CS22C - Concurrent Session 22C: Century Old, Run of the River Dam, Inspected for the First Time In 70 Years

The Public Utility District No. 1 of Okanogan County (District) owns and operates Enloe Dam on the Similkameen River, in Washington. Construction of the 57-foot high, 276-foot long concrete arch dam was completed in 1920 and generation ceased in the late 1950’s. In the absence of functioning diversions around the dam, the river has passed freely over the dam crest since then. Though the dam was designed to overflow and serve as a spillway, the absence of diversion structures prevented inspection of the dam face, abutments, and tailrace, leaving the condition of the dam largely unknown. In 2019, the District committed to constructing facilities to inspect the dam. This presentation describes the efforts taken to inspect the dam.

To dewater the dam for inspection, the District invested $10 million over two years, replacing the original gates and wood stave pipelines with vertical 72-inch diameter gates and two 200-foot-long sections of steel pipe. The diversion structures were designed to route late summer flows around the dam and the tailwater. Though construction was not without challenges, the work was completed in compliance with applicable permits and the diversion structures performed as intended for the late summer inspection.

In September 2022, the District, led by McMillen Inc., successfully and safely conducted the dewatered inspection and condition assessment. The multi-disciplinary team used unmanned aerial vehicles to capture photogrammetry, underwater side-scan and blue beam sonar for bathymetric surveys and traditional survey and rope access to complete geologic mapping. Field surveys seamlessly stitched the elevation data together. The work yielded dimensional drawings of the dam and appurtenant structures, flaw characterization, bathymetry and geologic maps of the abutments and immediate area. Installation of survey pins on the dam crest, and survey information, confirmed resolution of photogrammetry, and ensured accurate stitching of above-water and submerged maps. Three-dimensional imaginary with millimeter accuracy will be used for future planning and monitoring purposes.

This inspection is an example of a dam owner, taking initiative, investing, and committing to meeting dam safety objectives, on a unique facility, with multiple data collection techniques. The main findings from this inspection are that the dam is practically unchanged since 1950, and no emergency measures are needed to ensure continued safe operations in the near term. The District remains committed to dam safety at Enloe Dam.