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Chinook Water Supply Dam Failure

Location: Pacific County, Town of Chinook, near the mouth of the Columbia River

Owner: Chinook Water District

Type of Dam: 26 foot high, earthfill embankment built over an original 15 feet high concrete parapet dam. Concrete overflow spillway, 8 feet wide and 4 feet deep over embankment crest with plywood chute down the downstream face.

Date of Failure: Thanksgiving Weekend, November 1990.

Cause of Failure: During heavy rains on Thanksgiving Weekend, a flood occurred that exceeded the capacity of the project's "customized" spillway. At some point in the project's life, the 4 feet deep spillway channel was modified by the construction of a 2.5 feet high, concrete weir. The crest of the weir was flat and some 5 feet long. The poor geometry of the modification, i.e., a broad crested weir as opposed to a sharp crested weir, further reduced what limited hydraulic capacity remained. In short, 60% of the available freeboard was lost in raising the spillway crest; while the hydraulic capacity of the spillway was reduced by some 70% from that of the original engineer's design. Consequently, much smaller floods could fail the dam. Here, overtopping eroded the embankment around the spillway, dropping the spillway section into the void and breaking it into segments.

Damage Caused: Luckily, the downstream hazard setting was low. A logging road embankment was the only improvement between the dam and an estuary of the Pacific Ocean. Therefore, damages were limited to the dam; portions of the water transmission system; a breach of the road embankment and loss of an associated culvert; and environmental damage caused by siltation of the stream. Of most concern to the Town of Chinook was the fact that they had lost their water supply. Fortunately, Chinook is such a small metropolis that alternative arrangements could be made to temporarily meet their water needs. Even so, some $100,000 was spent in providing temporary service and reconstructing the dam.

Lessons Learned: Periodic inspections are necessary to confirm that life-safety components of dams have not been inappropriately modified.

The failure of even small, low hazard dams can have serious consequences, if they serve some vital public need, i.e., water supply.