DEBRIS FLOW RISK ASSESSMENT PRACTICE IN WESTERN OREGON - A CONSULTANT'S VIEW

Regardless of the state of science, time and budgetary constraints will probably always result in a use of proxy parameters (such as slope) to define risk of debris flow impact at a specific site rather than actual quantification of the risk. In most cases, the criteria developed by the Oregon Department of Forestry (ODF) provide for clear decisions regarding the legality of harvest of a particular forest unit. Therefore, consultants are only retained by the forest industry in borderline cases. Although the final decision still rests with the ODF, the author believes that it is the consultant's job to provide sufficient scientific evidence to either allow the ODF expert to decide that the risk of debris flow impact is sufficiently small to permit harvest, or to prove to the client that harvest of the unit presents a greater potential liability than the profits of harvest. In practice, the best evidence is found by examination of the geomorphology and primary sedimentary genesis in the vicinity of the potential impact zone. The underlying assumption is that the risk of future impacts at a specific location can be derived from evidence related to past characteristics and patterns of deposition. In using this approach, it is assumed that the main erosional and transport process active on the steep slopes of the Oregon Coast Range is localized removal of accumulated soil and weathered rock by debris flows at times of high precipitation and delivery of the material to streams. Whereas timber harvest may influence the timing of debris flow initiation, no significant change in the basic processes or dynamics is expected. As a result, the presence of poorly sorted sediments at the potential impact site is interpreted as an indication that debris flows have previously been deposited there. The absence of such deposits and presence of stratified, sorted sediments is taken as evidence that debris flows, if initiated higher in the drainage, have not reached the point of concern in the past and are not likely to do so in the future. Specific examples are used to illustrate this approach to debris flow risk assessment.