Water and Sediment Supply Requirements for Post-Wildfire Debris Flows in the Western United States

2021 ◽  
Vol 27 (1) ◽  
pp. 73-85
Author(s):  
Paul M. Santi ◽  
Blaire Macaulay
Keyword(s):  
Debris Flow ◽  
High Intensity ◽  
Debris Flows ◽  
High Volume ◽  
Sediment Supply ◽  
Time Intervals ◽  
Burned Areas ◽  
Sufficient Intensity ◽  
Water Intensity ◽  
Short Time

ABSTRACT This work explores two hypotheses related to runoff-related post-wildfire debris flows: 1) their initiation is limited by rainstorm intensity rather than cumulative rainfall depths and 2) they are not sediment supply limited. The first hypothesis suggests that it is common to generate more than enough rainfall to account for the volume of water in the debris flow, but to actually produce a debris flow, the water must be delivered with sufficient intensity. This is demonstrated by data from 44 debris flows from eight burned areas in California, Colorado, and Utah. Assuming a debris flow comprises 30 percent water and 70 percent solids, these events were generated during rainstorms that produced an average of 17 times as much water as necessary to develop a debris flow. Even accounting for infiltration, the rainstorms still generated an overabundance of water. Intensity dependence is also shown by numerous cases in which the exact timing of debris flows can be pinpointed and is contemporaneous with high-intensity bursts of rainfall. The hypothesis is also supported by rainfall intensity-duration thresholds where high-volume storms without high-intensity bursts do not generate debris flows. The second hypothesis of sediment-supply independence for the initiation of debris flows is supported by a significant increase in flow volume occurring directly after wildfire, compared to flows in unburned terrain. Also, repeated flows within short time intervals are only possible with an abundance of channel sediment, dry ravel, and bank failure material that can be mobilized. Field observations confirm these sediment sources, even directly after a debris-flow.

scholarly journals The temporally varying roles of rainfall, snowmelt and soil moisture for debris flow initiation in a snow-dominated system

2018 ◽  
Vol 22 (6) ◽  
pp. 3493-3513 ◽  
Author(s):  
Karin Mostbauer ◽  
Roland Kaitna ◽  
David Prenner ◽  
Markus Hrachowitz
Keyword(s):  
Soil Moisture ◽  
Debris Flow ◽  
High Intensity ◽  
Debris Flows ◽  
Regional Scale ◽  
Early Summer ◽  
Temporal Separation ◽  
Antecedent Soil Moisture ◽  
Trigger Mechanisms ◽  
Debris Flow Initiation

Abstract. Debris flows represent frequent hazards in mountain regions. Though significant effort has been made to predict such events, the trigger conditions as well as the hydrologic disposition of a watershed at the time of debris flow occurrence are not well understood. Traditional intensity-duration threshold techniques to establish trigger conditions generally do not account for distinct influences of rainfall, snowmelt, and antecedent moisture. To improve our knowledge on the connection between debris flow initiation and the hydrologic system at a regional scale, this study explores the use of a semi-distributed conceptual rainfall–runoff model, linking different system variables such as soil moisture, snowmelt, or runoff with documented debris flow events in the inner Pitztal watershed, Austria. The model was run on a daily basis between 1953 and 2012. Analysing a range of modelled system state and flux variables at days on which debris flows occurred, three distinct dominant trigger mechanisms could be clearly identified. While the results suggest that for 68 % (17 out of 25) of the observed debris flow events during the study period high-intensity rainfall was the dominant trigger, snowmelt was identified as the dominant trigger for 24 % (6 out of 25) of the observed debris flow events. In addition, 8 % (2 out of 25) of the debris flow events could be attributed to the combined effects of low-intensity, long-lasting rainfall and transient storage of this water, causing elevated antecedent soil moisture conditions. The results also suggest a relatively clear temporal separation between the distinct trigger mechanisms, with high-intensity rainfall as a trigger being limited to mid- and late summer. The dominant trigger in late spring/early summer is snowmelt. Based on the discrimination between different modelled system states and fluxes and, more specifically, their temporally varying importance relative to each other, this exploratory study demonstrates that already the use of a relatively simple hydrological model can prove useful to gain some more insight into the importance of distinct debris flow trigger mechanisms. This highlights in particular the relevance of snowmelt contributions and the switch between mechanisms during early to mid-summer in snow-dominated systems.

