Turbid Discharge Incident At Cannonsville Dam

On July 8, 2015, the New York City Dept. of Environmental Protection (DEP) discovered a turbid discharge at the toe of Cannonsville Dam, a 2,800 ft. long, 175 ft. max. height, earthen dam constructed from 1955-1965. The dam impounds the W. Branch Delaware River, has a storage capacity of 96.7 billion gallons and is an important component of the NYC water supply system which supplies 1.1 billion gallons daily to 9 million people (Figures 1 and 2). Reservoir was at full pool level.

The turbid discharge developed during test borings for foundation design of a proposed hydro- electric plant at the toe of the dam under a license issued by the Federal Energy Regulatory Commission (FERC). Turbid discharge resulted from open bore holes penetrating a confining geologic layer into an artesian formation in the dam foundation. DEP notified FERC and NY State Dam Safety officials and responded immediately to the potential dam safety incident. Actions included cessation of drilling at the dam, around the clock monitoring on site including parameters such as potential settlement, contingency planning in case the situation worsened, reservoir drawdown from full pool, public notification and emergency contracting for engineering consultant and construction contractor support.

DEP assembled a team of engineering consultants and additional recognized experts and received active assistance from FERC. The team used a collaborative process to develop, decide upon and implement a rapid mitigation strategy consisting of pressure relief wells and a repair strategy to permanently stop the turbid discharge. Seven pressure relief wells with deep well pumps were installed by the end of July to eliminate or neutralize upward flow from the artesian zone. The turbid discharge stopped by August 1, 2015, at which time DEP ceased active reservoir drawdown (up to 15 feet). Permanent closure of the open boreholes were successfully completed using compaction grouting methods, and the relief well pumps were turned off early in the morning of August 28, 2015. As of May 16, 2016, the reservoir returned to normal pool and the repairs continue to hold with no further turbid discharge.

This paper reviews the dam owner's response; monitoring and early warning mechanisms; efforts to warn and inform the public; the collaborative approach to analyze the situation and design mitigation and repairs; repair and test methods; the monitoring plan; and lessons learned. The following questions are explored: How can dam owners assure that prudent research and contractor preparations are taken before subsurface exploration on or around dams? What actions can dam owners take before an incident occurs that will assist them to respond and recover from an incident should one occur? What actions can dam owners take during an incident response and recovery that will help them achieve an acceptable solution?