INTRODUCTION

Much of the studies and literature on mass movement of soil and rock materials have focused on catastrophic landslide events. In many areas, the subtle, slow movement processes slump, creep, and earth flow can account for more erosion in the long run. These processes pose important land-management problems in the western Cascades and the coast range.

Road construction and timber harvest negatively influence slope stability and increase landslide activity. These processes have led to the examination of the processes, histories, and movement patterns in the terrains of the Pacific Northwest.

ABSTRACT

There are many mass movement processes that affect the terrain that we live in, and that surround us. This is extremely evident here in the Willamette Basin. We are surrounded by mountain ranges; to the east are the Cascade mountain range, and the west, the Coast Range. Steep mountain slopes combined with intense precipitation patterns render this region particularly prone to landslides and debris flow. These processes are in some cases deadly, and result in millions of dollars in property damage annually.

Mass movement processes in upland subbasins of the Willamette include creep, earthflow, landslide, slump, and debris flow. These processes have different magnitudes and frequencies, depending on terrain factors and landuse history. Climate, snowpack, road cuts, vegetation, timber harvesting, slope steepness, geology and soils are all factors controlling the occurrence of these mass movement processes. Understanding of these processes is critical for the development of regional watershed management and hazards mitigation plans.

TYPES OF MASS MOVEMENT

1.) Creep: A process of slow, down slope movement of mantle material in response to gravitational stress.

2.) Landslide: Masses of regolith and other earth material that slide downslope due to gravity and other natural or human induced causes.

3.) Earthflow: Where slow moving earth material slips downslope and is broken up.

4.) Debris Flows: Are gravity induced mass movements of generally course grained material.

FACTORS INFUENCING LANDSLIDES AND DEBRIS FLOWS

•Geologic, hydrologic, and vegetative factors control the spatial distribution and movement rates of occurrences of natural mass-erosion processes.

• Geology and Soils: The western Cascades are generally composed of Tertiary lava flows and volcaniclastic rocks. Which in some areas, undergo extensive weathering and erosion.

• Climate: The Willamette Basin is characterized by maritime climate consisting of wet, mild winters, and dry summers.

• Vegetation: Slope stability in the western Cascades and Coast Range is controlled in part by the extensive cover of forests of Douglas fir, western Hemlock and other plant species. •Modification of Forest Cover: Removal of forest cover by fire or timber harvest of man will lead to decreased rooting strength and an altered hydrologic regime at the site.

EXPERIMENTAL TECHNIQUES Scientific understanding of debris flows have been hindered by there unpredictable timing, location and magnitude which makes systematic observation and measurement of natural events both difficult and dangerous. Consequently, in 1991, the U.S.G.S. and the U.S. Forest Service constructed a flume to conduct controlled experiments on debris flows. To create a debris flow, up to 20 cubic meters of sediment is placed behind a steel gate at the head of the flume, saturated with water from subsurface channels and surface sprinklers, and is then released. Alternatively, a sloping mass of sediment can be placed behind a retaining wall at the head of the flume and is watered until slope failure occurs. These experiments are conducted to further understand the nature of debris flows, and get a better idea on how to predict when and where they will happen in the real world.

CONCLUSION Complex mass-movement terrains supply large quantities of sediment and

organic debris to streams, thus altering channel geometry. Knowing when and where landslides and debris flows are going to occur can

be very beneficial, and to further understand these processes will give us the knowledge to prevent further mass wasting, water contamination, property damage, and even human deaths.

To prevent future landslides and debris flows, there needs to be more knowledge, precautions, and regulations applied into new road construction, timber harvesting, and even urban development.

With better understanding of the geology, we can cut down on mistakes that we as humans make on a daily basis.

Figure 4: Experimental debris flow Flume

REFERENCES CITED Benda, L., and T. Dunne, 1997. Stochastic forcing of sediment supply to channel networks from landsliding and debris flow, Water Resources Research, 33(12), 2849-2864. Iverson, R.M., 1997. The physics of debris flows, Reviews of Geophysics, 35(3), 245-296. Sidle, R.C., 1992. A theoretical model of the effects of timber harvesting on slope stability, Water Resources Research, 28(7), 1897-1910. Carson, M. A., and Kirkby, M. J., 1972, Hillslope form and processes: London, Cambridge Press, 475.p. About. Com, 1997.

Landslide and Debris Flow in Headwaters of the Willamette Basin

Prepared by: Donnie Kasper, Earth Science Major, Western Oregon University

Figure 2: Above, landslide and its features. (About. com, 1997.) Figure 3: Below, debris track in the Western Cascades. (About. com, 1997)

Figure 1: Illustrations of mass movement processes. (Benda, 1997)