Objective: Identify and describe the different approaches to stormwater infrastructure development and the process for resilient infrastructure planning, as well as best practices for encouraging project implementation over a long timescale.
Author: Bridget Faust, Association of State Floodplain Managers.
Publication Date: June 30, 2014
Update Notes: May, 2016 - In the two years since this study was published, the city of Two Harbors has not experienced any severe storms events to further test the city’s improved stormwater system. Currently, the city is working to eliminate a variety of invasive species (e.g., buckthorn) from the Skunk Creek watershed and plant more trees across the landscape to lessen potential runoff and soil erosion.
The current Lake County watershed management plan is in the process of being replaced by the Skunk Creek watershed management plan. This is a cooperative effort between Lake County and Cook County. The new plan entitled, “Lake Superior North Watershed - Comprehensive Watershed Management Plan” has been completed and is now headed for public hearings and Minnesota state agency reviews before returning to Lake County and the Soil and Water Conservation District for adoption. This new plan is based on recent scientific principles and extensive watershed modeling. The new plan is projected to last ten years with annual assessments and a five year review.
Update source: Dan Schutte, District Manager, Lake County Soil & Water Conservation District
The city of Two Harbors, Minnesota, is located along the shore of Lake Superior approximately 25 miles north of Duluth. Although this city is home to just under 3,700 people ("Two Harbors, MN"), significant amounts of logging, railroad development, and urbanization upstream have altered the hydrology within the Skunk Creek watershed (Jaschke, 2012). While addressing the quantity and velocity of water flowing through the watershed had been identified as a high priority by managers in the region in 1997, wider acknowledgement of this need did not come about until 2 years later (Arrowhead Currents, 2002). In July 1999, Two Harbors experienced a 100-year storm event with catastrophic consequences (Seidel et al. 2012). Private property, city infrastructure, streets and highways were extensively damaged. In addition, because Skunk Creek flows into Lake Superior within one half mile from the public water intake for the City of Two Harbors, streambank erosion and pollutants from runoff threatened the quality of the water being supplied to residents' homes. With increasing storm-induced streamflows in the years since, additional water treatment has been required. Once taken into consideration, these real impacts and potential threats prompted city-wide support for immediate action in terms of stormwater management (Seidel Interview, 2014).
Recognizing that it was only a matter of time before a flood of the 1999 event's magnitude occurred again, Lake County Soil & Water Conservation District (SWCD) staff and University of Minnesota Extension personnel began assembling a team to create a stormwater management plan for the City of Two Harbors (Seidel Interview, 2014). With the help of a grant from the Minnesota Lake Superior Coastal Program, the plan was completed just two years after the flood. Over the course of the next ten years, the city invested over $80,000 into its stormwater infrastructure. With this and matching funds from various competitive grants, the following projects were completed: three flood control basins, two stream bank stabilization projects, and a rain garden. A Two Harbors Urban Forest Management Plan (with an emphasis on stormwater management) was also developed (Seidel et al. 2012). These stormwater system enhancements were designed to be resilient to conditions ranging from the 100-year to the 500-year flood event (Jaschke, 2012; Seidel Interview, 2014).
To learn more, click the map below and take a tour of the Two Harbors stormwater infrastructure system.
In June 2012, this newly reinforced stormwater system was tested when it sustained the now-infamous "Solstice Flood." In just 48 hours northeastern Minnesota saw eight to ten inches of rain fall onto its already-saturated soils. As a result, nearly all of this rainfall immediately ran as surface runoff down the moderate to steep hillsides for which the region is known, washing out roads and bridges, uprooting trees, dislodging boulders, and carrying massive quantities of debris and sediment directly into Lake Superior ("2012 Solstice Flood"). Owing to effective stormwater system design, the city sustained only minor damage to public infrastructure and private property (Jaschke, 2012). The purpose of this case study is to describe in detail the process for resilient infrastructure planning like it elsewhere and to identify some best practices for maintaining long-term support for these initiatives.
Different Types of Stormwater Infrastructure, Resilience Metrics and the Impacts of Urbanization
To understand the enhancement strategy used in Two Harbors, several critical questions need to be answered in regard to stormwater infrastructure systems:
1. What is Stormwater Infrastructure?
2. What is Green Infrastructure?
3. What are Flood Control Structures?
4. How do you Measure the Resilience of a Stormwater System?
5. What do These Infrastructure Systems seek to Mitigate and Enhance, and what Physical Characteristics Impact Their Effectiveness?
What is Stormwater Infrastructure?
