Dig It: Digging tips for your Rain Garden

Before Digging

            There are a couple steps you should keep in mind before you start. If you have currently have grass in that area, it would be easier to kill it first. You can either break it apart manually, or cover the space until the grass dies. The other important thing to consider is whether there are utilities running through that sector of the yard. It is much easier to call the municipality and check that there are no lines running through your property than pay for damages. 

To construct the garden, you will need the following: a tape measurer, shovels, rakes, trowels, carpenter’s level, wooden stakes at least 2 feet long, string, river stones. You will also need topsoil, mulch, and possibly grasses for the berm. From there you can add ornaments, decorative stones, or whatever you wish to improve your garden’s aesthetics. Residential gardens typically cost 3-5$ per square foot to install if you are not paying for labour. The plants chosen to occupy the garden are the most expensive portion.

 The Digging Process

 Outline the perimeter of your rain garden with string.  Now put stakes along the uphill and the downhill side of your garden approximately 1.5 meters apart (5 feet). Now tie a piece of string to the stake pairs and make sure that it is level, that way you can get a consistent depth measurement for the rain garden.

            Start to dig at the uphill stake until you reach the targeted depth you calculated before. Then move down the slope to the downhill side, digging and filling to reach the targeted depth. If you do the one section at a time, it is easier to use the displaced soil to level out the lower end of the garden and start outlining the berm with the surplus. Try to keep the base of the garden as level as possible.

            If you want to line the base with compost, add one or two inches to your target depth. Once you have the garden base dug, mix the compost with the soil at the bottom. If you want to add sand pockets to help filtration, do so now.

            Depending on the distance from your downspout to the garden, you may need to dig a shallow funnel to direct the flow. The side where the water enters the garden is the inlet. Inlets are susceptible to erosion because of the fast-moving water, so you should add larger, rough-edged river rocks to break the flow.

The Berm

            Heap soil around the garden, leaving the entryway bare and ensuring that the highest wall is at the downhill edge of the garden. Make the berm about 0.3 m (1 foot) across and stomp on it hard to make it compact. The berm should have very gently sloping sides to slow erosion and allow water to soak into the ground. To help the water to soak in, you can cover the slopes with mulch, grasses, or dry-tolerant plant species. If you expect to collect large volumes of water at a time, you can include a notch (a low point) in the berm that will allow water to pass during heavy rain events. Line the notch in landscape fabric and add river stones to slow the exit of the water. Angle the outlet away from structures and towards vegetation if possible.

How big should your rain garden be?

Rain gardens typically range from 30-90 square meters (100 to 300 square feet). They can be smaller or larger depending on your resources. Even if you reduce the recommended size of your rain garden by 30%, you will still control 90% of the runoff.

If your rain garden site is within 9 meters (30 feet) of a downspout, most of the storm water will come from the roof so you will only need to calculate its area to obtain the capacity. The area of the roof is the same as the area of the first floor. If you do not know it, use a tape measurer to find the length and width of your house. The product of the two is your area. Now take a walk around your house, how many downspouts do you have? Most houses have 4, each taking care of 25% of the roof runoff. By the shape of the roof, estimate the percent of water your downspout is responsible for. Now multiply the percent of water flowing down the downspout by the roof area. That is your roof drainage area.

Example: You have a building that is 5 m long and 5 m wide and has 2 downspouts that share the load equally. The ground floor area is: 5 m x 5 m = 25 m². If your 2 downspouts are equal, one will take care of 50% of the water. Your drainage area will then be: 25 m² x 0.5 =12.5 m²

If the site is over 9 meters away from a downspout you will need to consider drainage from the lawn. From your rain garden site, look up towards the house and identify which parts of the lawn slope towards your garden. You will need to measure the length and width of the uphill lawn and multiply them to get the lawn area. Now you simply need to add that lawn area to the roof drainage area (calculated in the previous step).

Example: Your building is surrounded by a lawn that slopes towards your garden. It measures 2 m long and 3 m wide. 2 m x 3 m =6 m² as your lawn area. Your total drainage area will then be: 12.5 m² + 6 m² = 18.5 m².

Now you need use your slope, drainage area, and suggested depth to get the surface area of your garden. Use the table below to determine your size factor, then multiply it by your calculated drainage area.

Location	Depth	Sandy	Silty	Clay
Within 9.1 m
	8-13 cm	0.19	0.34	0.43
	15-18 cm	0.15	0.25	0.32
	20 cm	0.08	0.16	0.20
Beyond 9.1 m	All Depths	0.03	0.06	0.10

Example: You have a drainage area of 18.5 m²  sandy soil, and a depth of 8-13 cm. The surface area of your rain garden should then be: 18.5 m² x 0.19 = 3.5 m².

Once you calculate the size of your rain garden, you can divide it by the intended width to find its length. You will want the garden to be twice as long as it is wide, with a max width of 15 feet especially for slopes greater than 8%.

Slecting the Best Site for the Rain Garden: Slope and Soil

Before you get started on building your rain garden, you want to know the answer to two basic questions: "What's my slope?" and "What kind of soil do I have?".  This article will help you get to the bottom of these two questions.

What’s My Slope?

 To calculate the slope of the intended rain garden, pound one stake uphill of the site and the other at the downhill end approximately 5 m (15 feet) apart. Tie the string to the stakes and make sure it is horizontal with a carpenter’s levels. Once the string is properly secured and level, measure the length of the string between the two stakes and then the height of the string from the ground at the downhill stake.   The percent slope will be equal to:

Percent Slope = Height x 100

                         Length

Slope <4%, easiest to build 8-13 cm (3-5 inch) deep rain garden.

