Siphon Design Under the West Fork of the Trinity River

One of the most challenging design problems for underground infrastructure, is to minimize disturbance/disruption, or 'pretend like we were never there'. In the case of the Bear Creek Relief and WF-G Interceptor improvements project, this challenge was further complicated with crossing the Trinity River, twice, with a 96-inch diameter gravity pipeline. To accomplish this, the owner and engineer collaborated with pipe manufacturers to design two inverted siphons with consideration to future system demands, eliminate the need for Portland cement concrete elements in the collection system, provide for cost-effective open cut installation, comply with USACE regulations, and navigate poor soil conditions - all the while maintaining uninterrupted service to customers and maintaining up to 84 MGD of average daily flow.

This presentation is intended to discuss the process and calculations taken to determine compliance of the siphon crossings underneath the Trinity River as well as showcase the manufacturing and construction of large diameter plastic pipe fittings.

Inverted Siphon Design:
The siphons were designed utilizing flow data provided by the Authority including 3-day hydrographs at selected locations from the Authority's model storm event. These hydrographs were provided for years 2040 and 2070, and the engineer designed for both worst-case scenarios, where the flow is the greatest and the lowest, at all locations. Low flow conditions are an equally important design consideration to ensure sedimentation in the siphon is kept to a minimum.

From this information, the engineer determined the Average Daily Minimum, Average Daily, Average Daily Maximum, and Peak Wet-Weather flows and used these values in the design of each siphon crossing based on their locations and characteristics within the system.

The initial designs for all siphon crossings were conducted through an intensive and iterative calculations process necessary to determine adequate siphon barrel diameters. InfoWorks ICM (Version 11.0) model was used to confirm the siphon design calculations. Following the preliminary design selection, the engineer modeled the siphon crossing designs to confirm the hydraulic capabilities of the siphon crossing designs with Peak, Daily Average, and Minimum Flows. (See Tables 1 & 2)

The models were later used to estimate the amount of expected sedimentation in each siphon during minimum flows over the course of 30-, 60-, 90-, and 365-days to confirm the design's reduction of sedimentation. These analyses were conducted independently of the preliminary design selection with the physical parameters of the siphon and the 3-day hydrograph (developed from flow information provided by the owner) used to corroborate the preliminary findings and design.

To model the sedimentation, the engineer modified the above mentioned 3-day hydrograph to remove the Peak Wet-Weather event and retain the dry weather flows to better approximate a worst-case scenario for sedimentation. The engineer determined the peak flow, occurring every third day, is unrealistic and that removing it would be a better approximation. The engineer repeated the dry weather flows from the hydrograph over 30-, 60-, 90-, and 365-days for sedimentation analysis. While sedimentation is shown in some cases in the results, the hydrograph analyzed does not include any Peak Wet-Weather events. Other assumptions were that the sediments are evenly distributed across the entire depth of flow, not near the bottom as it would realistically be expected, and that the sediment loading was approximately 24.4-mg/L (Metcalf & Eddy) with a specific weight of 2.65.

After confirming the siphon design met capacity requirements, the engineer designed an air jumper for each crossing in order to move odorous sewer gases across the length of the siphon crossing. Generally, the diameter of each air jumper duct was selected to best balance the available space in the cross-section of the siphon crossing and the air flow capacity of the duct.

Inverted Siphon Construction:
The engineer worked closely with the pipe manufacturer to ensure the constructability of the siphon fittings and lay schedule during the construction submittal process.

Multiple iterations of the lay drawings were reviewed to ensure the inlet and outlet inverts maintained their hydraulic requirements. (See Figure 1) During installation, care was taken to maintain flowline elevations to comply with the plans and submittal documents.