Mansfield Hollow Dam - Dike F Reservoir Report

Last Updated: December 6, 2025

Mansfield Hollow Dam - Dike F, also known as Mansfield Hollow Lake, is a gravity dam located in Willimantic, Connecticut, along the Nachaug River.


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Summary

Built in 1952 by the US Army Corps of Engineers, the dam stands at a height of 78 feet and serves the primary purpose of flood risk reduction. With a storage capacity of 76,000 acre-feet and a normal storage level of 2,800 acre-feet, the dam plays a crucial role in managing water levels and mitigating flood risks in the region.

The dam's spillway, with a width of 690 feet, is uncontrolled, allowing for the release of water to manage downstream flow and relieve pressure on the structure. With a high hazard potential, the dam undergoes regular inspections and has an emergency action plan in place to address any potential risks. The US Army Corps of Engineers actively manages the dam's flood risks by monitoring its condition, prioritizing maintenance activities, and collaborating with local emergency managers to ensure preparedness and response in the event of a dam-related emergency.

As a key infrastructure for flood control in the area, Mansfield Hollow Dam - Dike F plays a critical role in protecting surrounding communities from potential flooding events. With ongoing risk management measures in place, including regular maintenance and updates to the emergency action plan, the dam continues to be a vital asset in safeguarding the region's water resources and climate resilience.

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

Weather Forecast

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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