Detailed Representation of a Large Pumping Station using EXTRAN

Transcription

1 20 Detailed Representation of a Large Pumping Station using EXTRAN Mark Yeboah, Mitchell C. Heineman and Brent A. McCarthy Rigorous representation of''real world" structures and systems in hydraulic and hydrologic models is a nontrivial task. Since the 1960s and 1970s when SWMM and other EPA models were developed, representation of the physical characteristics of systems has often been unduly simplified due to the lack of features in the model necessary to simulate the real physical system, simulation duration, or level of detail (Wisner et. al., 1984). Current increased computational capability provided by significant advances in computer technology coupled with the availability of tools that can be used to better measure, monitor and simulate real world structures provides a sound foundation for detailed representation of complex structures in hydraulic and hydrologic models. The addition of an array of structural and operational capabilities to SWMM such as the various pump types and settings has significantly improved its ability to rigorously represent and simulate real world structures. For a flooding study of the South End neighborhood in Boston, Massachusetts, it was desirable to rigorously represent a 500 cfs (14m3 /s) pumping station which included a weir, three entry gates, screening channels, a large wet well, four pumps, pressure lines, and a tidally-influenced discharge chamber. A comprehensive flowmonitoring program was conducted to characterize inflow to the pumping station and field tests were conducted to develop head-capacity curves for pumps at the station. A SWMM model was developed and successfully used as a tool to evaluate facilities planning alternatives for the South End neighborhood served by the pumping station. Yeboah, M., M.C. Heineman and B.A. McCarthy "Detailed Representation of a Large Pumping Station using EXTRAN." Journal of Water Management Modeling R doi: /JWMM.R CH ISSN: (Formerly in Best modeling practices for Urban Water Systems. ISBN: )

2 326 Detailed Representation of a Large Pumping Station using EXTRAN 20.1 History The South End is a 659 acre (263-hectare) neighborhood in Boston (Figure 20.1) located on filled tidal flats of Boston Harbor. AB presented in Figure 20.2, the South End was created in the 19th to mid,,20th centuries byfiuing South Bay, a large tidal bay formerly between South Boston and the main part of Boston, and Back Bay, an estuary on the Charles River (Camp Dresser & McKee, 2001b). Filling of the South End began around 1785 with the construction of a seawall on the South Bay along what is now East Berkeley and Malden Street and was not completed until 1967 when the Roxbury Canal Conduit, a channel behind Albaay Street, was filled. AB the South End was filled over a 182-year period, the standards and elevations to which the land was filled varied. Much of the Back Bay was filled below mean high tide [10.23 ft (3.2 m) Boston City Base, (BCD)]. This area is now prone to flooding. ~ Location of MA io the U.SA Figure 20.1 South End location in Boston.

3 20.2 Union Park Pumping Station 327 Figure 10.1 Filling of the South End Union Park Pumping Station In 1915, the Union Park Pumping Station (UPPS) was constructed as a flood control facility for the South End. The original pumping station was equipped with four electric pumps providing a capacity of241 cfs (6.8 m3/s). The UPPS allowed the draining of low-lying areas during high tide. In 1977 the current Union Park Pumping Station was completed. This station was equipped with three 48 inch (1220 mm) turbine-powered pumps and a 30 inch (760 mm) electric powered pump. The renovation increased the station's nominal capacity to 557 cfs (15.8 m3/s). In addition to bigger pumps, bar screens were installed to remove floatablemateria1, and chlorination was provided to disinfect flows prior to discharge UPPS Schematic Overview The UPPS and SUlTOunding sewers are presented in Figure During dry weather, the main gates are closed. Flow bypasses the station and is conveyed by gravity to interceptor sewers that discharge to the Massachusetts Water

