Description: Catastrophic flooding occurred in Boulder County during the multi-day precipitation event that began on September 9, 2013. During this flooding disaster, many geologic hazards such as debris/mud flows, earth flows, and soil-slip landslides were triggered in Boulder County. Many of these events occurred well above and outside the mapped floodplains. Following the September events, the Colorado Geological Survey (CGS) conducted a preliminary survey of earth and debris/mud flows from post-disaster, high-resolution, georeferenced satellite imagery. CGS also evaluated of many of the sites that impacted private property. In 1978, areas along the Boulder foothills were identified has being susceptible to a variety of geologic hazards, including landslides. The Boulder County Geologic Hazard and Constraint Area map was developed and adopted by the county in 1978. The major and moderate hazard-area polygons include a linear band called the "Foothills Subprovince." Over the next few years, the risk of potentially damaging landslides may be greater in steep areas along the foothills where ground disturbance has occurred. This report identifies such areas and offers recommendations to mitigate short and long-term landslide risks.In addition, future earth, mud and debris flows may occur in both the Foothills Subprovince and mountainous regions of the county following significant precipitation events, and/or human-induced changes. This report also assesses the susceptibility of these geologic hazards, based on a combination of conditions, and offers recommendations to reduce potential risks. 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.MAP GENERATION PROCESSESThis dataset was generated by a multi-step process that included site investigations, review of existing hazard, maps and reports, mathematical modeling, and model verification. Debris flow susceptibility (source areas and run-out zones) was calculated using Flow-R (Flow path assessment of gravitational hazards at a Regional scale) (http://www.flow-r.org/), a MATLAB-based flow-path modeling program. Flow-R was developed at the University of Lausanne for empirical modeling of regional susceptibility assessments of debris flows. It has been used successfully in many projects around the globe and for a variety of different gravitational hazards, including rockfall and avalanche. Flow-R requires a digital elevation model (DEM) as the base dataset; in this case a mosaiced 10-meter U.S. Geological Survey National Elevation Dataset (NED) of Boulder County. From the 10-m DEM, Flow-R generated a stream network that was used to calculate the debris flow pathways. A stream curvature and slope map created from the 10-m DEM was used in the model. Mapped debris flows, resulting from the September 11-14, 2013 flooding events, were used to calibrate the model.The resulting raster dataset from Flow-R was converted into polygons using ArcGIS. The results were further verified by comparing modeled source areas and run out zones with the preliminary survey of debris flows and landslides that CGS inventoried from post-flooding satellite imagery and topography. Polygons were modified using the post-flooding imagery, topography, and geology. Within the foothills area, the CGS observed examples of landslide slumps in excavated slopes cut into rocky hillside colluvial soils, debris flows in steep drainage channels, and soil-slip earth flows on dip slopes not within drainage ways. Many of the soil-slip landslides occurred where the trend of highest precipitation crossed areas identified in the Boulder County Comprehensive Plan as major and moderate geologic -hazard zones along the Front Range Foothill Subprovince. These areas are included in the dataset.
Copyright Text: Matthew L. Morgan, Jonathan L. White, F. Scot Fitzgerald, and Karen A. Berry