The City of Delta wants to prevent an important salmon-bearing creek from bank erosion.
Cougar Creek is a watercourse with a mostly developed watershed within Delta and Surrey. It has a returning coho and chum salmon population, supported by a dedicated group of volunteers called the Cougar Creek Streamkeepers.
The stream has experienced an increase in peak discharge due to urbanization of the watershed and that increase has resulted in higher rates of erosion and sedimentation.
At moderate to high flows, the upper creek has sufficient stream power to erode the sand and gravel banks.
The city has issued a request for proposals for professional engineering consultant to design slope and erosion protection along the base of the creek’s slope in the 7000-block of Woodcrest Place.
In January 2022, a log jam in Cougar Creek near Cory Drive Park redirected the creek to the left bank.
The city notes that, as a result, an erosion scarp formed below 7030 and 7040 Woodcrest Place. Although the debris has been cleared and the creek diverted back to the centre of the channel, the slope remains susceptible to erosion during high precipitation events.
The city is hoping to have the detailed design work completed by no later than mid-October so that the remediation work can be completed as soon as possible.
Providing a preliminary assessment of the site, a recent geotechnical and hydrotechnical inspection report for the city recommended that further mitigative actions be taken prior to the onset of fall high flows.
The report notes that the log jam within Cougar Creek directed a concentrated flow along the left bank. The toe of the already steep bank subsequently eroded and the bed along the toe of bank scoured.
Based on field reports earlier this year, ongoing migration of the banks could threaten the residential properties located along the top of the steep slope at Woodcrest Place site.
The report goes on to note that due to the lower grade along the bank toe and potential for further channel obstructions, it is likely that the channel will shift back to the toe of the erosional scarp during a future high-water event.
Despite the sediments in the bank appearing consolidated, they underwent considerable erosion during the past high-water event and there is no indication that the bank will resist continued erosion when the stream shifts against the bank.
Ongoing erosion could result in bank retreat or slope destabilization which would threaten the houses in the 7000-block of Woodcrest Place.
A rapid response with mitigation of the hazard prior to upcoming fall-time high flows is warranted, the report notes, adding it can be expected that debris will continue to accumulate at the site of the log jam as there is still a substantial portion of woody debris in the channel.
Should additional wood accumulate at the site of the obstruction, it is likely the stream will be diverted again into the left bank which is likely to cause additional erosion.
A previous City of Delta staff report notes some of the key concerns identified in the Cougar Creek Integrated Stormwater Management Plan are that increased stormwater flows and velocities within the watershed contribute to erosion and scour.
“This leads to subsequent sedimentation of downstream reaches which are already constricted resulting in storage capacity issues, flooding and degradation of fish habitat. Efforts are ongoing through the implementation of stormwater best management practices, such as Delta's Rain Garden Program, to mitigate high stormwater flows and improve water quality,” the Delta report notes.
According to the Cougar Creek Streamkeepers website, the watercourse has good potential for restored stream health and wild salmon runs.
Culverting is not extensive, native streamside vegetation is fairly abundant, especially in Cougar Canyon Environmental Reserve and bordering the Delta Nature Reserve, and there are many potential rearing areas where juvenile salmon can take shelter.
“Miraculously, coho & chum salmon, cutthroat trout and other fish continue to survive in Cougar Creek. Given half a chance, nature heals itself!” the volunteer group says.