Water Water Everywhere!

On the second day of the new year, Austinist offers you an apocalyptic warning courtesy of the Lower Colorado River Authority (LCRA): much of Austin could one day be submerged in water. More specifically, it would arrive in the form of a 100-year flood, so named because it describes the 1% likelihood of it occuring in a given year and not, as we had initially imagined with shock and horror, the duration that it would last.

The press release issued by the LCRA is a rather colorful, largely fictional scenario involving a mini biblical flood lasting ten days and covering 10,000 square miles of the Hill Country:

Creeks, streams and rivers swollen by heavy rainfall pour into the Highland Lakes, causing high water, fast currents and dangerous conditions. LCRA, which is responsible for managing the Highland Lakes and dams, has opened floodgates at the dams along the upper Highland Lakes to move the floodwaters as quickly and safely as possible to Lake Travis. This is causing swift, high water in three “pass-through” lakes: Inks, LBJ and Marble Falls. Historic highs – 903 feet above mean sea level (msl) on Inks Lake and 760 feet msl on Lake Marble Falls – are recorded. Inks Lake remains above normal operating level for nine days.

Floodwaters spill over Starcke Dam, which forms Lake Marble Falls, and into Lake Travis at almost 400,000 cubic feet per second (cfs). To illustrate the power of the river at this stage, the volume of water approaching Niagara Falls during peak flow season is 202,000 cfs.

Mansfield Dam, which forms Lake Travis, does its job by holding back most of the floodwaters in its flood pool, the area above 681 feet msl. Holding the floodwaters in Lake Travis helps protect Austin and downstream towns from severe river flooding. Before the storm, Lake Travis was full at 681 feet msl. Now, floodwaters begin to fill up its flood pool above 681 msl.

Flood releases from Mansfield Dam also create a rise in Lake Austin above its normal elevation. Water levels rise as much as 10 feet immediately below the dam, but not quite as high further downstream on the lake. These flood releases are matched at Tom Miller Dam, which forms Lake Austin, with hydroelectric generation and floodgate openings. Below Tom Miller Dam, Town Lake in Austin peaks at 10 feet above its normal level and remains above normal for 12 days. Water from Town Lake flows over the hike and bike trail and Cesar Chavez Street almost to the First Street bridge, stopping traffic in both directions. It takes one and a half to two weeks for the downstream flooding to subside. Over the next several days, river levels below Austin crest above flood stage all the way to the Gulf of Mexico.