ASSESSMENT OF DEBRIS-FLOW HAZARDS, NORTH MOUNTAIN, PHOENIX, AZ

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Kathryn Jeanette Reavis (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: Population increases in many western cities have led to urban sprawl, which has been a constant issue in metropolitan Phoenix, AZ. The Metropolitan Phoenix Area (MPA) has sprawled exponentially since its initial development and in doing so has expanded into surrounding mountainous areas and onto alluvial fans. Alluvial fans, particularly in the western US, provide cooler, more scenic environments for people to live. Alluvial fans are conic-shaped features occurring in piedmont areas prone to floods and debris-flows. Debris-flows often supply large quantities of material to alluvial fans and are critical to the long-term development of alluvial fans. A key to understanding alluvial fan evolution is quantifying debris-flow dynamics above and below the fan apex. The addition of humans and built structures into these environments increases the risk of exposure to debris-flows, and therefore the vulnerability of people and their housing structures. This scenario magnifies the importance of understanding debris-flow hazards from a holistic multidisciplinary perspective. The scientific understanding of the evolution of alluvial fan systems needs to be instilled to protect society via engineered solutions and planning decisions. These management schemes must be grounded in a quantitative understanding of debris-flow behavior. A 2D debris-flow modeling approach, aided by high-resolution air-borne laser scanning (ALS) and terrestrial laser scanning (TLS) topographic data, is implemented to examine debris-flow behavior in a densely populated portion of the MPA and to assess the vulnerability of the built infrastructure to debris-flow damage. A calibrated 2D debris-flow model is developed for a recent debris-flow at an undeveloped site and applied to a developed site - Shaw Butte at North Mountain, a populated area with historical evidence of debris-flow activity. Several values were maintained from the calibrated model and these include: antecedent moisture conditions\; an estimated volume\; and sediment concentration by volume (Cv). Four separate scenarios were applied to the developed site with variations in precipitation events, including the historical rainstorm that initiated the debris-flow at Elephant Mountain, two higher magnitude seasonally average rainstorms, and one maximum event that made use of the largest recorded rainstorm for the area. Overall the results show that the highest debris-flow depths and velocities, and therefore the highest debris-flow intensities occur at the fan apex and within the existing debris-flow channel, directly below the fan apex. The complexity of the alluvial fan topography plays a major role in the flow direction. For instance, in all four scenarios, the flow did not exit into the developed neighborhood perpendicular to the fan apex (a northerly direction at the site), rather the topography steered flows in a northwesterly direction. Homes on the western side of the neighborhood are the most vulnerable to debris-flow inundation. The addition of impervious surfaces such as roadways on the alluvial fan also has an influence on debris-flow behavior as these areas provide an ideal surface for maintaining high velocities and therefore have an impact by amplifying the runout distances and the areal extent of inundation. The building vulnerability index results show that, depending on the magnitude of a potential debris-flow, a range of 22 to 44 homes are potentially in danger of damage. These results have scientific and applied merit. The 2D debris-flow modeling provides new insight into how drainage basin and alluvial fan topography influence debris-flow inundation, velocity, and runout. The 2D modeling coupled with the building vulnerability index provides a broader understanding of societal implications. These results are important in that they can provide communities and hazard management agencies with decision-making data and mitigation information based upon the degree of risk and therefore vulnerability associated with different debris-flow magnitudes.

Additional Information

Publication

Thesis

Language: English

Date: 2023

Subjects

Geomorphology;Geography;3-D laser scanning;Debris flows;Natural hazards

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