HARLINGEN — After years of negotiations with city hall, federal crews have dredged along a stretch of the Arroyo Colorado as part of a flood control plan.
The U.S. International Boundary and Water Commission dredged along bridges at Farm-to-Market Road 509, FM 499 and 77 Sunshine Strip, IBWC U.S. Secretary Sally Spener said.
Spener said the agency, which plans more extensive dredging, cleared sediment from the arroyo’s floor and banks to improve the floodway’s flow.
For years, city officials have requested the agency help clear the arroyo of sediment and brush that hinders its flow, pushing up water levels that could back up, flooding nearby areas.
“We’re grateful the IBWC is taking this task on,” Harlingen Assistant City Manager Carlos Sanchez, a hydraulic engineer, said. “We’ve been talking to the IBWC over the last several years to take up some of this. It is meant to clear some areas where we’ve seen some reduction in flow because of the accumulation of silt.
“The dredging and clearing that USIBWC is doing will remove some restrictions in the arroyo, which will cause water surface elevations in the arroyo to drop.”
From Monday to Wednesday, the agency’s crews used an excavator to dredge the floodway’s floor while clearing its banks of overgrown vegetation along the three bridges.
“This is the minor dredging we are undertaking as an interim measure … intended to increase conveyance capacity at these locations,” Spener stated.
“(Due) to sediment build-up and vegetation, currently the Arroyo Colorado cannot convey the full volume of water that it was designed to handle. The limited dredging at the three bridges being undertaken at this time is just one part of a larger effort to restore capacity.”
At city hall, officials are requesting the agency perform more extensive work to increase the floodway’s flow, Sanchez said.
“This is a starting point. It’s only the beginning of a larger project,” Sanchez said. “This is a great thing. It is important for the city that these activities be done. The city of Harlingen will continue to coordinate with the USIBWC to strategically address additional vegetation clearing and dredging of the arroyo.”
Spener said agency officials have not determined when they will perform more extensive dredging.
“At this point, I don’t know when this additional work will occur,” she stated.
Last year, the agency’s hydraulic study found that silt along the arroyo’s floor, along with overgrown vegetation, had reduced the floodway’s designed flow rate of 21,000 cubic feet per second by 45 percent, or 9,450 cubic feet per second, Sanchez stated.
“Based on a flow of 9,450 cubic feet per second, the water surface elevation in the arroyo would be much higher than 24 feet,” Sanchez stated, referring to the floodway’s 24-foot water surface elevation. “This poses a higher risk and would cause more water to back up into the city’s storm system, and cause significant flooding.”
In recent years, big storms have pushed the floodway’s water levels to dangerous levels.
This year, the June 24 storm spurred a peak flow of 5,226 cubic feet per second, pushing the arroyo’s water level to a height of 22.67 feet.
Meanwhile, a June 2018 storm forced a top flow of 5,069 cubic feet per second, driving the floodway to rise to 23.98 feet.
If the arroyo’s water level rises to 24 feet, floodwaters would spill its banks, Sanchez stated.
The arroyo’s dredging would slash the risk of flooding.
“With the proper dredging and clearing of vegetation in the arroyo, flows greater than 5,200 cubic feet per second can be sent down the arroyo without causing an impact to the city storm sewer system,” Sanchez stated. “Otherwise the city would not like to see flows greater than 5,200 cubic feet per second in the arroyo.”