Mansfield Hollow Dam - Dike A And B Reservoir Report

Last Updated: December 6, 2025

Mansfield Hollow Dam - Dike A and B, also known as Mansfield Hollow Lake, is a federal-owned structure located in Willimantic, Connecticut.


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Summary

Constructed in 1952 by the US Army Corps of Engineers, this gravity dam stands at a height of 78 feet and has a hydraulic height of 74 feet. With a dam volume of 1,458,000 cubic feet and a storage capacity of 76,000 acre-feet, it primarily serves the purpose of flood risk reduction along the Nachaug River.

Managed by the US Army Corps of Engineers, Mansfield Hollow Dam plays a crucial role in mitigating flood risks in the region by monitoring its structural health, conducting regular inspections, and engaging with local emergency managers to ensure preparedness in case of dam-related emergencies. The dam has a high hazard potential and a moderate risk assessment level, highlighting the importance of ongoing risk management measures to safeguard the surrounding communities and infrastructure. While the dam is equipped with an uncontrolled spillway and emergency action plan, efforts are continuously made to address potential flood risks and enhance public safety.

Despite its critical role in flood control, Mansfield Hollow Dam is not immune to the unpredictable nature of severe weather events and the inherent limitations of water storage capacity. As such, the US Army Corps of Engineers remains vigilant in its risk management approach, constantly working to address evolving threats and ensure the structural integrity of the dam. By collaborating with local stakeholders, maintaining emergency preparedness, and conducting necessary maintenance and repairs, the USACE strives to uphold the safety and resilience of Mansfield Hollow Dam for the benefit of the surrounding communities and environment.

Year Completed

1952

Dam Length

14050

Dam Height

78

River Or Stream

NACHAUG RIVER

Primary Dam Type

Gravity

Surface Area

450

Hydraulic Height

74

Drainage Area

159

Nid Storage

76000

Structural Height

78

Hazard Potential

High

Nid Height

78
       
Seasonal Comparison

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Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.
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