Cost-Effective Spillway Design: Adding Value Through Rock Scour Analysis

Salem Lake Dam was originally constructed in 1919 as a water supply dam for the City of Winston-Salem, North Carolina. The concrete gravity arch dam is roughly 40 feet high and includes a broad crested spillway weir 240 feet in length. The historic dam did not meet current recommended stability criteria for several loading conditions, had inadequate spillway capacity for the required design storm, and exhibited multiple structural and hydraulic deficiencies. An evaluation of alternatives by Schnabel Engineering recommended replacing the dam with a new straight concrete gravity dam to be built immediately downstream of the existing structure.
The project included an economical approach to the proposed spillway and stilling basin design, which resulted in considerable savings on the project. A two stage ogee crested weir was used to replicate the hydraulics of the existing spillway. The use of an ogee crest resulted in a reduction in the overall weir length from 240 feet to 200 feet. To provide energy dissipation for routine storms when there is little to no tailwater, a stepped downstream face was used. For the ½ PMP spillway design storm, tailwater submerges the spillway crest and energy dissipation was less of a concern. For intermediate storms, an incremental approach was used to size the stilling basin for various discharges at multiple locations across the spillway. Ultimately, the 500-year storm event was used to size the stilling basin.
The rock in the vicinity of the proposed spillway stilling basin encountered during our subsurface investigations was consistently hard, durable, and of high quality. Given these conditions, it was considered prudent to evaluate if the existing rock would provide sufficient resistance to scour such that a concrete stilling basin would not be needed.
An evaluation of the rock scour was performed using the Erodibility Index Method (Annandale 1995; 2007). The analysis entailed quantifying the erosive capacity of the flowing water and the erosion resistance of the rock. The rock is deemed to scour if the erosive capacity of the flowing water exceed the erosion resistance of the rock. A unique feature of the analysis entailed quantifying the erosive capacity of the water flowing over a stepped spillway. The analysis results indicated that the rock will adequately resist the erosive capacity of flows over the stepped spillway and that the stilling basin would not require additional protection. A concrete stilling basin was therefore not required.
This unique approach to spillway and stilling basin design resulted in a net savings when compared with a traditional concrete stilling basin. The paper discusses the overall hydraulic design methodology and the evaluation of rock scour to justify an exposed bedrock stilling basin. 13 pp. 7 references.