Objective: Learning from Toledo’s experiences and sharing their process to help other communities identify their flooding issues and understand the feasibility of using nature-based strategies (like green infrastructure) to help reduce flooding and what the costs and benefits of doing so would be.
Authors: Tashya Allen – The Baldwin Group at NOAA OCM, Rachael Franks Taylor – The Baldwin Group at NOAA OCM, & Lauren Long – The Baldwin Group at NOAA OCM.
Publication Date: June 23, 2016
Stormwater Flooding in Toledo, Ohio
The City of Toledo – situated in the low-lying area formerly known as the Great Black Swamp – is susceptible to flooding and drainage problems. Located at the southwestern crook of Lake Erie, the city spans both sides of the Maumee River just south of the Maumee Bay. Toledo and the surrounding area of Lucas County have numerous small creeks, tributaries and drainage ditches that flow into Lake Erie.
Nuisance flooding reports within the City of Toledo between 2005 and 2009.
Image Courtesy of: City of Toledo (archived image).
Although major flooding events get the most attention, areas that receive chronic flooding from short and intense rain events (sometimes an inch of rain within an hour or two) are affected in the same negative ways. Recent years have experienced more frequent and intense rain events leading to standing water, basement flooding, decreased water quality from either increased erosion or combined sewer overflows (CSO), a strain to public services and budgets, overloaded storm water sites, and property damage. The Toledo Waterways Initiative is addressing the CSO issues, but is not aimed at addressing storm-water flooding.
To help address the flooding issues, Toledo has been exploring the use of natural and nature-based strategies. The Office for Coastal Management, through a grant from the EPA Great Lakes Restoration Initiative partnered with the City, the Association of State Floodplain Managers, USACE, and Eastern Research Group, Inc., was tasked with developing a guide that helps communities work through the process of identifying their flooding issues and the feasibility of using nature-based strategies (like green infrastructure) to help reduce flooding and what the costs and benefits of doing so would be.
Through this process Toledo has been able to secure federal funding to implement bioswales into the study area, has established a private-public partnership with the local General Motors (GM) Powertrain Plant whereby green infrastructure was implemented on the property, and helped provide guidance on next steps for the regional green stormwater task force.
To see the locations, types, and even photos of some of the green infrastructure in the Silver Creek watershed and greater Toledo Metropolitan Area. Check out the Toledo Metropolitan Area Council of Government's (TMACOG) Green Stormwater Infrastructure Mapping Tool.
Screen shot of the TMACOG Green Stormwater Infrastructure Mapping Tool.
Factors and Conditions Affecting Stormwater Flooding
Flooding occurs when precipitation accumulates faster than it can be absorbed into the soil, evaporated by the sun, transpired by plants, stored in lakes, or conveyed by streams or stormwater infrastructure to receiving waters. Flooding occurs naturally. Floodplain areas, if left in their natural state, function to store and gradually release flood flows. This slows the flow of water and allows sediments to drop out which re-nourishes floodplains and bordering wetlands.
Several conditions can increase the likelihood of flooding: increased precipitation or increased intense precipitation events, large areas of impermeable surfaces like parking lots, insufficient natural resources like park lands and open spaces, land use, and the location of structures, for instance, where people build in the watershed and how close they are built to a stream.
Precipitation now and in the future
"Extreme rainfall events and flooding have increased during the last century, and these trends are expected to continue, causing erosion, declining water quality, and negative impacts on transportation, agriculture, human health, and infrastructure" (Key Messages about the Midwest from the National Climate Assessment, no date). Generally, annual precipitation increased during the past century (by up to 20% in some locations), with much of the increase driven by intensification of the heaviest rainfalls. This tendency towards more intense precipitation events is projected to continue in the future (National Climate Assessment, 2014).
