Photo by city of Austin
At its meeting on Wednesday, the Environmental Commission voted to approve a set of Land Development Code variances to accommodate the Watershed Protection Department’s Old Lampasas Dam #3 Modernization Project.
The Northwest Austin dam was built in the early 1980s and is classified by the Texas Commission on Environmental Quality as an intermediate size and high hazard due to the downstream infrastructure. The dam was already in disrepair in 2010 when it was further damaged by Tropical Storm Hermine.
Currently, the 60-inch corrugated metal pipe principal spillway has failed and water is passing under the pipe. Mike McDougal, city environmental policy program manager, explained that erosion exposed the pipe over time, and it has also cracked.
The proposed improvements to the dam are intended to improve the level of safety of the dam to both the public and the environment, as well as to mitigate potential future impacts. Both the city of Austin and TCEQ dam safety criteria require these improvements.
Credit: City of Austin
According to McDougal, the proposed improvements include removal of existing trees and boulders along the dam footprint, replacement of the existing 40-inch diameter principal spillway pipe with a new 42-inch diameter reinforced concrete pipe principal spillway and cradle. The project will also include installation of a new intake structure and low flow riser upstream of the dam to control the water level within the pond and provide extended detention for water quality benefits.
Additionally, the project will replace the existing wastewater line through the embankment of the dam with a new line parallel to the principal spillway. The city will remove the existing wastewater line upstream of the centerline of the dam, grade and resurface the upstream and downstream embankment slopes for increased stability, raise the top of the dam and auxiliary spillway elevations to meet city and state safety criteria, and install a new auxiliary spillway and a concrete headwall.
The city will also install a permanent maintenance drive to allow Watershed Protection Department Field Operations to access and maintain the structure, as well as a new Flood Early Warning System.
The project, on 9018 1/2 Old Lampasas Trail, requires the following variances:
- Request to vary from LDC 25-8-261 to allow construction in the Critical Water Quality Zone
- Request to vary from LDC 25-8-341 to allow cut up to 10 feet
- Request to vary from LDC 25-8-342 to allow fill up to 9 feet
- Request to vary from LDC 25-8-301 to allow driveway construction on slopes over 15 percent
Staff recommended the variance with the condition that the project include 609S native seeding and planting except where prohibited due to dam safety requirements. The specification 609S refers to seedbed preparation, seed planting, planting of rooted vegetation, watering, hydromulch, compost, and species selection.
“I mention ‘except for prohibited,’ because 609S can require the planting of woody vegetation – trees, things like that – that can be in conflict with the dam,” McDougal said. “So, staff is asking for 609S vegetation where it doesn’t conflict with dam safety requirements.”
In response to questions from the commission, staff said it has made the necessary arrangements to delay construction until the nesting season of the protected golden-cheeked warbler is completed, as well as to monitor the presence of the protected blind salamanders. The commission added one additional condition, which is that Watershed Protection consult with subject matter experts on how to minimize herbicide impacts on the Jollyville Plateau salamanders.
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Posted In: Environment, District 6
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This article First appeared in austinmonitor