Green Infrastructure – Manage Flood Risk

Green Infrastructure – Manage Flood Risk from EPA

Heavy downpours have increased in frequency and intensity worldwide in the last 50 years.  They are expected to become more frequent and intense as global temperatures continue to rise.  As a result, the risk of flooding is likely to increase dramatically across the United States.

The average 100-year floodplain is projected to increase 45 percent by the year 2100, while the annual damages from flooding are predicted to increase by $750 million.  Among the types of flooding that will likely become more frequent are localized floods and riverine floods.  Localized flooding happens when rainfall overwhelms the capacity of urban drainage systems, while riverine flooding happens when river flows exceed the capacity of the river channel.

By reducing stormwater runoff and protecting floodplains, green infrastructure can help manage both localized and riverine floods.  In areas impacted by localized flooding, green infrastructure practices absorb rainfall, preventing water from overwhelming pipe networks and pooling in streets or basements. 

Green infrastructure practices that enhance infiltration include rain gardens, bioswales, and permeable pavements.  In areas impacted by riverine flooding, green infrastructure, open space preservation, and floodplain management can all complement gray infrastructure approaches.  These practices reduce the volume of stormwater that flows into streams and rivers, protecting the natural function of floodplains, and reducing the damage to infrastructure and property. 

Green Infrastructure Maintenance Costs

September 2022

Green Infrastructure: Cost-effective solutions to flooding

To help communities weigh strategies for reducing flood risks, Headwaters Economics researched green infrastructure solutions with low maintenance costs.  These low-cost practices may help communities use existing land assets to protect public infrastructure, private property, and lives.  The cost estimates are based on industry-standard unit costs for construction in the Midwest. Geographic variability in construction costs and inflation will affect which solutions are ultimately the most cost-effective.  

Link to 2022HE-GreenInfrastructure

Recommendations

Communities can use maintenance cost data to help evaluate potential green infrastructure projects.  Understanding long-term costs can be especially important when the maintenance of green infrastructure will require interdepartmental collaboration.

Green Infrastructure Type /Upfront Costs/ Ongoing Annual Maintenance

Communities may also consider the following recommendations:

  1. Consider hybrid green-grey projects. For example, rain gardens can slow down stormwater and provide access to subsoils for water to seep into while also diverting excess water to grey infrastructure such as gutters to reduce the risk of localized flooding.
  2. Incentivize green infrastructure through regulations and redevelopment.  Stormwater management plans, standards, and subdivision regulations can inadvertently incentivize gutters and pipes instead of green infrastructure solutions like bioswales. Regulations and ordinances can be modified to prioritize green infrastructure. Communities can also work to incorporate green infrastructure into repair, upgrade, and redevelopment projects. For individuals, local governments can incentivize and provide information to property owners and property managers on lawn and landscaping alternatives that are responsive to the regional climate and weather patterns.
  3. Plan for alternative funding streams for maintenance of green infrastructure.  The type of skilled manual labor required to maintain green infrastructure is often different than that needed to maintain grey infrastructure. For example, green infrastructure may require annual landscaping crews on staff, whereas grey infrastructure is often maintained less frequently by contract road and construction crews. Communities need to plan for green infrastructure maintenance as an ongoing expense that requires a steady revenue stream, as opposed to planning for large capital expenditures for grey infrastructure maintenance. Programs that incentivize community monitoring of various types of green and grey infrastructure can support accounting for maintenance as a regular expense. The National Flood Insurance Program’s Community Rating System is an example of a program that incentivizes this work.
  4. Adapt and re-use community assets.  Property prone to flooding can be a great place to install green infrastructure. Green infrastructure can help reduce flood risk from localized stormwater management to an interconnected stream-side system. In cases where local governments have acquired low-lying land via property buy-outs and restrictive covenants limit redevelopment, green infrastructure can create a park-like asset out of a neighborhood eyesore.
  5. Enhance wildlife habitat in public spaces and parks with green infrastructure. The diversity of native plants and improved water quality can support habitat connectivity to facilitate wildlife movement, nesting, food, shelter, and shade; increase wildlife viewing opportunities, and help reduce the urban heat island effect by lowering temperatures. Communities can use tools like Neighborhoods at Risk to identify locations where green infrastructure would help combat urban heat and mitigate flood risk for the most vulnerable populations. 

