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Figure 8. 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.
Event #6 – Nor’easter with High Winds and Heavy Rain – March 2-8, 2018
Event Discussion:
Two distinct storms affected the study area from March 2-8, 2018, that were characterized by heavy rain and strong winds (fig. 9). The more intense of these two events was a Nor’easter that affected the area from March 2nd to March 4th, the second storm affected the area from March 7th to March 8th. Storm-total precipitation for the combined event was 4.62 in. at Long Island-Islip MacArthur Airport over the six days, with a maximum daily total of 3.11 in. at Long Island-Islip MacArthur Airport recorded as rainfall on March 2nd. An additional 1.35 in. at Long Island-Islip MacArthur Airport (0.79 in. at USGS station 01305575, unpublished) was recorded as rain and snow on March 7th during the second storm.
Winds were generally from the east to northeast at the beginning of the first storm, reaching a maximum sustained wind speed of 38 mph at Long Island-Islip MacArthur Airport (47 mph at USGS station 01305575, unpublished) on the afternoon of March 2nd, then shifting to a more north-northeasterly direction later that same day. Winds remained generally from this north-northeasterly direction until the morning of March 6th. As the second storm approached, winds shifted to the south by the afternoon of March 6th and then to a more easterly to north-easterly direction later that day and during most of the following day. A second sustained wind peak of 30 mph at Long Island-Islip MacArthur Airport (42 mph at USGS station 01305575, unpublished) was reached on the evening of March 7th. Winds then shifted to a north-westerly direction by the morning of March 8th.
This event began during a period of relatively high astronomical tides and ended during a period of relatively low astronomical tides as determined from NOAA station 8514779. The strong winds associated with the first storm produced coastal flooding that reached a maximum elevation of 3.17 ft (NAVD 88) at USGS station 01305575 in the early morning of March 4th. This maximum occurred well after the peak of the east to northeasterly winds from this event and coincided with an extended period of moderate north-northeasterly winds with high astronomical tides. As second peak in storm tide of 2.71 ft (NAVD 88) at USGS station 01305575 occurred in the early morning of March 8th as winds from the departing second storm shifted to a northwest direction. Tides throughout the duration of this 7-day event remained elevated, with peaks during each of the diurnal high tides reaching above 2.0 ft (NGVD 88).
Figure 9. Precipitation (solid blue), tide elevation (solid magenta), and maximum sustained wind speed (dotted grey), at USGS Great South Bay at Watch Hill on Fire Island, NY tidal gage (USGS station ID 01305575) for the period February 28, 2018, to March 18, 2018 (2 days before and 10 days after the storm). Storm period shaded in grey.
Ground-Water Levels:
During the event, all wells experienced significant increases in groundwater elevations (fig. 10). Water levels in the Marsh Well and Deep Well increased by about 2.5 ft during the event, with peak water levels of 2.79 ft (NAVD 88) and 3.19 ft (NAVD 88), respectively. The Deep Well reached a peak on the morning of March 4th at around the time of peak storm tide as recorded at USGS station 01305575, while the shallower Marsh Well reached a peak two days later in the early morning of March 6th at the time of high tide. This Marsh Well peak was only 0.01 ft above the peak water level reached at this well on March 4th at around the time of peak storm tide for the event. Only one of the three other inland wells (Inland Well 1) was monitored during this event. Inland Well 1 reached a peak water level of 2.77 ft (NAVD88) during the morning of March 8th, about 4 days after the storm-tide peak from the first storm and about 4 hours after the storm-tide peak of the second storm. Water levels returned to pre-storm levels for both the Marsh Well and Deep Well by around March 11th, while the Inland Well 1 remained elevated longer and did not return to pre-storm levels until around March 17th.
Parts of the shoreline above higher-high water were inundated during this event. However, none of the wells were believed to be overtopped by storm tide. Water levels did not peak above land-surface elevation during this event for any of the wells in the study area.
Specific Conductance:
The specific conductance at the shallow marsh well (Marsh Well) increased from about 500 µS/cm on March 2nd to almost 7,800 µS/cm on March 4th, generally coinciding with the recorded peak storm tide at USGS station 01305575 (fig. 8) presumably from aquifer infiltration of seawater from the storm-tide inundation of the marsh. Specific conductance after the event slowly declined to about 700 µS/cm by March 10th, then increased moderately during the second storm peaking at slightly above 2,300 µS/cm on March 13th. Specific conductance at the Marsh Well was then variable until around March 25th when another sharp rise occurred peaking at about 7,800 µS/cm on March 26th (not shown on hydrograph). It is not known if this March 26th peak is related to the effects of the two storms from this analysis period or are due to some other factors. It is also interesting to note that there was a semi-diurnal rise and fall of specific conductance at the Marsh Well during most of this event that closely coincides with the semi-diurnal (astronomical) tidal cycle. Specific conductance at the deeper marsh well (Deep Well) showed a delayed response to the coastal-flooding event. Specific conductance at this well began a gradual decline from about 800 µS/cm on March 2nd to about 520 µS/cm on March 12th. Specific conductance then increased sharply reaching a peak of about 4,600 µS/cm on March 20th, then declined until April 8th, which was the end of the period of data collection at this well.
One of the wells located further inland was also monitored for specific conductance during this event (Inland Well 1) with this well showing significantly less response to the event than the two marsh wells. Specific conductance at Inland Well 1 showed a sharp decrease from 400 µS/cm to 370 µS/cm on March 2nd around the time of the heavy precipitation event, then gradually increased to about 480 µS/cm on March 21st, and gradually declined to about 270 µS/cm by the end of April.
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Last updated: December 5, 2022