Spectral Wave Decay due to Bottom Friction on the Inner Shelf

(With Prof T. Stanton, Department of Oceanography, Naval Postgraduate School)

Wave dissipation due to bottom friction can be the dominant mechanism transforming waves across the shelf during times of significant waves. During our participation in the Shoaling Waves DRI, we will make measurements of wave dissipation due to bottom friction in collaboration with other wave transformation studies in October 1999 at a site approximately 2km offshore from the Army Corps of Engineers field research facility at Duck, North Carolina. Prior to the main experiment, field measurements of the wave forced boundary layer made during Sandy Duck in September / October 1997 and other sites will be analyzed to study wave dissipation, and the effect of lower frequency currents, ripple formation and sediment suspension in modifying the bottom boundary layer. Dissipation and structure of the bottom boundary layer are being measured using a hierarchy of acoustic instrumentation including a unique Bistatic Coherent acoustic Doppler Velocimeter (BCDV) to noninvasively measure three component velocity profiles and stresses at 0.6 cm scales up to 70cm from the bed. At larger scales, acoustic travel-time BASS current sensors and a BADCP will extend the current measurements to the surface to estimate the effects of low frequency currents (for example from tides and internal waves) on the wave-forced bottom boundary layer. Local bed morphology is measured by a newly developed scanning two axis altimeter which can measure 3 dimensional bed elevations over 4 by 4m areas with 4mm vertical resolution, to study the effects of ripple formation on the boundary layer dissipation.

A prototype BCDV and the scanning altimeter were successfully deployed at the Sandyduck nearshore experiment on an instrumented sled which was moved cross shore from 3m depth to the beach face each day for 7 weeks. The offshore stations, beyond the bar, provided useful measurements of the bottom boundary layer which are representative of inner shelf conditions over a very wide range of wave forcing. In the photo below, the BCDV is shown on the right deployed on the sled during Sandy Duck.

[Image of BCDV deployed on instrumented sled during Sandy Duck]
The BCDV deployed on instrumented sled during the Sandy Duck experiment.