BC Hydro Update: Puntledge River Operations

December 18, 2014

From BC Hydro:

"BC Hydro has been able to lower the Comox Lake Reservoir considerably since last Friday. The reservoir is now at 133.9 m, or down about 2.3 metres from the high of last week.

The discharges downstream into the Puntledge River are about 85 m3/s. There are some smaller storms through this weekend so these flows will likely be in place through the weekend. BC Hydro discharges from the dam may be up to 100 m3/s through the weekend. For public safety, BC Hydro requests people to continue to stay away from the Puntledge River through the weekend. Public warning signs have been updated.

[Today (December 18)], BC Hydro will reduce the discharge from the dam down to about 35 m3/s to safely remove log debris that’s built up against our log boom, located just upstream of the Comox dam, to protect debris from getting into the spillway gates. Higher flows will then be resumed. 

There looks to be a one-day subtropical storm tracking towards the West Coast and at this time it seems to be tracking to the south of us. We will keep a close eye on it.

BC Hydro would like to share a few statistical points of interest from last week’s record breaking storm event. This analysis has some uncertainty given how high the flows reached.

The two-day average water flow (213 m3/s over 48 hours) for the Tsolum River was the highest in 49 years of record, and about a 1 in 60 year event. The three day average flow was the second highest on record. The Tsolum also hit a new record peak flow of 282 m3/s. 

The two-day average flow (105 m3/s over 48 hours) for the Browns River was the second highest in 32 years of record, and about a 1 in 60 year event. The three day average flow was the highest on record, and about a 1 in 180 year event. Over the storm period, the Browns River hit a high of 254 m3/s.

The two-day average water inflow (475 m3/s over 48 hours) into the Comox Lake Reservoir was the highest in 51 years of record, and about 1 in 70 year event. The three day average flow was the highest on record, and about 1 in 50 year event. The water inflows hit a peak flow of 893 m3/s.

However, if you combine the Comox Lake/Puntledge, Browns and Tsolum rivers together, the two-day total average water flow (792 m3/s over 48 hours) was the highest in 29 years of record, and about 1 in 130 year event. The three day average flow was the highest on record, and about 1 in 110 year event. Of course BC Hydro’s Comox dam held back the majority of these water inflows during the storms and then safety released the excess water for days afterwards.

Flooding in Courtenay can begin at around 400 m3/s. There were a few flood events from Tuesday through Thursday.

On December 9, the Tsolum and Browns river were peaking or peaked at the same time as the high tide and about 40 cm of increased height from storm surge cause by winds. The Tsolum River was at 260 m3/s and the Browns was at 243 m3/s. BC Hydro’s discharge from the Comox dam at that time was reduced to only 32 m3/s. This was a high tide flood event.

On December 10, after backing off from the morning high tide, BC Hydro increased and then held steady the discharges from the dam at about 240 m3/s. The Tsolum and Browns river came up with the heavy rain and by around 2:30 pm the total river flow was a very high 710 m3/s. This was a low tide flood event.

[The following PDF] has two telling graphs and a third one for general background.

The first graph shows the water inflows into the Comox Lake Reservoir versus the discharges released downstream. You can see the dam held back a lot of water throughout the storm, and also where BC Hydro backed off the discharge considerably for the high tides to lessen the impacts of the downstream floods. The flooding could have been deeper and more far-reaching.

In the second graph, in addition to BC Hydro discharges from Comox dam, shows the Tsolum and Browns river flows over the storm period. Had BC Hydro not back off for the high tide on December 9 the flooding would have been significant, and even with the flood event on December 10, it could have been worse.

The third graph shows BC Hydro’s water supply year that goes from October to September. All 47 years of record are shown with the current year coloured. We are off to the second wettest on record. A major contrast to last year where we had record dry conditions.