According to the model used for this study, EPA's Climate Resilience Evaluation and Assessment Tool (CREAT), projected changes in average annual precipitation vary widely from a 3.2 percent decrease to an 8.9 percent increase in Toledo (see tables below). The projections are the range of values from the three CREAT model projections (hot and dry, central, and warm and wet). The warm/wet model projection was the climate scenario used to extrapolate precipitation data for Toledo in this study because it provides a worst case scenario with the greatest increase in precipitation throughout the year.
Toledo Climate Data Projections
Table Note a: For a detailed description of the methodology used to develop the
future climate scenarios, please see the Methodology Guide within EPA's CREAT software.
Table retrieved from page 3-6 of NOAA OCM's Economic Assessment of
Green Infrastructure Strategies for Climate Change Adaptation
Toledo Projected Change in the Frequency of the 100-Year, 24-Hour Storm Event
Table Note a: For a detailed description of the methodology used to develop the
future climate scenarios, please see the Methodology Guide within EPA's CREAT software.
Table retrieved from page 3-7 of NOAA OCM's Economic Assessment of
Green Infrastructure Strategies for Climate Change Adaptation
Land use and how it contributes to flooding
Image Retrieved from: Stream Corridor Restoration: Principles, Processes, and Practices; page 3-23.
Courtesy of: The Federal Interagency Stream Restoration Working Group.
Land use decisions can affect flood losses. Post-development hydrology is often very different from predevelopment hydrology. When watershed hydrology changes due to development, runoff is increased and floodplains may not be able to contain the increase in stormwater runoff. Consequently, flood elevations can increase and adjacent low-lying areas can become more flood-prone. This situation is exacerbated when development occurs within the floodplain and adjacent areas because natural flood storage capacity is displaced.
Structures built within a floodplain are at high risk of being damaged during a flood event. Additionally, development outside the floodplain can reduce the natural systems' ability to moderate flooding. Development increases flooding when pervious, vegetated land is replaced with impervious surfaces (e.g., pavement, buildings). This reduces evapotranspiration and prevents precipitation from slowly infiltrating into the soil and recharging groundwater, rivers, and streams.
Urban flooding can occur due to overbank flooding or when stormwater overwhelms drainage systems and ends up in basements, backyards, and streets. Impervious surfaces also increase stormwater runoff volumes, velocities, and peak discharges. Stormwater runoff can also affect water quality by increasing sediment, pathogens and nutrient loads as well as the temperature of receiving water.
Green infrastructure ability to slow and store floodwater
Rain Garden. Retrieved from US EPA Flickr.
Permeable Pavers. Retrieved from US EPA Flickr.
Green Roof. Retrieved from US EPA Flickr.
Natural resources like trees, native grasses, and wetlands soak up water. Green infrastructure (GI) is comprised of stormwater management techniques and practices that mimic natural hydrologic functions and incorporate landscape features to store or treat runoff. GI incorporates the natural environment to provide multiple benefits and support resilient communities. GI can include site-specific management practices like rain gardens as well as watershed-scale techniques such as land preservation and the restoration of wetlands and floodplains that naturally store water and reduce runoff.
A case study describing recent stormwater system upgrades for Two Harbors, MN titled Resilient Stormwater Planning Takes Time and Pays Off provides more information on stormwater infrastructure. In most communities where gray infrastructure is already in place, there are opportunities to design for or "retrofit" GI during infrastructure replacement and capital improvement projects. GI options are gaining widespread support as a credible approach that communities can use to manage stormwater sustainably and provide co-benefits. For more information, check out the publication Green Infrastructure Options to Reduce Flooding (NOAA, 2015).
Breakdown of Green Infrastructure Planning
Toledo wanted to be able to answer several questions about current and future flooding to help them better plan for projects that could reduce flood impacts. The Guide for Assessing Green Infrastructure Costs and Benefits for Flood Reduction (NOAA, 2016) provides the detailed steps that any community can use. The following steps outline the analysis done for Toledo to help them better plan for future flood impacts using green infrastructure approaches. For more details about the process, click on each step listed below the graphic or reference the extended methodology here.
Green Infrastructure Process Guide Framework.
