'Extremely dangerous': B.C. officials warn public not to approach or fly drones over Chilcotin River landslide

Provincial officials are urging people to respect the evacuation order that has been in place since a massive landslide blocked the Chilcotin River in central B.C. earlier this week.

While new modelling presented at a news conference Saturday suggests the eventual breach of the dam created by the landslide is likely to be less severe than previously feared, Emergency Management Minister Bowinn Ma warned that the situation remains "dynamic" and dangerous.

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"Yesterday evening we reissued the B.C. emergency alert," Ma said.

"This was necessary because we needed to renew our efforts to evacuate people from that landslide area. Despite significant efforts to evacuate and clear the area on Wednesday, we received reports that some people were coming back to see the landslide, some with drones."

Ma said the BC Wildfire Service has issued a Notice to Airmen (NOTAM) for the airspace above the landslide.

"This means that operation of any aircraft that is not part of the landslide response is illegal within this area," she said.

"For the safety of the air crew and of the public we are asking everyone to stay clear of the area both on the ground and in the airspace. This is extremely dangerous, and poses a threat to both the people that re-enter the area and experts who are assessing the situation on the ground."

The landslide occurred late Tuesday night or early Wednesday morning, but additional material from the hillside has continued to fall down the slide path since the initial incident, officials said Saturday.

The slide is roughly a kilometre long, 800 metres wide and 30 metres deep, according to the provincial government's latest mapping of the area.

B.C. officials said Saturday they expect the dam to give way once the lake forming behind it overtops it. It's unclear just how long that will take to happen, but officials presented two scenarios.

• Read more: Worst-case models point to reduced Chilcotin landslide downstream flood threat

One, described as the "worst-case scenario," would see the dam material substantially eroded in just one hour, causing a significant release of water well above the levels seen during a spring freshet on the Chilcotin River.

In that scenario, the water level would be four metres higher than its current level at the Chilcotin River's confluence with the Fraser River, a point the water from the breached dam would reach in about three hours.

The peak flow would be more than 5,500 cubic metres per second, according to the modelling. The typical freshet flow in the area is 300 cubic metres per second.

"I want to be clear that the valley could be extremely unstable and dangerous before, during and after overtopping of the barrier occurs," Ma said. "There is an evacuation order in place along the Chilcotin River. It has been in place since July 31. People need to stay out of the order area."

A second scenario would see the water behind the dam released over a 24-hour period, according to officials. This scenario would see peak flows of more than 3,600 cubic metres per second at the confluence of the two rivers, with a roughly 11-hour travel time for the landslide material to reach the Fraser.

While this scenario is less dangerous, especially farther downstream, the province continues to prepare for the potential worst-case scenario, and Ma said it needs the public's co-operation as the water behind the dam continues to rise.

"We are talking about water flows that could potentially exceed typical freshet volumes by 20 times," she said. "We are encouraging everyone – and requiring everyone – to stay off of the Chilcotin River and its banks."