Indianola Influent Trunk Sewer and Outfall Sewer Improvements

The City of Indianola has recently completed the construction of a new Water Resource Recovery Facility (WRRF) that replaces the City’s old plant. The new WRRF is located on a property approximately 2 miles north and west of the existing plant.  To convey wastewater flow from the existing plant to the new WRRF, a new trunk sewer was constructed. Additionally, the WRRF treated effluent needed to discharge to the Middle River about 3,000 feet away. In order to convey the treated effluent to the river, an outfall sewer was also constructed as part of the project. Finally, the City had been conveying treated effluent flow to the local golf course from their existing plant for purposes of irrigation water. This project also included several miles of irrigation force main piping back to the golf course pond so that treated effluent from the WRRF could be utilized for golf course irrigation water.

At the beginning of the design, HR Green developed a preliminary engineering report that provided several influent trunk sewer route alternatives and probable construction costs for each element of the project. The preliminary engineering report provided recommendations for all facets of the project, including capacity, design flows, scouring velocity, bury depth, manhole sizing & spacing, buoyancy, dewatering, material selection, impact to property owners, geotechnical considerations, and stream & road crossings. An in-house drone survey was used to determine the most economical and constructible sewer alignments. Once a sewer route was selected, HR Green proceeded with the final design, including the development of construction limits, permitting (County, IDNR, USACE, SHPO, & Wetlands), and coordination of easement acquisitions as necessary for construction and maintenance of the trunk and outfall sewers.

The influent trunk sewer and outfall sewer were designed for a peak flow of 19 mgd. The final trunk sewer alignment included the installation of approximately 2,700 LF of 36” sewer and 7,600 LF of 42” sewer. The majority of the trunk sewer alignment was constructed within a floodplain and required one major creek crossing and several tributary crossings. The selected alignment paralleled Cavitt Creek within the floodplain in order to minimize the sewer bury depth. When outside of the floodplain, the design of the project provided additional considerations and bid items for the installation of the sewer at buried depths greater than 20 feet. A portion of the alignment was located within the right-of-way of a local county road, and the project included reconstruction of the roadway.

The final outfall sewer alignment included the installation of approximately 3,000 LF of 42” sewer and a reinforced concrete headwall for discharge into the Middle River. The outfall sewer was almost completely within a farm field so special considerations were made with manholes and restoration of soils to farming suitability. Additionally, approximately 11,200 LF of 6” force main was constructed adjacent to the influent trunk sewer alignment to recycle plant effluent from the new WRRF to a local golf course for irrigation purposes.

HR Green also provided bidding and construction phase services. These services included evaluating bids, reviewing shop drawing submittals, reviewing pay applications and change orders, providing full-time construction observation, attending on-site progress meetings, and developing final record drawings. The final construction cost of the project was less than the bid amount, indicating a net negative change order percentage for the project.