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.
VIEWING:
Coordinates are for the weir itself. The weir can be glimpsed downstream from the Highway 77 bridge. Closer viewing can be had from parking behind several warehouse businesses off Spur 484 (Marlin Highway) N31 33.189 W97 5.660. Closer still, use the locals’ water access – unimproved dirt tracks down to the river from those lots.
AWARDS & RECOGNITION (a partial list):
ASCE Outstanding Civil Engineering Achievement (OCEA) Award of Merit 2009 (
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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! 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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