Dam Safety

Dam Safety


Seminary Hill Reservoir - Massive landslide

View of Breached Section of Reservoir

Reservoir #3 was evacuated in 3 minutes. The landslide was likely due to seepage into the rock foundation through continued deterioration of concrete panel seams, and a 2 foot raise in the reservoir pool elevation after holding it down for the previous year.

Location:  Within City of Centralia, on slope of Seminary Hill


Owner:  City of Centralia

Project Description:  Seminary Hill Facility consisted of two water supply storage reservoirs. Reservoir #3, constructed in 1914, contained about 3.5 million gallons of water. Reservoir #4 was constructed in 1926, and contained about 5 million gallons. Both reservoirs were 20 feet deep with 1H:1V side slopes, and were constructed by cut and fill methods into the steep hillside. With the exception of the west side of the reservoirs, the impoundments lie below original natural ground. The embankments forming the west side of the reservoirs were constructed by pushing excavation spoils out onto the hill slope. The reservoirs were lined with unreinforced concrete, with copper strips at panel seams to minimize leakage. An underdrain system was provided beneath the reservoir floors to collect leakage. The reservoirs had repeated problems with leakage over the years, and a number of different repair schemes were implemented over the life of the project to minimize leakage through the panel seals. 

Date of Failure:  October 05, 1991

Cause of Failure:  The failure occurred without warning. Reportedly, Reservoir #3 was evacuated in 3 minutes, with a peak channel discharge of 2500 cfs. Reservoir #3 failed first, causing the break of the service and drain lines for Reservoir #4. The 5 million gallons contained in Reservoir #4 drained for several hours after the failure. The failure was caused by a massive landslide in the siltstone rock formation that underlies the reservoirs. The cause of the landslide was likely due to increased seepage into the rock foundation through continued deterioration of concrete panel seams, and a 2 foot raise in the reservoir pool elevation after holding it down for the previous year.

Interestingly, the reservoirs had been drained and cleaned in August, and apparently no unusual conditions were noted. However, the cleaning crews did not document their inspections of the seams. Further, the city did not routinely inspect the underdrain outfall (which was in a deep manhole) to see if seepage was increasing or decreasing. Judging from the portion of the underdrain pipe exposed by the failure, the underdrains may have been clogged with fines, significantly restricting their ability to discharge seepage. This, in turn, may have led to increased seepage into the siltstone foundation.

Damage Caused:  The reservoirs were located on a steep hill above the city of Centralia. The flood first passed through an uninhabited park in a ravine then discharged into a crowded neighborhood, causing appreciable damage. Two homes were knocked off their foundations and destroyed. Several other homes were severely damaged by the water and mud flow. Many other homes had silt and mud deposited in their yards. Amazingly, no lives were lost in this failure. This may be attributed to the fact that the failure occurred at 10:15 a.m. on a Sunday morning, when few people were outside. A Boy Scout Troop was clearing debris in the ravine at the time of the failure, but fortunately were able to scramble out of the way of the flood wave.

Financial damages from the failure were significant, however, as damages to the neighborhood totaled over 3 million dollars. In addition, the City lost much of its online storage for several months, and replacement of the reservoirs with tanks was necessary.

Lessons Learned:  Inspection of seams and cracks in concrete lined reservoirs is critical. Unfortunately, maintenance crews are usually focused on cleaning, and only performed a superficial inspection of seams and cracks, finding only large-scale problems. All seams and cracks need to be examined closely by an engineer (including measurements), and the results of the inspection need to be thoroughly documented.

At Centralia, underdrain flow into the manhole was not measured or monitored. Diligent monitoring may have indicated decreased seepage and fines in the underdrains, thus increased seepage into the foundation.

Management's goal for the operation of the reservoirs was geared toward the product - water supply. Leakage was only addressed when it got large enough to affect the delivery of water. Minimal emphasis was placed on structural integrity and overall project safety.

A dam failure can occur when it is least expected. This failure occurred during a record fall drought in the state. The last thing people living below this reservoir were thinking of was a flood.

Just because a project has been in operation for 78 years without major problems doesn't mean that it can't fail. In fact, the project's advanced age may have contributed to its failure.


View of Breached Section of Reservoir (Looking Downstream)

Foundation Soils Composed of Fractured Sandstone With Underdrain Pipe Exposed

Aerial View of Flooded Residential Area

House Hit by Dam Break Flood, Note Mud Line on Front Window and Side of House

More Homes Hit by Dam Break Flood