Hydraulic Model Study of Waller Creek Tunnel Project for the City of Austin, Texas

This report provides the results of a series of model tests designed to understand the fluid dynamics involved with the Waller Creek Tunnel.Waller Creek is centrally located within the City of Austin, Texas, and has one of the most densely developed watersheds in the locality. The main stem is approximately seven miles in length and generally flows from north to south. The total drainage area for Waller Creek is 5.74 square miles (3700 acres) and the watershed lies entirely within the jurisdictional boundaries of the City of Austin and Travis County as shown in Figure 1 and Figure 2. Flood impacts to development adjacent to the creek have been a concern since the area was developed in the 1950’s. Central Texas (including the City of Austin) is prone to flooding, especially in creeks with highly impervious watersheds. The Waller Creek Tunnel Project (WCTP) will reduce the threat of flood damages to existing infrastructure and development, along the Lower Reach of Waller Creek. In addition to flood control, the proposed design will improve water quality, create ecological benefits, mitigate erosion problems, and provide safety to individuals and businesses located in the downtown Austin Waller Creek area. At the request of Crespo Consulting Service Inc., Austin, Texas (Crespo), Alden Research Laboratory (Alden) conducted a hydraulic model study using Computational Fluid Dynamics (CFD) and physical model studies for the proposed WCTP for the City of Austin, Texas. The objective of the study included evaluation of the flow patterns approaching and in the various hydraulic structures. Additionally, the model study was used to: Establish a rating curve for the morning glory spillway, Evaluate the potential air entrainment in the tunnel and the need for any air bleed structures, Establish junction loss coefficients for the 4th Street and 8th Street lateral junctions, Determine the fluctuating pressures due to flow induced excitations at the tunnel portal to the recirculation pump intake when the valve is closed and, Obtain a rating curve for the outlet spillway/weir. Inlet CFD Model -A CFD model of the inlet at Waterloo Park was used to design and evaluate the approach channel geometry, training wall and bar screens. The intake structure is comprised of a morning glory type inlet and includes six bar screen type trash racks which remove a portion of any debris before entering the vertical drop shaft and the underground tunnel. A training wall was designed to improve flow distribution approaching the structure. Model results show that 80% of the bar screen area has a velocity of less than 4 ft/s. The maximum velocity at any location on the screens is less than 5.5 ft/s. To address concerns that debris may accumulate along the training wall, modifications were made to fill in the backside of the barbs (the barbs were developed as part of the design to improve flow distribution and conditions at the screens). With the modifications the model results show that two screens did not have 80% of the bar screen area velocities of less than 4 ft/s and one screen exceeded the 50 % flow variation from the target flow. The final alternative was evaluated in the physical model. Lateral Junction CFD Models - Lateral junctions at 4th Street and 8th Street were evaluated using CFD models. The models were used to simulate flow conditions where the lateral flow is relatively large and the main conduit flow is relatively small. This condition results in the largest impact of the lateral junctions on the main conduit flow. Model results showed that the lateral junctions are not predicted to cause significant flow separations in the main conduit and cavitation potential is small. Outlet CFD Model -A CFD model of the outlet structure was used to design the riser shaft from the tunnel to the surface, and a flip bucket at the toe of the spillway. Based on the CFD model results, Outlet connection 2 as shown in Figure 17 was selected. The final structure design, based on CFD results, shows uniform flow distribution over the spillway. The flip bucket at the toe of the dam decreased the water velocity near the bed of the discharge channel as compared to a no flip bucket condition. Water velocity near the end of the spillway with the 2.25 ft high flip bucket is not predicted to erode the spillway apron. Inlet/Tunnel/Lateral Junctions/Outlet Physical Model - A 1:33 scale model of the Inlet, Tunnel, Lateral Junctions and Outlet of the Waller Creek Tunnel Project was constructed at Alden. The design flow for the model to simulate the friction losses and the expected Hydraulic Gradient Line (HGL) along the tunnel corresponded to the 100 year flood flow. The rating curves for the inlet spillway and outlet weir and the closed conduit flow loss coefficients for tunnel junctions were obtained from the model by testing a range of flows, as they were not affected by the tunnel HGL. Upon initial model start up, air entrainment at the morning glory vertical shaft was observed. The measured average Volume Fraction of air (VFa) in the model was about 5% for 25 year flow and 4% for the 100 year flow. Maximum Volume Fraction of air (VFa) in the model for the 25 year and 100 year flows were 8% and 6%, respectively. The volume fractions obtained from the model data could be corrected for any scale effects on generation of air entrainment using correction factors available in the literature. Also, as the Volume Fraction of air, VFa, can be a function of pressure and temperature, corrections need to be applied taking into account the expected pressure (from HGL calculations) and temperature in the field. The morning glory rating curve (Figure 64) was established in the physical model using inlet flows for the 2, 5, 10, 25, 50, 100 and 500 peak tunnel/ peak intervening events. During the 500 year event the morning glory spillway was submerged and an air drawing free-surface vortex was observed however it should be noted that 1) the building operations deck, which could interfere with vortex formation, was not included in the model and 2) the emergency spillway was not modeled which would result in lower water levels for the 500 yr event. Data was also recorded for two additional flows to determine the point at which the inlet weir becomes drowned out (approximately at a flow of 9,950 cfs at EL 483.2 ft water level). For the 100 year peak inlet flow condition (8,247 cfs) the average HGL was increased in the inlet shaft to above the morning glory crest elevation (474.0 ft) to elevations 478.2 and 479.7 ft to determine any effect on the inlet rating curve. For these submerged conditions, no change in the head on the morning glory spillway was observed. Therefore, the inlet rating curve is hydraulically disconnected from the inlet shaft tailwater up to at least 479.9 ft. Testing was conducted to determine any fluctuating pressures due to flow induced excitations at the tunnel portal to the recirculation pump intake when the valve is closed using the 100 year peak tunnel/ peak intervening condition. A plot of the prototype pressure versus time fluctuation referenced to EL 427 ft is included in Figure 65. The predicted maximum, minimum and average pressures were 16.9, 15.5 and 13.4 psi, respectively. This range of fluctuating pressure was not of concern to the JV in terms of design criteria for the valve. Tests were conducted using the model to determine the Minor Losses and corresponding junction Loss Coefficients at the 8th Street and 4th Street lateral junctions and the tunnel (tees combining main flow in the tunnel with flow from side inlet weir branch). Results indicating the Loss Coefficients determined from the model for the 100 year flood for both Peak Tunnel-Peak Intervening and Lagging Tunnel-Peak Intervening are shown below: 100 yr Peak Tunnel-Peak Int. 100 yr Lagging Tunnel-Peak Int. Loss Coefficient 8th St. 4th St. 8th St. 4th St. Tunnel: K2-3 0.3 0.2 0.3 0.3 Branch: K1-3 -0.6 -0.7 -0.4 -0.4 The branch loss coefficients (K1-3) are negative due to transfer of energy from the through flow in the tunnel to the flow from the branch as the branch flow is only about 10% or so of the tunnel flow. The outlet spillway rating curve was also established in the physical model using inlet flows for the 2, 5, 10, 25, 50, 100 and 500 year peak tunnel/ peak intervening events. The outlet spillway rating curve is shown in Figure 70.Waller Creek Working Grou

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