Downtown Houston flooded after Hurricane Harvey. Image by J. Daniel EscareƱo licensed under Creative Commons.

By now, Houston’s floodwaters have started to recede as Hurricane Harvey moves eastward. More than 70 people have died as a result of the storm and Houston is beginning its long road to recovery, as Hurricane Irma reaches Puerto Rico, Hurricane Jose increases in strength, and Hurricane Katia is picking up steam behind them.

One question that has come up is, why is Houston flooding so badly? Is it poor zoning? Poor environmental regulation? Sprawl? Geography? Climate change? Floodplains? The answer is very complex, and can’t be boiled down to one specific thing. Rather, it’s many factors working in combination to create a built environment that has been unable to handle Hurricane Harvey.

The most important thing to note is that no matter what factors are at play, they wouldn't have prevented Hurricane Harvey’s immense damage. The Houston region received more than 51 inches of rain, far beyond what cities can tolerate. But the way Houston has been built over the years certainly contributed to flooding issues, so there are still informative lessons - just not about preventing massive flooding like we've seen the past few days.

Zoning plays a part but isn’t the whole story

“Zoning” has come up a great deal in the media lately with regard to Hurricane Harvey. Houston doesn’t have municipal zoning laws, which has allowed developers to build wherever there is space (within reason; Houston does regulate other things like lot size, setbacks, and parking, and private property covenants effectively serve the same purpose as zoning). Population growth has fueled much of this development; Houston alone has grown to 2.2 million residents. Harris County, which surrounds Houston, has 4.4 million residents.

As it has grown, Houston’s city government hasn’t made many improvements to drainage.

A parking lot in Houston floods during the hurricane. Image by R. Crap Mariner licensed under Creative Commons.

Gray Kimbrough said, “Zoning specifically is not the issue, but lack of regulation absolutely is. A lot of what NIMBYs cry about around [the DC region] (e.g. "this is just a gift to developers!") is actually the case in Houston. Sure, there's relatively little zoning, but the much bigger issue is a lack of political will for any restriction of where and how new construction can be built. Combined with an unwillingness to properly engineer sewer systems and other public infrastructure, this is the result.”

Matt Johnson said Houston isn’t only an example of poor zoning, but of planning as well. It didn’t build densely, and “proper planning for this situation would say, how much land do we need to reserve for stormwater management and how many units do we need to accommodate? What is the best development pattern given those constraints?”

Matt added that if Houston (and other cities) had built with more density in mind, it wouldn’t have so much sprawl. This would have preserved the natural grasslands that would have absorbed a lot of water. Zoning should come in response to that question. But only after planning has occurred. But as a general rule, we don't really plan at that high a level in this country.”

Geography and climate change play a role, too

Houston is the most flood-prone city in the country. Its clay soil doesn’t absorb water very well. From 1992 to 2010, new construction paved over 30 percent of the region’s wetlands. The wetlands contained tall grasses that would have absorbed water at a much faster rate than the infrastructure Houston has built to address flood water, which - combined with sprawl - leads to a significantly reduced ability to handle flooding from other storms less severe than Harvey.

Scientists say climate change means storms will become more frequent. Storms are already affecting areas that are outside designated floodplains. For example, the Memorial City neighborhood in Houston was well outside FEMA’s 500-year floodplain and has seen severe flooding three times in the last 10 years, and Harris County does not plan to study climate change and its risks to flooding.

It matters where Houston builds homes

Another issue is that even though Houston’s two massive reservoirs, Addicks and Barker, are handling rainwater very well, there are thousands of homes built right alongside them and according to the Army Corps of Engineers’ maps, they are counted as being inside the reservoirs. Those neighborhoods are flooding because while water has not breached the reservoir edge, there has been enough water to overwhelm auxiliary spillways, which let water out of the reservoir to keep it from overflowing. The runoff from the auxiliary spillways is reaching neighborhoods nearby.

How we talk about storms affects how we plan for them

There are two ways in which we discuss floods. A 1-in-100-year flood refers to a 1 percent chance of a massive storm at any given year, based on an estimate of 100 years (so, a 1-in-100 chance). A 1-in-500-year flood refers to - you guessed it - a .2 percent change of a massive storm event at any given year, based on an estimate of 500 years.

