The dynamics of estuarine turbidity maxima

Stefan Talke * Huib de Swart * Victor de JongeGroningen Workshop

March 3, 2006

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 andbiological processes separately

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

Longitudinal Transects: 10 times since Feb. 2005

Cross-sectional Transects: Mar. 2005, Feb. 2006, Summer 2006?

Germany

Netherlands

MeasurementsMeasurements

ADCP (Acoustic Doppler Current Profiler)Velocity measured continuously in water column (~0.5 Hz)Backscatter used to estimate sediment concentrationBottom tracking used to estimate boat velocity

But, in turbid water, signal disappears!

Water

Fluid Mud

Consolidated Bed

600 kHz ADCP measures velocity and backscatter (turbidity) in 0.25 m increments

MeasurementsMeasurements

Solution:

Use external echo-sounders and differential GPS

Water

Fluid Mud

Consolidated Bed

210 kHz echosounder penetrates to the fluid mud layer

Boat Velocity from GPS

MeasurementsMeasurements

Solution

Use external echo-sounders and differential GPS!

Water

Fluid Mud

Consolidated Bed

15 kHz echosounder penetrates to the bed

Boat Velocity from GPS

Pump water into a bucket continuously

Measure: Turbidity, Fluorescence, Salinity,Temperature, Oxygen

Take care to prevent light, bubbles!

MeasurementsMeasurements

On-board Flow-thru system

Water

Fluid Mud

Consolidated Bed

MeasurementsMeasurements

Fixed Point Measurements

Consolidated Bed

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

Water Samples

CTD Casts

MeasurementsMeasurements

Long Term Fixed Point Measurements: “X”Monitored by NLWKN and WSA Emden

Measure: Tidal Stage, Salinity, Turbidity, Oxygen, pH, Velocity, Temperature, Sediment Concentration

CTD CastsGermany

Netherlands

X

XX

X

X

X

XX

X

Cross-Sectional Data at Pogum

Longitudinal Transects

Cross-sectional Transect: March 2005

Germany

Netherlands

Echosounder Data

Transect at Pogum

500 m

8:27 am

Fluid Mud

Echosounder Data

8:35 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

9:21 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

9:36 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

10:03 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

10:25 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

10:36 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

10:47 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

11:22 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

11:32 am

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

15:11 pm

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

15:14 pm

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

15:56 pm

Transect at Pogum

500 m

Fluid Mud

Echosounder Data

Question: What does sediment concentration from ADCP backscatter look like?

16:00 pm

Transect at Pogum

500 m

Fluid Mud

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

Sediment Calibration from Backscatter

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

Water attenuation

(range 0.05-0.2)

Sediment Calibration from Backscatter

With 2 ADCP’s of differing frequency, can estimate mean grain size

Sediment attenuation coefficient, 600 kHz

Maximum at ~ 2 microns

• Highly dependant on grain size, density, frequency

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

rct

tv EEKR

LP

RTCS

2log10

2

10

Fit regression line sed-conc to abs. backscatter

• Since attenuation depends on sediment concentration in profile…

1. make initial estimate of sed conc to calculate backscatter

2. Use backscatter to make linear regression

3. Re-estimate sediment concentration, recalculate attenuation, and repeat.

What about changes in floc size?

What about non-linear range?

Future Steps: 1. Calibrate non-linear range with Feb. 2006 data2. Estimate mean grain size using backscatter from 2ADCP’s on board one ship (Friesland)

Linear Range

Non Linear Range

Germany

Netherlands

Cross Section Sediment Concentration Profiles at Pogum

(March 8,2005)

Wa

ter

leve

l (m

)

Pogum

8:30 amflood

Wa

ter

leve

l (m

)

Time (hours)

mg/LHigh turbidity evident

Note structures in sediment profile

Non-linearRange:> 5 g/L

Wa

ter

leve

l (m

)

Time (hours)

mg/L

11:30 amslack

Turbulence collapses,Sediment settles

Sharp gradient between water and fluid mud

Fluid mud pools inchannel and shoal

Non-linearRange:> 5 g/L

8:30 am

Wa

ter

leve

l (m

)

Time (hours)

mg/L

Very high turbidityand fluid mud

Closer to ETMthan earlier measurements 16:00

Ebb

Non-linearRange:> 5 g/L

Vertical Observations: Interesting Salinity Profiles

Flood (morning)

Salinity often (but not always) decreases towards bed.

Need to investigate density profiles…

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?

