Outlet Channel Repairs - Milford Dam, Kansas

This paper presents an account of the repair design and construction of the outlet channel at Milford Dam damaged during the flood of 1993. Milford Dam was constructed and is operated by the Kansas City District, U.S. Army Corps of Engineers. The dam, which was completed in 1965, is located 4 miles northwest of Junction City, Kansas, on the Republican River. The dam consists of a zoned rolled earth and rock fill embankment, 6300 feet in length and a maximum height of: 147 feet, with an uncontrolled, fixed crest service spillway, and an outlet works comprised of an intake tower and a gated single 21 foot horseshoe conduit. Due to a sand foundation, underseepage control features, relief wells and an upstream impervious blanket are essential parts of the project.

During 1993, Milford Lake experienced record flooding that required large flood releases through the outlet works and several days of uncontrolled flows through the spillway. The outlet works was opened to maximum discharge (22,500 cfs) as the reservoir approached top of flood control pool elevation, and within a few hours, severe erosion had occurred on the left bank immediately downstream of the stilling basin and on the right bank several hundred feet downstream. Outlet releases had to be decreased, since continued erosion on the left bank could have provided an uncontrolled exit point for underseepage, possibly resulting in a piping situation. The District initiated an emergency repair and restricted releases to continue evacuating the flood pool.

The extensive erosion was determined to be caused by long term, downstream channel degradation which resulted in very high flow velocities immediately downstream of the stilling basin. Due to the unsymmetrical nature of the channel, a model study was conducted by Waterways Experiment Station, Corps of Engineers. The model indicated a skewed alignment of the channel and the lowering of channel would provide acceptable flow conditions for a full range of discharges including the maximum design flow. The model study also indicated that the tailwater and hydraulic channel control would need to be stabilized or else the future performance of the stilling basin would be compromised. Plans were developed for the repair based on the model test results, and a contract was awarded in October of 1996. Repairs consisted of reconfiguring the outlet channel, armoring banks, and constructing a channel stabilization structure. Excavation of the channel downstream of the dam was considered a risk to the integrity of the dam, and it was deemed critical that groundwater flows be properly controlled to prevent seepage and sloughing from occurring along the channel excavation. Plans required a dewatering system for predrainage to allow the excavation and construction of the channel slopes along the left bank. The work was completed by May of 1997, and a test release was made shortly after construction that indicated acceptable performance of the channel repair.