Debris flow initiation from ravel-filled channel bed failure following wildfire in a bedrock landscape with limited sediment supply

2021 ◽  
Author(s):  
Marisa C. Palucis ◽  
Thomas P. Ulizio ◽  
Michael P. Lamb
Keyword(s):  
Debris Flow ◽  
Debris Flows ◽  
Sediment Supply ◽  
Moderate Intensity ◽  
Sandy Sediment ◽  
Channel Network ◽  
Sediment Yields ◽  
First Order ◽  
Dry Ravel ◽  
Debris Flow Initiation

Steep, rocky landscapes often produce large sediment yields and debris flows following wildfire. Debris flows can initiate from landsliding or rilling in soil-mantled portions of the landscape, but there have been few direct observations of debris flow initiation in steep, rocky portions of the landscape that lack a thick, continuous soil mantle. We monitored a steep, first-order catchment that burned in the San Gabriel Mountains, California, USA. Following fire, but prior to rainfall, much of the hillslope soil mantle was removed by dry ravel, exposing bedrock and depositing ∼0.5 m of sandy sediment in the channel network. During a one-year recurrence rainstorm, debris flows initiated in the channel network, evacuating the accumulated dry ravel and underlying cobble bed, and scouring the channel to bedrock. The channel abuts a plowed terrace, which allowed a complete sediment budget, confirming that ∼95% of sediment deposited in a debris flow fan matched that evacuated from the channel, with a minor rainfall-driven hillslope contribution. Subsequent larger storms produced debris flows in higher-order channels but not in the first-order channel because of a sediment supply limitation. These observations are consistent with a model for post-fire ravel routing in steep, rocky landscapes where sediment was sourced by incineration of vegetation dams—following ∼30 years of hillslope soil production since the last fire—and transported downslope by dry processes, leading to a hillslope sediment-supply limitation and infilling of low-order channels with relatively fine sediment. Our observations of debris flow initiation are consistent with failure of the channel bed alluvium due to grain size reduction from dry ravel deposits that allowed high Shields numbers and mass failure even for moderate intensity rainstorms.

scholarly journals Relationship between the accumulation of sediment storage and debris-flow characteristics in a debris-flow initiation zone, Ohya landslide body, Japan

2017 ◽  
Vol 17 (11) ◽  
pp. 1923-1938 ◽  
Author(s):  
Fumitoshi Imaizumi ◽  
Yuichi S. Hayakawa ◽  
Norifumi Hotta ◽  
Haruka Tsunetaka ◽  
Okihiro Ohsaka ◽  
...  
Keyword(s):  
Debris Flow ◽  
Debris Flows ◽  
Laser Scanning ◽  
Flow Characteristics ◽  
Sediment Supply ◽  
Physical Analysis ◽  
Saturated Flow ◽  
Initiation Zone ◽  
Landslide Body ◽  
Steep Terrain

Abstract. Debris flows usually occur in steep mountain channels and can be extremely hazardous as a result of their destructive power, long travel distance, and high velocity. However, their characteristics in the initiation zones, which could possibly be affected by temporal changes in the accumulation conditions of the storage (i.e., channel gradient and volume of storage) associated with sediment supply from hillslopes and the evacuation of sediment by debris flows, are poorly understood. Thus, we studied the relationship between the flow characteristics and the accumulation conditions of the storage in an initiation zone of debris flow at the Ohya landslide body in Japan using a variety of methods, including a physical analysis, a periodical terrestrial laser scanning (TLS) survey, and field monitoring. Our study clarified that both partly and fully saturated debris flows are important hydrogeomorphic processes in the initiation zones of debris flow because of the steep terrain. The predominant type of flow varied temporally and was affected by the volume of storage and rainfall patterns. Fully saturated flow dominated when the total volume of storage was  <  10 000 m3, while partly saturated flow dominated when the total volume of the storage was  >  15 000 m3. Debris flows form channel topography which reflects the predominant flow types during debris-flow events. Partly saturated debris flow tended to form steeper channel sections (22.2–37.3°), while fully saturated debris flow tended to form gentler channel sections ( <  22.2°). Such relationship between the flow type and the channel gradient could be explained by a simple analysis of the static force at the bottom of the sediment mass.

scholarly journals Contribution of Excessive Supply of Solid Material to a Runoff-Generated Debris Flow during Its Routing Along a Gully and Its Impact on the Downstream Village with Blockage Effects

