Archaeological sites around the world are increasingly being affected by rising sea levels and higher storm frequency/intensity caused by climate change (Dawson et al. Reference Dawson, Nimura, López-Romero and Daire2017, Reference Dawson, Hambly, Lees, Miller and Fluck2021; Miller and Murray Reference Miller and Murray2018). Tracking erosion at coastal archaeological sites is a challenge, and organizations across the globe are working to document this loss using a variety of methodologies and technology (Hambrecht and Rockman Reference Hambrecht and Rockman2017; Holtz et al. Reference Holtz, Markham, Cell and Ekworzel2014). Many approaches involve technologically advanced methods of data collection and analysis that are beyond the scope of nonprofessionals (Hil Reference Hil2020; Howland et al. Reference Howland, Jones, Najjar and Levy2018; O'Rourke Reference O'Rourke2017; Pourkerman et al. Reference Pourkerman, Marriner, Morhange, Djamali, Amjadi, Lahijani, Beni, Vacchi, Tofighian and Shah-Hoesseini2018): this limits the involvement of dedicated contributors such as local communities and other interested nonprofessionals. Here, we present an example of a community-involved, low-cost, and low-tech method for documenting loss at the eroding Calusa Island Midden site (8LL45). Located on Florida's southwest Gulf Coast, the site provides an exemplary location to develop innovative approaches to monitoring and documenting coastal site loss for use in archaeological research and site management.
The site is named after its location on Calusa Island, a diminutive island within the Pine Island Sound region (Figure 1a). Calusa Island is regionally unique within southwest Florida in preserving evidence of people and environments from the Terminal Archaic period, 1200–500 BC (Marquardt Reference Marquardt and Marquardt1999; see Haney et al. Reference Haney, Veres, Walker, Marquardt and Newsom2016). “Calusa” is the name Spanish invaders recorded for the Indigenous communities that inhabited most of southwest Florida (Marquardt and Walker Reference Marquardt and Walker2012). Today, there are two federally recognized descendant tribes that have land and reside in Florida: the Seminole Tribe of Florida and the Miccosukee Tribe of Indians of Florida. The Calusa Island Midden site spans parcels privately owned by individuals and the Calusa Land Trust (CLT), an organization dedicated to the protection of “the natural diversity and beauty of the Pine Island region” (Calusa Land Trust 2023). Long-term collaborations in heritage preservation between the Florida Public Archaeology Network (FPAN), the Florida Museum of Natural History (FM) Randell Research Center (RRC), and the CLT provide the foundation of the work presented here. Future efforts in site monitoring will continue to include collaborative input from a variety of heritage management and preservation experts, as well as local and descendant communities.
Figure 1. (a) Comparison of tree lines (coastline) between the 1944 USGS aerial photograph and the 2016 Lee County aerial photograph (from Haney et al. Reference Haney, Veres, Walker, Marquardt and Newsom2016); (b) photo of Calusa Island North Beach, October 13, 2017; (c) map of Calusa Island North Beach, including rebar indicated by letters and major landmarks across the beach.
The North Beach area of 8LL45 is composed of a dense shell midden. Vegetation along the midden edge includes trees, brush, and mangrove stands, with gumbo limbo (Bursera simaruba) and mangrove dominating the vegetative diversity. Because of its coastal orientation, this portion of the site is extremely vulnerable to erosion due to broad-scale sea-level rise and catastrophic storm events, as well as day-to-day impacts such as kayak landings and tourist visits (Figure 1b). Aerial photography shows the loss of trees along the shoreline since 1944 as reported in Haney and colleagues in 2016 (Figure 1a). In 2016, the island's sole permanent resident reported that more than 12 m (about 40 ft.) of the midden had eroded since the late 1970s, averaging more than 0.3 m (1 ft.) of loss per year (Figure 1a; Haney et al. Reference Haney, Veres, Walker, Marquardt and Newsom2016). During this time, midden materials, including worked and unworked shells and pottery, were observed eroding onto the shoreline and into the water, sometimes being redeposited farther down the beach. These trends continue in the present.
