Mitigate Flooding

Green infrastructure can mitigate flood risk by slowing and reducing stormwater runoff and protecting floodplains. As precipitation events become more intense, this type of mitigation becomes even more crucial for communities. Heavy precipitation events have increased in recent years and 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 in many areas across the United States.

Among the types of flooding expected to become more frequent are localized floods and riverine floods. Localized floods happen when rainfall overwhelms the capacity of drainage systems, while riverine floods happen when river flows exceed the capacity of the river channel. By the year 2100, more areas are projected to be at risk of flooding, and the annual damages from flooding are predicted to increase by $750 million.^i,ii Communities with fewer resources to prepare for and recover from floods are vulnerable to flood damage and other negative impacts. 

Explore the sections below to learn how green infrastructure mitigates the risk of localized and riverine floods and to find resources for state and local government officials working in the field of stormwater management.

On this page:

• Using Green Infrastructure for Flood Mitigation
• Resources
• References

Using Green Infrastructure for Flood Mitigation

By improving stormwater and floodplain management, green infrastructure can build communities' resiliency to localized and riverine flooding. To learn about green infrastructure for coastal floods, visit the Protect Coasts page.

Model Impacts and Improve Infiltration to Mitigate Localized Flooding

In areas impacted by localized flooding, green infrastructure can help absorb rainfall—reducing the quantity of water pooling or flooding in streets—and reduce pollutants to improve water quality. Along with practices that temporarily store large volumes of stormwater (such as ponds or retention practices), green infrastructure can also help prevent stormwater from overwhelming pipe networks. Examples of green infrastructure that enhance infiltration (e.g., permeable pavements) and reduce flooding include rain retention (e.g., rain gardens, bioswales).

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

Communities can also use EPA's publicly accessible National Stormwater Calculator to estimate the annual amount of rainwater and frequency of runoff from a specific site. This knowledge can help communities plan for green infrastructure and stormwater management.

Preserve Land to Mitigate Riverine Flooding

Communities susceptible to riverine flooding can more effectively manage their flood risk by combining green infrastructure with floodplain management. Conserving land in or around the floodplain can complement gray infrastructure by protecting the natural function of floodplains, thus reducing the volume of stormwater that flows into streams and rivers, as well as reducing the damage to infrastructure and property.

Geographic information system (GIS) and hydraulic modeling can help with the following:

• Estimating the flood damage mitigation benefits of green infrastructure (e.g., the volume of stormwater that would be captured and infiltrated by green infrastructure).
• Comparing the benefits to the cost of conservation.
• Targeting investments in conservation toward the most cost-effective areas.

GIS can be used to identify areas that are flood-prone or cost-effective for conservation, and hydraulic modeling can quantify the amount of stormwater that can be managed by green infrastructure. EPA's National Stormwater Calculator allows users to analyze site hydrology, including future conditions, and estimate the cost of implementing green infrastructure. The Community-enabled Lifecycle Analysis of Stormwater Infrastructure Costs (CLASIC) tool is fully interfaced with GIS and allows users to analyze future precipitation rates and evaluate costs, reductions in runoff, and co-benefits of various stormwater management scenarios.

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 partnered with The Conservation Fund to protect land with those characteristics. As of 2022, the Greenseams program had protected roughly 5,000 acres of flood-prone land.iii Protecting that land reduced future flows and contaminants into receiving rivers, mitigating future flooding.

Resources

Funding:

• Building Resilient Infrastructure and Communities (BRIC) — This website details a funding opportunity for states, local communities, tribes, and territories through the Federal Emergency Management Agency (FEMA). If awarded it may be used for hazard mitigation projects to help with the associated risks of natural disasters such as wildfires, drought, hurricanes, earthquakes, extreme heat, and flooding.
• Flood Mitigation Assistance Grant — This website details a funding opportunity for states, local communities, federally recognized tribes, and territories through FEMA. If awarded, it may be used for project scoping, technical assistance, community flood mitigation projects, individual structure- or property-level flood mitigation projects, and management costs.

