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Swash Zone

Swash zone is where the last of the wave energy is dissipated.

Time stack of video data, at one cross shore profile, reveals:

Run-up distance

Swash frequency

Bore velocity

Run-up (Swash) and Backwash

Run-up (or Swash) is driven by the wave momentum and moderated by the beach slope and infiltration capacity of the sediment.

Return Flow (or Backwash) is driven by gravity and the volume of water remaining, so the beach slope and the infiltration again play a role.

Wave swash represents “cutting edge” of the ocean’s geomorphic buzzsaw.

We want to know how the wave conditions and beach morphology affect the run-up.

Total Run-up consists of: (1)  Set-Up (2)  Swash of Incident Waves (3)  Infragravity Motions

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Video approach, timestack, (ARGUS)

Runup wire

Field Data Collection of Wave Run-up

Thornton and Guza (1981, 82)

Holman and Sallenger (1985),

and Holland and

Holman (1993)

Calculate the speed of the broken wave bore.

v = Δx/Δt = (60-15)m/(10-2.5)s

= 6 m/s

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Swash Dynamics on Gently-Sloping Beaches

Wide swash zone

Low uprush velocities

Duration of swash-backwash cycle may be several multiples of the incident wave period

Several bores may be present in the swash zone simultaneously

Low gradient beach means water table is close to the surface, so infiltration is low.

Run-up distance varies over a time scale of 10’s – 100’s of seconds.

Swash Dynamics on Steeply-Sloping Beaches

Narrow swash zone

High uprush and backwash velocities

One (or at most, two) waves are present in the swash zone simultaneously.

High gradient beach (steep) means grain size is coarse, so infiltration is high.

Run-up temporal variability is on the order of the incident wave period.

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A Brief History of Run-Up Estimates

Hunt's (1959) estimate: R2%=8HsS, from the examination of run-up on rock rubble structures in The Netherlands

Battjes's (1971) estimate: R2%=HsC Ip, related run-up to Iribarren number, Ip=S/\sqrt{Hs/Lo}, and an experimentally calibrated coefficient, C, which ranges in value from 1 to 4.

Guza and Thornton's (1982) estimate of avg. of highest one-third of swashes: Rs=0.7Ho.

Effect of Substrate on Run-Up

Battjes (1971) hypothsized that Run-Up was a function of Ir #

This relationship was also used to explore the effect of substrate on run-up, presented in Komar's Fig. 6-31. Run-up was found to be approximately twice as great on a smooth surface, as compared to a rock-covered surface.