Hahn Arroyo Island Project:

Transcription

1 July 2010 Hahn Arroyo Island Project: Hydraulic and Debris Removal Performance Based on Physical Modeling Studies at U M Hydraulics Laboratory Prepared for Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA) Julie Coonrod, Ph.D., Associate Professor Nelson Bernardo & Kyle Shour, Research Assistants Department of Civil Engineering University of New Mexico

2 Introduction: Draining approximated 6.5 square miles before entering the North Diversion Channel, the Hahn Arroyo plays a vital role in conveying flood water in north eastern Albuquerque. Since its construction in the 1960s, the channel has deterioratedd significantly (Figure 1). The Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA) has begun rehabilitation of the Hahn. As part of this rehabilitation, AMAFCA would like to include measures to improve water quality best management practices (BMPs). AMAFCA engineers have designed a structure to be installed as an island in the channel (Figures 2-4). However, due to the unpredictablee nature of supercritical flow occurring in the Hahn Arroyo, AMAFCA engineers have requested the assistance of the University of New Mexico s (UNM s) Hydraulics Lab. AMAFCA Engineers and UNM research assistants have tested and improved three designs for the structure. The design objectives for the structure included efficient removal of debris up to the 1-year storm and conveyance of flows up to the 100-year storm (Table 1). Figure 1: Severely Deteriorated Concrete in Hahn Arroyo (from AMAFCA) Figure 2: Project Vicinity Map (from

3 Flow Figure 3: First Structure Installed in Flume Flow Figure 4: First Structure Installed in the Flume

4 Table 1: Hahn Arroyo Flow Recurrence Values at Project Site Recurrence Interval: Peak Flow (cfs): First Model AMAFCA engineers constructed the first structure and provided dimensions for fabrication of the channel sections. The model had a 1:16 scale. Construction of the first model was completed on April 8, The design intended to capture the majority of low flows and first flushes through a ramp preceding the structure inlet. The inside of the structure was to act as a detention basin, removing both buoyant and non-buoyant debris. Rapid reductions in velocity would cause debris to fall out in the structure. Typically, this was accomplished by a transition from supercritical to subcritical flow with a hydraulic jump occurring at the structure inlet. Treated water would then return to the arroyo through a series of weir wall and hanging wall pairs. Early testing of the structure revealed that, even at low flows, much of the flow bypassed structure s inlet ramp. Bypassed water rooster-tailed significantly as it passed the structure. Three-quarter inch tall walls (1 foot in actual design) were installed on either side of the ramp, and the ramp was lowered from 1.5 inches (2 feet) to 0.75 inches (1 foot) to increase flow into the structure and remove rooster-tailing. The modification was effective at lower flows; however, at high flows, the hydraulic jump forming at the inlet exceeded the height of the opening. The bottom of the ramp of dropped the remaining 0.75 inches (1 foot) and the lower hanging wall above the inlet was removed to increase the structure s inlet capacity (Figure 5). Flow Figure 5: First Structure after Ramp and Inlet Improvements After these modifications, the hydraulic jump occurred on the ramp instead of at the inlet. This caused the walls along the ramp to be readily over-topped. Additionally, the overflow rate out of the structure was too high (i.e. the retention time in the structure was too low) and debris was not retained. To

5 remedy this, sloped baffle walls were installed along the inside of the hanging walls, and the orifice area below the lowest, outermost hanging wall was significantly reduced (Figure 6). This did not significantly improve debris retention but did force the hydraulic jump further up the ramp causing the walls to overtop at even lower flows. Slope Baffle Walls Plugged Orifices Figure 6: Sloped Baffles Installed and Lower Orifice Plugged Second Model AMAFCA provided a second, 1:16 scale model (Figure 7). The dimensions of the model s footprint were the same as the previous model. One of the weir wall and hanging wall pairs was removed. The model featured adjustable walls around its entire circumference. The wall heights and orifice heights could be varied for all walls. UNM graduate students constructed multiple ramp configurations that could be quickly changed in the model. This included a ramp that was flush with the floor of the structure, a ramp ending in a 0.75 inch (1 foot) drop, and a ramping ending in a 1.5 inch (2 foot) drop. The second model was installed on May 6, 2010 and tested for the first time on May 7 th.

6 Flow Figure 7: Second Structure Installed in Flume Testing on the second model used the ramp with 1.5 inch drop. The second model was tested using the 1-year peak flow rate (Table 1) of 276 cfs which was significantly higher than flow rates analyzed in the first model ( cfs). The new model performed better hydraulically. The hydraulic jump moved back into the entrance of the structure, no water overtopped the ramp walls, and no splashing or rooster-tailing was observed (Figure 8). Figure 8: Second Structure Operating at 276 cfs

7 Optimum wall and orifice heights were determined by qualitatively observing hydraulics and measuring water depths within the structure. After finding optimum wall heights, the second structure s debris removal capabilities were tested. Addition of debris and dye revealed one very beneficial quality of the structure; water in the spaces between the weir and hanging walls flowed from the downstream to the upstream end of the model, implying that debris would have an increased retention time in the structure. The model performed well, removing floatable debris and most neutrally buoyant objects. However, smaller debris was only detained and not removed. Smaller debris was simulated with coffee grounds and paper confetti. To improve small debris removal, model alterations were made to decrease velocity in the structure. The first of these modifications was three rows of baffles installed immediately downstream of the ramp (Figure 9). However, testing showed no significant improvements in debris removal. Figure 9: Baffles Installed in Structure The baffles were removed. The orifice height under the center wall was reduced to 0.25 inches, the outer wall height increased to 2.25 inches (3 feet), the center wall height to 3.75 inches (5 feet), and the inner wall to 5.25 inches (7 feet). These adjustments provided hydraulic characteristics most conducive to debris removal. Finally, for both debris removal and liability, engineers decided to add bar screens to the entrance and top of structure (Figure 10).

8 Figure 10: Second Model with Screens Installed Third Model Using the final configuration of the second model, a third, and final, model was constructed with fixed wall heights (Figure 11). The final model included the wall and orifice heights described above with greater wall lengths caused by a lengthening of the structure s footprint. Final model alterations comprised creating a new screen over the structure and installing screens between inner and middle walls and in front of middle wall orifice (Figure 12). The orifice area below the middle wall was reduced by approximately 43 percent to decrease the flow over the outer weir and increase the flow over the middle weir. This forced the hydraulic jump upstream of the structure entrance at low flows, causing debris to flow over the ramp walls and bypass the structure. 6 inch (8 foot) long wingwalls were installed parallel to the flow to prevent water from overtopping the walls at low flows (Figure 13). The wingwalls also reduced rooster-tailing at the 100-year flow (Figure 14). AMAFCA engineers raised the ramp to match the slope of the adjacent portions of the cross-section and altered the ramp into the structure to match the slope of the drop on either side of the structure (Figure 13). This improved the design constructability and allowed for easier retrofit work in the future. This design improvement produced no adverse hydraulic effects.

9 Figure 11: Third Model Figure 12: Third Model with Screens Operating at 276 cfs

10 Figure 13: Third Model with Wingwalls and New Ramp Configuration Figure 14: Third Model Conveying 100-yr Event (1626 cfs)