Green Infrastructure Benefits For Floodplain Management A Case Study

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.

  1. Define the Problem
  2. Assess Flooding Scenarios without GI
  3. Identify How a Flood Reduction Target Can Be Met with GI
  4. Assess Flooding Scenarios with GI
  5. Estimate Benefits and Costs
  6. Identify and Communicate the Desired GI Strategy

Step 1: Define the Problem

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:

  1. Choose a watershed study area
  2. Characterize flooding issues and causes
  3. 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.

Step 2: Assess Flooding Scenarios without Green Infrastructure

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:

  1. Gather data
  2. Model current and future flooding
  3. 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.

Step 3: Identify How a Flood Reduction Target Can Be Met with GI

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:

  1. Select a flood reduction target
  2. Identify green infrastructure options
  3. 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.

Step 4: Assess Flooding Scenarios with GI

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:

  1. Model current and future flooding if the flood reduction target is met with green infrastructure
  2. 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.

Step 5: Estimate Benefits and Costs

The goal of Step 5 is to estimate annualized net benefits for your green infrastructure strategy and consists of the following tasks:

  1. Estimate green infrastructure option unit costs and strategy cost
  2. Estimate green infrastructure strategy benefits and co-benefits
  3. 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

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).

Step 6: Identify and Communicate the Desired GI Strategy

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:

  1. Finalize green infrastructure strategy
  2. 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
  • Buy-outs
  • 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.

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.

This page is the extended methology for the case study titled Green Infrastructure Guidance for Flood Reduction. For more background information on the economic assessment that was completed through this process please visit the full case study.

In Toledo, Ohio a six step process was utlized to complete an economic assessment of green infrastructure (GI). To learn more about a specific step in this process please click on it in the outline below.

  1. Define the Problem
  2. Assess Flooding Scenarios without GI
  3. Identify How a Flood Reduction Target Can Be Met with GI
  4. Assess Flooding Scenarios with GI
  5. Estimate Benefits and Costs
  6. Identify and Communicate the Desired GI Strategy

Step 1: Define the Problem

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:

  1. Choose a watershed study area
  2. Characterize flooding issues and causes
  3. Determine what’s at risk

Output: Definition of problem and study parameters

Task 1: Choose a watershed study area

While in Toledo, the team toured various watersheds within the city where there were flooding issues and opportunities for green infrastructure implementation. The main site selection criteria included:

  • Significant flooding damages reported within the watershed
  • Opportunities for future land use with GI within the watershed
  • Available data for Hydrologic & Hydraulic (H&H) modeling
  • Existing data to help with analysis
  • Community need

After considering the criteria, the project team chose to work in Silver Creek watershed. Silver Creek discharges into Lake Erie and has a total drainage area of 15.76 square miles, which includes the drainage area of Shantee Creek that flows into Silver Creek. The drainage area of Silver Creek alone (i.e., the area upstream of where Shantee Creek joins Silver Creek) was assessed in this study and is 7.41 square miles.

Land use within the Silver Creek watershed is primarily residential and commercial. Most of the watershed is developed and there are minimal areas with significant open space. There are opportunities for retrofitting existing land uses as they are redeveloped to incorporate green infrastructure to provide flood storage.



Land cover change in the Silver Creek watershed.
Courtesy of: NOAA Office for Coastal Management.

Task 2: Characterize Flooding Issues and Causes

The City of Toledo – situated in the low-lying area also 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.

Most of Toledo’s flooding is from chronic, short and intense rain events (sometimes an inch of rain within an hour or two). 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’s) and a strain to public services and budgets, overloaded storm water sites, as well as property damage. Read more about Toledo’s Chronic Urban Flooding.

Task 3: Determine what’s at risk

Many of Toledo’s people, homes, streets, parks, streams, and businesses are impacted by the chronic flooding. Due to limitations in Toledo’s data on money spent to repair, replace, and construct projects, the study focused solely on damages to buildings using the Hazus loss estimation model.