Stormwater infrastructure is a network of piping, catchments, and facilities that are designed to manage runoff from impervious surfaces such as roofs, roads, and parking lots into storm sewers and nearby surface waters. In general, the approach to stormwater management taken by any given community will be determined by the challenges it faces and the budget it has available. Chronic flooding, water conservation, and water quality are all common issues that are addressed through the construction of stormwater infrastructure. Such infrastructure currently falls into two categories: gray infrastructure, which relies on pipes and sewers to convey rainwater away from where it falls ("Green and Gray Infrastructure"), and green infrastructure, which uses vegetation, soils, and other natural features to capture and manage rainwater on-site ("Green Infrastructure").
What is Green Infrastructure?
Green infrastructure is a relatively new term that refers to a network of open spaces and natural areas (such as wetlands, parks, green streets, and forest preserves) that work together to naturally manage stormwater, reduce flood risk, and improve water quality. ("What is Green Infrastructure?", "Low Impact Development", "Vermont Watershed Management Division – Green Infrastructure"). Read more...
When constructing a new piece of stormwater infrastructure, Low Impact Development (LID) is one commonly applied approach to stormwater management. LID stormwater management strategies seek to recreate or mimic the hydrology of a region prior to human disturbance by working with on-site natural features to increase the infiltration, storage, filtration, evaporation, and transpiration capacity of the landscape ("Low Impact Development Resources"). Some of the more common structures one might see in a LID management strategy include rain gardens, sand filters, infiltration trenches, swales, permeable pavement, cisterns, rain barrels, green roofs, and detention ponds. Read more...
Two Harbors - Courthouse Rain Garden
Courtesy of: Wayne Seidel
However, existing structures frequently do not allow for the construction of new LID structures. Instead, communities are forced to work around or build upon existing structures. The process of upgrading or enhancing existing gray infrastructure with green infrastructure is called retrofitting. Retrofitting is often the most practical and cost-effective option for cities to implement, especially if they are already highly developed. In addition, because they help to 'soften' the landscape in areas that have already been highly urbanized, retrofit projects often allow more water to be stored in areas that need the additional storage the most. Retrofits thus provide critical stormwater storage for municipalities (Winters et al. 2012). Some examples of retrofitting include building a green roof when replacing one's roof, replacing concrete pavement with permeable pavement, and transforming an existing garden into a rain garden. Read more...
What are Flood Control Structures?
In addition to Low Impact Development strategies, flood control structures are another building method for mitigating the effects of flooding. This umbrella term refers to any type of stormwater infrastructure that seeks to alter the conveyance or storage of water within a system. In general storage these structures are designed to attenuate, or reduce, peak flows during a flood event. Structural improvements that are designed to alter the conveyance of water typically attempt to slow the rate at which water flows or the quantity of water that is able to easily flow through a system. Flood control structures can be considered either green or gray infrastructure depending on their purpose and the materials used to construct them. Flood control structures are commonly constructed from traditional gray infrastructure materials such as concrete. Read more...
City of Two Harbors Stormwater Detention Basin - Battaglia Blvd.
Courtesy of: Bridget Faust
Two Harbors - 19th Street Detention Basin
Courtesy of: Wayne Seidel
Two Habors - Stormwater Conveyance Pipes
Courtesy of: Bridget Faust
Typically, a combination of flood control structures, retrofits, and LID will be used when completing a major stormwater system improvement (Bloch et al., 2012). This is the true in the city of Two Harbors, in this case all three stormwater detention basins fall into the category of flood control, the two stream bank restorations are considered retrofits, and finally, the rain garden is an example of LID. See for yourself by viewing a storymap of Two Harbors infrastructure...
How do you Measure the Resilience of a Stormwater Infrastructure System?
The resilience of any stormwater infrastructure system can be measured based on the impacts it is designed to mitigate. The goals established in Two Harbors' stormwater management plan were to 1) decrease the turbidity and pollutant levels of water discharging into Lake Superior, and 2) become resilient to the 500-year flood event. This commonly used latter metric means that, in any given year, there is an approximately 0.2% chance of a flood of that magnitude occurring. This predictive value is called a flood recurrence interval, and it is used to describe the resilience of a stormwater management system. Read More...
