Managing Forested Watersheds for Hydrogeomorphic Risks on Fans
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LAND MANAGEMENT HANDBOOK 6 1 Managing Forested Watersheds for Hydrogeomorphic Risks on Fans 2 0 0 9 Ministry of Forests and Range Forest Science Program The Best Place on Earth Managing Forested Watersheds for Hydrogeomorphic Risks on Fans D.J. Wilford, M.E. Sakals, W.W. Grainger, T.H. Millard, and T.R. Giles Ministry of Forests and Range Forest Science Program The Best Place on Earth The use of trade, �rm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of any others that may also be suitable. Contents of this report are presented as information only. Funding assistance does not imply endorsement of any statements or information con- tained herein by the Government of British Columbia. Uniform Resource Locators (URLs), addresses, and contact information contained in this document are current at the time of printing unless otherwise noted. Library and Archives Canada Cataloguing in Publication Data Managing forested watersheds for hydrogeomorphic risks on fans / D.J. Wilford ... [et al.]. Includes bibliographical references. ISBN 978-0-7726-6119-7 1. Mass-wasting--British Columbia--Forecasting. 2. Landslide hazard analysis--British Columbia. 3. Forests and forestry--Environmental aspects --British Columbia. 4. Forest management--British Columbia--Planning. 5. Forest hydrology--British Columbia. 6. Alluvial fans--British Columbia. 7. Colluvium--British Columbia. I. Wilford, D. J. (David J.), 1950- II. British Columbia. Ministry of Forests and Range III. British Columbia. Forest Science Program SD387.E58M36 2009 634.961 C2009-909966-7 Citation Wilford, D.J., M.E. Sakals, W.W. Grainger, T.H. Millard, and T.R. Giles. 2009. Managing forested watersheds for hydrogeomorphic risks on fans. B.C. Min. For. Range, For. Sci. Prog., Victoria, B.C. Land Manag. Handb. 61. www.for.gov.bc.ca/hfd/pubs/Docs/Lmh/Lmh61.htm Prepared by D.J. Wilford T.H. Millard Ministry of Forests and Range Ministry of Forests and Range Smithers, BC Nanaimo, BC M.E. Sakals T.R. Giles Ministry of Forests and Range Ministry of Forests and Range Smithers, BC Kamloops, BC W.W. Granger Grainger and Associates Consulting Ltd. Salmon Arm, BC Prepared for B.C. Ministry of Forests and Range Research Branch Victoria, BC © 2009 Province of British Columbia Copies of this report can be obtained from: Crown Publications, Queen’s Printer PO Box 9452 Stn Prov Govt 563 Superior Street, 2nd Flr Victoria, BC V8W 9V7 1 800 663-6105 www.crownpub.bc.ca For more information on Forest Science Program publications, visit: www.for.gov.bc.ca/scripts/hfd/pubs/hfdcatalog/index.asp ABSTRACT Fans are linked to their watersheds by hydrogeo- In British Columbia, forest harvesting and road morphic processes—�oods, debris �oods, and debris building is associated with increased hydrogeomor- �ows. These processes move water, sediment, and phic hazards. The downstream effects of these for- debris from the hillslopes of a watershed through estry activities in source areas may be far-reaching channels to the fan. Fans in British Columbia are and extend beyond the scope of conventional site- often the site of residential developments, and trans- oriented planning. A �ve-step approach is presented portation and utility corridors, as well as high-value to assist land managers undertake risk analyses and habitat for �sh and high-productivity growing sites assessments that place their proposed developments for forests. Collectively, these features are termed within the watershed-fan system. The �ve steps are: “elements-at-risk” because they may be vulnerable 1) identify fans and delineate watersheds; 2) identify to watershed-generated hydrogeomorphic processes elements-at-risk on fans; 3) investigate fan processes; that issue onto the fan. These processes may be natu- 4) investigate watershed processes; and 5) analyze ral or result from land use activities, and can cause risks and develop plans. This scheme is applicable to the partial or total loss of some or all of the elements forested watersheds throughout British Columbia. on the fan. ACKNOWLEDGEMENTS The need for this handbook was raised by the For- sion has had the bene�t of very thorough reviews by estry Committee of the Council of Forest Industries Todd Redding, Robin Pike, Ian Smith, Steve Webb, and represents 3 years of collaborative research Rita Winkler, and David Maloney. This handbook involving many people throughout British Co- has bene�ted from a meticulous editorial review by lumbia. The concept for the �ve-step approach was Steve Smith. We are indebted to the Forest Invest- created by Bill Grainger and we are indebted to him ment Account–Forest Science Program, BC Timber for his foresight. This handbook has gone through Sales, and the B.C. Ministry of Forests and Range for many revisions over the past 3 years, incorporating �nancial support. suggestions from workshop participants; this ver- iii CONTENTS Abstract . iii Acknowledgements . iii Introduction . 1 The Five-step Method . 1 Step 1 Identify Fans And Delineate Watersheds . 2 Step 1.1 The Fan-watershed System . 2 Step 1.2 Fan Identi�cation . 4 Step 1.3 Watershed Delineation . 7 Step 2 Identify Elements-at-risk on Fans . 8 Step 2.1 Human Safety . 8 Step 2.2 Anthropogenic Features . 8 Step 2.3 Natural Features . 10 Step 3 Investigate Fan Processes . 10 Step 3.1 Hydrogeomorphic Processes . 10 Step 3.2 Event Frequency . 12 Step 3.3 Event Magnitude . 13 Step 4 Investigate Watershed Processes . 14 Step 4.1 Watershed-fan Process Linkages . 14 Step 4.2 Office Investigations . 16 Step 4.3 Field Investigations . 18 Step 4.4 Synthesis of Watershed Processes . 19 Step 5 Analyze Risks and Develop Plans . 20 Step 5.1 Understanding Risk Analysis and Risk Assessment . 20 Step 5.2 Consequence . 20 Step 5.3 Hazard . 21 Step 5.4 Risk Analysis . 23 Step 5.5 Assessing Risk and Making Management Decisions . 23 Step 5.6 Document, Monitor, Evaluate, and Report . 25 Case Studies . 25 Summary . 26 Literature Cited . 57 appendices 1 Wathl Creek case study . 27 2 Eagle Summit Creek case study . 33 3 Shale Creek case study . 42 4 Hummingbird Creek case study . 46 iv Tables 1 Characteristics of hydrogeomorphic process deposits . 11 2 Forest management focus for different hydrogeomorphic processes . 15 3 Predictive models for dominant hydrogeomorphic processes using the relative relief number and watershed length . 17 4 Example of long-term probabilities . 22 5 Qualitative frequency de�nitions . 23 6 Example of qualitative hazard analysis matrix . 23 7 Qualitative risk analysis matrix . ..