Case Study: Little York Lake Dam Replacement

Little York Lake Dam Replacement

Cortland County, NY (West Branch of the Tioughnioga River in Homer and Preble, NY)

 

Gomez and Sullivan Engineers was retained by Cortland County to design repair measures for the Little York Lake Dam. Little York Lake is located in Homer and Preble, NY on the West Branch of the Tioughnioga River. The Lake is approximately 11 acres in area and is impounded by the Little York Lake Dam. The Dam is comprised of two concrete structures used to retain the water in the pond; a 60‐foot± long overflow section and a 20‐foot± long gated section with two weir sections. To meet NYSDEC Guidelines, the proposed repair alternatives were required to more than double the hydraulic capacity of the existing dams. Gomez and Sullivan developed four conceptual alternatives for replacing the water retaining structures at Little York Lake. The conceptual alternative chosen (and advanced to final design) included replacement of the two existing structures with two concrete labyrinth spillways. This was considered the most attractive alternative based on the estimated construction cost. Additionally, this design allowed the proposed dam to pass the design flood without owner intervention (i.e. – gate operation or stop log removal).

Gomez and Sullivan’s responsibilities during the design included; hydrologic and hydraulic evaluation of existing and proposed conditions, development and oversight of a subsurface drilling program, hazard assessment/dambreak analysis and design flood evaluation, structural analysis of proposed design alternatives, design of sheet pile system beneath proposed concrete structures, development of an inspection and maintenance plan, development of construction cost estimates, preparation of permit applications and development of construction drawings, technical specification and bid documents for the chosen design alternative.

The hydraulic analyses performed in conjunction with this design considered the impacts of the culverts and bridges located immediately downstream on the expected hydraulic capacity of the proposed labyrinth structures. A 3‐dimensional computational fluid dynamics (CFD) model was developed to confirm the computed hydraulic capacity of the proposed labyrinth structures and evaluate alternative labyrinth layouts. A HEC‐RAS model simulation of each dam was developed to evaluate the hazard potential of both dams and to estimate an appropriate design flood.

Final design was completed in 2014. Construction was completed in 2015.