Coastal Hydrodynamics

Understanding the physics of waves, tides, currents, and sediment transport in nearshore and estuarine environments

Overview

Coastal hydrodynamics is the study of water motion in the nearshore zone, including interactions among waves, tides, wind-driven currents, and the morphological features of the coastline. This discipline is fundamental to predicting coastal erosion, designing coastal structures, managing sediment budgets, and understanding how pollutants and nutrients are transported in coastal waters. Modern approaches combine field observations (ADCPs, wave gauges, pressure sensors), remote sensing (satellite altimetry, SAR), and numerical models (ADCIRC, ROMS, Delft3D, FVCOM).

~70%
World's coasts are retreating
3.3 mm/yr
Global mean sea level rise
>600M
People in low-elevation coastal zones
~40%
Population within 100 km of coast

Key Concepts

Wave Mechanics

Surface gravity waves are generated by wind stress. Linear (Airy) wave theory provides fundamental relationships between wave height, period, wavelength, and water depth. In shallow water, waves shoal, refract, and eventually break, releasing energy that drives longshore currents and sediment transport.

η(x,t) = (H/2) cos(kx − ωt)
Dispersion: ω² = gk tanh(kd)

Tidal Dynamics

Tides are long-period waves driven by gravitational forces of the Moon and Sun. In coastal regions, tidal propagation is modified by bathymetry, friction, and Coriolis effects, creating amphidromic systems. Tidal constituents (M2, S2, K1, O1) are separated via harmonic analysis for prediction.

h(t) = H₀ + Σ Aₙ cos(ωₙt − φₙ)

Longshore & Cross-shore Transport

Breaking waves generate radiation stresses that drive longshore currents, transporting sediment along the coast. Cross-shore transport is driven by undertow, wave asymmetry, and infragravity waves. The CERC formula and Bailard equation are commonly used for estimating littoral drift.

Qℓ = K (ρg / 16κ²) H²b sin(2αb)

Storm Surge

Storm surge is the abnormal rise of water generated by strong winds and low atmospheric pressure during tropical cyclones. Surge height depends on storm intensity, forward speed, angle of approach, and coastal bathymetry. Models like ADCIRC and SLOSH simulate surge for forecasting and planning.

Estuarine Circulation

Estuaries exhibit complex circulation patterns driven by river discharge, tidal mixing, and density gradients. Salt-wedge, partially-mixed, and well-mixed classifications describe the degree of stratification. The competition between tidal energy and buoyancy input controls mixing.

Numerical Modeling

Models like ROMS, Delft3D, FVCOM, and ADCIRC solve the shallow water equations on structured or unstructured grids. Spectral wave models (SWAN, WAVEWATCH III) simulate wave generation, propagation, and dissipation. Coupled wave-current-sediment models capture complex feedback loops.

Interactive Visualizations

Wave Propagation: Shoaling & Breaking

Tidal Constituents & Predicted Tide

Coastal Bathymetry & Current Velocity Profile

Research Methods & Tools

Remote Sensing

Satellite altimetry (Sentinel-6, Jason-3) measures sea surface height; SAR imagery reveals internal waves and surface currents; optical sensors track turbidity and sediment plumes.

Modeling Frameworks

ADCIRC for storm surge, ROMS/FVCOM for 3D circulation, SWAN/WW3 for spectral wave modeling, XBeach for nearshore morphodynamics, and Delft3D for coupled processes.

In-Situ Instruments

ADCP for current profiling, wave buoys (NDBC), pressure sensors, CTDs, turbidity probes, and GPS-tracked drifters for Lagrangian observations.

Key References

  1. Dean, R.G. & Dalrymple, R.A. (1991). Water Wave Mechanics for Engineers and Scientists. World Scientific.
  2. Komar, P.D. (1998). Beach Processes and Sedimentation. Prentice Hall.
  3. Luettich, R.A. & Westerink, J.J. (2004). Formulation and numerical implementation of the 2D/3D ADCIRC finite element model. ADCIRC Theory Report.
  4. Shchepetkin, A.F. & McWilliams, J.C. (2005). The regional ocean modeling system (ROMS). Ocean Modelling, 9(4), 347–404.
  5. Wright, L.D. & Short, A.D. (1984). Morphodynamic variability of surf zones and beaches. Marine Geology, 56, 93–118.
  6. Booij, N., Ris, R.C. & Holthuijsen, L.H. (1999). A third-generation wave model for coastal regions. J. Geophys. Res., 104(C4), 7649–7666.