Courtesy of: City of Lincoln, NE
In many cases the need for development of additional stormwater infrastructure results from changes in land use within or upstream of a community. In general, increases in the extent of impervious surfaces are associated with higher rates of flooding. Impervious surfaces are problematic because they prevent rainfall from infiltrating into the ground. As a result, rainwater collects and rapidly runs off, carrying with it sediments and other contaminants that wash into nearby streams and other water bodies ("Impervious Surfaces and Flooding"). As ever larger areas of a watershed are converted from vegetation to impervious surfaces, less water is allowed to infiltrate into the ground. Although these effects may not lead to serious flooding in the short term, over time even minor rain events begin to culminate in larger quantities of water flowing into stormwater systems at faster rates. Eventually, system capacities are exceeded, thereby necessitating widespread system enhancements to prevent future erosion and flooding. Low Impact Development, flood control structures, and retrofitting can mitigate the deleterious side effects of urbanization by creating or enhancing areas within the developed landscape that provide opportunities for increased infiltration, storage, evaporation, transpiration, and filtration.
What Natural Factors Impact the Effectiveness of and Demand for Stormwater Infrastructure?
Besides land use and land cover, natural features within the watershed can also have significant impacts on the stormwater management needs of individual communities.
The quality and type of soils on which a city lies can have significant impacts on the quantity and rate at which water infiltrates into the ground. In general, soils with a larger particle size like sand or gravel, low water content, and low levels of organic matter allow for faster infiltration rates. As a result, urban areas with higher quality soils may not require as many Low Impact Development structures to mitigate the effects of a heavy rainfall event because of their increased capacity to absorb and hold water. Conversely, urban areas with poor soils may need additional LID structures to mitigate the effects of a significant rainfall event. Within Two Harbors, the predominant soil type present had significant implications for the city's stormwater management needs. Because the region has underlying heavy clay soils, infiltration rates are very slow. Due to increased surface runoff, there arose a growing demand for additional storage within the watershed to help lessen the impacts of the associated severe flood events. Read more...
Site topography also has a significant influence on the types of LID techniques implemented in a community, as steep hillsides present different challenges than flatter lands. In general, steeper hillsides require management techniques that will slow water down as it flows over the land. Reducing the velocity of runoff as it flows down a hillside can help to promote infiltration into soils, and ultimately reduce the amount of water entering channels downstream. Conversely, flatter landscapes require enhanced drainage in order to prevent excess water from ponding on-site. The city of Two Harbors and the surrounding Skunk Creek watershed are characterized as having moderately steep topography. Combined with the heavy clay soils, the stormwater management plans developed after the 1999 flooding addressed this challenge by ensuring that detention ponds were constructed in the upper reaches of the watershed. By increasing storage upstream, the quantity of water reaching downstream channels was significantly reduced and the moderately high flow was maintained over a longer duration. This ultimately reduced the flow reaching Lake Superior after subsequent heavy rainfall events, in turn reducing the sediment and contaminant loads being deposited in the lake. Read more...
What are the Co-Benefits of Low Impact Development Structures and Retrofitting?
Retrofit and Low Impact Development infrastructure have been shown to, in most cases, both reduce capital cost by between 15% and 80% and require less annual maintenance than traditional gray infrastructure (EPA, 2008). Further, they have the potential to significantly reduce municipal water treatment costs by effectively reducing the pollutant loads entering the water supply. For example, for every 10 percent increase in forest cover in the source area, treatment and chemical costs will decrease by approximately 20 percent. Finally, retrofit and LID infrastructure are proven to increase property values by improving site aesthetics and recreational value (EPA, 2008; ERG, 2014).
To date, much is still unknown about the value of the ecosystem services these natural structures provide. In addition to cost reduction, these management strategies have been shown to mitigate urban heat island effects, increase available habitat for wildlife, promote groundwater recharge, reduce erosion rates, and improve air and water quality. If a true monetary value could be assigned to these natural structures, more informed decisions could then be made about what types of stormwater infrastructure provide the most return on investment. This is an expressed research need among the management community (EPA, 2008; ERG, 2014).
The Stormwater Planning and Implementation Process
Infrastructure development poses a unique challenge for communities in that projects take a significant amount of time and funding to complete. Communities frequently need help from consulting engineers to determine what actions should be taken in order to make their stormwater infrastructure more resilient. In addition, working to complete projects that can take a decade to bring to fruition requires that strong relationships be created and maintained between planning entities, local decision-makers, managing agencies, and other stakeholders. Strong relationships between diverse partners can help to keep issues like stormwater infrastructure salient within the community and, because of their different constituencies, encourage engagement from more of the general public. Although any given community will have its own individual process for planning and implementation of stormwater system developments, there is much to be learned from the success stories of others. To learn more about the process followed by the City of Two Harbors, please click through the following diagram.
