lowermost Mississippi. River & sand delivery to the river mouth. Water-surface
profile for the Mississippi. River at low water discharge. •RK160. •RK367. •RK487
.
Connecting the Flow and Sediment-Transport in Coastal Rivers to Short- and Long-Term Patterns of Delta Sedimentation RK487 RK367 RK160
RK0
D. Mohrig; J. Nittrouer; K. Straub1; J. Shaw; & M. Allison Jackson School of Geosciences, Univ. of Texas at Austin, Austin, TX, USA 1. Earth and Environmental Sciences, Tulane University, New Orleans, LA, USA National Center for Earth-surface Dynamics, a NSF STC
Connecting hydraulics to sediment transport in the lowermost Mississippi River & sand delivery to the river mouth
RK487 RK367 RK160
Water‐surface profile for the Mississippi River at low water discharge
RK0
60
water surface elevation at 5000‐10000 m3/s
50 40
river bottom profile
Backwater Zone defined by a divergence of water surface and bed slopes.
Elevation (m)
30 20 10 0 ‐10
river bottom profile
‐20 ‐30 ‐40 0
100
200
300
400
500
600
700
800
Mississippi River Kilometer
900 1000 1100
Control of Mississippi River Backwater on Flooding
3 x low flow 6 x low flow 9 x low flow
[Significant flooding above RK400, almost no flooding at RK 0] 60
100 200 300 400 500 600 700 Mississippi River Kilometer
Water‐surface profiles for the Mississippi River at low, intermediate and high water discharge
Elevation (m)
0
water surface elevation at 5000‐10000 m3/s 50 water surface elevation at 15000‐20000 m3/s water surface elevation at 30000‐35000 m3/s 40 river bottom profile 800 1000 1100 30 900
m
Rise is Water Surface Elevation (m)
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
20 10 0 ‐10 ‐20 ‐30 ‐40 0
100
200
300
400
500
600
700
800
Mississippi River Kilometer
900 1000 1100
3
2008 flood of Upper Mississippi River
April 2008 flood of Mississippi River at RK 47
< 1m
Flood April Velocity Profile (RK 47)
Low Flow January Velocity Profile (RK 47) (Nittrouer, Mohrig & Allison, in prep)
Down river water‐surface slopes at low, intermediate and high water discharge
Water‐Surface Slope
0.0001
> 10X change in slope
0.00001 water‐surface slope at 5000‐10000 m3/s water‐surface slope at 15000‐20000 m3/s water‐surface slope at 30000‐35000 m3/s 0.000001 0
100 200 300 400 500 600 700 800 900 1000 1100 Mississippi River Kilometer
How does a time varying backwater condition influence sediment transport in this coastal river?
6
(J. Nittrouer et al., 2008)
What about bed‐material load in lower Mississippi River?
Estimating Bed‐Material Load from Dune Tracking High water discharge: 34000 m3/s Water Depth (m)
24 26 28
t=0hr t=8hr
30
Water Depth (m)
32 24 0
100
200
300 400 500 600 700 Application of 8‐m translation
800
900
1000
1100
Downstream Distance (m)
26 28
t=0hr t=8hr
30 32 0
100
200
300
400
500
600
700
800
900
1000
1100
Downstream Distance (m)
qs b C
8m 4.1m 2 0 . 7 1.4 m hr 2 8hr 2
H
1.4 m 2 hr 700m 1005 m 3 hr
Water Depth (m)Water Depth (m) Water Depth (m)
Estimating Bed‐Material Load from Dune Tracking Low water discharge: 6000 m3/s 16.7 16.9 17.1 17.3 17.5 17.7 17.9 18.1
t=0hr t=24hr
16.7 20 16.9 17.1 17.3 17.5 17.7 17.9 18.1
20
40Application of 2.5‐m translation 60 80 100
120 t=0hr140
160
120
160
Downstream Distance (m) t=24hr
40
60
80
100
140
Downstream Distance (m) H 2.5m 0.47 m 2 2 qs b C 0. 7 1.7 10 m hr 2 24hr 2 1.7 10 2 m 2 hr 700m 12 m 3 hr
Bedmaterial Discharge (m3/hr)
Water Discharge (m3/s) N. Loup R.
8.8
8 10m
DAY 1
10m Dashed lines connect the same points in each mosaic.