Step 2: Assess Flooding Scenarios without Green Infrastructure

The goal of step two is to answer what are flooding impacts now and in the future without a green infrastructure strategy. Step two consists of the following tasks:

  1. Gather data
  2. Model current and future flooding
  3. Quantify current and future flood damages

Output: Financial impacts of flooding under current and future scenarios without green infrastructure strategy

Task 1: Gather data

The study looked at four precipitation and land use scenarios:

  1. Current precipitation and current land use
  2. Future precipitation and future land use
  3. Current precipitation and current land use with provided flood storage
  4. Future precipitation and future land use with provided flood storage

Since Toledo had just updated their comprehensive plan, they were interested in using a 20 year planning horizon. 2035 represents their future. We worked closely with Toledo’s geographic information system department to acquire data. A list of the major datasets we used is available in this Green Infrastructure Data Checklist resource.

Task 2: Model current and future flooding

To help us answer the question “How much rain now and in the future?” We determined historical rainfall data from NOAA’s Technical Paper 40 (TP 40) “Rainfall Frequency Atlas of the United States.” The hydrology modeling approach used in this study required TP40 input. Although NOAA has updated its precipitation frequency information for most states (NOAA Atlas 14 precipitation frequency estimates).

For projected climate data EPA’s Climate Resilience Evaluation and Awareness Tool (CREAT) was used. CREAT precipitation outputs are provided in inches for the 5-, 10-, 15-, 30-, 50- and 100-year, 24-hour storm events. CREAT provides downscaled climate model projections. All model runs used to develop future climate scenarios within CREAT use the A1B emissions scenario from the IPCC Special Report on Emissions Scenarios (SRES).

CREAT provided three pre-loaded scenarios for Toledo, based on GCM results, which capture a range of possible future climate conditions: 1) hot and dry, 2) central, and 3) warm and wet model projections. The hot and dry, central, and warm and wet model projections each vary the change in precipitation and temperature differently. Data for these pre-loaded scenarios were available for the 2035 and 2060 time periods only. The data were derived as 30-year averages, centered on the time period year. This means that the 2035 future precipitation values used in this study are a 30-year average from 2020 to 2050.  Projected conditions were calculated as a change from an existing condition using the historical climate data set previously selected for Toledo.

For this study, the “warm and wet” model projection was used to extract future precipitation data for Toledo. The warm and wet model projection was chosen because it predicts the largest change in increased precipitation, which allowed the project team to assess a worst-case future flooding scenario.

According to the CREAT data for Toledo, under the warm and wet model, both average annual precipitation and the frequency of the 100-year, 24-hour storm event will increase.

For additional details on the climate data and study analysis, view the Study’s Technical Report.

Hydrologic and hydraulic (H&H) modeling was conducted to determine peak discharges and flood depths grids for several frequency storms for the Silver Creek sub-watershed. The hydrologic model used USGS regional regression equations and HEC-RAS was used for the hydraulic modeling. The project team chose these two models since the study is meant to help inform a green infrastructure strategy and not site-specific projects. This modeling resulted in flood depth grids.

Task 3: Quantify current and future flood damages

Hazus was used to estimate flood damages to buildings within the Silver Creek watershed based on the flood depth grids developed through H&H modeling. Hazus is FEMA’s nationally applicable standardized methodology that contains models for estimating potential losses from earthquakes, floods, and hurricanes. Hazus uses GIS technology for potential loss estimates such as:

  • Physical damage to residential and commercial buildings, schools, critical facilities, and infrastructure
  • Economic loss, including lost jobs, business interruptions, repair, and reconstruction costs
  • Social impacts, including estimates of shelter requirements, displaced households, and population exposed to scenario floods, earthquakes, and hurricanes

Only physical damage to buildings was estimated for this study. Hazus produces loss estimates for vulnerability assessments and plans for flood risk mitigation, emergency preparedness, and response and recovery.

