South Diversion Channel Project

Transcription

1 Physical Modeling Report II October 23, 2009 South Diversion Channel Project Recommendations on Flow Diversion System and Structures Based on Physical Model Studies at UNM Hydraulics Laboratory Prepared for Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA) Julie Coonrod, Ph.D., Associate Professor Seyed Mahmood Hosseini, Ph.D., Visiting Associate Professor Nelson Bernardo, Kyle Shour, Research Assistants Department of Civil Engineering University of New Mexico ,

2 1. Introduction The Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA) has actively introduced structural debris removal to storm water facilities as a Best Management Practice in recent years. The upstream portion of AMAFCA s South Diversion Channel (SDC) collects storm water runoff from the area east of Interstate 25 and south of the University of New Mexico area. The SDC receives storm water from storm water drains, the Genievas Arroyo, and the Kirtland Arroyo, before crossing Interstate 25. A concrete baffle chute is located on the channel approximately 800 ft downstream of its Interstate 25 crossing as show in Figure 1. SDC is a trapezoidal channel with an earthen bottom width of 30.0 ft and side slopes of riprap. Channel has a longitudinal slope of 0.14% just upstream of the baffle chute. The Manning roughness coefficient of the channel is estimated to be Although the channel has been designed for a discharge of 3450 cfs, the more frequent discharges in the channel vary between 100 cfs to 600 cfs. AMAFCA s goal is to divert flow at the upstream end of the existing concrete structure and to remove debris from that flow before allowing the flow to re-enter the SDC. A model has been constructed. Several improvements have been made to the structure to achieve desired hydraulic performance. The physical model was built in the UNM hydraulics laboratory by Research Assistants Nelson Bernardo and Kyle Shour, with assistance from undergraduate Michael Shorter, visiting professor Mahmood Hosseini, and AMAFCA engineer Kevin Daggett. 2. Physical Model Development The outfall structure model was constructed at a 1:30 scale, using plans (Figure 2) provided by AMAFCA. Original designs are documented in a preceding report. First, flow enters the sedimentation basin. Then flow is divided evenly between the two orifice plates; any excess flows pass into the baffle chute and moves downstream. After passing through the orifices, debris-laden water passes over wedge wire screens where water and debris are separated. Treated water returns to the baffle chute and moves downstream.

3 Figure 1. Image Map of the Project Site

4 RETURN CHANNEL (UNDER SCREEN) SEDIMENTATION BASIN DUMPSTER PARKING AREA ORIFICE PLATE Figure 2. Model Plan and Profile view with elevation drops (flow is right to left) The majority of model improvements made have been concerned with returning treated water to the channel. Initially, large cut-out orifices in the baffle chute walls and pipes connecting the return channels to the main channel were used to re-introduce water. Though this design was effective at returning water to the main channel without flooding the dumpster parking area, it would have been costly and difficult to construct because of conflicts with an existing underdrain system. The next design to be modeled abandoned the pipe and included a false floor in the return channel. The false floor was sloped at 8.3 percent. This was the steepest constructible slope, given return channel length and height. This idea was not effective because the return channel walls were too easily overtopped. The return channel was at maximum capacity at approximately 500 cfs (coincides with main channel flow of 1000 cfs) and overtopped at 600 cfs (coincides with main channel flow of 1700 cfs). For the final design alternative, the pipe remained covered and the false floor was removed. This left a level floor in the return channel. In this scenario, the return channel performed better than with the false floor. Improved performance was likely the result of increased hydraulic head on the cut-out orifices. However, when approximately 900 cfs

5 flowed in the return channel the water flooded the dumpster parking area. Four, ¾ inch diameter orifices were constructed in the model to simulate dumpster parking area drainage. Under these conditions, the dumpster area never overtopped. Despite the preferable hydraulic performance of the initial design, the final design alternative is recommended for final construction. The final design is much less costly and conveys water sufficiently. 3. Velocity Comparisons: Proposed and Existing Structure In order to compare the recommended design alternative to the existing structure, AMAFCA asked UNM graduate students to construct an existing model of the SDC Outfall. The existing model (Figure 3) was created by retrofitting the proposed model without damaging the proposed model. A SonTek FlowTracker Handheld ADV velocity meter was used to take measurements in both the existing and proposed design models. Multiple velocity measurements were taken at five locations (Figures 4-5 & Tables 1-3) in and around the models. The velocity just upstream and downstream of these measurements is not affected by the proposed design. Velocity measurements were recorded at three different prototype flows: 1700 cfs, 3400 cfs, and 4400 cfs. Using measured (model) velocity data, prototype velocities were calculated using similitude relationships for open channel flow. Prototype velocity is the velocity that exists in the actual SDC Outfall. Errors came from velocity measurement techniques and the limitations associated with the velocity meter. The velocity meter had the tendency to slow down the flow in locations that were supercritical or had small cross-sectional area. Similarly, measurements at locations with low depth tended to have high standard deviation. In one instance, flow depth was insufficient for velocity measurement. Finally, high turbulence made velocity measurements difficult and resulted in high standard deviation of measured velocities. For these reasons, meter readings were validated by timing a ping pong ball as it flowed through a segment of channel. This method validated the velocity meter.

