Hurricane Harvey has dumped massive amounts of water on the Houston area, causing widespread, calamitous flooding. Some context on the tremendous growth of Houston can show how many more people are at risk from hurricanes now than from hurricanes that have periodically struck that general area in the past.
Reports from Houston have described Houston as the fourth largest city in the United States with a population of 2,300,000 (estimated for 2016). As urbanists understand, this will always be just a portion of urban area population. Some reports have mentioned the size of the metropolitan area (MSA), which is much larger. But metropolitan areas include substantial exurban and rural territory outside of the built-up urban area. Certainly the populations included in both areas are at risk from hurricanes. But the built-up urban area is of special significance because urban development exacerbates the risks of flooding due to increased impervious surface and man-made structures that retard runoff. And flooding in urban areas poses different types of risk (not necessarily more severe) than flooding in rural areas.
My focus here is on what I have defined as the Houston urban area in my urban patterns research. This is an effort to delineate the more-or-less built-up portions of the larger metropolitan area for each census year from 1950 to 2010. These urban areas are similar to the census Urbanized Areas that delineate the area and population officially considered to be “urban,” but have the advantage of being defined in a consistent manner over time (along with other research advantages given that they are defined using consistent areal units).These urban areas consist of the contiguous census tracts having at least the urban density of 3 acres per housing unit or 213.33 housing units per square mile. This is generally equivalent to the census threshold of 500 persons per square mile currently used to delineate Urbanized Areas. (For more detail, see this earlier blog post.)
First, I’ll show maps of the extent of the Houston urban area in 1950, 1980, and 2010, showing the tremendous expansion:
Houston urban area 1950
Houston urban area 1980
Houston urban area 2010
Now for some basic statistics describing the urban areas at these three points in time. This table shows the land area of the urban area, the number of housing units, and the housing unit density for each census year:
|| Area (sq mi)
The Houston area grew 10 times larger in terms of land area, becoming the ninth largest urban area in 2010. Surprisingly it had the fourteenth largest land area in 1950 despite its much lower ranking in terms of housing units because many other urban areas were far denser at that time. This demonstrates the vast increase in the land area that could be vulnerable to urban flooding now as opposed to the middle of the last century. And the much greater impervious surface area and amount of structures that would contribute to greater flooding.
Housing units increased by 12 times. Houston was tenth among the large urban areas in 2010 in terms of housing units while it was only twentieth in 1950. So again, a huge jump in the number of dwellings, households, and people at risk in urban Houston. And from 1980 to 2010, the population of the Houston urban area increased from 2,100,000 to almost 4,900,000. (Due to the nature of the data sources used, I do not have population figures for 1950.)
The housing unit density did not change a great deal over the sixty-year period, increasing somewhat from 1950 to 1980 and then dropping back a bit to 1,066 housing units per square mile in 2010. This density was almost exactly the mean housing unit density for the 59 large urban areas in my study in 2010, which was 1,080 (and was even closer to the median value of 1,055). This is in contrast to Houston’s density in 1950, which was forty-second out of 59 with a value of 945 units per square mile compared to the mean for all areas of 1,268. Many other areas saw decreases in density over this period; Houston, with a modest increase, did not. (See this post and this paper for more on changes in density for large urban areas over this period.)