4 328 Detailed Representation of a Large Pumping Station using EXTRAN

5 20.2 Union Park Pumping Station 329 Resource Authority's (MWRA) Deer Island treatment facility. The interceptors have sufficient capacity to handle dry weather flows without the need for pumping. During large storms, the main gates are opened to allow flow into the pumping station's wet wen. Flow is pumped from the wet well and discharged to Boston Harbor via the pumping station's discharge chamber. Reversed flow can also enter the station via the UPPS annex from the downstream interceptors if the interceptor sewers are surcharged Flow Path to the UPPS Wet Well The primary flow path into the UPPS is through the main entrance. This entrance has three channels. Flow enters these channels from the High Level Sewer loeatei in Union Park Street and from the Low Level Sewers in Union Park and MaWen Streets. The High Level Sewer spills over a weir into the Low Level Seweron Union Park Streetimmediatelyupstreanl ofa sluice gate and tide gate. These gates prevent flow from continuing down Union Park Street in the High LeveJ Sewer. The combined flows are conveyed to the main entrance of the station via a 7ftby6 ft(2.1 m by 1.8m) conduit in front of the station. The station is placed online when the water depth in this channel reaches 2.3 ft (0.7 m). The pumping station's three entrance channels are labeled A, B and C. Channels A and B are usually opened when the station is placed online. Channel C is reserved for use as a backup channel. There is a 1.5 ft (0.5 m) weir at the ent:rance to each channel. The channels each measure 11 ft by 8.5 ft (3.5 m by 2.6 m) and are approximately 46.5 ft (14.2 m) long. Each channel is gated with a 9 ft by 9 ft (2.7 m by 2.7 m) sluice gate which must be opened to allow flow into the station. Channels A and B have automatically-cleansed coarse and fine bar screens to remove debris. The coarse screens have 0.75 inch (19 mm) bars with 3 inch (76 mm) spaces between bars and the fine screens have 0.75 inch (19 mm) bars with 0.75 inch (19 mm) spacing. Standby channel C has manually cleaned screening facilities, with 0.5 inch (13 mm) bars and 1.5 inch (38 mm) spacing between bars. Flow entering the station is conveyed through the bar screens and discharged to the wet well. The wet well has an average cross-sectional dimension of 46 ft by 81 ft (14 m by 24.7 m) and has an invert elevation eight feet below the invert of the entrance channels. Figure 20.4 presents a detailed representation of the pumping station. Figure 20.5 presents a profile through the pumping station. The third flow path into the station is via the UPPS annex. The annex entrance receives storm water from drains in Albany Street, as well as flow

6 330 Detailed Representation of a Large Pumping Station using EXTRAN I Figure 20A Pumping station detail. Figure 20.5 Pump station profile. reversed from the interceptor sewer. After overflowing a weir at the entrance to the UPPS annex, the flow enters the station via the 6 ft by 11 ft (1.8 m by 3.4 m) UPPS annex sluice gate. This gate is opened whenever the station is in operation. As is the case at the main entrance, the UPPS annex's entrance

7 20.2 Union Park Pumping Station 331 channel has bar screens for debris removal. These screens have 0.75 inch (19 mm) openings. Flow is conveyed through the annex channel and discharged to the wet well Flow Path from the UPPS Wet Well Flow is pumped ftom. the wet well using four pumps mounted behind the wet well. The tbj:ee turbine-powered pumps have an effective operating capacity of 395 cis (11.2 m3/s) based on 2000 field measurement. The electric pump has a capacity of 50 cfs (1.4 m3/s), The electric pump is started when the wet well depth reaches 10 it (3 m). If the wetweuis risina'rapidly. a turbine pump is also started when the wet wen depth reaches 10 it (3 m). The water depth in the wet well is maintained near 12 it (3.7 m}'!>y decreasing turbine pump speed or startin& additional turbines. If the wet wei depth is being maintained and the turbine pumps are 1'Ulllling at less than full speed, the electric pump can 1>e turned off. After the storm, the turbine powered pumps are taken otlline and the electric pump used to pump dowu the wet well to a depth of 6ft (1.8 m). Stripping pumps are used to dewater the remaining wet well volume. Flow pumped ftom. the wet well is discharged into a discharge cham1>er located 25 ft (7.6 m) above the invert of the wet well. Wbilethepumpingstation is online during severe storms, the discharge cl1an'l1>er can become pressurized. A 4 ft (1.2 m) circular pipe and a 7.3 ft (2.2 m) horseshoe pipe convey flow ftom. the discharge cham1>er via Union Park Street to Boston Harbor. The flow path through the discharge chamber is shown in Figure Representation of the UPPS in EXTRAN A RUNOFF-EXTRAN SWMM model (USEPA Stonn Water Management Model; Huber and Dickinson, 1988; Roesner et. al., 1988) was developed and used to simulate the hydraulics and operating rules at the UPPS and within the collection system. RUNOFF was used to represent the smaller pipes in the system.. Approximately 16 miles (25.7 Ian) of pipes were represented in the RUNOFF model. Non-circular pipes in RUNOFF were simulated as box culverts, as RUNOFF does not allow more complex shapes such as egg-shaped sewers. The EXTRAN model included over 200 pipes ranging in size from 15 to 240 inches (380 to 6090 mm). Approximately half of the pipes represented in the EXTRAN model were circular, while the others were horseshoe, egg, or box culverts. Each major sewer was represented in its entirety in EXTRAN. EXTRAN was also used to represent weirs, gates and outfalls within the

