What Controls Bed Erodibility in Muddy, Partially-Mixed Tidal Estuaries?

Carl Friedrichs, Grace Cartwright, Bob Diaz, Pat Dickhudt, Kelsey Fall, Lindsey Kraatz, Larry Sanford, Linda Schaffner

1. Introduction – What is bed erodibility? How do we measure it? Why do we care?

2. Large increases/decreases in erodibility are due to major deposition/erosion of muddy flocs

3. Gradual decrease in erodibility is due to armoring by muddy aggregates and consolidation

4. Short-term increases in erodibility follow short-term resuspension (e.g., tides, storms)

(Images from Dickhudt)

1. Introduction – What is bed erodibility? How do we measure it? Why do we care?

-- Bed erodibility characterizes of how sediment leaves the bed in response to bed stress.

-- We measure erodibility with Gust microcosms which exert step of nearly uniform bed stress on the surface of cores. The rate and mass of sediment eroded is then recorded.

-- In the absence of fluid-like mud, beds of almost all muddy, partially-mixed tidal estuaries are characterized by “Type 1”, i.e., depth-limited (or equivalently, mass-limited) erosion.

-- Gust experiments show erosion at each step in stress effectively stops within about 20 min; yet the M2 tidal time scale of change is much longer at 12.4 hr/(2p) = 2.0 hours.

-- Thus erosion rate is less relevant to erodibility in many muddy partially mixed tidal estuaries. Erodibility instead is characterized mainly by M = eroded mass in kg/m2 at a given stress.

(Images from Dickhudt)

1/17

-- Along most “drowned river” coastal plain estuaries, tidal velocity (and thus tidal bed stress) is relatively uniform with distance along the estuary.

-- Thus, as described nicely by Henk Schuttelaars, changes in sediment concentration (C) along estuaries must be due at lowest order mainly to changes in bed erodibility.

-- So if you care about sediment concentration (C) in estuaries, you need to care about bed erodibility, which in turn is characterized by M = eroded mass in kg/m2 at a given stress.

1. Introduction – What is bed erodibility? How do we measure it? Why do we care? 2/17

Bed ErodibilityBed Stress Near-Bed C

Daily averaged magnitudes from 3-D ROMS model of York River Estuary (Fall et al., 2014) utilizing depth-limited, temporally-evolving cohesive erodibility model of Sanford (2008).

Red =0.1 Pa

Red =1 kg/m2

at 0.2 Pa

Red =150 mg/L

7 Jul 2007 7 Jul 2007 7 Jul 2007

10 km10 km10 km

What Controls Bed Erodibility in Muddy, Partially-Mixed Tidal Estuaries?

Carl Friedrichs, Grace Cartwright, Bob Diaz, Pat Dickhudt, Kelsey Fall, Lindsey Kraatz, Larry Sanford, Linda Schaffner

1. Introduction – What is bed erodibility? How do we measure it? Why do we care?

2. Large increases/decreases in erodibility are due to major deposition/erosion of muddy flocs

3. Gradual decrease in erodibility is due to armoring by muddy aggregates and consolidation

4. Short-term increases in erodibility follow short-term resuspension (e.g., tides, storms)

(Images from Dickhudt)

Sample Collection 2006-2007

• Bed samples collected with Gomex box corer on anchor at slack tide

• Sub-sampled for: - Water content, Grain size, - Organic content, EPS - Digital X-radiography - Erodibility

Large spatial & seasonal variability in erodibility

“Biologically dominated” Near estuary mouth

“Intermediate site” Middle estuary

3/17

(Dickhudt et al. 2009)

R = 0.129

p > 0.10

Classic bed properties vs eroded mass

R = 0.188

p > 0.10

Total solids fraction

R = 0.0526p > 0.10

• Solids fraction (i.e., bulk density, ) water content) and sand fraction not correlated to erodibility.