In the following sections we describe the monitoring methodology used at the North Beach area of Calusa Island to document coastal erosion, including the changes made over seven years based on experience with and evaluation of the methods; we also present the valuable data collected and briefly comment on how the data have informed academic research, storm response, and management at the site. This methodology can be used by site managers and citizen scientists to assess the condition and stability of other coastal archaeological sites and as the basis for any number of academic and management projects worldwide based on individual site needs, data, and goals. Moreover, this methodology can complement more high-tech site documentation efforts such as aerial lidar and shoreline remote sensing by providing detailed data of shoreline changes best observed in person (e.g., undercuts not visible from aerial perspectives).
Methodology
Initial Installation
The initial monitoring protocol was designed to require few technical tools and to have instructions clear enough to be implemented by a diverse group of volunteers, some with little to no archaeological experience. In April 2016, the RRC partnered with the CLT to establish an ongoing monitoring project to document and define site loss at the North Beach area of 8LL45 (Figure 1a–c). With the permission of the CLT, archaeologists installed half-inch-diameter steel rebar vertically into the midden at fixed reference points for measuring the distance to the midden's edge and to divide the approximately 100 m (328 ft.) stretch of shoreline into eight distinct sections to better document erosional patterns and distribution of artifacts from the midden (Figure 1c). Rebar were placed approximately 10 m apart and 2 m inland of the eroding edge and adjusted to fit seamlessly into the existing landscape. Rebar C, for example, was placed abutting a palm tree, making it less likely to cause a safety risk to island visitors. The rebar locations were tied into a local grid system established by island residents and then referenced to prominent landscape features along the shoreline, providing the spatial foundations for the monitoring protocol (Figure 1c). Some rebar were since removed by natural forces or visitors; however, because the original locations were documented they were easily replaced. In 2019, the rebar locations were geolocated using a total station as part of a site-wide survey.
As erosion occurs, the distance between the rebar and the edge progressively diminishes, enabling a calculation of the rate of change. Following the establishment of the monitoring protocol, and with support of the CLT and private Calusa Island landowners, the RRC and FPAN partnered to continue site monitoring and documentation of artifacts and material loss. To maximize documentation and engagement, a group of RRC volunteers were trained by FPAN and RRC archaeologists and empowered to visit the site independently.
Subsequent Monitoring
Working with a group of 26 people—13 trained archaeologists and 13 volunteers—since 2016, the site has been recorded 31 times, with visits made based on the availability of personnel. Initial plans included monitoring once a month; however, because of the fluctuation of volunteers who live in the area seasonally, hurricanes, and a global pandemic, monitoring visits were less consistent. Although ideally site visits would be regularly scheduled, the opportunistic timing of monitoring visits does not diminish the value of data collected, especially when evaluated over time.
Since 2016, the methodology has evolved to reduce the potential for measuring error. In the initial protocol, a pair of monitors were instructed to pull a measuring tape from the rebar to the erosional edge and record the measurement. This simple technique had the potential for varied techniques that would introduce error. These variations included how each individual held the tape in relation to the rebar, subjective decisions of where the erosional edge began, the angle of the tape to the erosional edge, and the horizontal level of the measuring tape across the area between the rebar and the edge. Each of these differing interpretations and subsequent decisions can result in centimeters of difference in measurements from one visit to another that may inaccurately reflect changes at the site.
To resolve these challenges, we then created a protocol that is printed and kept in a kit that volunteers and professionals use for monitoring along North Beach. It was detailed enough that monitors could visit the site with no recollection of their training and could still collect consistent and accurate data. FPAN staff met with volunteers at the site to train them in the protocol, the site, and the necessary techniques (using a compass and plumb bob, taking photos, etc). Although straightforward, the protocol does require a degree of skill and coordination that should be understood by anyone seeking to replicate the methodology and the volunteers they enlist. The combination of the complexity of the protocol and the difficulty in accessing the island (requiring a boat or kayak trip) required the careful selection of volunteers for the Calusa Island project. The protocol is abbreviated here to focus solely on the monitoring technique; the full protocol, which includes photos and details for documenting artifacts, is on file with the southwest and southeast regional FPAN offices and the RRC. All data collected during the monitoring are reported to and filed with the FPAN.
Calusa Island North Beach Monitoring Protocol
Measure the midden edge using rebar. There are nine rebar along the midden (Figure 1c). Carefully follow the protocol in Table 1 and see Figure 2a–c to ensure accurate and consistent measurements.
