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65
Italian Journal of Engineering Geology and Environment, 1 (2011)
© Casa Editrice Università La Sapienza
www.ijege.uniroma1.it
DOI: 10.4408/IJEGE.2011-03.B-078
S
COTT
W. McCOY
(*)
, J
EFFREY
A. COE
(**)
, J
ASON
W. KEAN
(**)
, G
REG
E. TUCKER
(*)
,
D
ENNIS
M. STALEY
(**)
& T
HAD
A. WASKLEWICZ
(***)
(*)
University of Colorado - CIRES & Department of Geological Sciences - Boulder, Colorado 80309, USA - Email: scott.mccoy@colorado.edu
(**)
U.S. Geological Survey - Denver Federal Center - MS 966, Denver, Colorado 80225, USA
(***)
East Carolina University - Department of Geography - Greenville, North Carolina 27858, USA
OSSERVAZIONI SUI DEBRIS FLOWS DI CHALK CLIFFS, COLORADO, USA: PARTE 1,
MISURE IN SITU DELLA DINAMICA DI FLUSSO, TRACCIANTI DEL
MOVIMENTO PARTICELLARE E VIDEO IMAGERY DELL’ESTATE 2009
(‡)
OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO, USA: PART 1,
IN SITU MEASUREMENTS OF FLOW DYNAMICS, TRACER PARTICLE MOVEMENT
AND VIDEO IMAGERY FROM THE SUMMER OF 2009
(‡)
ABSTRACT
Debris fl ows initiated by surface-water runoff during short dura-
tion, moderate- to high-intensity rainfall are common in steep, rocky,
and sparsely vegetated terrain. Yet large uncertainties remain about the
potential for a fl ow to grow through entrainment of loose debris, which
make formulation of accurate mechanical models of debris-fl ow rout-
ing diffi cult. Using a combination of in situ measurements of debris-
fl ow dynamics, video imagery, tracer rocks implanted with passive
integrated transponders (PIT) and pre- and post-fl ow 2-cm resolution
digital terrain models (terrain data presented in a companion paper by
S
TALEY
et alii, 2011), we investigated the entrainment and transport
response of debris fl ows at Chalk Cliffs, CO, USA. Four monitored
events during the summer of 2009 all initiated from surface-water run-
off, generally less than an hour after the fi rst measurable rain. Despite
reach-scale morphology that remained relatively constant, the four
fl ow events displayed a range of responses, from long-runout fl ows
that entrained signifi cant amounts of channel sediment and dammed
the main-stem river, to smaller, short-runout fl ows that were primarily
depositional in the upper basin. Tracer-rock travel-distance distribu-
tions for these events were bimodal; particles either remained immo-
bile or they travelled the entire length of the catchment. The long-
runout, large-entrainment fl ow differed from the other smaller fl ows
by the following controlling factors: peak 10-minute rain intensity;
duration of signifi cant fl ow in the channel; and to a lesser extent, peak
surge depth and velocity. Our growing database of natural debris-fl ow
events can be used to develop linkages between observed debris-fl ow
transport and entrainment responses and the controlling rainstorm
characteristics and fl ow properties.
K
EY
WORDS
: debris fl ow, initiation, entrainment, sediment transport, monito-
ring, instrumentation, runoff, tracer particles
INTRODUCTION
Debris fl ows are rapidly gaining recognition as a serious hazard
to expanding development, as important agents of mountain land-
scape evolution, and as a signifi cant mode of sediment transport to
rivers in rugged terrain. This recognition has made apparent the need
for complete and quantitative datasets from well-constrained natural
debris fl ows to test theoretical and computational approaches, con-
fi rm experimental results and stimulate new ideas.
In steep, rocky, and sparsely vegetated terrain, debris fl ows initi-
ated by surface-water runoff in response to short duration, moder-
ate- to high-intensity rainfall are common (e.g. F
RYXELL
& H
ORBERG
,
1943; W
OHL
& P
EARTHREE
, 1991; M
EYER
et alii, 1995; C
ANNON
et alii,
2001; B
ERTI
& S
IMONI
, 2005; L
ARSEN
et alii, 2006; C
OE
et alii, 2008).
Runoff-initiated debris fl ows can form from small quantities of rain,
with very short lag times from the beginning of the rainstorm to the
beginning of the debris-fl ow event (B
ERTI
et alii, 2000). They start as
water-rich fl ows that rapidly entrain and concentrate large quantities
of debris and sediment into hazardous debris-fl ow fronts (C
OE
et alii,
2008) and can have peak discharges many times that of a comparable
water-fl ood (V
ANDINE
, 1985). Yet, for this type of debris fl ow, large
uncertainties remain about the mechanics of initiation and the poten-
tial for a fl ow to grow through entrainment of loose debris. These un-
certainties make it diffi cult to formulate accurate mechanical models
of debris-fl ow initiation, routing, and deposition in natural settings
and thereby add large uncertainties to hazard assessments.
To relate the entrainment and transport response of a debris-fl ow
event to potential controlling factors such as physical fl ow proper-
ties, rainstorm characteristics, and basin topographic parameters we
used a combination of in situ measurements of debris-fl ow dynamics,
video imagery, tracer rocks implanted with passive integrated trans-
ponders (PIT) and pre- and post-fl ow 2-cm resolution digital terrain
models acquired through terrestrial laser scanning (TLS). Terrain data
are presented in a companion paper in this volume by S
TALEY
et alii.
