Trump’s border wall needs storm gates which will be left open for months each summer to prevent it being knocked down by flash floods

Barrier will likely need hundreds more openings in order to prevent being washed away

Debris will build up in the gaps in the barrier as floodwater puts pressure on the wall during 'monsoon season' (AFP/Getty)

Donald Trump's border wall will require the installation of storm gates that must be left open for months every summer to prevent flash floods from undermining or knocking it over during “monsoon season” in the desert, according to US border officials, agents and engineers familiar with the plans.

The open, unmanned gates that are already in place in remote areas already have allowed for the easy entry of smugglers and migrants into the United States. US Customs and Border Protection (CBP) will have to instal many, likely hundreds, more.

At locations along the US southern border where such gates already are in operation, Border Patrol agents must manually raise them every year before the arrival of the summer thunderstorms that convert riverbeds into raging torrents that carry massive amounts of water and debris, including sediment, rocks, tree limbs and vegetation. Mr Trump's wall, which features 30ft metal bollards spaced four inches apart, effectively acts as a sewer grate that traps the debris; when clogged, the barriers cannot withstand the power of the runoff.

Because the gates typically are located in isolated areas that lack electricity, they cannot be operated from afar. That requires the Border Patrol to leave the gates open for months, increasing the need for US agents to monitor the sites because smugglers and other border-crossers can enter through the large gaps and advance northward following stream channels and narrow canyons to avoid detection.

The flooding risks are one of the biggest engineering challenges to the president's vision of a linear man-made structure spanning hundreds of miles of desert, canyons and mountains. But the Trump administration has said little about how it plans to manage the hydrology of the border region.

Though Mr Trump has boasted that his new "border wall system” will be an impermeable force against illegal crossings and drug trafficking, the need for open gates is another notable weakness that smugglers and migrants can exploit to slip through the barrier and evade capture.

Smugglers have learned how to cut through the new steel bollards using common tools they can buy at hardware stores, and some have demonstrated that the wall can be climbed with handmade ladders and rope. Most of the hard narcotics that enter the United States via Mexico pass through official border crossings, hidden in vehicles and among cargo, not through the remote areas where Mr Trump's new barriers are being erected.

Roy Villareal, chief of the Border Patrol's Tucson sector, which covers most of Arizona, described the addition of the floodgates as an example of how his agency has learned to adjust to the realities of the southwest's extreme weather and topography.

“The border is so diverse,” Mr Villareal said. “You have to plan for water flow . . . People think it's just this monolithic wall, sort of like the Great Wall of China, where you drop it into place and that's all there is to it. And that's not the reality at all.”

John Ladd, whose cattle ranch extends along the border for about 10 miles west of tiny Naco, Arizona, said the Border Patrol and the Army Corps of Engineers began installing 18ft bollards on his property in 2008, adding “lift gates” that could be opened during the summer to allow floodwaters through.

Mr Ladd, who supports Mr Trump and his wall project, said his span of the border now has about 70 gates, and US agents use a forklift to raise them at the beginning of every summer. They initially were designed to be hoisted by agents using the winch on their Border Patrol vehicles, Mr Ladd said, but the gates were so heavy that “the front end of their trucks would start lifting off the ground”.

When the gates were installed on Mr Ladd's ranch, smugglers would drive through the openings with loads of marijuana, he said, so the Border Patrol lowered the height of the opening to four feet. The vehicle incursions have stopped, but illegal crossings and smuggling increase along his property during the summer months, when the gates are left open, the rancher said.

“They know as soon as the Border Patrol opens them,” he added, referring to traffickers in Mexico.

Veteran Border Patrol officials acknowledge that the government would be foolish to place vertical metal bars in the direct path of rivers and creeks that swell to dangerous volumes during summer storms. The US southern border is crisscrossed by hundreds of drainage channels and several rivers. One Arizona river, the Santa Cruz, starts in a US valley, flows through the mountains into Mexico and returns to cross the border again with more water.

Outside of the high-traffic areas, much of the US-Mexico border through New Mexico and Arizona is lined with vehicle barriers that are welded from old rail tracks. Although they would stop a car or truck from crossing, they allow water and debris - as well as wildlife and people - to pass through.

Mr Trump's border project is replacing those barriers with the steel bollards, which act like a sieve and can impede water flow.

