We have yet another small scale residential design ready to go into grading plan check at Los Angeles Department of Building and Safety. The grading plan had some options for once, and at the end the client preferred the rain garden to the other alternatives.
Rain Garden as an Option
The idea of the rain garden is to bring all of the stormwater from the new impervious areas to the rain garden to saturate a portion of the yard. If the rain garden overflows, there are raised area drains to bring the excess stormwater into the drainage system and outlet to the street's storm drain system. That sounds much easier than it is to design, sometimes.
For the initial design I try to incorporate raised planter boxes as these are easiest to satisfy the Low Impact Development (LID) requirements. The client did not want these taking up precious room so I gave an option of using drywells, or infiltration tanks. The client also did not want these in the backyard, even though they are hidden. So we were left with placing a rain garden somewhere on the property. At first we were told to not touch the front yard, but the rain garden would be perfect in that area. After many emails and a few phone calls the rain garden was a go.
Rain Garden Design
One trick to putting a rain garden in is to line the bottom so the soil will not infiltrate the ground. But this causes the design to take into account a perforated pvc pipe to drain any water that reaches the bottom. So elevations are key to getting the water from this area out. We bring the entire impervious area (roof, concrete areas, etc), to the rain garden. Currently the City of Los Angeles Sanitation wants to see how the outlet pipe into the rain garden daylights. Paying special attention to inverts of the pipes we are showing the drainage pipe outlet near the top of the slope of the garden. From there the garden will fill and eventually overflow into area drains on top. to flow out to the street. One important aspect to the design of grading plans is what happens if it fails. The nice thing about this design is that it can't fail. If the pipes clog the rain garden will overflow and find their least resistance down the front stairs.
Rain Garden Detail
The proof of the concept working is in the detail. For some reason the plan checkers are looking really closely at their own supplied details. Nothing special to see here, except how the plan view looks in a profile view.
We are designing all of the different LID mitigation requirements as we see the siteplan, and like in this case designed in a few options. There is no one size fits all when it comes to designing a grading plan. The basic concept is always the same, drain the stormwater offsite in a safe manner. But bringing the stormwater to one of these devices can be quite complex. This project was the only straightforward single family residential project that we have touched in the last four years. How's that for a stat of what type of jobs we are taking on?