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CYCLICAL SHORELINE EROSION: THE IMPACT OF A JETTIED RIVER MOUTH ON THE DOWNDRIFT BARRIER ISLAND

Andy Fallon, Chris Hein, Peter Rosen, Haley Gannon, Virginia Institute of Marine Science, College of William and Mary, Northeastern University


Study Area: Plum Island, MA, USA

Merrimack River

Gulf of Maine

Plum Island N 0

2 km

Cape Ann


Plum Island Beach Sediment Source & Distribution Grain Size (ф)

Dist. from Merrimack Mouth (km)

0

FitzGerald et al., 1994 Very Coarse Sand and Gravel (<0 φ) Coarse Sand (0-1φ) Medium Sand (1-2 φ) Fine and Very Fine Sand (>2 φ)

1

2

Dec. grain size

10

20


Plum Island shoreline change and mitigation, 1827 - present • Long-term erosion rate: 0.09 ± 0.6 m/yr

A

(Thieler et al. 2013)

• Stabilized inlet  periods of erosion on N. Plum Island A. Coir bags: 2010 • Groin construction B. Revetment construction: 2014 • Rock wall revetments C. Plum Island shoreline change, 1827-1940; Newburyport News • Jute bag dune stabilizers C. C • Historical shoreline analysis • 100yr; focus on last 25 yr • Recent analysis: Dec ’13 - Mar ‘15

B


2010

Shoreline Highest Year Erosion Location 1912 200 m N tombolo 1928 Tombolo 1953 Right Prong

2013

Erosion Mitigation response distance to steady state 110 m unknown 100 m 115 m

1978-79 Center Island to 95 m south groins 2008Center Island to 110 m 2014 Annapolis Way

unknown Beach Nourishment 425,000 m3, groin (4) construction Intermittent rip rap revetments Coir bags, rip rap revetments

Steady-state position Erosive shoreline


2010

Shoreline Highest Year Erosion Location 1912 200 m N tombolo 1928 Tombolo 1953 Right Prong

2013

Erosion Mitigation response 2015 distance to steady state 110 m unknown 100 m 115 m

1978-79 Center Island to 95 m south groins 2008Center Island to 110 m 2014 Annapolis Way

unknown Beach Nourishment 425,000 m3, groin (4) construction Intermittent rip rap revetments Coir bags, rip rap revetments

Steady-state position Erosive shoreline


Evidence of a Shifting Erosion Hotspot N Haley’s figure

“Tombolo”

Center Island

Annapolis Way 0

100m


Seasonal Beach Variability Groins

December 2013

Vol = 255,376 m3

Groins

June 2014

Vol = 435,436 m3

Ex: 5x

Ex: 5x

Average Change in cross shore profile (m3/m) 200 150 100 50 m2 0 -50 -100 -150 -200 Nov-13

Tombolo Annapolis 2 S. Groins Refuge Center Is. whole beach Jan-14

Mar-14

Apr-14

Jun-14

Jul-14

Sep-14

Nov-14

Dec-14

Feb-15

Apr-15


Annapolis Way Erosion Hotspot Shift September 2014

Vol = 50,478 m3

Vol = 80,617 m3

5x Vertical exaggeration

5x Vertical exaggeration

Normalized % change

100

% Change

March 2015

50

Tombolo

0

Annapolis

-50

2 S. Groins

-100

Refuge

-150

Center Is.

-200 Nov-13

Jan-14

Mar-14

Apr-14

Jun-14

Jul-14

Sep-14

Nov-14

Dec-14

Feb-15

Apr-15


Annapolis Way Erosion Hotspot Shift September 2014

Vol = 50,478 m3 Sep 2014

March 2015

Vol = 80,617 m3

5x Vertical exaggeration

Mar 2015

5x Vertical exaggeration


Conceptual Model for the Erosion “Hotspot”


Conceptual Model for the Erosion “Hotspot”


Conceptual Model for the Erosion “Hotspot”


Conceptual Model for the Erosion “Hotspot”


Conceptual Model for the Erosion â&#x20AC;&#x153;Hotspotâ&#x20AC;?

December, 2004

Southern extent of migrating bar Accretion

0

250m


Conceptual Model for the Erosion “Hotspot”


Conceptual Model for the Erosion “Hotspot”

December, April, 2008 2004 Southern extent of migrating bar

Southern extent of migrating bar Accretion Erosion “hotspot”

00

250m 250m


Conceptual Model for the Erosion “Hotspot”

October, 2014 December, 2004 April, 2008 Southern Southern extent of extent of migrating bar migrating bar New erosion “hotspot”

Southern extent of migrating bar Accretion Erosion “hotspot”

00

250m 250m


Summary & Implications • Human impacts on both large & small spatial and temporal scales: • Large Scale: jetties stabilize shoreline and allow for development • Small scale: wave refraction around jetty, ebb-tidal delta and offshore bars drives 25-40 yr erosion cycle • Groin placement likely exacerbates natural erosion cycles • High natural seasonal variability 2012, Boston.com


Acknowledgements

• Advisor and co-author – Christopher Hein • Co-authors – Peter Rosen, Haley Gannon • Committee members – Courtney Harris, James Perry, Porter Hoagland • Local homeowners – Bob and Kathy Connors

Fallon Coastal Seds Presentation 2015  

Presentation from Fallon's presentation on erosion/accretion patterns in northern Plum Island from the Coastal Sediments 2015 conference.

Fallon Coastal Seds Presentation 2015  

Presentation from Fallon's presentation on erosion/accretion patterns in northern Plum Island from the Coastal Sediments 2015 conference.

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