 

Flood Avoidance Green Infrastructure

December 14, 2015

To address the impacts of excess stormwater, the U.S. Environmental Protection Agency (EPA) evaluated potential scenarios for managing stormwater from new development and redevelopment.  The purpose of this study is to examine one of these impacts: flood loss avoidance.  This study generated an estimate of the monetary value of flood loss avoidance that could be achieved by using distributed stormwater controls to capture a specified volume of runoff. 

This stormwater management approach retains on-site small storm events in an attempt to simulate predevelopment runoff conditions.  This approach is referred to as Low Impact Development (LID) or Green Infrastructure (GI) for stormwater management and is an integrated approach that uses site planning and small engineered stormwater controls spatially distributed throughout a development site to capture runoff as close as possible to where it is generated.  In this document, the term Green Infrastructure is used for bioretention filters, landscaped roofs, rainwater cisterns, and infiltration trenches are examples of stormwater controls commonly found in GI applications.  These controls infiltrate and evapotranspire runoff, or capture and store rain for beneficial uses like landscape irrigation and other non-potable uses.  The approach in this study considered the application of GI only to new development and redevelopment, not as retrofits to mitigate the impact of existing imperviousness.

The study approach consists of estimating flood depths and the associated flood losses with and without GI.  The benefits are the losses that are avoided by watershed-wide implementation of GI.  In this report, the terms “damages” and “flood losses” are used interchangeably.  The timeframe of analysis is from 2020 to 2040.  The extent of GI application assumed for this study is small initially, because the assumption in this study is that GI would be implemented only on new development and redevelopment starting in 2020.  The extent of GI application, and the associated benefits, would increase with development over time.  Therefore, maximum benefits are realized in 2040, the last year of this study period.  At the time of this report, several states have already adopted on-site retention practices; therefore, benefits of wider adoption nationwide are the focus of this study (i.e., the study focuses on areas that have not adopted retention policies to date).

Download the Flood Loss Avoidance Benefits of Green Infrastructure for Stormwater Management.

 

Localized Flooding

Communities susceptible to localized flooding can use models to learn more about the impact green infrastructure can have on managing the flood risk.  Hydrologic and hydraulic (H&H) modeling can help identify green and gray infrastructure practices that will meet flood reduction and water quality goals.

For example, the Capitol Region Watershed District in Ramsey County, Minnesota  developed an H&H model to select a set of green infrastructure practices to address localized flooding and phosphorus loading.  Selected practices included:

  • rain gardens,
  • underground infiltration trenches,
  • an underground storage and infiltration system, and
  • a regional stormwater pond.

All of the practices—except the stormwater pond, which was not completed at the time—reduced runoff volumes by 99–100 percent Installing green infrastructure—at an estimated cost of $2 million—was more cost-effective than installing an all-gray infrastructure alternative—at an estimated cost $2.5 million for a new 60-inch diameter storm sewer pipe. 

See: Arlington Pascal Stormwater Improvement Project Presentation from Saint Paul, Minnesota (PDF)(7 pp, 728 K, and Arlington Pascal Best Management Practices Performance and Cost-Benefit Analysis Project 2007-2010 (PDF)(16 pp, 10 MB

 

 

Riverine Flooding

Riverine Flooding: Communities susceptible to riverine flooding can more effectively manage their flood risk by combining green infrastructure practices with conserving land in or around the floodplain.  Geographic-information-based models can help with the following:

  • estimate the flood damage benefits of green infrastructure
  • compare the benefits to the cost of conservation
  • target investments in conservation towards the most cost-effective areas

When preserving open space throughout a watershed, communities may want to target areas with water-absorbing soils in regions experiencing high growth.  For example, the Milwaukee Metropolitan Sewerage District (MMSD) partnered with The Conservation Fund to protect land with those characteristics.  As of 2013, the Greensteams program had protected over 2,700 acres of land capable of storing an estimated 1.3 billion gallons of water.  Protecting that land reduced future flows and contaminants into receiving rivers, mitigating future flooding.

 

Technical Assistance Examples

Design for Resilience in Brattleboro’s Lower Whetstone Brook Corridor — The Town of Brattleboro, Vermont applied for EPA technical assistance to address flooding in the neighborhoods along the stretch of Whetstone Brook that flows through the downtown.  The design solutions developed through a public process in 2016 identify options for  creating resilient redevelopment and recreational opportunities within flood prone areas of the town while protecting water quality and connecting people with the Whetstone Brook.

