1

Surf Zone Wave Decay and Energy Dissipation

Steep, reflective beaches -

Wave breaking (and energy dissipation) is concentrated through plunging breakers.

Broken wave surges up the beach as runup.

Low-slope, dissipative beaches -

Extensive, wide surf zone over which spilling breakers dissipate energy.

At any time, several broken wave bores, and smaller unbroken waves, are visible.

Villano Beach

Anastasia Island Wav

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Reasons for understanding surf zone wave decay

Understanding the patterns of wave decay in the surf zone is important for two significant reasons:

1. Wave energy dissipation is inversely related to the alongshore pattern of wave energy delivery -- so it can help identify relative vulnerability of coastal property.

2. Wave energy expenditure is partially transformed into nearshore currents, which are responsible for sediment transport and beach morphologic modification.

2

Waves in the Surf Zone – Thornton & Guza (1982) - Torrey Pines Beach

Torrey Pines Beach – fine sand with minimal bars and troughs.

Wave staffs and current meters - measurements from 10 m water depth to inner surf zone.

Published the distributions of wave breaking within the surf zone on a natural beach

Measurements of Wave Breaking Distributions

1. Constant H and T - Lab channel - easy to determine breaking conditions.

2. Natural setting - range of H and T, so surf zone witnesses range of broken/unbroken waves/bores.

Unbroken = Rayleigh distributions

Broken = Modified distributions €

p(H) =2H

KsH∞( )2exp − H

KsH∞

⎛

⎝ ⎜

⎞

⎠ ⎟

2⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

€

p(H) =Hrms

γh⎛

⎝ ⎜

⎞

⎠ ⎟

n

1− exp − Hγh⎛

⎝ ⎜

⎞

⎠ ⎟

2⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

⎧ ⎨ ⎪

⎩ ⎪

⎫ ⎬ ⎪

⎭ ⎪

Histograms of breaking wave height distributions illustrate a greater fraction of broken waves at shallower depths.

Histograms of all waves in nearshore (broken and unbroken) show skewed distributions -- many small waves and few large waves -- comparable to that of a Rayleigh distribution, which also describe the distribution of deep water wave heights.

Histograms of broken waves only (cross hatched pattern) show a more uniform distribution.

3

The Surface Roller vs. The “Main Part” of the Breaking Wave

Wave Height in the Inner Surf Zone or The Height of Surf Bores

Broken wave height is controlled primarily by water depth

4

Energy Saturation of Broken Waves

Waves shoaling here

Broken Waves here

Surf wave heights after initial breaking

5

Waves over a barred beach profile

Bar (induces breaking)

Trough (breaking ceases)

Beach (breaking resumes)