This combination of readily available technologies captured a com-
prehensive record of the four debris-fl ow events that occurred during
the summer of 2009 in a natural debris-fl ow basin at Chalk Cliffs,
Colorado, USA. We documented the initial and fi nal topographic
state, the hydrologic conditions leading up to the event, net transport
distances of tracer particles and continuous time series of physical
fl ow properties and fl ow dynamics for each debris-fl ow event.
In this paper and the companion paper by S
TALEY
et alii (2011),
(‡)
This work is part 1 of the research presented at the 5
th
International Con-
ference on Debris-Flow: Mitigation, Mechanics and Assessement Prediction-
Padua, 14-17 June 2011-Italy
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OSSERVAZIONI SUI DEBRIS FLOWS DI CHALK CLIFFS, COLORADO, USA: PARTE 1,
MISURE IN SITU DELLA DINAMICA DI FLUSSO, TRACCIANTI DEL MOVIMENTO PARTICELLARE E VIDEO IMAGERY DELL’ESTATE 2009
66
S.W. McCOY, J.A. COE, J. W. KEAN, G.E. TUCKER, D.M. STALEY
& T.A. WASKLEWICZ
downstream from the drainage divide. The middle station is 92 m
downstream of the upper station and 54 m downstream of the east-
west channel junction. The lower station is 319 m downstream of the
middle station and is 220 m from the mouth of the basin and the apex
of the depositional fan (Fig. 1H).
As of May 2009 each station was equipped with: an ultrasonic
stage sensor (accuracy +/- 1 cm) suspended over the channel to meas-
ure the sediment and/or fl ow height above a fi xed datum (the datum is
the stable bedrock channel at the upper and middle stations and a buried
pressure transducer at the lower station); one or two unvented pressure
transducers (accuracy +/- 3 mm water pressure head) to measure the
pore-fl uid pressure at the bedrock-sediment interface (upper and mid-
dle stations) or at ~ 0.25 m depth in the bed sediment (lower station); a
siphoning rain gauge (accuracy +/- 2%); and a temperature sensor (see
M
C
C
OY
et alii, 2010 for complete sensor specifi cations). All these sen-
sors were sampled every 2 seconds during a rainstorm.
At the upper station only, we mounted a 15.24 cm by 15.24 cm
(232 cm
2
) metal plate attached to a single-axis load cell (accuracy +/-
2.5 kg) fl ush with the bedrock channel to measure total normal stress
at the sediment-bedrock interface. This load cell was sampled at 250
Hz through June 2009; we subsequently reduced the sampling rate to
100 Hz. Additional measurements of rainfall and soil moisture (ac-
curacy +/- 5% volumetric water content) were made on adjacent hills-
lopes within the basin and within the channel below the middle station.
Near-realtime data and a daily photograph from the upper station
were sent to a web server using a cell phone modem. This system
allowed us to know when a debris-fl ow event had occurred (http://
landslides.cr.usgs.gov/monitoring/chalk_cliffs/index.php
).
METHODS
TRACER ROCKS
We implanted 32-mm passive integrated transponders (PIT) in
99 rocks to measure the travel distances of cobbles during a debris-
fl ow event. PIT tags “labelled” each rock with a unique identifi ca-
tion number. Using radio-frequency identifi cation (RFID) technology,
we could fi nd and identify these tracer rocks, buried up to ~ 0.25-0.5
m depth, without disturbing the bed surface (B
RADLEY
, 2010). We se-
lected the tracer rocks from the area surrounding the upper station.
Rock strength determined the size of selected rocks; rocks that were
too small broke when drilled with the hammer drill. The mean length
of the a, b, and c axes of the tracer rocks were 13 cm, 9 cm, and 6 cm,
respectively. For comparison, the mean length of the a, b, and c, axes
of 100 randomly selected rocks from an October 2008 debris-fl ow de-
posit forming the levees and bed at the upper station were 10 cm, 7 cm,
and 4 cm, respectively. We prepared the tracer rocks by drilling a 4.76
mm diameter hole with a hammer drill, inserting a 32 mm long PIT tag
and then sealing the hole with marine epoxy. To aid in the recovery of
surface rocks and the initial survey, we spray-painted all tracers red.
We then placed the rocks in groups of three, along the centerline of the
channel, in the reach by the upper station, with each group separated
we compare the four debris-fl ow events that occurred during the sum-
mer of 2009 (2 June, 26 July, 6 September, 15 September) to highlight
dramatic differences in entrainment and transport behavior possible
from a single basin with similar topographic initial conditions for
each event, yet driven by rainstorms of different intensities and dura-
tions.
CHALK CLIFFS STUDY SITE
CATCHMENT DESRIPTION AND GEOLOGIC SETTING
The Chalk Cliffs study area is located on the southern fl ank of
Mount Princeton in the Sawatch Range of Central Colorado, USA.
The steep, 0.3 km
2
semi-arid basin is incised into pervasively frac-
tured and hydrothermally altered quartz monzonite, adjacent to the
range-bounding Sawatch normal fault (M
ILLER
, 1999). The steep
headwaters of the basin are exposed bedrock (generally > 45°) (Fig.
1A). Unconsolidated colluvium and debris-fl ow deposits (0°-45°)
cover the remaining 40% of the basin (C
OE
et alii, 2008). Between
debris-fl ow events dry ravel from the steep colluvium and rock fall
from the bedrock cliffs rapidly fi ll the channels with debris. Bedrock
slopes in the basin are devoid of vegetation, and colluvial slopes are
sparsely vegetated. This basin has a high frequency of debris fl ows
(one to four events per year between the months of May and October)
generated by rainfall related surface-water runoff (C
OE
et alii, 2008),
making it ideal for long-term monitoring. Basin topographic charac-
teristics are summarized in Table 1. A more complete description of
the geologic setting and site selection can be found in earlier work
(C
OE
et alii, 2008, C
OE
et alii, 2010).
