Low Impact Development (LID) has been all the rage in stormwater design lately. Implementing rain gardens, pervious pavement, and bio-retention swales into site stormwater design is almost a must in many jurisdictions, at least in Washington State, the epicenter of all things stormwater. Low impact means low impact to the natural order of things, the natural drainage patterns. It means trying to mimic the drainage patterns of an undeveloped site as much as possible when designing a stormwater system to serve the developed site. How does this translate to stormwater maintenance once the site is complete?
The Maintenance Requirements of LID
Low Impact Development may reduce the impact to the environment, but the maintenance requirement is typically significantly more than old-school systems. There are several reasons for this.
One major driving idea behind low impact design is that there should be multiple smaller storm drainage systems rather than a single, site-wide storm drainage system. Putting in multiple smaller systems allows the system to be designed to more closely mimic the runoff characteristics of the site before development occurred. While that may be good for the natural environment, it is not so good for your maintenance crew. There isn't one storm drainage system to maintain, there are several. Not only that, but each system is more complex than the systems we used to implement. Read on.
Types of LID Storm Structures
All earthen-built stormwater systems, LID or not (e.g. rain gardens, bio-retention swales, ponds) are intended, at least to some degree, to be depositories for sediment transported by stormwater flow. No problem - we know that. The additional complication with LID-style earthen-built systems is that there is a lot of engineering that goes into the subsurface soil at the bottom of the facility. A lot of engineering typically equals a lot of money, and that dirt at the bottom of your rain garden is no exception. Pay special attention when removing sediment that you aren't removing soil that was intended to be there and that will cost a lot of money to replace.
Another issue with both of these types of facilities is that the engineered dirt, the stuff that is so expensive, is also high in nutrients so that it can support the types of plants necessary for the facility. Those nutrients, however, can allow a lot of unwanted stuff to grow, stuff that isn't intended to be in the rain garden. Keep ahead of the maintenance of those areas, including removing weeds and unwanted vegetation regularly to keep them from getting established.
What about Pervious Pavement?
Pervious (or porous) pavement also requires special maintenance to continue to be porous. Porous pavement is created by reducing the binder that is typically included in traditional pavement mixes; it is that binder that plugs the voids in pavement and gives it the binding characteristics that makes it strong and long-lasting. Removing or greatly reducing the quantity of that binding material, which is how porous pavement is created, opens up the pavement pores to allow storm drainage to pass through it into the sub-surface below. Unfortunately, however, something else can get through into those pores - sediment. That sediment must be removed from the pavement surface quite often, before it lodges in the pores, or it becomes impounded in the porous pavement, impeding the flow of stormwater and creating ponding on the paved surface.
How do you remove it? Well, here's how not to remove it - don't use a pressure washer. While some companies are adept at removing sediment with a pressure washer, more often their use results in forcing the sediment further into the voids, thus reducing the useful life of the pavement. A vacuum sweeper is really the only way to effectively remove sediment from porous pavement, and that is not a piece of equipment that most owners have lying around.
Low Impact isn't Low Maintenance
I wish I could say that low impact development techniques equal low maintenance requirements, but it isn't true. In fact, the opposite is much closer to the truth. Keep ahead of LID stormwater maintenance or find a professional to help you keep ahead of it and ensure you are maintaining it as cost-effectively as possible. If you would rather put your LID storm drainage system on auto pilot, click here to be contacted by one of our stormwater maintenance professionals.
Jeff McInnis is an engineer who knew there had to be a better way to help owners keep their systems in compliance.