PLSO The Oregon Surveyor March/April 2022

18 The Oregon Surveyor | Vol. 45, No. 2 Colorado determined that if a heavy runoff were to occur, it was probable that the existing spillway would come into play as the pool filled. This outflow structure also had a design flaw. The spillway, as it existed, simply let the floodwater spill onto the ground at the toe of the dam. They were afraid the backwater could erode the base of the damand cause a failure frombelow. BOR ranmodels in their lab in Denver and came up with a plan to construct a large bowl at the downstream end of the spillway, called a “stilling basin.” This required that the bowl be excavated to a precise dimension, then linedwith roller-compacted concrete. On the floor of the basin were baffles designed to slow themovement of the water as it passed through the structure. A rock-lined channel then carried the water out to the existing creek. We must have done an okay job with the dam as we were rehired to stake the stilling basin. You can see by the pictures that this was not a straightforward construction staking job. Therehad tobe a lot ofmaterial removed and the whole thing was a weird shape. Slope stakes needed to be set high up on the side of the hill. Grades had to be carried down all the way to the bottom of the basin. I don’t remember the back slope but it was probably a one half to one, so pretty steep. They hired a grade hopper, but unfortunately he allowed some creep, so the back slope flattened slightly. By the time they reached bottomgrade, thewidth of the basin was too narrow. The contractor was mostly able to resolve the flaw. The BOR engineers ran another flowmodel and they were okay with what they ended up with. As we were reaching final grades and because of the configuration of the basin, it became difficult to determine fromfield calculations where final grades should be. I sent and asked for the digital terrainmodel from the bureau. I could pick any subgrade design X,Y, Z off of the DTM, shoot that point in, and thus determine what the contractor needed to do to get to the design grade. When final grades were achieved, the basin was lined with a 10-foot-thick layer of roller compacted concrete. A suitable rock source was found just below the dam. They set up a batch plant, and were able to produce the concrete on site. Finishing this project couldn’t have been timed more perfectly. A year after completing the stilling basin, a significant flood occurred in that drainage. The spillway accepted the overflow, and the basin and damperformed as designed. Unfortunately, there was still some major flooding in downtown Prineville. After all the time I have spent in Central Oregon, 1998 was the only time I can recall that the spillway and stilling basin has come into play to that extent. All of you surveyors out theremust have a favorite job that you have participated in sometime during your career. How about sharing? I’m sure Vanessa and the Publication Committee would love to hear of some. [Editor’s Note - Vanessa agrees! She would love to hear of some of your favorite jobs. If not favorite, then perhaps the most challenging.] Tales of your exploits might even spur some fledgling surveyors to want to get serious about this fun game called “surveying.” x Dick Bryant was recently featured as our Member Spotlight in the November/December 2021 Oregon Surveyor. He celebrated 50 (+1) years of licensure on September 22, 2021. Dick was licensed as Oregon Registered Professional Land Surveyor #920 in July 1970. He was voted PLSO Surveyor of the Year and was awarded Life Membership in 2004. In 1967, he and Tom McCullough formed McCullough, Bryant and Associates. Preparing the upstream face of the dam for curtain drain and impervious cover. Note breach to the left. Aerial view of completed dam with reconstructed spillway and new stilling basin. continuedT Featured Article