Reports and Studies:

• Flood Loss Avoidance Benefits of Green Infrastructure for Stormwater Management (pdf) — This modeling study estimates the flood loss avoidance benefits from applying small storm retention practices for new development and redevelopment nationwide. Over time, using green stormwater infrastructure can save hundreds of millions of dollars in flood losses while just applying the practices to new development and redevelopment only. If retrofitting were to occur, the avoided losses would be even more significant.
• Planning for Flood Recovery and Long-Term Resilience in Vermont: Smart Growth Approaches for Disaster-Resilient Communities (pdf) — 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, and using green infrastructure to manage stormwater.
• Conley, G., R.I. McDonald, T. Nodine, T. Chapman, C. Holland, C. Hawkins, and N. Beck. (2021). Assessing the influence of urban greenness and green stormwater infrastructure on hydrology from satellite remote sensing. Science of the Total Environment, 817, 152723. Retrieved February 23, 2023.
• A Flood of Benefits – Using Green Infrastructure to Reduce Flood Risk — This study by the Nature Conservatory describes how green infrastructure can reduce flood risk and restore connections between rivers and flood plains while also providing other benefits associated with green infrastructure.
• Green Stormwater Infrastructure for Urban Flood Resilience: Opportunity Analysis for Dallas, Texas. (pdf) — Using hydrologic modeling and spatial analysis, this study by The Nature Conservancy investigates effective enhancement of urban flood management in the city of Dallas, Texas. Key findings include cost-effective opportunities to deploy green stormwater infrastructure (GSI) for improved stormwater management, reduced modeled overflows by GSI, less costly GSI implementation compared to upgraded gray infrastructure, and other benefits to GSI.
• Kousky, C., S.M. Olmstead, M.A. Walls, and M. Macauley. (2013). Strategically Placing Green Infrastructure: Cost-Effective Land Conservation in the Floodplain. Environmental Science and Technology, 47(8), 3563. Retrieved May 5, 2023.
• The Potential of Green Infrastructure in Mitigating Flood Impacts on the Mobility of Low Income and Minority Neighborhoods — This report from the National Institute for Transportation and Communities, led by Courtney Crosson of the University of Arizona, advances national research methods for assessing flood vulnerability and prioritizing transportation improvement investments to ensure that no community is left stranded when the next flood occurs.

Guides and Fact Sheets:

• Why Connect with Your State Hazard Mitigation Office? (pdf) — EPA fact sheet on the benefits of, and opportunities for, integrating Watershed Plans with FEMA Hazard Mitigation Plans.
• Reducing Damage from Localized Flooding: A Guide for Communities (pdf)) — This guide was produced by FEMA to help U.S. cities, towns, villages, and counties reduce damage, disruption, and public and private costs caused by localized flooding within their jurisdictions.

Tools:

• Green Infrastructure Modeling Toolkit — EPA has developed innovative models, tools, and technologies for communities to manage water runoff in urban and other environments. The resources in this toolkit incorporate green or a combination of green and gray infrastructure to help communities manage their water resources in a more sustainable way.

Webinars:

• Rain Garden Reserve (pdf) — This webinar presentation 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.
• Localized Flood Management — This webinar focuses on chronic flooding in urban areas and addressing the flooding with green infrastructure solutions.

References

1. AECOM; Michael Baker Jr., Inc.; and Deloitte Consulting, LLP. (2013). The impact of climate change and population growth on the National Flood Insurance Program through 2100. Retrieved July 1, 2014.
2. Wobus, C., M. Lawson, R. Jones, J. Smith, and J. Martinich. (2013). Estimating monetary damages from flooding in the United States under a changing climate. Journal of Flood Risk Management (pdf). Retrieved July 1, 2014.
3. The Conservation Fund. (2022). Greenseams Milwaukee. Retrieved November 30, 2022.