8 332 Detailed Representation (?l a Large Pumping Station using EXTRAN

9 20.2 Union Park Pumping Station 333 system. These structures play an important role in energy dissipation and flow uniformity (Wang and Rudavsky, 1979). Figure 20.6 presents the RUNOFF and EXTRAN pipe networks. Because the desired use of the model was flood prediction, it was imperative that the hydraulics at the pumping station be represented as accurately as possible. This is because the model was used to provide predictive capabilities across a spectrum of alternative pumping station and collection system configurations. Also, a detailed representation of the station was ~ because the pumping station serves as the flood control facility for the Sov.th End. A riaorous approach was used to represent the pumping station in the BXTRAN model. The turbij1e..powered pumps and.tnc pump were fieldtested to develop head-capacity curves for input to EXTRAN.1be electric and two of the three tut'bine--powered pumps were tested d:w.iag this program.. These pumps were fitted with pressure reading transducers and recorders at both the pump suction and discharge flanges. Pump suction and discharge pressures and discharges for these pumps and corresponding wet well depth were recorded at one-minute intervals during a storm on June 6-7, The pump speeds were also recorded. This information was used to generate plots of total dynamic head vs. discharge for use in the model representation. Head losses at the station entrance, in the screen channels, pressure lines and the discharge channel leading out to the outfall were also computed. These losses were incorporated into the representation of the station. A few modifications were made for the model's physical representation. These included consolidation of the control gates and screen channels as EXTRAN orifices, and combination of screen channel A and B as a single equivalent pipe. The suction lines leadingto the fourpwnps and the pressurized discharge lines from the pumps were also lumped as a single equivalent pipe. All other structures were directly represented. These included the entry weirs, wet well, discharge channel, and 2000 feet of tidally influenced conduit above the outfall to Boston Harbor. Operational Representation of the UPPS in EXTRAN The operating rules associated with gates and pumps were implemented in the model. This was accomplished primarily with the use of orifices and pumps with head dependent rules (EXTRAN FIIF4 and HI data groups). The operating rules at the pumping station were based on water level elevations in the station's wet well or at a location within the collection system (Camp Dresser & McKee, 2000). Table 20.1 presents the operating rules for the pumps when the wet well depth is rising. As presented in the table, all pumping operations are based on

10 334 Detailed Representation of a Large Pumping Station using EXTRAN Table 18.1 Union Park pumping station operation during rising wet well level. Wctwell Wctwell Operation Depth(ft) Depth(m) Operate bar screens Start 30" electric pump Start III turbine Stop 30" electric pump Start 'J!'" t:utbine pump Start 3td turbine pump Start 30" electric pump the depth of flow in the wet well. This allowed the modeling of the pumps in EXTRAN using TYPE 3 head dependent pumps with tivepoint head-discharge curves. The EXTRAN input lines used to represent the three turbine pumps are presented below. H1 3 21KP1UPPS 21KPlDISC H1 3 21KP1UPPS 21KPlDISC H1 3 21KP1UPPS 21KPlDISC The EXTRAN input lines used to represent operation of the gates at the entrance to the pumping station are presented below. EXTRAN side outlet rectangular orifices with TYPE 23 head-dependent gated control orifices were used. Using Channel A as an example, the input lines describe a 9-ft by9-ft (2.7- m by 2.7 m) orifice with an invert 1.51 ft (0.46 m) above the invert of the node, 21KREUPPS. The gate is controlled by the depth at node 19LMHP143. It is fully open when the depth at the control node exceed 4.5 ft (1.37 m) and fully closed when the depth is below 3.6 ft (1.1 m). Between these two depths, the gate opening is interpolated. *Channel A F1 2lXREOPPS 2lKPUUPPS F4 19LMHP *Channel B Fl 21KREUPPS 21KPUUPPS F4 19LMHP *Channel C Fl 21KREUPPS 21KPUUPPS F4 21UMH

11 20.3 Calibration Calibration The collection system model was calibrated to water levels and flows monitored at311ocationswithin the South End. Calibration dataalso included the pumping station flows, wet well levels, and pumping operation at the pumping station. A rain gage located at the pumping station was used during the model calibration. Themodel calibration comprised rainfall event screening, review of sewer response to rainfall, selection of representative events for model detailed calibration and computer simulations. These are typical tasks involved in model cah'bration (Steiss and Watters, 2001). Detailed calibration was performed using six storms in Jaauary, September and October of At the pumping station where the physical representation and operation were closely met, very little calibration was required. This is generally the case when conditions for similarity are closely met (French 1985). Figure 20.7 and 20.8 present a rainfall hyetograph for a major storm on September and associated flow hydrograph for the pumping station's discharge channel. The flows shown in the figure are a function of the calibration at the pumping station and at the other meters within the collection system as the calibration at those meters have a direct impact on flows at the pumping station. ~'r " "." ~ tu t I Ojj JUllI1! 4' 6.13 ' ~~~NN~NNNNmNmmm~~~~~ -o. a._lgoltlllp Figure 20.7 Rainfall hyetograph for stood on September 15-16, 1999.

