Modeling the Interaction of Landslides, Debris Flows, and the Channel Network

Stephen T. Lancaster1 and Gordon E. Grant2

1. Dept. of Geosciences, Oregon State University (

2. Forestry Sciences Laboratory, Pacific Northwest Research Station, U.S. Forest Service


Landscape-level models are increasingly being used to evaluate effects of alternative management practices on key geomorphic and biological processes. Where forest practices are of interest, as in the Oregon Coast Range, landscape models need to be sensitive to the wide range of potential influences and feedbacks between vegetation and geomorphic processes such as landsliding, debris flows, and channel evolution. We are developing such a model that incorporates vegetation influences on landslide initiation, debris flow runout, and meso-scale (i.e., decadal to century) channel morphologic change. The model data structure attempts a balance between conflicting needs: to model large areas while retaining as much information about the landscape as possible. Landslide modeling requires detailed knowledge of the landscape, e.g., topography, vegetation, and soil. Modeling the effects of debris flows on the channel network requires knowledge of channel input from large debris flow source areas. Despite advances in computing power, detailed process modeling of large areas at fine discretization remains intractable. In our model, the landscape is represented by a mesh with variable discretization. Contiguous, channel-adjacent debris flow source areas are aggregated, and debris flows from these areas are routed through the channel network and interact with the sediment and wood stored in the channel. Preliminary simulations reveal the importance of management history, cutting patterns, and drainage network architecture on the pattern and timing of debris flows within the basin.

Most forests are managed. As a result, landslides and debris flows are managed implicitly. Our objective is to model landsliding and debris flow runout in some detail over large areas and a century in time as part of the Coastal Landscape Analysis and Modeling Study (CLAMS). We want to capture the interactions between the forest and the landslide/debris flow process. What are the process implications of management decisions?

conceptual diagram of models in area-descriptiveness space

``Area-Descriptiveness'' Space:

  • Landscape modeling usually involves a trade-off between the descriptiveness of the model and the area modeled.

  • Process models usually describe a small area in detail over geologic time.

  • GIS-based index models usually describe a large area in less detail at a static moment in time.

  • We attempt a compromise in area-descriptiveness space by lumping landslide source areas and resolving the channel network at high resolution.

Aggregation of landslide/debris flow source areas:

  • Initial calculations on each small node (e.g., 10m x 10m) result in a fine-scale map of landscape characteristics, e.g., soil depth, vegetation age, and landslide susceptibility.

  • Channel source basins and channel-adjacent areas form aggregates.

  • In each aggregate, areas with similar landslide susceptibility are binned, and average values of, e.g., soil depth, etc., are stored for each bin.
conceptual diagram of basin aggregation

Trial watershed: small tributary to Knowles Creek, Oregon Coast Range:

color-coded conceptual diagram of process regimes

The forest's influence on landslides and debris flows:

Effect of stand age on landslide initiation and debris flow runout and deposition:

  • Aggregate boundaries for the trial watershed are shown at right. Small blue lines represent aggregate outlets to the channel/valley network. Nominal aggregate size is 1 ha.

  • System driven with 10 yr. stochastic storm series, exponentially distributed storm intensities and durations and inter-storm durations (Eagleson, 1978): mean storm intensity 1.7 mm/hr; mean storm duration 20 hrs. (Benda and Dunne, 1997); mean interstorm duration 5 days (Duan, 1996).
plan view of basin aggregates
side view of network deposition
  • Over time, trees grow. Trees also fall, where the number of tree falls at a node is exponentially distributed and depends on the ratio of storm intensity to root strength. Tree fall re-distributes wood among neighboring nodes, e.g., to channel nodes from valley nodes.

  • Channel/valley networks shown in profile above for various initial forest ages. Line color represents deposit composition, and line thickness represents deposit thickness. Each line is associated with the node at its upstream end and connects that node to its downstream neighbor.

  • Maximum deposit thickness and total sediment and wood volume output (i.e., leaving the basin) are plotted vs. initial forest age at right.

  • For youngest stand, thickest deposits are near the outlet. Older stands result in thicker, woodier deposits further upstream along the channel network.

  • Sediment and wood output with the 20 yr.-old stand are dramatically lower than with the 10 yr.-old stand. Likewise, sediment output with the 40 yr.-old stand drops to zero (zero is plotted at 100 on the logarithmic axis). With older stands (but younger than 200 yrs.), outputs are greater. Outputs with the 200 yr.-old stand are lowest of all.
max. deposit depth and basin output




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Last Modified: June 25, 1999