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High-Resolution Analysis of Debris Flow–Induced Channel Changes in a Headwater Stream, Ashio Mountains, Japan∗

2009; Routledge; Volume: 61; Issue: 2 Linguagem: Inglês

10.1080/00330120902743225

1467-9272

Thad Wasklewicz, Tsuyoshi Hattanji,

Landslides and related hazards

Abstract Coupled hillslope and channel processes in headwater streams (HWS) lead to rapid changes in channel dimensions. Changes in channel size and shape caused by a debris flow event along the length of a headwater stream in the Ashio Mountains, Japan, were captured with the aid of repeat high-definition surveys using terrestrial laser scanning (TLS) techniques. The HWS was classified into three distinct reaches below the debris flow initiation zone. A large knickpoint separated an upper bedrock reach from a colluvial reach along the midsection of the drainage. The colluvial reach transitioned to a lower bedrock reach that terminated at the master stream. Cross-sectional and morphometric analyses revealed no statistically significant changes in channel size or shape along the upper bedrock reach. Debris flow erosion generated significant differences in channel size and shape along a colluvial reach. Sediment bulking associated with erosion along the colluvial reach led to increases in channel size along the lower bedrock reach, but no statistical differences in channel shape. Morphometric analyses from the TLS point cloud revealed that debris flow erosion produced a distinct nonlinear change in channel dimensions in the downstream direction within the HWS. Variations in channel substrate along the length of HWS contributed directly to this nonlinear response. The episodic nature and nonlinearity of erosion associated with the current debris flow event highlights the importance of debris flows in general in understanding the transport of sediment, coarse to fine particulate organic material, and large woody debris, which are critical to the long-term management of riverine environments. TLS sampling methods show promise as one component of a multianalytical approach needed to continuously monitor and manage the dynamics of HWS. La acción conjunta de procesos en las laderas montañosas y en el lecho de las fuentes tributarias (HWS) conducen a rápidos cambios en las dimensiones del lecho de la corriente. Los cambios de tamaño y forma del lecho causados por un flujo de debris a lo largo de las fuentes en los Montes Ashio, Japón, fueron captados con la ayuda de repetidas observaciones de alta definición, utilizando técnicas de escaneo terrestre con láser (TLS). El sector de las fuentes del río se clasificó en tres tramos diferentes por debajo de la zona de iniciación del flujo de debris. Una gran ruptura de gradiente separó el tramo superior de estructura rocosa de otro tramo coluvial, a través de la sección media del área de avenamiento. El tramo coluvial hizo transición hacia una estructura rocosa inferior que termina en la corriente principal. Los análisis transversos y morfométricos no revelaron cambios estadísticamente significativos en el tamaño o forma del lecho del río en el tramo superior de roca firme. La erosión del flujo de debris generó diferencias significativas de tamaño y forma del lecho a lo largo del tramo coluvial. El abultamiento de sedimentos asociados con erosión en el tramo coluvial llevaron al aumento de tamaño del lecho en el tramo inferior de roca firme, pero no produjeron diferencias estadísticas en la forma de aquél. Los análisis morfométricos de la nube puntos del TLS revelaron que la erosión del flujo de debris produjo un claro cambio no lineal en las dimensiones del lecho en dirección de la corriente dentro de la HWS. Las variaciones en el substrato del lecho, a lo largo de la HWS, contribuyeron directamente a esta respuesta de falta de linealidad. La naturaleza episódica y la ausencia de linealidad de la erosión asociadas con el evento del flujo de debris destaca la importancia de estos flujos en general para la comprensión del transporte de sedimentos, de material orgánico particulado tanto grueso como fino y debris leñoso grande, lo cual es crítico para el manejo de ambientes fluviales a largo plazo. Los métodos de muestreo TLS son prometedores como uno de los componentes de un enfoque multianalítico, necesario para monitorear y manejar continuamente la dinámica de las HWS. Key Words: debris flowheadwater streamsterrestrial laser scanning关键词: 泥石流上游流地面激光扫描Palabras clave: flujo de debrisfuentes tributariasescaneo terrestre por láser THAD A. WASKLEWICZ is an Associate Professor in the Department of Geography at East Carolina University, Greenville, NC 27858. E-mail: wasklewiczt@ecu.edu. His research interests include headwater streams and debris flow processes and forms and the application of terrestrial laser scanning in geography. TSUYOSHI HATTANJI is an Assistant Professor in the Graduate School of Life and Environmental Sciences and a faculty member in the Geomorphology Lab at the University of Tsukuba. E-mail: hattan@geoenv.tsukuba.ac.jp. His research examines the effect of underlying lithologies on channel initiation processes landslides (analyses and experiments). Notes ∗∗∗indicates significance at 0.05 level. aR = average rebound value of Schmidt hammer. N is number of shots. ∗The authors would like to acknowledge that this material is based on work supported by the National Science Foundation under Grant No. 0239749. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Further support was provided through Japanese Society for the Promotion of Science Short-Term Fellowship No. 04004. Special thanks are in order to Professor Takashi Oguchi and the Center for Spatial Information Science (University of Tokyo) for making the initial connection between Professors Wasklewicz and Hattanji and hosting Professor Wasklewicz during his stay in Japan. Yuichi Hayakawa, PhD (JSPS Fellow, Geomorphology Lab, Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan), aided with the fieldwork. Dennis Staley, PhD (United States Geological Survey, The Geologic Hazards Team, Landslide Hazards Program, Denver Federal Center), assisted with the fieldwork and provided an initial review of the manuscript. Two anonymous reviewers provided comments that greatly improved the final product.