Table 1. Instructions for Measuring Midden Edge Using Rebar and for Measuring Undercut.
Figure 2. (a–c) Procedures for measuring midden edge using rebar; (d,e) measuring an undercut.
Measure the undercut, if present. In some areas, the midden has eroded more at the base where it meets the beach than at the top where you measured the edge. This is called an undercut (Figure 2d–e). Carefully follow the protocol in Table 1 to ensure accurate and consistent measurements.
Rebar Photos. Take two photos of each rebar; these will help us track how the beach looks over time at each location.
(1) Facing due north and including the cap of the rebar that denotes the rebar's letter (Figure 3a).
(2) Facing due south from the beach, including the rebar and surroundings (Figure 3b).
Figure 3. Rebar photos: (a) top of rebar G, facing due north; (b) view of beach and Rebar G facing due south.
Artifact Photos. Take at least two photos of each artifact, both with a scale north arrow.
(1) Close-up with scale north arrow and artifact. This is to help visually identify the artifact with some detail (Figure 4a).
(2) Context with scale north arrow and artifact. This photo is to tie the artifact in with large or fixed landmarks that are easy to find, like large trees, the shed near the picnic area, the stairs, or a rebar (Figure 4b).
Figure 4. Artifact photos: (a) close-up; (b) context.
Evolution of Methodology over Time and Common Mistakes to Avoid
This methodology produced data that document trends when analyzed over numerous years and anomalies after a large erosional event, such as a hurricane (Figure 5). The varying techniques that introduced error prompted us to update our protocol multiple times (Walker Reference Walker2016). Many issues we encountered would be easily avoidable for new projects. The following issues caused inconsistencies in the data and were addressed in the following ways:
Figure 5. Calusa Island rebar data from April 2016 through January 2023. Each rebar measurement is represented by a single dot, and trend lines show the trends for each rebar. The y axis indicates measurements in cm; the x axis notes January of each year (for example, Jan 16 represents January 2016). For exact dates and measurements see Table 2. Note anomalous measurements but overall trends of declining measurement indicating erosion.
Issue: Orienting the measuring tape along a consistent line from rebar to erosion edge
Solution: We added the use of a compass to ensure systematic collection of measurements from each rebar due north to the eroding edge (Figure 2a,b)
Issue: Ensuring a consistent angle at which the monitors looked from above the measuring tape down to erosion edge
Solution: We introduced use of a plumb bob to control the vertical line from tape to the erosion edge to ensure it is consistently vertical (Figure 2c).
Issue: Using the top of the rebar for measurement, as described in Table 1; over time the rebar moved, whether from natural (e.g., wind, erosion) or anthropogenic causes (e.g., boats, kayaks, people lifting/pushing them).
Solution: We began measuring from the bottom of the aboveground rebar, where it was less likely that the angle of the rebar would affect measurement; this is not yet reflected in our protocol because we recently implemented the change and monitoring is now paused. It will be updated before any subsequent visits and is a valuable solution.
Issue: Differentiating between rebar when a rebar is missing. Some have very clear proximity to a notable landmark, but some can be more difficult to differentiate. These difficulties are exacerbated in a project that uses volunteers who train once a year but may not visit the site regularly and who are reliant on the map.
Solution: We covered the length of the aboveground rebar in white PVC pipe; we then capped the PVC and soldered the letter of each rebar into the cap to make each easier to identify in person and in photographs (Figure 3a,b).
Issue: Need to replace repar; with multiple rebar running the length of the beach, some were removed by visitors and discarded them in nearby vegetation. Rebar F, which was placed in a fairly open area near a bench and likely attracted more attention than the other ones, was particularly affected.
Solution: Although inconvenient, accurate GPS locations were taken when installed, so they were easy to replace. New projects should consider concealing the rebar.
Alternate solution: An elegant solution for most of these issues was developed by the FPAN West Central office: using a set of two markers flush with the ground to create an easily repeatable straight line when a measuring tape is run from one marker, across the second, and to the erosional line. This methodology was used in 2021 in Pinellas County, Florida, and has proven useful.