Retrieved from pg. 4 of A Guide to Assessing Green Infrastructure Costs and Benefits for Flood Reduction.
Image courtesy of: NOAA Office for Coastal Management.
- Define the Problem
- Assess Flooding Scenarios without GI
- Identify How a Flood Reduction Target Can Be Met with GI
- Assess Flooding Scenarios with GI
- Estimate Benefits and Costs
- Identify and Communicate the Desired GI Strategy
The goal of Step 1 is to define the flooding problem, project scope, study area, and information needs and availability. Step one consists of the following tasks:
- Choose a watershed study area
- Characterize flooding issues and causes
- Determine what's at risk
After considering the criteria, the project team chose to work in Silver Creek watershed, which discharges into Lake Erie. The drainage area of Silver Creek alone (i.e., the area upstream of where Shantee Creek joins Silver Creek), which was assessed in this study is 7.41 square miles.
Flooding is an issue in Silver Creek under current conditions. Land use within the Silver Creek watershed is primarily residential and commercial. 9,370 acres or 92% of Silver Creek watershed is covered in paved materials (e.g., concrete). This is a problem because rooftops and streets do not absorb rain water. Rainwater runs off into nearby streams, ditches, and storm drains. If there is too much water, too fast, then these systems become overwhelmed and flooding happens. Data and land use characteristics (both current and historic) can be retrieved from NOAA's CCAP Land Cover Data.
Land cover change in the Silver Creek watershed.
Courtesy of: NOAA Office for Coastal Management.
The goal of Step 2 is to answer the question: What are the estimated flooding impacts now and in the future without a green infrastructure strategy? Step two consists of the following tasks:
- Gather data
- Model current and future flooding
- Quantify current and future flood damages
The outcome of step 2 is to provide an estimate of the financial impacts of flooding under current and future scenarios without green infrastructure. Both scenarios are modeled using a 100-year, 24-hour storm event. For Toledo the financial impacts for these two scenarios are:
Current Flood Damages:
It was estimated that 253 structures in Silver Creek were damaged, totaling $738,000 in costs under current precipitation conditions.
Future Flood Damages for 2035:
Economic losses from flooding increase by more than 30 percent in the future (2035) scenario with a 4.85 percent annual increase in precipitation, compared to existing conditions. It is estimated that 293 structures in Silver Creek were damaged, totaling $980,800 in costs under future (2035) precipitation conditions.
Future land use plans in the Silver Creek watershed include additional development; primarily in areas that were previously developed therefore this will not have a large impact on hydrology and increase the percent impervious area in a manner that would increase stormwater runoff. The primary cause of increased stormwater runoff would be increased precipitation rather than an increase in impervious area. Because of this, the future land use data (i.e., percent imperviousness and land cover) is assumed to be the same as the current land use data for this study. The future precipitation was estimated based on data extrapolated from CREAT.
The goal of Step 3 is to establish a flood reduction target and design a green infrastructure strategy to meet the target, and consists of the following tasks:
- Select a flood reduction target
- Identify green infrastructure options
- Determine how much storage the green infrastructure options can provide
The City of Toledo chose a ten percent flood reduction target. A ten percent reduction in the 100-year, 24-hour storm peak discharge is a significant reduction. To reduce the peak discharge flooding by ten percent for three hours means Toledo needs to store 31 acre-feet of water under the current land use and precipitation scenario and 33 acre-feet of water under the future land use and precipitation scenario.
The following tools can be valuable resources if you are interested in learning more about the efficacy, benefits, and challenges associated with different types of green infrastructure:
The goal of Step 4 is to answer the question: What are the estimated flooding impacts now and in the future if a green infrastructure strategy is implemented to meet your flood reduction targets? Step four consists of the following tasks:
- Model current and future flooding if the flood reduction target is met with green infrastructure
- Quantify current and future flood damages if the flood reduction target is met with green infrastructure
The outcome of step 4 is to provide an estimate of the financial impacts of flooding under current and future scenarios with a green infrastructure strategy. Both scenarios are modeled using a 100-year, 24-hour storm event. If GI was implemented to reduce the peak discharge in Silver Creek by 10 percent (which corresponds to 31 acre-feet of flood storage under current conditions and 33 acre-feet of storage under future conditions), estimated economic losses from flooding associated with a 100-year storm would decrease by 39 percent under current precipitation conditions and 46 percent under future precipitation conditions.
