The Minerva Debris Flow Model
Intense and persistent rainfall infiltrates bedrock and surficial material, increasing pore water pressure, which can result in slope failure. Debris flows are generated when such a landslide intersects a flowing body of water, or when saturated bed sediments are mobilized and begin flowing downstream. Debris flows can be very destructive, frequently damaging buildings and roads constructed on debris flow fans.
For this study we define a “debris flow” model, intended as a conceptual abstraction of a watershed described by the properties and the terms that a geoscientist may use to determine which creek is more likely to generate debris flows. With debris flow, we refer to the class from the updated Varnes classification (Hungr et al., 2014). Table 1 summarizes the properties used to define the debris flow model, drawn from the scientific literature (Bovis and Jakob, 1999; Friele, 2012; Goudie, 2014; Holm et al., 2016; Howes and Kenk, 1997; Hungr et al., 2014; Jackson, 2019; Jackson et al., 2014; Segoni et al., 2018; Strahler, 1957; Wilford et al., 2004)
| Instance Property-Value-Frequency | Model Definition Source | Comments |
| has surficial form -Fan(s)-always | (Harvey A. 2013, Alluvial Fan, in Goudie A. (Eds) Encyclopedia of Geomorphology, Routledge, London and New York, p 1202) | Fans are where debris flows deposit. |
| has surficial form -Terrace(s)-usually | (Bridge J. S., 2013, Alluvium in Goudie A. (Eds) Encyclopedia of Geomorphology, Routledge, London and New York, p 516-521) | Terraces are formed by downcutting and lateral erosion of alluvial sediments by streams. Debris flows can generate terraces (Bridge 2013), hence, terraces can be indicator of debris flow activity. |
| has surficial form -Hummock(s)-always | Howes and Kenk 1997 | Hummocky topography may be indicator of landslide debris (Howes and Kenk, 1997) |
| has water -River/Stream-always | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has rainfall -Extreme Rainfall-always | Segoni 2018, Friele 2012 | Debris flows are triggered by intense rainfall (Segoni et al., 2018). Rainfall threshold for this study are derived from Friele (2012). |
| has rainfall -Severe Rainfall-usually | Segoni 2018, Friele 2012 | Debris flows are triggered by intense rainfall (Segoni et al., 2018). Rainfall threshold for this study are derived from Friele (2012). |
| has rainfall -Moderate Rainfall-sometimes | Segoni 2018, Friele 2012 | Debris flows are triggered by intense rainfall (Segoni et al., 2018). Rainfall threshold for this study are derived from Friele (2012). |
| has rainfall -Mild Rainfall-rarely | Segoni 2018, Friele 2012 | Debris flows are triggered by intense rainfall (Segoni et al., 2018). Rainfall threshold for this study are derived from Friele (2012). |
| has geomorph process -ErosionalProcess-always | Bovis and Jakob 1999 | Streams with active erosional processes are more likely to experience debris flows than streams with less active erosional processes (Bovis and Jakob, 1999). |
| has geomorph process -MassMovement-always | Guzzetti et al 2012 | Landslides are more likely to occur on slopes or valleys that have experienced landslides before (Guzzetti et al., 2012) |
| has been logged within years -5-10 years-always | Jackson 2019 Literature Review | Landslides are extremely likely by 5 to 10 years after tree harvesting. Most of tree roots have died, and new trees are too small to provide anchoring effect with their roots on the slope (Jackson 2019) |
| has been logged within years -10-20 years-usually | Jackson 2019 Literature Review | Landslides are likely by 10 to 20 years after tree harvesting as new trees are starting to provide anchoring effect with their roots on the slope (Jackson 2019) |
| has been logged within years -0-5 years-usually | Jackson 2019 Literature Review | Landslides are likely by 0 to 5 years after tree harvesting as the trees are dead but some roots are still providing anchoring effect on the slope (Jackson 2019) |
| has fire within years -0-2 years-always | Jackson 2019 Literature Review | Debris flows are very likely for 2 years after a wildfire. Water cannot infiltrate, runoff and erosion increase as the soil becomes water repellent and loses cohesion because of the fire heat (Jackson 2019). |
| has fire within years -3-5 years-usually | Jackson 2019 Literature Review | Debris flows are likely between 3 to 5 years after a wildfire. The water-repellent soil horizon degrades but the roots of dead trees are starting to rot and they do not support the slope with their anchoring effect anymore (Jackson 2019). |
| has fire within years -5-10 years-always | Jackson 2019 Literature Review | Debris flows are very likely between 5 to 10 years after a wildfire. Roots of dead trees decay, and they are not supporting the soil anymore as for the case of tree harvesting logging (Jackson 2019). |
| has fire within years -10-20 years-usually | Jackson 2019 Literature Review | Debris flows are likely between 10 to 20 years after a wildfire. The roots have lost anchoring effect and the new trees are still too small to support the slope (Jakson 2019). |
| has transport line -Road Resource-always | Jackson 2019 Literature Review | Logging roads are the greatest aggravating factor for landslide activity as compared to undisturbed slopes (Jackson 2019) |
