Rain Gardens
Rain gardens are used to treat stormwater runoff by capturing runoff from impervious areas (i.e., roofs, driveways, parking lots, sidewalks, and patios) through sheet flow and stormwater conveyors. Rain garden is mostly recommended to be used in the environmentally sensitive areas (i.e., locations near rivers, lakes, beaches, and springs).
Content Table
- Rain Gardens
Introduction
A rain garden is a planted depression that allows rainwater runoff from impervious urban areas like roofs, driveways, walkways, and compacted lawn areas the opportunity to be absorbed. This reduces rain runoff by allowing stormwater to soak into the ground (from Wikipedia).
On the surface, a rain garden looks like an attractive garden. It may support habitat for birds and butterflies, it may be a formal landscape amenity or it may be incorporated into a larger garden as a border or as an entry feature. What makes it a rain garden is in how it gets its water and what happens to that water once it arrives in the garden.
There are two basic types of rain gardens – under-drained and self-contained. Both types of rain gardens are used to improve stormwater quality, reduce runoff volumes and generally facilitate infiltration of cleaned water. Which type of garden is selected to be built is a balance of volumes of water to be treated, existing soil conditions, available space, and budget for the project (from Rain Garden Design Template).
Rain gardens can be used everywhere human activity has hindered infiltration, and were vegetation would fit in. It is a flexible tool to both keep water for some time, and to create a more diverse landscape.
Mitigation Strategy
This flood risk mitigation measure is usually aiming to mitigate the first part of the rainfall, e.g., the first 25 mm. It will reduce the effect of pluvial floods. Rain gardens can often retrofit into already developed areas as an alternative to increase the capacity of storm severs. It can be located in gardens receiving precipitation from roofs, parking areas, at sag-curve along roads and other areas were impermeable surface will increase the runoff intensity.
This measure is predominantly aiming to reduce flooding risks at the head catchment, the rise of floods.
Time to Take Effect
The effect of rain gardens will start immediately after construction. As plant cover increase evapotranspiration increase in the summer season. Within 1-2 years full capacity could be reached. Properly maintained, it will never stop. If used as measure against diffuse pollution, the soil media may be changed after some time.
Pro & Contra
Pro
• Flexible tool, possible to retrofit many problem areas.
• Very popular in areas were RG are known.
• Inexpensive and easy to make on sandy and silty soils.
• Multifunction: Less runoff; some purification of particles, nutrients entering from paved areas; beautiful to look at; more diverse landscape; microhabitat for insects - food for birds?
Contra
• Rain gardens on impermeable ground like clay and shallow soil on mountain will need large area to function. The best way is to exchange clay with sand and drainage, however, this will increase the cost.
• Rain gardens shall not have permanent water table. If to slow infiltration/evotranspiration, mosquito may form.
• Maintenance needed: weeding; get rid of silt entering, change soil after some time if input water is very polluted.
Possible Construction Guidelines
Guidelines give different size (3-40% of catchment depending of soil type and depth of rain garden). A role of thumb could be to make the rain garden 7 % of the catchment on sandy soils. On clay soil are exchanged with sand and sandy soils and drained. In this way the RG is sandy. Depth depends on the size of water that we want to hold back. The RG needs to get the water out within hours to hinder death of some plants, and possible mosquito production. In the US one inch (25 mm) rainfall is an often used design criteria.
Rain gardens are easy to fit into the landscape. However, they should have some distance to houses with basement to mitigate water entering, e.g., 3-4 m.
Case sites
Norway
In the EU Interreg 4b project, SAWA, 4 raingardens have been constructed in Norway. We measure storm water input in two of the raingardens, and groundwater table in all. Our aim is to get information for construction of RG in the cold Nordic climate. Registration started in 2010. We will get back with results later.
