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Cornet Creek Watershed and Alluvial Fan Debris Flow Analysis

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Cornet Creek Watershed and Alluvial Fan Debris Flow Analysis Cornet Creek Watershed and Alluvial Fan Debris Flow Analysis Submitted to: Town of Telluride P.O. Box 397 Telluride, Colorado 81435 Submitted by: 1730 S. College Avenue, Suite 100 Fort Collins, Colorado 80525 MEI Project 07-14.4 May 15, 2009 Table of Contents Page 1. INTRODUCTION ................................................................................................................1.1 1.1. Scope of Work............................................................................................................1.3 1.2. Authorization and Study Team...................................................................................1.4 2. HISTORICAL EVENTS AND MODELING ..........................................................................2.1 2.1. Existing Studies and Background Material.................................................................2.1 2.2. Mudflow Characterization and Processes..................................................................2.1 2.3. FLO-2D Model............................................................................................................2.4 2.4. Previous Mudflow Modeling .......................................................................................2.5 2.5. Historic Mudflows on the Cornet Creek Alluvial Fan ..................................................2.5 3. FLO-2D Modeling ...............................................................................................................3.1 3.1. FLO-2D Model Development......................................................................................3.1 3.1.1. FLO-2D Grid.......................................................................................................3.1 3.1.2. Channel Cross Sections.....................................................................................3.1 3.1.3. Bridges and Culverts ..........................................................................................3.3 3.1.4. Channel and Overbank Roughness Values .......................................................3.3 3.1.5. Streets ................................................................................................................3.3 3.1.6. Width Reduction Factors and Area Reduction Factors ......................................3.6 3.1.7. Clearwater Flow Hydrographs............................................................................3.6 3.1.8. Mudflow Hydrographs ........................................................................................3.8 3.2. Flood Hazard Index..................................................................................................3.10 3.3. Flood Simulations.....................................................................................................3.11 3.4. Model Results...........................................................................................................3.11 3.4.1. Scenario 1 ........................................................................................................3.12 3.4.2. Scenario 2 ........................................................................................................3.16 3.4.3. Scenario 3 ........................................................................................................3.21 3.5. Summary..................................................................................................................3.21 4. DEBRIS FLOW PREDICTION, WARNING, AND MITIGATION.........................................4.1 4.1. Hazard Impact Assessments......................................................................................4.3 4.2. Advance (early) Warning Systems .............................................................................4.4 4.3. Event (real-time) Warning Systems............................................................................4.5 4.4. Structural Mitigation Measures...................................................................................4.8 4.5. Summary..................................................................................................................4.13 5. SUMMARY AND RECOMMENDATIONS ..........................................................................5.1 5.1. Summary....................................................................................................................5.1 i 5.2. Recommendations .....................................................................................................5.2 6. REFERENCES ...................................................................................................................6.1 List of Figures Figure 1.1. Vicinity map of Telluride showing the general location of the study reach ......... 1.2 Figure 2.1. Classification of hyperconcentrated sediment flows (modified from FLO-2D manual, 2006) .................................................................................................... 2.3 Figure 2.2. Deposition limits of the 1914 and 1969 Cornet Creek mudflows (SLA, 1989).... 2.6 Figure 2.3. Mudflow deposits during the 1914 event........................................................... .2.7 Figure 2.4. Mudflow deposits from the 1914 event. Note the size of the boulders transported and deposited by the mudflow................................................... 2.7 Figure 3.1 Extent of 50-foot FLO-2D grid. ........................................................................... 3.2 Figure 3.2. Location of the hydraulic structures along Cornet Creek.................................... 3.4 Figure 3.3. Distribution of Manning’s n-values applied to the FLO-2D model ...................... 3.5 Figure 3.4. Distribution of width reduction factors and area reduction factors to the FLO-2D model.................................................................................................... 3.7 Figure 3.5. Developed hydrographs for the 10- and 25-year peak flow events. ................... 3.8 Figure 3.6. Sediment concentration hydrographs for the 10- and 25-year peak flow events............................................................................................................... 3.10 Figure 3.7. Predicted maximum inundation depths for Scenario 1 (25-year peak flow event with the Townsend Street Culvert blocked). ........................................... 3.13 Figure 3.8. Predicted maximum velocities for Scenario 1 (25-year peak flow event with the Townsend Street Culvert blocked). ............................................................ 3.14 Figure 3.9. Predicted Hazard Potential for Scenario 1 (25-year peak flow event with the Townsend Street Culvert blocked). .................................................................. 3.15 Figure 3.10. Comparison of the extent of flooding between Scenarios 1 and 3 with the 1914 and 1969 mudflow events........................................................................ 3.17 Figure 3.11. Predicted maximum inundation depths for Scenario 2 (25-year peak flow event with all bridges and culverts blocked)..................................................... 3.18 Figure 3.12. Predicted maximum velocities for Scenario 2 (25-year peak flow event with all bridges and culverts blocked)............................................................... 3.19 Figure 3.13. Predicted Hazard Potential for Scenario 2 (25-year peak flow event with all bridges and culverts blocked)............................................................... 3.20 Figure 3.14. Predicted maximum inundation depths for Scenario 3 (10-year peak flow event along the east side of the alluvial fan). ................................................... 3.22 Figure 3.15. Predicted maximum velocities for Scenario 3 (10-year peak flow event along the east side of the alluvial fan). ............................................................. 3.23 Figure 3.16. Predicted Hazard Potential for Scenario 3 (10-year peak flow event along the east side of the alluvial fan)........................................................................ 3.24 ii Figure 4.1. Historic photograph taken within Town of Telluride showing debris and damage caused by 1914 debris-flow event........................................................ 4.2 Figure 4.2. Historic photograph taken within Town of Telluride showing debris and damage caused by 1969 debris-flow event........................................................ 4.2 Figure 4.3. Looking west along Dakota Avenue at sediment deposited during July 2007 flood on Cornet Creek. .............................................................................. 4.3 Figure 4.4. Aerial photographs of a segment of Cornet Creek between Galena Avenue and Colorado Avenue showing mudflow inundation during the 1969 flood as well as the subsequent development and encroachment along the channel during the last 40 years. ..................................................................................... 4.4 Figure 4.5. Example plot of critical rainfall threshold for debris-flow initiation ...................... 4.6 Figure 4.6. Schematic of geotechnical mass flow sensor (Image based on promotional information obtained from Zostrich Geotechnical Monitoring)............................ 4.8 Figure 4.7. Concrete slit-type sabo dam. Steel bars were installed at the slit after debris flow was found to pass through the slit (Photo 5; Mizuyama [No 1], 2008). ............................................................................................................... 4.10 Figure 4.8. Concrete and steel pipe sabo dam. (Photo 4; Mizuyama
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