The Science of Beach Erosion

Lauren ElmegreenESP Seminar

December 1, 2006

Outline

What is natural?Sand BudgetsNormal changes

How have humans intervened?Changing sources and sinks of sand

Case studiesSouthern California BeachesOcean City, MD

Sand BudgetsSources: runoff, cliff erosion, onshore transport, longshore transport

Sinks: offshore transport, longshore transport

+ Qc + Qon - Qoff

Qc

Qon

Qoff

Willis and Griggs, 2003

It’s normal for beaches to change!

Sand dissipates the energy from waves

Seasonal changes

Beaches move landward as sea level rises

www.coastalchange.ucsd.edu

Unintentional human changes: damsDams in California greatly reduce the amount of sand that reaches the beaches from runoff

Average annual Sand and Gravel Flux (m3/year)River Natural

(no dams)Actual (with dams)

Santa Margarita

44,500 30,488

San Dieguito River

45,000 9,563

San Diego River

55,000 5,031

All of California

11,360,812

8,471,233 (25% reduction)

Willis and Griggs, 2003

Intentional human changes

Seawalls and breakwatersJetties/groinsBeach nourishment

www.coastalchange.ucsd.edu

Sea WallsReduce the amount of wave energy that reaches cliffs/dunes

Can cause more beach lossCan cause more cliff destruction

www.soest.hawaii.edu/SEAGRANT/CEaBLiH.html

Jetties/GroinsDesigned to prevent sand loss from longshore currents

Result: accumulation of sand on one side, erosion on the other

www.vsv.cape.com/~harharb/allenbeachnourishment2005.html

Beach NourishmentMakes beaches widerNot a natural state, not necessarily sustainable, but good for tourism

Littoral Cell

River Yield (m3/yr)

Nourishment(m3/yr) (1940-90)

Santa Monica 46,000 440,000

Oceanside 112-203,000 190,000

Mission Bay 5-84,000 75,000Flick, 1993

Case Study: Ocean City, MD

Jetties were built to protect the inlet The Northern part of Fenwick island is moving landward, leaving Ocean City behind

Assateague Island is rapidly moving landward- reduced sand supply

http://www.mgs.md.gov/coastal/osr/ocsand2.html

Conclusions

Beaches erode through weather processes, sea level rise, and human actions

Erosion is bad for tourism and buildings near the shore, so actions are taken to prevent erosion

Some of these actions can cause even more erosion in the long run

References Flick, RE (1993) The Myth and Reality of

Southern California Beaches. Shore and Beach: 61 (3), 3-13.

Pinet, PR (2003) Invitation to Oceanography, 3rd ed., Sudbury, MA: Jones and Bartlett.

Willis, CM and GB Griggs (2003) Reductions in Fluvial Sediment Discharge by Coastal Dams in California and Implications for Beach Sustainability: 111, 167-182.

http://www.soest.hawaii.edu/SEAGRANT/CEaBLiH.html

http://coastalchange.ucsd.edu/index.html http://www.vsv.cape.com/~harharb/allenbeachno

urishment2005.html http://www.mgs.md.gov/coastal/osr/ocsand2.htm

l