Portsmouth, NH Coastal Erosion Rate
Shoreline change 4.6 miles south of Portsmouth, NH
Good news, and an honest one: USGS's data can't distinguish this stretch, 4.6 miles south of Portsmouth, NH, from a flat trend. Whatever small movement it measured here is statistical noise, not a reliable direction, so none is claimed.
The grain of this number
USGS surveys the coast in transects spaced a few hundred feet apart; this page groups the nearest ones into a single ~3.5-mile reading for the stretch closest to Portsmouth, NH, rather than showing one arbitrary line. Segment center: 43.00, -70.76. The survey behind it runs to 2007-2009, in USGS's New England / Mid-Atlantic assessment, using the LRR method. Shoreline surveys aren't repeated every year, so this is the most recent long-term measurement USGS has published for this coast, not a live reading.
What backs this reading up
This reading comes from 77 individual transects averaged together, 31% of which measured erosion rather than growth. USGS's confidence interval here is ±0.15 ft/yr. The unadjusted figure here, 0.02 ft/yr, would place it 792 of 1211 if ranked directly - but an unreliable rank isn't a useful one, which is exactly why "no significant trend" is the headline instead. Source: USGS Open-File Report 2010-1119. Coronado, CA sits at national rank 1158 for growth (2.04 ft/yr), one reference point among the 1211 scored segments, 2597 miles from here. 1.7 miles up the coast, Hampton, NH shows 0.24 ft/yr of eroding.
One address near Portsmouth: the Coastal Change Hazards Portal zooms past this stretch to the underlying map.