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Lake Brazos Labyrinth Weir ASCE Texas Section Outstanding Civil Engineering Achievement (OCEA) Award, 2008
IN A NUTSHELL:
The Lake Brazos Labyrinth Weir was commissioned by the City of Waco to replace the Lake Brazos Dam, which had posed maintenance and operational problems and was unable to provide the stable lake levels that would be ideal for downtown development. Freese & Nichols refined the initial concept and were able to design the project in a way that eliminated the costly need to divert the river, empty the lake for construction, or to build downstream of the existing dam by constructing the weir in two phases over the foundation of the existing dam. The concept underwent extensive modeling to determine the most advantageous configuration and to develop optimal phased construction processes and river management practices. Ultimately, the design and construction of the weir was successful, allowing the City of Waco to benefit from reliable lake levels at nearly half the cost of earlier estimates.
In addition to minimizing cost, engineers were able to limit disruption to lake levels during construction through the phased construction process. Existing hydraulic gates remained operable while the left labyrinth was constructed and work started on the right labyrinth only when the left was completed and river flows were passed over it. Additionally, city-approved lake lowerings were accommodated at pre-determined times to facilitate specific work items. Although initial progress put the project five months ahead of schedule, heavy rains late in the construction process caused river flows to exceed 20,000 cfs and delayed completion of the right labyrinth weir, which had originally been scheduled for September, until October 2007. Despite the inconvenience, the floods in March and July offered a full-scale test of performance of the left labyrinth weir, which completely validated expectations while still in the construction phase. The weir has continued to perform as designed and has enabled city planners, developers, and businesses to look to downtown for events and development as steady lake levels have at long last become a reality in Waco.
Records:
Owner- The City of Waco,Water Utilities Director, Ricky Garrett
Engineer- Freese & Nichols, Inc.
Contractor- Archer Western Contractors, Ltd.
DETAILS! YOU WANT DETAILS!
The Brazos River Labyrinth Weir – just downstream of the Highway 77 bridge at the Baylor recreational sports fields – is not only fascinating to look at, but an innovative design that solved a 30 year eye-sore and maintenance problem.
THE PROBLEM:
The Brazos River plays a vital role in the commerce and appeal of downtown Waco. Aesthetics, however, require that the river/lake through town maintains a near constant reservoir level.
The original 1970 dam fell far short of promise, plagued throughout its thirty-plus years with operational problems. The original drum gates, a great idea on paper, proved unable to maintain constant reservoir level. Modifications in 1985 replaced the drum gate system with leaf gates using hydraulic cylinders, which showed a frustrating habit of failing; too often resulting in lowered and unappealing reservoir conditions while repairs were conducted. And, of course, the cost of constant repairs adds up.
Something HAD to be done.
THE SOLUTION:
Studies by Freese and Nichols, Inc. in 1992 and The U.S. Army Corps of Engineers in 2000 examined the feasibility of replacing the dam with a fixed-weir structure to eliminate the gates entirely. The fixed-weir solution was compelling, as it would eliminate the ‘moving parts’ and their chronic maintenance costs. Ultimately an innovative design was selected: a labyrinth spillway which would utilize the existing footprint of the dam, significantly reducing project costs for a number of reasons. (Ultimately, innovative approaches, both in design and project management, trimmed almost $14 million from a $30 million budget. From design through construction, the interested are encouraged to read the referenced papers at bottom for a wealth of fascinating detail – with photos!)
WHY A LABYRINTH WEIR?
Holding water back isn’t the issue. Letting flood waters flow without inundating lakeside property upstream is the trick. The advantage of a labyrinth over a straight weir is that it increases the discharge capacity for the same channel width by increasing the weir length. True, a labyrinth weir’s benefits decrease if water levels rise significantly above the design head. When too much water washes over the weir, cross flows from adjacent weir walls mix, reducing efficiency to such a point, as volume increases, that it performs essentially identically to a linear broad-crested weir. Therefore, the geometry of the weir needs tweaking to expected conditions.
