Planning for the “Great Storms”

With less than three months left in 2007, the year has already won a place in Texas record books.

Unusual weather that brought a record three inches of snow as far south as Austin over Easter weekend continued into the summer and supplied a large part of the state with a full year of rainfall by July.

Now, large areas in central, south, and east Texas find themselves more than 20 inches above normal rainfall for the year officially ending Texas’ 10 year drought.

How did the Brazos basin fare through the great storms of 2007?

Though some areas experienced flooding, many areas of the basin that historically have been devastated by flooding fared extremely well due, in part, to planning that occurred more than 75 years ago.

Massive floods in 1913, 1921 and 1934 took the lives of hundreds of Texans and left thousands homeless. These events spurred state and federal leaders to begin planning a means to slow and store flood waters during major rain events. Their answer was the construction of flood control reservoirs, which would also provide water supply benefits.

Over the next 50 years, the Brazos River basin has seen the construction of numerous water supply lakes including nine major reservoirs designed specifically to provide flood control benefits.

How does the system of reservoirs prevent flooding?

As rain falls on the northern most areas of the Brazos River basin from Lubbock to Abilene, the parched ground absorbs and accepts as much moisture as it can hold. The remainder, or runoff, flows downhill into creeks and streams eventually entering the Salt Fork, the Double Mountain Fork, or the Clear Fork of the Brazos.

This flow continues downstream moving into Possum Kingdom Lake (PK), northwest of Fort Worth. As a water supply lake, PK was not designed or constructed to retain or store floodwaters. As a result, when the lake is full at its normal capacity, engineers and hydrologists must begin opening gates at the dam to allow for a controlled release of the floodwater downstream.

Throughout this process, a series of gages installed and maintained by the United States Geological Survey (USGS) measures the rise of the river level and provides estimates of the flow rate at the various gage locations. These gages provide valuable information to river forecasters, reservoir operators, and other communities and individuals along the river. BRA engineers and hydrologists consult the USGS stream flow gage system to determine the flow rate of the river at the various gage locations upstream of the lake. This information aids in determining the timing and number of gates to open. The goal is to maintain a balance of releasing the same amount of water through the gates that is flowing into the lake to keep floodwaters from overfilling the reservoir.

From PK, the flow of the river continues downstream to Lake Granbury, another water supply lake. Lake Granbury is fed by the Brazos River and other intervening creeks and streams downstream of the PK dam. Like PK, once the lake is full, the engineers and hydrologists must open gates at the dam allowing the same amount of water to be released from the dam as is flowing into the lake.

As a result of the rain events from late June into early July, the PK dam passed approximately 450,000 acre-feet of water through its gates in a 13-day period (one acre-foot is approximately 325,851 gallons of water). That amount is nearly equal to the full capacity of the lake.

The Lake Granbury dam released 805,000 acre-feet of water – or 6.5 times the full capacity of the lake during this same 13-day period. This included all of the water that was released from PK, plus the additional runoff from rain that fell downstream of the PK dam.

What happened to the water that is now an equivalent of more than 7 full lakes?

It flowed downstream into the largest flood control reservoir in the Brazos River basin, Lake Whitney. Lake Whitney was built by the United States Army Corps of Engineers (Corps) in the 1950s specifically to hold floodwater. Though Lake Whitney was slightly above its normal level when the large rain events began in June, the lake’s flood pool, which provides over 1.3 million acre-feet of storage, was able to hold the entire amount of upstream lake releases as well as local rainfall – minimizing impacts to the Cities of Waco, Bryan, and Richmond and preventing a repeat of the early 20th century flooding.

In addition to Lake Whitney, there are eight other flood control lakes in the Brazos River basin that were built and are operated by the Corps. These reservoirs are located on tributaries of the Brazos River.

Lake Aquilla controls floodwater from Aquilla Creek, which flows into the Brazos downstream of Lake Whitney about 10 miles north of Waco.

Lake Waco, which is fed by the Bosque River system, controls floodwater that would pour into the Brazos just north of Waco.

Lakes Proctor, Belton, Stillhouse Hollow, Georgetown, and Granger store floodwater from tributaries that feed the Little River, which flows into the Brazos just above Bryan/College Station.

Finally, Lake Somerville controls floodwater from Yegua Creek that would join the Brazos near the town of Navasota.

In early July at the conclusion of most of the heavy rains, the nine Corps reservoirs had captured and retained more than 2.5 million acre-feet of water in flood storage. This water would be gradually released at a controlled rate throughout the summer and into early fall until the reservoirs were back down to their normal operating levels.

Once the water is flowing in the Brazos downstream of Lake Whitney, it eventually flows into the Gulf of Mexico unless it is diverted for use.

Though several areas of the Brazos basin did experience significant flooding upstream of the Corps lakes, without this system of lakes the result of these events could have been devastating to some downstream areas.

In addition to the Corps, BRA, and USGS, several other organizations play a large role in coordinating information during major rain events.

The National Weather Service provides weather forecasts and issues flood warnings. The West Gulf River Forecast Center produces forecasts of expected river flows which are used to warn areas of possible flooding. Links to these predictions are available on the BRA home page during flood events.

A Warning Coordination Meteorologist at the National Weather Service contacts local emergency management officials and the State Emergency Operations Offices to inform and update them of possible flooding emergencies.

The State Ops Center, which operates under the Governor’s Division of Emergency Management, serves as the coordinating hub for all counties in the state. It is charged with the responsibility of providing state assistance such as additional manpower and helicopters for high water rescues. During widespread extreme events such as those experienced earlier this year, the State Ops Center hosts daily coordination conference calls with all affected areas, officials, and agencies involved.

Local emergency management is typically headed by the county judge and city officials who work with law enforcement to provide local emergency assistance and coordinate evacuations as necessary.

Each of these organizations work together to ensure proper information is relayed, dams are operated in the most efficient manner possible, and emergency aid is available where it is needed. A system, first imagined and built over a half-century ago, continues to work as originally intended -- saving property and lives, and meeting the area’s water needs.