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
• Processes: debris flows have two faces• Debris flow properties• Regulatory definitions
– Brief historical context • Hazard identification• Frequency and magnitude• Examples
Highlights
• 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
• 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
• 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
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
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
• 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
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
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
Debris Flow Hazards (pre-typing)• Identification Methods
Debris Flow Hazards (pre-typing)• Identification Methods
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
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
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
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
• 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
• 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
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