| has transport line -Road Resource Demographic-always | Jackson 2019 Literature Review | Logging roads are the greatest aggravating factor for landslide activity as compared to undisturbed slopes (Jackson 2019). |
| has transport line -Road Unclassified Or Unknown-always | Jackson 2019 Literature Review | The ‘Road Unclassified Or Unknown’ in this area of BC are mostly old inactive logging roads. This assessment has been done by visual evaluation of the data. Logging roads are the greatest aggravating factor for landslide activity as compared to undisturbed slopes (Jackson 2019). |
| has bed rock -volcanic igneous rock-always | Bovis and Jakob 1999 | Quaternary volcanic rocks in BC have usually weak geotechnical properties. Basin underlain by these weak rocks are likely to experience frequent and large debris flow events (Bovis and Jakob 1999). |
| has fire within years ->20 years-sometimes | Jackson 2019 Literature Review | After 20 year since a wildfire, trees have regrown and the wildfire effects on slope stability have diminished (Jackson 2019) |
| has surficial material -Colluvium-Usually | Bovis and Jakob 1999 | Debris flows are common is areas with easily erodible material. |
| has surficial material -Morainal Material (Till)-Always | Bovis and Jakob 1999 | Debris flows are common is areas with easily erodible material. |
| has stream order -1-Always | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has stream order -2-Always | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has stream order -3-rarely | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has stream order -4-rarely | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has stream order -5-rarely | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has been logged within years ->20 years-sometimes | Jackson 2019 Literature Review | By 20 year since logging, trees have regrown and the roots are anchoring the soil again (Jackson 2019) |
| has geomorph process -Debris Flow-must_be | Wilford et al., 2004, Bovis and Jakob 1999, Holm 2016 | Melton ratio (number that takes into account relief and area of a watershed) and watershed length allows discrimination of debris flow, debris flood, and flood prone fans (Jackson 2019). |
| has landslide type-debrsi flow-Always | Hungr et al 2014 | Debris flows occur periodically on established path. Determining the frequency of event is a non-trivial task, but the fact that someone mapped a debris flow in a specific channel, indicates the channel as prone to debris flows events. |
| has landslide type-Fall -usually | Bovis and Jakob 1999 | Any landslide types may accumulate debris in a channel that can be then mobilized into a debris flow |
| has landslide type-Flow-usually | Bovis and Jakob 1999 | Any landslide types may accumulate debris in a channel that can be then mobilized into a debris flow |
| has landslide type-Slide-usually | Bovis and Jakob 1999 | Any landslide types may accumulate debris in a channel that can be then mobilized into a debris flow |
| has landslide type-Spread-usually | Bovis and Jakob 1999 | Any landslide types may accumulate debris in a channel that can be then mobilized into a debris flow |
| has landslide type-Topple-usually | Bovis and Jakob 1999 | Any landslide types may accumulate debris in a channel that can be then mobilized into a debris flow |
| has landslide type-Slope deformation-usually | Bovis and Jakob 1999 | Any landslide types may accumulate debris in a channel that can be then mobilized into a debris flow |
| has slope -Very steep-always | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has slope -Steep-always | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has slope -moderately steep-usually | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has slope -moderate-usually | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has slope -gentle-rarely | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has slope -plain-never | Hungr et al 2014 | Debris flows occur periodically on established paths, usually gullies and first- or second- order streams (Hungr et al., 2014) |
| has surficial form -cliff-always | Howes and Kenk 1997 | Cliffs indicate steep terrains where sediments may be mobilized as debris flows. |
| has surficial form -cones-always | Howes and Kenk 1997 | Cones store sediments that may be re-mobilized into debris flow. |
| has water -permafrost-always | Hungr et al 2014 | Permafrost degradation can destabilize sediments |
| has texture -blocks-always | Howes and Kenk 1997 | The presence of blocks can be indicator of landslide processes |
| has texture -rubble-always | Howes and Kenk 1997 | The presence of rubble is an indicator of landsldies processes. |
Table 1 The debris flow Minerva Intelligence model. Model name, property, property value, information source and rationale are shown in the table
References
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Segoni, S., Tofani, V., Rosi, A., Catani, F. and Casagli, N.: Combination of Rainfall Thresholds and Susceptibility Maps for Dynamic Landslide Hazard Assessment at Regional Scale, Front. Earth Sci., 6(June), doi:10.3389/feart.2018.00085, 2018.
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Wilford, D. J., Sakals, M. E., Innes, J. L., Sidle, R. C. and Bergerud, W. A.: Recognition of debris flow, debris flood and flood hazard through watershed morphometrics, Landslides, 1(1), 61–66, doi:10.1007/s10346-003-0002-0, 2004.