Prepared for a ‘25-year storm’

When it rains as hard and for as long as it did this week, even the best city infrastructure is going to have flooding issues, Costa Mesa city officials said Thursday.

A prime example was Monday, when the first — and so far the worst — flooding from this week’s series of storms took its toll on Costa Mesa roads and buildings.

For most of Monday, a few streets in Costa Mesa were inaccessible, cul de sacs became small lakes, and people had to use sandbags to protect their homes from the gathering water.

Costa Mesa storm drains are prepared for some deluges, said Costa Mesa City Engineer Ernesto Muñoz, but downpours like Monday’s are rare and too costly to build infrastructure for.

The city’s storm drains are big enough for a “25-year storm,” meaning the storm is so big, it should happen only once every 25 years or so. That means there is only a 4% chance a storm like that would occur this year.

“When we have a storm, like the one on Monday, I would venture to say that was in excess of a 50-year storm. That’s just something that’s not anticipated,” Muñoz said.

The size of the storm comes down to how much it rains and in how short a period of time. Muñoz said it wasn’t the amount of water that dropped on Costa Mesa that flooded roads, it was how fast it all came that made it so overwhelming.

Residents near Pomona Avenue and 17th Street know all about being overwhelmed in a rainstorm. Cars parked there were almost fully submerged.

The drains there are the exception to the city’s drainage system, Muñoz said. In easily the most flooded part of the city, the storm drains are about half as big as they are everywhere else, he said.

Water in much of the west side of Costa Mesa eventually flows under Superior Avenue into Newport Beach, where Muñoz said the pipes simply can’t handle so much water at one time.

There are few options, he said. The city could build a second storm drain through Newport Beach to relieve some of the burden, costing more than $2 million, or upgrade their own pipes’ capacity, so as the water gets backed up, it has more storage space before flooding into the streets.

“As funding becomes available, we work on those priorities,” he said. “Funding for storm drains is really limited. It’s all driven by development and development fees. As development goes down, so does the revenue available to do these projects.”

In the meantime, as the rain continues to pour into Orange County, keep those sandbags handy.