An Innovative Approach to Pump Station Design
The Trinity River Main Stem Pump Station and Pipeline Project was recognized with an Engineering Excellence Gold Medal from ACEC Texas.
The Trinity River Main Stem Water Supply project consists of a 100 MGD capacity pump station and 16 miles of 72-inch transmission pipeline that provides additional raw water to the East Fork Raw Water Supply Project – a constructed wetland where raw water is polished via naturally occurring processes – and then is further pumped to Lavon Lake and the Wylie Water Treatment Plant complex. This project provided needed water supplies between 2017-2022 and also secured and solidified another long-term water supply source for the North Texas Municipal Water District (NTMWD).
The Main Stem Pump Station is a reliable, renewable source of water because the upstream wastewater treatment plants continuously discharge effluent. NTMWD may only draw up to 70% of the effluent discharged from the contracted wastewater plants, allowing for an environmental base-flow to maintain aquatic life in the Trinity River.
Innovative Applications in Modeling
The Trinity River Main Stem Project is the first to use all of our available modeling tools to reduce cost, minimize risk and maximize value to the owner. Listed below are a few innovative components that were utilized:
2-D River Model: A 2-D HEC RAS model of the Trinity River was developed for the project site to verify the design flood elevations. During the analysis, the team discovered that the previous analysis of the site did not account for the flood protection levees along the river. These levees were incorporated into the new model and resulted in a 100-year flood elevation approximately seven feet higher than originally thought. Discovering this discrepancy utilizing the best available modeling tools allowed the team to adjust structure heights early in the design process and provide the required flood protection.
River and Intake Box CFD Models: The CFD models for the river and intake box were performed to improve the intake design and minimize sediment traveling into the pump station. The Trinity River model illustrated flow patterns near the intake and the typical location of sediment flowing down the river. This allowed the design team to locate the intake screens to limit the sediment intake into the pump station facilities. The intake box model showed sediment flow patterns and deposition. This was utilized to design baffle walls and floor sloping to trap sediment, prevent it from entering the pump station, and funnel it to removal pumps. Reducing the amount of sediment entering the pump station will allow for less maintenance and removal cost for the owner.
Forebay and Intake Piping CFD and Physical Models: The CFD models for the forebay and intake piping, combined with a 1:5.793 scale physical model of the pump station intake, were performed to improve flow patterns into the pump and limit turbulence at the pumps. The forebay CFD model showed flow patterns approaching the intake area. This was used to locate the weir into the forebay for even flow distribution to the pumps and less turbulence for more residence time of sediment in the water. The CFD and physical model for the intake area and piping showed the flow patterns into the suction bells of the intake piping and flow to the pump. The models supported removing the splitter walls from the design and identified uneven flow that lead to improving the initial design of the suction bells and intake piping. The improvements helped save the owner approximately $600,000 and will provide better pump performance.
Discharge and Flow Meter CFD Models: The CFD model of the discharge piping and flow meter was performed to verify flow patterns leaving the pump station and traveling through the flow meter. The model showed that flow was evenly distributed through the cross section of the pipe, which will provide better accuracy of the flow meter. The flow meter station was changed from a below-ground meter to an above-ground meter station to save the cost of building a large vault. The CFD model confirmed that the design would provide good flow meter performance while providing cost savings.
Surge Model: A surge model was also constructed for final air valve sizing and ensuring pump and valve operation would not cause adverse effects in the event of power loss. The surge model identified several locations of vacuum conditions in a complete power loss and allowed for the proper sizing of air valves to control the effects of pressure waves. This safeguards the pump station against critical failures from surge effects, which would be damaging to the pump station and result in the loss of ability to pump water for the owner.
Groundwater Uplift Pressure Design: To limit groundwater uplift pressures on structures, a system of gravel drainage blankets, well pumps, and a bentonite slurry trench were constructed. The slurry trench proved critical during construction allowing the excavation to take place with minimal dewatering required.