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Identification of Debris Flow 'Mudflow' Hazards for Assessment of Alluvial Fan Flooding

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Identification of Debris Flow 'Mudflow' Hazards for Assessment of Alluvial Fan Flooding Identification of Debris Flow ‘Mudflow’ Hazards for Assessment of Alluvial Fan Flooding Flooding Aspects on Alluvial Fans Floodplain Management Association – Annual Conference September 10, 2015 ---------------------------- Jeremy T. Lancaster California Geological Survey Highlights • Processes: debris flows have two faces • Debris flow properties • Regulatory definitions – Brief historical context • Hazard identification • Frequency and magnitude • Examples Defined • 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 Debris flow processes – Starts as a solid • 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 with H20 • 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 Source: USGS 2015 http://landslides.usgs.gov /research/wildfire/whatto do.php Sediment-water ratios, deposits Flow Type Sediment Load‡ Sedimentary Structures Deposits and Landforms Percent By Percent By weight* volume† Well to moderately sorted, stratified to massive; weak to Bar and swale, fans, sheets, Streamflow 1-40 0.4-20 strong imbrication; cut-and-fill splays; channels have high structures; ungraded to width-to-depth ratios graded Similar to streamflow; Poorly sorted and weakly transitioning to sheets, splays Hyperconcentrated stratified to massive; thin 40-70 20-60 and lobes at the higher end of flow gravel lenses; clast supported; the sediment/water normal and reverse grading continuum Marginal levees, terminal Very poor to extremely poorly lobes, boulder fields (in sorted; no stratification; weak coarse-grained viscous flows); Debris flow 70-90 >60 to no imbrication; matrix sheets, lobes, and splays (in supported; inverse grading at finer-grained fluidized flows base; normal grading near top 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 <10 percent clay content. †Values from Pierson and Costa, 1987 Regulatory framework 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 waters 2) The unusual and rapid accumulation of runoff of surface waters from any source 3) 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 <50% Fines -Debris flows are caused by intense surface flow and erosion -Debris flows may initiate from other types of landlslides (USGS Fact Sheet 2004-3072 [after Varnes, 1978]) NRC definition: 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. Current definition of debris flow = NRC definition of mudflow = NFIP definition of flooding Buking factors in practice 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) 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 Max Meltons # 0.51 0.12 1.03 Relief Ratio 0.24 0.05 0.71 Mean Slope (%) 57.8 18.7 84.8 Watershed Burn (%) 81.7 5 100 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 (R = Meltons#, WL = Plannimetric Length; Welsh and Davies, 2011) (Jackson et al., 1987) Debris flow hazards: Watershed Specific • Sediment availability – Supply limited Event Frequency – Supply unlimited Supply unlimited + Transport unlimited High Supply limited + Transport Limited Low • Hydroclimate – Transport limited – Transport unlimited • Slope processes – Landslides – Colluvium • Channel reach morphology – Constrictions/confinement – Plunge pools – Broad gentle reaches – Bedrock presence Camarillo Springs Oct./Dec. 2014 Drainage Basin Name Camarillo Springs • May 2013 Springs fire Area (square miles) 0.09 Mean Slope (%) 61.7 Max Elevation (ft) 1,777 – 24,250 acres Annual Precip (in) 15.8 Relief Ratio 0.55 – 22 Structures Meltons # 0.99 Plannimetric Length (km) 1 Watershed Geology Granite/Volcanics Frequency and magnitude – assessing potential 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) Fire frequency, Drought and Precipitation • 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). Closing Remarks • 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. .
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