There is now a significant concern for the high tides, storm surge and the subsequent potential risk for flooding in the Courtenay area, BC Hydro said Friday afternoon.
BC Hydro monitors a water survey gauge (Gauge 10) located near the Fifth Street Bridge. Thursday, the combined river flows for the Puntledge, Browns and Tsolum rivers was only about 85 cubic metres per second (m3/s), yet the gauge hit 3.86 metres at 3:30 p.m. due to storm surges from the high wind.
This is only about 30 to 40 centimetres from localized flooding thresholds that we saw in November 2009, and January 2010, Stephen Watson of BC Hydro said in a news release.
Typically, total river flows need to reach about 400 m3/s to hit the 4.2-metre threshold to cause flooding, he explained. Considerably less than that would have caused flooding Thursday, he added.
The five-metre high tide that took place during the 3.86 m gauge 10 peak had a 0.67 storm surge to go with it, for a total high-tide level of 5.67 m.
BC Hydro forecasting is continuing to see significant rainfall for Saturday and early Sunday, along with significant snowmelt along mid-elevations from the warmer temperatures. This will impact local rivers and the rate of rise — they are very flashy systems.
The major concern is the forecasted high tide on Sunday morning, at about 8 a.m. of 5.3 m. That's 0.3 m higher than Thursday.
"The high tide timing will be very close to the peak river flows," Watson said. "If we have significant storm surges, with winds forecasted to be from the same direction as yesterday but of less intensity, this will also add on to the situation."
This looks to be a situation where literally every drop counts in terms of flood risk, Watson stated. BC Hydro is changing its operations strategy.
Due to the lower-than-normal reservoir water levels for the time of year, BC Hydro is taking unprecedented action by shutting down the 24-megawatt powerhouse from around midnight Saturday through mid-morning Sunday to limit as much water as possible from moving downstream for the 8 a.m. high tide.
"Typically we operate within the 32- to 45-m3/s range during flood risk management conditions at high tide, and 110 m3/s or higher at low tide. For this storm, BC Hydro will provide only about 20 m3/s down the Puntledge River for the Sunday high tide.
"The only reason why BC Hydro is able to do this action is because of the low reservoir level, and the room to let it rise up to 1.5 m as a result. Given this is an intense but short-term storm event, we will have the ability to move water out of the system post-storm."
BC Hydro will adjust its operations with ocean tides, and as a result, advises the public to stay away from the river through Sunday.
The concern is that the Browns River and Tsolum River, and tributaries that feed into the Courtenay River, will react and peak at the worst time. The Tsolum River, for example, if the weather forecast holds, may reach the 200 m3/s range.
BC Hydro held a conference call with the City of Courtenay earlier Friday about the increased risk of flooding. Although BC Hydro’s dam cannot eliminate the possibility of flooding, it can reduce the size, frequency and impact of such events.
Any questions on potential risk should be directed to Sandy Gray, City of Courtenay administrator, at 250-334-4441.
— BC Hydro