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Analysis of Post-Wildfire Debris Flows: Climate Change, the Rational Equation, and Design of a Dewatering Brake

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Analysis of Post-Wildfire Debris Flows: Climate Change, the Rational Equation, and Design of a Dewatering Brake ANALYSIS OF POST-WILDFIRE DEBRIS FLOWS: CLIMATE CHANGE, THE RATIONAL EQUATION, AND DESIGN OF A DEWATERING BRAKE. by Holly Ann Brunkal A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geological Engineering. Golden, Colorado Date ________________________ Signed:_____________________________ Holly Ann Brunkal Signed:_____________________________ Dr. Paul M. Santi Thesis Advisor Golden, Colorado Date _______________________ Signed:_____________________________ Dr. Paul M. Santi Professor and Head Department of Geology and Geological Engineering ii ABSTRACT This dissertation presents the results of two lines of inquiry into the frequency and magnitude of post-wildfire debris flows, and a third investigation into the design parameters for a debris-flow mitigation structure are presented as an advancement of the current body of knowledge on the hazards and risks of post-wildfire debris flows, and the consideration of a potential mitigation design. Increasing areas burned by wildfire and increasing intense precipitation events with predicted climate change will produce a significant increase in the occurrence of post-wildfire debris flows in the western United States. A positive correlation is shown between an increase in wildfire area and number of debris flows. The probability of a debris flow occurring from a burned watershed is influenced by climate change. With conservative model interpretation, post-wildfire debris-flow probabilities for individual drainage basins increase on average by 20.6%, with different climate scenarios increasing the probability of post-wildfire debris flows by 1.6% to 38.9%. A predictive debris-flow volume equation for the Intermountain West is influenced by factors that will be affected by climate change in the coming decades, and debris-flow volumes are calculated to increase with changing conditions by 3.7% to 52.5%. Understanding the future implications of increased incidence of wildfire-related debris flows will help agencies and communities better manage the associated risk. 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; where C= an infiltration coefficient, I is the rainfall intensity, and A is the area of the basin. 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 iii event when compared with water flooding. It was shown that unburned and burned basins constitute two distinct populations for debris–flow bulking, and that the bulking factors for burned areas are consistently higher than for unburned basins. Previously published bulking factors for unburned areas fit the dataset in about 50% of the cases. Conversely, the bulking factors for burned areas that were found in the published literature were well below the increases seen in over half of the cases investigated in this study, and 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. Knowledge of the potential increase to the peak discharge from a basin during a debris flow event will help workers better design conveyances and thus will reduce risk to proximal infrastructure. While the first two studies address gaps in knowledge for design events, the third study considers the design elements in the debris –flow mitigation process. The investigation looks specifically at a mitigation structure whose design elements are not well-documented in the literature. A small-scale flume experiment was conducted to assess the design considerations for a horizontal dewatering debris flow brake. A design sequence, which was previously unavailable in the published literature, is developed from comparison to other mitigation design strategies and from results of laboratory flume experiments. It is concluded that the most important input parameters into the design of a debris-flow dewatering brake are the expected thickness of the debris flow deposit and the channel shape. The volume of debris that can be stopped and stored by this mitigation design is a function of the debris flow depth and the channel slope. The thickness of the debris that is arrested on the grate depends on the depositional properties of the debris-flow mass, such as the unit weight of the material, but was not affected by volume of the debris available. The ideal brake is a free-draining surface with an aperture smaller than the D90 iv value for the debris-flow grain size distribution. An easily implemented design that could be rapidly installed in the channel to reduce the velocity and volume of a debris flow is the goal of this dewatering structure. This design has the potential of being implemented in recently burned areas to reduce the debris-flow risk to areas downstream. Evaluation of the hazard posed by and the potential risk of, a debris-flow event involves many variables. Two variables in risk assessment are the likelihood of an event happening and the severity of that event, in addition to the likely extent of the losses if a particular event takes place. This research shows that there is an increase in the likelihood of post-wildfire debris flows happening with climate change, and that the magnitude of debris –flow events, with respect to the peak discharge measurement, is more severe in a post-wildfire setting than in an unburned basin. Post-wildfire debris flow risk could be reduced for communities at the wildland urban interface by the implementation of a horizontal debris-flow dewatering brake. v TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ..................................................................................................................... viii LIST OF TABLES ......................................................................................................................... ix ACKNOWLEDGMENTS .............................................................................................................. x CHAPTER 1 INTRODUCTION .................................................................................................... 1 CHAPTER 2 EXPECTED INCREASE IN POST-WILDFIRE DEBRIS FLOWS WITH CLIMATE CHANGE .................................................................................................................. 6 2.1 Abstract ................................................................................................................................. 6 2.2 Introduction ........................................................................................................................... 6 2.3 Climate Models ................................................................................................................... 12 2.3.1 Climate and fire models ................................................................................................ 14 2.3.2 Climate and precipitation models ................................................................................. 17 2.4 Debris flow initiation in burned areas ................................................................................. 19 2.5 Methods ............................................................................................................................... 21 2.5.1 Data acquisition, post-wildfire debris flows ................................................................. 22 2.5.2 Database analysis .......................................................................................................... 23 2.5.3 Analysis of better-documented post-wildfire debris flows ........................................... 23 2.5.4 Analysis of post-wildfire debris-flow probabilities ...................................................... 24 2.6. Results/Discussion ............................................................................................................. 26 2.6.1 Data set analysis ........................................................................................................... 28 2.6.2 Predicting post-wildfire debris-flow potential and volume estimates with climate change models ....................................................................................................................... 30 2.7. Conclusions ........................................................................................................................ 34 CHAPTER 3 CONSIDERATION OF THE APPLICABILITY OF DEBRIS-FLOW BULKING FACTORS............................................................................................................... 36 3.1 Abstract ............................................................................................................................... 36 3.2 Introduction ......................................................................................................................... 36 3.3 The rational method/rational equation ................................................................................ 39 3.4 Debris flow peak discharge ................................................................................................. 44 3.5
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