USACE begun the annual draw down of Fern Ridge Dam three weeks ahead of schedule due to the high amount of sediment coming out of the dam's drains. Officials are concerned because sediment levels draining out of the dam are more than ten-times higher than they were last month.
Daily amounts of sediment released from the drain known at Station 45 averaged six-tenths of a pound in August and September, compared with three-tenths in May, June and July. USACE believes the cause of the sediment leakage is due to damaged drainpipes installed more than 60 years ago when the dam was built.
Problems were first seen on the 60-year-old dam, when three depressions on the face of the dam were discovered. USACE is monitoring these depressions on the downstream slope of the dam on a daily basis. The depressions, discovered in July 2002 and February 2003, range in size from less than a meter in diameter to 4m. Testing conducted in 2003 had ruled out any instability problems that could cause the depressions at the dam.
In January this year, following inspection of the internal drain system with a remote video camera, USACE found the dam to be in a safe condition. Prior to the January inspections, engineers had proposed operating the reservoir at 0.8m below full pool due to internal erosion concerns.
Construction of the Fern Ridge dam was completed in 1942. The dam is an earthfill structure with a gated concrete spillway to regulate reservoir levels. It is about 7.2km long and about 13.7m high.