MONITORING SYSTEM
The automated monitoring system, established through collab-
oration between the U.S. Geological Survey and the University of
Colorado, consists of three instrumented cross sections (upper, mid-
dle and lower stations) and two video cameras fi lming at 2 frames per
second (one fi lming the middle station, the other fi lming the upper
station) (Fig. 1A-D and H). The upper station is located in the west
channel, 38 m above the junction with the east channel and ~590 m
Tab. 1 - Chalk Cliffs study basin topographic characteristics at each moni-
toring station shown in Fig. 1A and H. The Chalk Creek location is
on the fan where the debris-fl ow channel enters Chalk Creek (not a
station). Elevation, slope, and length parameters correspond to the
west channel profi le in Figure 1H (inset). Local slopes at the sta-
tions were taken over 10-horizontal meters
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OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO, USA: PART 1, IN SITU MEASUREMENTS OF FLOW DYNAMICS,
TRACER PARTICLE MOVEMENT AND VIDEO IMAGERY FROM THE SUMMER OF 2009
67
Italian Journal of Engineering Geology and Environment, 1 (2011)
© Casa Editrice Università La Sapienza
www.ijege.uniroma1.it
Fig. 1 - Chalk Cliffs study area with photographs of the monitoring stations and shaded relief images (from 1 m ALSM data provided by NCALM) with tracer-
particle positions marked by white circles. White rectangle in H encompasses zoomed-in view shown in E, F, and G. A) Line drawing of study basin show-
ing station locations and the distribution of bedrock and colluvium (redrawn from M
CCOY
et alii, 2010) B) Upper station with sensors. C) Middle station.
D) Lower station. E) Starting tracer-particle positions 20 May 2009. F) Positions 11 June 2009, 9 days after 2 June 2009 event. G) Positions 4 August
2009, 9 days after 26 July 2009 event. H) Positions 26 September 2009, 11 days after 15 September 2009 event (note no survey was completed after the 06
September 2009 event). Inset: Long profi le of east and west channel
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OSSERVAZIONI SUI DEBRIS FLOWS DI CHALK CLIFFS, COLORADO, USA: PARTE 1,
MISURE IN SITU DELLA DINAMICA DI FLUSSO, TRACCIANTI DEL MOVIMENTO PARTICELLARE E VIDEO IMAGERY DELL’ESTATE 2009
68
S.W. McCOY, J.A. COE, J. W. KEAN, G.E. TUCKER, D.M. STALEY
& T.A. WASKLEWICZ
by ~2 m (Fig. 1E), and surveyed their positions using a total station.
After each debris-fl ow event, we searched the entire travel-path
of the debris fl ow using a 0.5 m diameter RFID detection antenna.
When a tracer rock was passed over by the antenna, the capacitor in
the PIT tag was charged, which enabled the tag to transmit its unique
identifi cation number to the receiver to be recorded. Once the tracer
rock was recorded, we surveyed its location using a total station.
Many of the tracer rocks were buried below the deposit surface and
never visually identifi ed. The resulting error, +/- ~0.5 m, in the sur-
veyed position was primarily due to the read-range of the antenna.
To measure the total curvilinear distance travelled (streamwise
and cross-stream) by a tracer rock during a single debris-fl ow, we
projected tracer-particle positions onto the longitudinal profi le of the
debris-fl ow channel. The longitudinal profi le was extracted from a 1
m digital elevation model derived from airborne laser swath mapping
(ALSM) data collected by the National Center for Airborne Laser
Mapping (NCALM). We then calculated along slope travel distance
by differencing the post-event and pre-event slope distance along the
longitudinal profi le. All tracer rocks that moved less than 1 m were
put in the 1 m bin for plotting on the log-log scale in Fig. 2.
STATION-DATA PROCESSING
To remove much of the +/- 2.5 kg noise in the force time series,
we downsampled the 250 Hz and 100 Hz force plate data to 10 Hz by
binning data in 0.1 s bins and taking the mean of each bin. The stress
axis in Figs. 3-6 was scaled such that stage H and basal stress σ su-
perpose if σ
=
ρgH cos(θ) with ρ=2040 kg/m
3
, where ρ is the wet bulk
density of the sediment bed and fl ow combined, g is the gravitational
acceleration, H is the bed-normal height of material above the force
plate, equal to stage, and θ is the bed inclination (i.e., σ plots above
H when ρ
actual
>2040 kg/m
3
and σ plots below H when ρ
actual
<2040 kg/
m
3
). Note that accurate bulk densities can only be determined from
this method when the assumption of a lithostatic stress state is valid.
The load cell was buried beneath sediment for the majority of
2009, which precluded taking a new tare value before each event.
To minimize error associated with a tare value that is a function of
environmental factors (primarily temperature), we took advantage
of the fact that before rainstorms the bed sediment was dry (~10%
volumetric water content) and thus should have a relatively constant
bulk density before each event. We set the force tare value using a
pre-storm dry bed sediment bulk density of 1750 kg/m
3
; this value
was determined just after installation. We estimated the approximate
uncertainty resulting from this assumption as +/- 100 kg/m
3
or +/- 500
Pa if 0.5 m of sediment is present.
