THE TALE OF TWO MODELS RCC SPILLWAYS FOR OVERTOPPING PROTECTION

摘要

To increase the spillway capacity of many old embankment dams, stepped chute RCC spillways have been constructed over the embankments. Several of these spillway designs have: a long, straight overflow crest to minimize the flood rise in the reservoir; a tapering (or converging), stepped chute over the downstream face of the embankment; and a narrow stilling basin in the river bed. However, flow conditions in the convergence zones at supercritical flow along the sides of the tapering chute and energy dissipation of the converging jets in the stilling basin cannot be accurately calculated. To evaluate the flow behavior in converging spillways of existing and upcoming dam rehabilitation projects throughout the United States, the NRCS contracted the Agricultural Research Service Hydraulic Engineering Research Unit to conduct a hydraulic model study of a typical RCC converging, stepped chute spillway over an embankment dam. The spillway selected for the study was one of the conceptual design layouts developed in feasibility studies for upgrading the Big Haynes Creek Watershed Structure No. 3 (H-3) in Gwinnett County, Georgia. The model tested the effects of vertical and stepped chute walls for converging spillways with a 53 degree angle of convergence. The testing illustrated that the flow conditions in chutes with vertical side walls were better than in chutes with laid back side walls, and that energy dissipation in the stilling basin was better with the vertical side walls. The model also showed that a large degree of convergence for the spillway can be achieved with high, vertical side walls. However, in spillways with laid back side walls, the high velocity flow at the sides of the chute will jump over the walls, causing severe erosion downstream of the dam and undermining of the spillway. Recognizing these serious consequences, Golder developed a design layout for the Yellow River Watershed Structure No. 15 (Y-15) RCC overtopping spillway that will force the flow convergence to occur within the central area of the stepped chute, rather than at the side walls as with the H-3 model. The Y-15 spillway was modeled by the USBR at the Water Resources Research Laboratory in Denver, Colorado. This model showed that the major problems revealed by the H-3 model have been mitigated at Y-15 and that all high velocity and turbulent flows are contained within the lined RCC spillway. The model also identified minor modifications that improve flow conditions in the chute and energy dissipation in the stilling basin.