1 downstream hydraulic geometry of alluvial rivers pierre y. julien colorado state university new...
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DOWNSTREAM HYDRAULIC DOWNSTREAM HYDRAULIC GEOMETRY GEOMETRY
of of ALLUVIAL RIVERSALLUVIAL RIVERS
Pierre Y. Julien Pierre Y. Julien Colorado State University
New Orleans New Orleans December 2014
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Objectives
Discuss the downstream hydraulic geometry of alluvial rivers in terms of:
• Level I – Alluvial river equilibrium
• Level II – Spatial width changes
• Level III – Temporal width changes
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I – Alluvial river equilibrium
Objectives: Review the Downstream Hydraulic Geometry (DHG) equations for alluvial rivers
• Define Downstream Hydraulic Geometry
• Review DHG equations
• Compare with field measurements
• Discuss some limitations of DHG
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Cross-section geometry
Sand
GravelSilt
Wetted perimeter P
Bankfull widthTop width W
Q = 270 ft /s3
Area A
0 50 100 150 ft
Left bank
Mean flow
depth h
0
2
4
6
10
12
14
Ele
vat
ion
(ft
)
89.5 ft ASL
Flood plain
15 30 45 m
(1 m = 3.28 ft)
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A
Q = A V
Width W
Wetted perimeter P
Velocity V1
222
2
2
222
2
Mean flow depthh = A /W
Flood plainW 1
Flood plain
1h = A /W
2
P1
1 1
1V
Q = A V1 1
1A
1
Downstream Hydraulic Geometry
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Some DHG equations
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Julien-Wargadalam equations
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Bankfull width and depth
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Bankfull velocity and slope
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Performance of different equations
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I – Alluvial river equilibrium
Conclusion:• Downstream Hydraulic Geometry equations provide
very good first order approximations of alluvial river equilibrium conditions.
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Limitations of DHG equations
Limitations of the DHG equations include:
• What is the dominant, bankfull or effective discharge?
• What grain size (d50 or d90…) should be used?
• Can DHG equations predict meandering or braiding?
Is it also appropriate to ask:
• Do rivers have constant W, h or S?
• Does equilibrium exist?
Lets discuss further the width changes in space and time
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Objectives: illustrate the changes in river widths and explain how river width can decrease
• Illustrate channel width changes in space
• Explain the concept of equivalent channel width
• Show an example on the Rio Grande
II – Spatial width changes
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narrow
14
wide
Reach of the Rio Grande, NM
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Relationship between channel width and sediment transport
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Concept of equivalent widths
The concept of equivalent channel width stems from the decrease in sediment transport with increased channel width:
Hypothesis:
• The channel width of river reaches can be different from the equilibrium channel width from the DHG equations – however, to maintain the same sediment transport level, an increase in channel width requires an increase in channel slope.
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Example on the Rio Grande, NM
From Leon et al. ASCE-JHE 135(4), 2009
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Slope vs width
From Leon et al. ASCE-JHE 135(4), 2009
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Slope vs width-depth ratio
From Leon et al. ASCE-JHE 135(4), 2009
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Wider reaches are steeper!
From Leon et al. ASCE-JHE 135(4), 2009
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II – Spatial width changes
Conclusion:• Channel widths can be different from equilibrium
conditions, but to satisfy continuity in sediment transport, wider channels require steeper slopes.
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Objectives: explain how alluvial river widths change over time
• Illustrate that the channel widths can change with time
• Explain the concept of deviation from equilibrium
• Show and example on the Rio Grande
• Estimate the time scale for river width adjustments
III – Temporal width changes
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Temporal Changes in Hydraulic Geometry Rio Grande below Cochiti Dam, NM
Braiding Transition Meandering
1935 1972 1992
From Richard et al. ASCE-JHE 131(11), 2005
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Hydraulic Geometry of the Rio Grande
From Richard et al. ASCE-JHE 131(11), 2005
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CO-29
1,537
1,538
1,539
1,540
1,541
1,542
0 50 100 150 200 250Distance from left bank reference point (m)
Ele
vatio
n (m
) Sep-71
Sep-74
Oct-82
Nov-86
Aug-95
Aug-9843 km downstream from Cochiti Dam
How do rivers decrease their channel width?
