Seawalls are often used to protect structures from the ocean onslaught of storm driven surf. These artificial barriers have been shown to focus the ocean energy in areas around the sides of the seawall structure.
In this case study done by the USGS and US Army Corps of Engineers, this section of coastline near Fort Ord retreats at an astonishing 4-6 feet per year. (1)
This had an obvious impact on the attempts to save old Stillwell Hall on the CSUMB campus. Eventually natural coastal erosional processes won out with the building being removed and remaining sea wall cleared away from the beachfront.
This timeline of photos shows the erosion taking place around the sides of the seawall (rip-rap boulders & rocks). Over time the seawall itself will become isolated (like a “block” cut out from the coastline) from the shore as the coastline retreats inland, sometimes referred to as the peninsula effect. Over time, the former seawall turn wave deflector to offshore reef and eventually buried below the advancing sands. (credit below)
This NOAA imagery shows the Monterey Beach Hotel “peninsula effect” as a clear rectangular-shaped cut out feature as the shore nearby advances inland over time.
The seawall to the front of the hotel has helped to “focus” energy to the sides of the feature, increasing the rate of erosion near the sides of the sea wall.