Houston Flooding

We better figure this out. Maybe its time to build up instead of out.

There are two parts to a flood, only one of which we have control over. First there’s the rain, which we can’t control, and second is the flood, over which we have limited control.

When I say “flood” I really mean the response of the watershed to the rain. That involves an incredible number of different pieces, from the size of our bayous and channels, to storm sewers, street inlets, roadside ditches, culvert sizes, to elevation of houses, and down to the micro-level like putting a flower bed in the wrong place in your backyard.

Drainage design standards in the Houston area have been evolving over the last 100 years. The standards we currently build new projects to are pretty robust and minimize flood risk to new structures. Unfortunately there is a massive amount of infrastructure and houses that were built to lesser standards. There are a lot of “sins of the past” that need to be fixed…but that requires time and $$$, both of which are limited.

In four of the last five years (2015, 2016, 2017 and 2019), Houston has seen incredible rain events. Harvey was the biggest rain storm in US history, and I’ve read that last week’s Imelda was the 5th biggest ever. Almost makes you forget about the massive 2015 Memorial Day and 2016 Tax Day events. All of these storms produced greater than “500-year” rainfalls (or 0.2% annual exceedance probability rainfalls.) In many parts of the Houston area there were greater than 1000-year rainfalls (or 0.1% annual chance rain).

That’s a pretty incredible run on the odds…I could put it in terms of poker. The probability of getting a full house dealt to you in any given hand is about 0.14%. So consider that our rainfall the last five years has roughly the same probability of the dealer dealing you a full house in four out of five consecutive hands.

So while we are studying and building cost-effective ways to handle more water, Mother Nature keeps throwing more than we thought was possible. How many “once in a lifetime” storms are we going to keep getting? We don’t have control of that, but we’re certainly in a new world when it comes to rainfall amounts.

What I’d like to know, is this just an incredible run of luck or is this the new normal? Seriously, can one of you let me know?

Also, send your kids to the Cullen College of Engineering…we need more Civil Engineers in Houston. :wink:

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What did the investigation past Harvey revealed?
What is the correlation of Harris County issuing building permit in flood prone areas?
What is the correlation of builders building in flood prone areas?
Was the Harris County judge ultimately responsible for issuing such permits?
What happened after the investigation?

Timmy I fear these storms are the new normal. So as an engineer what would you recommend the city and county do?

These storms are not new. I stopped counting the number of times underpasses were flooded. This was in the mid 80’s to mid 90’s for me. The difference since then is the amount of new construction in flood prone areas. It is basic physics. The water has nowhere to go so it settles in lower areas. The water reaches a certain level and floods the next neighborhood that had never flooded before. Harvey was illustrated it clearly. The no zoning law does not help either.

https://www.hcfcd.org/flooding-floodplains/harris-countys-flooding-history/

I would think that this would mean that Houston’s future growth is up and not out. It doesn’t make sense to continue to put cement down over a flood plain. If we continue to grow out, I would think it would force us to dredge all of the bayous and make them deeper.

I wouldn’t call this an “investigation”, but here are a few interesting statistics I picked up a few weeks ago at the Texas Floodplain Management Association conference:

  1. There are 1.5 million structures in Harris County. 62% of them were built before 1989.(The year 1989 is a point of reference as most new developments in Harris County were built with storm water detention ponds after 1989. HCFCD implemented storm water detention requirements in 1984, but various grandfathering rules allowed most developments to avoid detention for a few years.)

  2. 90% of the structures flooded in the 2015/2016/2017 (Memorial Day/Tax Day/Harvey) storms were built before 1989.

  3. In Harvey, 11% of all structures in Harris County were flooded (154,000 structures). 68% of them are located outside the 100-year floodplain.

  4. There’s another statistic that I can’t put my hands on, so don’t have the exact numbers, but something like <5% of all homes flooded in Harvey were built after 2008, the most recent update to Harris County Floodplain Regulations before Harvey.