The Texas National Guard rescues a family from a flooded home in Houston. Image by Texas Military Department.

Predicting future storms and their potential damage is difficult, not just because of climate change but also because of how our built environments change over time. One problem is that people erroneously believe that a 1-in-100-year storm will only happen once in 100 years, when the likelihood is the same year to year and could happen in back to back years.

This affects how cities plan around storm risks because cities can’t just change their policies and physical infrastructure as quickly as FEMA changes its definitions and probabilities of when storms may hit. Cities can also increase protection, and then not see severe storms for years, potentially making it difficult to justify the cost. It also affects how residents think about hurricane chances and whether they decide to live on a floodplain or buy insurance (if they can even afford it); for example, 80 percent of people with flood damage from Hurricane Harvey don’t have flood insurance.

What can Houston do about its issues in the future?

“It should be noted that no matter how well planned a city is, there is no way it could accommodate [more than 51 inches] of rain within a short span of time. Not even sure a rain forest could handle it!” said Kristy Cartier.

The key here isn’t to do a complete about-face and implement strict zoning laws. It’s to implement zoning laws that keep environmental risk in mind, and to plan better for future hurricanes and other storms. To start, that could mean restricting sprawl and encouraging denser neighborhoods with adequate infrastructure, or find better uses for the land on top of reservoirs.

One contributor (who asked not to be named) said, “My take is that the articles on growth/zoning are painting with too broad a brush. It's true that Houston lacks much of the types of zoning regulations that cities like DC have, and this leads to unusual growth patterns [and] distributions and probably to overall growth. And certainly some of that growth, if not done in a way that addresses runoff and stormwater management, is driving the current crisis (although at [50 inches] of rain, I suspect one house in the middle of a large switchgrass field is still doing to experience some amount of flooding).”

The contributor continued, “My concern is that talk of "lack of zoning" being the "problem" here could lead cities like Houston to implement wholesale changes in zoning, when what they really need are some form of pervious surface requirements. What I'm seeing in our work [on supply-based drivers of housing affordability and about the flooding] is suggestive evidence that Houston's lack of zoning really does improve affordability, and zoning reform could jeopardize that if zoning changes make building more expensive in a way that doesn't address an environmental externality (and I would argue that little of DC's zoning regulations can be justified on the basis of internalizing externalities).”

Dan Reed adds, “The flip side is that Houston also has fewer restrictions on infill development, which could actually address the issues caused by sprawl. They can do something most American cities cannot—build up in the right places with little pushback—and can use that to reduce future flood risks.”

Another contributor suggested that if Houston and Harris County had coordinated better for environmental risks 20 years ago, both city and county might be better off today. Going forward, that might be a lesson learned for the metropolitan area.

Tracy Loh says, “You can argue that the entire city of Houston should not be where it is, or that it shouldn't be "allowed" to grow. That argument is unrealistic at best, and elitist/fascist at worst. We don't "build" resilient cities out of magical nowhere. The work is to build them out of the cities we have, many of which are in riparian areas, dating to the days when "surface transportation" meant a boat.”

Another way to look at the issue is achieving a more balanced ratio of concrete to natural land. Urban environments are full of surfaces that won’t absorb water; The Atlantic estimated that about 40 percent of a typical city had these hard, non-absorbent surfaces, like asphalt.

Houston is notoriously car-centric; more than 90% of Houston’s residents drive to work and roads have flooded, preventing residents from leaving the city. The Texas Department of Transportation calculated that there are 3,252 centerline miles of freeways and expressways in the Houston district. In 2012, Streetsblog ranked Houston 3rd on the list of cities with the most highway miles per capita, and ranked the District 10th on the list of cities with the least amount of highway miles.

Obviously, Houston can’t bring back all of the green areas it paved over, and more natural land wouldn’t have prevented flooding, but more natural land and protecting the natural land that remains could help with stormwater management for other, less severe storms in the future.

Joanne Tang is a Northern Virginia native and a graduate student in public administration and policy, focusing on resiliency and emergency response. She lives in Alexandria and enjoys learning about pretty much everything, including the history of pencils.