Water Stewardship Information Sources

Citation Northwest Hydraulic Consultants Ltd. 2013. Dry Creek Flood mitigation study: Final Report. Prepared for City of Port Alberni.
Organization City of Port Alberni
URL http://www.portalberni.ca/sites/default/files/doc_library/300111%20Dry%20Creek%20Final%20Report%20r1%20NWH%202013.pdf
Abstract/Description or Keywords The City of Port Alberni (City) retained Northwest Hydraulic Consultants Ltd. (NHC) to conduct a
flood investigation on the lower section of Dry Creek, in Port Alberni, BC. The study area extended
over a 1.6 km reach, starting 400 m upstream of 4th Avenue down to Alberni Inlet. The objective of
the study was to assess ways to reduce flooding and improve the conveyance of the channel. The US
Army Corps of Engineers (USACE) HEC‐RAS program was used to develop a one‐dimensional
numerical flow model to investigate the creek’s response to different flood scenarios generated by a
combination of high ocean levels and peak discharges.  
Topographic surveys of the channel were made in 2012 by Koers & Associates Engineering Ltd. and
this information was used to develop the base hydraulic model of the creek. The model was run to
simulate a range of flood scenarios generated by a combination of high ocean levels and peak
discharges. The model results showed that the key bridges in the study reach surcharged at
relatively low discharges (2‐year to 10‐year return period).  Road bridges in British Columbia
commonly are designed to have a clearance of 1.5 m during a 200‐year flood. The structures would
need to be raised by an amount between 1.6 m and 1.97 m (without channel improvements or
bridge lengthening) to meet this requirement.
Three flood mitigation options were developed through discussions at meetings with the City. The
analysed options included:
 Option 1: Napier Street Diversion
 Option 2: Bridge Improvements
 Option 3: Bridge and Channel Improvements.
Of the three options that were modelled, Option 3 was found to be the most effective option to
lower water levels in the study reach. The most complicated project from a hydrotechnical
perspective is Option 1. Some potential challenges with Option 1 would include: balancing the flow
split between the creek and the diversion pipe to match design targets; minimising hydraulic losses
at the inlet and outlet of the diversion; and handling trash, wood debris and sediment effectively.
The least effective option was Option 2.   
All options presented in this report would benefit from a sediment management plan. Conceptually,
the plan may include a sediment trap several hundred metres upstream of 4th Avenue. The trap
would require periodic excavation to maintain its effectiveness.
Additional hydraulic studies would be required for detailed design of the selected option. Example
optimizations include channel dimensions, final channel and bank grades, bridge deck heights, and
others.  We recommend that the model be calibrated and verified at higher flows than were
observed in 2012. If post‐2006 historical flood limits cannot be identified to calibrate the model then
one method that could be used is to measure the flow and stage at various cross sections during a
significant flood in 2013/14. 
Information Type report
Regional Watershed Vancouver Island North
Sub-watershed if known Dry Creek
Aquifer #
Comments
Project status complete
Contact Name
Contact Email