The Buoyant Plume Lift-off Zone as a The Buoyant Plume Lift-off Zone as a Test of Coastal Sediment Transport Test of Coastal Sediment Transport

ModelsModels

David A. Jay, Philip M. Orton and Douglas J. WilsonOGI 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 --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) --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 WS classes

• Objectives:– Analyze SPM balance by WS 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:The Fraser River Setting:• Compact glaciated basin, <10 of latitude• Summer, single-event hydrograph• Flows higher in cold PDO/La Niña years -- 1999 flow was the largest since 1974• 2000 was within the average range• Average total load is 18.5x106 mt with 6.5x106 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

1999-2000 Fraser 1999-2000 Fraser River Data Sets --River Data Sets --

• 300 and 1200 kHz ADCP, ABS (coarse material) & velocity• Towed salinity, OBS (for fines)• WS from Owen tube, size from LISST (2000 only), Coulter counter size• Bed material, pump/Niskin bottles• Bedforms from echo sounder• LMER Chl, zooplankton (1999)• 1999 -- extreme flows, few aggregates in river, short Tresidence

• 2000 -- 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, Fraser River Supplies a Broad Range of SPM Sizes, modified by Estuary/Plume:modified by Estuary/Plume:

• Sand and floc have similar settling velocities WS, 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 --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

SalinityX-velocity

A A

BB

C C

E

DD

E

Evolving Plume SPM Distributions --Evolving Plume SPM Distributions --

• Large (acoustically visible) particles settle out due to reduced vertical mixing. Small particles dominate in pycnocline, freshwater.

OBS FinesABS Flocs/Sand

Sand&Floc 2-D Dynamic Balance --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 --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

ConclusionsConclusions

• 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 WS 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