1. 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)

2. 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 M 2 tidal time scale of change is much longer at 12.4 hr/(2  ) = 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/m 2 at a given stress. (Images from Dickhudt) 1/17

3. -- 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/m 2 at a given stress. 1. Introduction – What is bed erodibility? How do we measure it? Why do we care? 2/17 Bed Erodibility Bed 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/m 2 at 0.2 Pa Red = 150 mg/L 7 Jul 2007 10 km

4. 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)

5. 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)

6. R = 0.129 p > 0.10 Classic bed properties vs eroded mass R = 0.188 p > 0.10 Total solids fraction R = 0.0526 p > 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 SOLIDS FRACTION SAND FRACTION EPS 4/17 (Dickhudt et al. 2009)

7. ADV at deployment -- ADVs provide continual long-term estimates of: Suspended mass conc (C ADV ):From ADV backscatter calibrated by pump samples. Bed Stress (τ b ): τ b = ρ = ρ u * 2 Bulk Settling Velocity (W S ): W S = /C ADV_SETT with C ADV_SETT = C ADV - C WASH Eroded Mass (M):From integrating C SETT (z) = C ADV_SETT (z/z ADV ) -P over 6-m water column during accelerating tide, where P = W s /(0.41 u * ) ADV after 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

8. Biological (GP) site Intermediate (CB) site Agreement between GUST and ADV estimates of Eroded Mass vs. Bed Stress allow us to use the ADV to calculate a continual time-series of bed erodiblity. 6/17 (Cartwright et al. 2009)

9. Biological (GP) site Intermediate (CB) site Eroded mass at 0.2 Pa Agreement between GUST and ADV estimates of Eroded Mass vs. Bed Stress allow us to use the ADV to calculate a continual time-series of bed erodiblity. 6/17 (Cartwright et al. 2009)

10. Biological site Intermediate site Nov 06 May 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

11. Nov 06 May 07 Oct 07 12 cm -- Although noisy, mean W s at biological site is generally higher – probably resilient aggregates (worm fecal pellets). -- At intermediate site, mean W s is bimodal and varies seasonally between (i) flocs and (ii) flocs mixed with pellets. Biological site W s ~ 1 to 2 mm/s Intermediate site Bulk W s varies from ~ 0.5 mm/s to 1 mm/s 3-day Mean W S = /C ADV_SETT using ADVs 2 1.5 1.0 0.5 0 Settling velocity (mm/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

12. Physics favoring seasonal floc deposition Seasonal secondary turbidity maximum & seasonal floc deposits Convergence in transport and settling at seasonal salinity front at “Intermediate” site. (Dickhudt et al. 2009) (Kuo & Neilson 1987) 9/17

13. 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)

14. What does the bed look like when fresh floc layer isn’t present? Answer: “Worm Cam” 10 April to 06 May, 2006 (1 frame/hour) York Estuary “Intermediate Site” Video by R. Diaz 10/17 Video from R. Diaz 1 cm

15. What does the bed look like when fresh floc layer isn’t present? Answer: “Worm Cam” 10 April to 06 May, 2006 (1 frame/hour) York Estuary “Intermediate Site” Video by R. Diaz 1 cm RESILIANT PELLETS 10/17 Video from R. Diaz

16. 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.)

17. 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 – Be 7 Isotope Activity (From Kraatz et al.) 12/17

18. Discharge (m 3 /s) Salinity (PSU) SPM (mg/L) Cruise Period 1 m above bed 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”s ite Conditions at “Intermediate” site Month in 2010 Q (m 3 /s) S (PSU) C (mg/L) Winter/Spring Freshet 13/17 (From Kraatz et al.) (Data sources: USGS & EPA monitoring programs)

19. 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’s missing? 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 shading p ≤ 0.1 < 0.2 light shading 14/17 (From Kraatz et al.)

20. 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)

21. Salinity (PSU) TSS (mg/L) Cruise Period 1 m above bottom Near surface 1 m above bottom Conditions at “Intermediate” site Month in 2010 Tidal range (m) April May 15/17 (From Kraatz et al.)

22. Days starting from 1 April 2010 Gust eroded mass (kg/m 2 ) 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.)

23. 17/17 (From Kraatz et al.)

24. 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)