Water Stewardship Information Sources

Citation Northwest Hydraulic Consultants Ltd. 2006. Lower Fraser River Hydraulic Model - Final Report. Prepared for Fraser Basin Council.
Organization Fraser Basin Council
URL http://www.env.gov.bc.ca/wsd/public_safety/flood/pdfs_word/2006nhc_fraser_flood_profile.pdf
Abstract/Description or Keywords In September 2005, the Fraser Basin Council (FBC) retained Northwest Hydraulic Consultants
Ltd. (nhc) to undertake a program of one-dimensional hydraulic modelling on the lower Fraser
River using MIKE11 software developed by Danish Hydraulic Institute (DHI). The overall
objective was to generate an up-to-date design flood profile based on the following two scenarios:
• The estimated flow during the 1894 Fraser River flood combined with high spring tide
• The 200 year winter storm surge with high tide combined with a Fraser River winter
The two profiles were then overlaid and the higher of the two values was used to develop an
overall design flood profile for the river. Initially, the study reach covered the 100 km distance
from Sumas Mountain to the Georgia Strait, encompassing the North, Middle and South Arms,
including Canoe Pass, as well as Pitt River to Pitt Lake inlet. Later on, the study was extended
upstream to re-assess flood levels upstream of Mission in the reach up to the mouth of the
Harrison River.
The hydraulic model was developed using field data collected in 2005. The field work included
detailed bathymetric surveys of the channel, LIDAR surveys of the floodplain and ADCP velocity
measurements to estimate flow splits at major channel branches. The model was calibrated and
verified initially using recorded data from 2002, 1999 and 1997 flood events. Peak discharges
from these floods ranged between 11,300 m3
/s and 12,200 m3
/s. Later on, a secondary “historic
model” was developed for the reach between Mission and New Westminster, using channel and
floodplain topography from 1951 to 1953. This secondary model was used to estimate the
channel roughness during floods in 1948, 1950, 1969 and 1972.
The adopted design discharge for the model is based on the 1894 flood of record estimated to
have had a peak discharge of 17,000 m3
/s at Hope. To account for inflow from tributaries, flow is
estimated to increase to 18,900 m3
/s at Mission and 19,650 m3
/s at New Westminster. The
adopted design discharge assumes containment of the river by the existing dike system
downstream from Hope under current and future floodplain conditions. Due to variations in
tributary flows and flow attenuation from overbank spilling and floodplain storage the actual
1894 flows at Mission and New Westminster may have been considerably less. An assessment of
floodplain conditions in 1894 suggested that the flow at Mission may have been only
16,500 m3
/s. For this reason, the 1894 historic flood profile is not directly comparable to the
computed design flood profile.
Channel roughness along sand bed rivers may vary with changing flow conditions due to the
formation of sand dunes on the river bed. During very high flows the roughness may decrease
substantially if the dunes wash out and flat bed conditions develop. Field observations on the
river during relatively high floods in 1950, 1986 and 1997 showed no evidence that the dunes
wash out in the 12,200 m3
/s to 14,500 m3
/s Mission flow range. Results of the model calibration
runs showed there is a weak trend for the channel roughness to decrease at high discharges. The
channel roughness was estimated to average 0.03 in the Douglas Island to Mission reach using the
2002 flood data (maximum discharge of 11,300 m3
/s at Mission). Based on flow estimates for the
1948 flood (maximum discharge estimated to be 15,500 m3
/s), the average channel roughness
was found to be 0.027 or approximately 10% lower. Based on this assessment, a value of 0.027
was adopted for the design flood profile computations in the reach between Douglas Island and
Information Type report
Regional Watershed Lower Fraser
Sub-watershed if known
Aquifer #
Project status complete
Contact Name
Contact Email