Erosion of steepland valleys by debris flows

Abstract

Episodic debris fl ows scour the rock beds of many steepland valleys. Along recent debris-fl ow runout paths in the western United States, we have observed evidence for bedrock lowering, primarily by the impact of large particles entrained in debris fl ows. This evidence may persist to the point at which debris-fl ow deposition occurs, commonly at slopes of less than ~0.03-0.10. We fi nd that debris-fl ow-scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river-incision models. Much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris fl ows. The degree of weathering of valley fl oor bedrock may also decrease abruptly downstream of such junctions. On the basis of these observations, we hypothesize that valley slope is adjusted to the long-term frequency of debris fl ows, and that valleys scoured by debris fl ows should not be modeled using conventional bedrock river-incision laws. We use fi eld observations to justify one possible debris-fl ow incision model, whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the fl ow and the number of upvalley debris-fl ow sources. The model predicts that increases in incision rate caused by increases in fl ow event frequency and length (as fl ows gain material) downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes, and stronger, less weathered bedrock. These adjustments lead to a spatially uniform lowering rate. Although the proposed expression leads to equilibrium long-profi les with the correct topographic signature, the crudeness with which the debris-fl ow dynamics are parameterized reveals that we are far from a validated debris-fl ow incision law. However, the vast extent of steepland valley networks above slopes of ~0.03-0.10 illustrates the need to understand debris-fl ow incision if we hope to understand the evolution of steep topography around the world. Figure 1. Debris-fl ow valley network in an Oregon Coast Range clear-cut. The combination of elevated water pressure during a 1996 storm and reduced root strength initiated landslides at valley heads that mobilized as debris fl ows, scouring sediment and the Eocene Tyee sandstone (white areas) along the runout. Road at top right indicates scale.

Correction to “Experimental study of bedrock erosion by granular flows”

“…Hence the predicted erosion rate depends not only on local physics of grain collisions with the bed, but also the network structure and associated frequency of landsliding. Stock and Dietrich [2006] summarize field and literature observations that indirectly support this hypothesis. Key untested assumptions in the model are that bedrock wear rate is proportional to grain collisional stresses, and that these stresses can be approximated by the product of the squares of both particle grain size and shear rate (inertial stresses, in the sense of Bagnold [1954] and Iverson [1997]).…”

“…According to Bagnold [1954], grain size enters the problem in several roles: the scale for the mass of the particle of interest, the scale for the vertical spacing between shear layers, the scale for the number of particles per bed area, and the scale for the number of collisions per unit time. In heterogeneous grain mixtures, it is not obvious that one grain size would serve as the scale for all of these terms, though one could argue that there is an effective grain size, D e , whose mass and concentration capture the characteristics of the size distribution [Stock and Dietrich, 2006]. Since the D p term is squared, the choice between using the smaller size fraction or the larger size fraction as the representative diameter can change the inertial stress estimation by several orders of magnitude.…”

“…[10] For low-slip, collision-dominated wear, Stock and Dietrich [2006] proposed a rate law to express the long-term erosion of successive debris flows:…”

“…For bouldery debris flows, Stock and Dietrich [2006] have proposed that the coarse, fluid-poor snout should exert the greatest collisional stress on the underlying bedrock, while the relatively fluid-rich, finer-grained tails may be less important to channel wear. In the coarse-grained snout, the large boulders impact the bedrock channel, causing abrasion and surface fatigue wear.…”

“…[2] As rock avalanches and debris flows surge down steep valleys, they entrain loose material [e.g., Dietrich and Dunne, 1978;Pierson, 1980;Suwa and Okuda, 1980;Hungr et al, 1984;Benda, 1990] and generate granular collisions with the bed, which can cause visible wear and plucking of the underlying bedrock [Stock and Dietrich, 2003;Papa et al, 2004;Stock et al, 2005;Stock and Dietrich, 2006]. These mass failure events fall under the general category of granular flows, gravity-driven masses of discrete solids with an interstitial fluid [Campbell, 1990].…”