Floodwaters spill over banks into city riddled with rivers and bayous

There is enough rain in one of Houston's counties to fill up 448 Superdomes; officials say the odds of a flood happening at this magnitude is 1 in 500.
2:25 | 08/28/17

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Transcript for Floodwaters spill over banks into city riddled with rivers and bayous
Every part of the Houston metro area is struggling in the rain. Look at this map. This is a city riddled with rivers and bayous. This is why there's such a problem here. Every red dot, a spot where water has spilled over the banks. Here's rob Marciano on why Houston is under special risk. Reporter: Tonight, Harvey has Houston under siege. Torrential rains engulfing America's fourth largest city. With an exploding population that defines urban sprawl. Extra prone to flooding. Look at this sinkhole, the result of what is normally a trickling creek turned into a torrent. The culvert, unable to handle that volume of water. Not helping is this. Concrete, a lot of it southwest of Houston and it does not absorb water. In league city, southeast of Houston, an astounding 30 inches has already fallen, and over 2 feet recorded around the metro area. That's enough rain in Harris county to fill up 448 Superdomes. An event of this magnitude, rare to say the least. Odds of a flood this bad? 1 in 500. And the geography here doesn't help. Rainwater is forced onto the streets, then into small bayous, easily overwhelmed in heavy downpours. Buffalo bayou runs through the middle of the city. Right now it looks like this. 15 feet over flood stage. A level never been seen before. Unlike other floods that we've experienced, the sheer volume of water that has fallen is going to take time to run off. Rob is live in west Houston. Part of the legacy of the storm will be how much it didn't move. Is there a track tonight, and any relief for the people of Houston? Reporter: The short-term relief, lighter rainfall periods like we're getting now. The rainfall rates have been absolutely staggering. And also the tornado risk, a tornado watch out for this area and southern Louisiana through tonight. And it's going to spiral in more bands. Tomorrow afternoon, another pulse of heavy rain for Houston. The center of Harvey gets over water, maybe strengthening before tomorrow. We expect the rain to be done by then. And another 20 inches of rainfall, on top of the 20 or 30 we already got, in a city that has nowhere else to put the water. This rain is relentless. Rob, thank you. The entire region here is threatened by days of rain in the forecast. As we just heard rob report.

This transcript has been automatically generated and may not be 100% accurate.

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