Nearshore Impacts of Dredging for Beach Nourishment

A methodology is presented for predicting the impacts of nearshore dredging on shoreline change, using numerical models for waves, currents, and shoreline change. The dredge pit, outside of the surf zone, was assumed to be steady state, but alters waves reaching the surf zone. The methodology accounts for the transformation of directional wave energy spectra due to spatial gradients in both mean currents and bathymetry, wind inputs, and energy dissipation via several mechanisms. The wave transformation model SWAN (Simulating Waves Nearshore) was used to simulate waves at Folly Island, S.C., which served as the test case. Wave model results were validated using short-term field measurements at three locations. Longer-term (1 year) simulations were driven using hindcast wave and wind data. Velocity fields and tidal stages were simulated using the hydrodynamic model Environmental Fluid Dynamics Code in two-dimensional mode, after calibrating with field measurements. Wave model results were used to estimate longshore sediment transport and shoreline change due to longshore gradients of the longshore sediment transport rate. Empirical sediment transport coefficients were chosen to match model results to measured beach volume changes derived from beach profile data. The calibrated model was then used to predict impacts of dredging on long-term shoreline change. Results reproduced observed trends of erosion and accretion along approximately 90% of the shoreline.

[1]  Jerome P.-Y. Maa,et al.  Physical impact of waves on adjacent coasts resulting from dredging at Sandbridge Shoal, Virginia , 1998 .

[2]  Jingping Xu,et al.  Ship Shoal as a Prospective Borrow Site for Barrier Island Restoration, Coastal South-Central Louisiana, USA: Numerical Wave Modeling and Field Measurements of Hydrodynamics and Sediment Transport , 2004 .

[3]  Robert G. Dean,et al.  The Reconstruction of Folly Beach , 1995 .

[4]  E. Wei,et al.  Potential Impacts of Sand Mining Offshore of Maryland and Delaware: Part 1—Impacts on Physical Oceanographic Processes , 2004 .

[5]  N. Booij,et al.  A third-generation wave model for coastal regions-1 , 1999 .

[6]  N. Booij,et al.  A third‐generation wave model for coastal regions: 2. Verification , 1999 .

[7]  R. Brooks,et al.  Hindcast Wave Information for the U.S. Atlantic Coast: Update 1976-1993 with Hurricanes. , 1994 .

[8]  M. Kassas.  Coastal Processes with Engineering Applications, by R.G. Dean and R.A. Dalrymple, 2002 , 2004 .

[9]  David B. Snyder,et al.  Physical and Biological Effects of Sand Mining Offshore Alabama, U.S.A. , 2004 .

[10]  Norman W. Scheffner,et al.  ADCIRC: An Advanced Three-Dimensional Circulation Model for Shelves, Coasts, and Estuaries. Report 1. Theory and Methodology of ADCIRC-2DDI and ADCIRC-3DL. , 1992 .

[11]  John M. Hamrick,et al.  A Three-Dimensional Environmental Fluid Dynamics Computer Code : Theoretical and computational aspects , 1992 .

[12]  T. Barnett,et al.  Measurements of wind-wave growth and swell decay during the Joint North Sea Wave Project (JONSWAP) , 1973 .

[13]  R. Dalrymple,et al.  Coastal Processes with Engineering Applications , 2001 .

[14]  J. Smagorinsky,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS , 1963 .

[15]  John M. Hamrick,et al.  User's Manual for the Environmental Fluid Dynamics Computer Code , 1996 .

[16]  Fukuzo Tasai,et al.  Observations of the Directional Spectrum of Ocean WavesUsing a Cloverleaf Buoy , 1975 .