Investigating The Impact of Approach Channel Sediment Removal On Spillway Performance

Garrison Dam near Riverdale, North Dakota on the Missouri River is the largest dam in the US Army Corps of Engineers (USACE) inventory, impounding over 23 million acre-feet of water at the top of exclusive flood control zone. The spillway is designed to pass over 800,000 cfs during the Spillway Design Flood. Over the past several decades, continued erosion of the left bank of the spillway approach channel has resulted in a significant deposition of material in the spillway approach channel, averaging 9 feet, reducing approach depth from 15 feet to 6 feet. According to research published by the US Bureau of Reclamation (USBR), the reduced approach depth would reduce the efficiency of the spillway crest by nearly 7% at the design head with gates fully open. The same research suggests that spillway efficiency would be reduced by roughly 20% if the approach were to fill with sediment to the height of the ogee weir. The research applies only to flows at the design head; it does not address flow at lesser heads, nor does it discuss the impact on flow with gates controlling the flow. Hydraulic model studies done during design demonstrated that less than ideal approach conditions resulted in overtopping of the downstream chute walls and poor energy dissipation in the stilling basin. Funding has been obtained to procure riprap to stabilize the approach channel right bank as well as dredge material from the spillway approach channel. A 2D HEC-RAS model and a sectional CFD OpenFOAM model are being developed to assess the impact that varying degrees of sediment removal may have on spillway efficiency and performance. Pressures along the ogee crest of the spillway from physical model testing from 1956 were compared to the OpenFOAM results to verify the ogee crest weir pressure profile closely matched the physical modeling. Preliminary results from the 2D model suggest a 34% increase in flow if all the sediment were removed from the spillway approach channel, assuming sediment had increased to the level of the spillway crest. We will investigate the impacts that varying degrees of sediment removal may have on spillway efficiency and performance with both the 2D and 3D model, ensuring that the limited funding available for dredging provides the greatest increase in efficiency and performance that can be attained.