In this study, it enabled visualization of the spatial relationships between populations located in flood-prone areas and how the shape and size of the flood-prone areas would change depending on the scenarios examined in this study. In addition to the visualization tool, outputs can be displayed as tables of social and economic losses. Most outputs can also be mapped or exported in various GIS data formats.

How HAZUS Was Used in This Study

FEMA’s Hazus 2.0, Service Pack 2 (Release 11.0.2) on Environmental Systems Research Institute (ESRI) ArcGIS 10.0 with Service Pack 2 (Build 3200) was used for all flood damage estimates in this study. Although Hazus can be used to estimate several types of flood damages noted above, it was used in this study to estimate the physical building damage associated with selected flood model scenarios only. It is important to remember that the physical building damage estimated by Hazus is only one component of all economic or structural damages likely to occur from flooding.

A Level 2 Hazus analysis was completed for Toledo by importing parcel-level data and attributes and flood depth grids generated by the HEC-RAS models to show the relationship between building locations and flood areas. The basic steps for this study’s Hazus analysis were:

  1. Identify and acquire parcel and/or building data and assessment attributes
  2. Format building datasets and attributes
  3. Import HEC-RAS flood depth grids (raster datasets) – a.k.a. “user-defined depth grids”
  4. Delineate inundated areas
  5. Import building data into Hazus as User-defined Facilities (UDFs)
  6. Run UDF analysis for each scenario that varied precipitation, land use, and the implementation of GI

Current Flood Damages

It was estimated that 253 structures in Silver Creek were damaged, totaling $738,000 in costs during the 100-year, 24-hour storm event under current precipitation conditions.

HAZUS Results: Silver Creek Current Land Use and Current Precipitation Flooding Damages (Scenario 1)

Future Flood Damages

The future year for this modeled scenario is assumed to be 2035. 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.

In Toledo’s Silver Creek, economic losses from flooding increase by more than 30 percent in the future (2035) land use scenario with a 4.85 percent annual increase in precipitation, compared to existing conditions. It was estimated that 293 structures in Silver Creek were damaged, totaling $980,800 in costs, during the 100-year, 24-hour storm event under future (2035) precipitation conditions.

HAZUS Results: Silver Creek Future Land Use and Future Precipitation Flooding Damages (Scenario 2)

Step 3: Identify How a Flood Reduction Target Can Be Met with GI

The goal of Step 3 is to establish a flood reduction target and design a green infrastructure strategy to meet the target. Step three consists of the following tasks:

  1. Select a flood reduction target
  2. Identify green infrastructure options
  3. Determine how much storage the green infrastructure options can provide

Output: Preliminary green infrastructure strategy for meeting the flood reduction target

Task 1: Select a flood reduction target

The flood reduction target is the amount of runoff that needs to be managed (stored or reduced) in order to protect assets at risk. Choose a design storm that most commonly causes significant damage or is a threshold for when damage occurs. Since this study was interested in identifying whether green infrastructure is a feasible way to reduce flooding from extreme events, Toledo chose the 100-year, 24 hour storm event as their design storm. Refer to Step 3, Task 1 in the Guide to Assessing Green Infrastructure Costs and Benefits for Flood Reduction for more information on how to choose a flood reduction target.

The City of Toledo chose a ten percent flood reduction target since the Silver Creek watershed is almost completely developed.  A ten percent reduction in the 100-year, 24-hour storm peak discharge is pretty 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. 

Task 2: Identify green infrastructure options

The project team spent several meetings learning about the green infrastructure options that could work in Toledo based on its land use and community needs. The project team had a green infrastructure expert that provided insight on what options could work in Toledo and their pros and cons. Having this subject matter expert provide guidance helped Toledo identify some potential green infrastructure options of interest.

Because Toledo is minimal open space, they were interested in green infrastructure that could be incorporated into smaller spaces or work in a retrofit. Some specific green infrastructure opportunities include:

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.

