TRIGGER
MECHANISMS
FOR
Groundwater
percolates through
weathered bedrock
ribs
• Insitu field instrumentation suggests that runoff seeps into the exposed bedrock ribs and collects within thebedrock depression, beneath the overlying colluvium
Emergent seepage
forces
•
Four common
conditions that
promote
development of
excess uplift
pressures due to
elevated seepage
levels
•
Pore pressures
develop quickly in the
pervious bedrock ribs,
then dissipate quickly
(upper left).
•
Pore pressures
within
the colluvium
continue to rise due
to areal accretion, and
dissipate very slowly
(middle left)
Explosive Failures
•
When
pore water
accumulates in these
spoon-shaped
bedrock
depressions
(upper
right), explosive
‘blow-out’ failures can ensure,
as shown in the photo
at upper left.
PHYSICAL
ATTRIBUTES
OF
DEBRIS FLOW LOBES
Debris flows coalesce in first-order and second order drainages. They usually deposit debris on slopes of around 10% grade. These
attributes can be programmed into a topographic recognition model.
•
Colluvium is typically
stored within bedrock
ravines and depressions
until
external
and
environmental factors
trigger erosive cycles,
such as those shown
here.
Repeated Cycles of
Erosion and Filling
•
C14 dating within
colluvial filled
hollows suggests
that they
periodically fill and
develop soil
horizons, then
undergo erosive
cycles.
• Physical factors diagnostic of hillsides experiencing clastic debris flows. Debris is usually deposited on a slopes between 11o and
Traction Erosion of
Channel
Channels are easily eroded by traction and buoyancy of
debris flow mixtures in
confined channels where the hydraulic grade exceeds 10%
Destructive
Snouts
Debris flows arrest themselves and cease moving when they roll onto a slopes less than 10 degrees and/or are allowed to disperse laterally. This dispersion bleeds off excess pore
water pressure and engenders shear strength to the one fluid mass,
Inverse Sorting
Debris flows are easily recognized by
matrix support and inverse sorting of coarse clastic fraction, as sketched at left, in Gypsum Canyon, UT, along the upper reaches of Lake Powell.
SIEVING OF FINES
In semi-arid regions fines are gradually sieved from the parent debris mass by runoff between extreme events which trigger debris flows. This often results in the development of ‘desert pavement’.
MECHANICS OF
DEBRIS FLOWS
• Orographic lifting is a common trigger for debris flows.
• Precipitation is retained while the storm fronts rise at a rate > 27 fps, the average velocity of rainfall.
• When the lifting slows near the crest of the range, the retained moisture is suddenly dumped onto the upper slope, causing short bursts of intense precipitation which spawn debris flows and debris torrents.
Soil Regolith
• The relatively thin soil regolith often separates from the underlying
bedrock with severe rainfall intensity and duration, as shown at upper left.
• Debris flows tend to favor shaded slopes where there is greater soil moisture retention
• Empirically-derived threshold for debris flows generally
compare rainfall intensity and duration, as shown here. These thresholds vary from place to place, across the United States. • The 60-day antecedent moisture (precipitation) is also a key
Erosion and
Deposition
• Debris is typically
eroded from steep faces in zero and first order basins, as shown in upper image
• Deposition typically
occurs on coalescing fans, when the
hydraulic grade drops to something less than
11% slope, as shown in the lower image
Formation of
destructive snout
•
The killer aspect
of debris flows is
their tendency to
develop a
destructive flow
snout
, sketched
here
MATRIX SUPORTED
MATERIALS
• Debris flow deposits can be
deposited along channels, when their hydraulic grade drops to something around 10%
• They are characterized by fine
grained matrix between clasts, large variation in clast and particle sizes, and often, by inverse sorting
• This intra-channel debris can be
swept out of the channel by much larger events at some later date, and deposited on a fan
Freighting of Large Blocks
Large boulders can be transported on a matrix of dense debris through
buoyancy. The submerged weight of the boulder is its dry weight minus the weight of the debris flow it displaces.
Buoyancy and Effective Stress
•
Debris flows can freight large
quantities of rocky debris and
enormous boulders.
•
In clear water, a rock only weighs
64%
of its dry unit weight because
of
buoyancy
•
In a debris flow with 65%
entrained solids, the same rock
would only weigh
22%
of its dry
weight
•
The 1978 debris flows at
Wrightwood, CA had 65%
entrained solids, typical for a mica
schist source area
Dave Rogers standing in debris
• Debris flows tend to impact small portions of alluvial fans each time they occur, as sketched at left.
• They are the dominant physical process by which large volumes of sediment are deposited on alluvial fans.
Erosive
Capacity
• Debris flows have enormous erosive capacity in steeper reaches of the channels from which they evolve, as shown here.
• This shows the aftermath of the Big Thompson (debris flow) Flood in Colorado on July 31,1976, which killed 144 people.
Debris flows can envelop structures like a fluid and fill them without destroying
them. People are killed by suffocation of the heavy debris against their chests.
Talus
Fans
•
Talus fans are not matrix
supported material
•
They accumulate in
mountainous areas by dry
gravity fall, with sporadic
debris flows emanating
from specific watercourses
Closer views of the 2005 Beartooth debris flows, showing deposition along flatter gradients above the highway and erosion of the steeper gradients below the highway. This highway will be closed for several years while repairs are undertaken.
MONITORING
SYSTEM
Installation of twelve items in the field
Debris detection sensor
Line 3 Line 1 Line 2 GeoPhone Tiltmeter Master logger Water contents sensor Water contents sensor
＊Master logger : 1 set (rain gauge included) ＊Water contents sensor : 4 sets (RF incl.) ＊Debris flow detection sensor : 3 sets (RF logger included) ＊Geophone : 1 set
1st check dam
RF logger
Water contents Slope displ.
Debris flow Detection logger Geophone logger
Wire sensor
Detailed photographs of the system
2nd check dam
Wire sensors
Detailed photographs of the system
Wire sensor Tensioning of the wire sensor Wire sensor fixation device
GeoPhone and wire sensor logger GeoPhone sensor
Excavation of the ground Sensors and RF logger
Master logger
Installation of web camera
Detailed photographs of the system
Window of the site image by the web camera
Sensor mapping
Tables of the measured data
Graphs of the data