Current flood damage results with GI
Flood Storage Volume
Flood Storage Volume = (1,255 ft3/sec - 1,130 ft3/sec)(3 hours)(60 sec/min)(60 min/hr)(acre/43,560 ft2) = 31 acre-feet. It was assumed that the peak flow is reduced by 10 percent for three hours. The three-hour reduction time was chosen based on engineering judgment and is somewhat arbitrary; however, it does provide an order of magnitude estimate of the storage volume needed for peak flow reduction.
It was estimated that 159 structures in Silver Creek were damaged, totaling $453,700 in costs under current precipitation conditions. Reducing the peak discharge by 10 percent resulted in a 30 to 44 percent reduction in total structural flood damages for various storm events. The same peak discharge reduction also resulted in 19 to 37 percent fewer structures being damaged in a storm event.
Future flood damage results with GI
It is estimated that 179 structures in Silver Creek were damaged, totaling $527,500 in costs under future (2035) precipitation conditions. Reducing the peak discharge with the implementation of GI reduces the flood losses. Reducing the peak discharge by 10 percent resulted in a 21 to 50 percent reduction in total building flood damages for various storm events. The same peak discharge reduction also resulted in 19 to 39 percent fewer structures being damaged in a storm event.
Structural Damage Estimates (100-year storm event)
Table retrieved from p. 20 of A Guide to Assessing Green Infrastructure Costs and Benefits for Flood Reduction.
The goal of Step 5 is to estimate annualized net benefits for your green infrastructure strategy and consists of the following tasks:
- Estimate green infrastructure option unit costs and strategy cost
- Estimate green infrastructure strategy benefits and co-benefits
- Annualize costs and benefits over a specific time frame
Cost is a large factor to consider when deciding what GI practices should be implemented on a site. In general, GI costs vary widely between geographic areas and are extremely site-specific. The project team performed a literature review of available GI costs nationwide. The team looked at both capital and operations and maintenance (O&M) costs per square foot of surface area of the practice and per cubic foot of water storage of the practice.
Green Infrastructure Estimated Unit Costs
Green Infrastructure Estimated Unit Costs
Unit costs retrieved from pg. 3-22 of NOAA OCM's Economic Assessment of
Green Infrastructure Strategies for Climate Change Adaptation.
Please see the extended methodology for table notes.
The amount of reduced damages associated with flood mitigation strategies is represented as "benefits" (i.e., the difference between the economic impact of flooding without flood mitigation and those impacts with the implementation flood mitigation infrastructure). These economic benefits will likely be widespread; however, due to data limitations, only the largest benefit of flood mitigation in Toledo is quantified: reduced building damages.
Reduced building damage is only one component of the overall benefits provided by implementing GI. Other potential benefits such as improved water quality, increased habitat, increased green space, reduced infrastructure damage, reduced land damage, and increased property values are important to consider, but were not able to be monetized in this study.
The Present Value (PV) of benefits from avoided building damage is estimated to be approximately $700,000 over 20 years (roughly $38,000 annually). The expected annual benefits increase over this 20-year period because the expected damages of storms will increase as expected precipitation increases.
Communities may want to consider longer benefit timelines in order to more accurately reflect the benefits provided by GI throughout its entire life cycle. If these benefits were extended to reflect a 50-year period, the PV would increase from $698,539 to $1,769,644 (roughly a 150% increase since the number of years considered increases by 150% and the benefits do not exhibit diminishing returns).