  • Installing bioretention in the form of bioswales along the approximately 70 miles of unimproved roadway. These projects can be sequenced over a 20-year period and synchronized with roadway improvements to reduce costs.
  • Working with local industries to install blue roofs on large commercial buildings, which are estimated to have roof areas totaling 2.5 million square feet within Silver Creek.
  • Installing permeable pavement where sidewalks or bikeways need to be replaced or built.
  • Installing underground storage beneath parking lots, roadways, and other developed areas.
  • Installing stormwater tree trenches along existing and new sidewalks as they are built or as opportunities arise.
  • Installing stormwater retention ponds in open areas.
  • Building an extended detention wetland in the upstream portions of the watershed.
  • Consider buyouts (two buy-outs have occurred in the past) of chronic flood areas, possibly in conjunction with installing GI to increase flood storage on the approximately 7.8 acres of tax-forfeited parcels in the watershed. There may be opportunities to examine the connectivity of these areas to design community co-benefits such as public open space, bike paths, walkways, or community gardens into the design.

Task 3: Determine how much storage green infrastructure options can provide

In order to achieve approximately 30 acre-feet of flood storage, a variety of GI could be implemented on multiple sites. Many types of GI practices can be designed to have small-scale applications, which is advantageous because they can be implemented as a retrofit or as part of new construction on almost any property. There are multiple ways that a community can mix and match different types of flood storage options in order to achieve the necessary acre-feet of storage. Each community will need to determine the best combination of practices and sites.

Flood storage can be achieved in a variety of ways. The type of GI implemented on a site depends on factors such as:

  • Site hydrology (permeability, soil, slope, ground cover)
  • Available open space
  • Community preference/acceptance
  • Presence of underground of obstructions such as utility lines or natural features such as public shade trees
  • Cost

The project team worked through an activity where we used aerial maps to assess potential locations for green infrastructure implementation. This activity consisted of working with the stormwater and the water resources engineers who know the study watershed very well to be able to determine where opportunities exist.

Step 4: Assess Flooding Scenarios with green infrastructure

The goal of Step 4 is to assess how current and future flood damages change if green infrastructure strategy is implemented to meet your flood reduction targets. Step four consists of the following tasks:

  1. Model current and future flooding if the flood reduction target is met with green infrastructure
  2. Quantify current and future flood damages if the flood reduction target is met with green infrastructure

Output: Impacts of flooding under current and future scenarios with a green infrastructure strategy

Task 1: Model current and future flooding if the flood reduction target is met with green infrastructure

Reducing peak discharges and accounting for 31 acre-feet and 33 acre-feet of storage, respectively, changes the flood depth grids generated by HEC-RAS. HEC-RAS was re-run assuming that 31 acre-feet (current) 33 acre-feet (future) of storage was added, which resulted in depth grids for the Silver Creek watershed that represent flooding when storage is provided.

Current Land Use and Current Precipitation with Flood Storage

In scenario 3 we looked at current land use and precipitation with flood storage (a 10 percent increase in flood storage). All of the land use and precipitation assumptions are the same as in scenario 1, the difference in this scenario is that it is assumed that peak discharges are reduced by 10 percent at River Station 8071 through the implementation of stormwater management and green infrastructure upstream. A 10 percent reduction in peak discharge correlates to an associated storage volume of runoff. This scenario looks at the flooding damage caused if the current conditions in Silver Creek remain the same, except for a 10 percent increase in flood storage.

The 100-year, 24-hour peak discharge at River Station 8071 in Silver Creek is 1,255 cfs under scenario 1. That flow was reduced by 10 percent to 1,130 cfs for the scenario 3 analysis. Reducing peak discharges by 10 percent results in a four-to-five percent decrease in velocity for all storm events. A 10 percent peak discharge reduction for scenario 3 is equal to 31 acre-feet of flood storage. This means that if a community wanted to reduce peak discharges by 10 percent during a 100-year, 24-hour storm event, 31 acre-feet of storage would need to be provided upstream of River Station 8071.

