Utilizing dam freeboard for additional storage at Blunn Dam/Arvada Reservoir

Abstract Only - Blunn Dam is at an existing 25-year-old zoned earthfill dam that impounds Arvada Reservoir, an integral water storage facility for the City of Arvada, Colorado. The Blunn Dam Arvada Reservoir Enlargement Project was designed by GEI Consultants, Inc. (GEI) to provide Arvada with an additional 5 feet of storage in the reservoir (1,000 ac-ft) without raising the existing 73-foot-high high-hazard dam. Construction of the modifications began in October, 2004 and was completed by May, 2005.
To facilitate raising the normal reservoir pool level, modifications to the existing ogee crest service and emergency fuse plug spillways were designed and constructed to pass a larger design flood than the original design without encroaching on the minimum 1-foot design flood freeboard. A labyrinth service spillway configuration was selected for the service spillway because of its hydraulic efficiency. Even though the new labyrinth crest elevation is set 5 feet higher than the old ogee crest, GEi's modifications allow routing the PMF, a larger design flood than was required during initial construction of the facility. An improved emergency spillway fuse plug design was used, permitting 5 feet of water to be stored against the fuse plug embankments at the normal maximum reservoir pool.
Design and construction of the project was completed with a minimal amount of environmental permitting because the dam footprint was not changed. As part of the emergency spillway reconfiguration design, GEI performed scour analyses of the earth emergency spillway discharge channel and found that operation of the emergency spillway, with or without the raised reservoir, could cause significant erosion that might threaten the dam. A 425-foot-long soil cement apron and cutoff was designed and constructed to address this issue. The walls of the existing service spillway chute were also raised to contain the additional flow during the PMF.