selecting the right drainage solution

To find the best solutions for drainage issues in your yard, you can start by assessing the existing conditions.

• Identify the sources and approximate relative quantity of the water including off-site run-off draining onto your property; downspout drainage (consider what portion of roof drains to each downspout), and on-site rainfall.

• Identify where the surface runoff is currently draining and existing low areas where it could potentially outfall. The goal is to provide adequate surface drainage away from the house and direct that as well as surface drainage in yard areas to low areas that provide adequate outfall.

Next you can determine the best solution for each area. There are various tools in our toolbox, and you can use a combination of several of these that is best adapted to the amount of run-off and the existing options for outfall. First select the least intensive, lowest cost (surface drainage); then surface systems (dry stream); then infiltration and combination infiltration/ piped (French drains or rain garden), then the more intensive inlets and subsurface piped drainage.

• As mentioned above, always select the simplest solutions first, such as surface grading. The ideal grading would allow preferably 4% slope (2% minimum) away from the house, to a shallow swale that drains at 2% minimum slope, to outfall at a lower area off-site or to the street. You can measure the slope using a 3 to 4 foot long board resting on grade and a hand level (1/2" per foot is approximately 4%). A 2% slope (1/4" per foot) would be the minimum slope that can allow for drainage on lawn areas.

• If there are areas that cannot be drained by surface grading, then consider a series of solutions for individual areas on-site. such as adding a dry-stream, a rain garden, vegetated swales, or installing a series of French drains. Each of these solutions is sustainable (and relatively low cost) because it involves small solutions that allow small areas of drainage to be detained after a large storm and infiltrate (or slowly soak) into the ground.

• If the above solutions do not address all of the site drainage issues, then a solution for some areas may need to be sub-surface piping individual downspouts, and / or adding yard inlets with sub-surface pipes to drain to an outfall in a lower area. Subsurface solid PVC pipes can be sloped at minimum 1% slope so can work in locations where you do not have adequate vertical fall to allow for the minimum 2% slope that is required for surface drainage across lawns. Piping drainage is less sustainable because it does not allow drainage to soak into the soil. It is also more labor and cost intensive, and does require maintenance to keep the system free of debris.

• A last resort is collecting all downspout drainage and surface drainage and piping it off-site. This is the most expensive solution and least environmentally responsible. It does not allow for water to soak into the soil and can cause erosion and excess water volume at the outfall.