|Citation||Kerr Wood Leidal. 2015. Lower Mainland Flood Management Strategy - Analysis of Flood Strategies. Prepared for Fraser Basin Council.|
|Organization||Fraser Basin Council|
|Abstract/Description or Keywords||This project is part of a larger initiative to develop a Lower Mainland Flood Management Strategy. The Fraser
Basin Council is facilitating this collaborative process, which includes 26 local governments, the provincial and
federal governments and numerous other partners. Following consultation with a number of key stakeholder
communities in 2013, it was identified that a collaborative approach will be most effective in addressing the current
conditions and future impacts from flooding on a regional perspective.
Phase 1 comprises analysis, planning and consultation with the coordination and support of the Government of
Canada, Provincial ministries, and 25 local governments within the Lower Mainland. Phase 2 will focus on
development of options assessment and implementation.
Phase 1 of this Lower Mainland Flood Management Strategy is comprised of the following projects:
Project 1: Analysis of Flood Scenarios;
Project 2: Regional Vulnerability Assessment; and
Project 3: Flood Protection, Policies and Plans.
This study focuses on Project 1 – Analysis of Flood Scenarios.
The study area extends from White Rock to Squamish and from Hope to Richmond, and includes regional
organizations such as Metro Vancouver, the Fraser Valley Regional District and select provincial ministries. The
primary hazards considered for this study were coastal and Fraser River flooding. Due to the large extent of the
study area and the data collection effort required on a regional level analysis, this study focused primarily on the
use of existing flood hazard information from studies, reports and models that were readily available. Gaps and
transfer of pertinent information, data and findings was analysed.
Two coastal and two Fraser River flooding scenarios were identified to support the regional vulnerability
assessment. Consultations were held with the Fraser Basin Councils’ technical committee on February 11 and
March 10, 2015 and the criteria for selection of flood scenarios and associated flood levels were agreed. The
flood scenarios selected acknowledge the impacts from sea level rise, climate change and site specific
uncertainties on flood levels.
The following flood scenarios are recommended for the purposes of the Lower Mainland Flood Management
Strategy’s regional vulnerability assessment.
Coastal Flood Scenarios
1-in-500 AEP still-water ocean state with current sea level; and
1-in-500 AEP still-water ocean state with 1 m sea level rise.
Fraser River Flood Scenarios
High tide with current sea level and 1894 design flood conditions in Fraser River; and
High tide with 1 m sea level rise and “moderate” climate change for 1-in-500 AEP freshet flow conditions in
Coastal and Fraser River flood levels were determined for the selected flooding scenarios for individual
communities from White Rock to Squamish and are presented in this report. Flood levels were mapped and a
standalone GIS Portal with salient information was developed. Key gaps in existing data and studies in
conjunction with the regional nature of this study were reviewed.
Some of the key conclusions and recommendations relevant for this study are provided below. The primary objective of the Lower Mainland Flood Management Strategy Project – Analysis of Flood Scenarios
is to support Project 2 of Phase 1 – Regional Vulnerability Assessment and should not be used for design of
flood protection measures (i.e., Dike Design, Flood Protection).
The simplified site specific joint probability analysis conducted in this study highlights the importance of the
effect of local conditions on flood levels. As well, there are a number of gaps in the current understanding of
Canadian Hydrographic Service (CHS) data, local conditions such as subsidence, datum adjustments, and
wave generation. These gaps also explain the uncertainties in the final flood level estimates.
Variations in flood level estimates due to local conditions may affect flood protection design, i.e., dike design,
but are unlikely to impact the regional scale vulnerability assessment significantly. Therefore, a uniform geodetic
coastal water surface elevation was selected for all locations, incorporating an acceptable allowance to address
uncertainties (Table 3-4).
It is recommended that policy and design decisions from individual communities consider separate site specific
analysis incorporating the combined effects of all processes to establish an appropriate level of safety for flood
protection design studies. This may include evaluation of different approaches (i.e., combined, joint probability
analysis, hindcast) for flood level estimates.
In addition to impacts from the Fraser River flooding, communities may experience catastrophic consequences
from flooding from local rivers and creeks, including debris flooding and urban flooding due to ineffective
drainage infrastructure. The consequences from these additional sources of flooding are not addressed for this
regional study, but are recommended for future site specific flood risk assessment by individual communities for
effective flood protection.
|Regional Watershed||Lower Fraser|
|Sub-watershed if known|