Water ◽  
2019 ◽  
Vol 11 (1) ◽  
pp. 169 ◽  
Author(s):  
Ming-liang Chen ◽  
Xing-nian Liu ◽  
Xie-kang Wang ◽  
Tao Zhao ◽  
Jia-wen Zhou
Keyword(s):  
Debris Flow ◽  
Debris Flows ◽  
Large Scale ◽  
Residential Buildings ◽  
Solid Material ◽  
Sediment Supply ◽  
Mitigation Measures ◽  
Dry Land ◽  
Field Investigations ◽  
Abundant Supply

On 8 August 2017, a runoff-generated debris flow occurred in the Puge County, Sichuan Province of southwestern China and caused huge property damage and casualties (25 people died and 5 people were injured). Emergency field investigations found that paddy fields, dry land, residential buildings and roads suffered different degrees of impact from the debris flow. This paper reveals the formation process of the debris flow by analyzing the characteristics of rainfall precipitation and sediment supply conditions in the study area and it approaches the practical application of hazard prevention and mitigation constructions. Doppler weather radar analysis indicates that a very high intensity rainfall occurred in the middle and upper zones of the basin, illustrating the importance of enhancing rainfall monitoring in high-altitude areas. The abundant supply of deposits in gully channels is among the significant causes of a transformation from mountain floods to large-scale debris flows. It was also found that the two culverts played an important role in the movement affecting the processes of debris flows which has substantially aggravated the destructive outcome. The excessive supply of solid material and local blockage with outburst along a gully must receive significant attention for the prediction of future debris flows, hazard prevention and mitigation measures.

Towards real-time global assessment of post-fire debris flow hazards with remotely sensed data

2021 ◽  
Author(s):  
Elijah Orland ◽  
Dalia Kirschbaum ◽  
Thomas Stanley
Keyword(s):  
Remote Sensing ◽  
Debris Flow ◽  
Real Time ◽  
Debris Flows ◽  
Remotely Sensed Data ◽  
Model Framework ◽  
Burned Areas ◽  
Early Results ◽  
Fire Debris ◽  
Debris Flow Initiation

&lt;p&gt;As the risk of wildfires increases worldwide, burned steeplands are vulnerable to the secondary hazard of widespread sediment mobilization through debris flows. Following an initial burn, sediment and soil previously restrained by vegetation are no longer consolidated, allowing for easy mobilization into channels and along steep hillslopes through runoff. &amp;#160;Sufficiently powerful rainfall incorporates entrained material into turbulent flows and serves as the primary trigger for debris flow initiation. There is thus an ongoing need to establish the relationship between rainfall and debris flow initiation based on a variety of spatiotemporal preconditions. Previous work establishes regional and local thresholds to constrain the effect of rainfall in recently burned areas, but no empirical or numerical solution has worldwide application. Building from regionally-based efforts in the U.S., this work considers how remote sensing data can be applied to better approximate the post-fire debris flow hazards worldwide using freely available global datasets and software. Our work assesses the utility of remote sensing resources for analyzing burn characteristics, topography, rainfall intensity/duration, and, thus, debris flow initiation. Early results show that global observations are sufficient to delineate background rainfall rates from storms likely to cause debris flows across a variety of burn severity and topographic conditions. However, the dearth of publicly-available post-fire debris flow inventories globally limit the ability to test how the model framework performs within different climatologic and morphologic areas. This work will present preliminary analysis over the Western United States and demonstrate the feasibility of a global, near-real time model to provide situational awareness of potential hazards within recently burned areas worldwide. Future work will also consider how global or regional precipitation forecasts may increase the lead time for improved early warning of these hazards.&lt;/p&gt;

Consideration of the Validity of Debris-flow Bulking Factors

2017 ◽  
Vol 23 (4) ◽  
pp. 291-298
Author(s):  
Holly Brunkal ◽  
Paul Santi
Keyword(s):  
Debris Flow ◽  
Debris Flows ◽  
Rainfall Intensity ◽  
Peak Discharge ◽  
Data Set ◽  
Burned Areas ◽  
The Given ◽  
Basin Area ◽  
Bulking Factor ◽  
Significant Underestimation

