CS10A - Concurrent Session 10A: Narrows Bypass Channel CFD Analysis – Modeling Challenges for Large CFD Models with Large Flows and Interpretation of Their 3-D Results

The Narrows Dam is located on the Yadkin River in Badin, North Carolina. The dam consists of a main
dam section and a bypass spillway section. The dam’s bypass spillway structure is located on the left side of the dam when looking downstream and consists of ten (10) Stoney gates. Each gate is 28 feet high and 35 feet wide. The bypass spillway structure is approximately 430 feet long and has a concrete crest elevation of 480.84 feet NAVD 1988 (513.0 Yadkin datum). The bypass spillway chute (channel) is approximately 1,200 feet long and approximately 103 feet wide at its narrowest point. The Probable Maximum Flood (PMF) for Narrows Dam is 414,500 cfs, which includes the capacity of the bypass spillway (about 242,455 cfs) and main dam (about 172,045 cfs). The reservoir water surface elevation during the PMF event is 517.64 feet NAVD 1988 (549.8 feet Yadkin datum). The normal pool level is 508.94 feet NAVD 1988 (541.1 Yadkin datum).

Following a high flow event occurred between November 13-15th in 2020, that the bypass channel experienced about 118,000 cfs, operations staff performed an inspection in accordance with the Narrows Dam Safety Surveillance and Monitoring Plan and noticed a few rocks on top of the east wall of the bypass chute located at the downstream end of the rock backfill section. Recognizing the need to investigate further to ensure the structural integrity, CFD modeling was proposed to reevaluate the capacity of the spillway and evaluate the turbulent dynamic pressure forces on the spillway chute training walls and highly turbulent plunging pool.

The discussion highlights the essential steps needed to develop a complex skewed spillway chute geometry with a large vertical elevation drop between upstream and downstream. The model is further complicated by the necessity to model the full domain with large discharge capacity, gate operational sequence, and high speed thin sheet flows in the chute and associated flow splashing due to bends. A major part of the modeling effort was calibrating the CFD model against a physical model and identifying the model sensitivity to externally applied Manning’s Roughness values. The discussion also explains the challenges faced during the model development and how they were resolved using efficient CFD modeling and 3-D meshing techniques using the FLOW-3D software and its numerical tools.