Green Street Charrette and Concept Design Report for Huntington, West Virginia — EPA-supported technical assistance for a green street charrette for the Huntington Stormwater Utility in West Virginia.  This effort helped the community identify concerns related to stormwater and opportunities for implementing green infrastructure concepts.

Regional Design Assistance in the Mid-Atlantic Webinar — In 2019, EPA worked directly with stakeholder communities including Huntington WV to identify and facilitate policy and planning measures that promote green infrastructure and help address flood management and water quality needs using the Resilient Design Assistance Tool developed by EPA’s Office of Research and Development.

Smart Growth Implementation Assistance for Caño Martín Peña, San Juan, Puerto Rico — This EPA technical assistance project supported the community in developing design options to address stormwater management and flooding problems in tandem with the proposed improvements to the area’s traditional drainage infrastructure system and ecosystem restoration project while also providing parks and plazas, and making the neighborhoods more walkable and bikeable. 

Rio Reimagined – Rio Salado Urban Waters Partnership — EPA-supported project to protect, restore and revitalize the Salt and Middle Gila River Watershed by restoring ecosystem functions and balancing revitalization with issues of urban resilience and public safety including flood mitigation.

 

 

Resources

Regional Resilience Tool Kit Presentation — EPA recently released its Regional Resilience Toolkit, for addressing regional resilience planning needs to address hazards from natural disasters and is working with a handful of pilot communities: https://www.epa.gov/smartgrowth/regional-resilience-toolkit#2020.

Two Webcasts: Lessons Learned Integrating Water Quality and Nature Based Approaches for Hazard Mitigation Webinar  and Building Resilient Communities with Green Infrastructure and Hazard Mitigation Planning Webcast— EPA and FEMA worked with three regions – Ashland OR, Albany NY, and the Commonwealth of MA – to pilot projects that have successfully integrated watershed planning, green infrastructure practices and source water protection into FEMA hazard mitigation plans. 

Including Watershed Planning and Green Infrastructure into State Hazard Mitigations Plans — EPA fact sheet on the four main benefits of integrating planning and resources for getting started.

Reducing Damage from Localized Flooding: A Guide for Communities (PDF (180 pp, 9 MB, —This guide was produced by the Federal Emergency Management Agency to help U.S. cities, towns, villages, and counties reduce damage, disruption, and public and private costs caused by localized flooding within their jurisdictions.

A Flood of Benefits – Using Green Infrastructure to Reduce Flood Risk  —This report describes how restoration or conservation of forests, wetlands, rivers, and floodplains can reduce flood risk while supporting an array of other benefits.

Strategically Placing Green Infrastructure: Cost-Effective Land Conservation in the Floodplain —This paper estimates the flood damage and costs that can be avoided by preventing development of floodplain parcels in the East River watershed of Wisconsin’s Lower Fox River Basin.  The analysis demonstrates how to use a geographic-information-based model to estimate the benefits of green infrastructure in reducing flood damage, compare the benefits to the costs, and target investments to develop cost-effective nonstructural flood damage mitigation policies.

Rain Garden Reserve (PDF)(2 pp, 2 MB, —This case study from the city of Cuyahoga Falls, Ohio, demonstrates how using a series of rain gardens can mitigate localized flooding.  The 24,000-square foot park drains an approximately 3-acre residential area and enhances outdoor recreational opportunities for the community.

Envision® Sustainable Infrastructure Rating System – Business Case Evaluator (BCE) for Stormwater  —This tool from the Institute for Sustainable Infrastructure includes estimates for the value of green infrastructure benefits, including reduced flooding resulting from green infrastructure improvements.

Economic Assessment of Green Infrastructure Strategies for Climate Change Adaptation: Pilot Studies in the Great Lakes Region  —This National Oceanic and Atmospheric Administration study assesses stormwater management practices that can help communities reduce flooding by capturing, storing, or absorbing more water from precipitation events.

Planning for Flood Recovery and Long-Term Resilience in Vermont: Smart Growth Approaches for Disaster-Resilient Communities (PDF)(64 pp, 390 K, – This report describes strategies that communities can use to become more flood resilient, including:

  • conserving land in flood-prone areas
  • directing new development to safer areas
  • using green infrastructure practices to manage stormwater.

 

 

 

 

 

 

 

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