We will take advantage of the 2m spring tide range
by installing the fixed array at spring low tide. This results in
the surf zone sweeping past the fixed measurement location over a tidal
cycle, so that the entire surf zone and swash is measured over tidal cycles.
Steep Beach Objectives: Our objectives
are to measure wave transformation and set-up/down, breaking wave and current-induced
turbulent bottom and surface boundary layers, and sediment flux in the
surf zone. It is hypothesized that the plunging and collapsing/surging
wave breaking processes occurring on a steep beach are significantly different
than plunging/spilling breaking wave processes previously measured at Duck
over a moderately sloping bar. Plunging breakers inject bubbles and turbulence
deeper into the water column, sometimes penetrating to the bottom and suspending
the sediments. Wave reflection can be significant on steeper beaches, and
it is hypothesized that the breaking wave location is modified by reflected
waves owing to modulation of the water depth and the kinematics in the
breaking wave. The modulation of the breaker location can then modify
the dynamics of the nearshore.
Measurement objectives include utilizing fixed arrays
of new high resolution instruments to measure the surface and bottom boundary
layers, suspended sediments in 3-dimensions and bottom morphology under
breaking waves. An array of pressure sensors, PUV and ADV systems
will measure cross-shore gradients of waves, velocity, set-up and suspended
sediments, and an alongshore array will measure shear instabilities of
longshore currents and infragravity waves.
Measurements include a cross-shore array of 8 pressure
sensors, 2 PUVs, 3 Sontek ADV Hydra systems (3 components of velocity,
suspended sediments, waves and bottom elevation) and the BCDV2, and an
alongshore lagged array of 5 PUVs (Figures 2 and 4). A fixed central array
will be deployed (Figure 3), including vertical arrays of 8 em's and 12
conductivity cells, and a x-y scanning altimeter, a co-located surface
piercing wave staff and pressure sensor, along with the new high resolution
velocity, sediment and morphology instruments
Additional measurements may include scanned LIDAR
from a 10 m tower located in the surf zone, and an upward looking BCDV2
at central array.
RIPEX Objectives: The objective is to comprehensively
measure in the field a rip current system. The approach is to expand and
leverage on the funded ONR-funded steep beach dynamics experiment (Figure
2). The observations will include an expanded array of current meters and
pressure sensors to measure currents, cross and long-shore pressure gradients
to resolve wave-induced dynamic forcing, infragravity and shear instabilities
of the longshore current, vertical velocity profiles of the rip current,
video imaging of the wave and current fields, and rapid surveys of bathymetry
and current profiles. The proposal for RIPEX is available here
.
Experiment participants:
Naval Postgraduate School to provide fixed array:
Thornton, Stanton, Gallagher, Reniers, Wyland, Stockel, Orzech, Morichon,
Cowen, Anderson, students
Ohio State University to measure surface currents
using PIV from video images from a blimp: Lippmann
University of Florida to
measure vertical velocity profiles of the rip current from a moving water
craft: MacMahan and Dean
Experiment Dates:
Installation of video cameras on Beach Hotel and Pumping
Station: Early 2001
Initial beach survey: Tues 6 March
01, Low tide, 1421PST, -0.34m
Model simulations to define final array configuration:
March
Installation of fixed array form beach: Tues
5 April 01, Low tide, 1421 PST, -0.34 m
Installation of offshore with divers: Weds
30 March 01
Commence data taking: Thurs 7 April 01
Remove fixed array from beach: Tues 3 May
01, Low tide, 1403 PST, -0.20m
Remove offshore equipment with divers: Thurs
5 May 01
Climatology for April:
Beach: crescentic bars intercepted by rip channels,
beach slope 1:10.
Waves in deep water outside Monterey Bay:
Hs = 2.2 m (range 1- 6 m)
Figure 2. Plan view schematic of proposed array to
measure rip currents at Sand City, CA. The bathymetry is based on historical
profile data. The Steep Beach Experiment array already funded by
ONR is delineated.
Figure 4. Cross-shore array of instruments on steep beach (1:10) at Sand City, CA scaled with a typical 14 second breaking wave. The various tidal datums are shown.