Sioux Falls Collector Well 72 Helps Optimize Water Supply

Challenge: The City of Sioux Falls needed to find a new well site to increase its raw water supply and maximize its return on investment. Sioux Falls is a community of approximately 200,000 people that utilizes several wells to provide groundwater to the Sioux Falls Water Purification Plant. The groundwater is treated and distributed throughout the City of Sioux Falls as potable water.

Solution: A geological study determined the ideal location for the Sioux Falls Collector Well 72 to be near water bodies, and in 2020, HR Green was hired to provide design and bidding services for the new well. Several special details needed to be considered during design, including long-term flood evaluations, proximity to environmentally sensitive areas, and routing of well discharge piping.

It was determined that a specialized horizontal collector well would give the city the best long-term withdrawal rate performance while maximizing the runtime of the well before the need to recondition the well screens. The well consists of a 16­ft inside diameter caisson with well lateral screens that project horizontally from the caisson through the aquifer. The well is completed with a pump and a well house on top of the caisson. Subsequently, HR Green was hired in 2021 to provide construction services for the project.

Due to the location of the well near water bodies, several special details were considered. For the well to be able to operate during flooding conditions and to keep the electrical equipment and instrumentation safe from flood waters, the well house portion of the well was located on a berm that was set at an elevation above long­term flood elevations and was sized to incorporate the city’s maintenance equipment. The environmentally sensitive area included a wetland, which dictated how the well discharge piping was routed to minimize disturbance to the surrounding wetland.

In addition to the routing, the design evaluated the need for casing pipe near the wetlands and where the water main crossed under railroad tracks. The discharge piping design included sizing the pipe for a range of well discharge rates and the potential for a future well, which would also discharge into a portion of the same piping. The piping was also located within corrosive soils, which required sizing and determining the type of cathodic protection needed for the main if ductile iron were to be utilized and for the steel casing pipes.

With the design of the well itself, considerations had to be taken for determining the diameter, length, location, and number of the lateral screens to minimize the entrance velocity of the groundwater through the screens while maximizing the potential of groundwater withdrawal. The pump had to be sized for a potential range of withdrawal rates from a lower range to an anticipated high-end rate. Finding a good-fitting pump curve with this large range of operation was somewhat challenging while keeping the pumping rate within a highly efficient operating range.

The well design also factored in ease and safety for maintenance personnel while providing a safe location for sophisticated and sensitive instrumentation and equipment. Finally, this well was also engineered to include an additional room and SCADA equipment utilized as one of the city’s centralized well field communication hubs. Though the project looks simple from what is seen above the surface, for a single well, numerous complicated processes and precise equipment layouts are required to provide the City with an easy-to-operate, high-performing well.

During construction, the project experienced delays with equipment and valves due to supply restraints. Journey, their Sub­Contractors, the City of Sioux Falls, and HR Green worked together to provide temporary interim solutions to keep the project moving forward while providing a great finished product.

Benefit: Although the Sioux Falls Collector Well 72 is in a low-traveled area and may not be noticed by the casual passerby, its significance to the Sioux Falls public is invaluable. The result was a well outperforming its anticipated design withdrawal rate by about 66%. This new horizontal collector well is one of the top-performing groundwater wells for the City of Sioux Falls.