Best Practices for Ensuring Long-Timescale Project Completion
As is true with almost any landscape-level challenge, there is no single solution to the problems of the type overcome by the City of Two Harbors. No two communities are alike, and a multitude of factors—social, financial, meteorological, and geological—will determine what approach to stormwater infrastructure development (green infrastructure, gray infrastructure, Low Impact Development, flood control structures, retrofitting, or a mix) is most appropriate. The cost-efficiency and effectiveness of potential solutions should be taken into account before deciding which alternative(s) to pursue.
Beyond there being no one-size-fits-all solution, additional barriers to implementing system-wide stormwater infrastructure upgrades are widely known. Landscape-level initiatives not only take a considerable amount of time to realize, but also require that deep and diverse partnerships be built and maintained to have a chance of success. With this in mind, a few best practices for planning and implementing long timescale projects can be drawn from the efforts of the partners who carried out the stormwater infrastructure upgrades in Two Harbors, Minnesota.
1. Connect – with city officials, interest groups, and concerned citizens to start a dialogue around the issue and encourage grassroots support.
Critical partners include anyone from soil, water, and coastal resource managers, urban planners, utility commissions, watershed alliances, task forces, community groups, local decision-makers, impacted residents, county and state agencies, and regional non-profit organizations. Building connections between stakeholders that do not necessarily interact with one another frequently can present a major challenge. Simply waiting for the correct moment to take action can greatly increase the success of this effort. In Two Harbors, for example, what started out as a recommendation from the county water planning commission quickly grew into a large-scale initiative by building on the concerns raised after the flooding of July 1999. Because of this opportune accident of timing, many new critical partners were brought to the table with little effort on the part of those advocating for the project. Building these connections early on laid the foundation for strong partnerships down the road.
2. Coordinate – all partners who need to be present to implement on a community or county level.
Implementation is a strategic process, especially for large geographies and long-term initiatives like stormwater infrastructure enhancements. It is rare that a community enters into a planning process with the resources already on hand to implement all of the proposed retrofits, Low Impact Development, and flood control projects at the same time. Because of this, identifying and involving partners who will have the needed influence and/or expertise during certain phases of the process can greatly increase the likelihood that it is completed in a timely manner. For example, project personnel in Two Harbors looked to Lake County's master gardeners and others to assist in the design and construction of the courthouse rain garden. An additional benefit of this targeted approach to partner involvement is that it helps to build a sense among stakeholders of perceived ownership of the individual project and the broader initiative.
3. Capitalize – on opportunities when they become available.
Not surprisingly, funding is one of the most commonly cited barriers to the implementation of stormwater infrastructure. While competitive funding is difficult to obtain, the use of a few key techniques can help increase the likelihood of receiving funding for projects. For example, Lake County SWCD and UMN Extension personnel remained adaptable with regard to the order in which projects were implemented. Although they knew that building the detention basins first would be critical to the success of the project, when funding opportunities materialized that suited any of their project objectives, they made efforts to apply for them. In addition, the project partners took on the grant writing responsibilities that typically fall to city staff. This was done with the recognition that other obligations would likely prevent them from completing the task on a short timescale. By taking charge of the grant writing process they provided critical support to city staff. Finally, the project partners worked to ensure that projects were completed on time and to a high standard of quality. This helped establish a trusting relationship between the City and the project's funders and effectively increased the likelihood that the same funding entities will work with them again. The due diligence and conscientious effort shown was critical to the success story told here. Finding a local champion to take on the grant writing process, remaining flexible in the approach to implementation, and ensuring that projects are completed on time are all essential to funding stormwater infrastructure projects over the course of a long timescale.
4. Communicate – impacts and outcomes to project partners and the general public.
When working on long project timescales, keeping the central issue to be addressed salient and relevant in the minds of decision-makers, funding entities, and the general public can be a significant challenge. As time passes and progress is made, long-term initiatives like stormwater infrastructure frequently get tabled as more immediate, pressing issues are addressed by communities. Continual effective, interactive communication is another best practice for ensuring that a community's green infrastructure initiative remains a community-level priority. The City of Two Harbors embraced this concept at every step of the implementation process: giving tours to all interested parties, ensuring that credit was given to funding entities and that they were made aware of the finished work their contributions made possible, working with local media outlets to showcase the progress made within the community, and hosting public meetings to educate residents on the work being done in the community. Ultimately, local decision-makers, funding entities, and residents were never allowed to lose sight of the task at hand, which in turn guaranteed that the stormwater infrastructure update initiative never lost momentum before completion.