DOI

10.17077/etd.ehdswuwz

Document Type

Dissertation

Date of Degree

Fall 2016

Access Restrictions

Access restricted until 02/23/2020

Degree Name

PhD (Doctor of Philosophy)

Degree In

Geoscience

First Advisor

Weirich, Frank H

First Committee Member

Campbell, David

Second Committee Member

Dorale, Jeff

Third Committee Member

Eichinger, William

Fourth Committee Member

Barnhart, William

Abstract

Ground Penetrating Radar (GPR) methods have been used to evaluate the presence, extent, and spatial variability of hydrophobic soils in Southern California Watersheds. It has been shown that high frequency ground penetrating radar equipment, under certain conditions, has the ability to determine the presence, depth, and persistence of post fire hydrophobic soils. As part of this study an extensive investigation was undertaken to not only evaluate the capability of this approach but also to understand under what conditions the method can be applied successfully and what are the limitations of the approach. The investigation includes use of computer simulations and modeling, laboratory investigations in sand boxes with native soils, and multiple field trials spanning a five year time period. Of particular significance is the finding that using GPR it is possible to: locate the interface between the uppermost burnt soil layer, and soil horizons below; quantify the depth at which the hydrophobic layer forms; and quantify the spatial extent of the layer. As part of this study best practice methods for both field and lab experimentation have also been developed and are presented in the body of the thesis. Based on this study it is concluded that the use of GPR can provide a much more accurate and comprehensive method of evaluating the nature of hydrophobic layers in such environments than the current point specific manual methods. As a result the use of GPR has significantly advanced our capacity to assess the potential for increased erosion and the generation of debris flows in such environments after rainfall events.

Public Abstract

Wildfires are responsible for the creation of water repellent soils (also known as hydrophobic soils) that can inhibit or completely prevent the infiltration of water into the soil profile, which increases surface runoff dramatically. Enhanced surface runoff over de-vegetated and water repellent land surfaces, especially at locations with a large amount of topographic relief, can lead to very high levels of sediment transport, and in some instances dangerous mass wasting events such as debris flows, both of which have dramatic and long lasting effects for the watersheds in which they occur. Current methods to evaluate post fire hydrophobic soils do exist, however they are limited in their evaluation capabilities and cannot spatially image the depth and variability at which a hydrophobic soil layer is created. In this study, Ground Penetrating Radar (GPR) methods have been used to successfully evaluate the presence, extent, and spatial variability of hydrophobic soils in selected Southern California Watersheds. Evaluation of both the applicable conditions as well as capability constraints associated with using high frequency GPR to evaluate the presence, extent, and spatial variability of hydrophobic soils are presented. The development of this entirely new method to identify, spatially image, and quantify the depth of hydrophobic soils should fill an acknowledged capability gap that currently exists in the study of and assessment of post wildfire hydrophobic soils. The methods outlined in this research may potentially lead to significant improvements over existing practices. The use of GPR can provide a much more accurate and comprehensive method of evaluating the nature of hydrophobic layers in such environments. As a result, the use of near surface GPR may significantly advance our capacity to assess the potential for increased erosion and the generation of hazardous debris flows in fire impacted watersheds in Southern California.

Keywords

GPR, Ground Penetrating Radar, Hydrophobic Soils

Pages

xxiv, 204 pages

Bibliography

Includes bibliographical references (pages 202-204).

Copyright

Copyright © 2016 William John Neumann III

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