In late 2007, the Brazos River Authority (BRA) discontinued hydropower generation at Possum Kingdom (PK) Lake. In the absence of hydropower releases from Possum Kingdom, an interim water management protocol was established by BRA in 2008 based on operating Lakes PK and Granbury on a 1:1 elevation drawdown basis (equal drawdown). In other words, releases are made from PK as needed so that Lake Granbury is never lower in terms of depth below full than PK.

Absent a detailed study, this method of operation was simple, easily explainable, and justifiable in the short-term. However, it did receive scrutiny from various interest groups, particularly those around Lake Granbury that had been accustomed to the historical flows provided by PK hydropower generation.

The drought conditions of 2008-2009 contributed heavily to this scrutiny, as well as concerns related to the proposed addition of two new nuclear generating units at Luminant's Comanche Peak Nuclear Power Plant that would divert additional water from Lake Granbury for cooling.

In the spring of 2010, BRA, in conjunction with Halff Associates, began a water management study of the PK - Granbury - Whitney portion of BRA's reservoir system. Lake Whitney was included in the study because it is physically located downstream of the other two reservoirs. However, due to the relatively small amount of storage space controlled by BRA in Lake Whitney, the focus of the study effort was on PK and Granbury.

The goal of the study was to formally evaluate and develop a water management procedure for the PK-Granbury-Whitney portion of the BRA reservoir system that meets water supply needs and considers facilities affected by lake levels and water supply management.

Extensive field work, physical inventorying, and data collection were used to determine lake bottom elevations at over 1,600 features (docks, ramps, marinas, etc.) on Possum Kingdom Lake and nearly 3,500 features on Lake Granbury. This analysis indicated that the Equal Drawdown plan did not produce "equal" impacts to water features at Possum Kingdom Lake and Lake Granbury. A higher percentage of features proved to be out-of-service at Lake Granbury than at Possum Kingdom Lake for an equal drawdown level.

Based on these results, a Zonal Drawdown plan was developed with a goal of balancing the percent of features out-of-service at each lake for a range of elevations (drawdowns). Conclusions from the study suggested that operating the system dependent upon "lake depth zones" (Zonal Drawdown) can produce a more equitable balance of impacts at both PK and Granbury without sacrificing the water supply yield of the upper Brazos River system.

Based on this more comprehensive analysis, the BRA Board of Directors voted in April, 2011 to implement a recommended 1.75:1 (PK:Granbury) drawdown ratio when PK's lake elevation is above 992.0 ft-msl, meaning releases would be made as needed from PK so that for every 1 foot of drawdown experienced at Lake Granbury, a corresponding 1.75 feet of drawdown would be experienced at PK. Additionally, when PK's lake elevation falls below 992.0 ft-msl, the drawdown ratio changes to 1:1. Based on the study, this practice provides a "balanced" impact (percent of time facilities out-of-service) between PK and Granbury.

This recommended operational procedure continues to allow the BRA to meet contractual lakeside and downstream water supply obligations while balancing impacts that may be experienced at the lakes during dry times.

For the completed PK-Granbury-Whitney study results, please click the Completed Report button at the right.


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