Vertical Observations—density profiles

Including sedimentconcentration essentialfor water column stability

Note again sharp transition to fluid mud

Vertical Observations—density gradients

Positive means unstable

Salinity profile dominatesupper water column

Sediment profile dominateslower water column

Vertical Observations—Richardson number

Richardson # measuresratio of shear (turbulence)to density gradient (buoyancy)

>0.25 density dominates

< 0.25 shear dominates

< 0 Unstable

Turbulence highly damped

9:00 am (endof flood tide)

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

Longitudinal Data

Longitudinal Transects—ADCP measurements in March, April, June, July, September 2005, and Feb. 2006

Germany

Netherlands

Large horizontal salinity and turbidity gradient (Turbidity not yet calibrated)

Question: Are there density driven currents from both salinity and turbidity?

From NPAanderaa ProbeIn flow-through system

Distance downstream from Herbrum (km)

Longitudinal Results—Turbidity and Salinity

Upstream Downstream

Longitudinal Results—Oxygen and Fluorescence

Not yet calibrated—Fluorometer

How are Dissolved oxygen and fluorescence related to the physical parameters of system?

Longitudinal Results--Salinity

Fixed NLWKN salinity measurements (note different scales)

Knock

Pogum

Terborg

Longitudinal Results--Sediment

Fixed NLWKN sediment concentration measurements

Note concentrations of up to 10 g/L; in summer, > 25 g/L measured

Knock

Pogum

Terborg

Longitudinal Results—Density Gradients

Residual circulation proportional to density gradient

Note that sediment density gradient changes sign

Knock-Pogum

Pogum-Terborg

Salinity gradient

Combined gradient(salinity + sed. Conc.)

Thought Experiment

• Consider “Bath Tub”

Next, add turbid water to center

What happens?

Fresh Water (Salinity = 0)

Heavy, turbid water flows along bottom

Fresh water circulates to conserve mass

Circulation cell

Thought Experiment

• Consider another situation

Fresh Water(River)

Now, consider case in which density differencesoccur only from salinity

What Happens?

Salt Water(Ocean)

Circulation cell from density difference dueto salinity (gravitational circulation)

Thought Experiment• Now, consider both together

Turbidity induced circulation

+Fresh Water

SaltWater

Salinity induced circulation

HypotheticalCombinedSalt + turbiditycirculatation

SaltWater

Fresh Water=

Thought Experiment

Analysis:

Fresh Water HypotheticalCombinedSalt + turbiditycirculatation

SaltWater

Fresh Water

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

Development of Simple Model

• We make the following assumptions:– No Tides—Consider only averages– Constant horizontal salinity gradient– Salinity well-mixed vertically– Sediment Concentration is prescribed– Balance between settling velocity and

turbulent mixing

Development of Simple Model

• Following equations solved analytically:

• Basically, classical gravitational circulation model with longitudinal sediment gradients as a forcing mechanism

dz

duA

zg

dx

dpz

sin0 Horizontal Momentum

gdz

dp

Csso )( 0

0)(

z

CKCw

z zs

H

Qubdz0

Vertical Momentum

Prescribed Density variation

Sediment Mass Balance

Water Mass Balance

Development of Simple Model

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

Preliminary Results• Downstream of ETM at maximum gradient:

Flow reversedat bottom for largesediment gradients

Not reversed whensediment gradient notlarge

Saline flow shiftedupwards

Fresh Water out

Saline Water into estuary

Salinity driven flow

Sal. + Sed. driven flow

Flow reversal

Preliminary Results• Upstream of ETM at maximum gradient:

Flow enhanced at bottom becausesalinity and sedimentgradients in same direction

Could turbidity currents explain upstream shift of turbidity zone and asymmetrical profile?

Flow into estuary

Flow out of estuary

Salinity driven flow

Sal. + Sed. driven flow

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 testthis hypothesis (currently being worked on)

And, havn’t forgotten biology…

• Modeling scalars– Model of H.M. Schuttelaars being adapted to model algae– Important processes—interaction 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

Growth

Growth

No Growth

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?

Summary—Numerical Modeling

Fluid Mud

Consolidated Bed

Saline Water

Exchange

Turbulence vs. Bouyancy

Turbidity CurrentBed friction

Fresh(er) Water

Gravitational Circulation

Flocculation andsettling

Algae

Periodic Stratification

Bacteria withhigh oxygen consumption

Summary—Next 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

Analyze recent cross-sectional measurements over tidal period 2 ADCP’s on one ship (Friesland)—estimate mean grain size Calibrate non-linear range Analyze vertical mixing and turbulenceAnalyze residual currents, fluxes over tidal periodAnalyze near bed turbidity currentsFeed results into Models to make more realistic

Water

Fluid Mud

Consolidated Bed

1200 kHz ADCP

600 kHz ADCP

Summary—Next steps

Thanks for listening!