Hey guys, we’re tackling the engineering hot topic Low Impact Development, and I’m happy to introduce Dr. Thomas Soerens as our guest blogger on the topic. Check out my short Quick Tips Video on the subject, and then read on!
What is Low Impact Development (LID)?
The most accurate answer is probably that low impact development is development. When I first
started teaching LID nearly a decade ago, I thought it was this cool new approach to managing storm
water and runoff. In the years since, proposed LID concepts have become law over much of the USA and
in fact some concepts were already making their way into laws back then. Among the changes in the
past decade is that term “low impact development” has mostly given way to the broader term “Green
Infrastructure”. But I still use the term LID.
The EPA defines LID this way:
The term low impact development (LID) refers to systems and practices that use or mimic
natural processes that result in the infiltration, evapotranspiration or use of storm water in
order to protect water quality and associated aquatic habitat. (1)
What’s So Important about Storm Water Management?
The important part is that LID manages water. Other design and green building practices will try to
lessen the impact on the environment in other ways, but LID is about the management of storm water.
LID approaches seek to manage storm water in a way that more closely matches the natural hydrology
(how much water goes where) of the developed site. Managing the water naturally
preserves the physical and biological integrity of the streams and water bodies that receive the runoff.
Water Needs to Infiltrate
The first time I taught a seminar on low impact development to a group of PEs in a professional development seminar, I got alot of push back. They said, “We don’t want water by our building foundations. We want to get the water off the site.” But LID seeks to keep the water on the site longer and infiltrate the water into the subsurface.
And as I mentioned previously, this is now required by law in many places in the US.
For decades now, developers have been required to keep the peak runoff rate from a developed site no
higher than the peak runoff rate before development. Today, laws also require volume control as well as
peak control. As vegetation is removed and ground is paved, infiltration is reduced and runoff volume is
increased. Under volume control, we have to find some way to get that extra volume to infiltrate or
evapotranspire.
In the sensitive watershed where I work in Pennsylvania, the law actually requires that post-development runoff volume be 75% (i.e., less than) the pre-development runoff volume.
Best Management Practices to Manage Water Runoff
A number of technologies and best management practices (BMPs) have been designed to manage water
by LID approaches. A couple of the sexier BMPs are green roofs and permeable pavements. Green roofs
are fairly rare because they require extra structural considerations in the design process and
maintenance costs, but there are some and they are really cool. Likewise, permeable pavements are not
widely used, but they’re amazing to watch. We have some on our campus.
The more commonly used BMP include bioswales, bioretention facilities, and rain gardens. These allow
water to infiltrate instead of just run off the site. Under an LID approach, instead of parking lots having
built-up islands and boundaries that channel water to storm sewers, the grassed areas are lower and the
water is channeled to the grassy areas. Even breaking up a parking lot with grass strips rather than having one paved expanse can have benefits.
When I was a kid, our street had grassed ditches that handled the street runoff but those were replaced by storm sewers. These days, storm sewers are being replaced by grass ditches, although the grass “ditches” actually have engineered underdrains and high flow bypasses.
Conclusion
Low impact development may sound simpler, but often LID technologies are more complex and more expensive than just channeling the runoff and getting rid of it.
I suspect some of you practicing engineers will do or maybe are already doing low impact development
without even knowing that it’s a thing with a name. It’s just the storm water management that you’ve been
taught and been required to follow. If any of this is new to you and you will be working with storm
water, these practices are coming your way.
If you have questions, feel free to contact me. Or maybe take my online LID course through the
University of Arkansas M.S. Engineering program.
Thomas Soerens, PhD, PE
Professor, Messiah College
Adjunct, University of Arkansas