We fi ltered the 2 s ultrasonic stage data based on echo return
strength to remove low strength returns. Persistent increases or de-
creases in stage were due to sediment deposition or erosion.
To measure pore-fl uid pressure using the unvented, temperature-
compensated pressure transducers, we subtracted the pre-storm pres-
sure (i.e., atmospheric pressure) from the total pressure. For most of
the events, the bed sediment between the fl ow and the pressure trans-
ducer was not saturated. Given these conditions, the pressure trans-
ducer did not measure positive pressure head due to the overriding
fl ow (not shown in our fi gures) or the pressure head was attenuated.
The only exception to these behaviours was recorded at the middle
station on 2 June.
We calculated surge front velocities (each event was composed
of multiple surge fronts) by manually tracking surge front positions
in the spatially referenced video imagery recorded by the upper sta-
tion camera. Surge-front velocities Ū were then plotted as a function
of maximum surge depth h to develop a rating curve for the upper
Fig. 2 - Histograms of tracer-particle travel distance between consecutive
surveys. Solid line is the travel distance from the mean starting posi-
tion on 20 May to the junction with the perennial stream Chalk Creek.
Dashed line is the distance travelled by the fl ow front. In C, both lines
overlap. A) Distance travelled due to 2 June 2009 event. B) Distance
travelled due to 26 July 2009 event. C) Distance travelled due to com-
bination of 6 September 2009 and 15 September 2009 events
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OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO, USA: PART 1, IN SITU MEASUREMENTS OF FLOW DYNAMICS,
TRACER PARTICLE MOVEMENT AND VIDEO IMAGERY FROM THE SUMMER OF 2009
69
Italian Journal of Engineering Geology and Environment, 1 (2011)
© Casa Editrice Università La Sapienza
www.ijege.uniroma1.it
station (ū
=
3.6h
1/2
). To calculate maximum fl ow depth, the height of
the bed sediment and the maximum stage during the surge must be
known. We used the period of no or little fl ow in between successive
surges to determine the bed height through time. We automated this
procedure by taking a moving-window minimum of the stage data,
with a window width equal to the maximum surge duration for each
event. It has been demonstrated by others (P
IERSON
, 1986; S
UWA
et
alii, 1993; A
RATTANO
et alii, 2000) that the peak fl ow velocity can
occur after the peak fl ow depth and coarse-grained fl ow front have
passed. Since many of the surges were of short duration and we only
measured the surge front velocities, we did not detect this phenom-
ena, but if present, it would make our rating curve underestimate the
actually velocities in the more shallow, dilute portions of the surge.
For each debris-fl ow event, we calculated an estimate of the total
event volume passing the upper station (sediment plus water) using
where the summation was taken from the beginning of the fl ow event
t
i
to the end t
f
,
ū
is the mean cross sectional velocity from the rating
curve, A(t) is the current active cross sectional area at a given time
calculated using the surveyed bedrock cross section, the current bed
height, and current fl ow depth, and Δt is the length of time between
two stage measurements (typically 2 s).
To determine the total time spent above certain transport stages
during the coarse of a debris-fl ow event we summed the total time
over which the fl ow depth was greater than 0.1 m, 0.2 m, and 0.3 m.
RESULTS AND DISCUSSION
Monitoring results from the four debris-fl ow events indicated that
2 June and 26 July events had relatively short travel distances, stayed
within the basin and were primarily depositional in nature. In sharp
contrast, the 15 September event was primarily erosional in nature
and traveled all the way to the modern fan at Chalk Creek, where it
temporarily dammed the stream. The 6 September event was interme-
diate to these two end members, exiting the basin, but not reaching
Chalk Creek. Below, we fi rst provide details on the tracer-particle
travel distance distributions and then describe the conditions that
caused this dramatic difference in entrainment and transport behavior
between the 4 events.
TRACER ROCKS
Of the initial 99 PIT-tagged tracer rocks (Fig. 1E), the number of
tracer rocks with known pre-event and post-event positions was 84,
68, and 33 after the 2 June, 26 July, and 15 September events, respec-
tively. We were not able to conduct a survey after the 6 September
event do the rapid arrival of the 15 September event. These recovery
rates are much smaller than those obtained using the same techniques
and technology in a fl uvial environment, where the rates can be up to
98% (B
RADLEY
, 2010). At Chalk Cliffs, the high losses were due to
burial of many of the tracers, especially in the 15 September event,
to depths greater than the read range of the antenna (~0.25-0.5 m).
In our early surveys, we found most of the tracer rocks in the
seed reach near their origins (Fig.1 E-G). However, our fi nal survey
on 26 September 2009, following the 15 September event, showed
that the highest concentration of tracer rocks had shifted ~1.6 km
downstream to the fan at the junction of the debris-fl ow channel and
the perennial stream Chalk Creek (Fig. 1H). The 6 September event
did not reach county road 162 (Fig. 1H), however 67% of tracer par-
ticles were found below the road on 26 September. Because extended
sediment transport was due to the 15 September event, and only small
amounts of erosion were measured at the monitoring stations after the
6 September event (Fig. 5), we grouped the 6 September event with
the earlier lower-transport events.
The fraction of particles that moved greater than 1 m after the 2
June, the 26 July, and the 15 September events were 67%, 25% and
94%, respectively. Distributions of travel distance are plotted in Fig.
2 and summarized in Table. 2. Travel distances measured after the 2
June event and the 26 July event (Fig. 2A and B) differ greatly from
the distribution for the 15 September event (Fig. 2C). Travel distances
< 1 m dominate the distributions for the low transport events (Fig. 2A
and B), whereas travel distances > 1500 m dominate the distribution
for the 15 September event (Fig. 2C).