• Extracellular Polymeric Substances (EPS, i.e., “bio-glue”) – no correlation

• Standard bed properties are not obvious control on erodibility

SOLIDSFRACTION

SANDFRACTION

EPS

4/17

(Dickhudt et al. 2009)

ADV at deployment

-- ADVs provide continual long-term estimates of:

• Suspended mass conc (CADV): From ADV backscatter calibrated by pump samples.

• Bed Stress (τb): τb = ρ <u’w’> = ρ u*2

• Bulk Settling Velocity (WS ): WS = <w’CADV’>/CADV_SETT with CADV_SETT = CADV - CWASH

• Eroded Mass (M): From integrating CSETT (z) = CADV_SETT(z/zADV)-P

over 6-m water

column during

accelerating tide, where P = Ws/(0.41 u*)

ADVafter retrieval

Estimates of erodibility determined by an Acoustic Doppler Velocimeter (ADV)Sensing volume ~ 35 cm above bed, recording at 10 Hz for 2 min burst every 15 minutes

(Images from Cartwright)

5/17

Biological (GP) site Intermediate (CB) site

Agreement between GUST and ADV estimates of Eroded Mass vs. Bed Stressallow us to use the ADV to calculate a continual time-series of bed erodiblity.

6/17

(Cartwright et al. 2009)

Biological (GP) site Intermediate (CB) site

Eroded mass at 0.2 Pa

Agreement between GUST and ADV estimates of Eroded Mass vs. Bed Stressallow us to use the ADV to calculate a continual time-series of bed erodiblity.

6/17

(Cartwright et al. 2009)

Biological site

Intermediate site

Nov 06 May 07May 07 Oct 07-- Erodibility based on ADV at biological site is generally lower.

-- At intermediate site, erodibility is bimodal and varies seasonally. -- Deposition is presumably highly transient and patchy.

Newly deposited floc layer present at surface in May.

3-day fits to ADV eroded mass at 0.2 Pa

12 cm

(Cartwright et al. 2009)

7/17

Nov 06 May 07May 07 Oct 07

12 cm

-- Although noisy, mean Ws at biological site is generally higher – probably resilient aggregates (worm fecal pellets).

-- At intermediate site, mean Ws is bimodal and varies seasonally between (i) flocs and (ii) flocs mixed with pellets.

Biological siteWs ~ 1 to 2 mm/s

Intermediate siteBulk Ws varies from

~ 0.5 mm/s to 1 mm/s

3-day Mean WS = <w’CADV’>/CADV_SETT using ADVs

2

1.5

1.0

0.5

0

S

ettlin

g ve

loci

ty (m

m/s

)

Newly deposited floc layer present at surface in May.

What makes us think the high erodibility is due to floc deposition?

(Cartwright et al. 2009)

8/17

Physics favoring seasonal floc deposition

Seasonal secondary turbidity maximum& seasonal floc deposits

Convergence in transport andsettling at seasonal salinity frontat “Intermediate” site.

(Dickhudt et al. 2009)

(Kuo & Neilson 1987)

9/17

What Controls Bed Erodibility in Muddy, Partially-Mixed Tidal Estuaries?

Carl Friedrichs, Grace Cartwright, Bob Diaz, Pat Dickhudt, Kelsey Fall, Lindsey Kraatz, Larry Sanford, Linda Schaffner

1. Introduction – What is bed erodibility? How do we measure it? Why do we care?

2. Large increases/decreases in erodibility are due to major deposition/erosion of muddy flocs

3. Gradual decrease in erodibility is due to armoring by muddy aggregates and consolidation

4. Short-term increases in erodibility follow short-term resuspension (e.g., tides, storms)

(Images from Dickhudt)

What does the bed look like when fresh floc layer isn’t present? Answer: “Worm Cam”

10 April to06 May, 2006(1 frame/hour)

York Estuary“IntermediateSite”

Video byR. Diaz

10/17

Video fromR. Diaz

1 cm

What does the bed look like when fresh floc layer isn’t present? Answer: “Worm Cam”

10 April to06 May, 2006(1 frame/hour)

York Estuary“IntermediateSite”

Video byR. Diaz

1 cm

RESILIANT PELLETS

10/17

Video fromR. Diaz

Microscope image of resilient mud pellets from bed of York River “Intermediate Site” as caught between 63 and 90 micron sieves, January 2013

11/17

(Image from Schaffner et al.)

