AN INTERDISCIPLINARY APPROACH TO EVALUATION OF POTENTIAL INSTABILITY IN ALLUVIAL CHANNELS

A modular procedure to assess the magnitude, distribution, and potential for channel instabilities at a large number of sites has been designed and implemented. The procedure, based on diagnostic interdisciplinary criteria of alluvial channel morphology and associated riparian vegetation, is presented. The modules include (1) initial site evaluations, (2) GIS-based data input and management, (3) ranking of relative channel stability, (4) identification of spatial trends, (5) ranking of socio-economic impacts and identification of most ''critical'' sites, and (6) collection of additional field data for more detailed evaluation of the magnitude and type of future instabilities and the effects of proposed mitigation measures. The procedure, using site evaluation forms as the fundamental means of data collection, takes a trained person 1 to 1.5 hours to complete. Site evaluation forms can be altered according to the specific environment being studied and the objectives of the study. An objective ranking scheme based on physical attributes extracted from the GIS data base permits the identification of the most unstable channel sites and, thereby, focuses attention on potentially ''critical'' sites. If a significant concern about a bridge or adjacent lands arises, or if mitigation measures are considered, various methods to estimate future channel changes are proposed. These include (1) numerical alluvial channel modeling, (2) empirical models of channel evolution, (3) regime equations, and (4) empirical relations based on process dominance in different fluvial environments. The methods require differing amounts of additional field data and provide results of varying detail and accuracy. The decision on which method to use must be based on the objectives and resources of the agency involved in the evaluation study.