• Flow characteristics
• Causes
• Deposits
• Flow mechanisms

• Slurries of fine and coarse materials
• Audible from a distance
• Closely follow topography
• High range of yield strength

• Pinatubo, 1991
• Caused by typhoon after eruption
• Average velocity of 12 ms^-1
• Carried boulders > 1.5 m diameter
• ~ 100 people killed

• Kelut, 1919 and 1990, > 5000 people killed
• Ruapehu, 1953
• Some loss of life and property
• Possible to control by lake size?

• Nevado del Ruiz, 1985
• Small eruption
• 6 x 10⁷ m³ of lahar produced
• Mud flows traveled > 60 km
• Average v = 36 ms^-1
• Q = 5 x 10⁴ m³s^-1
• 90 minutes following eruption
• 22,000 killed in Armero

• V = 3.8 km³
• Age = 5600 yBP
• Runout > 120 km
• Area > 200 km²
• Velocity at least 20 m/s
• Q_max = 2.5 * 10⁶ m³/s
• Bulking of sand and gravel

• Debris avalanche
• Non-cohesive lahar
• Cohesive lahar
• Stream flow

Flow fronts

Marginal levees

Ogive surface ridges

Unsorted

No bedding

Basal shear layer?

Laminated fine-grained top

High range in yield strength

Slight density contrast matrix/clasts

Intergranular lubrication

Little clast friction

k = shear strength
c = cohesion
s = normal stress
f = friction angle
s = (s - P)
P = pore pressure

Fluid phase

Frictionless mixture water and fine particles

Responsible for cohesive strength

Granular phase

Coarser particles

Determines frictional strength

• Debris avalanches
• Debris flows flows
• Hyperconcentrated flows
• Stream flows

Flow characteristics vs. time

Stage height (m)

Discharge

Q (m³/s)

Area under the curve is total volume

Maximum discharge

Duration

• Source
• Size and shape
• volume
• Deposit
• Cross sectional area
• Planimetric area

• General failure body
• Saucer shaped geometry
• Terzaghi (1931) slip surface
• Volume approximation
• cone formula
• volume ~ 1/3 p r²h ~ A h/3

•   Coulomb (1773) establlished the modern criteria for slope failure
•   Failure will occur at two intersecting planes for which:

• In two dimensons, intersecting lines at 45^o- f/2 define the stable zone
• A plot of t vs. s defines the Mohr stress space
• A Mohr circle passing through s₁ and s₃ that lies within the two lines indicates a stable material.
• If s1 increase to the point where the circle touches the two planes, the material fails as a Columb plastic (Tersaghi, 1943)

                    F = (SS_uil)/(SW_i sin a_I)

• Iverson et al. 1999 model
• Cross sectional area (A)
• Stream profile
• Top of deposit/flow
• Planimetric area (B)
• Outline of deposit
• Cumulative volume calculation
• LAHARZ code
• GIS method for delineating lahar hazard zones
• Empirically based on 27 lahar paths on 9 volcanoes
• Predictive equations based on expected lahar volumes
A = 0.05 V^2/3
B = 200 V^2/3
• Typical large lahars in the ~ 1,000 years frequency have volumes of ~10⁸ m³

Lahars of 100-year frequency have volumes > 10⁵ m³.

Iverson, RM, Shilling, SP, and Vallance, JW, 1998, Objective delineation of lahar-inundation hazard zones, Geological Society of America Bull., 110:972-984.

Scott, KM, Vallance, JW, and Pringle, PT, 1995, Sedimentology, Behavior, and Hazards of Debris flows at Mount Rainier, Washington, USGS Professional Paper 1547, 56 p.