What you need to know about possible flooding in New Brunswick

If the spring of 2018 taught us anything, it's that you can never be too prepared for flood season.

What started as a St. John River swollen by a typical snowmelt in the north, quickly changed into a historic flood in the south.

Water levels destroyed homes, cottages and businesses from Fredericton to Saint John.

And as the water rose, flooding displaced more than 1,600 residents, in some cases for months.

Now officials with New Brunswick's Emergency Measures Organization are advising people to plan for the worst again.

What's going to happen this year?

The short answer? Nobody knows for sure.

Jasmin Boisvert, a water resources specialist with the province, says significant flooding depends on day-to-day weather.

When the water rises, Boisvert uses data and modelling expertise to come up with a daily five-day forecast.

"The St. John River is almost always going to reach flood stage somewhere," he said.

But officials hope for a gradual melt, made possible bywarm days, cool nights and little or no rain.

Ryan Snoddon, a CBC meteorologist, said seasonal temperatures are expected for the end of March into April, which is good news.

With all that snow and all that water equivalent, the gun is loaded.-Ryan Snoddon, CBC meteorologist

Temperatures are expected to rise above 0 Celsius during the day and drop to below freezing at night.

This will bring a slow, natural melt this time of year.

But things can change quickly,Snoddonwarned.

"It's kind of the nature of the beast here in Atlantic Canada," he said. "You never know what's going to come around the corner from Mother Nature."

How the St. John River floods

6 years ago

Duration 1:26

How high will the St. John River rise this spring, and does the Mactaquac Dam have anything to do with it? A New Brunswick flood, explained.

What are the different types of flooding?

Every spring, Boisvert said, the entire St. John River is susceptible to two types of flooding: ice-jam flooding and open-water flooding.

A spring flood usually has its origins in the snowmelt in northwestern New Brunswick,along the Quebec and Maine borders.

It travels down the province through Edmundston, Grand Falls andPerth-Andover and into the Woodstock area. Those areas are typically more susceptible to ice-jam flooding.

Open-water flooding typically happens below the Mactaquac Dam in the Fredericton, Maugerville, Jemseg and Saint John areas.

What's ice-jam flooding?

Over the winter, ice develops on the river, andsnow eventually freezes on top of the ice.

Come spring, the water level rises, and the ice above isn't strong enough to resist the pressure.

The ice starts to break into chunks, which flow down the river until they catch onto something, such asbridge piers, a bend in the river, shallow areas within the river, or ice cover that's too entrenched to move.

"It kind of catches onto that and piles up and then the water can't go through," Boisvertsaid.

• 'It's heartbreaking': Business owners struggle to stay afloat in midst of flooding
• Tips for taking control of your finances before another flood hits
• 'It's devastating': Fredericton, Grand Lake residents deal with flooding, damage

"It basically acts like an instant dam that just goes right into the middle of the river."

Ice jams can last for several days. But enough water flow can push an ice jam through a channel in an instant.

These are the two main types of floods along the St. John River

6 years ago

Duration 1:35

Ice jams and open-water are the main types of spring floods along the St. John River.

An ice jam can also catch a few more times and then eventually dissipate in water.

Ice jams typically happen in the Beechwood headpond in Carleton County and then again in the Mactaquac headpond just before Fredericton, where they start melting.

"There's only so far they can go," he said.

What aboutopen-water flooding?

This happens when the river is so overwhelmed by water, it doesn't have enough room to handle the flow.

Snow in northern parts of the province melts into streams and eventually into the St. John River. The melt is also known as the freshet.

"It just comes to a point there's too much water, [the river] can't handle it," Boisvert said.

That extra water travels from the north down the rivertoward the Bay of Fundy.

• New Brunswick flood damage seen from above
• Mill Cove family still dealing with aftermath of historic spring flood
• Worst floods in New Brunswick history: how 2018 compares

Along the way, it pours over the riverbanks, often across highways and roads, and into fields and flood plains inMaugerville and Jemseg.

Close to Saint John, sea levels and storm surges depend onthe tide.

When the sea is at high tide, the river is lower than sea level, so the ocean is actually draining back into the river. But when the Bay of Fundy is at low tide, the river is higher than sea level, draining the river backinto the ocean.

Is the Mactaquac Dam to blame?

Last year, many people questioned the role the Mactaquac Dam plays in flooding.

The dam, opened in 1968, is operated by NB Power and is a key part of the system that regulates the flow of water into the lower basin of the St. John River.

But Boisvert said the dam has nothing to do with spring flooding. There would be flooding even without the dam, he said.

"The dam has practically no storage capacity," he said. "They can't hold back water, they're just letting go of whatever's coming in."

In fact, the dam actually blocks ice jams from flowing down river. Instead, chunks of ice will sit in the Mactaquac headponduntil they eventually melt.

Water in snow matters

In mid-March, Snoddon said, the snowpack wasdeeper than normal, particularly in northern New Brunswick, where three to five feet (about one to 1 metres) of snow was waiting to melt.

But a lot of snow doesn't mean a major flood.

Snoddon said major flooding is less likely if temperatures are really cold and the snow on the ground is dry and fluffy.

Significant flooding, however, can happen when snow has a high water content.

This year, the water content is higher than usual.

"With all that snow and all that water-equivalent, the gun is loaded," Snoddon said.

Snow also melts faster when it rains and when temperatures are unseasonably warm over a long period of time.

This could include double-digit temperatures lasting a week or a huge weather event that brings significant rainfall.

For more than a weeklast spring, temperatures sat in the double digits, then moved upward into the 20s, which is above average for the time of year. At night, temperatures fell to a low of 0 to 1 C.

"If there's that much melting, that quickly, the basin just can't handle it," Snoddon said.

"It all has to go the same route."