The Buoyant Plume Lift-off Zone as a Test of Coastal Sediment Transport Models David A. Jay, Philip M. Orton and Douglas J. Wilson OGI School of Science and Engineering Oregon Health & Science University, Portland, OR Research Supported by the National Science Foundation Thanks to Rocky Geyer and Dan MacDonald, WHOI, Denise Reed, LSU and and Ray McQuin, Captain, R/V Barnes, UW

Concept -- “Challenging” data sets are needed to test models. Desirable properties -- –strong sediment transport in a simple geometry –tractable experimental site with definitive data –broad range of particle sizes –departure from local vertical equilibrium (e.g., strong advection) –aggregation and settling/erosion alter the size distribution –strong gradients in salinity, bedstress and vertical mixing –wave-current interactions (not in this data set) –variable sediment transport direction –supply limitation, variable bed properties –poorly understood, but geologically relevant! Fraser River mouth as a valuable experimental site

Concept (Continued) -- New types of data need new data analyses methods! Tools: –High spatial resolution allows a variety of calculations –Joint acoustic/optical methods to encompass broad size ranges –Need better data-driven treatment of vertical turbulent mixing –Use a multi-sensor inverse method to determine W S classes Objectives: –Analyze SPM balance by W S class (ideally), for now, by fines vs. flocs –Understand SPM transport in advection-dominated systems –Match data analyses with simple theoretical as well as numerical models

The Fraser River Setting: Compact glaciated basin, <10  of latitude Summer, single-event hydrograph Flows higher in cold PDO/La Niña years flow was the largest since was within the average range Average total load is 18.5x10 6 mt with 6.5x10 6 mt of sand Fraser delta is growing, unstable? Delta is sandy, fines in Straits of Georgia Flows monitored, 30+ yrs sediment transport data, multiple stations in river

Fraser River Data Sets and 1200 kHz ADCP, ABS (coarse material) & velocity Towed salinity, OBS (for fines) W S from Owen tube, size from LISST (2000 only), Coulter counter size Bed material, pump/Niskin bottles Bedforms from echo sounder LMER Chl, zooplankton (1999) extreme flows, few aggregates in river, short T residence more normal year with aggregation, retention of SPM on the neaps Anchor stations, lateral and longitudinal sections, with two vessels (1999 only) 1999 anchor calibration stations 1999 mouth drifts 2000 bed sizes Drifts D&E  25/7/1999 Drifts  Drifts A,B,C

Fraser River Supplies a Broad Range of SPM Sizes, modified by Estuary/Plume: Sand and floc have similar settling velocities W S, but distinguished by location, dis-aggregated sizes, distinct acoustic responses Low bedstress, salt wedge sample -- Coarse fraction is flocs LISST-100 Settling Tube River sand Plume Flocs ABS Response

Drift Tracks at ~35 min Intervals -- Along-drift velocity (left) and salinity (right) at end of ebb. Very strong shear and stratification, leading to SPM advection Salinity X-velocity A A B B CC E DD E

Evolving Plume SPM Distributions -- Large (acoustically visible) particles settle out due to reduced vertical mixing. Small particles dominate in pycnocline, freshwater. OBS Fines ABS Flocs/Sand

Sand&Floc 2-D Dynamic Balance -- Strong alongchannel advection at plume lift-off  C/  t, vertical diffusion, and settling smaller -- NOT a local balance in z W important in upper water column at lift-off Need better vertical mixing representation, which includes interfacial mixing near pycnocline Lateral advection? Not measured here, can be evaluated in lateral sections

2-D Dynamical Balance for Fines -- Strong advection in pycnoclne, settling not important What balances advection if not settling? Need to estimate loss to aggregation Need better vertical mixing representation! Lateral advection? No! V/U

Conclusions SPM balance strongly advective at plume lift-off Mixing, advection, settling, aggregation all important Broad size distribution requires multiple sensors, inverse analysis techniques to separate W S classes Need to evaluate turbulence field from data using mixing efficiency, salt and momentum balances (Kay and Jay, submitted to JGR) Particle field evolves rapidly in lift-off zone -- sampling scales must be on consistent with processes Particle settling, transformations in the lift-off zone affect delta and shelf processes Analytical, conceptual and modeling challenges abound