There is this about the ocean’s role in its transport and disposal of sand being problematic. Breaking waves can diminish beaches under one set of conditions and restore them under others. Differences in turbulence and wind direction make for outcomes.
There are, also, the results of mankind’s involvement.
Newbury — and later Salisbury and Newburyport — lived without the jetties from 1635 to their construction in the late 1880s. Prior to that nature had extended Plum Island’s northeastern end at a loss of Salisbury’s southeasterly one, and the building of the jetties stabilized what nature had done.
Had the natural growth northerly of Plum Island not been stopped by the jetty construction, it’s reasonable to expect that the river’s mouth would be much as it had been, but we can’t be certain of that. Barrier beaches can, and sometimes do, change their settings.
Their construction at the mouth of the Merrimack resulted in safe depths of the river’s channels, but they must be maintained regularly because the river is the island’s major source of sand.
As for attending beach erosion, there was a time when Plum Island structures were, in the main, built for summer use. Most cottages, compared with those of today, were modest, and there was little year-around occupancy right up to, and following, the Second World War.
There were, of course, losses, but there was much less official involvement in the aftermaths. Owners built at their own risk, as they do today, with losses much greater and layers of regulatory authority considerably more involved.
The realty is self-evident. Building on a barrier beach involves risk, and solutions do not come with guarantees. Indeed, a case can be made that corrective action taken to protect one area can result in problems elsewhere because of nature’s unpredictability.