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Identification of Debris Flow ‘Mudflow’ Hazards for Assessment
of Alluvial Fan Flooding
Flooding Aspects on Alluvial Fans Floodplain Management
Association – Annual ConferenceSeptember 10,
2015----------------------------Jeremy T. LancasterCalifornia
Geological Survey
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• Processes: debris flows have two faces• Debris flow
properties• Regulatory definitions
– Brief historical context • Hazard identification• Frequency
and magnitude• Examples
Highlights
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• Debris flow: a form of rapid mass movement in which a
combination of loose soil, rock, organic matter, air, and water
mobilize [and liquefy] in a slurry the flows down slope
Defined
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• Slope translational failure* *starts with a landslide
– Initiation by failure of discrete landslide masses occurs on
hillslopes – Results from infiltration into colluvial and weak
geologic deposits– Prolonged rainfall, commonly a day or longer
(Cannon and Gartner,
2008)– Short runout, load channel networks– Pore pressure
increases and reduces effective stress– Gartner (2008) analyzed 210
debris flow occurrances after fire,
finding that only 16% of the debris flows initiated by this
process
Debris flow processes – Starts as a solid
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• Runoff initiated* debris flows*starts with H20 becomes a
landslide
– Post-wildfire impacted watersheds– Progressive bulking of
surface runoff – Some sediment entrained by rilling on canyon
slopes– Most sediment entrained by channel scour and bank collapse–
Threshold location in channel network– Long runout events, up to
1,500,000 cubic meters– Gartner (2008) analyzed 210 debris flow
ocurrances after fire,
finding that 76% of the debris flows initiated by this process –
Triggering rainfall thresholds are achieved in minutes
Debris flow processes- Starts with H20
Source: USGS
2015http://landslides.usgs.gov/research/wildfire/whattodo.php
http://landslides.usgs.gov/research/wildfire/whattodo.php
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Sediment-water ratios, deposits
Flow Type Sediment Load‡ Sedimentary Structures Deposits and
LandformsPercent By
weight*Percent By volume†
Streamflow 1-40 0.4-20
Well to moderately sorted, stratified to massive; weak to strong
imbrication; cut-and-fill
structures; ungraded to graded
Bar and swale, fans, sheets, splays; channels have high
width-to-depth ratios
Hyperconcentrated flow 40-70 20-60
Poorly sorted and weakly stratified to massive; thin
gravel lenses; clast supported; normal and reverse grading
Similar to streamflow; transitioning to sheets, splays and lobes
at the higher end of
the sediment/water continuum
Debris flow 70-90 >60
Very poor to extremely poorly sorted; no stratification;
weak
to no imbrication; matrix supported; inverse grading at base;
normal grading near top
Marginal levees, terminal lobes, boulder fields (in
coarse-grained viscous flows); sheets, lobes, and splays (in
finer-grained fluidized flows
with lower viscosity)
‡These values are general guidelines used to classify flow types
in a continuum of sediment, debris and water mixtures.*Values are
provided in Costa, 1984, reportedly assuming
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FEMA definition of flooding• NFIP –Title 44, CFR 59.1,
Definitions :
– Flooding Means: • A) A general and temporary condition of
partial or complete inundation of normally dry
land areas from:1) The overflow of inland tidal waters2) The
unusual and rapid accumulation of runoff of surface waters from any
source3) Mudslides (i.e. Mudflows) which are proximately caused by
flooding as defined in
paragraph (a) (2) of this definition and are akin to a river of
liquid and flowing mud on the surface of normally dry land areas,
as when earth is carried by current of water and deposited along
the path of a current
– (Graf 1988): Components of a fluvial system:• Surface Waters•
Stream Waters • Flood Waters“Once [surface waters are] collected
into a watercourse, the flow is designated a stream
water (Martinez vs. Cooke), and if it leaves the channel through
overflow, it is designated as floodwater (Mader vs. Mettenbrink,
Maricopa County Municipal Water Conservation District No.1 vs.
Warford, Southern Pacific Company vs. Proebstel)
- “A watercourse…[has] a definite bed and well marked
banks.”
Regulatory framework
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• Where does ‘Mudflow’ come from (NRC, 1982)
• Mud flows – A subset of landslides whose dominant transport
mechanism is that of a flow having sufficient viscosity to support
large boulders within a matrix of smaller sized particles.
