The Failure Mode

A post-fire debris-flow hazard assessment reaches a county emergency manager as a map of shaded basins, and the legend names the storm behind the shading. Peak 15-minute intensity of 24 mm/h. The unit is millimeters per hour. The window named in the same phrase is fifteen minutes long. Both halves of that label are accurate, and together they describe a rainstorm that puts six millimeters of water on the ground. Six millimeters is a shade under a quarter of an inch. A reader who takes 24 mm/h at face value is picturing an hour of rain and close to an inch of water. A reader who notices the fifteen-minute window is picturing a burst that is over before a kettle boils. The statistical model behind the shading was fitted to the second picture. The sentence the public reads on a weather office page is built around something closer to the first. One number, two readings, and a factor of four of water between them. Eight Rainstorms, Named by the Hour and Measured in Fifteen Minutes The g...

A Flood With No Storm Attached Strong ground shaking is over in well under a minute. The flood it sets up can arrive a week later, or two months later, in clear weather, with nothing in the rainfall record to account for it. The object standing between the two events is a pile of rock and soil that fell into a river channel during the shaking and stayed there. Geologists call it a landslide dam , or a barrier lake when the emphasis falls on the water rather than the debris. The U.S. Geological Survey studied the general case in The Formation and Failure of Natural Dams , Open-File Report 87-392, which compiled landslide dams worldwide and tabulated how long they lasted. Its central finding is uncomfortable: these structures do not fail on any schedule that can be pinned down, and among those that do fail, most fail well after the response has wound down. The failure mode is not the landslide. The landslide is the visible event, the one that gets counted. The failure mode is the i...