2007 Flood Report

Summer 2007 Flood Report LCRA Water Services River Operations Center NEXRAD image of the June 27 storm that triggered the Summer 2007 Flood Executive Summary Flood Facts: The Summer 2007 Flood this event, as did relation- The Summer 2007 Flood Rainfall intensity near Marble Falls was unexpected, sudden, ships with other agencies did not break the severe (18 inches in 6 hours) was in ex- cess of a 500-year event, based on severe and a great test of that work with LCRA during drought of 2006. Actually, depth-duration-frequency analysis. LCRA assets, both in terms flood emergencies. the drought had ended of facilities and people. before then, thanks to The greatest intensity of Unit-peak discharge on Hamilton rains earlier that spring This event demonstrated rainfall was in the Marble Creek, 722 cubic feet per second which filled lakes Bu- the value of remote- Falls area. The peak flow (cfs) per square mile, exceeded the chanan and Travis. But historical record. Unit-peak flow controlled floodgates at on Hamilton Creek sur- public attention was riv- was even higher on Backbone Starcke Dam, dedicated passed that of the previ- eted by the June 27 rain Creek in Marble Falls. floodgate hoists at Wirtz ously documented extreme event. The public became Dam, a refined computer peak discharge set in more aware of floods and Lake Travis reached its fifth highest simulation model to fore- 1936. The worst flooding droughts, and of the value level: 701.52 feet above mean sea cast flood conditions with occurred in Marble Falls of the Highland Lakes to level (msl). greater accuracy, a higher and the headwaters of the people of the lower state of readiness, and Lake Travis, qualifying the Colorado River basin. LCRA received so many hits on its depth at key positions. area for federal disaster Flood Summary page on the LCRA Communications continued assistance. web site, that the method of gener- to improve as a result of ating graphics had to be changed to keep up with demand. Summer 2007 Flood Report Weather Pattern The phenomenally heavy rich supply of moisture off counties. The storms en- rain event on June 27 was the Gulf of Mexico. The trained a very warm, moist part of an unusually wet May to August period was flow of air off the Gulf of weather pattern that began one of the wettest on re- Mexico (low-level Jet). The After 18 months in May and lasted into cord. These rains filled the intersection of the Jet with early August. During that conservation pool of Lake outflow in the upper atmos- of drought, wet time, a trough of low pres- Travis. phere caused the storm sure in the middle and complex to stall. Rainfall On June 26, a complex of upper atmosphere over intensity increased as indi- weather in the thunderstorms developed Texas was surrounded by vidual storms repeatedly in the unstable atmos- spring of 2007 ridges of high pressure to moved over the same ar- phere over Central Texas. the east and west. Atmos- eas. Heavy rain began to Late that night, the storms filled Lake Travis. pheric disturbances track- fall after midnight. reached Llano, Burnet and ing south from the Plains northwestern Williamson Then the flood hit states interacted with a in June 2007. Hydromet Gauge Marble Falls 6 ENE Between 11 p.m. on June Rainfall continued at a rate Here, totals of 14 to 18 26 and 5 a.m. on June 27, of about 3 inches per hour inches were recorded. The the storms moved slowly for 6 hours. This amount highest total of rain meas- east across southern Bur- of rain in such a short time ured during the event was net County with heavy rain. ranks among some of the 19.06 inches at the Marble Between 12:45 a.m. and 2 highest rain rates ever Falls 6 ENE Hydromet sta- a.m., the rainfall intensity observed in Texas. The tion. Approximately 18 became extremely heavy very heavy rain around inches of rain fell in a six over southeastern Burnet Marble Falls began to re- hour period. That rate of County. An LCRA tipping- cede, with most of the rain rainfall intensity exceeded bucket rain gauge located ending around 5 a.m. the 500-year return fre- six miles east-northeast of quency for Central Texas. The core of the heaviest Marble Falls captured the rain occurred across south- For much of the region, the intensity of this extremely ern Burnet County near the summer of 2007 ended up heavy rain, at one point City of Marble Falls and being one of the wettest measuring nearly 2 inches areas just to the east. and coolest on record. of rain in 15 minutes. Page 2 Rainfall Distribution LCRA Water Services In the watersheds feeding three inches west of Mason, covers the area uniformly directly into the Highland from 3 to 5 inches between and fills the gaps between Lakes, the June 27 rainfall Mason and Llano, and from rainfall gauges, and the rain- totals were very impressive. 