Portsmouth, NH Coastal Erosion Rate
What USGS measures 3.1 miles southeast of Portsmouth, NH
The coast 3.1 miles southeast of Portsmouth, NH is actually gaining ground, not losing it: USGS's transects measure 0.32 feet of accretion a year here, sediment building up rather than washing away. At that rate, the shoreline has gained roughly 3.2 feet over the last decade.
What "Portsmouth" means on this page
This is a roughly 3.5-mile stretch of coast, not one address or parcel - USGS's own transects run every few hundred feet, but that's finer than any one property needs, so this page reports the stretch nearest Portsmouth, NH. Centered near 43.03°N, 70.74°W. Survey years: 2007-2009, method LRR, from USGS's New England / Mid-Atlantic shoreline-change assessment. That's how current the data is here, real, but not a live feed, since a shoreline survey is a major undertaking, not an annual check-in.
25 years at this rate
Holding the 0.32 ft/yr accretion rate 3.1 miles southeast of Portsmouth, NH for 25 years would add roughly 8 feet of shoreline, again assuming nothing about the local sediment supply changes.
What backs this reading up
Averaging 12 separate transects produced this segment's rate, with 0% individually reading as erosion, rather than relying on any single survey line. The published margin on this reading runs ±0.29 ft/yr. Sorted from fastest erosion to fastest growth across all 1211 measured segments nationally, this one lands at 969. Source: USGS Open-File Report 2010-1119. Rank 963 nationally for growth belongs to Albion, CA, 2727 miles from this stretch, measured at 0.3 ft/yr, 0.9x this stretch's rate. Portsmouth, NH, 2.5 miles off, is the adjacent measured stretch, itself no significant trend.
Not this whole stretch, just Portsmouth itself: the Coastal Change Hazards Portal is USGS's own live version of this data.