24AGR060 - Condition Assessment of Water Management Structures Bigelow Dam Low Level Outlet Assessment - 2023

Background
The Bigelow Dam is located in central Alberta northwest of Trochu as shown in Figure 1.0 of the TOR. The main purpose of the system is to regulate flow on Threehills Creek for agricultural purposes, provide breeding habitat for waterflow, and create recreational opportunities.

The Bigelow Dam consists of a 216m long, 10m high embankment dam, concrete drop inlet spillway structure, low level outlet, sheet pile auxiliary spillway, and natural spillway. The main dam was constructed in 1973 and is a homogeneous dam with an upstream pervious toe.
The drop inlet structure is a 3.05m square drop inlet that exits through a 2.13 diameter corrugated steel pipe (CSP) low level outlet. The CSP was originally 2.28m in diameter but was later lined with a 2.13m steel pipe due to deterioration of the original pipe. The Bigelow Dam Site and Drawing are shown in Figure 2.0 of the TOR.

Low Level Outlet
The 2.13m diameter low level outlet (LLO) conduit does not appear to have a coating system and corrosion has been observed along the length of the pipe. The downstream end of the conduit has light pitting while the upstream end has moderate pitting and some scaling as shown in Figure 3.0 of the TOR. While all 19 welds were inspected and determined to be in satisfactory condition (2021 DSR), there were dents observed near the crown of the conduit.

A survey of the interior of the conduit to measure deformation and the thickness of the liner is required to set a quantitative baseline that can be referenced to determine the rate of deterioration of the steel liner. The design drawing of the Bigelow Dam Low Level Outlet is available in Appendix D.

The 2022 PMFEA report commented that while the available drawings indicate that the riprap cover extends to the crest on the upstream face of the dam, it could not be confirmed due to the soil and grass coverage on site. A review needs to be done to verify the extent of rip rap on the upstream slope of the dam and determine if the riparap on the upstream slope requires raising beyond the current coverage.