The Department of the Army, U.S. Army Corps of Engineers (USACE), Northwestern Division, Omaha District has a requirement for Garrison Dam Safety Modification Study.

• The project consists of a zoned earthfill dam, outlet works, spillway, and reservoir. The main embankment generally runs east-west and is approximately 12,000 feet long with a maximum height of 210 feet. The dam has a crest width of 60 feet with a design crest elevation of 1,876.2 feet NAVD88. The outlet works includes an approach channel, intake structure, powerhouse with five power tunnels, three regulating (flood-control) tunnels, a regulating tunnel stilling basin, and a tailrace discharge channel.  The crest and spillway bridge occupy ND State Highway 200 which is a major highway through central North Dakota.
• The approximately 3,000-foot-long spillway is excavated in the left abutment with a design crest elevation of 1,826.2 feet NAVD88. The chute-type spillway structure consists of a crest structure (including Tainter gates and a bridge), paved chute channel, and a stilling basin. The chute transitions from a width of 1,336 feet at the crest to 800 feet wide in the upper half of the spillway and maintains the 800 feet width through the lower half of the chute and stilling basin. The updated Probable Maximum Flood (PMF) in 2020 resulted in decreased freeboard at the dam from 13.8 to 1.4 feet and increased spillway peak discharges from the original design discharge of 827,000 cubic feet per second (cfs) to 1,150,000 cfs.
• The spillway underdrain system consists of a herringbone pattern of nine lateral vitrified clay pipe drains connected to three longitudinal drains situated under the frost blanket (drainage material) beneath the reinforced concrete spillway chute slab. There are 27 manholes in the chute at the intersections of the lateral and longitudinal drains.
• The Recommended Plan approved in the DSMS for PED consists of:
  • A full replacement of the spillway drainage system through selective demolition of the existing chute slab at designated new lateral drain locations.
  • A reinforced concrete overlay throughout the entire chute and stilling basin.
  • Armoring the area behind the chute walls.
  • Raising the spillway abutment monoliths.
  • Constructing a deflector beam to deflect the overflow nappe from gates and modifying the gate trunnion hubs.
  • Rehabilitation of the spillway gate deicing system and additional intervention measures to facilitate gate deicing.
  • An additional line of drains on the west abutment above the powerhouse.