Houston-area officials defend decision not to evacuate ahead of Harvey: 'Nonsensical thing to do'

[Igel]

36. It's more complicated.

I know people outside of 500-year floodplains that are flooded simply because the local drainage system failed. It's not that there wasn't room for the water to fit into the bayou system, but because man-made structures failed. In one case, the drainage ditch was simply too narrow even though there was white water from it's mouth down to the ditch leading to the bayou. (There'll be hell to pay at the next HOA meeting.) In this case, it's really just a floodplain when rainfall exceeds 5"/hr for two hours, otherwise it's way outside danger. Call this the "North" case.

I've watched, not this time but during a previous flood, water gush out of storm drains in one part of Houston. They were all linked uphill as the city grew north and west, except that drainage pipe system was adequate when community M was built, but when communities N through W were added on it didn't work. In some cases they have large pipes connected to smaller diameter pipes. By the way, when I watched the storm drains upchuck their floodwaters the sun was shining and it wasn't raining where I was. It's like a desert arroyo, you get a flood if it rains upstream. (Houston's working on fixing this, but it'll take years. We're talking thousands of miles of drainage pipe, and if you don't work from down-flow up you're a fool. The area I watched flood is years away from help.) This was north-central Houston.

Cypress Creek's flooding neighborhoods now. Yesterday there was some street flooding, stopped storm drains and just too much water to run off the roads at once. Cypress Creek's cresting later today at record levels. (This hits close to home, I live south of here but think of the area north as my home base.)

Greenspoint is a weird case. It flooded early this morning, Monday. It didn't flood when it got 16" of rain Saturday night, including some times when rainfall exceeded 5"/hr. It flooded when it got 4" in the same amount of time and rainfall never exceeded 1.5"/hr. Now, a large chunk of Greenspoint is in the 100-year floodplain, making this even weirder.

Here's background info to make sense of this. The Greens Bayou watershed (it's mine, so I know it) empties into a river shortly before the river reaches Galveston Bay, so it's long. However, upstream from Greenspoint there's not much watershed, and it's fairly narrow. So think long and skinny. The heavy rains Saturday night occurred in bands that cross-cut it so when it was raining 5"/hr in one place it was drizzling a mile away. The average rainfall for the watershed upstream of Greenspoint was a bit less than 1"/hr, even if it was a lot higher at times in certain places. Sunday night there were two hours of about 1.5"/hr, but the rain band responsible for that was parallel to the watershed's length, so that 1.5" inches hit pretty much every square inch of the watershed. After half an hour it all hit Greenspoint. Downstream there was no flooding because the bayou widens out below Greenspoint.

So let's talk evacuation orders. In the case of Cypress Creek I think you're right, generally. They can look at the flood gauge info and rain patterns and the contour maps and say what's going to flood.

In the case of Greenspoint, no evacuation order could have been announced. It would have been pointless Saturday night, and unexpected Sunday night. It's really chance. If you say "anyplace could have this happen" then you'd evacuate 10 million people, 6+ from around Houston and the other millions from Victoria over to central Louisiana. And the "North" case is just stupidity in central planning.

Doing things residence by residence is difficult. We have revised flood plain maps after Allison, so the newest are perhaps a decade old. Even if we ignore changes since then (subsidence, new structures), a lot of houses are built up. It's a requirement here, that new construction is at least a certain height above flood level. Older houses, naturally, aren't, but many have been raised (esp. in nicer areas). So if you look where I live, it's maybe 3 feet from street to the concrete pad this thing's built on, and about 4" from where dirt ends to where flooding would begin. It's built since the floodplain map was issued, and I have no idea what the relationship between anything here is and the floodplain. Was the street gouged out? The yard built up? Both? Argh.

For hurricanes I like large evacuation orders. For things like this, you really need to know what's happening. Most people here are safe. Some are flooding today that didn't flood Sunday. It's largely predictable, given inputs, but you need those.

Even then, I think residence-by-residence won't work until we know the elevation of each building. It's a theoretical possibility, but it's just not going to happen. Nobody's going to collect the data.

As for merging bayou levels, rainfall levels, and flood-plain contours, that should be done. I've suggested here before, you've suggested it, but I'm not sure anybody's suggested it to the counties. It hasn't been a big issue or much thought of before. But still you need those inputs, not just the floodplain maps.