Regulatory framework
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USGS definition (Current Geological Terminology)Debris Flow: “…a
form of rapid mass movement in which a combination of loose soil,
rock, organic matter, air, and water mobilize [and liquefy] in a
slurry the flows down slope.” Typically have
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Bulking Factor = 1/(1-CV)Where CV is equal to the sediment
volume expressed in decimal percent (Hamilton and Fan, 1996)
Values used for long-term design (from LADPW, 2006; Gusman,
2011):• Ventura County: 1.2 – 1.75 • Los Angeles County: 2.0
(DPA-1)• San Diego County: 1.5 - 2.0• San Bernardino County: 2.0•
FEMA: 1.1 - 1.5• AFTF: Suggest using 2.5 for debris flow • Shuirman
and Slosson (1992) reported as high as 3.2 following a fire
*Error ranges in debris basin cleanout volumes: -45% to +80%
(Santi and Morandi, 2012)
Buking factors in practice
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Debris Flow Hazards (pre-typing)• Identification Methods
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Debris Flow Hazards (pre-typing)• Identification Methods
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Debris Flow Hazards (pre-typing)Watershed Morphometric
Factors
– Relief Ratio– Meltons #– Meltons # + Plannimetric Length
Watershed Factors used in USGS Debris Flow Models (sounthern
Cal)Gartner et al. (2014) Mean Min MaxMeltons # 0.51 0.12
1.03Relief Ratio 0.24 0.05 0.71Mean Slope (%) 57.8 18.7
84.8Watershed Burn (%) 81.7 5 100
Wilford (2005) Non-FireMeltons # >0.30Meltons # and
Plannimetric Length >0.60 and ≥ 2.7km
Bovis and Jakob (1999) Non-fire Meltons # >0.52
(R = Meltons#, WL = Plannimetric Length; Welsh and Davies, 2011)
(Jackson et al., 1987)
Sheet1
Drainage Basin NamePushwallaOak CreekHanesSan Jacinto
Area (square miles)19.524.30.90.3
Mean Slope (%)36.652.948.747.7
Max Elevation (ft)5,21513,2182,1763,681
Annual Precip (in)10.420.519.817.7
Relief Ratio0.0750.170.140.39
Watershed Geology Granite/GneissGraniteShale/SstGranite
Watershed Factors used in USGS Debris Flow Models (sounthern
Cal)
Gartner et al. (2014)MeanMin Max
Meltons #0.510.121.03
Relief Ratio0.240.050.71
Mean Slope (%)57.818.784.8
Watershed Burn (%)81.75100
Wilford (2005) Non-Fire
Meltons #>0.30
Meltons # and Plannimetric Length>0.60 and ≥ 2.7km
Bovis and Jakob (1999) Non-fire
Meltons #>0.52
Pak (2009) Debris prediction Model
Relief Ratio
Sheet1
Drainage Basin NamePushwallaOak CreekHanesSan Jacinto
Area (square miles)19.524.30.90.3
Mean Slope (%)36.652.948.747.7
Max Elevation (ft)5,21513,2182,1763,681
Annual Precip (in)10.420.519.817.7
Relief Ratio0.0750.170.140.39
Watershed Geology Granite/GneissGraniteShale/SstGranite
Watershed Factors used in USGS Debris Flow Models (sounthern
Cal)
Gartner et al. (2014)MeanMin Max
Meltons #0.510.121.03
Relief Ratio0.240.050.71
Mean Slope (%)57.818.784.8
Watershed Burn (%)81.75100
Wilford (2005) Non-Fire
Meltons #>0.30
Meltons # and Plannimetric Length>0.60 and ≥ 2.7km
Bovis and Jakob (1999) Non-fire
Meltons #>0.52
Pak (2009) Debris prediction Model
Relief Ratio
Sheet1
Drainage Basin NamePushwallaOak CreekHanesSan Jacinto
Area (square miles)19.524.30.90.3
Mean Slope (%)36.652.948.747.7
Max Elevation (ft)5,21513,2182,1763,681
Annual Precip (in)10.420.519.817.7
Relief Ratio0.0750.170.140.39
Watershed Geology Granite/GneissGraniteShale/SstGranite
Watershed Factors used in USGS Debris Flow Models (sounthern
Cal)
Gartner et al. (2014)MeanMin Max
Meltons #0.510.121.03
Relief Ratio0.240.050.71
Mean Slope (%)57.818.784.8
Watershed Burn (%)81.75100
Wilford (2005) Non-Fire
Meltons #>0.30
Meltons # and Plannimetric Length>0.60 and ≥ 2.7km
Bovis and Jakob (1999) Non-fire
Meltons #>0.52
Pak (2009) Debris prediction Model
Relief Ratio
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Debris flow hazards: Watershed Specific
• Sediment availability– Supply limited– Supply unlimited
• Hydroclimate– Transport limited– Transport unlimited
• Slope processes – Landslides – Colluvium
• Channel reach morphology– Constrictions/confinement – Plunge
pools– Broad gentle reaches– Bedrock presence
Supply unlimited + Transport unlimitedSupply limited + Transport
Limited
Event FrequencyHigh Low
Sheet1
