Domingo Baca Arroyo crossing at Jefferson

Transcription

1 Modeling Report October 4, 2000 Domingo Baca Arroyo crossing at Jefferson Prepared for the Albuquerque Metropolitan Arroyo Flood Control Authority Julie Coonrod, Ph.D. Department of Civil Engineering The University of New Mexico

2 Introduction The Domingo Baca Arroyo is located in northeast Albuquerque. The arroyo crosses Jefferson just north of Paseo del Norte. Smith Engineering is designing improvements to the arroyo which includes a concrete lined channel upstream of the existing four barrel 10 feet wide by 6 feet tall concrete box culvert at Jefferson. At the time the box culvert was designed, the estimated peak 100-year design flowrate was 5314 cfs. Because of development in north Albuquerque, the current design of the arroyo improvements are for a peak 100-year design flowrate of 7075 cfs. The HEC-RAS analysis performed by Smith Engineering indicated that the existing culvert could handle the increased flow. (The HEC-RAS model was not reviewed as part of this project.) However, the design engineers were not confident that the existing culvert would behave as the computer model predicted. Modeling Objective The objective of this project was to build a scale model of the Domingo Baca Arroyo crossing at Jefferson, to use laws of similitude to determine the capacity of the crossing, and to alter the entrance conditions to the crossing to maximize the capacity. "As-built" Model Froude number similitude is required for open channel models so that the ratio of inertial and gravitational forces is the same for the model and for that which is being modeled. The pump in the lab has a capacity of approximately 2000 gallons per minute (gpm). A scale model, one twentieth of the actual size, was built so that the design flowrate of 7075 cfs could be modeled using 1775 gpm. A larger model would have required a model flowrate larger than the pump's capacity. As-built drawings of the existing structure were provided by Smith Engineering, as were plan and profile sheets for "Alt. #1, V.4: Sta to " The plan and profile sheets provided the basis for the channel and all of the slopes set in the model. In addition to the as-built drawings, Dr. Coonrod visited and photographed the existing structure. Tom Escobedo, UNM Technician, and Gene Valdez, UNM Civil Engineering student, constructed the model according to the instructions given by Dr. Coonrod. The constructed model measures one-twentieth of the actual structure so that the model barrels of the box culvert measure 6 inches wide by 3.6 inches deep. The upstream end of the box has an increased opening of 1 foot in height (or 0.6 inches in the model.) Three splitter walls extend upstream 12 feet (7.2 inches, model) from the culvert into the arroyo to assist in distributing the flow into all four barrels. Because Jefferson does not cross the arroyo at a 90 o angle, the most northern splitter wall extends further into the flow in the arroyo than the most southern splitter wall. 1

3 Model Calibration Although the pump is equipped with two flow meters, the meters consistently vary from each other. To calibrate the flow meters, the time to fill a large tank was taken at a number of different flow rates. Because larger flowrates were of interest for this model, eight points representing a flowrate range of approximately 1000 gpm to 1900 gpm were used to develop a regression equation relating the flowrate on the gray meter in the lab to an actual calibrated flowrate. Figure * shows the points with the regression equation. To best represent the computed peak 100-year flowrates of 5314 cfs and 7075 cfs, the gray flow meter should read approximately 1059 gpm and 1426 gpm Calibrated Flowrate (gpm) y = 1.207x R 2 = Gray Meter Reading 2

4 Model Scenarios Table 1 summarizes the different scenarios and corresponding capacity of the crossing structure. Table 1. Summary of Model Scenarios Scenario Max flow Failure* "As built" 5200 cfs southern most splitter wall Extended splitter walls Adjusted south wingwall Angled splitter walls 5600 cfs 6000 cfs * see text for more explanation of the failure northern most splitter wall, backflow in southern most barrel backflow in southern most barrel, backflow in northern most barrel 6400 cfs backflow in all barrels The as-built structure had overflow above the southernmost splitter wall at approximately 5200 cfs. The water followed the shape of the splitter wall to the head wall and shot straight up. With the exiting structure and a flow of approximately 5200 cfs, it is likely that some flow could enter the street. It should be remembered that a design storm peaks at the design flowrate; whereas the model is run at steady state (or quasi-steady.) It is difficult with the current pumping system in the lab to actually model the storm so that the peak is instantaneous. The first change made to the model consisted of extending the two most southern splitter walls so that each splitter wall extended to the same station in the channel. The longer southern most splitter wall was no longer the point of failure. In this scenario, the water followed the most northern splitter wall to the headwall where possible splashing in the street could occur at about 5600 cfs. At this point it was concluded that the splitter walls needed to be longer that the original design and that the shape of the splitter walls should be altered so that the top of the splitter wall would extend all the way to the base of the headwall rather than 1 foot (0.6 inches, model) below the base of the headwall. The next change made to the model involved the southern most wingwall. In the previous runs of the model, there was consistently more flow in the southern most barrel causing the flow to the hit the top of the barrel and then backflow. The existing southern most wingwall is vertical and parallel to the splitter walls for approximately 10 feet. Then at approximately Station (Alt.1, V.4, Smith Engineering) the north and south wingwalls are symmetric with respect to the channel. The northern most wingwall was kept as shown in the design plans where the vertical wall transitions from completely 3

5 vertical sidewall (downstream) to a 2:1 side slope (upstream) over a distance of 160 feet. The southern most sidewall was altered so that the approximately 10 feet of vertical wall was omitted. Instead the southern most sidewall uses this additional 10 feet along with the 160 feet to transition from a vertical wall to a 2:1 side slope. Therefore, the wingwalls are no longer symmetrical with respect to the channel. This absence of symmetry is very slight. The alteration proved successful because the backflow no longer occurred in the southern most barrel first. At approximately 6000 cfs, backflow in the two outside barrels started. The final alteration to the model consisted of placing the most northern and most southern splitter walls at a slight angle to help evenly distribute the flow into the boxes. Both walls were set at a 2-1/2 o angle away from the center of the channel. The flow was better distributed between barrels. At approximately 6400 cfs, backflow started. In addition to the scenarios presented above, it was thought that an upstream slope the same as the box (1%) might provide uniform velocities and help prevent backflow. The scenario failed at a lower flow rate. In fact, normal depth in the barrels (if treated as an open channel) reaches the full 6 feet in height when the flowrate is 5400 cfs (using n =0.013). Conclusions & Recommendations The capacity of the existing structure can be increased by approximately 23 % from 5200 cfs to 6400 cfs. To increase the capacity requires construction of three new splitter walls. In addition, the southern most wingwall shown on Smith Engineering Company's "Plan & Profile Alt. #1 V.4:Sta to 32+00" needs to be altered so that the wingwall starts at the culvert. The dimensions of the wingwalls should be as shown on Figure *. The wingwalls that provide the transition from 2:1 side slopes to vertical side slopes should start at approximately Station and end at the entrance to the culvert (Station on the north side and Station on the south side.) 4