COWICHAN VALLEY NOVEMBER 2009 FLOODING – DOCUMENTATION AND ASSESSMENT FINAL REPORT DISCLAIMER This document has been prepared by Northwest Hydraulic Consultants in accordance with generally accepted engineering and geoscience practices and is intended for the exclusive use and benefit of the client for whom it was prepared and for the particular purpose for which it was prepared. No other warranty, expressed or implied, is made. Northwest Hydraulic Consultants and its officers, directors, employees, and agents assume no responsibility for the reliance upon this document or any of its contents by any party other than the client for whom the document was prepared. The contents of this document are not to be relied upon or used, in whole or in part, by or for the benefit of others without specific written authorization from Northwest Hydraulic Consultants and our client. Report prepared by: Original signed by: Original signed by: Vanessa O’Connor, EIT Tamsin Lyle, P.Eng Project Engineer Senior Engineer Reviewed by: Original signed by: Dave McLean, P.Eng. Principal TABLE OF CONTENTS 1 Introduction ....................................................................................................... 1 1.1 Event Summary ............................................................................................ 1 1.2 Integrated Flood Hazard management Plan ................................................. 1 1.3 Purpose of Event Assessment...................................................................... 1 2 Data Collection .................................................................................................. 2 2.1 Photo and Video Records............................................................................. 2 2.2 Climate and Hydrology Data......................................................................... 3 2.3 Survey........................................................................................................... 3 3 Climate and Hydrology ..................................................................................... 4 3.1 Climate.......................................................................................................... 4 3.2 Hydrology...................................................................................................... 4 3.3 Ocean Level.................................................................................................. 5 3.4 Historical Context.......................................................................................... 5 4 Mapping ............................................................................................................. 6 4.1 Methodology ................................................................................................. 6 4.2 Discussion .................................................................................................... 6 5 Numerical Modelling ......................................................................................... 8 5.1 Model Development...................................................................................... 8 5.2 Model Boundary Conditions.......................................................................... 8 5.3 Model Results ............................................................................................... 9 5.4 Discussion .................................................................................................. 10 6 Event Assessment .......................................................................................... 12 6.1 Lakes Road................................................................................................. 12 6.2 JUB Lagoons .............................................................................................. 14 6.3 RCMP Building ........................................................................................... 15 6.4 Lee Street ................................................................................................... 15 7 Recommended Actions .................................................................................. 17 7.1 Priority Actions............................................................................................ 17 Upgrade of JUB Lagoon Dike and Outfall..................................................................... 17 Lakes Road Dike and Beverly Street Pump Stations.................................................... 18 Channel Maintenance Program .................................................................................... 19 7.2 Alternative Actions and Best Management Practices ................................. 19 Property Acquisition...................................................................................................... 19 Property Elevation ........................................................................................................ 20 Cowichan Valley November 2009 Flooding – Documentation and Assessment i Final Report Invisible Flood Control Walls......................................................................................... 20 Dry and Wet Floodproofing........................................................................................... 20 8 Conclusions..................................................................................................... 22 9 References....................................................................................................... 23 Appendix A - Dike Inspections ............................................................................... 1 A.1 Cowichan River Dike .................................................................................... 1 A.2 JUB Lagoon Dikes ........................................................................................ 2 A.3 Cowichan River South Side Dike .................................................................. 3 A.4 South Side Spur Dike ................................................................................... 3 A.5 Quamichan Dike ........................................................................................... 4 A.6 Hatchery Dike / Mission Road Dike .............................................................. 6 A.7 Lee Dike........................................................................................................ 8 Appendix B - 2009 Integrated Flood Management Plan Goals and Strategies........................................................................................................... 1 Cowichan Valley November 2009 Flooding – Documentation and Assessment ii Final Report 1 INTRODUCTION 1.1 EVENT SUMMARY In late November 2009, a series of frontal systems hit coastal British Columbia. Flood warnings were issued across Vancouver Island as several rivers experienced very high flows. The worst flooding occurred in the Cowichan Valley on Friday, November 20. Following more than a week of rain, the Cowichan and Koksilah Rivers and several creeks overflowed their banks. Over 50 home were flooded in North Cowichan and the City of Duncan; dozens of homes were flooded “up to the doorknobs”. Residents were evacuated, roads and schools were flooded and closed, and property damage was extensive. Between November 15 and 26, the MNC raised dikes and filled gaps and low spots in certain critical areas near the flooded areas around Lakes Road. 1.2 INTEGRATED FLOOD HAZARD MANAGEMENT PLAN In 2008, Northwest Hydraulic Consultants (NHC) was retained by the Cowichan Valley Regional District (CVRD), in partnership with Cowichan Tribes, the City of Duncan and the Municipality of North Cowichan (MNC) to update existing floodplain mapping and to develop an Integrated Flood Management Plan (IFMP) for the Lower Cowichan-Koksilah River floodplain, including major tributaries. This study was completed in September 2009 and included the development of a flood model and floodplain mapping. Beyond the technical tools, this plan promoted an integrated approach to flood management with the goal of “reducing risk to all communities on the floodplain, while protecting aquatic and riparian habitat and addressing the cultural values of the river”. Ten planning strategies were developed to support the project goals, and over twenty priority and long-term actions were recommended. Of particular relevance to this project, was the recommendation that the lagoons dikes be upgraded and that a new dike be constructed along Lakes Road as a priority project. 1.3 PURPOSE OF EVENT ASSESSMENT On November 26th, 2009 Northwest Hydraulic Consultants (NHC) was retained to complete an assessment of the November 20th flooding event. The assessment includes data collection and surveying of high water marks, running of the previously developed MIKEFlood model to simulate the 2009 flood, assessing the value of the temporary “no-post” barrier instalment on Lakes Road, and to provide recommendations based on the assessment. This report outlines our methodology and findings. Cowichan Valley November 2009 Flooding – Documentation and Assessment 1 Final Report 2 DATA COLLECTION A comprehensive data collection effort was made that involved collecting primary and secondary source data. The data collection methodology and results are outlined in this chapter. 2.1 PHOTO AND VIDEO RECORDS On November 20, 2009, NHC personnel visited the affected area and took photos around the peak of the flood. Photos and video footage collected on or shortly after the flood event were mainly supplied to NHC by the relevant jurisdictions.