Regional debris‐flow and debris‐flood frequency–magnitude relationships

Construction of frequency-magnitude (F-M) relationships of debris floods and debris flows is challenging because of few direct observations, discontinuous event occurrence, loss of field evidence, the difficulty of accessing the sediment archive and the challenge of finding suitable statistical methods to analyse the dataset. Consultants often face budget limitations that prohibit application of the full gamut of absolute dating methods, stratigraphic analysis and analytical tools necessary to fully resolve the F-M legacy. In some cases, F-M curves are needed for watersheds without local information, or where obtaining this information is prohibitively expensive. For such watersheds, the F-M relationship may be estimated where several F-M curves have already been assembled in a specific region. Individual F-M curves are normalized by fan area or fan volume, then stratified by process type and geomorphic activity level. This paper describes the development of regional F-M curves for debris flows in southwestern British Columbia and debris flows and debris floods in the Bow River valley near Canmore, Alberta. We apply the regional relationships to other cases in Canada and the United States and demonstrate that the method can be globalized. The regional approach is compared to cases where detailed F-M relationships have been established by other means. Strong negative deviations from the regional debris-flow or debris-flood magnitude trends could signal inherent watershed stability, while strong positive deviations could signal extraordinary landslide processes, or suggest that the fan may be largely of paraglacial origin. We highlight some of these outlying cases and develop a method whereby the regional curves can be meaningfully adjusted, or reliance can be placed on lower or upper confidence bounds of the F-M curves. We caution against the indiscriminate use of the regionally based F-M curves, especially in watersheds where multiple geomorphic processes are active. (c) 2020 John Wiley & Sons, Ltd.