In a race to fix the damaged Oroville Dam’s main spillway by November, the California Dept. of Water Resources, the operator of the country’s tallest dam, is going to bid with a 65%-complete design that breaks recovery efforts into three parts, with an ultimate goal of doubling the main spillway’s release capacity to 270,000 cu ft per second.

Four contractors—Granite Construction, Barnard Construction Co., Kiewit and ASI Constructors Inc.—have until April 12 to bid; awards will be announced on April 17, with work starting in May. DWR declined to provide cost estimates.

Unless the gated spillway is operational by November, “a very significant risk would be incurred,” warned an independent board of experts for the Division of Dam Safety and Inspections.

To accommodate increased flows, a more robust design calls for removal and replacement of the upper main-spillway chute’s intact portion. 

The lower spillway’s damaged structure will be replaced using roller compacted concrete (RCC) for faster installation.

Adjacent eroded slopes will be reinforced. Any work not completed will be buttressed and completed the following year.

The higher main spillway’s flow capacity is intended to eliminate emergency spillway use, which, in February, eroded the unreinforced hillside for the first time in the dam’s almost 50-year history and led to the evacuation of almost 200,000 people.

As a safety measure, DWR plans to insert a cutoff wall in the emergency spillway to prevent “head-cutting” erosion before the fall wet weather begins. Other improvements that could be completed after Nov. 1 include buttressing the emergency spillway and placing an RCC apron downstream of the weir to prevent bedrock erosion.