Misconception About Bioretention

On March 10th and 11th, I attended an Inspection and Maintenance Certification Course for Stormwater Control Measures in Ohio. This course taught me many essential installation and long-term post-installation maintenance issues for detention basins, wetlands, swales, permeable pavers, green roofs, and bioretention basins.

One debate all of us taking the class had was, if perimeter controls are used (such as silt fence or compost filter sock) around bioretention basins, can they be installed (with the intended bioretention soil, and the filter layers of concrete sand and gravel) prior to the completion of construction or not? Well, that question could be answered from a stormwater inspection I conducted at a construction site on April 1st.

During a stormwater inspection, I am looking to see if installed sediment and erosion control measures (such as silt fence, and inlet protection) need maintenance or added in certain areas. Additionally, I am always looking for areas of the site which could have temporary stabilization (such as grass seed and straw) installed on areas of exposed soil which will be lying idle for some time. Installing grass seed and straw on these areas of exposed soil is critical to help minimize erosion and sediment-laden surface runoff during rain events; and for many construction sites, it is needed in order to remain in compliance with Ohio EPA Permit No.: OHC000005. At the end of an inspection I notify the site supervisor or project manager of sediment and erosion control measures that need addressed.

So, back to April 1st. Well, to set the scene, we actually have to go back to Saturday March 28th and Sunday March 29th. Remember the weather? It rained – a LOT! In fact, according to Weather Underground, in Cleveland, OH it rained .96 of an inch on the 28th, and another 1.56 inches on the 29th - most of which occurred overnight from 12AM until 3AM. With all that rain, I had a feeling some inspections during the week were going to have some items which would need addressed.

As soon as I arrived on this one site which has several bioretention basins installed and exposed soil throughout much of the site, I went straight to one of the bioretention basins. This site does have silt fence installed around the bioretention basins and in previous inspections I noticed a bioretention basin or two with a slight amount of sediment accumulation in it. This is likely due to an area where concentrated surface flow during a rain event caused a slight amount of sediment to under-cut an area of silt fence, or over-top the silt fence in a section where the staples had ripped free from a wood stake, and therefore causing the fabric part of the fence to be damaged in a section.

However, on April 1st, things at these bioretention basins were not good. It was evident that the magnitude of concentrated surface flows causing erosion and sediment-laden water to over-top and under-cut the silt fence was much greater than what had occurred at this site in the past. Several of the installed bioretention basins had been completely covered by sediment which appeared to be “caked” on top of the mulch within the basins. In order to function properly, these bioretention basins where this occurred will have to have the plants, mulch layer, and at least the first few inches of the special bioretention soil removed, then new bioretention soil followed by the plants and new mulch added back into the basin.

While looking at the bioretention basins on site, I could not stop thinking about our little debate that was occurring at the certification course I attended the previous month. The condition of the bioretention basins after the heavy rain events which occurred March 28th and 29th prove that bioretention basins cannot be installed prior to vegetative stabilization (significant grass growth) in the area on site which will flow into a bioretention basin, or the area which will flow into storm drains which outlet into a bioretention basin.

Blog Author: Chris Vasco, Urban Technician

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