Erosion of the dam’s main spillway

The gated main spillway of a dam became damaged following a period of heavy precipitation. At 230 m high, it is the tallest embankment dam in the United States. Its spillway capacity is 4,4 .10e9 m³. Its main gated spillway is located on the northwest shoulder of the dam. It controls the reservoir level by releasing large quantities of water via a chute (open concrete channel) that measures 930 m long and has a 200 m drop in height. It also has a 530 m wide concrete emergency spillway located alongside the main spillway. The open channel above the emergency spillway starts at 6 m below the overflow level of the dam’s main structure.

On 7 February 2017, the dam’s operator noticed an unusual flow pattern of 1540 m³/s in the main concrete spillway. The main spillway’s eight valves were closed. A crater was found halfway down the spillway. Its concrete lining was eroded through and water was escaping out of the spillway and into the chute.

Test flows were conducted over the next two days. The length of the crater increased from 76 to 91 m. Runoff created an erosion pit on the left bank. Confronted with the rising level of the reservoir, on 9 February the operator opened the main spillway’s valves, discharging water a rate of 2000 m³/s through it and releasing the excess via the emergency spillway. On 10 February 2017, the operator took necessary measures on the shoulder to allow the emergency spillway to be used. The power transmission lines were moved and the trees on the hillside were cleared. On 11 February, the lake level reached the emergency spillway’s overflow level for the first time since the dam was commissioned back in 1968.

Shortly after 8:00 a.m., the emergency spillway began carrying water at a rate of 360 m³/s. Although this flow rate was well below the design rate, erosion at the dam’s base quickly progressed. Damage at the base of the emergency spillway threatened to collapse it.

On 12 February the Butte County Sheriff’s Office issued a mandatory evacuation order. A total of 188,000 people living in the Feather River Basin were evacuated. A fish hatchery was relocated. The operator increased the flow rate in the main spillway to 2800 m³/s in order to lower the water level in the reservoir and protect the emergency spillway. At 9:00 p.m., the water level dropped to below the emergency spillway top. The downstream area was immediately inspected. The damage was considered considerable. On 13 February, helicopters began to drop sandbags and large boulders in the area of the emergency spillway to protect its base from erosion. The water released by the main spillway lowered the reservoir level but significantly eroded its adjacent edge. A total of 110,000 m³ of erosion debris clogged the channel of the Feather River downstream of the dam, preventing release of water from the hydroelectric plant. The plant was shut down, reducing the overall capacity of the dam to release water and resulting in operating losses. At 2:45 p.m. on 14 February, the reservoir level dropped enough to lift the evacuation order.

On 27 February, the flow to the spillway was temporarily shut off, allowing crews to begin removing debris in the river in preparation to restart the power plant. The damage to the spillway was considerable, with 200 m of it having been completely swept away. The erosion area caused by the open channel, which was very heavily marked, extended over a few hundred square metres.

Flaws in the spillway’s design were found:

• the minimum thickness of the concrete slabs was insufficient;
• the steel reinforcement of the base slabs was insufficient and continuous between slabs;
• lack of waterstop barriers between the slabs;
• ineffective ground anchors (stripping down to the rock not properly executed).

Cracks and joint displacements on the main spillway had been regularly repaired. Vegetation that had been allowed to grow adjacent to the spillway and its drainage pipes increased the spillway’s permeability. Massive floods in 1986 and 1997 may have contributed to the weakening of the dam.

The cost to repair the damage to the dam was estimated at €1.1 billion (nearly €1 million).