6 Additionally, the movement of the ball allowed for observations of flow trends. In the existing model, the following trends were observed at each location. A: Flow displayed minimum turbulence and was sufficiently deep to use velocity meter. B: Uniform flow existed, but low depths made consistent velocity measurements difficult. As a result of this, measured velocities were lower than theoretically calculated velocities. C: Flow was of adequate depth for consistent measurement but was highly turbulent across the entire cross-section. As a result of this, measured velocities were lower than theoretically calculated velocities. D: Flow at location D was considerably less turbulent than the flow at location C. E: Flow was virtually uniform in the cross-section. In the proposed model, the following trends were observed at each location. A: Flow was virtually uniform in the cross-section; the water surface profile varied longitudinally approaching the structure. B: Flow was not uniform in the cross-section due to the turbulence created in the sedimentation basin; however, flow was virtually one-dimensional. The flow at location B undergoes a hydraulic jump caused by the baffles immediately downstream. As a result of these things, measured velocities were higher than theoretically calculated velocities. C: Flow was highly turbulent. At lower flows, flow measurement was not impaired by the turbulence. At high flows, there was a significant increase in standard deviation for measured velocity data. As a result of this, measured velocities were higher than theoretically calculated velocities. D: Flow was still fairly turbulent, but more uniform in the cross-section and better approximated one-dimensional flow. The persistence of turbulence was likely the result of the contraction of the channel section. E: Flow became less turbulent and was uniform in the cross-section.

7 Overall, the proposed model (Figure 3) performed better than the existing model. This was most apparent in the baffle chute and contracting channel section immediately downstream of the baffle chute. Water depth was greater in the baffle chute for the existing model. Under high flow conditions in the existing model, the baffles were submerged; however, the baffles were not submerged in the proposed model. This would suggest lower relative roughness during high flows in the existing model. Therefore, the hydraulic performance of the baffle chute improves in the proposed model. In the contracting channel section downstream of the baffle chute, supercritical flow persisted for a greater length downstream in the existing model for all flows. This suggests that the existing structure is more likely to scour the channel than the proposed model. Figure 3. Proposed Model Operating at High Flow

8 Figure 4. Existing SDC Outfall Model and Measurement Locations Figure 5. Proposed SDC Outfall Model and Measurement Locations

9 Table 1. Summary of Velocities Prototype Flow = 1700 cfs Existing Avg. Velocity (fps) Proposed Avg. Velocity (fps) Location Description Model Prototype Model Prototype A Channel Section U.S. of Outfall Structure B Zero Slope Section on U.S. End of Baffle Chute C Zero Slope Section on D.S. End of Baffle Chute D Narrowing Section D.S. of Outfall Structure E Narrowed Section D.S. of Outfall Structure Table 2. Summary of Velocities Prototype Flow = 3400 cfs Existing Avg. Velocity (fps) Proposed Avg. Velocity (fps) Location Description Model Prototype Model Prototype A Channel Section U.S. of Outfall Structure B Zero Slope Section on U.S. End of Baffle Chute C Zero Slope Section on D.S. End of Baffle Chute D Narrowing Section D.S. of Outfall Structure E Narrowed Section D.S. of Outfall Structure Table 3. Summary of Velocities Prototype Flow = 4400 cfs Existing Avg. Velocity (fps) Proposed Avg. Velocity (fps) Location Description Model Prototype Model Prototype A Channel Section U.S. of Outfall Structure B Zero Slope Section on U.S. End of Baffle Chute C Zero Slope Section on D.S. End of Baffle Chute D Narrowing Section D.S. of Outfall Structure E Narrowed Section D.S. of Outfall Structure Conclusion The proposed design alternative to the SDC consists of routing frequent flows around the existing baffle chute and through wedge wire screens. The debris-free flow re-enters the baffle chute. The proposed design alternative relieves the baffle chute of some flow during the design storm allowing for a shorter distance of supercritical flow downstream than in the original design. Based on the physical model, the proposed design alternative will be effective at removing debris for frequent storms while having no adverse impacts on the hydraulic performance of the baffle chute during extreme events.