Abstract Compilation of a database of debris-flow peak discharges (Q) allowed for a comparison with the expected basin discharge, as computed using the rational equation, Q=CIA. The observed values of Q for debris flows in unburned and burned areas were divided by the computed Q values of runoff using the rational method. This ratio is the bulking factor for that debris-flow event. Unburned and burned basins constitute two distinct populations; analysis shows that the bulking factors for burned areas are consistently higher than those for unburned basins. Previously published bulking factors for unburned areas fit the data set in about 50 percent of the observed cases in our compiled data set. Bulking factors for burned areas that were found in the published literature were well below the observed increases in peak discharge in over 50 percent of the cases investigated. If used for design purposes, these bulking factors would result in a significant underestimation of the peak discharge from a burned basin for the given rainfall intensity. Peak discharge bulking rates were found to be inversely related to basin area.

scholarly journals Headwater sediment dynamics in a debris flow catchment constrained by high-resolution topographic surveys

2016 ◽  
Vol 4 (2) ◽  
pp. 489-513 ◽  
Author(s):  
Alexandre Loye ◽  
Michel Jaboyedoff ◽  
Joshua Isaac Theule ◽  
Frédéric Liébault
Keyword(s):  
Debris Flow ◽  
Debris Flows ◽  
Laser Scanning ◽  
Early Spring ◽  
Sediment Dynamics ◽  
Sediment Supply ◽  
Power Law Distribution ◽  
Sediment Reworking ◽  
Sediment Budgets ◽  
Sediment Transfer

Abstract. Debris flows have been recognized to be linked to the amounts of material temporarily stored in torrent channels. Hence, sediment supply and storage changes from low-order channels of the Manival catchment, a small tributary valley with an active torrent system located exclusively in sedimentary rocks of the Chartreuse Massif (French Alps), were surveyed periodically for 16 months using terrestrial laser scanning (TLS) to study the coupling between sediment dynamics and torrent responses in terms of debris flow events, which occurred twice during the monitoring period. Sediment transfer in the main torrent was monitored with cross-section surveys. Sediment budgets were generated seasonally using sequential TLS data differencing and morphological extrapolations. Debris production depends strongly on rockfall occurring during the winter–early spring season, following a power law distribution for volumes of rockfall events above 0.1 m3, while hillslope sediment reworking dominates debris recharge in spring and autumn, which shows effective hillslope–channel coupling. The occurrence of both debris flow events that occurred during the monitoring was linked to recharge from previous debris pulses coming from the hillside and from bedload transfer. Headwater debris sources display an ambiguous behaviour in sediment transfer: low geomorphic activity occurred in the production zone, despite rainstorms inducing debris flows in the torrent; still, a general reactivation of sediment transport in headwater channels was observed in autumn without new debris supply, suggesting that the stored debris was not exhausted. The seasonal cycle of sediment yield seems to depend not only on debris supply and runoff (flow capacity) but also on geomorphic conditions that destabilize remnant debris stocks. This study shows that monitoring the changes within a torrent's in-channel storage and its debris supply can improve knowledge on recharge thresholds leading to debris flow.

scholarly journals Interactions between the accumulation of sediment storage and debris flow characteristics in a debris-flow initiation zone, Ohya landslide body, Japan

2017 ◽  
Author(s):  
Fumitoshi Imaizumi ◽  
Yuichi S. Hayakawa ◽  
Norifumi Hotta ◽  
Haruka Tsunetaka ◽  
Okihiro Ohsaka ◽  
...  
Keyword(s):  
Debris Flow ◽  
Debris Flows ◽  
Laser Scanning ◽  
Flow Characteristics ◽  
Sediment Supply ◽  
Small Scale ◽  
Physical Analysis ◽  
Initiation Zone ◽  
Landslide Body ◽  
Steep Terrain

Abstract. Debris flows often occur in steep mountain channels, and can be extremely hazardous as a result of their destructive power, long travel distance, and high velocity. However, their characteristics in the initiation zones, which could possibly be affected by temporal changes in the channel topography associated with sediment supply from hillslopes and the evacuation of sediment by debris flows, are poorly understood. Thus, we studied the interaction between the flow characteristics and the topography in an initiation zone of debris flow at the Ohya landslide body in Japan using a variety of methods, including a physical analysis, a periodical terrestrial laser scanning (TLS) survey, and field monitoring. Our study clarified that both partly and fully saturated debris flows are important hydrogeomorphic processes in the initiation zones of debris flow because of the steep terrain. The predominant type of flow varied temporally and was affected by the volume of storage and rainfall patterns. The small-scale channel gradient (on the order of meters) formed by debris flows differed between the predominant flow types during debris flow events. The relationship between flow type and the slope gradient could be explained by a simple analysis of the static force at the bottom of the sediment mass.

