East Fork Water Supply Project, Conveyance Pipeline: Review of Design Options for Flow Metering and Lake Lavon Outfall

Since its formation in 1951, the North Texas Municipal Water District has been charged with developing a reliable water supply source for its 13 member cities and 46 other customers (some direct and some indirect), including more than 1.3 million people in portions of Collin, Dallas, Denton, and Kaufman Counties and all of Rockwall County. Historically, municipal water supplies have been obtained from surface water. However, recent long-range water supply planning efforts have identified significant increases in water demands that must be met through conservation, water treatment plant expansions, and improvements to both the raw water and potable water transmission systems. The District has elected to pursue augmentation of current water supply reservoirs with recycled water by the use of wetlands in which river water is treated and recycled. The East Fork Raw Water Supply Project is designed to transfer water from the East Fork of the Trinity River near Combine in Kaufman County, Texas, to the north end of Lake Lavon near Farmersville in Collin County. This system uses a river intake and diversion pump station to transfer water out of the East Fork and into a constructed wetland. It also uses a conveyance pump station and a 44-mile conveyance pipeline to transfer the water from the wetland to an outfall that discharges into Lake Lavon. The conveyance pipeline’s nominal diameter was set at 84 inches to convey flows ranging from 36 to 250 MGD. The focus of this paper is to discuss options considered to optimize the hydraulic performance of the last 5,000 linear feet of the proposed conveyance pipeline while considering impacts to the overall project cost. Constraining factors on the design included variable flow ranges, existing topography (creeks and ridge lines), outfall elevations at Lake Lavon and steep grades upstream of the outfall, long gravity flow sections with high flow velocities, deep trench excavations to minimize partial pipe flow velocity, full-pipe flow for flow metering facilities, impacts to lifecycle costs of adjusting the system high point, and overall construction costs. Options considered to address the many design constraints included lowering the pipeline, weir/drop structures, throttling valves, orifice plates, cast-in-place box culverts, multiple pipe systems, baffle chutes, and various combinations of these options.