The South Jetty Breach on Plum Island

Slide 1: 

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The South Jetty Breach : 

The South Jetty Breach

The beach was divided into 5 study sectors for satellite image analysis. The period studied was from April 1992 - April of 2007. Square acreage of each study sector was followed over time… : 

The beach was divided into 5 study sectors for satellite image analysis. The period studied was from April 1992 - April of 2007. Square acreage of each study sector was followed over time… Ocean Beach Study Sectors PI Point

During the study period, the Ocean Beach Sectors, as a group, lost atotal of 23 square acres of Shoreline…(where did the sand go?) : 

During the study period, the Ocean Beach Sectors, as a group, lost atotal of 23 square acres of Shoreline…(where did the sand go?) 109 86

During the study period, Plum Island Point gained some 22 square acres of beach… : 

During the study period, Plum Island Point gained some 22 square acres of beach… 30 53 Clearly sand migrated from the ocean facing Beach to Plum Island Point.

At it’s smallest size (30 Sq. Acres) within the study period, Plum Island Point looked like this in 1998… : 

At it’s smallest size (30 Sq. Acres) within the study period, Plum Island Point looked like this in 1998… By 2004 it grew to 53 sq acres & has remained at about 50 sq acres in size through 2007…

But how does the sand get around the Merrimack River South Jetty to build PI Point? : 

But how does the sand get around the Merrimack River South Jetty to build PI Point? The sand passes through an on shore breach in the jetty SE Ground Swell Constantly Impacts Jetty Over time, Rocks topple Waves drive sand Through breach

The persistent pounding of the SE ground swell coupled with the pressure of the migrating sand, have, over the years, pushed the Jetty granite into the Merrimack River… : 

The persistent pounding of the SE ground swell coupled with the pressure of the migrating sand, have, over the years, pushed the Jetty granite into the Merrimack River… March 6th 2008

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…about 7200 waves in 24 hours! Waves push that sand along every 10 – 12 seconds, as long as there is a swell …

Gravity returns water, along with some sand, back to its level…in the river. : 

Gravity returns water, along with some sand, back to its level…in the river.

After passing through the jetty breach, the sand forms a pile, or “wash over terrace”, as it has for decades… : 

After passing through the jetty breach, the sand forms a pile, or “wash over terrace”, as it has for decades…

Slide 12: 

Then, smaller ocean waves penetrating the river entrance scour the sand away… Incoming Wave Emerging Reflection Wave Erosion …transporting it westward to build Plum Island Point… Sand Transport

Slide 13: 

Small beach building wavestransporting sand…

Slide 14: 

On the incoming tide, sea water enters the river through the breach… The Tide Carries a plume of sand with it Wave action kicks up sand