RELATIONSHIPS BETWEEN ERODIBILITY AND FINE-GRAINED SEABED PROPERTIES
ON TIDAL TO SEASONAL TIME-SCALES, YORK RIVER ESTUARY, VA
Lindsey M Kraatz, Carl T. Friedrichs, Grace M. Cartwright, Kelsey A. Fall, and Carissa N. Wilkerson
Physical Sciences Department, Virginia Institute of Marine Science/ College of William & Mary, Gloucester Point, VA 23062
The complex bio-geo-physical nature of muddy particles in coastal and
estuarine environments has limited our understanding of fine-grained
sediment dynamics. An ongoing sedimentalogical study within the York
River Estuary is investigating controls on cohesive bed erodibility by
assessing changes in seabed properties over varying timescales. During
the spring of 2010, multiple GOMEX box cores were collected and
subsampled for grain size, fecal pellet content, erodibility, and water
content. Initial findings suggest that erodibility of the seabed increased
during periods following seasonal deposition events. Higher erodibility
was also generally found to coincide with peak spring tides and
following storm events, perhaps in response to enhanced physical bed
disturbance. In addition, a higher percentage of flocculated sediment
relative to resilient pellets were found to makeup the surface of the
seabed during high erodibility depositional periods.
Resilient Fecal Pellet Presence
•Analyzed 2 sets of 10.00g from the same sediment
•Samples were sieved through 4 mesh sieves
150 µm, 90 µm, 63 µm, and 45 µm
Methods
Field Sampling:
Sediment sampling cruises were taken to coincide with
spring/neap in 2010
•
Spring ~ 3 samples
•
Neap ~ 2 samples
Gust Microcosm:
Applied increasing shear stress
to the seabed within the core at
20 min. increments
Samples collected using a Gomex Box Core
•
Sliced at 1 cm intervals
•
Sampled for
•
water content
•
grain size
•
resilient pellet presence and concentration
•The 1st set of sediment was sieved using traditional
sieving methods. Sediment was disaggregated and
sieved through each sieve, capturing the original grain
size (φoriginal) before any agglomeration, flocculation, or
pellet production had occurred
0.01, 0.05, 0.1, 0.2
0.3 0.45, and 0.6 Pa
Filtered eroded material to
determine initial erosion rate
(E0), along with an erosion rate
constant (M), and the initial and
final critical shear stresses (τce)
actually applied to the surficial
sediment
Study Location
The York River Estuary (Figure 1) is classified as a partiallymixed estuary, formed at the confluence of the Pamunkey
and Mattaponi Rivers in southeastern Virginia. The estuary
is linear and narrow, stretching 50 km from Goodwin
Islands, at the mouth of the estuary, to West Point.
• The
set was not disaggregated and careful
attention was paid as to not disturb the original integrity
of the sediment. Seawater (15‰) was used in the
gentle sieving process, in order to not cause any rapid
breakdown of the biological material from the fecal
pellets.
Fecal pellet weight g=φcollectedtot(g)-φoriginaltot(g)
Fecal pellet % of sample= Fecal pellet weight (g)/φoriginaltot(g)
Results
Discharge
occurs in
winter/
early
spring
350
300
U*
River flow
(m3/s)
200
150
Salinity in main
channel near
coring site
(PSU)
1 m above bottom
Near surface
100
50
•ETM located at West Point
(Estuarine Turbidity Maximum)
3/1/2010
4/1/2010
Date
5/1/2010
6/1/2010
•
•
•
Figure 4. York River Discharge for the first 6 months of 2010. The York River
typically has a wet spring and during 2010, there were steady increases during
this time as the freshet was released and peaked in early April. This study
focused on the sediment bed characteristics, just as the river discharge began to
decline, in order to capture the transition between wet spring and dry summer
conditions. Data was acquired from USGS river gauges in the Mattaponi and
Pamunkey Rivers.
r = - 0.51
Gust
eroded
mass
(kg/m^2)
at 0.2 Pa
•STM found seasonally at
Clay Bank (Secondary Turbidity Maximum)
r = + 0.75
Background & Objectives
Tidal
range
(m)
Wetter in
Spring
Boxes are tidal range for previous 5 days
Objectives
To investigate seabed properties and surface erodibility
on a short-term timescale. Specifically observing:
1. The transition between periods of high and low river
flow
4/29/10
r=+
0.89
r2 =
0.79
Figure 6. A least squares fit, combing both time elapsed since peak discharge and
tidal variation, shows that during periods of higher erosion the tidal range is low,
allowing the bed to consolidate. Sampling periods when tidal range was high were
correlated to high erosion, as sediment was not able to settle and consolidate.
5/5/10
0
0
0-1
cm
3.3
5/11/10
3
0
0
12.5 cm
Fecal
Pellet %
by mud
weight
dpm/g
1 2
0
2
4
6
8
dpm/g
1 2
Fecal
Pellet %
by mud
weight
0-1
cm
12.9
1-2
cm
15.2
4
6
8
10
12
dpm/g
1 2
0-1
cm
9.8
1-2
cm
14.2
4
6
8
10
12
9.7
12
14
14
(d) Mud (% dry wgt.)
(e) Mud matrix % solids
C1 + C2 x (Time) – C2 x (Tide Range)
Pellet abundance increases with time and decreases with tide range
Figure 9. Fecal pellet abundance vs. time and tidal range. Pellet abundance
was found to increase with time presumably because as the sediment bed
was reworked, smaller components of the fines were winnowed away leaving
pellets behind. Conversely, pellet abundance decreased in response to spring
tide, perhaps because they were dispersed or broken up during periods of
highest stress.
r = + 0.35
Conclusions
(c) Organics (% dry wgt.)
r = - 0.97
Only significant
1-component
regression
(f) Pellets (% dry wgt.)
