Transportation crews have been working to repair wash outs and damage to more than 12 roads across P.E.I. (CBC)

Highway crews are struggling to reopen flood-damaged roads two days after heavy rains hit the province but some watershed groups say that’s because culverts used in highway ditches just aren't big enough.

Crews have replaced the damaged culvert, and are now rebuilding dozens of trouble spots around the province.

"We're out cleaning up culvert ends; we’re out cleaning up ditches, trying to get the water where it's supposed to go. But it all just takes time," said Darren Chaisson, director of highway maintenance.

Repairs could take six weeks or more.

This week’s flooding has overwhelmed bridges and culverts across the Maritimes.

Now some watershed groups say the old, small steel culverts simply aren't big enough to do the job.

"There’s a lot of old infrastructure out there," said Sean Hill, with the P.E.I. Watershed Alliance.

He said road planners need to increase the capacity of culverts.

"In the past, we might have seen events that happened periodically and they called them one in 100 year events.  Now we're seeing events like once in every two years. Big, big flash flows."

Highways officials say culverts across the Island are being upgraded, but only as they need to be replaced.

"Knowing with the weather patterns we're getting hit with, we're erring on the side of caution, going with even bigger than what's recommended most times," said Chaisson.

The cost of repairing the damage from this week's flood could reach $1 million.