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Morphodynamics of the North Fork Toutle River Near Mount St. Helens, Washington John Pitlick 1 , Jon Major 2 and Kurt Spicer 2 1/Geography Department, University of Colorado, Boulder, CO 80309 2/US Geological Survey Cascades Volcano Observatory, Vancouver, WA 98683. Methods, continued - PowerPoint PPT Presentation
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Morphodynamics of the North Fork Toutle River Near Mount St. Helens, Washington
John Pitlick1, Jon Major2 and Kurt Spicer2
1/Geography Department, University of Colorado, Boulder, CO 80309
2/US Geological Survey Cascades Volcano Observatory, Vancouver, WA 98683
Introduction
More than 25 years have elapsed since the eruption of Mt. St. Helens, yet the North Fork Toutle River
(NFTR) continues to carry probably the highest sediment loads of any river of comparable size in the
conterminous United States [Major et al., 2000]. Much of the sediment carried by the NFTR is derived
from the debris avalanche deposited during the May, 1980 eruption (Fig. 1). Alluvium stored in terraces
along the NFTR , as well as sediment stored behind an Army Corps of Engineers retention dam, represent
significant additional long-term sources of sediment.
AcknowledgementsField work for this project was completed while the first author was on sabbatical at the US Geological
Survey-Cascades Volcano Observatory; very little of the field work could have been completed without
the advice and logistical support of CVO staff. We would also like to acknowledge the people who
assisted us in the field, including Tom Hale, Dennis Saunders, and especially Rebecca Thomas.
ReferencesMajor, J.J., T.C. Pierson, R.L. Dinehart, and J.E. Costa, 2000, Sediment yield following severe volcanic disturbance- A
two-decade perspective from Mount St. Helens, Geology, v. 28, p. 819-822.Mueller, E. R., J. Pitlick, and J.M. Nelson, 2005, Variation in the reference Shields stress for bed load transport in gravel-bed streams and rivers, Water Resources Research, v. 41, W04006, doi:10.1029/2004WR003692Parker, G., Hydraulic geometry of active gravel rivers, J. Hydraul. Div. Am. Soc. Civ. Eng., 105, 1185-1201, 1979.
Figure 1. North Fork Toutle River near Mt. St. Helens, WA
Figure 2. Study site along the N. Fork Toutle River
Figure 4. Regional relations for mean annual flood as a function of (a) drainage area (b) main channel length.
Continued erosion in the headwaters of the NFTR will likely affect downstream reaches of the Toutle-
Cowlitz River system for decades to come, however, current rates of erosion and sediment transport are
poorly constrained, and the information needed to model the evolution of the system is lacking. This
poster summarizes data obtained in 2006 to (a) assess present-day trends in slope, grain size and channel
morphology, and (b) develop preliminary estimates of the bed load sediment yield of the upper NFTR.
97.5
98.0
98.5
99.0
99.5
100.0
100.5
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Distance (m)
Elevation (m)
NF 300
Figure 3. Cross section of the dominant channel.
1
10
100
1000
1 10 100 1000 10000
Q = 1.00*A0.87
R2= 0.62
Drainage Area (km2)
1
10
100
1000
1 10 100 1000
Length of Main Channel (km)
Q = 0.91*L1.38
R2= 0.58
Methods, continued
Hydrologic Data: To estimate channel-forming discharges we developed empirical relations between
high-flows and basin characteristics, using data from 40 gaging stations in SW Washington. Figure 4
shows relations between the mean annual flood and (a) drainage basin area and (b) channel length.