 

Future Land Use and Future Precipitation with Flood Storage

This scenario looks at how the community could reduce flooding damage under future conditions if a 10 percent increase in flood storage was provided. In scenario 4, all of the land use and precipitation assumptions are the same as in scenario 2. The difference in this scenario is that it is assumed that peak discharges are reduced by 10 percent at River Station 8071 through the implementation of stormwater management and green infrastructure upstream. A 10 percent reduction in peak discharge correlates to an associated storage volume of runoff.

The 100-year, 24-hour peak discharge at River Station 8071 in Silver Creek is 1,318 cfs under scenario 2. That flow was reduced by 10 percent to 1,187 cfs for the scenario 4 analysis. Reducing peak discharges by 10 percent lead to a four to five percent decrease in velocity for all storm events. A 10 percent peak discharge reduction for scenario 4 is equal to 33 acre-feet of flood storage. This means that if a community wanted to reduce peak discharges by 10 percent during a 100-year, 24-hour storm event in 2035, 33 acre-feet of storage would need to be provided upstream of River Station 8071.

Task 2: Quantify current and future flood damages if the flood reduction target is met with green infrastructure

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), Hazus shows 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

Reducing the peak discharge by 10 percent at River Station 8071 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

Reducing the peak discharge with the implementation of GI reduces the flood losses. It was estimated that 60 structures in Silver Creek were damaged, totaling $181,800 in costs during the 10-year, 24-hour storm event under future precipitation conditions with the implementation of GI. It was estimated that 179 structures in Silver Creek were damaged, totaling $527,500 in costs during the 100-year, 24-hour storm event under future precipitation conditions.

Reducing the peak discharge by 10 percent at River Station 8071 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 

Step 5: Estimate Benefits and Costs

The goal of Step 5 is to estimate annualized net benefits for your green infrastructure strategy. Step five consists of the following tasks:

  1. Estimate green infrastructure option unit costs and strategy cost
  2. Estimate green infrastructure strategy benefits and co-benefits
  3. Annualize costs and benefits over a specific time frame

Output: Annualized net benefits

Task 1: Estimate green infrastructure option unit costs and strategy cost

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.

Task 2: Estimate green infrastructure strategy benefits and co-benefits

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 flood storage 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.

EAD and the PV of the benefit of reduced building damages are assessed over 20 years. When evaluating the costs and benefits of investing in flood mitigation in Toledo it is important to keep in mind that the estimated benefits are an underestimate of the true benefits since many benefits are not monetized. The PV of benefits from avoided building damage is estimated to be approximately $700,000 (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. See Technical Report Appendix D for annualized benefits.

The benefits calculated in this assessment are predicted to be much lower than those that would actually be provided for two main reasons: 1) additional benefits outside of avoided building damages were not monetized and 2) the benefit analysis ended at 20 years. Because non-monetized benefits such as increased habitat and improved water quality were not included in this study’s assessment, the calculated benefits are likely to be greatly underestimated. Not all benefits are tangible and placing a value on an intangible benefit is difficult and subjective. It should be understood that the GI recommended in this study provides numerous benefits outside of the costs avoided from building damage. These non-monetized benefits should be acknowledged and considered by the community so that they are at least qualitatively incorporated into any cost-benefit analysis.

Additionally, many GI practices have benefits that continue beyond a 20-year time period. Ending a benefit analysis at 20 years assumes that at year 21 the benefit is zero dollars, which is not true for many GI practices. Because the economic benefit analysis in this study only went out 20 years, the overall benefits are further underestimated. 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).

