Research - Category I: Hydrology, Climatology & Hydraulics

A Stochastic Sediment Supply Model for a Mountainous, Semi-Arid Landscape
(Funded 1999-2000)

Principal Investigator: Thomas Dunne
Environmental Science and Management
UC Santa Barbara
(805) 893-7557
tdunne@bren.ucsb.edu

Executive Summary:
This study will model the delivery of sediment from hillslopes into river channels in a mountainous, semi-arid landscape by directly linking climatic events and fires with sediment transport processes. Current theory hypothesizes that chronic, low magnitude erosional processes, such as sheetwash, transport sediment directly into channels but also concentrate sediment into convergent areas on hillslopes (hollows). Over time, the soil in these hollows thickens, becomes unstable, and may fail catastrophically as debris flows during rainstorms. These debris flows are the main source of sediment to river channels in steep watersheds.

This study will investigate both the chronic and catastrophic erosional processes at and around Sedgwick Ranch, a UC Natural Reserve near Santa Barbara. The area has a cover of rangeland and chaparral on a wide range of hillslopes gradients and two dominant lithologies. Much of the research will be done through rainfall simulation experiments performed on a variety of terrain characteristics (e.g. lithology, vegetation, and slope). During these experiments, the project will investigate the mechanics of sediment transport by sheetwash and rainsplash. Furthermore, these rainfall experiments will allow the examination of the partitioning of rainfall between surface and subsurface pathways, which determines the relative importance of sheetwash and shallow landsliding. The results of these experiments will be complemented by a thorough mapping and surveying of the dozens of debris flows that occurred at Sedgwick Ranch during the El Niño-driven rainstorms of winter 1997-98.

The results from the fieldwork and rainfall simulations will be used as the basis for a computer model that simulates watershed-scale erosion. Random sequences of rainstorms and fires will drive the erosional processes and establish the relationship between climate and landscape response. This model will help land managers estimate the influx of sediment into river channels according to different land use practices (e.g. grazing intensity, controlled burns, etc.) and will help city planners to identify areas prone to debris flows and floods. This issue has economic as well as scientific relevance because, as the urban fringe in California continues to climb higher into the mountains, the cost of these hydrologic and geological hazards will continue to escalate.