Flooding is nothing new to Houston residents. Over the past five decades no other urban area in the country has experienced as many casualties and property loss from flooding.

In 2015, eight people died in what local media described as “historic flooding”. In 2016, what became known as the “tax day floods” saw three waterways in the city exceed their “500-year flood levels”. Six people died.

The size of tropical storm Harvey – some experts estimate it has already dumped 12tn gallons of rain in Houston and south Texas – is unusual. But there is no single reason for the extent of the impact it has had at street level.

“It’s a combination of factors,” said Sam Brody, a professor in the department of marine sciences at Texas A&M University. “It’s a very low-lying coastal plain, with clay-based soils that do not drain very well. The city is subject to very heavy rainfall, as well as flooding from tidal events.

“You take that flood-risk landscape and you put 6 million people on top, with prolific amount of pavement and roadways and a lack of collective and regional thinking about what that does to the natural drainage of that landscape, and you end up with disasters.”

Brody – who himself has been trapped in his western Houston home for the past three days – said Houston’s rapid expansion is part of the problem. The city has added hundreds of square miles of pavement and vastly increased its housing stock, but the flooding infrastructure has, in some areas, not kept pace.

“Many neighborhoods are still using roadside ditches as drainage. It’s like the Middle Ages almost,” Brody said.

When the city experiences heavy rain – which happens a lot due to its humid subtropical climate – the water has nowhere to go. In new developments where modern drainage systems are installed, Brody said they are often “treating the symptoms, not addressing the underlying problem”.

Those developments themselves might have systems in place to prevent them flooding. But those systems can involve channelling additional water into streams or bayous already at capacity. In severe storms that water can gush downstream and threaten older neighborhoods that traditionally may have been less prone to flooding.

“We hear a lot from folks who have lived in one neighborhood for 20 or 30 years and they say: ‘I have never flooded before, I’m flooding,’” said Lisa Gonzalez, president and chief executive officer of the Houston Advanced Research Center – a not-for-profit which analyses energy, air and water issues.

“When we’re making these development decisions in the Houston region we tend to think of it project by project, and they don’t really think of the cumulative impact of development as they’re making these decisions.”

One of the reasons for the lack of joined-up thinking is that the metropolitan area of Houston is governed by nine different local county governments. Gonzalez said much of the tax income generated by local authorities comes from property tax – “so the economic incentive is to allow development to occur basically where development wants to occur”.

The sheer scale of Harvey – some parts of Texas may experience a year’s worth of rainfall in just over a week – has undoubtedly contributed to the extent of the flooding. The storm has become wedged between two weather systems, one trying to push it north, the other trying to push it south, which means it is likely to continue to pour on Houston for days.

And with that rain the storm is essentially feeding itself, according to the Scientific American. Hurricanes normally pull in moisture from the ocean. But Harvey has dumped so much on Houston that at this point it is reabsorbing that water from the flooded city and hurling it back down.

But experts say that even less volatile storms in the future will cause widespread damage to Houston.

The city’s impermeable roads and sidewalks are built on the coastal prairie and wetlands of the western Gulf, and its topography is flat, with no run-off area for excess water. At the same time, the changing climate means these heavy storms are becoming more frequent.

“Houston is going to flood,” Gonzalez said. “What we need to do is identify those areas that are prone to flood. And we need to work to make sure we don’t develop in those areas, and if we do have housing in those areas we may want to think about a buyout program.”