12 336 Detailed Representation of a Large Pumping Station using EXTRAN dr ~ I-Modlll. *"1.. :.. o~~~~~~-=~~~~~~~ ~~ ~ II t e I) ,.. ~~~~~~~~~~~~~~~~~~~~~... INr.._ Flpre 10.8 Flow hydrograph for pumping station's discharge channel, stonn on September 15-16, Application The model was used to evaluate alternatives for facilities planning in the South End. Potential improvements forthe collection system were developed and their impacts on flows, hydraulic grade lines (HOL), and pumping station performance evaluated using the model. A storm occurring on June 13, 1998 that caused flooding in the South End was used for this evaluation. Potential improvements based on the June 13, 1998 simulations were then simulated for each major storm from to evaluate their performance. Twenty-seven storms were selected from this period for simulation after careful review of storms occurring in Boston in that period. This provided a rigorous and realistic basis for evaluation of the alternatives (Farrell et al ). The 27 storms were arranged in sequence with dry intervals to allow the system to return. to dry weather conditions. The dry intervals had an average duration of24 hours. The historic measured tides coincidental with each storm were also arranged in sequence to match the storms. The 27 storms were simulated over a 56-day period using the arranged storms and tides as input. This allowed the identification of conditions that lead to flooding. Improve-

13 20.5 Conclusion 337 ments evaluated included collection system modifications such as pipe enlargement and separation in some combined areas, tributary area reduction using gates and weirs, and pumping station improvements such as pump rehabilitation and the installation of new pumps. Scenarios involving the failure of one or more pumps were also simulated (Camp Dresser & McKee, 2001a) Conclusion A SWMM (R.UNOFF-EXTRAN}model was developed to evaluate flooding in the South &d. This model included a detailed representation of the Union Park Pumping Station, which serves as the flood control facility for the South End. The model was successfully used as a tool to evaluate facilities planning alternatives for the South End. Performance of the station was evaluated for each ~or storm occurring between 1948 and The rigorous representation of the pumping station was found to work well under most conditions. In some conditions, it was necessary to remove the control gates to obtain robust solutions using the dynamic head pump option. The dynamic head pumps provided accurate assessment of pumping capacity during flood conditions when the downstream tail water affected pump performance. However, variable speed pumps yield better model performance with no loss of accuracy during conditions when downstream tail water does not limit pump perfonnance. References Camp Dresser & McKee, Inc Union Park Pumping Station - Updated Sections of Operation and Maintenance Procedures Manual. Prepared for the Boston water & Sewer Commission. Camp Dresser & McKee, Inc a. South End Facilities Plan Final Report- Volume I. Prepared for the Boston water & Sewer Commission. Camp Dresser & McKee, Inc b. South End Facilities Plan Final Report- Volume ll. Prepared for the Boston water & Sewer Commission. Farrell, A., R.B. Scheckenberger and R.T. Guther "A Case in Support of Continuous Modeling for Stormwater Management System Design." Journal of Water Management Modeling R doi: /JWMM.R French,R. H.l985. Open-Channel Hydraulics. McGraw-Hill, Inc. ISBN# pp.660. Huber, W. C. and Dickinson, R. E Storm Water Management Model User's Manual, Version 4. EP A/600/3-88/00la (NTIS PB / AS). Environmental

14 338 Detailed Representation of a Large Pumping Station using EXTRAN Protection Aaency. Athens, Georgia. Roesner, L.A., Aldrich, J.A. and Dickinson, Storm Water Management Model User's Manual, Version 4: Addendum I, EXTRAN. EPA/600/3-88/00lb (NTIS PB882366S8/AS). Environmental Protection Agency. Athens, Georgia. Steiss, G. and W.D. Watters "Developing a Model for Basement Flood Relief Works for the New Millennium." Journal of Water Management Modeling R doi: /JWMM.R Wang J. C. andrudavsky, A.B Physical Modelina of Inflow Structures in Urban Drainage Systems. U of Kentucky Urban Hydrology, Hydraulics And Sediment Control Symposia, Kentucky, pp Wisn«, P., Rampersad, C.A. and Lam, A Realistic Simulation of Sewer Surcharge and Prevention of Basement Floodins. Proceedings of 3rd International Conference on Urban Storm Drainage. Goteborg, Sweden. pp