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Shoreline Erosion Problems & Approaches
Karen Duhring Virginia Institute of Marine Science
College of William & Mary
June 10, 2015 Living Shoreline Contractor Training Workshop
Camden County College, New Jersey
Shoreline Erosion Problems & Approaches
• Typical erosion situations
• Typical responses
• Why a different approach is now popular
– ‘Living Shorelines’
What causes shoreline erosion?
Wind
Energy
Water • Rainfall
• Surface water runoff
• Groundwater
• Tidal waters
• Wind waves
• Boat wakes
Where does erosion occur?
Upland Riparian area
http://pages.unibas.ch/environment/Forschung/Current_projects/Gully_erosion.png
Where does erosion occur?
Top of Bank
By VHB in NPS-CNHP Env Assess PMIS 145520
Where does erosion occur? Base of Bank
Where does erosion occur? Marsh Edge
Where does erosion occur?
Beaches and Dunes
FEMA/Ty Harrington
Epod.usra.edu/Jim Phillips
WCU Program for the Study of Developed Shorelines, psds.wcu.edu
Where does erosion occur? Shallow Water
Why control erosion?
• Loss of land
Why control erosion?
• Loss of land
• Infrastructure at risk
Why control erosion?
• Loss of land
• Infrastructure at risk
• Loss of topsoil
http://www.eurosem-soil-erosion.org/
Why control erosion?
• Loss of land
• Infrastructure at risk
• Loss of topsoil
• Pollution
http://ga.water.usgs.gov/edu/characteristics.html
Some erosion is good
• Erosion is a source of sand for estuary beaches
Some erosion is good
• Erosion is a source of sand for estuary beaches
• Erosion is a source of sediment for marshes
Coastal development drives demand for shoreline protection
2007
Typical Approach for Shoreline Protection
• All erosion can be stabilized if you want to
• Stabilization usually achieved with structures as long as they are ‘necessary’ & impacts are ‘minimized’
• Minimized, unavoidable wetland impacts should be compensated through wetland mitigation
• Mitigation typically only for direct impacts
On-site Off-site Mitigation bank In lieu fee
Types of Shoreline Impact
• Direct impacts
– Immediate result of activity, permitted footprint
• Secondary or incidental impacts
– Occur after the activity or in a nearby area
• Cumulative impacts
– Combined impacts of many projects over time and space
Bulkheads Vertical structures that act as retaining walls
Usually built parallel to shoreline using various materials
Timber
Vinyl
Concrete
Gabions
Dry-Stack Blocks
Bulkheads Ecosystem Services Impacts
• Direct impacts:
– Riparian buffer clearing & fill
– Slope change due to fill
– Tidal wetland fill & loss
– Shallow water fill & loss
• Secondary impacts:
– Reflected wave energy
– Wildlife movement disrupted
– Landward migration of wetlands interrupted
Secondary Bulkhead Impacts Reflected Wave Energy
Wave Action
Wetland & Beach Impact Area
Reflected wave energy may increase erosion of the backfill area, adjacent wetland, beach, &/or shallow water areas. This increases water depths plus re-suspends & re-distributes sediments.
“End effect” erosion can occur on adjacent, unprotected shorelines over time.
Benthic & Shallow Water Impact Area
Secondary Bulkhead Impacts
Fish and wildlife movement is disrupted between habitats when gradual slopes and integrated vegetation communities are sharply divided by vertical bulkheads.
