Fire Island study conclusion

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Fire Island National Seashore (hereafter referred to as FIIS) in New York is approximately 31 miles long and varies from about 500 to 1,300 ft wide. The study area for this project was located on the Great South Bay side of the barrier island at the Sunken Forest (fig. 12). The U.S. Geological Survey (USGS) installed a set of shallow wells in low-lying forest and near-forest areas likely to experience saltwater inundation during a coastal storm-tide flooding event, and a single well in a low-lying inland forest area likely to experience fresh groundwater flooding of the tree roots. Wells were installed at various distances from the shoreline to help assess the influence of tidal forcing, coastal flooding, and precipitation.

Figure 12. Location of the study area on Fire Island.

Five wells were monitored for this project in the Sunken Forest: Marsh Well (USGS station ID 403922073064401) and Deep Well (USGS station ID 403922073064402) are both approximately 80 ft from the shoreline of the Great South Bay and are 60 ft away from each other; Middle Well (USGS station ID 403921073064801) is 180 ft from the shoreline; Inland Well #1 (USGS station ID 403921073070401) is approximately 300 ft from the shoreline; and Inland Well #2 (USGS station ID 403919073064801) is approximately 380 ft from the shoreline. The location of wells in the Sunken Forest is shown in figure 13.

Depth to the water table in the Sunken Forest area is generally less than 10 ft. Four of the five wells (Marsh Well, Middle Well, Inland Well 1, and Inland Well 2) were installed 1 to 3 ft below the water table. The fifth well (Deep Well) was installed 4 to 8 ft below the water table.

Figure 13. Location of wells in the Sunken Forest, Fire Island National Seashore, N.Y.

A sensor measuring water-levels and (or) specific-conductance was installed at the wells in September 2017. Data were measured every 15 minutes at most wells from September 2017 to February 2019 to record water-level and specific-conductance changes in the shallow aquifer caused by non-storm (astronomical) semidiurnal tidal forcing, coastal flooding, and (or) precipitation events. Fluctuations in water levels and specific conductance were highly dependent on a variety of factors such as precipitation, wind speed and direction, storm intensity, astronomical and storm-tide levels, wave climate, and proximity to the coast.

Responses to tidal forcing, coastal flooding, and precipitation were examined using the timeseries of water-level and specific-conductance data collected at the wells, along with precipitation, maximum sustained wind speed and direction, and tidal elevation data compiled from nearby USGS and National Oceanographic and Atmospheric Administration stations. During the monitoring period, two primary types of storms were observed: coastal flooding and precipitation-only events. Three events – one heavy rain, one short duration high wind and heavy rain, and one long-duration high wind and heavy rain (Nor’easter) – were reviewed in more detail in the data summary section of this webpage.

For the duration of the study, significant water-level responses to tidal forcing were observed in both marsh wells (shallow and deep). Water levels at these wells responded to semidiurnal and storm tides with little or no lag as shown in figure 14 below for the October 29-30, 2017 short duration heavy precipitation and coastal-flooding event. Water levels at wells located further inland responded much less to tidal forcing throughout the study period but did show minor responses to some of the heavy precipitation events.

Figure 1.

Figure 14. Precipitation (dashed blue) and tide elevation (dashed magenta) at USGS Great South Bay at Watch Hill on Fire Island, NY, tidal gage (USGS station ID 01305575) and water-level elevations at USGS wells 403922073064401 (Marsh Well – solid brown), 403922073064402 (Deep Well – solid teal), 403921073064801 (Middle Well – solid dark red), 403921073070401 (Inland Well 1 – solid pink), and 403919073064801 (Inland Well 2 – solid blue) for the period October 27, 2017, to November 10, 2017 (2 days before and 10 days after the storm). Storm period shaded in grey.

Significant increases in specific conductance during or shortly after the storm tide were recorded at both marsh wells as shown in figure 15 below for the October 29-30, 2017, short duration heavy precipitation and coastal-flooding event. The shallow marsh well showed the greatest increases in specific conductance throughout the period of study, generally occurring during or just after the peak storm tide. The deep marsh well showed less of a response with a much greater time lag than that of the shallow well. Wells located further inland from the Bay showed more modest or no specific conductance response to storms during the study period.

Figure 15.

Figure 15. Precipitation (dashed blue) and tide elevation (dashed magenta) at USGS Great South Bay at Watch Hill on Fire Island, NY, tidal gage (USGS station ID 01305575) and specific conductance at USGS wells 403922073064401 (Marsh Well – solid brown), 403922073064402 (Deep Well – solid teal), 403921073064801 (Middle Well – solid dark red), and 403921073070401 (Inland Well 1 – solid pink) for the period October 27, 2017, to November 10, 2017 (2 days before and 10 days after the storm). Storm period shaded in grey.

Data References:

National Oceanographic and Atmospheric Administration, Center for Operational Oceanographic Products and Services (CO-OPS). Tides and Currents website, https://tidesandcurrents.noaa.gov/stationhome.html?id=8514779, accessed on 2/11/2022.

National Oceanographic and Atmospheric Administration, National Centers for Environmental Information (NCEI). Climate Data Online (CDO) website, https://www.ncdc.noaa.gov/cdo-web/datasets/LCD/stations/WBAN:04781/detail, accessed on 2/15/2022.

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