What does all this mean? To me, items 1, 2 and 4 confirm my assumption that there are a lot of “sins of the past” that need to be corrected, and also that we are building new developments in a way that greatly reduces flood risks.

However, #3 points out that there is a huge gap in our floodplain maps showing true flood risk. First of all, we didn’t get a 100-year storm, so the 100-year floodplain line is nearly meaningless for Harvey. The map below shows that all of Harris County received at least a 2,000-year rainfall over 4 days, with probably 80% of the county getting a >5,000-year rainfall, and a significant portion of the county getting >20,000-year rainfall.

We just don’t design or map to those levels of rainfall. Historically we’ve said it doesn’t make sense to spend the money to protect against something that’s so statistically unlikely. That said, our statistics are based on our historic records, which only go back 100-150 years in this area, and with varying degrees of accuracy over that period. The new Atlas 14 rainfall data released in 2018 indicates that our previous assumptions about 100-year rainfall depths are about 40-50% too low.

It’s not an exact science. No one knows how much it will rain next time. Even the State Climatologist said that Harvey was worse than what was known to be the worst-case scenario. Now that it’s happened, we now that it CAN happen, and it can happen again. The Imelda storm showed that this maybe isn’t a freak occurrence but a much more likely event than previously known.

When you mention flood-prone areas, the fact is, when Houston gets a year’s worth of rain in 4 days, everything is a flood-prone area. There is no static, one-size fits all map that can show you what is flood prone. Houston was flood prone before man ever put his first tepee, lean-to shelter, or cabin here. Flooding wasn’t a problem until we put things of value in the way.

So what is being done to prevent this damage in the future? There are a number of things:

  • New criteria for minimum floor elevations on new developments, redevelopments or substantial improvements of existing development. Floors must be built at least 2’ above the 500-year floodplain.
  • New floodplain maps are being developed. Believe it or not, Harris County has the most technologically advanced floodplain maps in the nation, we’ve been a model and test ground for FEMA as it updates flood maps across the rest of the nation. Now Harris County/HCFCD is making the next leap in technology, and bringing FEMA along for the ride. New maps will be based on the new Atlas 14 rainfall data (assuming 100yr rain is 16" - 18" in 24 hours, instead of previous standard of 12-13" in 24 hours). New maps are also using 2D floodplain modeling to better represent overland flooding after water breaks out of channel banks. No one in the US has done this modeling and mapping on a community-wide basis.
  • Infrastructure improvements are being planned and constructed across the county. Between the Harris County bond issue and federal grant money, there is expected to be about $5 billion of drainage improvements built over the next +/- 10 years.
  • Buyouts of flood prone properties are occurring. Buyout money often provides the best “bang for the buck”. If there’s nothing in the way of flood waters, there’s no damage to worry about. No harm no foul. Why spend money for expensive structural improvements when it’s cheaper to just remove homes.

I could keep going and going and going about this. Definitely not a simple issue. There’s a ton of science involved, which is tough enough, but the things that make this even more difficult are the economics, the politics, and the emotions of the problem.

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It’s not a new phenomenon that Houston gets tropical rainfall, but the intensity and areal extent of these recent storms are unprecedented.

Harvey was the worst rainfall in US history by most measures. Absolutely unprecendented. This report from HCFCD goes into great detail about the rainfall: https://www.hcfcd.org/media/2678/immediate-flood-report-final-hurricane-harvey-2017.pdf

What it shows is that we didn’t just break the national rainfall records, but obliterated them. We didn’t just get a “little” more rain than previous records, didn’t beat it by an inch or so, but by a foot in some cases. And when we’re measuring rainfall in FEET you know it’s a big event. Here’s a small excerpt from the report:
image

Think about that last statistic for a second. Enough rain fell in four days to supply all water needs in the US for more than nine months!!!

We didn’t just barely break those records…Harvey was generally 50% greater than the previous national record rainfalls. That would be like someone breaking Barry Bonds’ HR record (73) by hitting 110 HR in a season!

