IMPORTANCE OF BORE-GENERATED TURBULENCE

19 June 2001

Source: http://postoffice.nrlssc.navy.mil/abstract%2060.htm

IMPORTANCE OF BORE-GENERATED TURBULENCE
TO SWASH ZONE SEDIMENT TRANSPORT

J. Puleo
Naval Research Laboratory
Stennis Space Center, MS 39529-5004
J. Allen R. A. Holman R. Beach
Oregon State University
Corvallis, OR 97331

Abstract

The swash zone is a region where intense fluid/sediment interactions cause sediment suspension and transport that continually reshape local morphology. Swash zone sediment transport is frequently addressed using an energetics argument that was first developed in the 1960's. These simple Bagnold-type formulations were developed for steady, uni-directional river flow where the energy to support the suspended load against the fall velocity was a direct result from bottom shear, and so scaled with the flow velocity. A major source of energy dissipation in the uprush, however, is bore-derived. Hence it might be expected that the uprush suspended load will not scale with the flow velocity. Data from a swash zone sediment transport study conducted on a steep beach has shown the leading edge of the uprush and bore front to be dominated by very high sediment suspension. This present study examines sediment transport in relation to energy dissipation across a bore. A significant drop in energy flux across the bore (most likely manifested through turbulence) was documented and showed a strong correlation to the observed immersed weight sediment transport rate across the bore. Standard Bagnold-type sediment transport formulations did not show strong correlations with either instantaneous immersed weight transport rates or total integrated transport rates as observed in the uprush or backwash. These findings support the concept that standard Bagnold-type formulations are not, in themselves, adequate for describing swash zone sediment transport. Rather, sediment suspension and transport in this region may be significantly influenced by bore-generated turbulence.