Feds pick cut-and-dredge option to improve flood flows

HARLINGEN — A long-awaited proposal to improve the Arroyo Colorado to lessen flooding in the city is moving forward with the most aggressive option.

The International Boundary and Water Commission, which is responsible for flows in the 89-mile arroyo, has selected the fourth option among those available which will mean dredging the arroyo and severely pruning back vegetation along its banks.

If the plan, which is now in the public comment stage, goes ahead, work should start in December or January on a nine-mile section of the arroyo which begins at the South 77 Sunshine Strip bridge and meanders downstream to just above the Port of Harlingen.

Flow rate disputed

The options the IBWC rejected were do nothing, build retention reservoirs and ponds, and sharply cut back vegetation which impedes the flow of floodwater but without dredging.

Federal officials say plan No. 4 “would provide properties in Harlingen with the greatest decrease in flood risk.”

“Over time, increases in vegetation and sediment caused the Arroyo Colorado to lose more than 50 percent of its capacity to carry floodwaters, which increases flood risk,” IBWC officials said.

The IBWC insists the arroyo can carry up to 21,000 cfs, which is its design maximum. City officials, who have lobbied for improvements to the arroyo for more than 15 years, have long disputed that number, noting that during recent flood events it appears the arroyo can handle only about 10,000 cfs, less than half of what federal engineers say it can accommodate.

City officials have noted previously the flood-control system of which the arroyo is a key component was designed decades ago when most of the area was still agricultural.

“And the economy has diversified tremendously, the region has grown tremendously, but we’re using a 70-year-old system that doesn’t work,” Mayor Chris Boswell said in 2019.

The 1968 standard

Jim Chapman, a longtime environmentalist in the Rio Grande Valley who formerly served as chair of the Lower Rio Grande Valley chapter of the Sierra Club, has been studying water flows in the Valley for decades.

Chapman, who noted he does not speak for the Sierra Club now, said that in 1968 post-Hurricane Beulah the IBWC determined that a 250,000 cubic feet per second flow on the Rio Grande at Rio Grande City would constitute a 100-year-storm event.

“Everybody is absolutely glued to this 1968 number and ignores the fact the situation now is very different,” Chapman said.

At the time, he said, it was probably a reasonable guess for a 100-year-storm event.

“But the thing is, since 1969 the storage capacity on the watershed, on the Rio Grande watershed, has increased by 60 percent because there have been six or seven reservoirs built on the watershed since then, including Amistad, which is the largest reservoir of all.”

What all this means, Chapman said, is there’s no reason to divert 21,000 cfs into the Arroyo Colorado during a flood event since upstream retaining capacity has been increased by the new reservoirs, and the 100-year-flood standard has not been updated to reflect it.

“My point is, I don’t think they need to plan for 21,000, but until they change the divisor (the number factor) back in Mercedes, that area in Harlingen’s at-risk for flooding, because right now they’re going to let 21,000 go into the arroyo at Mercedes, and when it gets to Harlingen, it’s going to flood,” Chapman said.

“If they come up with a lower number (via a new engineering flow survey), which to me is almost guaranteed they will, then they can modify the divisor so that the arroyo only gets 10,000 or 12,000, which I think is around what you can handle now,” he added.

City likes option

No dollar amount for the project has been divulged by IBWC, city officials say, no doubt because the project hasn’t yet gone out for bids.

“We’re pleased with the option,” said Carlos Sanchez, assistant city manager. “I think it’s the better option in terms of restoring the capacity, or at least improving the capacity, for the Arroyo Colorado to convey storm water.

“It’ll make a big difference,” he added.

But Sanchez and other city officials remain dubious about the IBWC’s claim that this work on the arroyo will allow the waterway to reach its maximum flow set by engineering studies.

“We are of a different opinion at this point with the IBWC because in their report they’re saying that this will restore the capacity to 21,000 cfs,” Sanchez said. “We don’t neccesarily agree with that, but what’s promising is that after this work is done, there is a hydraulic study coming. So we will be able to verify that information by having actual field data and putting that data on a modeling software system that’s going to be able to give us a better understanding of the capacity.”

That new hydraulic study, however, will only cover the area between Mercedes and the point where the arroyo dumps into the Laguna Madre, and not the full flood-control system from Rio Grande City down.

Sanchez said the city has sites for the spoil, the dredging mud, which will come out of the arroyo. But first three sites have been selected for temporary storage areas where the spoil will dry out, making it easier to transport.

The choke-point

The beginning of the project at South 77 Sunshine Strip’s arroyo bridge illustrates one of the biggest problems the city is facing. Above the bridge, along the hike-and-bike trail, the arroyo’s bottom-land is quite broad and free of vegetation, allowing floodwaters to spread out.

But where McKelvey Park ends just downstream, the channel narrows abruptly, and the banks become steep.

“It’s not a very long stretch, I would guess probably a quarter of a mile. … that’s the real choke-point and the problem there is the banks are so steep that if they really try to clear those banks, they may cave in, and I think even when you dredge down below, that’s going to cause simply faster erosion from those banks,” Chapman said.

“If they really want to tap 21,000, I think they would have to buy or condemn some of the houses along there and make it a broader channel,” he said.

How to Comment

Public comments on the final environmental assessment are due Nov. 5, and should be submitted to Mr. Kelly Blough, Environmental Protection Specialist, via email at Kelly.Blough@ibwc.gov or via mail at Mr. Kelly Blough, International Boundary and Water Commission, 4191 N. Mesa, El Paso TX 79902.

rkelley@valleystar.com