Data
Based on the data collected at Calusa Island to date, we can make two important assertions. First, there is a clear negative correlation between the distance from the rebar to the edge of the midden and time (Table 2; Figure 5), which suggests severe erosion at the site. Data gathered from April 2016 to January 2023 show that the rebar-to-edge measurement is declining at an average rate of 77 cm (2.5 ft.) per year (see trendlines in Figure 5). This illustrates the threat to the site posed by major storms and day-to-day erosion. Second, although the midden covers the entire North Beach area, there is significant variability in terms of in situ artifact exposure and distribution documented using the section letters assigned to each rebar, suggesting differing archaeological site components (Figure 1c data available on request from the FM).
Table 2. Calusa Island Measurements for Eroding Edge and Undercut.
Note : Dates are mm/dd/yyyy. All measurements are in cm. The first column of each letter indicates measurement from the lettered rebar to the eroding edge; the second column of each letter documents undercut measurements. Em-dashes indicate no measurement taken due to missing rebar. 0 indicates no undercut noted. See Figure 5 for graphs and trend lines.
Although this methodology has demonstrated the ability of trained volunteers to collect useful data, we acknowledge that more consistent data collection would allow for additional, more refined analyses and conclusions. For example, monthly monitoring would allow for tracking temporal—monthly or seasonal—trends in North Beach shoreline change. Regular monitoring would also show how different causes of erosion may variably affect different midden areas / rebar locations, such as storm events, weather patterns, and visitor traffic.
Using Monitoring Data to Inform Research and Understand Storm Impacts
Archaeological Research
The data from our monitoring program have been foundational for archaeological research on Calusa Island. In 2016, a bulk sample of the eroding midden edge along North Beach was collected, and the first professional archaeological profile was documented (Haney et al. Reference Haney, Veres, Walker, Marquardt and Newsom2016). Three years later, in 2019, an assessment of monitoring data indicated high concentrations of in situ and displaced artifacts (e.g., pottery, worked shells, animal bones) along the east side of North Beach between Rebars B and D. In early 2020, in collaboration with the CLT, the RRC and FPAN conducted the first systematic archaeological shovel test transect across the interior of Calusa Island. In addition, a bulk sample was collected, and a midden profile drawn from the same location as Haney and colleagues’ (Reference Haney, Veres, Walker, Marquardt and Newsom2016) work to enable comparison of data through time. One of the goals of this work was to understand what cultural resources stand to be lost by erosion over the next five to 10 years.
Although the 2020 field season was cut short by the COVID-19 pandemic, nine shovel tests were completed, the bulk sample was collected, and the midden was profiled. Current results suggest that the shell midden extends across Calusa Island and is up to 1.3 m (4 ft.) deep across portions of the island, and the North Beach area includes cultural components representing Late Archaic to Caloosahatchee I and II (about 1000 BC–AD 1200) occupations, representing some of the earliest and least documented time periods of Indigenous history in the Pine Island Sound region.
Documenting Storm Damage
The North Beach monitoring data and protocol are especially useful for assessing the dynamic and devastating impacts of storms on the site. Most recently, on September 28, 2022, Hurricane Ian struck Pine Island Sound directly, profoundly affecting Calusa Island. Powerful winds, rain, and waves, as well as storm surge reaching 2.7 m (9 ft.), accelerated erosion at parts of the site (as documented in monitoring data) and uprooted trees left newly exposed portions of the midden vulnerable to future erosion (Figure 6a,b). Additionally, a significant amount of shell material from the sound was deposited on parts of the beach. These deposits impeded our ability to define the full extent of erosion prior to their deposition; however, they now provide a temporary measure of protection by partially covering erosional edges of the midden.
Figure 6. Rebar E and staircase images from 2017 to 2023. Notable hurricanes during that time were Irma on August 18, 2017, and Ian on September 28, 2022. The staircase was no longer present after Ian, and the midden had eroded 100 cm past the rebar.
Most Calusa Island volunteers were displaced from their homes, lost property during the storm, or both, and Calusa Island was inaccessible for weeks. FPAN archaeologists were not able to monitor the site until December 6, 2022, followed by another site visit with RRC archaeologists and CLT members in January 2023. The first visit focused on locating and assessing the contextual integrity of the rebar and documenting overall damage to the North Beach midden area. Many of the rebar survived the storm without alteration, but all experienced corrosion and showed signs of deterioration. Rebar F had been displaced before the storm (likely by visitors to the island), Rebar D and G were missing or displaced after the storm, and the midden surrounding Rebar E and I had eroded past the rebar.