scholarly journals The temporally varying roles of rainfall, snowmelt and soil moisture for debris flow initiation in a snow dominated system: the compound trigger concept

2017 ◽  
Author(s):  
Karin Mostbauer ◽  
Roland Kaitna ◽  
David Prenner ◽  
Markus Hrachowitz
Keyword(s):  
Soil Moisture ◽  
Debris Flow ◽  
High Intensity ◽  
Debris Flows ◽  
Late Summer ◽  
Early Summer ◽  
Temporal Separation ◽  
Antecedent Soil Moisture ◽  
Trigger Mechanisms ◽  
Debris Flow Initiation

Abstract. Debris flows represent a severe hazard in mountain regions. Though significant effort has been made to predict such events, the trigger conditions as well as the hydrologic disposition of a watershed at the time of debris flow occurrence are not well understood. Traditional intensity-duration threshold techniques to establish trigger conditions generally do not account for distinct influences of rainfall, snowmelt, and antecedent moisture. To improve our knowledge on the connection between debris flow initiation and the hydrologic system and to overcome the above limitations, this study explores the use of a semi-distributed conceptual rainfall-runoff model, linking different system variables such as soil moisture, snowmelt, or runoff with documented debris flow events in the inner Pitztal watershed, western Austria. The model was run on a daily basis between 1953 and 2012. Analyzing a range of modelled system state and flux variables at days on which debris flows occurred, three distinct dominant trigger mechanisms could be clearly identified. While the results suggest that for 68 % (17 out of 25) of the observed debris flow events during the study period high-intensity rainfall was the dominant trigger, snowmelt was identified as dominant trigger for 24 % (6 out of 25) of the observed debris flow events. In addition, 8 % (2 out of 25) of the debris flow events could be attributed to the combined effects of low-intensity, long-lasting rainfall and transient storage of this water, causing elevated antecedent soil moisture conditions. The results also suggest a relatively clear temporal separation between the distinct trigger mechanisms, with high-intensity rainfall as trigger being limited to mid- and late summer. The dominant trigger in late spring/early summer is snowmelt. Based on the discrimination between different modelled system states and fluxes and more specifically, their temporally varying importance relative to each other, rather than their absolute values, this exploratory study demonstrates that already the use of a relatively simple hydrological model can prove useful to gain some more insight into the importance of distinct debris flow trigger mechanisms in a compound trigger concept, highlighting in particular the relevance of snowmelt contributions and the switch between mechanisms in early- to mid-summer in snow dominated systems.

scholarly journals Dry sediment loading of headwater channels fuels post-wildfire debris flows in bedrock landscapes

Geology ◽  
2019 ◽  
Vol 48 (2) ◽  
pp. 189-193 ◽  
Author(s):  
Roman A. DiBiase ◽  
Michael P. Lamb
Keyword(s):  
Debris Flow ◽  
Sediment Yield ◽  
Debris Flows ◽  
Shallow Landslides ◽  
Airborne Lidar ◽  
Sediment Supply ◽  
Burn Severity ◽  
Sediment Loading ◽  
Sediment Delivery ◽  
Flood Model

Abstract Landscapes following wildfire commonly have significant increases in sediment yield and debris flows that pose major hazards and are difficult to predict. Ultimately, post-wildfire sediment yield is governed by processes that deliver sediment from hillslopes to channels, but it is commonly unclear the degree to which hillslope sediment delivery is driven by wet versus dry processes, which limits the ability to predict debris-flow occurrence and response to climate change. Here we use repeat airborne lidar topography to track sediment movement following the 2009 CE Station Fire in southern California, USA, and show that post-wildfire debris flows initiated in channels filled by dry sediment transport, rather than on hillsides during rainfall as typically assumed. We found widespread patterns of 1–3 m of dry sediment loading in headwater channels immediately following wildfire and before rainfall, followed by sediment excavation during subsequent storms. In catchments where post-wildfire dry sediment loading was absent, possibly due to differences in lithology, channel scour during storms did not occur. Our results support a fire-flood model in bedrock landscapes whereby debris-flow occurrence depends on dry sediment loading rather than hillslope-runoff erosion, shallow landslides, or burn severity, indicating that sediment supply can limit debris-flow occurrence in bedrock landscapes with more-frequent fires.

Sign in / Sign up

Export Citation Format

Share Document