Did I mention that I would like to convert our parkway into a rain garden? In case you haven’t read the previous posts, let me mention it again.
A rain garden is a shallow excavated and vegetated area that allows stormwater runoff to infiltrate into the ground. That begs the question: where will the stormwater for my parkway rain garden come from?
The contributing area
The adjacent concrete sidewalk will contribute some runoff, but not much, because the sidewalk surface area is actually smaller than the parkway rain garden.
The stormwater that I would really like to manage in the rain garden comes from the street and gutter. Surface drainage on our street is poor and we always end up with standing water in the gutter – sometimes for a couple of days, and sometimes for a couple of weeks. I would like to drain and infiltrate the street runoff into the parkway.
With that goal established, I could look at the appropriate rain garden depth and storage capacity. The elevation of our curb averaged around three inches above the gutter. The top of the curb roughly equaled the grade elevation in the parkway.
To manage the street runoff from the half of the street bordering the parkway (924 square feet), I should plan for a six inch ponding depth across the 470 square feet of rain garden. At a tested infiltration rate of two inches per hour, the parkway rain garden should be able to handle the 100 year design storm for the 924 square feet of contributing area.
The issue of conveyance
The next big question was, how do I get the runoff into the rain garden? I have a street curb that is in the way.
Typically one would rely on curb cuts to allow the water to flow from the street and gutter into the rain garden. But this is not my curb and I don’t want to get into trouble with the City. Whatever I do has to be easily reversible in case they end up not liking what I am doing.
I think I’ll start by drilling a number of one inch holes through the curb at the gutter low spots. Those holes can be easily filled and patched if needed. But because the holes only allow a limited amount of volume to flow through at any time, I may not be able to get all the runoff from intense downpours into the rain garden. So even though the rain garden could manage a 100 year design storm, the limited conveyance capacity may reduce that effectiveness.
I also will have the issue of maintenance. There is always a lot of debris in the gutter, which could clog the one inch holes. I will probably have to check on those holes a couple of times a month to keep them clear.
And I have two paths crossing the parkway, which leaves me with three rain garden cells. To connect these cells hydraulically, I incorporated PVC pipes under the path. This way runoff can easily flow from one rain garden cell into the other.