A case for investing in marshes

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There is growing evidence for the beneficial role that wetlands can play in reducing flood risk, but in many urban estuaries, coastal development has resulted in dramatic habitat loss and fragmentation. In the past several decades, marsh restoration has emerged as a core management objective in the San Francisco Bay. Our study quantifies the social and economic flood risk reduction benefits of salt marsh restoration currently and with climate-driven changes in storms and sea levels for the bay side of San Mateo County. We identify where stakeholder-identified potential salt marsh restoration could have the greatest socio-economic impacts in reducing flood risk and the role that habitat restoration may play in building community resilience to climate change. 

Results indicate that marsh restoration has an important role to play in building resilience to sea level rise and storm-driven flooding, in addition to providing community and ecosystem co-benefits. While restoration can meaningfully reduce flood risk to people and property in San Mateo County and the San Francisco Bay Area, it is only part of a set of hybrid solutions needed to provide sufficient flood protection. Furthermore, flood protection is only one of many benefits that salt marshes provide, such as carbon sequestration, fish and wildlife habitat, improved water and air quality and recreation. This first-of-its-kind valuation of marsh restoration benefits has the potential to advance the protection and restoration of this important form of natural infrastructure in San Mateo County, the California county with the greatest exposure to sea level rise, via the following pathways: 

Restoration project design 

Marsh restoration does not reduce flood risk evenly across the landscape—some locations in San Mateo County would benefit more than others, as demonstrated by the restoration scenario south of the San Francisco International Airport, which would provide an estimated $350 k/year in benefits with current sea level, a benefit which would increase tenfold with half a meter of sea level rise. The detailed flood model developed in this study evaluates the risk of flooding with and without salt marshes at a local scale, a tool that can be highly useful for the planning and design phases of marsh restoration projects in San Mateo County. 

In the top row, reduction in flood heights during a 100-year storm due to habitat restoration with 0 meters of sea level rise in subplot A and 0.5 meters of sea level rise in subplot B. Shaded blue areas show flood extent of a 100-year storm with change in depth as a result of marsh restoration. In the bottom row, present value of marsh restoration with 0 meters of sea level rise in subplot C and 0.5 meters of sea level rise in subplot D. Darker green colors signify higher present values, and paler green colors signify lower present values of flood reduction benefits.

Flood insurance applications 

A key next step will be to explore potential applications with both the insurance industry and FEMA’s National Flood Insurance Program (NFIP). FEMA’s Community Rating Strategy, a voluntary program where community investments in flood risk reduction, such as preserving open space, result in discounted premiums for NFIP policyholders, may serve as an analog. There is also the potential to incorporate the modeling of salt marshes into industry risk models used for pricing private flood insurance products, providing a more detailed assessment of the flood risk reduction benefits of salt marshes and potentially, premium reductions. 

Investment in marshes 

Monetizing the flood risk reduction benefits of salt marshes has the potential to help identify where funds for hazard mitigation, disaster recovery and/or climate adaptation would be most cost-effective. The additional quantification of benefits may focus greater investments in restoration projects throughout San Mateo County by further building the case that project benefits outweigh the costs. Such benefit-cost analyses may be used to bolster applications for public investments, such as through FEMA’s BRIC grant program or Regional Measure AA, which provides approximately $500 million for marsh restoration throughout the San Francisco Bay. Private property owners along the Bay may be motivated to invest in salt marshes if they directly benefit from reduced flood risk, in addition to co-benefits such as recreation and habitat protection. 

A primary goal going forward will be to explore these pathways and determine the opportunities for leveraging the value of salt marshes for flood protection to expand their protection and restoration in San Mateo County and the San Francisco Bay more broadly, which has historically relied on gray infrastructure, such as levees. Wetlands play a valuable role in reducing flood risk, and marsh restoration can, therefore, be an important tool for increasing community resilience through nature-based flood defense. 

As climate change progresses, and sea level rise and flooding put communities at increasing risk, protecting and restoring wetlands can ameliorate the social and economic impacts to coastal communities in San Mateo County and beyond. While this study was focused on a single county, and site-specific conditions, our approach may be applied to a diverse array of communities in estuaries worldwide. Quantifying the benefits of nature-based solutions such as salt marsh restoration will allow for greater opportunities for incorporating them into a holistic set of climate resilience investments.

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Climate Change
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Climate Sciences > Climate Change
Coastal Sciences
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Earth System Sciences > Coastal Sciences

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