Dunes move ~2.5m/hr Bars move ~10m/day. DAY 3
(Mohrig & Smith, 1996)
Punctuated transport of sand through the lower river, RK 40 During 2008 flood: ~40 % of sand transport as bed‐ material load, ~60 % measured in suspension At low flow all sand transport as bed‐ material load
Empire Reach (RK40)
Water Discharge
Sand Discharge
(m3/s)
(m3/hr)
April 2008
41,000 January 2008 10,000
3960 32.4
Distance Below Water Surface Flow Depth
Essentially all of the sand delivery to river mouth happens during floods Suspended Sand
April 2008
(J.Nittrouer, 2010)
Channel Radius of Curvature/Channel Width
Punctuated Sand Transport and Minimal Storage in Lower Mississippi River: Mixed Bedrock‐Alluvial Channel
500 m
~RK 160, 2003
= Exposed Pleistocene substrate
(J. Nittrouer et al., accepted)
Visit J. Nittrouer poster, T-12, to view the spatial accelerations during flood that minimize alluvial cover of river bed.
Punctuated sand transport drives growth of Mississippi River delta & others in GOM by channels that erode their beds & aggrade B their margins.
~ RK150
(J. Nittrouer et al., accepted)
A
Rip-up clasts of channel substrate in bed-material load
B A
13
Growth of Wax Lake Delta, Louisiana, by channels that erode their beds and aggrade their margins.
Pre-delta substrate
Visit J. Shaw poster, T-14, on Tuesday.
Moving from evaluation of Topographic Source-to-Sink System to Geologic Source-to-Sink System: Extraction of sediment tied to net subsidence Delta Lobes & Channel Avulsion of delta surface. Dynamic delta top is constructed via sedimentation tied to “active” channels. Subsurface preserves sufficient number of delta lobes and channels to constrain space-time-rate distributions of delta-building elements.
(Day et al., 2007)
Visit A. Petter poster, T-31, & K. Straub poster, T-27, on Tuesday.
To what degree is delta lobe sedimentation distinct from depositional patterns of other depositional systems?
t1
z x
t2
y
sedimentation pattern
t3
subsidence pattern
z y The transport system must rearrange itself in order to fill space produced by subsidence. (following method of Sheets et al, 2002; Lyons, 2004; Straub et al, 2009)
Sedimentation versus subsidence along cross‐section Fit between sedimentation and subsidence patterns can be measured by standard deviation in the following ratio…..
r (T ; x, y ) Sedimentation rate at a point for given time increment rˆ( x, y ) Long-term sedimentation rate at that same point
x, y = horizontal coordinates; A = area
Seismic Volume & Well Data Provide Best Possible Resolution of: 1) Spatially Varying Subsidence Field 2) Long-term Subsidence Rates 3) Arrangement of Channel-Fills
WesternGeco seismic survey Quantifying depositional process on delta by comparing short-term sedimentation patterns (B & C) against long-term pattern (A).
(Straub et al., 2009)
Depositional stacking patterns for Mississippi Delta and other sedimentary environments Depositional Systems
(Straub et al., 2009)
Deposit Thickness Decay in the standard deviation of sedimentation/subsidence collapses onto a single power-law trend when the measurement window is standardized by the mean channel depth of each system.
Thickness
deposit thickness ss 0.33 channel depth
0.75
Visit K. Straub poster, T-27.
Final Points on Modern Delta System (Topographic source-to-sink): 1. Transport in the MRD cannot be accurately predicted without proper treatment of the river plume and marine boundary condition. 2. Spatial changes in cross-sectional area of transporting flows drive sediment-transport patterns that cannot be accurately characterized using water discharge or contributing area. 60
water surface elevation at 5000‐10000 m3/s water surface elevation at 15000‐20000 m3/s water surface elevation at 30000‐35000 m3/s river bottom profile
50 40
m Elevation (m)
30 20 10 0 ‐10 ‐20 ‐30 ‐40
2008
0
100
200
300
400
500
600
700
800
900 1000 1100
Mississippi River Kilometer
Final Points on Geologic Delta System (Mass extraction Source-to-sink): 3. Power-law collapse in deposit-stacking data suggests that a stacking behavior ~midway between purely random & prefect compensation in a good starting point for any channelized depositional system. J. Nittrouer poster, T-12, M.Lamb poster, T-28, & K. Straub poster, T27.
