Rare and dangerous: Recognizing extra-ordinary events in stream channels

Extreme-value statistics has taught us that flood flows can be estimated reasonably well, and that while extreme flows are rare, they are certain to occur. The physical process of flooding is reasonably well understood. However, this knowledge does not extend to steep creeks with potentially highly mobile beds. Most infrastructures on such creeks have been designed for clearwater floods with return periods of up to 200 years. This does not account for hydrogeomorphic processes such as debris floods and debris flows in which parts of, or the entire, channel bed sediments are mobilized and lead to massive erosion of channel bed and banks and debris inundation on terminal alluvial fans. Similarly, the potential for outburst floods - many times larger than normal floods - related to failure of landslide, glacier, moraine, beaver or man-made dams is not systematically included in standard hazard assessments. This paper has the objective of bridging science and practice by highlighting some of the most threatening hydrogeomorphic hazards to which people and infrastructure in mountain regions are exposed, and provides suggestions on how practice can be improved to properly diagnose and analyze the potential for such unusual floods. It is hoped that this will reduce potential losses in spite of the continued encroachment of urban and industrial development into mountain terrain.