Geotechnical and Civil Engineering: Historical Dam Compliance Assistance

Geotechnical and Civil Engineering: Historical Dam Compliance Assistance

Why GeoInsight

While the Castle In The Clouds (CITC) facility and setting is among New Hampshire’s most beautiful, the operations are costly and must address maintenance of infrastructure and compliance with regulations. As a non-profit, CITC needs to make very careful use of their limited funds. GeoInsight was brought in to provide engineering evaluation and design assistance for the dam at the property and help CITC decision makers enable a more prioritized approach of expenditures related to the dam.

Achievements

Multi-service approach: The interaction of dams with the water they hold back and the land on which they are built is more complicated than most people think. GeoInsight was able to provide CITC with all the technical support required to better plan their expected funding needs for the dam. Our team working with CITC brings full service civil engineering, hydraulic and hydrologic modeling, geotechnical engineering, and construction engineering to the project, resulting in a holistic view that ensures cost efficiency.

Beginning with the end in mind: The dam is classified as a Significant Hazard category and is an earthen embankment 600 feet long and up to 30 feet tall. It Is part of the attractions that draw visitors to the CITC each year. The first major assessment we performed for CITC was to evaluate whether the dam, which was originally constructed in 1913, should remain as part of the facility or be removed. This decision involved GeoInsight preparing a comprehensive Feasibility Study to compare tangible and intangible costs and benefits associated with making needed improvements to the dam and its operations to comply with applicable regulations, or whether the dam should be removed to eliminate future liability and costs. Based upon the findings of the study, the CITC Board of Directors decided to keep the dam and invest in its preservation.

Client: Castle In The Clouds
Location: Moultonborough, NH
Services: Engineering
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Scope of Services

  • Comprehensive Dam Evaluation
  • Emergency Action Plan Preparation
  • Geotechnical Stability and Seepage Analyses
  • Hydrologic and Hydraulic Flow Modeling
  • Trash Rack Design for Normal Outlet

Project Details

While the dam was originally constructed in approximately 1913, it was reconfigured in 1949 as part of the redevelopment of parts of the property. The pond impounded by the dam contains trout and is one of the attractions at the property for tourists. The impounded water is not otherwise used by the property, nor is the dam actively operated in a manner to manage flooding. The top of the embankment is approximately 10 to 12 feet wide and serves as a vehicle access road across the top of the dam and a pedestrian access for numerous hiking paths, including the famous Brook Walk trail with multiple waterfalls.

The dam retains 15-acre feet of water at the normal pool elevation. The pond’s primary outlet consists of a 60-inch diameter drop inlet spillway connected to a 48-inch diameter corrugated metal pipe (CMP) that discharges at the southwest downstream toe of the dam. Catastrophic and partial failures have occurred at the dam in the past. In recent years, several storm events caused flows over the emergency spillway, resulting in significant erosion of the woods downstream of the dam.

GeoInsight has attended meetings and given presentations to the CITC Board of Directors, attended meetings the NHDES evaluated seismic risk relative to the dam for various conditions, studied potential flood path limits for various flow conditions, addressed toe seepage and monitoring, and evaluated and recommended solutions for localized voids in a portion of the downstream face. We also designed a new emergency spillway to safely accommodate the design flow event without losing the use of the access road.