By Carrie Schuman, PhD, SCCF coastal resilience manager & Paul Julian, PhD, SCCF hydrologic modeler

The following is part of Coastal Resilience Connections, a regularly occurring series that explores how building coastal resilience is interwoven with a variety of local issues and topics.

Hurricane Ian’s passing in late September brought record amounts of storm surge to our area, resulting in a tumble of water and waves that washed over Sanibel and Captiva Islands, significantly impacting our natural systems and human communities. The storm’s wind and water left innumerable visible fingerprints, many of which are still evident and may be for some time.

One of the more conspicuous signposts are high water lines or marks. These marks — which consist of water stains or mud, debris, and seed lines — provide a collective record of the highest points water reached above ground.

High water marks appeared on the outside of buildings on Captiva, as highlighted by the included arrows. The left-most is an example of a seed/debris line, the middle of a mud line, and the right is of a water stain line.

Shortly after the hurricane hit Southwest Florida, the United States Geological Survey (USGS) deployed teams to gather data in the region, including high water marks. This data was uploaded to the USGS Hurricane Ian Flood Event Viewer. Although the USGS undertook an extensive collection effort in the region, including Sanibel, they did not cover the majority of Captiva. Therefore, SCCF took additional measurements across Captiva to supplement USGS results and to better understand the storm.

When we surveyed Captiva, we found high water marks were most evident on the outside of built infrastructure. While we didn’t use the same level of surveying equipment as USGS, we were able to collect basic data at these locations.

SCCF Hydrologic Modeler Paul Julian combined the Captiva data with comparable portions of the USGS dataset for Sanibel to paint a more holistic picture of storm surge across both the barrier islands. For technical and scientific uses, these measurements are often expressed as height above mean sea level. However, we present the height of high water marks as measured relative to the ground (as seen in the maps below).

The resulting analysis is presented in two different ways. The first map shows high water lines measured at specific sites in feet above ground level. The second map represents the interpolation of the data across Sanibel and Captiva. Interpolation is a method that allows someone to estimate a value between two or more points and its accuracy is determined by the amount of data supplied.

Point (top) and interpolated (bottom) representation of high water line (feet above ground) for both Sanibel and Captive (left) and just Captiva (right).

These figures reveal certain patterns about how water affected both islands. Portions of Sanibel were exposed to higher levels of water — possibly up to 10 feet — than Captiva, which experienced 0-4 feet of flooding above ground in most places. There is also a gradient of flooding that runs approximately north to south across the islands, highlighting that ocean-facing shorelines experienced more flooding than those on the bayside.

Factors like surge and wave direction (which was approximately southeast as confirmed by the SCCF RECON wave buoy), the orientation of the islands, elevation gradients, and protection from natural features like mangroves and dune systems likely played a role in how water inundated Sanibel and Captiva. However, it can be hard to make firm interpretations without more information.

High water marks can help us understand how high the water got in certain areas, but they only represent static snapshots in time that do not provide information on how long the water persisted at any given site. Furthermore, high water marks are not an exact reflection of storm surge, which is the rise of water above normal levels due to forces like wind. As storm surge moves inland, it interacts with the landscape, including natural systems and built infrastructure, in ways that can modify where and how it flows.

Storm surge diagram from the National Oceanic and Atmospheric Administration’s SciJinks. See what 9 feet of storm looks like over time by exploring this animation.

Information on high water marks is valuable and may have additional utility when combined with other Hurricane Ian datasets or when compared with the impacts of different storm events, as no one storm is the same as another. However, from a resilience perspective, this high water data — even absent supplemental information — can provide starting guidance on which areas of the islands may be particularly vulnerable to flooding from storm-surge dominated hurricanes with similar pathways through our region. This can help inform aspects of pre- and post-emergency management, while also highlighting locations that could benefit from hurricane resilience measures like elevated infrastructure and preserving coastal wetlands, dunes, and mangrove systems.