What Planners Need to Know
2011 Spring Conference
Meeting in the Middle
GEOLOGIC HAZARDS AFFECTING SOUTHWESTERN
UTAH
William R. Lund
Utah Geological Survey
Cedar City, Utah
SOUTHWESTERN UTAH’S MOST COMMON
GEOLOGIC HAZARDS
• Floods – riverine and flash floods/debris flows
• Problem Soil and Rock
Expansive soil or rock
Collapsible soil
Soluble soil or rock
Piping
Wind-blown sand
• Slope Failure
Landslides
Rock falls
• Ground Subsidence
• Earthquake Hazards
Ground shaking
Liquefaction
Surface fault rupture
FLOODING
Flooding is southwestern Utah’s most frequent and
destructive geologic hazard
Two types of floods commonly occur in southwestern Utah: (a)
riverine floods, and (b) flash floods. Riverine floods are usually
regional in nature, last from hours to days, and typically are
confined to the flood plains of perennial rivers and streams.
Flash floods result from cloudburst thunderstorms, are
localized, quickly reach maximum flow, and then just as quickly
diminish. Flash floods can occur in any drainage and may
transition in to debris floods and debris flows.
Numerous major riverine floods have affected southwestern
Utah since the area was settled; the most recent were in 1966,
1983, 1984, 2005, and 2010. Flash floods have repeatedly
damaged every major town in southern Utah, with the first flash
flood recorded in Cedar City in 1853.
RIVERINE FLOODS
FLASH FLOODS/DEBRIS FLOWS
PROBLEM SOIL AND ROCK
Expansive soil or rock – clay-bearing soil or rock that swells
when wetted and shrinks when dried. Chiefly associated with the
Petrified Forest Member of the Chinle Formation in
southwestern Utah
Collapsible soil – soil that has considerable strength in its dry,
natural state, but that settles significantly when wetted. Usually
associated with young alluvial fans, collapsible soil has caused
damage in Cedar City and Zion National Park.
Soluble soil or rock – soil or rock containing minerals that
dissolve in water, such as calcium carbonate and gypsum, which
can cause subsidence and formation of sinkholes.
Piping – subsurface erosion of soil or rock by ground-water flow,
forming narrow voids. Pipes can remove support of overlying soil
and rock, resulting in collapse.
Wind-blown sand – Shifting sand moved by wind. May present a
hazard to existing structures (burial) or roadways (burial, poor
visibility).
EXPANSIVE SOIL AND ROCK
Petrified Forest Member
Chinle Formation
"Blue Clay"
COLLABSIBLE SOIL
GYPSUM DISSOLUTION
Quail Creek Dike Failure
PIPING
Piping occurs when groundwater percolates to a free face through
permeable, fine-grained unconsolidated or semi-consolidated deposits.
The percolating water slowly removes silt and fine sand, and forms
narrow, subterranean conduits that grow over time both in diameter
and length until they finally collapse.
WIND-BLOWN SAND
SLOPE FAILURE
LANDSLIDES –includes slides, spreads, and slow-moving flows.
Due to the arid climate in southwestern Utah, spreads and slowmoving flows, which typically depend on a high water content to
mobilize, are rare, and as a result the landslide category in
southern Utah consists almost exclusively of rotational or
translational slides and slumps.
ROCKFALL - is a natural mass-wasting process that involves the
dislodging and downslope movement of individual rocks and small
rock masses. Rock falls are a hazard because a large boulder
traveling at high speed can cause significant damage. Rock falls
are the most common form of mass wasting (slope failure) in
southwestern Utah.
LANDSLIDES
ROCK FALL
GROUND SUBSIDENCE
Ground subsidence (aquifer compaction) due to ground-water
pumping in excess of annual recharge is becoming an
increasing problem in some southwestern Utah valleys. First
identified in 2005 in the Escalante Valley when erosion by
floodwaters revealed four earth fissures near the small
community of Beryl Junction. Since then, active earth
fissures also have been identified in Cedar Valley where one
fissure trends through a new subdivision displacing curb and
gutter, cracking asphalt, and reversing the flow direction on
sewer lines. Additional fissures have been identified north
and west of Quichapa Lake on the west side of Cedar Valley.
Although still comparatively small and causing little current
damage in Utah, earth fissures related to ground-water
withdrawal in Nevada, Arizona, and California have destroyed
homes and businesses and extensively damaged roads, canals
and dams causing $100s of millions in damage.
ESCALANTE VALLEY
CEDAR VALLEY
EARTHQUAKE HAZARDS
Surface faulting – propagation of an earthquake-generating
fault rupture to the ground surface, displacing the surface
and forming a fault scarp.
Ground Shaking –shaking or vibration of the ground during an
earthquake.
Liquefaction – sudden, large decrease in shear strength of a
saturated, cohesionless soil (generally sand, silt) caused by a
collapse of soil structure and temporary increase in pore
water pressure during earthquake ground shaking.
Liquefaction may induce ground failure, including lateral
spreads and flow-type landslides.
INTERMOUNTAIN SEISMIC BELT
Map of the Intermountain
seismic belt showing the largest
historical earthquakes in
southwestern Utah; the ~M 6
Pine Valley earthquake in 1902
and the M 5.8 St. George
earthquake in 1992.
ACTIVE FAULTS IN SW UTAH
Parowan
Panquitch
Cedar City
Hurricane
fault
Sevier fault
St. George
Washington
fault
Kanab
From Quaternary Fault and Fold Database and Map of Utah
(Black and others, 2003)
Quaternary
faults in
southwestern
Utah
SURFACE FAULT RUPTURE
Washington
Fault
Fault Scarp
Near
Cedar City
GROUND SHAKING - LIQUEFACTION
Earthquake ground shaking Utah
Earthquake ground shaking China
Liquefaction along the Virgin River
Liquefaction in Japan
http://geology.utah.gov/
UGS
• Conducts
geologic hazard
studies
• Disseminates
geologic/hazards
information
• Assists with
geologic hazard
ordinance
preparation.
• Makes limited
consultant
report reviews.
• Responds to
emergencies.
CONTACTS
William Lund or Tyler Knudsen
Utah Geological Survey – Southern Regional Office
88 East Fiddler Canyon Road, Suite C
Cedar City, UT 84721
435-865-9034 (Lund)
435-865-9036 (Knudsen)
billlund@utah.gov
tylerknudsen@utah.gov
UGS webpage
www.geology.gov