Earlier today, we told you how a section of Donald Trump's border wall collapsed, just hours after he bragged about how powerful the structure is, and how fast it's going up.

Now, the president's top campaign promise—which remains unfulfilled— appears to have hit another snag.

The Washington Post reported Thursday afternoon that plans for the wall require flood gates that must remain open for months at a time, allowing easy entry for migrants and smugglers. Without the open gates, flash floods would undermine the wall or knock it over.

From WaPo: At locations along the U.S. 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. Trump's wall, which features 30-foot 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 U.S. 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.