AESI provided engineering and surveying services for the Narrows Pond Dam Reconstruction project located in the Beaverhead-Deerlodge National Forest at over 6,800 feet in elevation. Montana FWP and the Madison Ranger District proposed reconstructing the Narrows Pond Dam to provide additional storage to allow supplement of flows into Narrows Creek and Spring Creek. The existing dam is a small earthen structure with sparse vegetation, approximately 7-feet tall, and assumed to be built with unconsolidated materials. The project goal was to remove the existing dam and construct an engineered structure to increase dam stability, lower risk of future dam failure, and ensure enough water is impounded annually to support native species reproduction in downstream Narrows and Spring Creeks.
After conducting a site visit and geotechnical explorations, AESI developed a detailed reconstruction design, plans, and specifications. The reconstruction design included the following elements:
- 5H:1V rock-lined downstream embankment slope designed to overtop during the Inflow Design Flood.
- Low-level outlet works consisting of 18-inch reinforced concrete pipe and a 5-foot diameter concrete outlet tower with an integral overflow weir and slide gate to control the pool elevation.
- Rock-lined auxiliary spillway to handle more frequent runoff events.
- Pre-cast concrete flow augmentation system diversion box at the toe of the dam to direct water to the existing flow augmentation system 10-inch pipe and/or to Narrows Creek.
- 3H:1V upstream embankment slope with rock riprap erosion and wave protection.
- Embankment core built from select on-site recompacted soils from the existing embankment and supplemented from a borrow source on the upper end of Narrows Pond.
A dam breach analysis was also performed to estimate the flood inundation extents that would result from a failure of the reconstructed dam. The analysis was completed with the computer program HEC-RAS version 5.0.3.