Bren School of Environmental Science & Management University of California, Santa Barbara Post-Fire Sedimentation and Flood Risk Potential in the Mission Creek Watershed of Santa Barbara A Group Project submitted in partial satisfaction of the requirements for the degree of Master’s in Environmental Science and Management By Leslie Abramson Milli Chennell Erica Eisch Alicia Glassco Thomas Holley Faculty Advisor: Thomas Dunne Post-Fire Sedimentation and Flood Risk Potential in the Mission Creek Watershed of Santa Barbara As the authors of this Group Project report, we are proud to archive this report on the Bren School’s website such that the results of our research are available for all to read. Our signatures on the document signify our joint responsibility to fulfill the archiving standards set by the Bren School of Environmental Science & Management. _______________________________________________________ Leslie Abramson _______________________________________________________ Milli Chennell _______________________________________________________ Erica Eisch _______________________________________________________ Alicia Glassco _______________________________________________________ Thomas Holley The mission of the Bren School of Environmental Science & Management is to produce professionals with unrivaled training in environmental science and management who will devote their unique skills to the diagnosis, assessment, mitigation, prevention, and remedy of the environmental problems of today and the future. A guiding principal of the School is that the analysis of environmental problems requires quantitative training in more than one discipline and an awareness of the physical, biological, social, political, and economic consequences that arise from scientific or technological decisions. The Group Project is required of all students in the Master’s of Environmental Science and Management (MESM) Program. It is a four-quarter activity in which small groups of students conduct focused, interdisciplinary research on the scientific, management, and policy dimensions of a specific environmental issue. This Final Group Project Report is authored by MESM students and has been reviewed and approved by: ____________________________________________ Thomas Dunne, Advisor ____________________________________________ John Melack, Acting Dean March 2009 ABSTRACT Runoff and erosion rates rise dramatically in the rainy seasons following wildfire, increasing the risk of destructive floods, sediment accumulation, and debris flows. Watershed assessments are typically performed post-fire, but risk quantification and determination of mitigation treatments are limited when a fire occurs immediately before the rainy season. In highly vulnerable areas, such as Mission Creek watershed in California, pre-fire analysis of post-fire risk is warranted. This study examined potential post- fire runoff and erosion risk by simulating the effects of fire on upper Mission Creek watershed. The project utilized field observations of an analogous burned watershed, spatially explicit data on watershed characteristics, historic rainfall and runoff measurements, and accepted modeling techniques to estimate post-fire changes in hydrologic and sedimentary processes in Mission Creek watershed. Results estimate flood discharge associated with the 100-year storm is four times more likely after fire, and even small storms will flood areas of downtown Santa Barbara. Sediment yield from the upper watershed may increase by several orders of magnitude, depending on precipitation, increasing flood risk through channel aggradation. These estimates were used to assess various pre- and post-fire mitigation projects that reduce risk for downstream communities in Santa Barbara. iii Acknowledgements We would like to extend sincere thanks to the people who helped make this project possible. In particular, we thank our advisor, Tom Dunne, whose expertise and guidance was invaluable to the creation and development of the project. We would also like to thank our external advisors for their guidance and feedback during the course of project development and execution: Arturo Keller ~ Bren School of Environmental Science and Management Edward Keller ~ Dept of Geography, U.C. Santa Barbara Raymond Smith ~ Mission Canyon Association In addition, thanks to those organizations and individuals who provided data and support during project development and analysis: Santa Barbara County Flood Control District Candice Constantine Maureen Spencer Shawn Johnson Matt Griffin U.S. Forest Service William Elliot Cindy Chojnacky Mary Moore Slippery Rock Ranch ~ Madars Ozonlins S.B. Office of Emergency Services ~ Yolanda McGlinchey S.B. County Office of Emergency Services ~ Jay McAmis Santa Barbara County Fire ~ David Neels National Weather Service ~ Eric Boldt iv TABLE OF CONTENTS Abstract..............................................................................................................................iii 1. Executive Summary........................................................................................................ 1 2. Introduction..................................................................................................................... 3 2.1 Project Significance .................................................................................................. 4 2.2 Research Questions................................................................................................... 4 2.3 Research Objectives.................................................................................................. 4 3. Background..................................................................................................................... 5 3.1 Regional California Climate ..................................................................................... 5 3.2 Fire Regime............................................................................................................... 6 3.3 Effects of Fire on Hydrology and Sedimentation ..................................................... 7 3.4 Study Area ................................................................................................................ 8 3.5 Current Post-Fire Management Policies in Santa Barbara ..................................... 14 4. Approach....................................................................................................................... 17 4.1 Conceptual Approach.............................................................................................. 17 4.2 Scenario Development............................................................................................ 19 5. Methodology & Results: Hydrology (HEC-HMS)....................................................... 21 6. Methodology and Results: Sedimentation .................................................................... 29 6.1 Surface Erosion Model (ERMiT)............................................................................ 29 6.2 Shallow Landslide/Debris Flow Model (SHALSTAB).......................................... 37 7. Analysis of Post-Fire Risk ............................................................................................ 42 7.1 Flood Risk............................................................................................................... 42 7.2 Effects of Sedimentation on Flooding .................................................................... 44 8. Recommendations......................................................................................................... 49 8.1 Emergency Post-Fire Actions ................................................................................. 49 8.2 Long Term Planning Actions.................................................................................. 52 9. References..................................................................................................................... 55 Appendix 10.1: HEC-HMS............................................................................................... 62 Appendix 10.2: ERMiT .................................................................................................... 71 Appendix 10.3: SHALSTAB............................................................................................ 78 1. EXECUTIVE SUMMARY Wildfires pose a complex management problem, especially in fire-prone areas such as southern California. Flood, sedimentation, and debris flow hazards increase in the years following fire when precipitation falls on bare hillslopes and unprotected, sometimes hydrophobic soils. Communities downstream of burned areas are at higher risk when storm runoff and erosion are intensified. Mission Creek watershed, a small coastal watershed in southern California, has high wildfire potential indicated by regional fire history and current fuel accumulation. Mission Creek begins in steep, chaparral-covered mountain slopes and flows to the Pacific Ocean after winding through flood-prone downtown Santa Barbara. Fire in the upper watershed would greatly increase water and sediment supplies to the channel, increasing flood risk to downstream urban areas. This project uses spatially and temporally explicit data and watershed modeling programs to quantify increases in runoff, sedimentation, and risk of debris flows in Mission Creek watershed following a potential wildfire. Current observations of post- fire hydrologic and sedimentary response in the nearby Gap Fire