The maximum possible travel distance for a tracer particle trans-
ported by a debris fl ow is set by its starting position and the down-
stream end of the debris-fl ow dominated portion of the catchment,
which in this case is near the junction of the debris-fl ow channel and
the perennial stream Chalk Creek (which fl ows down a low-gradient
glacial valley). The distance from the mean tracer starting position
on 20 May to the end of the debris-fl ow dominated system (the junc-
tion of the debris-fl ow channel and Chalk Creek) was 1620 m. For
the 2 June and 26 July events, the mean travel distances were 10 m
and 4 m, or only 0.6% and 0.2% of the channel length dominated
by debris fl ows. The mean tracer travel distance was also consider-
ably shorter than the maximum fl ow-front travel distance for these
two fl ows, which was on the order of 300 m. In contrast, the mean
tracer travel distance for the 15 September event, which had a maxi-
mum fl ow-front travel distance of 1620 m, was 1100 m or 68% of the
1620 m debris fl ow dominated channel length. Sixty-seven percent
of the recovered population travelled a distance equal to the debris
fl ow dominated channel length in one event. Moreover, both the
mean travel distance of 1100 m and the percent of particles that trav-
elled the entire length of the debris fl ow dominated channel length
are underestimated for the 15 September event due to undercount-
ing of tracer particles. Upstream of the fan, few debris-fl ow deposits
had depths greater than the read range of the antenna (S
TALEY
et alii,
2011), whereas the reach from county road 162 to Chalk Creek had
signifi cant areas of aggradation greater than 2 m. Had all particles
been found, the measured 15 September distribution would be even
more heavily skewed toward travel distances closer to the length of
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OSSERVAZIONI SUI DEBRIS FLOWS DI CHALK CLIFFS, COLORADO, USA: PARTE 1,
MISURE IN SITU DELLA DINAMICA DI FLUSSO, TRACCIANTI DEL MOVIMENTO PARTICELLARE E VIDEO IMAGERY DELL’ESTATE 2009
70
S.W. McCOY, J.A. COE, J. W. KEAN, G.E. TUCKER, D.M. STALEY
& T.A. WASKLEWICZ
the debris fl ow dominated portion of the catchment.
Travel-distance distributions were bimodal: particles either re-
mained nearly immobile or they travelled the entire length of the de-
bris fl ow dominated portion of the catchment (Fig. 2). Although we
only have tracer data from three events, if this bimodal distribution
is characteristic of travel distances for particles transported by debris
fl ows, it sets them apart from many other modes of sediment trans-
port, in which the single-event travel distance is often a small fraction
of the system size (B
RADLEY
, 2010). The existence of particle travel
distances that are equal to the system size calls into question the abil-
ity to write sediment transport equations that depend only on local
topographic and fl ow parameters (T
UCKER
& B
RADLEY
, 2010).
STATION DATA
Monitoring station data for the four debris-fl ow events are shown
in Figs. 3-6. Key storm characteristics, fl ow properties, and tracer
Fig. 3 - Sensor data 2 June 2009. A) Upper axis: 10-minute rainfall intensi-
ties measured at the upper station. Lower axis: Complete event dura-
tion measured at the upper and middle station. Flows stopped up-
stream of the lower station. The fi rst debris fl ows initiated in the west
channel, arrived at the upper station at 16:22:54 and stopped before
reaching the middle station. The second set of debris fl ows initiated
in the east channel and arrived at the middle station at 17:09:14.
Pre-fl ow sediment depth at the upper station was 0.47 m and 0.26 m
at the middle station. Net change in sediment depth was +0.54 m and
+0.78 m at the upper and middle station, respectively. Due to rain at
11:00, pre-event pore pressures were not zero. B) 5-minute time slice
encompassing the arrival of the fi rst 5 debris-fl ow surges at the upper
station. C) 20-minute time slice from the middle station
Fig. 4 - Sensor data 26 July 2009. A) Upper axis: 10-minute rainfall intensities
measured at the upper station. Lower axis: Complete event duration
measured at the upper and middle station. Flows stopped upstream
of the lower station. The fi rst surge front reached the upper station at
7:12:42 and the middle station at 7:17:32. Pre-fl ow sediment depths
at the upper and middle stations were 1.04 m, and 1.01 m, respectively.
Net changes in sediment depth at the upper and middle stations were
+0.19 and +0.07 m, respectively. B) 25-minute time slice from the up-
per station. C) 25-minute time slice from the middle station
background image
OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO, USA: PART 1, IN SITU MEASUREMENTS OF FLOW DYNAMICS,
TRACER PARTICLE MOVEMENT AND VIDEO IMAGERY FROM THE SUMMER OF 2009
71
Italian Journal of Engineering Geology and Environment, 1 (2011)
© Casa Editrice Università La Sapienza
www.ijege.uniroma1.it
travel distances are summarized in Table. 2. By comparing the meas-
ured rainfall and fl ow properties between the four 2009 events, we
can gain insight into why the September 15 fl ows were so much more
effective, in terms of travel distance, clast transport, and bed entrain-
ment, than the previous three events.