5 to 10.5 inches between fall gauges provide data that The storm reached its full Llano and Lake LBJ. The refines the map of rainfall intensity in the early morning pattern was higher rainfall at distribution. of June 27, with rainfall locations closer to the lakes. The following figure, showing rates as high as two inches The area around In the Pedernales River wa- the 24-hour rainfall accumu- in 15 minutes. The Marble tershed, there were lesser lation on June 27, can be Falls area received as much Marble Falls rainfall totals than the Llano compared to the gauge- as 18 inches of rain in six River watershed, from less adjusted radar rainfall distri- hours. Many gauges in received 18 than 0.5 to 4.5 inches. bution shown in the NEXRAD southern Burnet County, image on Page 1. This grid- inches of rain in north and south from the LCRA uses this rainfall data ded data is used as input to Colorado River, measured in two ways: as direct input LCRA’s flood simulation six hours. more than 8 inches of rain to estimates of runoff result- models. The models accu- on June 27. ing from heavy rainfall, and rately predict rates of flow in as an adjustment to radar In the Lake LBJ watershed, streams and rise of lake rainfall estimates. Radar rainfall totals were less than levels through time. Page 3 Summer 2007 Flood Report Streamflow Intense rainfall in southern Backbone Creek near Mar- land on Sycamore, Camp Burnet County caused tribu- ble Falls has a contributing and Hickory Creeks. taries in the area to rise watershed of 35 square Cow Creek near Lago Vista rapidly. Most of the runoff miles. The estimated peak entering the north side of experienced devastating flow on Backbone Creek was Lake Travis came from Ham- peak flows where the stream ilton Creek with a watershed 26,000 cfs. The flow on washed out FM 1431 on the north side of Lake Travis. Many bridges of 84 square miles. The Backbone Creek combined peak flow on Hamilton Creek with Whitman Branch and Cow Creek had a peak flow of approximately 25,000 cfs were washed out was approximately 56,000 caused extensive flood dam- cfs, exceeding the previously age in the City of Marble from a watershed of 46 square miles. There were documented peak discharge Falls. Many bridges were and many homes two fatalities attributed to that occurred in 1936 washed out, and many (USGS, WRI Report 96- the flood in that area. and businesses homes and businesses were 4072). Incredibly, the FM flooded. That water flowed Flood damages in southern 1431 bridge over Hamilton into Lake Marble Falls, then Burnet County qualified for were flooded in Creek was undamaged by federal disaster assistance. the event despite massive into Lake Travis. Marble Falls. scouring in the floodway. Further east of Marble Falls That water entered the along FM 1431, there was headwaters of Lake Travis. significant flooding of ranch- Tributaries upstream from Lake Travis - June 27, 2007 60000 50000 Hamilton Creek Backbone Creek 40000 Cow Creek Honey Creek 30000 Flow(cfs) 20000 10000 0 0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 Page 4 Streamflow (continued) LCRA Water Services South of Marble Falls, tribu- To the east along the south taries to the Highland Lakes side of Lake Travis, Double also had high flows. In the Horn Creek experienced early morning hours on June similar conditions and washed out a county road 27, Little Flat Rock Creek crossing on the south side of quickly rose at State High- Lake Travis. way 71 and U.S. Highway 281, flowing over the high- Peak flow on the Llano River way at both crossings. There was about 69,000 cfs, which was a 12-foot rise on Flat was less than a 5-year flood magnitude (USGS WRI Re- Rock Creek at the Hydromet port 96-4307). The peak gauge at FM 2147. flow on the Pedernales River Flat Rock Creek came within at Johnson City reached a few feet of flooding the 18,000 cfs. These rivers are typically large contributors to Huber underground mine floods in the Texas Hill Coun- south of Marble Falls. Just try, but in this case most of downstream, the flood the water came from other caused damage to the tributaries. bridge crossing on the road leading to Starcke Dam. Washout of FM 2147 at Double Horn Creek southeast of Marble Falls Flat Rock Creek, June 27, 2007 1.4 882 Stream rise = 12 ft 1.2 880 1 878 Rain (in) Stage (ft) 0.8 876 0.6 874 Stage (ft) Stage Rainfall Rainfall (in) 0.4 872 0.2 870 Total rainfall = 10 in 0 868 20:56 1:56 6:56 11:56 16:56 21:56 2:56 Page 5 Summer 2007 Flood Report Streamflow (continued) Prior to the flood in the Mar- again to a level of 4 feet storm in Marble Falls was ble Falls area on June 27, over the spillway on June 29.

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