The Longmont Wtp Forebay Embankment, also known as the Nelson Flanders Forebay, is a crucial water resource infrastructure located in Boulder County, Colorado. Owned by the local government, this embankment serves the primary purpose of water supply and was completed in 2005 by Lyman Henn Inc. The structure is designed as an earth dam with a height of 20 feet and a hydraulic height of 19 feet, providing a storage capacity of 129 acre-feet with a maximum discharge of 369 cubic feet per second.

Situated along the St. Vrain Creek, the Longmont Wtp Forebay Embankment is state-regulated and inspected regularly by the Colorado Department of Water Resources to ensure its safety and functionality. With a moderate risk assessment rating and a high hazard potential, it is crucial for water resource and climate enthusiasts to monitor and maintain this critical infrastructure. The embankment has a satisfactory condition assessment as of June 2020, but emergency preparedness measures, such as an Emergency Action Plan, may need updating to meet current guidelines.

In the event of a breach or failure, the embankment has an uncontrolled spillway with a width of 28 feet to manage excess water flow. With its strategic location and vital role in water supply for the city of Hygiene, the Longmont Wtp Forebay Embankment serves as a significant asset in the management and distribution of water resources in the region.