Shades of Green: Flood Control Study Focused on Duluth, Minnesota
Maintaining and restoring the wetlands near the headwaters of Chester Creek and within the higher portions of the watershed may be the first and biggest step to managing floodwater. Map generated by Tom Hollenhorst, ecologist, EPA Mid-Continent Ecology Division, Duluth, MN.
The City of Duluth has built a rapport with staff from the National Oceanic and Atmospheric Administration's Coastal Services Center (CSC) over a 4,548-acre combination of forests, homes, businesses, bluffs and streams. This swath, defined as the Chester Creek Watershed, lies primarily above a waterfall and is part of the Lake Superior basin.
In the aftermath of the economically and environmentally painful flood of 2012, the City and the CSC examined ecologically based options to reduce runoff velocities and flood volume in the watershed with assistance and input from Minnesota Sea Grant, the University of Minnesota Duluth's Natural Resources Research Institute, the Mid-Continent Ecology Division of the Environmental Protection Agency (EPA), and others. The resulting report, Economic Assessment of Climate Change Adaptation: Pilot Studies in The Great Lakes Region, synthesizes a year's-worth of data gathering, modeling and discussions into a springboard for what happens next: incorporating “green infrastructure” into the City's plans for the watershed.
Green infrastructure relies on vegetation, soil and natural landscapes to manage rainwater where it falls. By contrast, traditional gray infrastructure invokes systems that quickly dispose of stormwater, such as pipes, pumps and reservoirs. Tashya Allen, Coastal Hazards Specialist with CSC, said the Duluth study concentrates on green infrastructure options because they help protect against flooding while providing recreational, aesthetic and ecological benefits.
"This study lays out a path for the City of Duluth. It is not the only path, but it is at least one way to prepare for future floods," said Allen.
An open house hosted by Minnesota Sea Grant attracted residents, city officials and others in July. The focus of the open house was to share information about green infrastructure and the newly minted Coastal Services Center report.
Duluth is special, but it's not the only city where green infrastructure and economics have been jointly studied. Duluth split the limelight with Toledo, Ohio, in the final report based on the CSC study made possible through Great Lakes Research Initiative funding. The EPA also recently published a report* about 13 cities in which green infrastructure was actually implemented and fiscal analyses were conducted. Both the CSC report and the EPA report were written so that other communities can glean ideas from a framework founded on science.
The ideas for Duluth, documented in the CSC report, are based on flooding simulations. The simulations indicate that today a "100-year storm" (a storm that has a 1 percent probability of occurring every year) is a storm that produces a peak flow of 1,530 cubic feet per second in the lower reaches of Chester Creek. By 2035, the modelers suggest that a storm producing this rate of flow could have a 1.84 percent chance of occurring in a given year due to minor increases in extreme weather and average rainfall, and development projections (adding 0.1 percent and 0.74 percent to the probability, respectively). In essence, within two decades our "100-year storm" will look like it is occurring every 54-years at the water gages in the lower reaches of Chester Creek.
Today, according to the modelers, a "100-year storm" would accompany a volume of water that would spill over the banks of Chester Creek to damage 21 structures to the tune of $405K; in 2035, 22 structures could be damaged adding $15K to the bill. If the City could design a way to reduce current peak discharge by 20 percent — an achievable goal, says the City — the ferocity of the water rushing down Chester Creek in such a deluge might be tamed to that of a more manageable 25-year storm. Green infrastructure suggestions to reduce current peak discharge by 20 percent include a combination of:
- Underground storage beneath parking lots and roads (estimated $84 per cubic foot of flood storage)
- Tree trenches along sidewalks ($ estimate not available)
- Bioswales along unimproved roads (estimated $21 per cubic foot of flood storage)
- Permeable pavement sidewalks along unimproved roads (estimated $17 per cubic foot of flood storage)
- Roofs that store precipitation on commercial buildings (estimated $6 per cubic foot of flood storage)
- Retention ponds in open areas (estimated $3 per cubic foot of flood storage)
- Wetland preservation and restoration in the upper section of the watershed
($ estimate not available)
- Stream "re-meandering" ($ estimate not available)
- Vegetation management in the upper portions of the watershed ($ estimate not available)
Through the process of examining the Chester Creek Watershed — collecting data, running the computer simulations, discussing results and then documenting them — it became clear that reducing flood damage with green infrastructure and the costs associated with installing green infrastructure are highly site-specific. Since cost-effectiveness will be a determining factor in how the City can manage stormwater, the study team sees preserving such preexisting green infrastructure as an obvious first step in achieving a 20 percent reduction in peak water flow in Chester Creek. Such places exist in wetlands near the headwaters of Chester Creek and within the higher portions of the watershed.
Allen and her colleagues with the CSC will provide technical support as Duluth moves further toward implementing green infrastructure ideas; so will Minnesota Sea Grant's Hilarie Sorensen and Brent Schleck, who serve as the study's information coordinators and community educators.
"In February of 2014, the City of Duluth was the first city ever to receive a Great Lakes Restoration Initiative Shoreline Cities grant through the EPA," said Sorensen. "The funding is being used to implement three green infrastructure projects. It would be exciting to have a fourth one going on in the Chester Creek Watershed. There is so much potential here."
To access Economic Assessment of Climate Change Adaptation: Pilot Studies in The Great Lakes Region, contact Hilarie Sorensen (email@example.com, 218-726-7677) or go to: www.csc.noaa.gov/digitalcoast/publications/climate-change-adaptation-pilot.
*Case Studies Analyzing the Economic Benefits of Low Impact Development and Green Infrastructure Programs. August 2013. EPA 841-R-13-004.
By Sharon Moen