EQUILIBRIUM BEACH PROFILE Conceptually the result of balancing constructive and destructive forces. Really a misnomer because equilibrium never reached. WHY? Sediment dynamics happen much slower than ever- changing hydrodynamics

EQUILIBRIUM BEACH PROFILE - APPROACHES Kinematic: determine motion of each grain: impractical Empirical: Purely descriptive and data driven Dynamic: balance constructive and destructive forces What about details of processes? Don’t necessarily know

DESTRUCTIVE FORCES Turbulence

DESTRUCTIVE FORCES (Komar, 1998) Undertow

DESTRUCTIVE FORCES Gravity mg b mgsin(b)

CONSTRUCTIVE FORCES Net onshore stresses result from non-linear profile Non-linear wave profile

CONSTRUCTIVE FORCES Intermittent suspension u t Wave Breaking Velocity variation under broken waves t Sed concentration Very rough sketch Largest onshore velocities coincide with highest suspension

CONSTRUCTIVE FORCES Boundary layer streaming δ1δ1δ2δ2δ3δ3 Flow is non-uniform in flow direction Boundary layer thickness varies in flow direction Induces small vertical velocity component Time average of uw not zero since u and w not perfectly 90 degree out of phase Finite but small additional shear stress induced

EQUILIBRIUM BEACH PROFILE THEORY Turbulence is major destructive force F is wave energy flux h is water depth y’ is cross-shore coordinate (onshore-directed) D * is energy dissipation per unit volume (dependent on grain size) Solve for h(y) h varies as cross-shore coordinate to 2/3 power A is the profile scale factor (function of grain size)

EBPs Larger particles have steeper slopes: Can withstand energy better d = 0.1 mm d = 0.5 mm d = 1.0 mm

EBPs, BEACH SLOPE d = 0.1 mm d = 0.5 mm d = 1.0 mm UH OH. Slope goes to infinity as shoreline approached