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