RAINFALL
Cumulative rainfall amounts since the beginnings of each debris-
fl ow producing storm event are plotted as a dashed line in panel A
of Figs. 3-6. On the upper axis of panel A in Figs. 3-6, 10-minute
rainfall intensities are plotted. With exception of the 2 June event,
all storms had durations less than 2 hours. The 15 September event
had the largest cumulative storm rainfall (24.6 mm) and the highest
peak 10-minute intensity (38.1 mm/hr). The 6 September event had
the smallest storm total (6.3 mm), and the 2 June event had the low-
est peak 10-minute intensity (9.1 mm/hr). The fi rst debris-fl ow surge
generally reached the upper station in less than 45 minutes after the
onset of rain. There was a poor correlation in time between the storm
Fig. 5 - Sensor data 6 September 2009. a) Upper axis: 10-minute rainfall inten-
sities measured at the upper station. Lower axis: Complete event dura-
tion measured at the upper, middle and lower stations. Stage at middle
station shifted -0.3 m in all plots. The fi rst surge front reached the upper,
middle and lower stations at 14:05:18, 14:06:38, and 14:09:41, respec-
tively. Pre-fl ow sediment depths at the upper, middle and lower stations
were 1.14 m, 1.06 m, and 0.22 m, respectively. Net change in sediment
depth at the upper, middle and lower stations were -0.04 m, - 0.06 m,
and 0.0 m, respectively. b) 6-minute time slice from the upper station. c)
10-minute time slice from the upper, middle and lower stations
Fig. 6 - Sensor data 15 September 2009. A) Upper axis: 10-minute rainfall
intensities measured at the upper station. Lower axis: Complete event
duration measured at the upper station. B) Complete event duration
measured at the middle and lower stations. Stage at middle station
shifted +0.2 m. The fi rst surge front reached the upper, middle and
lower stations at 17:38:18, 17:38:40, and 17:42:41, respectively. Pre-
fl ow sediment depths at the upper, middle and lower stations were 1.1
m, 1.0 m, and 0.22 m, respectively. Net changes in sediment depth at
the upper, middle and lower stations were -1.1 m, -0.28 m, and +0.14
m, respectively C): 20-minute time slice from the upper station
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OSSERVAZIONI SUI DEBRIS FLOWS DI CHALK CLIFFS, COLORADO, USA: PARTE 1,
MISURE IN SITU DELLA DINAMICA DI FLUSSO, TRACCIANTI DEL MOVIMENTO PARTICELLARE E VIDEO IMAGERY DELL’ESTATE 2009
72
S.W. McCOY, J.A. COE, J. W. KEAN, G.E. TUCKER, D.M. STALEY
& T.A. WASKLEWICZ
maximum 10-min intensity and fl ow initiation, but a tight correlation,
often less than a few minutes, with local peaks in 10-minute rainfall
intensity and fl ow initiation (Figs. 3-6). The short temporal lag be-
tween the beginning of rain and the fi rst debris-fl ow surge (as short as
10 minutes) and the close correlation in time with local peaks in the
10-minute rainfall intensity reinforce the role of surface-water runoff
as the debris-fl ow generation process, as opposed to longer response
time, infi ltration related, land sliding. Similar short response times
from the beginning of rain to the initiation of a debris fl ow have been
observed at other fi eld sites (e.g. B
ERTI
et alii, 2000)
To quantify the role of cumulative rainfall in determining event
volume, we normalized the event volume (sediment plus water)
measured at the upper station by total rain volume that fell upstream
of the upper station (calculated as the product of upstream accumula-
tion area and measured rainfall). This ratio was not constant (range of
6% to 57%) with the large 15 September event having a ratio of 54%
and the smallest (2 June event) having 6% (Tab. 2). During small fl ow
events, a larger part of the water potentially available for overland
fl ow and debris transport was not measured. In larger events, a greater
part of available water passed the station as surface fl ow of measur-
able depths, often as debris fl ows or water-dominated fl ow vigorously
transporting sediment.
VIDEO OBSERVATIONS
Although each debris-fl ow event varied in size and duration, all
events consisted of multiple surges with distinct longitudinal sort-
ing of sediment grainsize. Each surge began with a steep and deep
granular front, composed of boulders and other coarse-grained mate-
rial (often without visible interstitial fl uid), which was followed by a
shallower, water-rich tail of relatively fi ne-grained material. The fi n-
er-grained tail rapidly changed from a mud-rich slurry of intermedi-
ate thickness with a moderate concentration of visible coarse-surface
clasts directly behind the granular front, to a shallow, water-domi-
nated, turbulent fl ow in which intense bed load transport was evi-
dent. The duration and depth of the water-dominated tail fl ows varied
greatly. In small events like that of 2 June, the water- dominated fl ow
between surge fronts was a trickle, compared to the 15 September
event in which the water-dominated fl ow vigorously transported sedi-
ment between surge fronts and continued transporting and eroding
sediment for tens of minutes after the last well-developed granular
surge front had passed. Flow front velocities measured at the upper
station had a well-defi ned h
1/2
scaling, and were best fi t by
ū
=
3.6
h
1/2
. We measured maximum front velocities of 3.7 m/s during the 15
September event.
STAGE
Stage data from the upper, middle and lower stations are shown
in Figs. 3-6 for all four events. Neither the 2 June event nor the 26
July event reached the lower station. Both of these events had maxi-
mum fl ow depths < 0.35 m at the upper and middle stations and peak
discharges at the upper station less than 2 m
3
/s. The two events that
travelled past the lower station, 6 September and 15 September, had
maximum fl ow depths >= 0.45 m at the upper and middle stations and
peak discharges at the upper station > 5 m
3
/s. Peak fl ow depth of the
15 September event was not consistently larger than the 6 September
event (Tab. 2).
