The dynamics of estuarine turbidity maxima

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The dynamics of estuarine turbidity maxima

• Stefan Talke Huib de Swart Victor de Jonge
• Groningen Workshop
• March 3, 2006

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Overview

• Experiments
• Analysis
• Modeling

• Ultimate Goal Understand the effect of biology
  and physical
• processes on each other and morphology.
• Preliminary Goal Describe and analyze physical
  and
• biological processes separately

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Overview of Experiments

• Measurements at Longitudinal and Cross-Sectional
  Transects
• Boats from RWS, WSA Emden, and NP GmBH used
  (Thanks!)
• Both fixed station and continuous measurements

Germany
Netherlands
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• Measurements
• ADCP (Acoustic Doppler Current Profiler)
• Velocity measured continuously in water column
  (0.5 Hz)
• Backscatter used to estimate sediment
  concentration
• Bottom tracking used to estimate boat velocity
• ?But, in turbid water, signal disappears!

600 kHz ADCP measures velocity and backscatter
(turbidity) in 0.25 m increments
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• Measurements
• Solution
• Use external echo-sounders and differential GPS

Boat Velocity from GPS
210 kHz echosounder penetrates to the fluid mud
layer
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• Measurements
• Solution
• Use external echo-sounders and differential GPS!

Boat Velocity from GPS
15 kHz echosounder penetrates to the bed
Water
Fluid Mud
Consolidated Bed
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• Measurements
• On-board Flow-thru system

Pump water into a bucket continuously Measure
Turbidity, Fluorescence, Salinity, Temperature,
Oxygen Take care to prevent light, bubbles!
Water
Fluid Mud
Consolidated Bed
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• Measurements
• Fixed Point Measurements

CTD Casts with OBS Oxygen sensor ? measure
Salinity, Temperature, Turbidity, Depth,
Oxygen Water Samples (surface and water
column) ? Analyzed for SSC, Organic Carbon,
Nitrates, Silicates, Phosphorous, pH, Algal
counts and types
CTD Casts
Water Samples
Consolidated Bed
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• Measurements
• Long Term Fixed Point Measurements X
• Monitored by NLWKN and WSA Emden
• Measure Tidal Stage, Salinity, Turbidity,
  Oxygen, pH, Velocity, Temperature, Sediment
  Concentration

Germany
CTD Casts
Netherlands
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Cross-Sectional Data at Pogum
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Echosounder Data
Transect at Pogum
827 am
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Echosounder Data
Transect at Pogum
835 am
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Echosounder Data
Transect at Pogum
921 am
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Echosounder Data
Transect at Pogum
936 am
15
Echosounder Data
Transect at Pogum
1003 am
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Echosounder Data
Transect at Pogum
1025 am
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Echosounder Data
Transect at Pogum
1036 am
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Echosounder Data
Transect at Pogum
1047 am
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Echosounder Data
Transect at Pogum
1122 am
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Echosounder Data
Transect at Pogum
1132 am
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Echosounder Data
Transect at Pogum
1511 pm
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Echosounder Data
Transect at Pogum
1514 pm
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Echosounder Data
Transect at Pogum
1556 pm
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Echosounder Data
Transect at Pogum
1600 pm
Question What does sediment concentration from
ADCP backscatter look like?
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Research Questions

• What are the sediment concentrations
• Analysis of ADCP data
• What does mixing and turbulence look like over a
  tidal period
• Research project of Robbert Schippers (Msc)
• Analyze field data to estimate turbulent mixing
• Apply GOTM 1-D vertical turbulence model

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Sediment Calibration from Backscatter

• Need to estimate attenuation of sound due to
  water and sediment

Water attenuation (range 0.05-0.2)
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Sediment Calibration from Backscatter

• Highly dependant on grain size, density,
  frequency

Sediment attenuation coefficient, 600 kHz
Maximum at 2 microns
With 2 ADCPs of differing frequency, can
estimate mean grain size
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Calculate Absolute backscatter

• Loss is due to spreading of beam and attenuation
• R distance along beam to adcp bin (20 degrees
  offset from vertical)
• E measured backscatter
• Alpha combined, integrated attenuation
• C,L,Kc,Er are instrument constants

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Fit regression line sed-conc to abs. backscatter

• Since attenuation depends on sediment
  concentration in profile
• make initial estimate of sed conc to calculate
  backscatter
• Use backscatter to make linear regression
• Re-estimate sediment concentration, recalculate
  attenuation, and repeat.
• What about changes in floc size?
• What about non-linear range?

Non Linear Range
Linear Range

• Future Steps
• 1. Calibrate non-linear range with Feb. 2006
  data
• Estimate mean grain size using backscatter from 2
• ADCPs on board one ship (Friesland)

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Cross Section Sediment Concentration Profiles at
Pogum (March 8,2005)
Pogum
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Water level (m)
Time (hours)
High turbidity evident Note structures in
sediment profile
mg/L
830 am flood
Non-linear Range gt 5 g/L
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Water level (m)
Time (hours)
Turbulence collapses, Sediment settles Sharp
gradient between water and fluid mud Fluid mud
pools in channel and shoal
mg/L
1130 am slack
Non-linear Range gt 5 g/L
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Water level (m)
Time (hours)
Very high turbidity and fluid mud
mg/L
Closer to ETM than earlier measurements
830 am
Non-linear Range gt 5 g/L
1600 Ebb
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Vertical Observations Interesting Salinity
Profiles
Flood (morning)
Salinity often (but not always) decreases towards
bed. Need to investigate density profiles
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Vertical Observations Interesting Salinity
Profiles
Ebb (afternoon)
As fluid mud is approached, salinity goes down.
Measurment artifact? Or real physics? Why the
low salinity? Perhaps not mixed with rest of
water column? Are low salinities evidence of
turbidity currents? How does mixing change
between flood and ebb?
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Vertical Observationsdensity profiles
Including sediment concentration essential for
water column stability Note again sharp
transition to fluid mud
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Vertical Observationsdensity gradients
Positive means unstable Salinity profile
dominates upper water column Sediment profile
dominates lower water column
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Vertical ObservationsRichardson number
900 am (end of flood tide)

