‘Off the charts’: Why catastrophic rains overwhelm city’s drainage system

Courtesy: National Weather Service Brownsville

HARLINGEN — That makes two.

As Harlingen residents continue ripping out carpet, drywall and a little bit of their hearts in the aftermath of a second catastrophic flooding, many are asking just how the Valley could be hit with two “500-year rain events” in just 12 months.

“Looking at the probabilities of a return interval from something like this, 12 inches of rain in a six-hour period happens every 100 to possibly every 500 years,” said Chris Birchfield, a meteorologist with the National Weather Service in Brownsville.

“That doesn’t mean that it’s going to take 500 years to get this amount of rain, it just shows how rare an event that is.”

Birchfield, who happens to live in Harlingen, was one of the speakers at Tuesday night’s town hall meeting on the flooding event held at Lee H. Means Elementary School. For some, the meeting was a catharsis, and a few members of the 4,000 households with flood-damaged homes vented some of their pent-up frustration.

But the meeting also took a deep dive into the city’s drainage system, the improvements made since Hurricane Dolly in 2008, and why creating a flood-proof city is practically impossible to engineer and certainly impossible to finance.

Rainy day blues

Area rainfall totals for the June 24 storm vary wildly, but the heaviest precipitation fell in a line roughly to the west of I-69E from western Willacy County in the north, western Harlingen to the east, south of La Feria in that direction and the Hidalgo County line to the west.

By comparison, in Harlingen during the June 2018 rain event, the city received 16.85 inches, but that was spread over nearly four days.

The National Weather Service’s official totals for the latest storm event put the most rainfall in Santa Rosa at 15.2 inches, with 11.6 inches in west Harlingen and 9.67 inches of rainfall in Raymondville. North Harlingen had about 8.6 inches, according to the federal numbers.

“Nothing like that (Santa Rosa) has been reported in Harlingen,” said Robert Frye, another meteorologist with NWS Brownsville who lives in Harlingen. “The most was 11.6 across the northwest part of town and that’s where the heaviest rain fell. But if you look at San Benito, 2.4? What happened? Well that’s where the cold front set up right over the city of Harlingen.”

The mini-cold front, or to be more specific, a frontal boundary layer coming from the north, stalled over west Harlingen and belched rainfall measured in feet.

“On the northwest side of town toward Bass and Stuart Place, they got almost a foot,” Frye added. “And that’s a very small area. Trying to forecast that, again, is impossible. It is just simply impossible; we can’t do it.”

Rain totals vary

The meteorologists with the NWS only use official rain gauges, operated either by the weather service or in cooperation with local volunteers signed up with CoCoRaHS, an acronym for the Community Collaborative Rain, Hail and Snow Network.

They readily concede in specific areas rainfall totals from the storm may be far higher than the official recordings indicate.

“The rain event of June 24 we had between 12 and 15 inches,” said Harlingen City Manager Dan Serna. “I know the meteorologists just talked about 11.6, but we have reports from citizens that tell us that they had over 15 inches and in some cases 16 inches of rain.

“About 4,000 homes were impacted,” he added. “That was 12 to 15 inches in three-and-a-half to four hours. That’s a lot of rainfall in a very short period of time.”

That 500-year label

Calling it a “500-year” rain event is a bit misleading. What the phrase is designating is the percentage of the likelihood of a 100-year or a 500-year rainfall event occurring in any single year.

Engineers measure it by inches of rain falling over a designated period. A five-minute deluge of 1.37 inches has a likelihood of occurring once every 100 years, and 1.66 inches falling in five minutes is designated as potentially occurring statistically just once in 500 years.

Under engineering standards in the Atlas of Depth-Duration Frequency of Precipitation Annual Maxima for Texas, 6.15 inches of rain falling in three hours is a possibility that would statistically occur once every 100 years, and 8.48 inches over three hours could be expected just once every 500 years. So those would be 100-year and 500-year rain events.

But in some areas in north and west Harlingen, and out in the county in places like Primera and Santa Rosa, rainfall totals far exceeded those benchmarks, with 15 to 16 inches falling in about a four-hour period.

That is a rain event which is statistically off even the biggest charts, and what occurred in Cameron County on June 24 could just as reasonably be called a thousand-year rain event or a two thousand-year event, given the numerical improbability of that much rain falling that fast in one spot.

Drainage goals

Serna went on to describe how the city engineers its drainage system to meet requirements to shed water from a five-year rain event, not a 500-year event.

For example, the city rates rain events by the depth-duration classification referred to previously. So much water falls in such-and-such a time, and the city’s target is to have a drainage system which can move 3.3 inches of rain falling in three hours — an event anticipated only once every five years — without significant flooding.

“In a three-hour period of time at 6.2 inches, you’re well above already the 100-year event,” Serna said. “That’s far exceeding the capacity of our interior storm drain system. So the 13 to 15 inches of rain? That’s off the charts. That’s somewhere in a 500-year storm frequency.

“And I know we say that, we had one last year, why is this happening so often?” he added. “These are weather events. Kind of the perfect storm, no pun intended, but these things happen. For some reason, they’re happening more frequently here.”

Costs prohibitive

Both Serna and Mayor Chris Boswell said at the town hall that making the city drainage system more efficient to handle a 100-year or a 500-year rain event is unrealistic both from an engineering and a financial standpoint.

“One of the reasons our system is designed for five years is it takes a certain type of pipe and infrastructure to carry that kind of water,” Serna said. “The larger you go, the more costs you run into. The more costs you run into, the more properties cost. So developers look at developing something that has a rain frequency that we normally see. And a five-year design typically handles that.”

Serna said the numbers back up creating a drainage system intended to handle a five-year rain event.

“A 500-year storm has a 0.2 percent probability of happening in any given year — 0.2,” he said. “Unfortunately for us, it happened twice in the last 12 months.”

For NWS meteorologist Birchfield, these tiny percentages are confirmation of the rarity of rainfall like that which occurred June 24. Perhaps the fact such a major storm popped up out of nowhere contributed to the sense of shock at what eventually transpired.

“You’re probably wondering why did we get a foot of rainfall and we didn’t forecast a foot of rainfall,” he said.

“It’s not a perfect science, and we’re doing our best to improve things,” he told the crowd. “And this event’s really going to help us in the future for forecasting.”

rkelley@valleystar.com