Sediment deposition occurred at the upper and middle station
during the short-runout fl ows of 2 June and 26 July (Tab. 2). In con-
trast, the 6 September event eroded the bed slightly (<= 0.08 m) at the
upper and middle stations. The 15 September event eroded to bedrock
at the upper station (1.1 m) and eroded signifi cantly at the middle
(0.34 m) and lower stations (0.11 m).
Tab. 2 - Summary of rainstorm characteristics, fl ow properties, and tracer travel distances for the four debris-fl ow events in 2009. Front velocities measured from
video at the upper station are denoted with Vus. Front velocities calculated from travel time between the middle station and the upper station are denoted
with Vtt. US, MS, LS are upper station, middle station and lower station, respectively
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OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO, USA: PART 1, IN SITU MEASUREMENTS OF FLOW DYNAMICS,
TRACER PARTICLE MOVEMENT AND VIDEO IMAGERY FROM THE SUMMER OF 2009
73
Italian Journal of Engineering Geology and Environment, 1 (2011)
© Casa Editrice Università La Sapienza
www.ijege.uniroma1.it
The volume (both sediment and water) of each fl ow measured
at the upper station varied signifi cantly between the long-transport
15 September event and the other shorter-transport events. The 15
September event had an event volume at the upper station of ~820 m
3
,
which was ~4 to 23 times larger than the other events (Table 2). Note
that these volumes are not totals for each event because material from
the other main tributary (east channel) joins the west channel down
slope of the upper station. We did not calculate volumes at the middle
station because the poor spatial referencing of video imagery there
precluded accurate velocity measurements.
The total length of time that the measured fl ow depth was great-
er than 0.1 m, 0.2 m, or 0.3 m over the coarse of each debris-fl ow
event varied signifi cantly between large and small transport events
(Tab. 2). The total time elapsed while the measured fl ow depth was
greater than 0.1 m, 0.2 m, or 0.3 m was largest for the 15 Sep-
tember event. Although the 6 September event had surges of com-
parable depth to the 15 September event, there were fewer surges
and the shallow fl ow between surge fronts only lasted for a short
time. The short duration of shallow fl ow observed during the 6 Sep-
tember event is in strong contrast to the 15 September event where
there were sustained periods of fl ow greater than 0.1 m between
the deeper surge fronts. This indicates that total event volume can
be strongly infl uenced not only by the largest surges, but also by
sustained periods of moderate fl ow.
For the two larger events, September 6 and September 15, we
observed changes in surge characteristics as the fl ows moved down-
stream past the three stations (Fig. 5 and 6). The coalescence of small
surges into larger, longer-duration surges is particularly pronounced in
the 6 September event (Fig. 5C). During the 15 September event, the
opposite behavior occurred after 17:50. At this time, fl ows measured at
the upper and middle stations became water-dominated and lacked the
deep surge fronts measured during the previous 10 minutes. However,
by the time these moderate-depth, water-dominated fl ows reached the
lower station, deep surges had developed (Fig. 6). Although there was
a range in surge depth (0.31 m to 1.1 m at the upper station) almost all
surges had the asymmetric shape characteristic of debris fl ows, with a
steep and deep front followed by a more gently decreasing tail.
TOTAL NORMAL BASAL STRESS
Total normal basal stress varied in phase with stage, with no con-
sistent time lag observed between changes in stage and changes in
stress. Assuming a one-dimensional static stress state, we calculated
that the mean wet bulk densities of the fl ows ranged between 1400
and 2200 kg/m
3
(see M
C
C
OY
et alii, 2010 for an explicit density time
series). We measured denser mixtures near the surge front (1800-2200
kg/m
3
), and less dense mixtures in the water-dominated tails (1400-
1800 kg/m
3
). A similar range of densities have been measured in other
fi eld and large-scale laboratory measurements of passing debris fl ow
surges (P
IERSON
, 1986; M
C
A
RDELL
et alii, 2007; I
VERSON
et alii, 2010)
PORE-FLUID PRESSURE AND SOIL MOISTURE
Over the four different debris-fl ow events, the pore-fl uid pres-
sure measured at the sediment-bedrock interface was variable and
depended largely on the thickness of overlying bed sediment and
the state of saturation of the bed sediment. For the majority of the
events, no pressure response due to the pressure head of the overrid-
ing fl ow was measured (presumably because the static bed sediment
between the overriding debris fl ows and the pressure transducer at
the bedrock-sediment interface was partially dry). When there was
no bed sediment covering the pressure transducer and the fl ow in-
teracted directly with the transducer (such as during the end of the
event on the 15 September) we measured large spikes in pore-fl uid
pressure. Only during the 2 June event did we measure a signifi cant
pressure response beneath some thickness of bed sediment. During
this event, positive pore-fl uid pressures were developed in the static
bed sediment at the upper and middle stations prior to the fi rst surge
front arriving. At the upper station, we measured an attenuated pres-
sure signal that generally followed the trend of fl ow stage, but sharp
increases in pore-fl uid pressure lagged sharp increases in stage and
normal stress by 4-8 s (Fig. 3B). We presume the pressure signal was
muted because the bed sediment may not have been complete satu-
rated. From video observations, it was evident that the measured lag
between increases in stage and increases in pore-fl uid pressure was
equal to the time the coarse-grained, visually unsaturated front spent
traveling over the sensors. Similar lags between the rise in stage
and the rise in pore-fl uid pressure have been repeatedly measured
in large-scale laboratory debris fl ows and related to the passage of
the dilated, coarse grain granular front (I
VERSON
et alii, 2010). At the
middle station during the 2 June event, we measured a less attenu-
ated pressure signal and peak measured pressures were up to two
times hydrostatic (M
C
C
OY
et alii, 2010, Fig. 3C).
