Why rigorous details in models matter: Modeling the impact of mangroves on hurricane risk

Author: Chris Thomas, Director, Analytics and Modeling - Insurance, Moody's

It is widely accepted that mangrove forests can protect people and property against flooding from the sea.

These coastal wetlands—prevalent in many tropical and sub-tropical areas—act to slow down the flow of water through them, and therefore to limit the inland extent of coastal floods, whether caused by hurricanes, tsunamis, or high tides [2], [3].

Moody’s RMS™ North Atlantic Hurricane Model Version 25 takes the presence of mangroves into account when modeling storm risk. This often-overlooked but important feature of the U.S. coastline can have a real impact on hurricane-related storm surge estimates and the level of damage caused. This can lead to mispriced risk and misuse of capital.

In the U.S., hurricane-driven storm surge is a major source of coastal flooding. As a hurricane approaches land, strong winds blowing against the shoreline can push vast amounts of seawater towards the coast, causing it to pile up at the coastline and, in some cases, overtop it, flooding overland areas with saltwater and waves. The resulting damage to communities and property can be catastrophic.

Southwest Florida’s Gulf Coast has seen its fair share of large, destructive hurricanes in recent years, with both Irma (2017) and Ian (2022) battering the region with strong winds and damaging storm surges.

The region’s flat topography and high concentration of exposure close to the coast combine to make it relatively vulnerable to coastal flooding losses. However, it is also home to some of the largest mangrove forests in the country, which provide an important layer of natural protection against coastal floods.

Florida has by far the largest coverage of mangroves in the U.S., accounting for over 90% of the country's mangrove forests. Estimates indicate that Florida has nearly 600,000 acres of mangrove swamp, most of which is in its southern peninsula, concentrated along the southwest coast, where the Everglades and Big Cypress Swamp drain to the Gulf.

Mangroves extend up to 30 miles (48 kilometers) inland on the southwest coast and spread along the more northern coasts (north of Cape Canaveral on the Atlantic Coast and north of Cedar Key on the Gulf Coast).

In a recent scientific study carried out alongside our academic partners [1], we decided to quantify the value Florida’s mangroves could provide in terms of protection against hurricane-driven storm surges. The results, published in a peer-reviewed article in Cell Reports Sustainability [4], are explored below.

How much higher would losses have been if mangroves weren’t there?

First, we wanted to ask the question: how much worse would Hurricanes Irma and Ian have been if mangroves weren’t there? And then more generally, what kind of annual average reduction in losses are mangroves currently affording to residents and to insurers of coastal properties, perhaps without them even fully realizing it?

To examine these questions, we used the Moody’s RMS North Atlantic Hurricane Storm Surge Model, which captures the extent of mangrove cover throughout the domain and dynamically models their effect on event flood footprints.

We ran a counterfactual experiment assuming that Florida’s mangroves were lost and replaced with a much smoother type of terrain, roughly characteristic of developed open space. The aim was to estimate the full value of Florida’s mangroves, rather than to create realistic alternative scenarios. We reran model storm surge simulations for the two different scenarios: ‘with’ and ‘without’ mangroves, to see how different the flood extents and losses are for Hurricanes Irma and Ian.

We found that removing mangroves caused the total economic storm surge losses for Irma (2017) and Ian (2022) to rise by 14% and 30% respectively.

Mangroves in the real world slow down the flow of floodwater from the sea through the effect of friction between the mangrove trees and the floodwaters flowing through them. This causes energy dissipation and therefore reduces the momentum of the flow.

In the counterfactual world, however, water could flow much more freely overland without encountering resistance, leading to bigger and deeper flooding footprints, driving higher losses.

Figure 1: The effect of mangroves in reducing economic loss during Hurricane Ian (2022). Positive values (Blue) mean that mangroves acted to reduce losses inside that cell, compared to the counterfactual scenario where mangroves were removed. Negative values (Red) mean mangroves acted to increase losses.

These headline figures disguise a high amount of geographic variability. For example, Figure 1 above shows the value of mangroves in Hurricane Ian.

We can see that while mangroves reduced flood losses for all locations built inland of the mangroves, the picture is much more complex for properties inside the mangrove forests themselves. In fact, at some locations seaward of the mangroves, their presence actually served to increase flood depths and losses by blocking the flow of water through the forests and onto the land behind, much as a concrete seawall can increase water depth on its seaward side.

Not all storms are equal: Mangroves protect better against flooding from less-damaging storms

To understand whether mangroves protect better against weaker or stronger storms, we also ran a selection of storms from our stochastic event set through our counter-factual ‘no-mangroves’ scenario.

These storms were all representative of physically possible storm events in Collier County, Florida, from Category 1 events through to the strongest Category 5 events. We focused on Collier County, which includes the coastal towns of Naples and Marco Island, as it combines a large mangrove extent with a high concentration of exposure.

First, we found that mangroves act to reduce overall average annual losses (AAL) by up to $67 million. In other words, if all mangroves in Collier County were removed, we would expect AAL from storm surges to increase by that amount. Much as we saw with Hurricane Ian, the situation is spatially complex. Some settlements located seaward of large mangrove forests actually experience higher losses due to the presence of mangroves behind them, for example, Marco Island (Figure 2), whereas all exposure located landward of mangroves receive significant benefits.

Figure 2 – The effect of mangroves on AAL in Collier County. Positive values (Blue) mean mangroves act to reduce loss inside those cells.

What if we take this $67 million total saving and analyze the proportion that comes from smaller-loss, high-frequency events, compared to larger, more infrequent events? We find that over half (55%) of the mangrove benefits come from protecting against events with return periods (RPs) below 30 years (Figure 3), whereas only 14% of the benefits come from events with an RP above 100 years.

In other words, mangroves are doing a great job of reducing flood risk from frequent, smaller-loss events, but ultimately make less of a difference to the risk from the stronger, tail events, which overwhelm the protection offered by these ecosystems.

Land use strongly affects storm surge flooding risk

This study builds on earlier research carried out by Moody’s and our academic partners, which estimated that salt marshes in the Northeastern U.S. reduced storm surge losses from Hurricane Sandy by over $600 million [5]. Taken together, these two case studies serve as a reminder of the important role of natural ecosystems in protecting against flooding, and also stress the importance of utilizing risk modeling solutions that account for the effect of mangroves, salt marshes, and other land types on overland flow. Using overly simplistic models, which do not adequately account for the effect of land use, can result in badly mispriced risk.

Figure 3 – The proportion of AAL reduction benefits contributed by events with RPs at or below the values shown in the x-axis. For example, 86% of the total AAL reduction benefit of $67M comes from events whose RPs are <= 100 years.

[1] This work was jointly carried out with colleagues at the University of California, Santa Cruz, and East Carolina University

[2] Chen Q., Li Y. et al. (2021). Improved modeling of the role of mangroves in storm surge attenuation, Estuarine, Coastal and Shelf Science, 260, https://doi.org/10.1016/j.ecss.2021.107515.

[3] Montgomery J.M., Bryan K.R. et al. (2019) Attenuation of storm surges by coastal mangroves. Geophysical Research Letters, 46, https://doi.org/10.1029/2018GL081636

[4] Narayan S., Thomas C.J. et al. (2025). The spatially variable effects of mangroves on flood depths and losses from storm surges in Florida, Cell Reports Sustainability, 100531, https://doi.org/10.1016/j.crsus.2025.100531

[5] Narayan S., Beck M. et al. (2017) The Value of Coastal Wetlands for Flood Damage Reduction in the Northeastern USA, Scientific Reports, 7, https://www.nature.com/articles/s41598-017-09269-z