Speaker: Jeffrey Obelcz Project Collaborators: Kehui Xu 1,2, Clay McCoy 1,2, Jeffrey Obelcz 1, Thomas Bierce 1, Jeffrey Rabil 1 1 Department of Marine Science, Coastal Carolina University, Conway, SC USA 2 Burroughs and Chapin Center for Marine and Wetland Studies, Coastal Carolina University, Conway, SC USA
Outline Background Statement of Purpose Field/Lab Methods Results Interpretation/Discussion Summary/Further Studies
Background Grand Strand is sedimentstarved region-rivers do not contribute significant amounts Tropical storm and hurricane events exacerbate erosion caused by limited sediment supply Myrtle Beach is huge tourist destination-$14 billion/yr Without intervention, erosion causes instability in beachfront structures *Image courtesy of visitsouth.com
Beach Nourishment Beach renourishment is used to retard erosion 50 year ongoing project by the Army Corps of Engineers 3 renourishments so far: 86-87, 96-98, 07-09 Previous nourishments used inland sediment, 07-09 used offshore borrow sites
Borrow Sites 4 in total: Little River, Arcadian Shores, Cain South, Surfside Total volume of sediment borrowed: 2.3 million m 3 Sediment distributed over 37 km of shoreline from Little River to Surfside Figure from McCoy et. al 2010 Surfside Borrow Site Arcadian Borrow Site Little River Borrow Site Cane South Borrow Site
Project Goals Difference between nourished and unnourished sediments not well known, hinders ability to track spread of nourished sediment Overall objective of project: to quantitatively discern differences between nourished and unnourished sediment Hypotheses: Nourished sediment will have poorer sorting and larger overall grain size than unnourished sediment Nourished sediment will be more organic-rich than unnourished sediment Nourished sediment will have higher carbonate content than unnourished sediment
Field/Lab Methods Waties Island 432 sediment samples collected along 72 shoreperpendicular transacts 4 regions of study area: North Myrtle Beach, Arcadian Shores, Myrtle Beach, Surfside Myrtle Beach Arcadian Shores North Myrtle Beach Surfside
Field Methods Cont d 6 samples taken at each transact: dune top, dune base, 20 cm below dune base, berm, 20 cm below berm, and swash 216 samples analyzed using sieving (every other transact) Dune Top (A) Dune Base (B1) 20 cm below Dune Top (B2) Berm (C1) 20 cm below Berm (C2) Sea Level Swash (D)
Results 0.35 0.3 Average of all Surface Samples -Grain size distribution (top right) shows avg. grain size of 1.5<φ<2, coarse skewness -Grain size distribution of sites show dune top sediment is finer than berm, dune base -Large presence of φ<-1 sed in dune base samples indicate presence of shell fragments (below) Grain Size Frenquency 0.25 0.2 0.15 0.1 0.05 0 phi < - 1 phi -1 to -.5 phi -.5 to 0 phi 0 to.5 phi.5 to 1 phi 1 to 1.5 phi 1.5 to 2 phi 2 to 2.5 phi 2.5 to 3 phi 3 to 3.5 phi 3.5 to 4 Site-Specific Averages phi > 4
Cross-Shore Variations -- Dune top is the finest -- Dune base and swash are coarse -- Standard deviations are close Mean Grain Size (Phi) 2.5 2 1.5 1 0.5 0 Seaward Dune Top Dune Base Berm Swash
Longshore Variation -Mean grain size (top left) greatly varies at dune base and berm; dune top: fine overall, linear decrease from northeast-southwest -Standard deviation (bottom left) of dune top lower than dune base, berm across study areaeolian transport -> finer grains, better sorting Mean Grain Size (phi) Standard Deviation (sorting, phi) 3 2.8 2.6 2.4 2.2 2 1.8 1.6 1.4 1.2 1 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Dune Top(A) Dune Base (B1) Berm (C1) Longitude (W E)
Spatial Variations -- Complex spatial variations of surficial grain size Phi Dune Top Dune Base Berm Swash Surfside Myrtle Beach Arcadian Shore N. Myrtle Beach 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0
Surface and Subsurface Variations -Dune base and berm: subsurface samples coarser than surface -Surface samples: mix of wind-blown seds and offshore sed -> smaller MGS -20 cm samples more offshore seds-larger grain size, poorer sorting -No apparent significant difference between MGS of DB and berm surface, subsurface samples Mean Grain Size (phi) Mean Grain Size (phi) 3 2.5 2 1.5 1 0.5 0 3 2.5 2 1.5 1 0.5 0 Berm (C1) Dune Base (B1) 20cm below Berm (C2) Longitude (W E) 20cm below DB (B2)
Comparison With Waties Island Waties Island: unnourished, north of study site Presumed to be natural sediment Waties Island samples finer, better sorted at both dune top (left) and swash (right) Longitude (E W) Longitude (E W)
Comparison with Borrow Site Sediments -Borrow sites coarser than Grand Strand sediment -Dune base samples have higher shell fragment % than borrow sites *Borrow site grain size distributions courtesy of Derk Bergquist, SCDNR
Interpreting Results Dune base, berm, and swash coarser than dune topsuggests dune tops not affected by nourishment efforts More shells in dune base than swash/berm-longshore transport moving nourishing sediment SW Linear decrease of grain size from SW-NE at dune top Spike in grain size across all transacts-nourished sediment not moving? Mean Grain Size (phi) 3 2.8 2.6 2.4 2.2 2 1.8 1.6 1.4 1.2 1 Dune Top(A) Dune Base (B1) Berm (C1) Longitude (W E
Summary Grand Strand sed is fine to medium grains, highly variable over region Dune tops universally finer, better sorted than other sample sites Finer, better sorted seds at surface-eolian seds mixed with offshore seds GS sed is middle ground between Waites and borrow site seds-suggests integration of borrow sediment
Further Studies Analysis of sediments for organic, CaCO 3 content: better indicator of dispersal of offshore seds? Laser grain size analyses (support by NSF) BERM (Beach Erosion Research and Monitoring) Numeric modeling of sediment transport (ROMS)
Thanks for your time! Acknowledgements: Professional Enhancement Grants through CCU for funding BERM for project background Derk Bergquist and SCDNR for borrow site data The Burroughs and Chapin Marine and Wetland Center s wonderful faculty and staff!