Integrated tidal marsh & riparian forest vegetation on gradual slope
Steep, vertical divide between riparian and wetland habitats
Before After
Secondary Bulkhead Impacts Habitat migration in response to sea level rise
Revetments Sloped structures with layers of stone or broken concrete
Placed along upland bank or failing bulkheads
Bulkhead toe revetment
Revetment Advantages over Bulkheads
• Long life expectancy
• Wave energy reduced
• Contour with shoreline
• Dimensions can vary with changing bank conditions
Revetment Advantages over Bulkheads
• Reef-like habitat in intertidal zone & shallow water
Juvenile fish at submerged area of revetment
Revetments
• Direct impacts:
– Loss of riparian buffer habitat
– Loss of tidal marsh production
– Loss of non-vegetated wetland and shallow water habitats
– Fewer benthic prey animals
• Secondary impacts:
– Tidal marsh fragmentation
– Less fish production
– Reflected wave energy
– Interrupted sand movement
– Wildlife movement disrupted
– Landward migration of wetlands interrupted
Direct Revetment Impacts
Before
After
Stable Riparian Buffer Clearing
• Less rainfall & runoff interception
• Less organic matter contribution
• Less water temperature regulation
• Less forest habitat
Direct Revetment Impacts
Before
After
Non-Vegetated Wetland and Shallow Water Habitat Fill
• Local habitat complexity reduced
• Less transfer of organic matter between habitats
• Benthic community covered
Less benthic prey available, less fish production
Secondary Revetment Impacts
Separating Riparian Buffer & Tidal Wetland Habitats
• Wetland habitat complexity reduced
• Wildlife movement interrupted along shoreline, between habitats
• Wetland migration into landward habitat area interrupted
Secondary Revetment Impacts
Interrupting sand movement & dune building on beaches
• Sediment supply for adjacent shorelines reduced or altered
• Onshore-offshore sand movement between beach & dune interrupted
Secondary Revetment Impacts
New erosion area
Wind & wave
direction
New erosion area
Reflected wave energy “End Effect”
• Increased erosion of adjacent, unprotected shorelines
• Continual sediment re-suspension and scour in adjacent intertidal and shallow water benthic habitats
Shoreline Hardening The placement of bulkheads and revetments along the shoreline “hardens” previously natural shoreline conditions.
Permit applications are approved for 16 - 18 miles of new hard structures each year in Virginia.
Hardening occurs on all types of shorelines, regardless of erosion and other risk levels.
Cumulative Shoreline Hardening
State or Region % Shoreline Armored
Source
New Jersey 17 % Lathrop and Love 2007
New Jersey – Barnegat Bay 45 % PDE report #11-04
Chesapeake Bay 18 % VIMS-CCRM 2013
Florida 21 % FDEP 1990
Southern California 30 % Griggs 1998
Source: Bilkovic et al 2015 in press
SHORELINE HARDENING TREND:
“hold the line” armoring approach is most popular in spite of other available alternatives
CUMULATIVE IMPACTS: Many small shoreline projects over time …can alter large geographic areas even where shoreline structures are absent National Research Council, 2007
National Review of Shoreline Management
SOME OF THE FINDINGS
Stormwater runoff - less infiltration and filtering
Riparian forest suppression & fragmentation
Tidal wetland loss & fragmentation
Disrupted riparian & wetland habitat connections
Altered sediment supply & transport patterns
Cumulative Impacts of Shoreline Development & Hardening
Static homogeneous shoreline, less biodiversity
Degraded water quality
Decreased fish & benthic animal diversity
Natural habitat shifts and migration interrupted
Cumulative Impacts of Shoreline Development & Hardening
Recent Shoreline Protection Approach
• Wetland productivity is directly linked in complex ways with riparian & shallow water habitats
• Cumulative development impacts are significant in spite of avoidance, minimization & mitigation requirements
• Development impacts are now combined with climate change impacts, especially sea level rise
• Renewed emphasis on vegetative stabilization, living resources, & dynamic rather than static shoreline ecosystem
Living Shorelines are Not New
Planted Tidal Marshes 1970’s - 1990’s
Ed & Joanna Garbisch - Maryland
C. Scott Hardaway, Jr. & others - Virginia
Stephen Broome, W.W. Woodhouse, E.D. Seneca & Spencer Rogers - North Carolina
Knutson & Others
Main Findings from Early Planted Tidal Marsh Research
It is possible to create tidal marshes
Offshore wave attenuation structures are sometimes necessary for planted marsh success
A “well-established” planted marsh can be an effective method of abating shoreline erosion
Source: Chesapeake Bay Foundation
Vegetation is dominant – Riparian Buffer + Wetland
Bank Grading & Sand create suitable slopes & elevations
Stone is minor element to support vegetation & sand
Living Shoreline Treatments
US States with Living Shoreline Programs due to concerns for shoreline hardening trend
• Northeast
– Connecticut
– Rhode Island
– New York
– New Jersey
– Delaware
• Mid-Atlantic & Southeast
– Maryland
– Virginia
– North Carolina
– Florida
• Gulf Coast – Alabama
– Mississippi
– Texas
• West & Northwest – California
– Oregon
– Washington
Shoreline Management Approaches
Summary
• Direct impacts get the most attention
• Secondary & cumulative impacts have as great or greater overall effect on coastal habitats
• Shoreline guidance has evolved
– based on the current scientific understanding of impacts on living resources & feasibility to use engineered habitats
• Living shoreline treatments are now the preferred approach in many coastal states & there are now many successful examples