Flow and Sediment-Transport Calculations: At each site, we estimated the discharge corresponding to
the mean annual flood, and calculated the bed load transport rate for that discharge. Flow conditions
(width, depth and velocity) were determined by simultaneously solving the equations for continuity and
flow resistance, using the measured channel geometry, reach-average slope, S, and surface grain size:
U
u*
=2.5 ln11Hks
q* =11.2τ * −τ r
*( )4.5
τ 3τ r
* = 2.18S + 0.021Methods
Field Data: We measured channel characteristics,
reach-average gradient, and bed material grain size at
12 sites, spaced 2-4 km apart from the base of Mount
St. Helens to the toe of the 1980 debris avalanche- a
total distance of ~25 km. The sites were located in
single-thread reaches (Fig. 2) where we could
identify a dominant channel with a well-defined
cross-section (Fig. 3). We surveyed three cross
sections at each site using an engineering level and
stadia rod; average gradients were surveyed over
distances of 100-300 m using the same equipment.
The grain size distribution of the surface bed
material was determined from point counts of 300
particles selected at evenly spaced intervals along
transects on exposed gravel bars. Rock sizes were
measured with a gravelometer, or in the case of large
boulders with a measuring tape. Sand was included
in the point counts, but eliminated from the overall
grain size distribution
Q =BHU
Results
Channel Geometry and Grain Size: The slope of the NFTR decreases systematically downstream (with
no obvious knickpoints, Fig. 5a), whereas there is little change in the median grain size, D50 (Fig. 5b).
The channel width and depth likewise increase downstream (Fig. 5c and 5d), however, there is a sharp
change in depth below site 9, where two tributaries- Coldwater Creek and Castle Creek- join the NFTR.
where B is width, U is mean velocity, H is depth, u* = (gHS)1/2 is the shear velocity (all with respect to
the mean annual flood), and ks is the equivalent roughness (3D84). Transport rates were calculated with
the relation of Parker (1979), using a variable reference Shields stress, τ*r (Mueller et al. 2005):
Figure 5. Downstream trends in (a) slope, (b) median grain size, (c) channel width and (d) channel depth.
0.00
0.02
0.03
0.05
0.06
1 2 3 4 5 6 7 8 9 10 11 12Site
Slope
0.00
0.02
0.04
0.06
0.08
1 2 3 4 5 6 7 8 9 10 11 12Site
D50
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10 11 12Site
Width
0.4
0.6
0.8
1.0
1.2
1 2 3 4 5 6 7 8 9 10 11 12Site
Depth
Results, continued
Hydraulic Geometry: The downstream adjustments in width and depth of the NFTR scale remarkably
well with the mean annual flood (Fig. 6), and the exponents of the resulting power law relations for
hydraulic geometry are essentially identical to the commonly cited values, B = aQ0.5 and H = cQ0.4.
Figure 6. Downstream hydraulic geometry relations.
Downstream Trends in Shields Stress and Bed Load Transport Capacity: The figure below left shows
that the difference between the Shields stress at the mean annual flood and the Shields stress at the
approximate threshold for motion increases slightly downstream (Fig. 7a). This effect, coupled with the
increase in channel width and nearly constant grain size, lead to an almost linear relation between the
instantaneous bed load transport rate and discharge corresponding to the mean annual flood (Fig. 7b).
1
10
100
10 100
Bankfull Width (m) B = 2.24*Q0.49
R2= 0.62
Mean Annual Flood (m3/s)
0.1
1.0
10.0
10 100
Bankfull Depth (m)
Mean Annual Flood (m3/s)
H = 0.16*Q0.40
R2= 0.77
Figure 7. Downstream trends in (a) Shields stress and (b) instantaneous bed load transport rate.
0.00
0.10
0.20
0.30
0.40
tau*
tau*ref
5 10 15 20 25 30
Shields Stress
Distance (km)
Conclusions
The North Fork Toutle River has the opportunity to reshape its channel almost every year in response to
high flows. The field data and transport estimates presented here suggest that downstream adjustments
in slope, width and depth are consistent with the theory that a channel with unlimited sediment supply
and no constraints on width will shape itself to maintain a constant sediment concentration, Qs Q.
0.01
0.10
10 100
Bed Load Transport Rate (m
3 /s)
Discharge (m3/s)
Qb = 0.0015*Q1.05
R2= 0.33
BA
A BA B
A B
C D