Harvey was unlike anything ever before recorded in the US. But what’s frightening is just the fact that it’s now a possibility. Until it happened it was the impossible, something we didn’t worry about. There’s definitely a new sense of urgency nowadays.

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Great info. Thank you for posting.
Question:
Let’s say the past developments were built with flawed flooding assumption. Would it help to now build in areas that will not affect the channeling of rain storms? You stats shows it is. I can’t seem to find the actual direction of the water affecting flooded areas. Holland has done an amazing job building below sea level and re-directing historical canals. Could Harris County re-engineer the multiple rivers and “redirect” water flow to large containers. It is easier said than done but other countries have done it. As you know water is the new gold. One of the major problems in California and the Southwest in general is the lack of real containment infrastructure. It makes no sense that monsoon waters in Arizona are not collected. This is a massive undertaking. Putting it together this is a tax that I would 100% support.
Thank you again for posting.

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It could be that we need to go up and not out. But I think that’s more determined by transportation and employment issues than flooding.

Your concern about more concrete is sort of correct…but as opposed to what most people think, it’s not that we’re covering soil with concrete. Our native soils are very fine-grained clay soils with very very low infiltration rates (think about our black gumbo clay). Our native clay soils aren’t that much better than concrete at absorbing water. And no natural landscape in the world can absorb 50" of rain in four days.

The biggest impact on increased runoff rates is actually our extremely effective drainage systems. The fact that we take rain directly from roof to downspout to driveway to gutter to inlet to storm sewer to bayou in a very efficient way. We’ve made a superhighway to get rid of rainwater, because water is our enemy, right? Roadside ditches don’t get rid of water nearly as fast, but they do provide a significant benefit: storage volume. They attenuate the peak flow of runoff by acting as mini-detention basins.

If we want to improve flooding conditions, I say we go back to roadside ditch drainage in every subdivision, and go back to building block and beam or pier and beam foundations to elevate homes as much as possible.

But I don’t get to make those decisions. It’s “the market” that makes those decisions. And people like their curb-and-gutter streets, and slab-on-grade foundations.

Not sure I understand every bit of your question, but I’ll say this: it’s generally pretty difficult to divert large amounts of water away from where natural topography wants to take them. For example, there’s a project nearing completion called the Luce Bayou Interbasin Transfer Project, which will bring water from the Trinity River watershed into Lake Houston (San Jacinto River watershed). It’s taken 50 years of planning, design, and construction to get that accomplished.

Here’s another excerpt from an HCFCD report, which graphically shows the types of improvements being planned in Harris County. We just have to find ways to handle huge amounts of water falling from the sky. The best tools are to build channel improvements, build detention storage to slow down the water a bit, and to relocate people out of harm’s way.

What you’re talking about, to store rainwater for later use, is done all the time with big reservoirs. Lake Houston and Lake Conroe are our biggest local examples. Those two lakes provide the majority of our area’s drinking water.

And yes, all of this is easier said than done. It’s much easier to come up with ideas than to pay for them. These are generally public projects for public good, and paid for with public money. But the public likes their property and income taxes to be reduced. Understandable, but not an easy answer.

Among many other publications you have one for example that brings up quite a few questions.
https://thegoldenhammer.net/the-flooding-catastrophe-from-tropical-storm-harvey-was-largely-man-made/
Are Lake Houston and Lake Conroe adequate reservoirs and do they need to be overhauled immediately? Is this being done?

I have lived in Houston pretty much my whole life and have come to realize that areas around Houston have a 100 year flood event every couple of years, but it is a different part. Any place can get 7-8 inches in 2-3 hours and very hard to handle so much rain so fast. Additionally, if you look around Houston area, I think it owns the 2 of the top 5 rain events in the north hemisphere (Harvey, 79 Alvin/ Pearland flood). It is very hard to design something to handle all the water.