Experiment designed to address the affect of resilient pellets on bed erodibility

• Sediments were collected with a Gomex box well after the winter/spring freshet in 2010

• Collection occurred:– Weekly – Allowed for consideration of

spring vs. neap tides– Well after spring/winter freshet

• Each week, cores were sub-sampled for:

– Water content – Grain size – Resilient pellets – Organic content – Digital X-radiography– Erodibility– Be7 Isotope Activity

(From Kraatz et al.)

12/17

Dis

char

ge (

m3 /

s)S

alin

ity (

PS

U)

SP

M (

mg/

L)

Cruise Period

1 m above bed

Near surface

Near surface

1 m above bed

Study Focus:Starts ~ 1

month after peak river discharge

Salinity stratification significantly decreases

Tendency to disperse flocs

away from“Intermediate”

site

Conditions at “Intermediate”

site

Month in 2010

Q(m3/s)

S(PSU)

C(mg/L)

Winter/Spring Freshet13/17

(From Kraatz et al.) (Data sources: USGS & EPA monitoring programs)

Correlations Between Observed Quantities

Highly variable significance (only 5 observations…), but consistent trends for signs of all correlations.

With time, non-pelletized mud and associated organic matter are winnowed, and finest sand and pellets are winnowed = BED ARMORING. Also water content of mud decreases = CONSOLIDATION. Result: ERODIBILITY ↓

But why is erodibility among the least correlated? What’smissing? ANSWER: Effects of recent resuspension.

“More consolidation/armoring” as

Time ↑Pellet abundance & size ↑Sand abundance & size ↑

Water content ↓Organic content ↓

Beryllium Activity ↓Erodibility ↓

p < 0.1 dark shadingp ≤ 0.1 < 0.2 light shading

14/17

(From Kraatz et al.)

What Controls Bed Erodibility in Muddy, Partially-Mixed Tidal Estuaries?

Carl Friedrichs, Grace Cartwright, Bob Diaz, Pat Dickhudt, Kelsey Fall, Lindsey Kraatz, Larry Sanford, Linda Schaffner

1. Introduction – What is bed erodibility? How do we measure it? Why do we care?

2. Large increases/decreases in erodibility are due to major deposition/erosion of muddy flocs

3. Gradual decrease in erodibility is due to armoring by muddy aggregates and consolidation

4. Short-term increases in erodibility follow short-term resuspension (tides, storms)

(Images from Dickhudt)

Sal

inity

(P

SU

)T

SS

(m

g/L)

Cruise Period

1 m above bottom

Near surface

Near surface

1 m above bottom

Conditions at “Intermediate”

site

Month in 2010

Tid

al r

ange

(m

)

April May

15/17

(From Kraatz et al.)

Days starting from 1 April 2010

Gust eroded mass

(kg/m2) at 0.2 Pa

Tidal range

(m)

r = + 0.76

r = - 0.57

Erodibility generally decreases with time since the end of the spring freshet. Erodibility also increases and decreases in proportion to the tidal range, with a maximum correlation associated with the average tidal range over the previous 5 days.

(a)

(b)

16/17

(From Kraatz et al.)

17/17

(From Kraatz et al.)

What Controls Bed Erodibility in Muddy, Partially-Mixed Tidal Estuaries?

Carl Friedrichs, Grace Cartwright, Bob Diaz, Pat Dickhudt, Kelsey Fall, Lindsey Kraatz, Larry Sanford, Linda Schaffner

1. Introduction – What is bed erodibility? How do we measure it? Why do we care?

2. Large increases/decreases in erodibility are due to major deposition/erosion of muddy flocs

3. Gradual decrease in erodibility is due to armoring by muddy aggregates and consolidation

4. Short-term increases in erodibility follow short-term resuspension (e.g., tides, storms)

(Images from Dickhudt)