A spillway capacity study revealed that the Bond Falls reservoir, constructed in the 1930s, had insufficient capacity to pass probable maximum flood (PMF) flows. The reservoir was drawn down to lower the risk of flooding in event the dam’s PMF level was exceeded during a storm. To meet Federal Energy Regulatory Commission (FERC) dam safety criteria, the dam needed a new spillway sized to pass a PMF event.
Barr developed a detailed design based on a physical scale-model study configuration tested by St. Anthony Falls Laboratory. The design consisted of two new gates, wider and higher than the original single gate. A new minimum flow pipe was also designed integral with the new concrete spillway structure to provide minimum discharge flows to the downstream river channel.
Our design included a staged cofferdam to minimize reservoir drawdown and provide a dry, stable work area for demolition and construction activities. Barr developed a water management strategy to stage the cofferdam sequencing to accommodate construction and maintain continuous minimum flows to the downstream river channel throughout construction.
The new spillway consists of the following features:
Two tainter-gate bays, each larger than the original bay
Replacement of the 35-foot-high earth embankment and steel sheet-pile seepage-cutoff walls
Remote tainter-gate monitoring and operation
An emergency generator in the control building, capable of powering one gate hoist
A new minimum-flow structure to provide flexibility in flow discharge location
Monitoring equipment to accurately measure discharge flows and reservoir levels
Raised embankment crests to address low points on the main and perimeter dam locations
Barr’s work also included documenting a mini–potential-failure-mode analysis (PFMA) conducted to identify potential failure modes, fatal flaws, and critical design issues to be addressed during development of a final design. The mini-PFMA served to supplement a previous PFMA, adding failure modes applicable during construction of the replacement spillway.
The completed Bond Falls spillway can now accommodate twice as much discharge as the preconstruction configuration. Barr also developed a commissioning and refill plan to foster safe, controlled startup and fill conditions.