What is storm water runoff?
Storm water runoff is precipitation that does not percolate into the ground but runs over impervious surfaces including paved streets, parking lots, and buildings. As the water flows across paved or impervious surfaces, it collects heavy metals (lead, zinc, copper, cadmium, mercury), hydrocarbons (oil. grease, gasoline, cleaning solvents), salt, and sediment. When storm runoff reaches a body of water, it releases these chemicals into our waterways. The EPA has more information on storm water.
What are problems associated with storm water?
Impervious surfaces can increase the total volume of runoff as well as the runoff rate. As the percentage of impervious surfaces increases, rivers and creeks are more susceptible to flooding and erosion. Flooding and erosion can lead to degradation of the riparian ecosystem and loss of habitat structure. Because of this degradation, ecosystems can experience a decrease in abundance of biodiversity.
How are storm water problems mitigated?
Low impact design (LID) is an approach to storm water management that aims to manage runoff as close to the source as possible. It also aims to use natural methods and cooperate with local ecosystems to effectively manage water while preventing unnecessary infrastructure. LID is not a specific set of stormwater techniques. Instead, it is the practice of finding methods of management that are best adapted to a place’s hydrology, soil, climate, and population density. LID can be used on a rural farm or a highly urbanized city.
What are some examples of storm water management?
Several storm water management techniques include:
- Swales, which are grassy channels that allow water to infiltrate into the ground. They are planted with erosion resistant grass. Swales are often used instead of curbs and gutters to transport water from impervious surfaces to a place where the ground can absorb the water.
- Permeable pavement, or porous surfaces that allow water to filter through and slowly infiltrate into the ground beneath the pavement. Permeable pavement looks similar to concrete and asphalt, but contains many small holes or spaces that store water and slowly release it. Despite its benefits, it may not be suitable for a cold climate like Minnesota. Water that travels through the porous openings and freezes would likely damage the pavement.
- Riparian buffers, which provide a variety of services. Vegetation can stabilize a river bank, preventing erosion from both the river and from runoff traveling into the river. They also trap some of the sediments and compounds present in the run off, keeping the waterways cleaner.
- Green roofs, which are roofs covered in vegetation. Roofs are normally impervious surfaces with gutters that transport water down to the ground and release it out of a spout at a high rate. Green roofs have plants that absorb some of the water that falls on the roof. Some extra benefits of green roofs include lower energy costs of the building and improvements in the air quality of the community. Regents Hall has a green roof that is accessible from the fourth floor near the greenhouse.
For more information about different types of storm water management, the EPA has created a list of popular management practices here.
Storm Water Management at St Olaf
An average 3-hour rainstorm in Northfield dumps approximately 1.5 inches of rain. Some of the rain is absorbed into the ground, but the majority of the rain flows into creeks or storm drains that eventually lead to the Cannon River. To prevent storm water on campus from flowing into the Cannon, St Olaf uses an integrated management approach: wetlands, basins, soil retention techniques, and Regent’s green roof. St Olaf’s use of storm water wetlands and other management techniques can lead to cleaner, safer water for Northfield’s community and ecosystems. To further contribute to the health of the local watershed, St Olaf could move towards decreasing the area of impervious surfaces on campus.
Direction of Storm Water Flow
Water flow on campus splits into five sections based off of the topography: north/northwest, northeast, south, southwest and east. The water flows mainly along roads, paths, and other impervious surfaces.
•North/northwest (Skoglund, Natural lands): Water flows through restored prairie and ponds.
•Northeast (Thorson, Practice Fields): Water travels down Thorson hill and is retained in the practice fields. Water either is absorbed into the ground or flows into a sewer in the far side of the fields.
•South (Regents, Old Main): Water is directed through a series of artificial ponds and wetlands designed to filter contaminants in the water.
•Southwest (Hoyme, Hill/Kitt): Water is retained through a rain garden and soil retention techniques
•East (St Olaf Ave, Sewer drains): Very little water is retained on this side of campus due to the high amount of impervious surfaces. Water follows the road and enters city storm water drains.
The map above shows the impervious surfaces, the slope, and sinks, giving you a sense of which direction water flows on campus and where it collects. Water flow is fairly simple due to the topography of the hill, but the placement of impervious surfaces can change the route of water flow.
Keep your eyes open as you walk around campus and see if you can find storm water management techniques. Can you see how the landscape is designed with runoff in mind? Where will water collect and infiltrate? Where will it flow along roads and pathways? The beauty of Low Impact Design is that management blends right into the landscape!
Data Layers and Images used to create maps:
(www.mngeo.state.mn.us/chouse/elevation/lidar.html, Rice County)
Campus raster data from Jason Menard, Campus map from St Olaf College (www.stolaf.edu)
We would like to thank Dr. Jason Menard and Professor Paul Jackson for their guidance and assistance in creating the maps.