Task 3: Annualize costs and benefits over a specific time frame

Although we do not know the true benefits and costs associated with implementing GI in Toledo, the previous two sections have presented some analysis of benefits and costs, which can be compared to demonstrate how the city may conduct a benefit-cost analysis. In Section 3.8, the cost of obtaining 31 acre-feet of storage using the least expensive GI practice, extended detention wetlands, was calculated to be $1,755,468. If a third of these costs were incurred in years two, four, and six of the analysis, then the PV of the cost would be $1,700,543 (Technical Report Appendix D). This PV of cost occurs regardless of whether a 20-year of 50-year time horizon is considered since all costs are incurred in the first six years. In Section 3.9, the PV of benefits associated with reduced building damages over 20 years was estimated to be $698,539; over 50 years the benefits would be $1,769,644.

However, when comparing the above benefits and costs it is important to keep in mind that these values may not reflect the true benefits and costs to the city. Federal funds, state funds, or grants may also be available for green infrastructure construction, which would reduce the cost to the city. The true benefits are greater than the estimated benefits since many benefits are not monetized. In addition, the cost per unit of GI may vary significantly. The city needs to proceed from planning scale to design scale to calculate site-specific costs. As shown in Table “Green Infrastructure Estimated Unit Costs“, there is a wide range of costs depending on the type of GI implemented. To minimize costs, the city can focus on cheaper solutions and sequence them to coincide with other capital projects or funding sources to reduce marginal costs. Additionally, the city must consider the lifespan of the GI project as an important factor in determining the timeframe over which to compare benefits and costs.

If benefits and costs over the 20-year time are compared, the costs ($1.7 million) exceed the calculated benefits ($700,000). However, when the time horizon is extended to 50 years the costs remain constant at $1.7 million but the benefits grow to $1.77 million. In this comparison, benefits exceed costs, providing evidence in favor of implementing the GI project, thus demonstrating the importance of determining the appropriate time horizon when calculating benefits and conducting a benefit-cost analysis.

Step 6: Identify and Communicate the Desired GI strategy

The goal of Step 6 is to identify and communicate the green infrastructure strategy. Step six consists of the following tasks:

  1. Finalize green infrastructure strategy
  2. Communicate the green infrastructure strategy and plan next steps

Output: Implementation of green infrastructure strategy

Task 1: Finalize GI strategy

As this analysis has shown, flooding can be mitigated through the implementation of 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, future development and, importantly, redevelopment patterns in Toledo (in general) and in Silver Creek watershed (in particular) are critical decisions that 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. All of these issues fall into the category of land use considerations. Although the project team suggested some of these approaches in community meeting, they were not the focus of our analysis. Thus, we remind our community partners to consider these in the course of their discussions about sustainability planning.

A wide variety of adaptive land use practices, policies, tools, and strategies are available to communities interested in planning for sustainable flood management. See Technical Report Appendix B for a more complete listing of strategies that can be considered. The following options were discussed for consideration in Toledo.

Urban Form Requirements: One policy option to reduce building damage would be to implement “urban form" requirements (which help shape and structure the future of the city) for development in critical flood storage areas. Such requirements could dictate that structures have floodable first floors (e.g., parking garage, structures elevated on stilts, no critical utilities in basements).

Buy-outs: Toledo has purchased homes as part of buy-out strategies in 2002 and 2006, so there is a history of using this method to remove chronically flooded properties from harm’s way. The city of Toledo estimated the average residential buy-out costs at $87,000 per home for property purchase and creation of green space. When considering that this cost provides flood mitigation for the future and reduces repeated flood damages (and insurance claims) to properties, this option may compare very favorably to other options. The disadvantage of this particular approach is that the FEMA buy-out program tends to be reactive (after major flood damages have already occurred). To be more pro-active, the city of Toledo could identify potential buy-out locations that are proximate to other optimal siting factors for reclaiming open space and restoring flood plain function. For example, criteria could include proximity to existing open space, proximity to tax-forfeited parcels, flood damage history, suitability for flood storage, etc. In conjunction with this planning, open space public amenities (such as community gardens, bike/walk ways, pocket parks) could be integrated to enhance the neighborhood and provide co-benefits. A portion of the estimated $9 million annual collection of stormwater fees could be considered to support this effort (within the eligibility guidelines of the fund) to start building pieces of the plan to implement this vision so that when opportunities arise to leverage funds from other sources, such as FEMA, and make strategic purchase of land parcels, the city has done the preliminary planning to expedite the process.

