Finding prime sites for development in Southeastern PA can be a challenge. When you do, there’s a good chance it has limestone or karst geology. This is particularly true in Chester and Montgomery Counties.
What is karst geology?
Karst geology is a topography formed by the dissolution of soluble rocks like limestone, dolomite, and gypsum. In these areas, water that infiltrates through the ground slowly erodes and dissolves the carbonate minerals forming an underground network of caves and cavities. When one of these underground cavities collapse it forms a sinkhole.
Can you still develop karst properties?
Yes, it is possible. In fact, many areas like the Swedesford Road/Route 202 corridor in Chester County and Plymouth Meeting in Montgomery County are popular locations for redevelopment projects – even though they are also areas prone to sinkholes.
How does karst geology influence stormwater management design?
These days, stormwater management, especially water quality, drives the design and timing of most development projects. The fact that you cannot infiltrate on sites with karst geology plays a big role in the process. Therefore, the need for creative stormwater management design is paramount. Meeting NPDES regulatory requirements while also maintaining budget and schedule is the challenge. Fortunately, there is a newly accepted method – Managed Release Concept.
What is Managed Release Concept (MRC)?
Per the PA DEP’s website, “Managed Release Concept (MRC) is a post-construction stormwater management (PCSM) strategy that comprises the collection, management, and filtration of captured runoff through a best management practice (BMP) that is preferably vegetated and includes release of a portion of the captured runoff through an underdrain within the BMP.”
DEP just started to allow this as an accepted BMP within the last two years. In particular, this alternative BMP treats stormwater and meets regulatory requirements on sites where infiltration is not feasible. In some cases, MRC is the only way to meet the water quality treatment requirements.
With MRC, you are capturing runoff, holding it, and releasing it at a very slow rate to reduce the risk of dissolution of the bedrock and erosion downstream. It is a lined and self-contained unit. Unless the liner is punctured, there is no discharge to subsurface soils – no way for water to get from the BMP and into soils. The runoff is safely conveyed, and ultimately discharged off-site.
Here are two projects where we recently utilized MRC in our stormwater management design:
- This project involved one of the first approved MRC designs in Montgomery County.
- Without MRC, a site surrounded by sinkholes like this one, wouldn’t be a viable development site.
- The final design includes two lined underground detention basins using MRC, along with two rain gardens with native plants, and non-structural BMPs including soil amendments and landscape restoration.
- When engineering design began on this project, MRC was not an option and DEP required seepage beds; which are good in theory, but challenging in practice.
- The original stormwater design also involved spray irrigation, a viable stormwater management option for karst geology.
- Halfway through construction, a sinkhole formed on site, causing us to abandon the idea of seepage beds. That’s when we teamed up with the Conservation District to talk about Managed Release.
- While the pivot in design during construction posed time challenges and increased costs, MRC combined with spray irrigation proved to be a good solution to reduce the chance of future sinkholes on site and provide water quality to meet regulatory requirements.
How to reduce the chance of future sinkholes?
As civil engineers in Southeastern PA, we know that managing the risks associated with karst geology is part of the development process. The design process in these areas is highly technical. It requires meeting very specific parameters. But we understand it, and have successfully implemented this BMP on a number of sites. By using MRC, we are able to meet the water quality, volume, and rate requirements set forth by the local municipalities, and DEP, as well as, reduce the chance of future sinkholes.
Have question about developing sites located on karst geology?