Figure 7. An analysis of parameters that are classically expected to effect erodibility were conducted.
Contrary to what was expected, many variables appeared to have no statistical correlation, except to the
presence of fecal pellets, with had a negative correlation of 0-.97
5/20/10
5/27/10
3
0
dpm/g
1 2
3
0
0
Fecal
Pellet %
by mud
weight
0-1
cm
7.0
1-2
cm
14.3
2
4
6
8
10
14
2. The role of spring and neap tidal currents on the
erodibility of cohesive sediments
3. Sediment bed properties (including particle types) to
decipher controls on bed erodibility
r = - 0.08
r = + 0.43
10
1-2
cm
(b) Sand (% dry wgt.)
(a) Water (% by vol.)
2
Fecal
Pellet %
by mud
weight
r = - 0.43
r = + 0.20
0
2
3
r = + 0.95
Month in 2010
C1 – C2 x Time (net erosion effect)
+ C3 x Tide Range (lower consolidation effect)
Depth (cm)
Figure 2. York River conceptual model (Dickhudt et al, 2009). During
high river flows, the estuary becomes stratified, causing sediment
convergences, allowing a STM to form and is characterized by a highly
erodible seabed. Conversely, during low river flow periods, the estuary
becomes well mixed and the seabed has low erodibility.
Depth (cm)
Drier in
Summer
Figure 8. Spring Tide and Neap Tide sediment erodibility schematic.
During spring tides, cohesive sediment beds were found to be more erodible
as there was less time for bed consolidation. Conversely, during neap tides,
when weak currents, were present, less sediment was eroded due to a more
compact bed that had time to consolidate.
r2 = 0.90
Figure 5. York River discharge, salinity, and total suspended solids at the Clay Bank region during 2010.
The green boxes indicate high river flow discharge during the winter and early spring, just before the
beginning of the study. The purple boxes delineate the 5 week study period, just as the river discharge
declined and began to equilibrate.
Days starting 1 April 2010
Figure 5. Eroded mass captured using the Gust Microcosm at a stress 0.2 Pa
showed a decrease over time, corresponding to minimal river discharge with an
r-value to -0.51. Due to the misfit around the line, tidal range for 5 days before
the sediment sampling was analyzed. As tidal range increased, erodibility
increased . Conversely as tidal range decreased , erodibility decreased as well.
The correlation of tidal range to erodibility gave an r-value of 0.75
Near surface
USGS & EPA monitoring)
Observed eroded mass at 0.2 Pa
•Shoals
dominated by fine sands from the Pleistocene
inactive oyster reef
• Tidal range ~ 0.8m (microtidal)
• Tidal currents ~ 1 m s-1 near surface
2/1/2010
Leads to
transition from
convergence to
net erosion of
flocs
Figure 8. X-radiography, 7Be activity, and fecal pellet percentage of the Clay Bank study site for the 5 week period.
12
14
0
Fecal
Pellet %
by mud
weight
0-1
cm
19.1
1-2
cm
11.8
2
4
Depth (cm)
The York River is characterized by:
•
1 m above bottom
Eroded Mass (kg/m2) at 0.2 Pa
0
1/1/2010
Stratification
favors:
TSS in
convergence of
main channel
sediment flocs
near coring site
net deposition
(mg/L)
higher erodibility
higher TSS
(Data sources:
Depth (cm)
Figure 1: The York River. The Clay Bank study site is indicated by the black circle.
Two long term monitoring stations, located within the York, are shown as red
boxes.
Salinity
stratification
significantly
decreases
Depth (cm)
Discharge (m3/sec)
Salinity
stratification
develops
(lagging
discharge)
Small Tidal Range
Decreased current velocities
Minimal water column mixing
Decreased bottom shear stresses
More time for bed consolidation
Less erodible material
Study Focus:
~1 month after
river discharge
peak
250
•Secondary channel
~ 5m depth
Larger tidal ranges
Higher current velocities
Increased water column mixing
Resuspension of bottom sediments
Less time for bed consolidation
Easily erodible material
2nd
Figure 3. Resilient Fecal Pellet Method Schematic
York River Estuary Discharge
• Main channel
~10m depth
silty clay sediments
U*
Pellet Abundance (% dry wgt.)
Abstract
dpm/g
1 2
3
Two main factors affecting bed erodibility
•The convergence and divergence of sediment due to
stratification
•The spring-neap effect on tidal velocity
Environmental factor analysis
•Erodibility was negatively correlated to lagged decreases in
river discharge and therefore stratification
•Erodibility was positively correlated to previous changes in
tidal range
•Spring Tide ~ Increases erosion potential
•Neap Tide ~ Decreases erosion potential
•The combination of the two factors of time and tidal range
leads to a correlation of .89
6
8
10
12
14
Sediment Bed Properties and Comparisons
•No classically expected bed parameters directly affect bed
erodibility
•EXCEPT…the abundance of resilient fecal pellets
Resilient Fecal Pellets may be serving as a proxy for other
parameters influencing the area
•Bed armoring
•Cohesion
•Winnowing of fines
Reference:
Dickhudt, P.J., C.T. Friedrichs, L.C. Schaffner, and L.P. Sanford, 2009. Spatial and temporal variation in cohesive sediment
erodibility in the York River estuary: a biologically-influenced equilibrium modified by seasonal deposition. Marine
Geology, 267: 128-140.