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A Parameter Space for Particle Trapping – Explorations in Two Estuaries

A Parameter Space for Particle Trapping – Explorations in Two Estuaries. David A. Jay, Philip M. Orton, Douglas J. Wilson, Annika M. V.Fain, Oregon Graduate Institute Daniel McDonald, and Wayne R. Geyer, Woods Hole Oceanographic Institute

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A Parameter Space for Particle Trapping – Explorations in Two Estuaries

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  1. A Parameter Space for Particle Trapping – Explorations in Two Estuaries David A. Jay, Philip M. Orton, Douglas J. Wilson,Annika M. V.Fain, Oregon Graduate InstituteDaniel McDonald, and Wayne R. Geyer, Woods Hole Oceanographic Institute Research Supported by the National Science Foundation and Office of Naval Research

  2. The Challenges -- • Define a parameter space for estuarine turbidity maxima (ETM) • Invent flexible observational and theoretical methods • Understand SPM advection, which is critical to formation of an ETM • Investigate potentially contradictory influence of riverflow on particle trapping

  3. ETM-Ecological Perspective CRETM-LMER project-http://depts.washington.edu/cretmweb/

  4. Approach -- • Use acoustical and optical methods to measure SPM properties by settling velocity (Ws) class • Use scaling analysis of SPM equations to bring out the role of advection • Understand how intratidal processes condition subtidal patterns • Define the tidal monthly and seasonal patterns • Use two estuaries (Fraser and Columbia) to increase dynamical range.

  5. To Determine SPM from Data -- • Single-frequency inverse method (Fain MS Thesis, 2000) for acoustic backscatter (ABS) data from moored ADP data • Multiple frequency method for ABS (from vessel ADCP) plus optical (OBS) data

  6. Single-Frequency Inverse Methods • Define profiles (basis functions) for known Ws classes (0.014, 0.3 , 2, 14 mms-1 in Columbia) • Use non-negative least squares to determine contribution of each basis function to each profile • Advantages: works well with aggregates -- does NOT assume a scattering law • Disadvantages: doesn’t account for size-variability of ABS or advection effects

  7. Stage 1 Inverse Analysis: • Calibrate and cor-rect ABS • Fit WS classes to ABS profiles via non-negative least squares

  8. Multi-Frequency Inverse Methods • ABS vs. SPM & OBS vs. SPM calibrations • Stage 1 consists of single-frequency analyses for ABS and OBS separately • Stage 2 provides an empirical scattering law to calibrate each Ws class for each sensor • Advantages: works well with aggregates AND with a broad size range of particles • Disadvantages: requires more input data, advection effects still problematic

  9. Flow Chart --Two-Stage, Multi-Frequency Inverse Analysis

  10. Calibrating the Two-Stage Inverse -- • C1 to C4, Ws = 0.01, 3, 15, 45 mms-1 for Fraser • Two-stage inverse recon-ciles OBS and ABS views of ETM • OBS responds to all Ws classes, ABS C2 to C4 only • Note that theory and analysis are forced to agree on C2 in table

  11. Scaling Analysis -- • Equations: • Local SPM conservation equation in 2-D (x and z), with boundary conditions • Integral SPM conservation over the ETM volume, averaged tidally (Jay and Musiak 1994) • Determine the governing parameters • Test relevance against data

  12. Local SPM Conservation -- • Non-Dimensional Parameters: • Rouse Number P = Ws /(kU) ~ 1-4 (ETM particles) • Time-change m <0.1(neglect) • Advection number A = PHm/H ~ 0.1- 500. Hm is the height of the SPM max off bed; cf. Hm/U of Lynch et al. (1991) • Aggregation number  (neglect for now)

  13. Integral SPM Conservation -- • ETM extends from X1 to X2, overbar = tidal average, subscript V refers to vertical deviations, subscript R refers to river • Tracks subtidal evolution of the SPM inventory on LHS, and supply, fluxes in and out, aggregation and erosion on RHS • Non-dimensional numbers -- • Trapping efficiency E = CE/CR >1 ratio of estuarine to fluvial SPM • Supply number SR = const P UR/( H) is thefluvial SPM input • Shear flux number FV = const E TP where: • Trapping potential TP = U/(kU) is in FV

  14. Summary of ETM Parameters: • Rouse Number P = Ws /(kU) • Advection number A = PHm/H • Trapping efficiency E = CE/CR • Supply number SR = P UR/( H) • Trapping potential TP =U/(kU) • Not Considered here: lateral exchanges with peripheral areas, aggregation, erosion/deposition • Salinity intrusion problem has only two non-dimensional numbers!