At several locations in Arizona where construction crews are racing to erect the structure, workers have been leaving gaps at creek beds and river channels because they do not yet have the new fencing panels with storm gates. Older vehicle barriers remain in place along those sections, the gaps akin to missing teeth.

CBP officials in Washington declined to respond to questions about how many storm gates they plan to install and where they would be located.

But Mr Villareal and other Border Patrol officials said the storm gates, and their need to be left open all summer, amount to a relatively minor challenge that can be compensated for using technology such as cameras and sensors - along with more agents.

They also say a barrier with openings is preferable to no barrier at all, and the gaps can help them by funnelling foot traffic into areas where they can concentrate their interdiction efforts.

“At the end of the day, you still need an individual to monitor and make that arrest,” Mr Villareal said. “What's been tested, and seems to work well for us, is opening them up at the beginning of monsoon season, and at the end of monsoon season, closing them back down. Which means for the patrol agent in charge of that particular area, he or she has to deploy manpower to cover that area when the gates are open.”

The storm gates are different from the much larger vehicle gates the government installs along the Rio Grande in Texas to allow farmers, US agents and others access to land that becomes cut off from the rest of the United States by the barrier. Those gates can be quickly opened and shut and do not need to remain open to manage flooding.

White House officials this month acknowledged that they are preparing to divert an additional $7.2bn (£5.4bn) from this year's Defence Department budget for wall construction, money that will allow the administration to complete nearly 900 miles of new barriers by 2022.

If that plan goes forward, it likely will include new barriers in mountainous areas where the force of floodwaters is even greater and engineers would need to install even more storm gates through canyons and creek beds.

In 2011, a 40ft span of mesh-style border fencing collapsed in Arizona's Organ Pipe Cactus National Monument after storm debris became trapped against the structure, causing floodwaters to back up.

“They came in and installed gates, but it was a challenge for the Border Patrol,” said Lee Baiza, who was the park superintendent at the time. “How do you man those gates during weather events?”

Mr Baiza said the president's border barrier project is likely to exacerbate the risk of damage because the crews building the structure and the adjoining road have loosened and disturbed soil, rocks and plant life, all of which can be picked up and carried away in sudden rushes of water.

“It's a fragile environment, so if you go drive across that country you will loosen up the soil and brush,” Mr Baiza said. “The more activity, the more it gets run over, the more debris you create.”

Another section of border barrier toppled in 2014 near Nogales, Arizona, when US agents failed to open the floodgates in time, sending mud and stones into nearby homes. And in 2008, two people in the Mexican city of Nogales, just across the border, were killed in catastrophic flash floods that inflicted millions of dollars' worth of damage, with some of the blame falling on a Border Patrol project that placed bars into cross-border culverts intended to block illegal crossings.

The gates have created logistical problems for US officials, who must physically lift and lower the gates with the changing of the seasons. In autumn, Border Patrol crews sometimes struggle to close the gates on Mr Ladd's Arizona ranch, he said, because the openings accumulate rocks and sand during the summer.

Despite what border officials said about sensors, Mr Ladd said agents have told him they cannot leave sensors in the stream channels and canyons while the gates are open because they, too, risk being swept away by flash floods. He said he has not seen an increase in summer patrols, either.

The San Pedro River, which starts in Mexico and flows north into Arizona at the edge of Mr Ladd's property, meanders for much of the year or dries up. But its sandy flood plain is as wide as a football field.

At the site where it crosses the border west of Naco, Border Patrol agents must remove everything in the river's path, even the permeable vehicle barriers, to prevent them from being swept away.

“There's no way you can put a bollard fence in here,” said Laiken Jordahl, an environmental activist with the Tucson-based Centre for Biological Diversity. On a recent afternoon, he counted 18 cottonwood trees next to the river at the border crossing that appeared to be tagged for removal, some of which were mature. “They're trying to take every inch they can,” Mr Jordahl said.

Crews were preparing to install new barriers, but border officials haven't said what type of gates they will use at the river.

“This is a wildlife superhighway,” Mr Jordahl said, listing the species that traverse the border through the stream channels: antelopes, deer, coyotes, bears, bobcats, ocelots, javelina.

On a bright January day, the river's cool waters ran just a few inches deep, but the San Pedro's latent power was visible a little higher on the riverbank. A tangle of logs, mud and other flood debris was snarled against a tree, 10 feet above the river channel.

The Washington Post