The New Hampshire and southern Maine seacoast received two major northeaster storms, only three days apart in mid-January 2024. The January 10 storm, as measured at the Portland NOAA tide station, had a 4-foot storm surge on top of a fairly normal predicted high tide, resulting in a double peak high tide, reaching elevation 8.58’ above NAVD88 datum (typical FEMA flood map datum), or 13.8’ above mean lower low water (MLLW) chart datum.

Figure 1 January 10, 2024 storm high tide (NAVD88 datum)

The following northeaster on January 13, had a slightly lower storm surge, but the underlying predicted tide was higher in its cycle, resulting in a higher flood level, reaching elevation 9.32’ above NAVD88 datum (typical FEMA flood map datum), or 14.57’ above mean lower low water (MLLW) chart datum.

This preliminary NOAA data, awaiting verification, and once verified, will break the previous highest recorded tide from the February 7, 1978, storm, which reached elevation 8.87’ NAVD88. These flood elevations are often close to the FEMA 100-year flood elevation for areas protected from significant wave action. Of note, the FEMA 100-year flood does not mean it happens every 100 years, but it has a 1 in 100 chance of occurring in any given year. For this reason, the 100-year flood is now often referred to as the 1% flood, for 1% probability in any year.

It should be expected that storm surge floods will reach higher elevation over time. The NOAA Portland tide gauge has been recording sea level since 1912 and this long-term trend is a sea level rise of 0.62’ per 100 years.

One positive note for houses or facilities in NH and ME that are built on bedrock, firm ground or piles, is that the earths crust here is slightly rising, reducing the apparent rate of sea level rise relative to the ground, as a long-term trend of the crust rising following the last ice age and removal of the ice sheet weight that pushed down the crust. If your house or facility was built on shallow foundations over soft soils, such as soft clay or filled marsh, it may be subsiding, thereby increasing the relative sea level rise.