We would like to take this opportunity to thank all of the participants of the Bloomington Green Streets for Blue Waters program for taking part in this important project and give everyone else a quick overview of all that’s been accomplished!
Over the last two years, Metro Blooms, in partnership with the City of Bloomington and the Lower Minnesota River Watershed District, worked with residents to design and install a vegetated bioswale and 18 curb cut raingardens in Bloomington neighborhoods that drain to the Minnesota River.
Curb cut raingardens are unique in that they not only capture runoff from the property itself but from the street as well. The project area was selected because it is adjacent to the Minnesota Valley National Wildlife Refuge. Individual sites were selected based on site suitability and resident participation. In total, during an average year with 30″ of rainfall, the completed practices:
- Treat stormwater runoff from 55.42 acres
- Remove over 3,500 pounds of solids
- Remove 8.5 pounds of phosphorous and
- Infiltrate 1.9 million gallons of runoff
The solids and phosphorus removed would otherwise end up in the Minnesota River, causing algae blooms which deplete oxygen and diminish aquatic habitat when they decompose. The millions of gallons of runoff infiltrated reduces flooding and recharges groundwater to ultimately reach the Minnesota River clean and cool. In addition the bioswale and raingardens beautify City neighborhoods and provide habitat for pollinators and other wildlife. This project was made possible by funds from the Clean Water Land and Legacy Amendment and local matching funds.
The backyard bioswale was built in the fall of 2012. It involved eight residential properties and Holy Emmanuel Church.
The bioswale filters stormwater from 44 acres, capturing 97% of the runoff (12,260 gallons), removing 713 pounds of solids and 2.5 pounds of phosphorous that would otherwise flow into the Minnesota River.
The bioswale is best viewed by the pedestrian path connecting E 105th St Circle to E 104th St.
Construction of the curb cut raingardens began in July of 2014 after a very wet spring. Rich Harrison, Metro Blooms’ Landscape Architect, worked with Steve Gurney, Water Resource Specialist with the City of Bloomington, to mark the raingardens. Then sod removal and excavation began! Construction was completed by Urban Companies and overseen by Metro Blooms and the City.
The Bloomington curb cut raingardens are all 12″ deep and range in size from 90 to 290 square feet depending on site conditions such as utilities, existing vegetation and topography. The raingardens were excavated to 24″ so soils could be amended with 12″ of compost and sand. This is the ideal growing medium for plant establishment and helps ensure the long term survival of the gardens.
The raingardens were designed with consideration for the sandy sub soils that are prevalent throughout the project neighborhoods. Soil type helps determine raingarden depth, so this was important to ensure that each basin drains completely within 24 hours of a rain event and doesn’t become a source of the dreadful mosquito.
Not your ordinary raingardens…Pre-Treament is key to longevity
Bloomington’s curb cut raingardens have evolved over the years. This version contains a concrete pre-treatment structure with overflow pipes to direct water into the raingarden. The purpose is to allow stormwater into the structure from the street and capture solids and organic material before it overflows into the raingarden. Without pre-treatment, solids would enter the raingarden directly and eventually clog the basin. This would decrease both infiltration and the basin’s capacity to hold stormwater.
The overflow pipes drain onto limestone blocks placed in the raingarden to prevent erosion and washout of the mulch. There are also holes in the bottom of the pre-treatment structure to slowly drain the stormwater down after it rains. The City of Bloomington is responsible for the removal of the solids that accumulate in the pre-treatment structures. They visit each garden twice per year with a vacuum truck to remove accumulated solids.
Double Shredded Hardwood Much
After the basins were formed, the pre-treatment was in place, the curbs were cut, and edging was placed, mulch was spread throughout each raingarden. The type of mulch used in raingardens is very important. Double shredded hardwood mulch is fibrous and binds together at the bottom. It forms a dense matrix that helps keep weeds from growing, keeps moisture in the soil during dry spells and it won’t float away during storm events.
What about the Plants?
Upon plant delivery, Rich worked with the landscape contractor to lay out the plants according to each design. A distinct and concise plant pallet was chosen for this project for ease of maintenance, variety of bloom times, visual interest and their ability to tolerate the extreme wet and dry conditions that occur in raingardens. Most of the raingardens contain 9 to 12 species, making it easier for homeowners to become familiar with and identify the beneficial plants. More importantly, it makes it easier to identify unwanted noxious weeds such as tree seedlings, thistle and dandelion.
908 perennials, 92 shrubs and 1 tree were installed in the curb cut raingardens. During the establishment period (2-3 years), homeowners are responsible for watering the plants. Once established, the plants’ deep root systems will allow them to survive on their own. Some of the raingarden plants include: Dwarf Bush Honeysuckle, Red Osier Dogwood, Little Bluestem, Prairie Dropseed, Butterfly Weed, Meadow Blazingstar, and Purple Coneflower, to name a few. Property owners were given a copy of their raingarden plan to be used as a guide when it comes time for maintenance. The Blue Thumb Plant Selector Tool is a very useful resource in identifying individual plant characteristics.
The first flush is what we’re after!
While many raingardens will fill up with stormwater during major rain events and cause water to bypass the inlet, we’re capturing the worst of it. The first flush of rain water contains the highest concentration of pollution during a storm event. That’s including phosphorous which leads to algae blooms in lakes and rivers. This project reduces the amount of phosphorus running into storm sewers (which lead directly to the Minnesota River) by 70%! Additionally, over 90% of all rainfall events are 1″ or less within 24 hours. Most of these typical rain events will be captured and infiltrated in the 18 raingardens. Overall, this project captures 64.5% of the total runoff from 55.42 acres in Bloomington, MN. But, it’s really the first flush that counts!
Making a Difference
Since 2008, the City of Bloomington has installed over 50 curb cut raingardens, 18 of which were part of the 2012-2014 Green Streets for Blue Waters Project. Metro Blooms is proud to be part of such a transformative process. Raingardens and bioswales are becoming an increasingly familiar landscape in Bloomington neighborhoods year after year. Take a stroll down a block next summer and there’s a good chance you’ll find a raingarden in bloom, teaming with bees and butterflies. When you see these know that the cumulative effect is making a difference in the neighborhood, the Minnesota River and beyond.