Environmental Commission Recommends Code Changes for Old Lampasas Dam Modernization Project

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Photo courtesy: City of Austin

Friday, November 3, 2023 Nina Hernandez

The Environmental Commission voted at Wednesday’s meeting to approve a series of land development code changes to accommodate the Watershed Protection Authority’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 moderate in size and high risk due to downstream infrastructure. The dam was further damaged by Tropical Storm Hermine in 2010 and was already in disrepair.

The main spillway, a 60-inch corrugated metal pipe, is currently damaged and water is passing underneath the pipe. Mike McDougall, the city’s environmental policy program manager, explained that over time, erosion has exposed and cracked the pipes.

The proposed improvements to the dam are aimed at increasing the level of safety of the dam for both the public and the environment, as well as reducing potential future impacts. Both the City of Austin and his TCEQ dam safety standards require these improvements.

Credit: City of Austin

Proposed improvements include removing existing trees and rocks along the dam’s footprint and replacing the existing 40-inch diameter main spillway pipe with a new 42-inch diameter reinforced concrete main spillway pipe, McDougall said. This includes replacing the waterway and cradle. The project also includes the installation of new water intake structures and low-flow risers upstream of the dam to control water levels in the pond and extend residence time to improve water quality.

In addition, the project will replace the existing drainage line through the dam’s embankment with a new line parallel to the main spillway. The city will remove existing drainage pipes upstream of the dam’s centerline and level and resurface the upstream and downstream levee slopes to improve stability and meet city and state safety standards. For this purpose, we plan to increase the height of the top of the dam and the auxiliary spillway, and install the dam. New auxiliary spillway and concrete headwall.

The city will also install a permanent maintenance drive to allow Watershed Conservation Authority field operations to access and maintain the structures, as well as a new flood early warning system.

This project, located at 9018 1/2 Old Lampasas Trail, requires the following variances:

  • Request to amend LDC 25-8-261 to permit construction in critical water quality zones.
  • Request modification from LDC 25-8-341 to allow cutting up to 10 feet
  • Request modification from LDC 25-8-342 to allow filling up to 9 feet
  • Requests to amend LDC 25-8-301 to permit construction of driveways on slopes greater than 15 percent

Staff recommended a variance from the condition that the project include seeding and planting of 609S native species unless prohibited by dam safety requirements. Specification 609S refers to nursery preparation, seed planting, rooted vegetation planting, watering, hydromulching, composting, and seed selection.

“I say ‘unless prohibited’ because 609S may require the planting of woody vegetation (such as trees) that may be inconsistent with the dam.” Mr. McDougall said. “As such, staff is seeking 609S vegetation that does not conflict with the dam’s safety requirements.”

In response to questions from the committee, officials said they had postponed construction until the nesting season for the protected golden warbler was completed and had made the necessary arrangements to monitor the presence of the protected blind salamander. The committee added a condition that Watershed Protection consult with subject matter experts on how to minimize the impact of herbicides on salamanders on the Jollyville Plateau.

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