Shoreward Migrating Sandbars

Shoreward Migrating Sandbars(Scroll down for Power Point Slides)

With the river delta bar & its sand supply now far offshore, sand migration back to shore via wave action, is slow & protracted. In the process of migrating back to shore, sandbars alter the angle at which waves approach the beach. This has resulted in erosion hotspots at various points along the shore.

In recent history, as Jetty Breaches have allowed sand to spill from the ocean shores into the river, this sand ultimately washed out to the delta bar at the river entrance. Here, wave action drove some of that sand south, growing a long sand bar, offshore of Plum Island. While Wave action expanded the bar to the south, it concurrently also drove it towards shore. As a consequence of this process, the southern tip of this bar refracted incoming wave energy, creating erosion hotspots on the beach. These Hotspots mirrored & traveled with the tip of this southerly expanding sandbar. For example, we've observed an erosion hotspot near the area of Mad Martha's around the year 2000. As the offshore bar expanded southward, this erosion hotspot also migrated south - to the center groin by 2004.

With a narrower overall shore, Plum Island's beach groins became uncovered. As this traveling erosion hotspot interacted with the now exposed center groin, we experienced extreme instances of beach erosion there. Currently this migrating bar is close to shore near Plum Island Center, providing protection from offshore waves & passing sand to the that shore, which was recovering in early 2013. The tip of this sandbar however, now lies south of the center, refracting wave energy there. Hence, the erosion hotspot moved further south once again, to the area between the Center Groin & Harvard Way. This sequence of events happened previously in the early & mid 1970's.

Offshore Sandbars Near Plum Island Center

SandBar Panorama North of Center 4 22 10

"Shoreward Migrating Sandbars"

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Mike Morris

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