Twin Valley Dam – Mechanical and Chemical Rehabilitation of Relief Wells

PROJECT OVERVIEW Project Background – Twin Valley Dam and Reservoir (formerly known as Little Bow River Dam project) are located in southern Alberta, approximately 100 km south-southeast of Calgary, and are operated and maintained by Alberta Agriculture and Irrigation (AGI), Water Infrastructure and Operations Branch (WIOB). Twin Valley Reservoir is used to regulate flows of the Little Bow River and Mosquito Creek for municipal water supply, irrigation, conservation, and recreational purposes. The project was completed in 2002, and the reservoir reached its full supply level (FSL) of 964.8 m for the first time in 2004. Its storage capacity at FSL is 61,700 dam3 and the maximum height of the dam is 26 m. Twin Valley Dam is classified as a “very high” Consequence Dam (Canadian Dam Association [CDA] Guidelines, 2007). The classification of the existing project is governed by the large economic damages that would result from a potential cascading failure of Twin Valley Dam with Travers Dam. The dam has an internal drainage system within the downstream slope of the dam embankment to alleviate impacts from water seeping through the structure. The internal drainage system consists of an inclined chimney drain and a horizontal blanket drain. A portion of the seepage which enters the system is carried to the downstream toe of the dam by six outlet finger drains which discharge into a gravel-lined collector drainage ditch. Additional seepage is managed by passive pressure relief wells which are located approximately 15 m downstream of the chimney drain. The current system consists of 11 relief wells installed in the alluvium and underlying bedrock. Well depths vary from 22 to 25 m, and each well is equipped with an upper and lower screened section. The lower screen allows water under pressure to enter the well from the underlying alluvial-bedrock aquifer(s). The upper screen is installed across the collector drain and allows the water to discharge from the well into the collector drain. The purpose of relief wells and finger drains are to maintain pore pressures in the dam foundation materials at acceptable levels, as elevated pore pressures have the potential to cause unfavourable seepage conditions and can compromise the overall stability of the embankment. Instrumentation at Twin Valley Dam was generally installed during construction from 2001 to 2003. Actively monitored instrumentation includes 35 standpipe piezometers, 87 pneumatic piezometers, 11 slope inclinometers (SIs), six finger drains, ten jointmeters, and 11 relief wells. Aside from finger drains which are monitored weekly, instrumentation data is collected at least three times per year. Standpipe piezometers were installed in till, alluvium, and bedrock to monitor groundwater levels, primarily for monitoring conditions within the foundation of the dam. Thirty-three of these standpipes were installed in the foundation while two were located on the west side of the spillway. Pneumatic piezometers were generally installed along three transverse lines on the downstream embankment referred to as Lines 4, 5, and 6 for the purpose of monitoring water levels in the dam foundation, embankments, and drains. SIs were installed through the dam crest and embankment slopes and were anchored to the dam foundation to monitor displacement. Six SIs on the upstream side of the dam were abandoned prior to first fill in 2003. Liquid level settlement gauges were installed along Lines 4, 5, and 6 on the downstream embankment to monitor settlement of the foundation. Magnetic plate settlement gauges were installed along Lines 4, 5, and 6 to monitor settlement of the foundation beneath dam shell material. Six finger drains comprising of two 300 mm diameter slotted pipes surrounded by a layer of gravel, further encased in fine filter sand, exit into a seepage collector ditch at the downstream toe of the dam. This seepage collector ditch feeds into a settling chamber and a weir flow measurement system to monitor seepage rates through the embankment. Eleven pressure relief wells were installed within the downstream shell materials of the dam to relieve porewater pressures in the foundation alluvium and upper bedrock. A similar relief well rehabilitation program was completed in 2016 for a portion of the relief wells. Recent data suggests that water levels within several relief wells either met or exceeded maximum levels recorded since the previous rehabilitation program completed in 2016 (RW-01, RW-02, RW-05, RW-06, RW-07, and RW-08). Relief wells RW-01 and RW-02 appear to be moderately correlated to reservoir elevation while the remainder of relief wells demonstrated a weak correlation or no correlation to reservoir elevation. Historical data suggests that water levels within a portion of the relief wells have been gradually increasing since 2016, potentially indicative of well inefficiencies related to biofouling, sedimentation, carbonate buildup, or otherwise. While the connection between piezometers and relief well performance has not been assessed or conclusively established, steadily increasing piezometric elevation trends have been identified for nearly all deep bedrock piezometers along Lines 4, 5, and 6, as well as a few non-deep bedrock instruments. Specific piezometers noted to have steadily increasing trends since the first few years of operation include PP-4-06, PP-4-13, PP-4-14, PP-4-16, PP-4-18, PP-5-06, PP-5-10, PP-5-13, PP-6-06, PP-6-10, PP-6-13, and PP-6-16. In addition to these trends which have been steadily increasing since the first few years of operation, several instruments had increasing trends after the completion of the relief well rehabilitation program in 2016/2017. Instruments with an apparently increasing trend since 2016/2017 include SP-8-03, SP-8-04, PP-4-03, PP-4-04, PP-4-05, PP-4-07, PP-4-08, PP-4-09, PP-4-12, PP-4-14, PP-4-15, PP-4-17, PP-4-25, PP-4-26, PP-5-07, and PP-5-14. In addition to increasing trends, numerous record high piezometric levels were recorded in 2025. After the omission of suspected erroneous readings, instruments which recorded a record high piezometric elevation in 2025 included SP-1-06, PP-4-03, PP-4-04, PP-4-05, PP-4-06, PP-4-07, PP-4-08, PP-4-09, PP-4-10, PP-4-12, PP-4-13, PP-4-14, PP-4-15, PP-4-17, PP-4-18, PP-4-20, PP-4-26, PP-5-06, PP-5-07, PP-5-10, PP-5-13, PP-5-14, PP-5-25, PP-6-06, PP-6-07, PP-6-10, PP-6-13, PP-6-16, PP-6-20, PP-6-24, SP-8-03, SP-8-04, and SP-8-05. The response of these piezometers, as well as those not mentioned, will be closely monitored by AGI personnel to assess if/how relief well rehabilitation impacts piezometer performance and overall water levels within, below, and around Twin Valley Dam. Project Objectives – The main objective of this project is to assess the 11 relief wells and select as many candidates for mechanical and chemical rehabilitation as the approved budget will cover. Project Structure/Business Model – The consultant will report to the AGI Contract Manager who will be located in the Bow Operations office in Calgary, Alberta. With offices in Calgary and Vulcan, the WIOB is responsible for the day-to-day operations and maintenance of Twin Valley Dam. A single point of contact will be established with the Contract Manager, and other technical and operations personnel will be assigned as necessary to work with the consultant to deliver the required outcomes.