Specially designed gardens treat parking lot runoff
Angie Hong
Stillwater, MN - October 15, 2009
It’s a good thing I don’t melt in the rain like the bad witch from the Wizard of Oz. This October has been rough on all of us Minnesotan’s, but especially so for whiny people like myself. I’ve got a month to go in training for a triathlon that takes place next month in Arizona and biking outside is quickly progressing from unpleasant to miserable.
Two weeks ago, a torrential downpour overtook me out in the country, ten miles away from my office in Stillwater. (Apparently the hour-by-hour weather forecast is not as accurate as I had previously assumed.) Freezing and struggling to stay on the road as the rain began blowing sideways in pulsating waves, I told myself that the experience would make a really funny story later – much later. A guardian angel appeared in the form of a woman in a minivan who picked me up on the north edge of town and drove me back to my car. I’ve forgotten her name by now, but am so glad that nice people like her exist in the world!
In the half an hour or so of rain-soaked riding before my savior arrived, I had plenty of time to observe the volume of water falling from the sky and pouring off the road into the ditch. It is the fate of a stormwater educator to notice these things on a daily basis. Nonetheless, I was amazed by how quickly the water accumulated on top of the trail I was riding on. Within minutes it was nearly two-inches deep and flowed from the pavement as if from a wide, wide waterfall.
After being rescued and then taking a long hot shower at the gym, I headed back to my office and learned that my coworkers had also been out in the rain, taking photos of the brand new raingardens installed alongside the parking lot at the Valley Ridge Mall. Significantly larger than most residential raingardens, these gardens treat polluted runoff from the parking lot before it flows into the ditch and eventually the St. Croix River. Considering how much rainwater had gushed over the top of the bike trail when I was riding, I could only imagine the torrents that must be coursing through the parking lot as the rain continued to fall.
Because the parking lot at the Valley Ridge Mall can generate so much runoff in a big rainstorm, only a fraction of this amount can soak into the ground in the three new raingardens. The central garden, for example, collects runoff from 1.4 acres of asphalt. If there is one inch of rain, nearly 40,000 gallons of water surge off this portion of the parking lot – enough to fill five large tanker trucks. The raingarden is able to capture about 4000 gallons, after which the remaining amount continues onward into the roadside ditch. As an added benefit though, the raingarden filters dirt and debris out of the stormwater before it reaches the ditch. Salt, sand and other sediments drop into a long concrete trough between the edge of the parking lot and the garden and mulch and plants in the raingarden further filter the stormwater as it passes through. In smaller rainstorms or on days when it drizzles steadily throughout the day, the raingardens are able to soak all of the runoff into the ground.
As I suspected, the flow of water from the parking lot into the Valley Ridge’s new raingardens was dramatic on that rainy day. Truth be told, it would require ten times as many gardens to capture every drop of water from even that small of a parking lot. The gardens were functioning valiantly in their first week of existence though, and when the plants are mature in a few years they will perform even better.
The Valley Ridge Mall project is a good example of a public and private partnership. The property owner paid for half of the cost of the new raingardens, while a grant from the Department of Natural Resources covered the rest. The Middle St. Croix Watershed Management Organization and the Washington Conservation District provided the design for the project and solicited an experienced contractor to do the grading and install the gardens. In addition to reducing stormwater pollution to the St. Croix River, the Valley Ridge Mall has three beautiful new gardens to showcase and the local agencies have a unique demonstration project.
I, meanwhile, have avoided another disastrous ride by sentencing myself to the monotony of a stationary bike at the gym on potentially rainy days. I continue to tempt frostbite and hypothermia as I ride outdoors on ever-colder days, though. These rides make for great stories after all – later, much later.
Angie Hong is an educator for the East Metro Water Resource Education Program, which includes Brown’s Creek, Comfort Lake – Forest Lake, Middle St. Croix, Ramsey Washington-Metro, Rice Creek, South Washington and Valley Branch Watersheds, Cottage Grove, Dellwood, Forest Lake, Lake Elmo, Stillwater, West Lakeland and Willernie, Washington County and the Washington Conservation District.
Posted: October 15, 2009