Texas National Guard helped evacuate Houston residents.

The system of bayous and reservoirs built by Houston in the 1930s fell short of protecting the city when Hurricane Harvey struck in late August, dumping as much as 50 inches of rain.

Analysis by the Associated Press says that experts blame too many people, too much concrete, insufficient upstream storage, not enough green space for water drainage and too little regulation.

Houston is the most flood-prone city in the United States, AP quotes Rice University environmental engineering professor Phil Bedient as saying.

The system is designed to drain 12 to 13 inches of rain per 24-hour period, said Jim Blackburn, an environmental law professor at Rice University. He called that an "obsolete" standard.

In early August, President Trump reversed Obama-era flood standards in a bid to get infrastructure projects approved more quickly. The rule was signed by former President Barack Obama in 2015 and had not yet come into effect. It meant that roads, buildings and other infrastructure needed to be constructed to take climate change into account and be built to withstand the effects of factors such as rising sea levels in coastal areas.

Trump believed the rule would slow down the permitting process. The president favored a faster route for infrastructure building.

Speaking from New York when he announced the rule rollback on August 15, Trump said, "We're going to get infrastructure built quickly, inexpensively, relatively speaking, and the permitting process will go very, very quickly."

According to the AP, Harris County, which includes Houston, has 2,500 miles of bayous and channels and more than 300 storm-water holding basins. Those are designed to fill during downpours and drain slowly as high waters recede.

Water is supposed to flow west to east through bayous. These are tidal creeks that often have concrete improvements to make water flow. They connect to the Galveston Bay on the Gulf of Mexico.

When big rains come, officials also activate two normally dry reservoirs to collect the water and keep it from overwhelming the downtown area.

But the main bayou through downtown Houston, Buffalo Bayou, remains largely a dirt mud channel, according to a U.S. Geological Survey hydrologist.

And because the coastal plain slopes about 1 foot per mile, the water doesn't flow out of the bayous fast. Harris County also didn't leave enough right-of-way space to expand its bayous.

After early 20th-century floods, Houston designed two dry emergency reservoirs that are activated in heavy rain, Addicks and Barker, both formed by earthen dams. Addicks is 11.7-miles long (18.8 kilometers) with a maximum elevation of nearly 123 feet (37 meters). Barker is 13.6-miles long (22 kilometers) and has a maximum elevation of 114 feet (34 meters).

Normally, the floodgates are open and the two areas are dry parkland with sports fields and biking paths. They were essentially dry on Aug. 25, the day Harvey struck. By the middle of the next day, the floodgates were closed and water levels were starting to rise.

The reservoirs overflowed on August 29, and officials are releasing some of the water potentially worsening the extreme flooding downstream in Houston.

But mostly the problem comes down to extensive development in a county with no zoning, leaving lots of concrete where water doesn't drain and little green space to absorb it.

Since the previous record flood, inflicted by Tropical Storm Allison in 2001, Houston's population has grown more than 23 percent, said Sam Brody, a Texas A&M professor in Galveston who studies coastal flooding.

Houston also is getting heavier rains because warmer air holds more water. Since 1986, extreme downpours—the type measured in double-digit inches—have occurred twice as often as in the previous 30 years, an Associated Press weather data analysis showed in 2016.