Highland Falls, New York, which is 40 miles north of Manhattan, stretches along the Hudson River and is populated by many lakes and ponds, including the Cragston Lakes (a.k.a. Lower Cragston). For the community’s 4,000 residents, living in an area where water is abundant has many benefits, but the benefits are not without flood risk.
The 9-acre Lower Cragston Lake, the second largest lake in the Highland Falls area, contains the Lower Cragston Dam, which is owned by the United States Military Academy at West Point and managed through the U.S. Army Corps of Engineers New York District (USACE NYD). According to the Office of the New York State Comptroller, Lower Cragston Dam is classified as a “High Hazard” dam. The dam is approximately 10 feet high and 210 feet long, and consists of an earthen embankment with a concrete core wall, a concrete ogee spillway, and a low level outlet.
In order to ensure safety to the surrounding community and mitigate any potential flood risk associated with the dam’s operations, Princeton Hydro was contracted by the USACE NYD to perform an Engineering Assessment for Lower Cragston Dam. Engineering Assessments and periodic safety inspections are intended to provide an independent review of an existing dam structure to ensure that all components are functioning properly and in compliance with current dam safety regulations.
Princeton Hydro utilized a multidisciplinary approach to perform the Lower Cragston Dam Engineering Assessment, which consisted of:
- Document Review: In order to understand the site and to develop a proper drilling scope and methodology, our team conducted a thorough review of existing documentation, including historic engineering plans, dam inspection reports, and an Emergency Action Plan.
- Geotechnical and Geophysical Investigation and Reporting: This is one of the most significant aspects of a dam safety evaluation and is often the most efficient means of obtaining critical subsurface information. The information obtained from these field studies is used to devise safety improvements if determined to be necessary.
- Bathymetric and Topographic Survey: The bathymetric survey entails the accurate mapping of water depths and the quantification of the amount of accumulated, unconsolidated sediment. The topographic survey looks at the height, depth, size, and location of the dam and surrounding area.
- Hydrologic & Hydraulic Analysis: This analysis looks at the watershed and spillway structure related to the extent of potential flooding from storm recurrence intervals within the study area. The data helps to evaluate measures that can reduce and mitigate existing and anticipated flood risk.
- Structural Analysis: Our team utilized various methods, to assess the structural integrity of the dam and to evaluate the internal stresses and stability under usual, unusual, and extreme loading combinations.
- Seepage & Stability Analysis: Seepage through an earthen dam generally correlates with the reservoir water level of the dam. A careful analysis helps to detect any abnormal seepage issues and associated consequences.
- Dam Break Analysis: This type of analysis is used to estimate the potential hazards associated with a failure of the dam structure and features.
The geotechnical investigation for the Lower Cragston Dam Engineering Assessment involved performing soil borings and rock coring within the dam embankment, for which Princeton Hydro developed a Drilling Program Plan (DPP) to ensure the activities were performed successfully and safely. The DPP, which also required our team to have a comprehensive understanding of bedrock and surficial geologic formations in the area, was ultimately approved by the USACE Dam Safety Officer and successfully executed in the field. The collected samples were tested at Princeton Hydro’s AASHTO accredited and USACE validated soil laboratory.
Ultimately, the geotechnical investigation and subsequent soil analysis were used to inform the slope stability and seepage analysis. The geotechnical analyses, hydrologic & hydraulic study, structural inspection, bathymetry, and dam break analysis were used to provide USACE and West Point with recommendations for repair options, replacement options, and decommissioning options for the dam.
Engineering Assessments are vital to the longevity of dams and the safety of the communities they protect. By providing detailed analysis, effective repair, and management programs can be designed and implemented efficiently. This helps to ensure dam systems are providing the level of protection they were designed to deliver.
Princeton Hydro has designed, permitted, and overseen the reconstruction, repair, and removal of dozens of small and large dams. Our Geoscience and Water Resources Engineering teams perform dam inspections and conduct dam feasibility studies throughout the Northeast. For more info, visit: bit.ly/PHEngineering.