Slope 5-7%, try to build it 15-18 cm (6-7 inches) deep.

Slope 8-12%, build it 20 cm (8 inches) deep.

If the slope is greater than 12%, it would be easier to install a garden elsewhere.

How Do I Know What Kind of Soil I have?

Before you even think of putting a shovel in the ground, you also need to figure out what type of soil your rain garden site has. Sandy soils have the fastest infiltration; clay the slowest. Sandy soils are described as gritty and coarse, silty as smooth but not sticky, and Clay as sticky and clumpy. Clay garden must be bigger to handle infiltration and should have sandy pits installed in them to help infiltration. Here is an easy test to help you see if your rain garden will work well at your selected spot:

Soil test: Dig a hole 15 cm (6 inches) deep and fill with water and allow it to filter away. If it takes more than 24 hours to drain, the soil is more clay-based and is not good for a rain garden. You can mitigate the lack of drainage by adding sandy soil pockets during installation, but that takes time and money.

Planning the Rain Garden

The first thing you need to know is where the water goes. Where do the downspouts from your gutters point? Do you see small rivulets of water passing through the grass? Do they join the stream from the downspouts? Does the water pool anywhere? Where can you catch the most water?

This downspout allows stormwater to flow away from the property.

Once you have an idea of where the water flows, you can start eliminating areas around your house. Do not install a rain garden where the water naturally pools, that is where filtration is slowest; think about installing it where the rain garden can catch the water before it pools. You also want to avoid large tree roots. If there are branches overhead, there are roots beneath your feet.

Try to put the rain garden close to a downspout but it is important that you keep the rain garden at least 3 meters (or 10  feet) away from the building to protect the foundation of your house. It is also important to keep your rain garden at least 7.6 meters (or 25 feet) away from your septic system.

After that, it is more a matter of practicality. Installing a rain garden in a flatter portion of land will be easier to dig and level. Make it easier for yourself and do not decide to dig a rain garden where the slope is more than 12%, you will need retaining wall or a high berm to keep the water in and it will take a lot of earth to level out the garden.

In terms of design, crescent-, kidney-, or teardrop-shaped gardens work well. You will want the longer side of the garden to face upslope. Water will hit the berm at the back of that long line, and then flow back towards the garden like a receding wave at the beach. A larger surface will catch the most water possible.

Consider the height, bloom times, colours, and textures of the plants you are planning to use. By choosing plants that bloom at different times, your garden will have a longer flowering season. If you want a traditional, more formal look, try clumping individual species in groups of 3-6 plants and repeat the pattern throughout the garden. Try mixing sedges, rushes, and grasses with flowering species to keep the root network strong and keep some plants from overpowering each other. 

Pioneer Park Stormwater Management Project

The Town of Richmond Hill, Ontario, created a stormwater management park to collect and treat runoff as well as provide flood and erosion protection to neighbouring communities. Originally built in the 1980's, Pioneer Park treated runoff from 26 ha of residential development. Redesigned in 2009/2010, the improved stormwater management park now provides water quality and quantity services to a 740 ha catchment area.

The park consists of a wet pond to treat runoff from everyday rain events, a dry pond to collect and treat runoff from major, intense rainfall events, an oil grit separator for pre-treatment and a channel for fish passage. This project was a 2010 winner of Insurance Bureau of Canada's Watershed Awards.

The completed stormwater management park. Photo: www.rmconstruction.ca

Under construction in 2010. Photo: www.rmconstruction.ca

Check out this great video to learn more about how Pioneer Park was created and the many environmental and economic benefits it provides to the Town of Richmond Hill. This video was provided by Insurance Bureau of Canada.

Retention Basins (Wet Ponds)

In a recent blog post we talked about dry detention basins which hold water temporarily and release it slowly into the environment.  A retention basin works in a similar way but retention basins can store water for longer periods of time, mimicking a real lake or pond.  Retention basins are effective in storing and slowing runoff, thereby improving water quality by allowing pollutants to filter out of the water column. 

To find some examples of retention basins, we visited Dartmouth Crossing to see how stormwater runoff was collected from the large roofs and parking lots at the shopping centre.

Retention pond behind Walmart

We were pleasantly surprised to see and hear a gurgling stream in this landscape with so much pavement and hard surface cover.  The stream, Grassy Brook, is largely fed by runoff collected off parking lots and rooftops. A drop of water that lands on Walmart’s roof or parking lot is collected underground then released into Grassy Brook which flows behind the building. Marsh vegetation and willow trees were planted to enhance habitat and slow the flow of water, thereby helping to remove pollutants. Since white surfaces absorb less heat and stay cooler on sunny days, Walmart's roof is painted white to keep water temperature cool, as warm water entering a natural system can harm fish and other species. The increased flow of water into the retention basin and into Grassy Brook has created an active spawning site for trout.

White roof

 The wet pond shown below has both aesthetic and functional purposes. The pond is surrounded by a walking path, and an amphitheatre, so people can listen to concerts and enjoy the scenery. Trout also use this pond as a spawning area.

Pondside amphitheatre retention basin.The pipes in the background (left)
collect overflow during extreme precipitation events.

Runoff is slowly released into the pond through this small opening
to avoid a high peak flow.