• Richardson measures
• ratio of shear (turbulence)
• to density gradient
• (buoyancy)
• gt0.25 density dominates
• lt 0.25 shear dominates
• lt 0 Unstable

? Turbulence highly damped
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Summary vertical and cross-section measurements

• High sediment concentrations observed
• How and at what tidal phase is sediment being
  mixed into upper water column?
• Periodic formation of fluid mud layer
• Collapse of turbulence, formation of flocs

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Longitudinal Data
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Longitudinal ResultsTurbidity and Salinity
Downstream
Upstream
From NP Aanderaa Probe In flow-through system
Distance downstream from Herbrum (km)
Large horizontal salinity and turbidity gradient
(Turbidity not yet calibrated) Question Are
there density driven currents from both salinity
and turbidity?
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Longitudinal ResultsOxygen and Fluorescence
Not yet calibratedFluorometer How are Dissolved
oxygen and fluorescence related to the physical
parameters of system?
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Longitudinal Results--Salinity
Knock
Pogum
Terborg
Fixed NLWKN salinity measurements (note different
scales)
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Longitudinal Results--Sediment
Knock
Pogum
Terborg
Fixed NLWKN sediment concentration
measurements Note concentrations of up to 10 g/L
in summer, gt 25 g/L measured
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Longitudinal ResultsDensity Gradients
Knock-Pogum
Pogum-Terborg
Residual circulation proportional to density
gradient Note that sediment density gradient
changes sign
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Thought Experiment

• Consider Bath Tub

Fresh Water (Salinity 0)
Circulation cell
Heavy, turbid water flows along bottom Fresh
water circulates to conserve mass
Next, add turbid water to center ?What happens?
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Thought Experiment

• Consider another situation

Salt Water (Ocean)
Fresh Water (River)
Now, consider case in which density
differences occur only from salinity ? What
Happens?
Circulation cell from density difference due to
salinity (gravitational circulation)
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Thought Experiment

• Now, consider both together

Turbidity induced circulation
Salinity induced circulation
Hypothetical Combined Salt turbidity circulatati
on

Salt Water
Fresh Water
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Thought Experiment

• Analysis

• Possible explanation for observed, asymmetric
  turbidity profiles?
• To be realistic, need freshwater flow Q, bed
  slope, friction, etc.

• Spread of turbid water critical for
  understanding
• depleted oxygen levels and other biological
  processes

• Next step Modeling

Hypothetical Combined Salt turbidity circulatati
on
Fresh Water
Salt Water
Fresh Water
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Development of Simple Model

• We make the following assumptions
• No TidesConsider only averages
• Constant horizontal salinity gradient
• Salinity well-mixed vertically
• Sediment Concentration is prescribed
• Balance between settling velocity and turbulent
  mixing

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Development of Simple Model

• Following equations solved analytically
• Basically, classical gravitational circulation
  model with longitudinal sediment gradients as a
  forcing mechanism

Horizontal Momentum
Vertical Momentum
Prescribed Density variation
Sediment Mass Balance
Water Mass Balance
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Development of Simple Model

• Preliminary Results
• Presribed Salinity gradient 1 psu/km
• Sediment Concentration

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Preliminary Results

• Downstream of ETM at maximum gradient

Fresh Water out

• Flow reversed
• at bottom for large
• sediment gradients
• Not reversed when
• sediment gradient not
• large
• Saline flow shifted
• upwards

Saline Water into estuary
Flow reversal
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Preliminary Results

• Upstream of ETM at maximum gradient

Flow out of estuary
Flow enhanced at bottom because salinity and
sediment gradients in same direction
Could turbidity currents explain upstream shift
of turbidity zone and asymmetrical profile?
Flow into estuary
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What about other processes?

• Tidal asymmetry

Could this be the cause of upward migration of
turbidity zone? --Suggested by C. Habermann,
others but never tested --2D Analytic model of
de Swart and Schutelaars will test this
hypothesis (currently being worked on)
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And, havnt forgotten biology

• Modeling scalars
• Model of H.M. Schuttelaars being adapted to model
  algae
• Important processesinteraction of sediment
  concentration with light availability

Light availability Io depends on time of day,
season, cloud cover, etc. Attenuation
coefficient k kw water attenuation kb
self shading by algae ks shading by sediment
(proportional to SSC) kd shading by detritus
No Growth
Growth
Growth
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Summary

• Questions to come out of measurements
• Why the funny vertical salinity profiles?
• Can we calibrate non-linear range of ADCP
  backscatter?
• Are there turbidity currents being driven by high
  sediment concentrations? How does this interact
  with longitudinal salinity gradient?
• What controls the position and longitudinal
  extent of the ETM plume?

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SummaryNumerical Modeling
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SummaryNext steps

• Continue developing models to investigate these
  questions
• Turbidity induced circulation
• Effect of tidal assymmetry on residual
  circulation
• Vertical mixing processes near ETM
• Both Salinity and Sediment induced density
  differences and stratification

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SummaryNext steps

• Analyze recent cross-sectional measurements over
  tidal period
• 2 ADCPs on one ship (Friesland)estimate mean
  grain size
• Calibrate non-linear range
• Analyze vertical mixing and turbulence
• Analyze residual currents, fluxes over tidal
  period
• Analyze near bed turbidity currents
• Feed results into Models to make more realistic

1200 kHz ADCP
600 kHz ADCP
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Thanks for listening!