Dam Safety

Dam Safety


Iowa Beef Processors Waste Lagoon - Erosive Breach

Derailed Train from Failure of Downstream Railroad Fill

Location: At Wallula in Walla Walla County, 12 miles SE of Pasco.

Owners: IBP Inc. and Simplot

Date of Failure: January 25, 1993

Project Description: The IBP Waste Pond was constructed in 1971, and was used for the storage of wastewater from the IBP Beef Processing Plant near Wallula. When full, the pond had a surface area of 37 acres and a maximum storage capacity of 270 acre-feet. The pond was located on a natural drainage course, and was impounded behind a 15 foot high, 1000 foot long earthfill dam.

In 1981, the dam was inspected as part of the National Dam Safety Program. The resulting "Phase 1 Inspection Report" concluded that the facility did not have an overflow spillway, and could thus not handle any floods from the 55 square mile drainage basin above the dam. Further, the embankment stability was considered questionable, and the dam was riddled with animal burrows.

In 1985, in response to a request by Ecology, IBP hired a consultant to perform a geotechnical investigation of the dam. The consultant found that the embankment stability was adequate, provided seepage remains at low levels within the embankment. The consultant also cautioned that animal burrows in the embankment were a problem, and could initiate a piping failure if the intercepted the seepage line within the dam. The consultant recommended that the burrows be filled and the animals be removed from the site.

Cause of Failure: According to newspaper accounts of the incident, the failure of the dam occurred sometime between 11:19 PM on the 24th and 3:40 am on the 25th, when a Union Pacific freight train derailed on a washed out section of track downstream from the dam. The breached section had a width of 60 feet and a depth of 19 feet below the dam crest. The cause of the failure was attributed to 3 events:

  1. The facility did not have an emergency spillway to pass flood flow out of the facility.
  2. The rapid melting of a record snowpack, coupled with higher than normal pond levels, filled the waste pond and slightly overtopped the west end of the dam. Also, the high pool level allowed water to flow into animal burrows that exited above the normal high water level on the upstream face.
  3. The dam most likely failed directly from uncontrolled seepage through the animal burrows, exiting on the downstream face and causing erosion. Evidence of similar erosion below animal burrows was found at the west end of the dam, and burrows were noted in the sides of the breach. The silty soil of the embankment was highly susceptible to erosion, and it is likely that the erosion backcut rapidly toward the upstream face, eventually breaching the dam.
Damage Caused: The dam breach flood passed down the natural drainage channel below the dam, washing out the dikes on two holding ponds owned by Boise Cascade located 1000 and 2000 feet below the IBP dam. The flood was then impounded behind the Union Pacific RR fill located about 1 mile downstream from the dam. The floodwater flowed over the south end of the fill, significantly eroding the downstream face. Sometime after the overtopping occurred, the railroad fill failed at the location of a culvert, about 700 feet north of the overtopped area. A northbound freight train derailed at the location of the overtopping where the fill had been washed from one side of the tracks. All 5 locomotives went off the tracks and into the flood waters, injuring the 3 crewmen.

The estimated cost of this failure was $5,000,000, which included the cost of the locomotives, environmental cleanup, and repair of the rail line. The cost for IBP to construct a new waste facility was several million more dollars.

Lessons Learned: Maintenance of animal burrows on embankment dams is critical. Burrows should be backfilled in as timely a manner as practicable, especially those near the dam crest. If at all possible, the animals should be removed from the area.

The owner claimed they inspected the dam weekly, but this only involved someone driving over the dam crest. Inspections must be performed regularly and thoroughly by knowledgeable representatives of the owner. If deficiencies are found, they need to be corrected promptly.

As in many other cases, from an operational standpoint, the pond was solely considered a place to put wastewater until spray irrigation could be resumed in the spring. Little thought was given to safe levels of operation to maintain the structural stability. Thus, the pond was allowed to fill to unsafe levels prior to the snowmelt event which precipitated the failure.

The siting of a the wastewater pond without a spillway across a natural drainage course with a 55 mi2 tributary area was a poor design. Apparently the original designers were primarily concerned about preventing surface water releases from the pond, given the nature of the reservoir contents. In defense of the original designers, the basin typically produces only a nominal amount of runoff. In the case of heavy rains on frozen ground, the runoff was sufficient to overwhelm the effective freeboard on the pond. Thus, where practicable, it is desirable to construct the impoundment off-channel where there is a water quality issue with the release of overflows.


Aerial View of Failed Structure - The white areas are snow. The dark area in the lower part of the picture is the empty reservoir. The dark swath in the upper middle right of the picture is the path of the dam break flood

Repairs Being Made to Failed Railroad Fill