Volumetric water content (i.e. soil moisture) measured at depths
of ~5 cm and ~45 cm in the channel sediment ~50 m downstream
of the middle station, where total sediment depths were greater than
2 m, remained unchanged from the pre-storm values of ~10% and
13%, respectively, during the short duration storms of 26 July, 6 and
15 September (Tab. 2). During the longer duration 2 June event soil
moisture increased from 15% to 18% at 5 cm, and hours after the end
of the fl ow event we measured soil moistures > 30% at 45 cm depth.
Our measurements indicate that relatively dry bed sediments are typi-
cal of storms in which the duration is short relative to the infi ltration
time. Only during longer duration storms and only locally where the
bed sediment was thin and surrounded by large areas of bedrock (as
was characteristic of the upper and middle station) did sediments ap-
proach saturation. The June 2 event also indicates that merely having
a saturated or nearly saturated bed (as was locally present at both the
upper and middle station) does not ensure entrainment. The June 2
event was depositional at both stations where the shallow bed sedi-
ments were locally saturated.
background image
OSSERVAZIONI SUI DEBRIS FLOWS DI CHALK CLIFFS, COLORADO, USA: PARTE 1,
MISURE IN SITU DELLA DINAMICA DI FLUSSO, TRACCIANTI DEL MOVIMENTO PARTICELLARE E VIDEO IMAGERY DELL’ESTATE 2009
74
S.W. McCOY, J.A. COE, J. W. KEAN, G.E. TUCKER, D.M. STALEY
& T.A. WASKLEWICZ
SUMMARY AND CONCLUSIONS
Using a combination of PIT tagged tracer rocks, automated moni-
toring, and terrestrial laser scanning, we measured the entrainment
and transport response of the four debris-fl ow events that occurred at
Chalk Cliffs in the summer of 2009. We also constrained the rainstorm
characteristics, topographic parameters, and fl ow properties that were
responsible for the observed responses. All the observed debris fl ows
were trigged by storms capable of generating surface-water runoff.
These storms generally had durations of less than two hours, but vari-
able intensities (10-40 mm/hr). The arrival of debris-fl ow surges gen-
erally followed the onset of rain by less than one hour. All fl ow events
consisted of multiple surges, with each asymmetric surge composed of
a steep granular front and a more watery tail. Despite these similari-
ties, the four events in the summer of 2009 were surprisingly variable
in terms total volume and, especially, runout length, and volume of
entrained bed sediment. The 15 September event was conspicuously
larger by these measures than the preceding three fl ows.
Using PIT tagged tracer rocks we quantifi ed the large variability
in particle transport distance and found a strongly bimodal transport
response. In the short-transport mode, the population of tracer parti-
cles had transport distances in which both the mean and maximum
distances were generally tens of meters and the tracers only travelled
a small fraction of the debris-fl ow dominated catchment length. In
the contrasting long-transport mode, the population of tracer particles
had mean and maximum travel distances on the order of thousands of
meters and the majority of the population travelled the entire length
of the catchment during a single fl ow event.
TLS surveys (S
TALEY
et alii, 2011) taken before and after fl ow
events showed little change in reach-scale topographic parameters
over the course of the summer. Therefore, the large variation in ob-
served transport and entrainment response must be controlled by
individual storm characteristics and fl ow properties, rather than by
changes in fl ow-path topography.
Large-entrainment, long-transport fl ows were best differentiated
from small-entrainment, short-transport fl ows by peak 10-minute
rain intensity, total elapsed time with fl ow of signifi cant depth in the
channel, and to a lesser extent, peak surge depth and velocity. Larger
events had higher 10-minute rain intensities and during these higher
intensities (20-40 mm/hr) a larger fraction of the precipitation exited
the basin as measurable debris-fl ow surges and overland fl ow. The
15 September event had the longest elapsed time during which fl ow
depths were measurable (>0.1 m), and the longest elapsed time dur-
ing which fl ow depths were deep (>0.3 m), in addition to having the
most rapidly moving surge fronts. With a growing dataset of events
in which both the effects of the debris-fl ow event and the triggering
rainstorm and fl ow properties are quantifi ed it will be possible to test
the strength of these correlations and gain additional insight into the
controlling mechanisms of initiation, entrainment, and transport by
debris fl ows in natural, uncontrolled settings.
ACKNOWLEDGEMENTS
This research was supported by the National Science Foundation
(NSF) Graduate Fellowship, NSF grants EAR-0643240 and 0934131,
NSF CAREER grant 0239749, and the U.S. Geological Survey Land-
slide Hazards Program. ALSM data was provided by NCALM. We
thank Joe Gartner, for fi eld assistance and Nate Bradley for advice
and technical help with the RFID survey equipment. Careful reviews
on an earlier version of the manuscript by Mark Reid and Nate Brad-
ley improved the clarity of presentation.
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OBSERVATIONS OF DEBRIS FLOWS AT CHALK CLIFFS, COLORADO, USA: PART 1, IN SITU MEASUREMENTS OF FLOW DYNAMICS,
TRACER PARTICLE MOVEMENT AND VIDEO IMAGERY FROM THE SUMMER OF 2009
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© Casa Editrice Università La Sapienza
www.ijege.uniroma1.it
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Received October 2010 - Accepted December 2010
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