  15. Overview of Columbia and Fraser River Systems and Analyses --

  16. Columbia and Fraser Data -- • Columbia: 7-8 mo data from four ADPs, largest spring freshet in 25 years (1997). Three 15d cruise for calibration data. Much aggregation. • Fraser: 20 d of vessel data in 1999, during extreme high flow. Currents to 4.5 ms-1(!), little aggregation. • Calibration data for both: • gravimetric (bulk) SPM calibration • known Ws spectra (Owen tube) • Coulter counter size spectraS

  17. The Columbia River Basin • Columbia basin spans >15° of latitude • Timing of snow melt in the Canadian and Snake parts of the basin strongly influences duration of freshet

  18. Columbia River Flow and SPM Supply • 1997 La Niña year -- highest total flow of century. • Largest daily flow: 20,000 m3s-1 in January -- a western basin rain-on-snow event (<2.7x mean) • Spring freshet (interior basin snowmelt) peaked in May at 16,000 m3s-1 (2.1x mean) • Natural freshet was ~25,000 m3s-1 (3x mean) • Pre-release of water began in January to cut freshet

  19. The Fraser River Basin A compact basin, spans <s10 of latitude

  20. 1999 Fraser River Flow • Peak Fraser flows were 4x times the mean • Freshet lasted ~50 d because of late, cold spring • Such flows have not occurred in the Columbia since 1948

  21. Intratidal Processes --

  22. Columbia River Stations - • Tansy, Am169 and Am012 are in the ETM • Red26 is on seaward edge of ETM

  23. Velocity and Total SPM at Tansy • Strong outward flow during freshet • High SPM during freshet • strong neap-spring SPM signal • Biofouling days 230- 290

  24. Advection vs. Vertical Motion in Columbia • Single-frequency inverse analysis; Ws classes C1 to C4 : 0.014 (washload), 0.3 , 2, 14 (aggregate+sand) mms-1 • Near-bed: advection +deposition/erosion of large particles • Surface: mostly advection of fines with some advection Surface C2 concentration Near-bedC4 concentration

  25. Intratidal Processes Spring Tide In Columbia: • A high on flood; ~0.3 on springs • sand not impor-tant 2m off bed • Peak SPM on ebb leads to SPM export • single stage inversion

  26. Neap Tide Intratidal Processes In Columbia: • A higher than on springs; ~0.4-0.6 • Maximum SPM on flood, not ebb • sand not impor-tant 2m off bed • single stage inversion

  27. Fraser River 1999 Stations -- • All data here are from bD11, at entrance • bL11 = upstream limits of salinity intrusion

  28. Fraser Intratidal Processes, A and  - • A >5 on flood, must include advection (under development) • U is very small P3 large, except on greater ebb • TP negative and sometimes very large (no trapping)

  29. Fraser River Freshet Season Salt Wedge-- • High stresses on ebb, U > 0.1 ms-1, rapid response to changes • Large particles on ebb, mostly sand, Ws = 0.01,3, 15, 45 mms-1 • Little stress on flood, SPM maximal at surface • No ETM particle trapping -- all SPM removed on each ebb

  30. Subtidal Processes --

  31. Freshet and Post-Freshet Transports • Freshet: outward transport at all stations-- SPM residence times short (<14 d) • Post-freshet: recirculation from South to North Channels -- SPM residence times as long as 60-100 d.

  32. SPM Residence Time Index RT • Low RT during the spring freshet, only ~14 d • After freshet, RT increases with time since the freshet • Since there is no seasonal storage on the channel bed, SPM is being supplied from peripheral areas • North Channel RT is much longer -- lack of export.

  33. Rouse Number P -- • Station Tansy was in mid-ETM during the freshet • Despite large variations in tides and QR, minimum tidal P for C4 is constrained within a narrow range • Pf > Pe during freshet yielded little particle trapping; • Spring values of Pf and Pe are closer toward the end of the record Maximum flood and ebb Rouse Numbers (P) in the Columbia over 8 mo. in 1997

  34. Subtidal Processes: E and A vs P: E vs. P • E is lagged by 7 days -- SPM in water column on springs was trapped on the bed on neaps. • Lagged E in Columbia is low on springs (P low) and high on neaps (P high), • Hm/H (therefore A) in-creases with P (on neaps) A=PHm/H vs. P 0.18P

  35. Subtidal Processes: E vs A and TP: • E is maximal at inter-mediate AF/AEbecause max A occurs on weak tides, when SPM is on bed • E is maximal at high TPF/TPEbecause max TP occurs on strong floods during periods of moderate stratification

  36. Subtidal: E vs. Supply Number Paradox: increased QR shortens estuary, but intensifies two-layer flow -- what happens? CR FR • As QR, E 0; all SPM is removed on each tide. As QR0, E 0, there is no shear to trap SPM • Maximum E at moderate flows • BUT: peripheral bay storage/supply partly determines E!

  37. Summary of Particle Trapping -- Hypothetical view of particle trapping with E as a function: P = Ws/(kU) TP=U/(kU) • Columbia is near optimal particle trapping, with moderate shear and bedstress • In Fraser, P is too small on ebb (washload limit) and too big on flood (bedload limit) with respect to flocs

  38. Conclusions -- • Inverse methods: promising as tools to analyze estuarine SPM dynamics, but advection must be included • Scaling analysis provides understanding of ETM dynamics; parameters need to be tested further • Advection (A) is a very strong factor in river estuary ETM formations • Moderate values of A, P and SR lead to max E • Max TPis associated with maximal E

  39. Subtidal Ws-Class Distributions • C1 and C2 dominant near surface, C4 minimal • C3 and C4 dominant at bed on springs, but more variable; low on neaps Surface time series Near-bed time series

  40. Time Series of A (left) and E (right) -- • A is consistently high on neap tides • spatial variations in A and E are consistent with seasonal migration of ETM • E is high in the North channel, sbecause QR goes to South channel

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