To learn more about economic assessments of green infrastructure, open spaces, and other natural resources visit the following data portals:
The goal of Step 6 is to identify and communicate the green infrastructure strategy and ultimately implement this strategy, which consists of the following tasks:
- Finalize green infrastructure strategy
- Communicate the green infrastructure strategy and plan next steps
As this analysis has shown, based on the models and past experience, flooding can be mitigated through the implementation of green infrastructure (GI), but this is just one tool that should be considered in the larger context of community land use and sustainability planning. Flooding can be worsened, negating gains made by implementing mitigation options, if preventing future flooding is not also part of the agenda. Thus, the decisions made about future development and redevelopment patterns in Toledo (in general) and in Silver Creek watershed (in particular) will impact future flooding. In the case of Toledo, which is largely built out, those decisions come primarily in the form of redevelopment—including where further density should be encouraged, how it is designed, where open spaces should be reclaimed, and where flood storage function should be restored and enhanced.
A wide variety of adaptive land use practices, policies, tools, and strategies are available to communities interested in planning for sustainable flood management. The following policy options were discussed for consideration in Toledo and are fully described in the Toledo Pilot Study, Section 3.11 (NOAA, 2014).
- Urban Form Requirements
- Transfer of Development Rights (TDR)
- Stormwater Ordinance Revisions
As a next step and follow-on to this project, it is recommended that Toledo refine the watershed-level analysis from this study and begin to hone in on specific locations and GI practices that can be implemented in the Silver Creek and other watersheds. A more refined analysis would include developing site-specific concept plans, calculating stormwater runoff reductions, estimating the cost of implementation for chosen GI practices, and developing a 20-year implementation plan that takes advantage of economies of scale and leveraging other capital improvement projects.
Toledo applied for funding through the EPA to implement several bioswales in the Silver Creek watershed based on the results of this study. Additionally, as a result of the NOAA technical assistance provided through this GLRI grant, the City of Toledo was able to increase its outreach about green infrastructure and the resulting community benefits seen from its implementation. The City was also able to partner with the General Motors Powertrain plant in the project study area to conduct a green infrastructure assessment, where the City was able to provide some technical assistance funds from our project to hire the consultant to develop site level green infrastructure options, leading to the implementation of bioswales on the plant's property to address flooding issues.
Improving the Process Approach Based on Toledo’s Experience
Anytime a new process is developed there are successes, challenges and lessons learned. This section describes the lessons learned from this process, based on the experiences and events that unfolded as the project team worked with staff and officials from the Toledo area. These lessons provide a strategy to improve this process guidance, which is then subsequently intended to be used by other communities going forward.
- Clarify needs and expectations up front, and repeat them at every meeting. Folks are busy and forget. This ensures that everyone is in the know and scope creep doesn't happen.
- Assess the benefits that ecosystem services provide. Toledo did not have the data needed to do this type of assessment. However in our other pilot study (Duluth, Minnesota) we were able to value recreational uses of the Chester Creek Park and stream bank land. Most communities have natural resources that are providing protection, aesthetics, food, recreation and these values should be quantified so you have a complete picture of the costs and benefits of all community resources.
- Identify as many damages as possible. Our study only looked at damages to buildings in Toledo. In our Duluth study we were able to assess damages to buildings, stream banks, stormwater infrastructure, and recreation. This was due to availability of data.
- Take a long-term planning approach and look for lasting solutions. The project team initially looked at a 20 year time horizon since that coincides with most land planning efforts. However, in the economic analysis, the costs outweighed the benefits; partially because we were not able to measure all flood damages. However when we extended the analysis out to 50 years, the benefits outweighed the costs. Since green infrastructure is part of a community's infrastructure, it makes sense to look at techniques that have a longer lifecycle.
Data, models, and information
- Start with best available data. We conducted the assessment using best available data and information. We wanted to have a basic method that the majority could use. However once we started the assessment, we found that some of the data and information either did not exist, was dated, or we couldn't use. For example, Toledo had existing hydrology and hydraulics models, however the models were too old to work and we had to go an alternate route which required the U.S. Army Corps of Engineers to run hydrologic and hydraulics (H&H) models for the project. This type of modeling can be costly.
