costs.

Maintenance: Varies according to product. Routine street sweeping will sustain the infiltration capacity of voids. Porous concrete/asphalt require annual vacuuming, to remove accumulated sediment and dirt.

Additional Benefits: Easily customized to various projects and land uses; enhances aesthetic value of site; easily retrofitted into existing paving configurations.

Design Specs and Supplementary Information:


Subsurface retention facilities are typically constructed below parking lots (either permeable or impervious) and can be built to any depth to retain, filter, infiltrate, and alter the runoff volume and timing. This practice is well suited to dense urban areas. Subsurface facilities can provide a considerable amount of runoff storage.

Figure 22 shows that the porous parking bay has an infiltration gallery (with 40% void space) below it for storm water retention. The water is filtered through the stone aggregate and infiltrates into the ground. An alternative strategy is to construct the subsurface facility with a filtering and pumping mechanism so that collected water can be reused for non-potable uses such as irrigation or flushing of toilets.

Figure 22: Cross section of porous asphalt pavement
Courtesy Cahill Associates, Inc.

Similar techniques include gravel storage galleries, sand filters, infiltration basins, and infiltration trenches (for areas with space constraints).

Typical Uses: Parking lots, sidewalks, and roads.

Land Use: Ideal for commercial, industrial, and residential (urban, suburban, ultra-urban); suitable for new construction and retrofit projects.

Approximate Cost: Costs are typically higher than conventional paving systems; however, they help reduce the overall storm water infrastructure costs (land allocated for ponds, cost of pipes, inlets, curbs/gutters).

Maintenance: Varies according to manufacturer; routine street sweeping and vacuuming will retain infiltration capacity of voids.

Additional Benefits: Easily customized to various projects and land uses; enhances aesthetic value of site; easily retrofitted into existing paving configurations.

Design Specs and Supplementary Information: These are specialized systems and should be designed by, or under the direct supervision of, an appropriate licensed professional.

The reduction of street widths (i.e., from 36' to 24') can result in a cost savings of approximately $70,000 per mile in street infrastructure costs (estimated paving cost = $15 per square yard).

Land Use: Residential, commercial, industrial.

Design Specs and Supplementary Information:

Figure 23: Reduced road widths and vegetated swales
Courtesy Pierce County, Washington and AHBL, Inc.


Tree box filters are essentially 'boxed' bioretention cells that are placed at the curb (typically where storm drain inlets are positioned). They receive the first flush of runoff along the curb and the storm water is filtered through layers of vegetation and soil before it enters a catch basin. Tree box filters also beautify the streetscape with landscape plantings such as street trees, shrubs, ornamental grasses, or perennials and can be used to improve the appearance of an area or to provide habitat.

Typical Uses: Positioned along the curb of a street; particularly effective at targeting point source pollution in urban areas by retrofitting/ replacing existing storm drains.

Land Use: Commercial, residential (urban, suburban, ultra-urban), and industrial areas.

Approximate Cost: Approximately $6,000 per unit per 1/4 acre of impervious surface. This estimate includes two years of operating maintenance and filter material and plants. Additional costs include installation and annual maintenance. Installation is approximately $1,500 per unit (varies with each site).

Maintenance: Tree box filters require more specialized maintenance to ensure filter media is not clogged and there is no accumulation of toxic materials, such as heavy metals. Maintenance is typically performed by Departments of Transportation or agencies responsible for storm drain maintenance. Annual manufacturer maintenance is $500 per unit; owner maintenance costs are approximately $100 per unit.

Additional Benefits: Improves water quality and enhances the community.





Bioretention Cells
Rain Gardens

LID rain garden diagram.jpg

 Rain gardens are designed to be self-sufficient.

Some weeding and watering will be needed in the first two years, and perhaps some thinning in later years as the plants mature, but a well-planned raingarden can be maintained with little effort after the plants are established.

A bioretention cell (strip or trench) is an engineered natural treatment system consisting of a slightly recessed landscaped area constructed with a specialized soil mixture, an aggregate base, an underdrain, and site-appropriate plant materials that tolerate both moist and dry conditions. The site is graded to intercept runoff from paved areas, swales, or roof leaders. The soil and plants filter and store runoff, remove petroleum products, nutrients, metals, and sediments, and promote groundwater recharge through infiltration. The cells are designed to drain in 24 hours, with no risk of standing water and breeding of mosquitoes.

The Solution

Slow the flow of water, so it can be soaked into and filtered through the soil to clean it before it gets to our waterways

Here's how it works...