A rain garden should not be designed to capture the most rain that could ever fall from one storm.
That would be like designing a restaurant for the most people that could ever go there on one night.
Is that a good idea?
Sure, on that one night.
But every other night the restaurant will have wasted their time, money, and resources.
By designing your rain garden to capture most, but not all, of the rain that could ever fall, you maximize the rain garden’s cost-to-benefit ratio.
This practice of economically sizing your rain garden will mean that there will probably be times that there is more water entering the rain garden than can be held inside the rain garden basin.
This excess water, or overflow, needs to be accommodated for at the design stage with a safe and long-lasting path out of the rain garden, or outlet.
There are two main types of overflow outlets, direct and indirect.
– The excess water flows directly out of the rain garden basin overland, preferably on to the existing stormwater system.
– This would most likely be used in a residential or small-scale commercial setting where the overflow would travel into a side yard drainage swale, ditch, stormwater basin, or street.
– Direct overflow outlets should be covered with gravel, sedges, or other erosion preventing material.
– The excess water flows into a riser pipe inlet that is located inside the rain garden basin below the elevation of any other point of the rain garden basin sides, thereby passing along any rain water beyond the rain garden’s designed capacity to another location.
– This is a good option if there is no direct path from the rain garden basin to the stormwater system, (i.e. center of yard, backyard),
– This could also be used when it is not desirable/possible to have the excess water flow onto adjust land (i.e. small-lot residential, parking lot island, commercial developments)
– If the design volume could potentially be much larger than the capacity of the rain garden this option should be used. (i.e. small-lot residential, parking lot island, commercial developments)
A direct overflow outlet is my preferred method and will usually meet the needs of a rain garden in a residential settings.
The costs of the piping needed for an indirect overflow outlet makes this option restricted to a need-only basis.
At this point we have our rain garden basin dimensions, plant selections, conveyance system, and overflow outlet all chosen, now we have one last decision to make, mulch.