Self-Contained Rain Garden (RG) on Sandy Moraine (Oslo, 220 m asl)
This RG was initiated and designed by Bent C. Braskerud, and built the summer 2006 when the family was excavating the basement for better drainage and heat insulation (This could be the first RG in Norway). The RG is rather small, only approx. 6 m2, compared to the runoff area of approx. 140 m2, 
which is the road to the house. It is located 8 m away from the house to hinder water infiltration into the basement, but only 1 m from the garage which have no basement. The RG is located on two properties. The neighbor was happy to contribute, even economically, as long as he did not need to maintain the RG. It took the excavator less then 2 hours to create the depression in the terrain needed, and make it good looking. Later Bent sowed grass on the berm and got some plants form a constructed wetland and a nursery (should have been more). He did not use mulch on the RG floor to prevent weeds. As a result, an undercover of grass, moss and garden weeds entered. It gives the RG a “wild” look, even though Bent does some weeding. The total cost was approx. 250 Euro. Hours for Bent are not included. The RG function well; a few times per year the storage capacity is overloaded. Infiltration of ca 15 cm water happed within few hours. The RG is irrigated as the ordinary flowering beds in dry periods.
Under-Drained Rain Garden on Clay Soils (Oslo , 90 m asl)
This RG was initiated by Bent C. Braskerud on his friend Eivind Fremstads garden after a flooding event in his basement in 2008. The exterior and 
plant choice was designed by landscape architect Elin T. Sørensen (City of Oslo). Bent did the technical design. Both Eivind and his wife were happy to try RG as an additional measure to keep water from the basement. As a SAWA project with 50 % refund, it was easy to try it out. The city of Oslo wanted to try this stormwater measure, and paied the last part. Eivind excavated his basement for better drainage, and let the outlets from the roof gutters enter a 7 m2 RG through pipes covered with 10 cm soil and grass. The roof is 100 m2. The RG was made the summer 2009, and is approx. 10 m from the house. Since the permeability of clay soils are very small, the original soil was replaced with sand and a top cover of sand, mature compost and 5 % of the top-soil in the garden. A drainage pipe goes from ca 15 cm over the RG floor down to 0.8 m and to a manhole were the water enters the storm water system. The drainage pipe capacity is reduced to approx. 4 l/min before entering the storm sewer. When the infiltration capacity of the RG is overloaded, some water will enter the drainage system from above, and infiltrate the soil from within. The small drainage capacity to the storm sewer prevents increased runoff to the network. Excess water will flow over the grass in the garden. Monitoring the ground water level and frequency of capacity overload in the RG will be done thought the SAWA project. The total cost was 5800 Euro and is rather high. 60 % of the cost was the pipe connection from roof to RG, and pipe from RG to storm sewer. Elin, Bent and Eivinds labour hours is not included.
Under-Drained Rain Garden on Clay Soils (Trondheim , X m asl)
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Under-Drained Rain Garden on Silty Clay Soils (Melhus , 67 m asl)
Rolf A. Grande is a senior citizen and member of the local horticultural society in Melhus. He is very interested in all sort of plants. When invited to make a raingarden through the SAWA project, he was very enthusiastic. The soil was silty clay, so drainage was needed. He dug out a wide 1 meter deep ditch for the drainage pipe by hand. A sand layer covered the bottom 20 cm before a fiber cover and the original soil was filled back. The top 20 cm was exchanged to a sandy soil type. The RG is approx 2.6 m2, and the roof surface 90? m2. The cost was 1200 Euro, incl. pipes from the house and drainage, labor (Rolf made the RG in 38 hours), but not plants.
Further Reading
1.) Rain Garden Design Templates (Low Impact Development Center)
2.) Rain Gardens ; A how-to Manual for Homeowners
3.) Guidance on the Permeable Surfacing of Front Gardens (Environment Agency, UK)
5.) Rain Gardens: Managing Roof Runoff in your Backyard
6.) Adding a Rain Garden to your Landscape
7.) Bioretention (Rain Garden) (U.S. Environmental Protection Agency)
8.) Rain Garden Keeps Rivers Clean
9.) How Can Create Rain Garden
Related Article
References
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Links
Rain Garden Network
Clean Water Campaign: Rain Garden
Rain Garden Alliance
