Complicated Bio-retention

[Eric Galvin]

Anyone ever deal with a bioretention situation where the flow coming into the retention areas is not from sheet flow? We want to pipe in the flow to the bioretention cell, while still looking nice. All i can seem to find details on are the standard sheet flow into the bioretention cell (via grass, swales, curb cuts etc.) Thanks for any help – Eric

[Andrew Garulay, RLA]

We do it from time to yime by using energy dissapation rocks below a flared pipe end and if it is a large area we section off the first area for sediment to drop out as well before the water spills over into the bigger area.

A lot of the commercial zoning in one of the towns that we work in requires us to use bioswales and retention ponds as much as feasible (sometimes to the point of ridiculous) including using every planting island in the parking lots in some cases. One thing that we worry about wth sheet flow into small islands is that if they fill up in a rain event, no one can tell that the swale is nothing more than a shallow puddle (we have a lot of cars go through the front windows of drug stores and post offices due to a large elderly demographic). Because of that, we often use curbing or cape cod berms to identify the islands which also “curbs” sheet flow. We have the water enter the small retention areas through asphalt swales when we can not let it sheet flow. Sometimes we connect the small swales with perf pipe in gravel trenches through a domed grate on a raised rim. We also do this to limit the depth of water since a fence is required in Massachusetts if the water can get 2′ deep or more which would be a problem in a lot of situations. These may continue through solid or perf pipe to connect larger retention ponds if the rain event is more than the small swales can handle.

[Tanya Olson]

Just as Andrew said – energy dissipation rocks below a flared pipe – exactly the detail I’ve used before. Cleanout is not ideal as sediment tends to drop in the rocks.

[Jason T. Radice]

I concur. You need to first dissipate the energy of the water coming out of the pipe. As mentioned above, rip-rap or concrete dissipation mats (look these up) are required at the pipe outfall. You will then need some sort of fore bay (or two) to further calm the water and allow sediments to drop out of suspension. The water can then be further dissipated by concrete dams that further slow the water flow and hold various filtration plants downstream. This actually is a pretty typical retention pond design. There are also many time-of-concentration issues as to the scale of your bioretention system may face, as most biorentention systems can’t handle the volume which piped systems create. These things can be huge because you are shortening the time it takes the water to collect, and you are concentrating the accumulation. Depending on the type of development, you may want to consider underground storage via cisterns or an under pavement gravel storage system to delay the outfall, and slow the water entering the biofilters. 12” in, 4” out.

[John Moe]

Eric;

I think there is a project in Minnesota; the City of Minnetonka’s new Government Center is using a solution which might work for you. A large bioretention area takes runoff from a large parking lot. Instead of permeable pavers, or curb cuts which would seem to follow recent trends, they opted for standard-issue storm sewer inlets, then routed them below grade to a “stand pipe” in the center of the bioretention area. It’s all but invisible there in the tall grasses. The water bubbles up and fills the retention area. This gets rid of the energy, eliminates scour and puts the plumbing where the sun doesn’t shine. I’m not exactly sure how they cycle the water that remains “charged” in the bottom of the storm drains, but I bet there’s some kind of permeability built in. Also, the new University of Minnesota football stadium has a lot of bioretention areas which may have this technique.

John Moe

[Eric Galvin]

Thanks to you all! I spent most of the day yesterday looking at examples and how well they were done and in minutes you came up with the best answers i have come across.
It seems like the two options we will evaluate will be the flared pipe/rip-rap & forebay option, as well as the “stand pipe” option. Thanks for the info on the stand pipe John, i ran across a detail in the Virginia Stormwater Guide that talked about then, and now i know enough to detail one out with our civil engineer. And thanks Jason for the info on disspertion maps, that was not something we had looked at yet. Hopefully i will be able to post some pics in the next 6 months or so of the completed project!

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