Meteorological and Hydrological analysis of July 31st flooding and debris flows over the Santa Catal

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Meteorological and Hydrological analysis of July
31st flooding and debris flows over the Santa
Catalina Mountains by Glenn Lader
Extreme Hydrometeorological Event From
Multi-Day Mesoscale Convective
Thunderstorms over the Santa Catalina Mountains,
Arizona
Craig Shoemaker, Michael Schaffner, and Erik
Pytlak, NWS TucsonChristopher Magirl, Robert
Webb, and Peter Griffiths, USGS
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Four day rainfall totals from Pima county alert
gauges over the Catalina foothills. What
implications might these amounts have if further
rainfall occurs ?
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Four day rainfall totals from Pima county alert
gauges over the Catalina mountains. What
implications might these amounts have if further
rainfall occurs ?
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This is the middle of the monsoonanything wrong
with this picture?
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Upper level streamlines and wind speeds. Pay
particular attention to flow pattern over
northeast Arizona and possible storm movement.
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What can be said about the atmosphere in place
over the lower deserts?
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Will the low level flow contribute to convective
development ? Will any topographical effects come
into play with this flow pattern?
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KTUS Forecast Sounding valid 12z July 31
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How will this thunderstorm complex likely evolve
as it moves south? What
is the key element of this image radar for
southeast Arizona?
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How will the activity moving south from northern
Arizona transition as it moves into this
environment?
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Gpm
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Kg/m2
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0z July 31 Tucson Sounding
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The Extreme Precipitation Event of July 31, 2006,
and Its Effects

• In the early morning hours of July 31, following
  4 days of rain, pulses of rainfall from mesoscale
  convective thunderstorms fell on the southern
  Santa Catalina Mountains.
• Record floods occurred in Rincon Creek, Pantano
  Wash, Tanque Verde Creek, Sabino Creek, and
  Rillito Creek.
• What was really spectacular, though, was what
  happened in Sabino Canyon and other nearby
  watersheds in the Front Range of the Santa
  Catalina Mountains.

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GOES Water Vapor Imagery from July 31, 2006
Upper-level low-pressure system over New Mexico
steered moisture into southern Arizona over a
five-day period. Early in the morning on July 31,
a series of thunderstorms moved southwards from
the Phoenix area to northeastern Pima County.
Half-hour images white indicates high
atmospheric water content
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Morning Sounding on July 31, 2006
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Upper -Level Shear From Northwest Lower-Level
Jet from Southwest
Wind and water vapor movement was like a firehose
aimed straight up over Sabino Canyon and other
Santa Catalina Mountains tributaries the morning
of July 31.
(not to scale)
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First Punch Cold-Top Mesoscale Thunderstorm
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Second Punch Warm-Top Mesoscale Thunderstorm
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Composite NEXRAD, July 31, 0000 0800
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Cumulative Storm Total from NEXRAD and ALERT
1.7 Mt Lemmon Whitetail
Sabino watershed
8-10
5-6
Ventana - 1.3
0.8 Green Mtn.
2.0 Sabino
4.6 - Molino
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July 31, 2006 Streamflow in Tucson Metro Area

• USGS Gage Peak Flow (cfs)
• Sabino Creek near Tucson 15,700 (record)
• Tanque Verde at Sabino Canyon Rd. 26,600
  (record)
• Pantano Wash at Broadway Blvd. 15,900 (record)
• Rillito Creek at Dodge Blvd. 37,900 (record)
• Rillito Creek at La Cholla Blvd. 39,000
  (record)
• Canada Del Oro near Ina Rd. 7,080
• Santa Cruz River at Congress St. 7,200
• Santa Cruz River at Cortaro 40,900
• Santa Cruz River at Trico Rd. 27,200
  (record)

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What is Impressive is the Multi-Day Storm Return
Periods
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Streamflow and Precipitation July 26-July 31
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Debris Flows in the Catalina Front Range n 250
slope failures Most failures are between 4,000
6,000 feet elevation
One debris flow exited and two made it to or
close to the mountain front.
86 of failures were in areas little affected by
the 2003 Aspen Fire
Rattlesnake
Sabino
Bear
Molino
Bird
Ventana
Soldier
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Debris-Flow History of the Santa Catalina
Mountains Little Known

• Before 2006, about five debris flows (all small)
  are known to have occurred in the Santa Catalina
  Mountains.
• In 2003, 2002, 1997, 1993, 1983.
• Depositional evidence south of the mountain front
  indicates a long (but perhaps dormant?)
  occurrence of debris flows.

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What is a Debris Flow?

• A slurry of sediment (70-90) and water (10-30
  by weight).
• Often initiated by the collapse of sediment on a
  steep slope during intense or prolonged rainfall.
• A dense, viscous matrix can support very large
  boulders and transport them easily over several
  kilometers.
• A debris flows can deposit levees of sediment on
  either side of the flow, keeping the flow
  channelized and maintaining forward momentum,
  even over open ground.

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What is a Debris Flow?
Some debris flows only travel short distances and
have large particles.
Debris flow in China
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What is a Debris Flow?
Some debris flows only travel short distances and
have large particles. Other debris flows can
travel relatively long distances on low slopes.
Debris flow in southern California
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Debris Flows in Sabino Canyon
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On the ground in Sabino Canyon
DF 4, 5
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New rest station (trash cans too)
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Sabino Canyon -Western wall
DF 17
DF 16
DF 10
DF 9
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Rattlesnake
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6
9
4
6
Sabino
10
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Sabino Canyon west wall
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Rattlesnake Canyon upstream
Rattlesnake Canyon debris flow
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On the ground in Rattlesnake Canyon
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Fresh debris-flow levees were deposited onto old
DF levees
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Debris flows coalesced and traveled down-canyon
over 2.5 miles (4 km).
Rattlesnake Canyon
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Max depth of the Rattlesnake debris flow was
modest
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but the damage was significant
Existed culvert plugged
New channel cutthrough road!
Sabino Canyon tram road at Rattlesnake
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1994
8/18/2006
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Post-storm aerial photo of Soldier Canyon
Multiple slope failures on west side of canyon
Infra-structure damage at Catalina Highway
crossing
(Pima County Flood Control)
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Soldier Canyon
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Soldier CanyonDebris flows and control
structures
Mt. Lemmon Short Road
Catalina Highway
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Whirlpool developed at the Catalina Highway
culvert
Little deposition above culvertmostly erosional
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Catalina Highway culvert damaged but worked
beautifully
2.5 m double box culvert

• Damage to metal plated divider
• Culvert acted as a 2.5 m sieve
• Steep slope sucked DF down

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Culvert at Mt. Lemmon Short Road quickly plugged
with debris-flow snout
Debris-flow levee topped the guard rail
Sands, gravels, cobbles, boulders all in matrix
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Debris flow fill channel and plugged culvert.
Subsequent flood followed the course of the
alluvial fan.
Culvert
Debris-flow snout
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Paleo debris-flow levees near Finger Rock trail
How old?
Q2 debris levee
Q3 debris levee
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at the top of Alvernon Road
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Summary and Some Questions

• The extreme storm event of July 31 caused gt250
  slope failures in the Front Range of the Santa
  Catalina Mountains.
• Before this event, about five debris flows are
  known to have occurred historically in this area.
• Deposits south of the mountain front suggest a
  long history of debris flow occurrence, but what
  relevance does that have to future debris flow
  hazard?
• The destabilized watersheds suggest high
  potential for more debris flows in the immediate
  future how real is this threat?
• NWS Tucson will work with USGS to determine
  rainfall thresholds used to provide debris flow
  specific info and call to action statements in
  existing flash flood watch and warning products.