Feature Photo: The view looking north toward the Lake Junction Dam, as seen from the Main Street bridge above the South Llano River in Junction. Image: Google Streets.
by Adolfo Pesquera
Junction (Kimble County) — The city of Junction has been approved for more than $4 million in financial assistance from the Texas Water Development Board, funding dedicated to the repair of Lake Junction Dam.
In addition, under a separate agreement, the city is to receive $906,000 to perform upgrades and repairs to the city’s water treatment plant.
Located on the South Llano River, east of Schreiner Park, the dam is a rock-filled, concrete covered gravity dam designed for over-topping. It was originally constructed in 1965 as a recreational impoundment dam.
It is 17 feet high and currently stores about 92 acre-feet of water and is permitted to store up to 312 acre-feet. It is the city’s sole source of water.
In October 2018, a major flood event occurred in the South Llano River watershed that impacted the dam. The city needs to repair it and install mitigation measures to prevent future catastrophic failure of the dam
If left unattended, the dam is capable of breaching during high flows, due to abutment failure or from excessive erosion on the downstream toe from sliding.
Scope of Work at the Dam:
VBX Project ID 2021-1695
The proposed project will repair the damage to the dam caused during the flood and protect its integrity for use as a critical municipal water supply.
The project will include placing roller compacted concrete along the downstream toe of the dam to provide energy dissipation and increase the factor of safety for the dam from sliding downstream; widening and lengthening the cutoff walls under the concrete abutments to reduce flow through the alluvium on each side of the dam, which will assist in mitigating potential piping in the abutments; and reconstructing the concrete abutments including toe walls to prevent erosion through the abutments.
The financial agreement totals $4,120,000 and TWDB is scheduled to close on the agreement with the city May 5, 2021. The funding sources are $2,060,000 in the form of a loan, and another $2,060,000 in the form of a gram from the Flood Infrastructure Fund.
However, the engineering report is not expected to be complete until June 30,2022. The project schedule calls for the design phase to be complete Nov. 30, 2022.
Constructed is scheduled to begin Feb. 1, 2023 and conclude Oct. 31, 2023. The portion of the project cost dedicated to actual construction is $3,004,224.
Scope of Work at the Water Treatment Plant:
VBX Project ID 2021-169C
The same October 2018 flood event deposited large sediment bed loads around the water treatment plant’s intake structure and surrounding area. The existing intake pipe is at risk of being damaged or rendered inoperable by another large sediment deposit that would effectively incapacitate the city’s ability to draw raw water to its water treatment plant.
The proposed project includes construction of an infiltration system using a 24-inch diameter steel pipe with three-quarter inch diameter holes. The system will be extended into the river approximately 80 feet and will have four 20-foot-long branches. The infiltration pipe will collect water into a new sedimentation wet well where it will flow by gravity to the existing pump station wet well.
The infiltration system would operate in the saturated gravel and not be susceptible to the evolving conditions of the river.
The existing sedimentation wet well and intake pipe is to be abandoned in place.
The financial agreement closing date is May 5.
The engineering report is not due until March 31, 2022. The design phase should conclude Aug. 31, 2022.
Construction would then being by Nov. 1, 2022 and is anticipated to be completed March 31, 2023. The actual construction cost portion of the project is estimated at $625,000, according to the budget summary.