Karen A. Duhring Marine Advisory Scientist Center for Coastal Resources Management Virginia Institute of Marine Science College of William & Mary March 20, 2009
Low-Impact Shoreline Stabilization Cumulative impacts of shoreline armoring Water quality Habitats Non-structural methods Bank grading Planted tidal marshes Hybrid methods Marsh sills Beach nourishment Offshore breakwaters
Cumulative Impacts of Shoreline Hardening Why we need more low-impact stabilization
Typical Erosion Protection Projects on Virginia s Tidal Shorelines Revetments Bulkheads Bank Grading with Toe Structures
Cumulative Total Permitted Shoreline Hardening in Virginia 2000 2007 Permit site 36.3 miles bulkhead 113.2 miles revetment Average 16.5 miles new hard shoreline permitted each year Direct and indirect losses Tidal wetlands Beaches and dunes Shallow water habitats Removal and suppression of vegetated buffers Source: VIMS Tidal Shoreline permit database
Cumulative Impacts of Stabilization on Water Quality - Pros Reduced bank sediment input Increased stability for upland buffers More stable buffers have greater potential for surface and groundwater filtering Sediment retention Organic matter and denitrification
Cumulative Impacts of Stabilization on Water Quality - Cons Direct and indirect loss of tidal marshes and beaches Strong disconnect between wetland and upland habitats Land disturbance and replacing sediment washed away from structures Landscape management made easier with stabilization structures Suppression of riparian buffer vegetation Lawn maintenance New sources and paths for non-point source pollution
Plants remove sediment and pollutants from runoff More plant communities = more opportunities for water quality protection More habitat complexity = more biological activity for nutrient capture and processing Integrated root system holds soil in place and intercepts groundwater
Less vegetation = Less sediment and pollutants removed Less underground root structure = Less nitrogen removed, more soil mobility
Main principle for low-impact stabilization Integrated succession of vegetated buffers & filters that provide ecosystem services Riparian buffers Tidal marshes Sand beaches Shallow water reefs Underwater grasses Model Shoreline
Instead of bulkhead or revetment... Gradual loss of intertidal area Reflected wave action and sediment resuspension Create or enhance vegetation buffers Graded bank + planted marsh + sills Connected habitats Dense plant cover Active biological community
Stabilization Alternatives general order of preference Minor erosion with low risk Maintain / enhance vegetation Minor erosion with some risk Non-structural Major erosion with risk, natural buffers present Hybrid Major erosion with high risk, natural buffers absent or not feasible Structural
Non-Structural Methods
Bank Grading Before grading, waves striking the bank toe caused collapse of entire bank After grading, there is an elevation gradient for wave run-up and dissipation of wave energy
Bank Grading with Riparian Buffer Restoration
Planted Tidal Marshes Enhancing or creating suitable conditions for tidal marsh plants May require grading the bank and/or filling into the water Biodegradable coir logs can provide temporary protection Narrow marsh provides habitat value, wide marsh >15 ft provides wave reduction
Planted Tidal Marshes using coir logs Coir logs placed for natural accretion Planting into logs not always necessary, more successful in freshwater Sand fill contained by coir logs until marsh plants are established Also effective for restoring vegetation on slopes and graded banks
Hybrid Methods strategically placed structures to enhance or create vegetation buffers
Marsh Sill Low profile revetment backfilled with sand to create or enhance tidal marsh Imported sediment from upland source or suitable bank grading material Offensive approach structures built in Marsh sill (foreground) compared to natural tidal marsh (background) not wide enough to prevent bank erosion sand transport region to address impinging waves before they reach upland areas
Typical Sill Cross-Section Planted tidal marsh Height near mean high water elevation Width Varies 10-30+ feet Construction access for hand or machine placement of stone required Construction-related impacts must be managed Graphic: Jefferson Patterson Park project, Maryland
Oyster Reefs Planted tidal marsh Habitat for native oyster restoration Not always effective for wave reduction P. Ponce Monitoring & research underway by VIMS, CBF, TOGA, et al.
Estimated Cost Comparison Low Energy Non-Structural Beach nourishment Planted marsh Medium Energy Hybrid Marsh sill High Energy Structural Revetment Bulkhead $50 - $100 per foot $150 - $ 500 per foot $500 - $1,200 per foot Source: MD Department of Natural Resources, 2007
Conclusions Low-impact Stabilization Projects Target specific erosion problems at the source Enhance natural buffers where they exist or create them where feasible Provide complex habitats for plants and animals that provide sediment trapping, bioturbation, nutrient processing Carefully planned and managed during construction
Thanks for your interest spread the word! Contact Information: Karen Duhring karend@vims.edu 804 684-7159