1971 1971
1998 1998
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Rio Grande – note the channel width changes 1996-2009
1996
26
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Rio Grande
2005
27
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Rio Grande
2006
28
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Rio Grande
2009
29
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Temporal width changes
Hypotheses:
1. The alluvial channel width gradually adjusts towards the equilibrium width.
2. The annual change in channel width is proportional to the deviation from equilibrium, measured as the difference between the current channel width and the equilibrium channel width.
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0
50
100
150
200
250
300
350
400
0 100 200 300 400
Measured Active Channel Width (m)
Ca
lcu
late
d E
xpe
cte
d C
ha
nn
el W
idth
(m
) Ju
lien
-Wa
rga
da
lam
(1
99
5)
19181935194919621972198519922001
19182001
Hypothesis 1. Channel widths change towards equilibrium
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Hypothesis 2. Width changes are proportional to the deviation from equilibrium
equilibrium
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Changes in active channel width Rio Grande, NM (after Richard et al., 2005)
Reach No.Reach No. kk11 kk11WWee WWe e (m)(m) RR22
EntireEntire 0.02910.0291 1.65811.6581 7575 0.420.42
11 0.02710.0271 1.06071.0607 3939 0.180.18
22 0.03130.0313 3.16153.1615 101101 0.570.57
33 0.04370.0437 3.70703.7070 8585 0.580.58
Sub-reach 1, y = -0.0271x + 1.0607
R2 = 0.1799
Sub-reach 2, y = -0.0313x + 3.1615
R2 = 0.5729
Sub-reach 3, y = -0.0437x + 3.707
R2 = 0.581
- 25
- 20
- 15
- 10
- 5
0
5
10
0 100 200 300 400 500 600Active Channel Width (m)
Change in A
ctive C
hannel W
idth
(m
/year)
.
Sub- reach 1Sub- reach 2Sub- reach 3Linear (Sub- reach 1)Linear (Sub- reach 2)Linear (Sub- reach 3)
)(1 eWWkdt
dW
Change in active channel width
equilibrium
Deviation from equilibrium
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Entire reach
R2 = 0.97
0
50
100
150
200
250
300
350
1980 1990 2000 2010 2020 2030 2040 2050
Time (year)
Active c
hannel w
idth
(m
)
.
MeasuredEquilibrium WidthExpon. (predicted)Expon. (Measured)
Prediction of Active Channel Widthby Exponential equation (after Richard et al., 2005)
tke0e
1e)W(WWW
Equilibrium width
half adjustment
Half adjustmentTime scale 0.7/k1
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Conclusions:• Channel widths gradually change toward equilibrium
• The annual width change is proportional to the difference between the actual width and the equilibrium width
• The time required to reach equilibrium is asymptotic, but half the width adjustment can be reached in 0.7/k years, with for instance k ~ 0.025-0.045 on the Rio Grande.
III – Temporal width changes
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Conclusions
I - Downstream Hydraulic Geometry equations• Give very good first order approximations of alluvial river equilibrium
II - Spatial changes in channel widths• Channel widths can differ from equilibrium channel widths, and wider reaches
require steeper slopes
III - Temporal changes in channel widths• Channel widths gradually change toward equilibrium at a rate proportional to
the difference between the actual width and the equilibrium width
• The time required to reach equilibrium is asymptotic, but half the width adjustment can be reached in 0.7/k years, and k ~ 0.035 on the Rio Grande
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J. Wargadalam, CSU and DID IndonesiaG. Richard, CSU and Mesa State University
C. Leon, CSU and RTIU. Ji, Y.H. Shin and K.Y. Park, CSU and K-WATER
D.C. Baird, R. Padilla and J. Aubuchon, USBRJ.S. Lee, Hanbat University
So many others…
Acknowledgments
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ByenByen Mersi !Mersi !