Little Bog Pond Dam and Lower Trio Pond Dam Alternative Analysis and Reconstruction
Deep in New Hampshire’s Nash Stream Forest, hikers and rugged vehicles could find two non-functional dams that originally supported the timber industry more than 100 years ago. The Little Bog Pond Dam and Lower Trio Pond Dam were simple timber crib and rockfill spillways with earthen embankments, useful engineering for their time but now out of date.
Over time, the dams’ spillway capacities became inadequate. The Little Bog Pond Dam had breached during a flood event in 2006 and was never fully repaired. This left the lake elevation approximately 2 feet lower than its historic pool elevation.
While classified as a low hazard safety risk, the dams’ age and design posed concerns for functionality, safety, and environmental resilience.
The state of New Hampshire’s Department of Environmental Services (NHDES) Dam Bureau contracted GEI for guidance to determine the best path forward for the dams. GEI’s task was to evaluate multiple alternatives to restore the dams’ performance, safety, and ecological function for the surrounding forest.
Our team performed a hydrologic and hydraulic analysis using HEC-HMS to calculate design flood inflows and HEC-RAS to evaluate different proposed spillway geometries. The team also used HEC-RAS to simulate dam failure scenarios and confirm the dams’ hazard classification. The team analyzed various types of spillway configurations, including a fixed crest concrete broad-crested weir, labyrinth, drop-inlet structure, and multiple concrete bays with stop-logs.
After careful analysis, GEI recommended NHDES remove and reconstruct both dams as modern-day concrete structures with fixed broad crested weirs. This approach would not only resolve spillway capacity issues but also modernize the structures for many decades.
GEI designed replacement dams featuring:
- Reinforced concrete stepped weir spillways
- Stoplog bays for flexible water level control and maintenance
- Modernized embankments to meet current dam safety requirements
- A pedestrian bridge at each dam across the concrete spillway
The team completed detailed design drawings, specifications, wetland delineations, site surveys, bathymetric surveys, and all necessary permitting—laying the groundwork for reconstruction.
As both dams near completion, GEI is continuing to provide construction inspection and support services to ensure the dams’ safe and sustainable renewal. Soon the dams will be ready to once again serve as functional structures and return healthy ponds to nearby communities and residents.