Drainage Basin NamePushwallaOak CreekHanesSan Jacinto
Area (square miles)19.524.30.90.3
Mean Slope (%)36.652.948.747.7
Max Elevation (ft)5,21513,2182,1763,681
Annual Precip (in)10.420.519.817.7
Relief Ratio0.0750.170.140.39
Watershed Geology Granite/GneissGraniteShale/SstGranite
Watershed Factors used in USGS Debris Flow Models (sounthern
Cal)
Gartner et al. (2014)MeanMin Max
Meltons #0.510.121.03
Relief Ratio0.240.050.71
Mean Slope (%)57.818.784.8
Watershed Burn (%)81.75100
Wilford (2005) Non-Fire
Meltons #>0.30
Meltons # and Plannimetric Length>0.60 and ≥ 2.7km
Bovis and Jakob (1999) Non-fire
Meltons #>0.52
Pak (2009) Debris prediction Model
Relief Ratio
Event Frequency
Supply unlimited + Transport unlimitedHigh
Supply limited + Transport Limited Low
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Camarillo Springs Oct./Dec. 2014
• May 2013 Springs fire– 24,250 acres– 22 Structures
Drainage Basin Name Camarillo SpringsArea (square miles)
0.09Mean Slope (%) 61.7Max Elevation (ft) 1,777Annual Precip (in)
15.8Relief Ratio 0.55Meltons # 0.99Plannimetric Length (km)
1Watershed Geology Granite/Volcanics
Sheet1
Drainage Basin NamePushwallaOak CreekHanesSan JacintoCamarillo
Springs
Area (square miles)19.524.30.90.30.09
Mean Slope (%)36.652.948.747.761.7
Max Elevation (ft)5,21513,2182,1763,6811,777
Annual Precip (in)10.420.519.817.715.8
Relief Ratio0.0750.170.140.390.55
Meltons #0.99
Plannimetric Length (km)1
Watershed Geology
Granite/GneissGraniteShale/SstGraniteGranite/Volcanics
Watershed Factors used in USGS Debris Flow Models (sounthern
Cal)
Gartner et al. (2014)MeanMin Max
Meltons #0.510.121.03
Relief Ratio0.240.050.71
Mean Slope (%)57.818.784.8
Watershed Burn (%)81.75100
Wilford (2005) Non-Fire
Meltons #>0.30
Meltons # and Plannimetric Length>0.60 and ≥ 2.7km
Bovis and Jakob (1999) Non-fire
Meltons #>0.52
Pak (2009) Debris prediction Model
Relief Ratio
Event Frequency
Supply unlimited + Transport unlimitedHigh
Supply limited + Transport Limited Low
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USGS data on debris flow frequency and magnitude• 344 events
(Gartner et al., 2014)
– Many watersheds with up to 10 events since the 1950’s
(Recurrence is on engineering timescales)
• Debris removal and overtopping are a concern– On February 6,
2010, debris flows produced in the Station fire
burn area overtopped sediment-retention basins and damaged or
destroyed 46 homes in La Crescenta, California (Gartner, 2013)
Frequency and magnitude – assessing potential
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• California droughts stress watershed vegetation, increase fuel
loads
• Warm El Nino phases enhance moisture availability, follow
periods of drought and heavy wildfire seasons
• Westerling (2006): fire season increase by 2-months since the
1980’s; frequency and size have also increased
• Enhanced precipitation extremes are generally expected due to
greater moisture availability in a warming atmosphere…(Gurshunov et
al., 2013).
• Enhanced precipitation associated with atmospheric rivers
yielding extreme precipitation, is projected by most current
climate models (Gurshunov et al., 2013).
Fire frequency, Drought and Precipitation
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• Post-fire debris flows fit with the FEMA definition of
fooding• Need to update terminology to fit our scientific
understanding of
the process• Where present, consideration should be given to
fire-flood
processes where • Watershed assessments may need to consider
higher bulking
factors• developments encroach on alluvial fan areas.
Closing Remarks
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Identification of Debris Flow ‘Mudflow’ Hazards for Assessment
of Alluvial Fan FloodingHighlightsDefinedDebris flow processes –
Starts as a solidDebris flow processes- Starts with
H20Sediment-water ratios, deposits�Regulatory frameworkRegulatory
frameworkRegulatory framework Buking factors in practiceDebris Flow
Hazards (pre-typing)Debris Flow Hazards (pre-typing)Debris Flow
Hazards (pre-typing)Debris flow hazards: Watershed Specific
Camarillo Springs Oct./Dec. 2014Frequency and magnitude – assessing
potentialFire frequency, Drought and PrecipitationClosing Remarks
Slide Number 19