THE LAKE BRAZOS SOLUTION:
The labyrinth developed for Lake Brazos not only included replacing the existing gated spillway with a labyrinth but, in order to match length with expected flood flow, extended the weir along the extant dam’s overflow embankment. The placement of the labyrinth on the embankment area resulted in an upstream shift across the labyrinth near mid-stream – in effect, two labyrinth weirs side-by-side. This pushed tried-and-true design into unorthodox territory, prompting extensive model studies (again see the articles, links at bottom). Fourteen sectional models and a full-width model were employed to evaluate the labyrinth weir concept: testing parameters such as wall angle, wall width, cycle width, cycle depth, apex shape, and crest shape, all while examining the effects of tailwater submergence on discharge capacity (e.g. that cross flow problem I mentioned).
The “winning” configuration consists of a compact cycle with an 8-degree wall angle – maximizing effective weir length across the relatively small footprint – and rounded apexes. The 2-foot thick cycle walls are topped with an ogee-type crest (ogee, pronounced “oh, gee” – think Zero-G or zero gravity. Spillways’ arcs are designed to mimic the path the typical volume of water takes over the top. This limits the water’s contact with the concrete – zero-g – reducing wear-and-tear on the spillway. Of course the arc varies with flow, so there’s always compromise…), which, in turn provides an increased discharge capacity and better performance, limiting upstream nuisance inundation during normal river flows.
In total, the Lake Brazos Labyrinth Weir has 25 segments -- technically called "cycles" (best design from testing to maintain the FEMA upstream inundation levels), divided, as noted, into two distinct sections: a left section with 13.5 cycles within the footprint of the existing overflow embankment and a right section with 11.5 cycles within the footprint of the existing gated spillway. The two sections have an upstream/downstream offset from each other as previously noted.
The other beauty of the design is that it could be constructed while, mostly, maintaining normal lake levels. This was abetted by Texas’ severe drought conditions throughout most of the construction period. However, late in the project substantial flooding provided a full scale test of the left labyrinth performance. Which, to everyone’s relief, validated the hydraulic modeling calculations.
AWARDS & RECOGNITION (a partial list):
ASCE Outstanding Civil Engineering Achievement (OCEA) Award of Merit 2009 (
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Nominee: ASCE's 2009 Outstanding Civil Engineering Achievement (OCEA) Award. (
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Engineering Excellence Honor Award from the American Council of Engineering Companies and Excellence in the Constructed Project from the United States Society on Dams, both in 2008. (
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The United States Dam Society’s award for the best constructed dam of 2008 and the American Public Works Association – Texas Chapter 2008 Project of the Year in Environment award.
ACEC Engineering Excellence Awards 2008 Honor Award (
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THUMBNAIL DESCRIPTIONS:
Freese & Nichols: Our Work: Lake Brazos Labyrinth Weir (
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New Lake Brazos Dam Complete (
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Kleinfelder Project Spotlights: Lake Brazos Dam (
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DETAILS! MORE DETAILS!
Full project descriptions – with pictures! and charts! – can be found in two excellent articles:
January 2009 Civil Engineering Magazine: “AN ELEGANT SOLUTION: The Lake Brazos Labyrinth Weir” (
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28th Annual USSD Conference Portland, Oregon, April 28 - May 2, 2008 “A LABYRINTH RISES IN THE HEART OF TEXAS” (
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FURTHER READING:
Waco Water Utility Services “Projects & Info Lake Brazos Dam” (
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“A LABYRINTH RISES IN THE HEART OF TEXAS -- 28th Annual USSD Conference Portland, Oregon, April 28 - May 2, 2008 (
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Kleinfelder “Lake Brazos Dam Project Spotlight” (
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Walsh Group “Lake Brazos Dam Replacement” (
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Freese & Nichols “Lake Brazos Labyrinth Weir” (
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Waymark Code: WMJ0TT Lake Brazos Labyrinth Weir, Waco (
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