Liberty Wastewater Treatment Plant Protected by New Floodwall

Challenge: The City of Liberty owns and operates one wastewater treatment plant (WWTP) that was constructed in the mid-1990s. The WWTP provides treatment not only for the City of Liberty but also for the two neighboring cities of Ames (east of Liberty) and Hardin (north of Liberty). The WWTP was flooded during two separate events in 2015 and most recently during Hurricane Harvey, when the City was unable to access the facility until nearly a week after the storm.

Solution: FEMA’s Hazard Mitigation Grant Program provided an opportunity for the City to mitigate the recurring flooding at its WWTP. HR Green helped the City secure the funding and designed an increase to the facility’s flood protection with the construction of earthen berms, concrete T-walls, steel sheet piles, a flood gate, a stormwater pump station, and site drainage improvements.

Due to space constraints, a floodwall was proposed on the north and west sides of the WWTP, the existing earthen levee on the east side was raised, and a new earthen levee was constructed on the south side of the WWTP. The floodwall is 14 inches thick and includes an underground footing and steel plate to provide seepage control and structural support. The floodwall ties into the earthen levee northeast of the bar screens, follows the WWTP property line to the west and south, and ties into the levee on the south.

A floodgate was added just south of the solids processing driveway and ties into the floodwall and levee. The earthen levees have 4:1 side slopes and an eight-foot-wide top. The clarifiers and aeration basins’ top elevation is 36.34 ft., an adequate elevation to mitigate floods; therefore, no improvements to the existing basins were proposed.

Benefit: By providing increased flood protection around the WWTP, critical components, including the gas chlorination disinfection system, solids processing, operations building, and electrical components, including emergency power and electrical feed and blowers, will be protected against future floods. Another component of flood mitigation was upsizing the existing flood-compromised, hydraulically deficient WWTP outfall pipe. The new 42-inch pipe will convey WWTP effluent at all flow conditions. Installation includes bore-in-case at roadway and railroad crossings.

The project included Army Corps permitting, and HR Green delivered the design of this fast-tracked project in just six months to meet the FEMA requirements. HR Green is currently providing construction management and inspection services for the construction, which is on schedule to be completed in February 2022.