19.5 Trillion Gallons in Four Days
Comparing floods in Texas and California
Refugees cling to a levee during the Great Mississippi Flood of 1927 - called the most destructive river flood ever in the United States by no lesser authority than Wikipedia. Time will tell how it will be compared to the current flooding in Houston and surrounding areas.
Rising Tide: The Great Mississippi Flood of 1927 and How it Changed America (John M. Barry, 1998) tells the flood's riveting story of relentless rain leading to controversial deliberate levee breaks which exacerbated racial and regional tension and led to the election of Huey Long as governor and Herbert Hoover as president. This book is well worth reading.
Hurricane Harvey dumped 19.5 trillion gallons of rain on Texas over the 4-day period from August 27-30.
That sounds like a lot, and it is. How does it compare to our recent wet winter in California?
19.5 trillion gallons converts to about 60,000,000 acre-feet - 12 times the amount of rain that fell in California's Central Valley from February 7-10 earlier this year, or 13 times the volume of Lake Shasta - - the largest reservoir wholly within California.
Admittedly, there is a lot of "Apples to Oranges" in this comparison. California also received a lot of snow and its runoff lasted into July, while the rain has eased in Texas. Also, the comparison is between the Houston area and California's Central Valley - not the whole of either state. Of course, the terrains of our two states are also very different.
Until California's record breaking precipitation this year, it was easy to forget that the large reservoirs in the Sierra Nevada foothills provide flood control as well as water supply. The Army Corps of Engineers does not allow these dams to fill until the rain and snowmelt seasons are over. Consequently about one half of California's runoff from February 7-10 was captured by large Central Valley dams. Capturing this water upstream reduced the pressure on the notoriously fragile levees in the Sacramento- San Joaquin Delta - which fared well amid the storms of 2017.
Given California's wet winter, there was minimal flood damage. The debacle at Oroville notwithstanding, large reservoirs did their job.
It would be grossly misleading, however, to say that large reservoirs are the only solution to floods. Downstream channel capacity, the placement of levees, and, of course, where we choose to build our homes and businesses are all critical factors in managing our exposure to floods. The Yolo Bypass, for example, over which motorists travel as they approach Sacramento from the west, is a wide flood control channel with "set back levees", that is also productive farmland.
Our favorite river, the Tuolumne (which will one day flow unimpeded through Yosemite's Hetch Hetchy Valley) did cause some minor flooding in Modesto in 2017 after Don Pedro Reservoir filled to capacity. San Francisco paid half the cost of constructing Don Pedro, in part to move its flood control responsibilities at Hetch Hetchy downstream and in part for a 570,000 acre-foot water bank in the Reservoir.
Don Pedro Reservoir's total capacity is 2,030,000 acre-feet, which includes storage for the Turlock and Modesto Irrigation Districts, as well as flood control space and San Francisco's water bank. It is almost six times as large as Hetch Hetchy Reservoir, and will play a key role in ensuring no loss of water supply when Yosmeite's Hetch Hetch Valley is restored.
There is no current flood control reservation at Hetch Hetchy Reservoir, although San Francisco has a unofficial policy of leaving 30,000 acre-feet of space in the reservoir during the rainfall and snowmelt seasons.
The Houston area has long been prone to flooding. When I was there several years ago, I saw huge huge channels alongside the freeway and wondered what they were. Even when I realized their purpose, they seemed impossibly over-sized. Obviously I was mistaken.
In response to flooding in 1935, Houston constructed the Addicks and Barker Reservoirs to capture future floodwater. The total capacity of these reservoirs is only 410,000 acre-feet, however, less that 1 percent of the Houston's four day rainfall. So reservoirs play a tiny role in controlling floods in Houston, but a significant role in California.
Addicks and Barker Reservoirs (shown in light green) are located west of downtown Houston. Together, they provided a veritable drop in the bucket toward protecting the city from the flood. Due to the area's flat terrain, additional flood control reservoirs are probably infeasible.
Our hearts go out to the people and communities affected by Hurricane Harvey. The effects of the Houston flood will not be forgotten, nor should they be. Solutions in Texas, California, or any other flood-prone area of the planet are neither cheap nor easy, and will always be incomplete. But there are things we can and should do. The Houston flood is a wake up call for all of us.