The large‐scale precipitation events between 11 and 14 September 2013 resulted in catastrophic flooding on the floors of existing stream and gulch valleys in Boulder County. In addition to the flooding within the 100‐ to 500‐year floodplains, there were also many upland flows that occurred well above and outside mapped floodplains. These types of flows are classified as earth flows and debris/mud flows, which are considered geologic hazards. This report and the accompanying map coverage address only these types of hazards. Hydrologic flooding within main stream channels and their floodplains were not included in this study. Includes map plate, GIS files, booklet. Digital ZIP download. OF-14-02D
From the Introduction:
Earth and debris/mud flows are generally rapid-moving, turbulent, hyperconcentrated flows that are bulked with entrained sediment that has been eroded from a slope. Conditions for these types of flows are: steeper channelized slopes, loose soils or disaggregated rock, and sufficient water from either rapid precipitation runoff and/or saturated ground conditions. The density of mud matrix of these hyperconcentrated flows gives them the hydraulic “lifting” capacity to mobilize and “float” very large boulders, trees, and any other large debris in its pathway. Other characteristics of debris/mud flow are lateral levees that form along the sides where flow rates slow and frictional shear forces of the entrained aggregate re-engage. Where slope grades decrease, debris flows can plug channels and form alluvial fans by channel migration and lateral spreading of successive flows. Initiations of these types of flows are usually a combination of high precipitation events, rapid erosive overland flows, and soil-slip type landslides. Triggering thresholds are lower, and risk higher, in locations where forest fires have occurred. Land-use impacts can vary widely depending on the site-specific velocity of the flow, depth or thickness, and bulking factor and debris entrained within it; from nuisance flooding of a thin low-viscosity mud to catastrophic bouldery debris with the consistency of wet concrete that can collapse walls, knock homes off foundations, or completely destroy structures.