Transfer of Development Rights (TDR): This is one tool that can transfer development density “credits” from one place (flood prone areas) to other areas that are more suitable for higher density development, for example, less flood-prone areas (and areas that can otherwise support additional development and redevelopment). By shifting development away from existing and future flood hazard areas and conserving those areas to restore their function for flood storage (floodplains), the city can realize several benefits. By reshaping development patterns for more open space along creeks and their associated floodplains, the city can provide opportunities for co-benefits such as improving water quality, creating open space corridors along streams and rivers for wildlife and fisheries, and utilizing these areas for multiple uses such as bikeways, parks, and walkways. This can also reduce flood damage costs by avoiding development in harm’s way. More sophisticated forms of TDR include a “Density Transfer Charge,” where money is deposited into a fund dedicated to purchasing easement, abandoned property, or development rights in the flood plain. The account can become self-sustaining, in the form of a revolving fund. For more information about components of successful TDR programs, see Technical Report Appendix E.

A larger area than the Silver Creek watershed (such as citywide) would be most appropriate for consideration of the above policies and could be done in conjunction with the city’s sustainability and climate adaptation planning.

Stormwater Ordinance Revisions: Toledo’s stormwater ordinance could be examined for possible modifications to more aggressively reduce runoff and increase flood storage. The city’s stormwater credit manual is in the process of being updated. Opportunities for revision include: incorporating best practices that have worked elsewhere; considering options such as impact fees for impervious cover (if this is not already required); and encouraging innovative design (such as LID) or onsite retention as conditions of permitting for new construction or redevelopment.

Some recommendations that the project team made to Toledo at community meetings included: conducting more outreach/awareness building on the existing stormwater credit program to developers since many developers do not know it exists, raising the baseline standard to qualify for credits, and adding specifications for green practices and guidance on how to design and build GI methods. The stormwater utility could also become more actively involved in helping fund projects and providing incentives. In this way, new development, including redevelopment, could make more of a positive impact on reducing stormwater runoff.

The comparison of current precipitation to future precipitation indicates that precipitation is expected to increase along with flooding damages in the Silver Creek watershed over the next 20 years. The following strategies are recommended to reduce flooding damages in the future:

  • Look for opportunities to increase flood storage and reduce runoff with green infrastructure:
    • Identify areas where the flood plain can be restored or new flood retention areas can be created within existing open space (e.g., tax-forfeited parcels).
    • Identify commercial/industrial rooftop areas that may be suitable for blue roofs.
    • Incorporate into roadway capital improvement plans the use of pervious pavement.
    • Incorporate into roadway capital improvement plans the use of curb cuts to direct runoff into vegetated islands and vegetated strips rather than into storm drains.
    • Install bioretention areas and swales, particularly along unimproved streets.
  • Remove buildings from the flood plain where flooding is severe (buy-outs) and consider doing so in combination with other land use strategies such as strategic purchase of tax forfeited parcels, transfer of development rights or other mechanisms to shift development density away from the most flood-prone areas and into other areas more suitable for sustainable development while restoring flood storage function.
  • Optimize community acceptance of GI by building on past successes and showcasing benefits (e.g., previously installed bioretention areas).
  • Look for opportunities for co-benefits of GI:
    • Create recreational trails along water features.
    • Create parks and open space on buy-out parcels.
  • Consider revising stormwater standards to incorporate more stringent requirements for onsite retention through revised policies.

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.

Task 2: Communicate GI strategy and plan next steps

Toledo’s outreach has consisted of the following:

Much of this work was done through technical assistance partnership with NOAA’s Office for Coastal Management, which was part of the EPA Great Lakes Restoration Initiative funded project.

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. 

NOAA and its partners continue to outreach Toledo’s work and the study framework.

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