Closure to Stop Logs for Emergency Spillway Gate Dewatering
Journal of Hydraulic Engineering
American Society of Civil Engineers
The writers appreciate the additional information on emergency stop-log deployment provided in the discussion by researchers from the U.S. Army Corps of Engineers. The studies noted were not found during the course of the research, and their inclusion would have been beneficial in the published paper. The writers also are grateful for the opportunity to respond to questions raised in a subsequent discussion of the article and are pleased to provide the following responses: Response #1: The observation made by the discusser is correct in that the data shown in Figs. 5 and 6 of the published article are not the actual forces that would be experienced by the hoist mechanism but rather represent the maximums to be expected. The most significant forces of concern for designing the hoist are the extremes that the hoist would experience during deployment of the stop logs, and those are what are shown in Figs. 5 and 6. The hoist forces induced by the stop logs would actually go to zero during the deployment process because the stop logs are dogged and left resting on the pier and the hoist used to retrieve and make ready the next stop log for installation. Response #2: The horizontal axes in Figs. 5 and 6 vary from 0 to 6 and 0 to 10 meters, respectively. The distance measured is the vertical distance from the bottommost point of the bottom stop log to the seating surface on the spillway. Response #3: The force reduction shown in Fig. 5 “EL310.39-0.005m/s EL 310.39-0.005 m/s” has been reviewed by the writers, and its cause is unknown. The writers’ best estimate of the reduction prior to submersion is related to seal friction between the rubber seal used on the model stop logs and pier. A flexible membrane was used to seal the sides of the stop logs to the piers. Response #4: The insight provided by the discusser is appreciated and useful. The intent of the referenced sentences was to describe common problems associated with the typical installation of stop logs in flowing water and not to convey all possibilities of uplift or downpull. The discusser is correct in that uplift can occur for other reasons, as shown by the data presented in the paper. Response #5: The 14- and 16-gate spillways use hollow structural sections (HSS) having a depth of 0.406 m, and the 7-gate spillway uses HSS having a depth of 0.610 meters, which agree with the discusser’s estimate. The depth was an important design consideration that was necessary for the design of the model stop logs and was considered by the licensed professional engineers performing the design. The weight of each log and the details of each stop log were carefully considered as the stop logs are presently being implemented on a specific structure. The paper was not intended to be a design guide but rather to introduce the concept and validate the performance of the specific designs. Response #6: The writers expect this seating force to be sufficient. The HSS tubes are being constructed to allow water ballast to be added to increase the seating force if necessary. Response #7: The discusser is correct; the friction forces are not governed by the same scaling laws that govern fluid flow for hydraulic modeling. For that reason, careful machining and high-quality stainless-steel axle bolts were used in conjunction with machined aluminum wheels. The purpose of the model was to validate whether the design was deployable under emergency conditions. The notion of using sealed bearings was considered and dismissed after a discussion about friction between the model and the prototype. The rationale was such that the prototype bearings will have less friction than the model and that the model would be conservative pertaining to stop-log stalling. One of the important considerations of the design was to size the hoists and not resolve component forces. In none of the cases investigated was the hoist force greater than the static weight of the stop logs. Certain tests, which were not included in the published paper, were completed with the stop logs stalled at some opening. In those cases, the hoist forces were similar to those with stop-log movement. Additionally, different rates of stop-log deployment showed very similar hoist forces, and it was determined that the hoist forces were a function of position and not deployment rate. Response #8: The writers apologize for the confusion and hope that the following sentences clear it up sufficiently. The intent was to set depths corresponding to approximately one-third and two-thirds of the free-flowing depth at the spillway crest. Therefore the gate was inserted to approximately one-third of the flow depth to create the two-thirds flow depth and the gate insertion of two-thirds created the one-third depth. In retrospect, it would have been better to simply list gate elevations and flow depths. Read More: http://ascelibrary.org/doi/full/10.1061/(ASCE)HY.1943-7900.0000556?mobileUi=0
Freckleton, Derek R., Johnson, M.C., Boyd, M.L. and Mortensen, D.G., 2012. Closure to Stop Logs for Emergency Spillway Gate Dewatering. Journal of Hydraulic Engineering, Vol. 138, No. 6, June 2012.