Stories & Features

Record inflows, but limited downstream flooding, thanks to BC Hydro operations and some luck

Breaks in the weather,  planning with local officials, limit flooding

It was an engaging few weeks for our generation operations team on Vancouver Island and everyone involved in forecasting river inflows, monitoring water levels at our dams and reservoirs, and observing what was happening downstream.

Storm after storm pounded the watersheds near Courtenay and Campbell River across October and November – and as water levels continued to rise in the rivers and behind our dams, concern about possible flood impacts to local communities was rising too.

Looking at the forecast this week, stakeholder engagement advisor Stephen Watson is very glad to see some sunny breaks.

"For the past six weeks or so, it's been a series of storms coming in and we were seeing incredibly high water run-off into our reservoirs. We got close to the levels where we would have needed to pass whatever's coming into the reservoir downstream but we just squeaked by without that happening – so it's fantastic to see the weather back off and the reservoir levels start to come down."

The big concern over the past few weeks was the high likelihood of downstream flooding in the local communities. Our dams and reservoirs were operating safely and as intended, but there comes a point when the reservoirs are full and have to simply pass natural inflows to respect dam safety requirements. With rising reservoir levels, communities downstream become increasingly exposed to the full potential and strength of natural river flows – something they don't see very often.

"There's a lot of factors that can affect whether downstream flooding occurs, not just the amount of rain we were getting, which was a huge amount. We're looking at the wind creating storm surge from the ocean coming up the estuary, the influences of an incoming high ocean tide, the impact of peaking water levels on rivers where dams aren't located, to how much water we can hold back during key times like high tides," says Watson.

As he explains, managing reservoir levels is a balancing act – with our team respecting dam safety requirements while exercising operational flexibility to consider public interests around the reservoir and downstream, power generation, and environmental impacts such as fish habitat.

It's a big job that gets trickier when you consider all the factors that come into play. Add record-setting rainfall, and the downstream municipalities had a potential recipe for some serious flooding.

Image of the John Hart Dam
To put it into perspective, the water spilling from John Hart Generating Station at the peak of the inflows would have filled an Olympic-sized swimming pool in four seconds.

Just how much water? Enough to fill an Olympic-sized pool in four seconds

Watson says to understand how much water was coming into both the Puntledge River and Campbell River watersheds, you need to put it into perspective. He's been involved with our reservoir operations for 17 years, and has never seen anything like it.

"The upper Campbell River watershed received the normal amount of precipitation for the whole month of November (400mm) in the first eight days of the month. In the six weeks since the start of October, Upper Campbell Reservoir/Buttle Lake received the volume that normally would occur between October to mid-January," he says.

We increased the spills from our three dams on the Campbell River system to manage the reservoir levels. And, at other times, decrease the spill from our John Hart facilities to hold back water during daily high tides. Total discharge (power generation and spill) from John Hart was a record-tying (1990) flow rate. At the height of the storms, ~600 cubic meters per second (m3/s) was being released with 480 m3/s over Elk Falls (120 times the normal Elk Falls Canyon flow rate) and 120 m3/s through the powerhouse, providing a downstream Campbell River flow that's five times the normal winter discharge.

That's enough discharge to fill an Olympic-sized swimming pool in four seconds. In contrast, inflow into the system peaked at about 1100 m3/s, the equivalent of that same swimming pool entering the reservoir about every two seconds for 24 hours. BC Hydro operations were able to reduce peak downstream flows by almost two-fold.

The story isn't much different near Courtenay on the Puntledge River.

Over a seven-day period and a 14 day period during the storms, we saw new 53-year water volume records for inflows into the Comox Lake reservoir.

October precipitation was a record high (537 mm) and the record precipitation from November 1 to 13, at our Eric Creek gauge in the upper watershed, recorded about 566 mm. That's 1.1 metres of rain.

The amount of water into the reservoir over the past two weeks was likely approaching a 1-in-100 year event for storm inflows, while in the Campbell River system, it exceeded a 1-in-100 year return period.

"It's all about the timing – the downstream communities were fortunate this time. When the Browns and Tsolum Rivers did peak, they peaked at low tide. The high tides were lower than where they could have been over the storm period, at only 4.7 metres versus 5.2 metres right now with the King tides."

If we had seen higher King tides during this series of storms, things could have ended much differently.

Across the watershed, record levels of precipitation kept our operations team and emergency officials on their toes determining the best way to mitigate flood risk and prepare for potential flooding, including in areas that had never flooded before.

Preparation, coordination, and anticipation

It wasn't just BC Hydro's team on high alert – municipal officials from Campbell River, Courtenay, Comox, and the Strathcona and Comox Valley regional districts were working hand in hand with Emergency Management BC. Flooding in other areas of Vancouver Island was taking place; Highway 4 was re-routed near Port Alberni and water levels were on the rise across the island.

Watson credits collaboration and coordination between all parties for the high level of awareness and preparation in the community. Daily calls, regular updates, and proactive communication by the municipalities to affected residents made sure everyone was ready for whatever was coming. Plan for the worst and hope for the best.

And it's a good thing. As he points out, it's only mid-November.

"We need to remain diligent as the reservoirs are high and we're now hitting mid-November which is typically when storm season starts. It will take some time for the upstream reservoirs to gradually lower to more normal conditions."

In other words – we're glad to see the water levels heading down – but we're ready for when they start to rise again.

comox-dam-spillway-gates-fullwidth.jpg
Intense rainfall from a series of storms hammered the Puntledge and Campbell River watersheds over the past week, breaking records and nearing the upper limit of normal reservoir operations.