HR: 0800h
AN: H51I-0896    [Abstracts]
TI: Tidal Bedform Morphology and Flow Field Characteristics in the Fraser River, British Columbia
AU: * Allison, M A
EM: mallison@mail.utexas.edu
AF: University of Texas, Institute for Geophysics, Austin, TX 78758-4445, United States
AU: Nittrouer, J A
EM: nittrouer@mail.utexas.edu
AF: University of Texas, Dept. of Geological Sciences, Austin, TX 78713-8902, United States
AU: Mohrig, D
EM: mohrig@mail.utexas.edu
AF: University of Texas, Dept. of Geological Sciences, Austin, TX 78713-8902, United States
AU: Venditti, J G
EM: jeremy_venditti@sfu.ca
AF: Simon Fraser University, Dept. of Geography, Burnaby, BC V5A 1S6, Canada
AU: Church, M
EM: mchurch@geog.ubc.ca
AF: University of British Columbia, Dept. of Geography, Vancouver, BC V6T 1Z2, Canada
AB: Channel floor morphology was mapped on a reach of the lowermost Fraser River (river km 2-8; 600 m wide single channel) near Steveston, BC using longitudinal profiles of swath bathymetry (Reson 8101 Seabat) and current velocity (RDI 1200 kHz ADCP) and bottom sampling in order to examine the flow field structure over tidal bedforms. Surveys of the channel reach were repeated after 24 h in order to differentiate tidal and riverine discharge controls on bedform evolution. The survey was conducted during a high river discharge phase (22-24 June 2007; 8,800-9,200 cms) ten days after the highest discharge (12,000 cms) observed since 1948. Mean annual flood is 9,600 cms. Preliminary examination of these data indicate reach morphology is a sand bed channel (mean grain size 0.23 mm) with two lateral bars on opposite banks that overlap by about 30/% of length. Downstream thalweg flow velocities were 1.3 m/sec at the surface with no evidence of salt wedge penetration in the reach. Transverse dunes are present on bar surfaces and extending across the entire channel in the overlap zone: irregular (relict?) morphology is present in thalweg pools (19-30 m deep) opposite the bars. Transverse dunes are asymmetrical (downstream slip face) and reach maximum size (1.5-2.5 m height, 40-100 m wavelength) on bar flanks (13-15 m water depth) and in the bar overlap (thalweg) zone. Repeat survey data, collected over a 3 h period centered around slack low semi-diurnal tide both days, showed dune crests had translated 1/5 to 1/10th wavelength downstream, with limited change in individual dune morphologies, suggesting river discharge was dominant in setting sand field morphology at this high discharge and neap (2 m range) tidal phase. This does not conflict with previous investigations in the lower Fraser suggesting that dune morphology lags behind changes in river discharge and tidal phase. These data will be used to calculate bedload transport rates for comparison with previous Fraser rates obtained upstream of this reach and with different methodologies.
DE: 1895 Instruments and techniques: monitoring
DE: 3020 Littoral processes
DE: 3094 Instruments and techniques
DE: 4558 Sediment transport (1862)
SC: Hydrology [H]
MN: 2007 Fall Meeting