STOCHASTIC FORCING OF SEDIMENT SUPPLY TO CHANNEL NETWORKS FROM LANDSLIDING AND DEBRIS FLOW

Sediment influx to channel networks is stochastically driven by rainst orms and other perturbations, which are discrete in time and space and which occur on a landscape with its own spatial variability in topogr aphy, colluvium properties, and state of recovery from previous distur bances. The resulting stochastic field of sediment supply interacts wi th the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that chara cterize the sedimentation regime of a drainage basin. The regime varie s systematically with basin area. We describe how the stochastic sedim ent supply is generated by climatic, topographic, geotechnical, and bi otic controls that vary between regions. The general principle is illu strated through application to a landscape where sediment is supplied by mass wasting, and the forcing variables are deterministic thickenin g of colluvium, random sequences of root-destroying wildfires, and ran dom sequences of rainstorms that trigger failure in a population of la ndslide source areas with spatial variance in topography and colluvium strength. Landslides stop in channels or convert to scouring debris f lows, depending on the nature of the low-order channel network. Sedime nt accumulates within these channels for centuries before being transf erred downstream by debris flows. Time series of sediment supply, tran sport, and storage vary with basin scale for any combination of climat ic, topographic, and geotechnical controls. In a companion paper [Bend a and Dunne, this issue] we use simulations of timing, volumes, and lo cations of mass wasting to study the interaction between a stochastica lly forced sediment supply and systematic changes of storage and flux through channel networks.