During our second visit, we toured the site with local CLT members and individual property owners. Using Hurricane Ian as an example, our goal was to consider how future hurricanes may affect the site and thus management decision-making. We discussed how the pattern of shoreline erosion followed by storm surge deposition along North Beach created deposits of mixed archaeological and more recent materials (e.g., oysters and sand) along the midden profile. It was clear that the exposed North Beach midden profile is vulnerable to a variety of taphonomic and site formation processes related to large storm events. Based on this observation, and within the context of previous monitoring data indicating rapid loss along Calusa Island's north coastline overall, we recommended against additional destructive (e.g., subsurface) archaeological investigations of the North Beach midden profile for the foreseeable future and the continuation of nondestructive monitoring efforts.
For Calusa Island, the monitoring data demonstrate that a single storm can be the catalyst for immense change and destabilization of a coastal site. Dozens of tropical systems have affected Pine Island Sound, and since 2016 two major hurricanes—Irma in 2017 and Ian in 2022—made landfall within 60 miles (95.5 km) of 8LL45. Our data provide CLT property managers and Calusa Island property owners with information on erosion rates they can apply to site management and preservation decisions. Moving forward, we are exploring the installation of more durable location markers to replace the corroding rebar and moving markers farther inland along their established compass bearings as needed. We, however, intend to continue using the established monitoring protocol for data consistency because of its adaptability for use by volunteer monitors.
Summary and Conclusions
Worldwide, tracking erosion at coastal archaeological sites is a cultural heritage preservation challenge exacerbated by accelerated sea-level rise and increasingly frequent and powerful storms. Documenting the rate of site loss is beyond the capabilities of many managers or entities, including federal, state, municipal, and nonprofit organizations charged with preserving archaeological heritage. However, it is essential that site change be documented to help understand the dynamic impacts of climate change, even if on a small scale. Here we described a low-tech, inexpensive, replicable, and community-accessible monitoring methodology developed at the Calusa Island Midden site to systematically measure shoreline erosion and cultural resource disturbance.
Over the past seven years, our results demonstrate not only the effectiveness of the approach in documenting the rates and impacts of erosion but also the relevance of monitoring data to archaeological research and to understanding the scope of site loss due to significant storm events. Our data show the normal rate of erosion at the site, the distribution of displaced artifacts across the site, and the increased impact of storm events. The photographic data are invaluable in communicating the erosional impacts to the site and the incredible changes in appearance after a storm, which reflect the change in site stability (Figure 6). This approach is adaptable to a variety of coastal locations, is affordable to implement and sustain independently, and can also complement any number of high-tech and more costly methods (such as lidar documentation). We hope our approach inspires more collaborative efforts to document and preserve vulnerable coastal cultural sites.
Acknowledgments
This work is the product of several years of planning and implementation and volunteer efforts carried out at the Randell Research Center. We thank Jen Haney, Karen Walker, William Marquardt, Victoria Lincoln, Drew Schneider, and Katie Higgins for their dedication during the early years of this project. However, this project would not be possible without the sincere passion and dedication to environmental and cultural heritage by the members of the Calusa Land Trust, especially Ed Chapin, Bill Spikowski, Allison Ackerman, and Robert Ballard. Finally, we acknowledge the resilience of Pine Island Sound communities and those from surrounding areas still recovering from Hurricane Ian. All images not otherwise credited are courtesy of the Florida Public Archaeology Network.
Funding Statement
Initial work at Calusa Island was supported through grants from the National Science Foundation (Award No. 1636872 to Newsom and Haney) and a Hill Fellowship Award through the Department of Anthropology at Penn State University. Archaeological investigations in 2020 and Hurricane Ian assessments in 2023 were supported by the Knight Professorship in Archaeology in South Florida Archaeology at the Florida Museum of Natural History and the National Science Foundation (BCS-2304807 to M. J. LeFebvre), respectively.
Data Availability Statement
Data that support the findings of the excavations are still being analyzed. The monitoring data that support the findings of this study are available from the Florida Public Archaeology Network, Florida Atlantic University.
Competing Interests
The authors declare none.