- Use what you have, but be willing to modify if additional information becomes available. We determined that we would use FEMA's HAZUS loss estimation software to estimate buildings losses. The limitation is that HAZUS does not estimate losses to stormwater infrastructure, natural resources, or roads.
- Create inventory of data and information used. Having a helpful data and information organizational structure can help you quickly identify what is missing or the quality of some information.
- Conduct a technical debrief to share and discuss all the technical aspects of the project. Although we worked directly with the community to obtain technical information and we shared our steps throughout, we never really had the chance to go into great details about the modeling. Near the end of the study, we held a technical debrief that enabled us to spend several hours discussing all the assessment steps in detail.
Partnerships, Community Engagement, and Technical Assistance
- Consider who you need on your project team. The project was done through a partnership team that consisted of staff from the U. S. Army Corps of Engineers, Association of State Floodplain Managers, Eastern Research Group, Inc., Horsley-Witten, Inc., City of Toledo's Division of Environmental Services, other state and local partners, and NOAA's Office for Coastal Management (OCM). Each partner provided different resources, skills, and knowledge to the project.
- OCM worked with the Ohio's Coastal Management Program to identify some locations that were having flooding issues and were interested in researching how GI could help reduce the impacts from flooding. At the same time, the City was researching resources to help them.
- In Toledo, OCM partnered directly with the City of Toledo's Division of Environmental Services. We worked with a smaller group on most of the project, then for larger decisions, we worked with several other City departments and several non-profits, like American Rivers.
- The benefits of this approach
- Working directly with municipal staff is beneficial because they are on the ground dealing with the issues every day and know what they need.
- They can also see direct applications of the study and put it to work quickly. Toledo did this with preliminary results from the analysis. They were able to secure EPA funding to do a bioswale installation in the pilot study area. American Rivers was able to use our study as a foundation for a Coastal Storms Program proposal to do more detailed analysis for three study areas.
- The limitations of this approach
- Changes in government left us without a commissioner who supported our work.
- Government staff are busy and this can prolong project timelines.
- A Champion is needed. Ensure someone can see the study through to implementation. We worked with American Rivers, Old Woman Creek NERR and Ohio Coastal Management Program and the City of Toledo. They are ensuring that the study results are being implemented, helping inform future proposals, and projects.
- Look for ways to incorporate local knowledge. For example, in both communities, we brought up a map and talked through potential places to find floodwater storage. This enabled our team to identify a few places and rough estimate potential storage. It also helped the communities see potential areas for future project proposals.
- Share how you have incorporated your community project teams' input into the analysis. We ensured our project had enough time to obtain local input and knowledge and expertise. We also found it went a long way with the communities when you discussed with them how you used their knowledge and ideas.
- Look for ways to show off the communities' hard work and expertise among their peers, leaders, other communities, and at national conferences. We accomplished this by asking city staff to open up meetings and share what has been happening, and present at conferences.
- Ensure all partners are at the table. We should have had some state agencies at the table more often and engaged with them more on the project.
- Listen and learn, and provide technical assistance throughout. If had we not educated all stakeholders to ensure the process was understood and provided technical assistance throughout/after the study, our study would not have been grounded in local needs.
- Ensure that all project stakeholders and bought-in to the process and product; it would probably have been another report that just sat on the shelf; Toledo may not have been able to secure funding for implementation.
- Conduct outreach throughout the project to showcase what is happening in the project. One good outreach medium is social media; Toledo has a Facebook page that links to the study results.
- Conduct an assortment of engagement opportunities for a variety of stakeholders. We held project meetings in the communities every few months. We also held regular calls with a subset of team members monthly.
- Cast a wider net to share with others. Part of our GLRI funding requirement is to extend the information to others outside the pilot communities. We have held several webinars, talked at many conferences, and held smaller meetings about the project. We have developed a Process Guide to help others use the framework and step them through what it takes to conduct an assessment.