Physical Model Investigation on optimum Design of U shaped Weir in combined Sewer

Transcription

1 1 Paper N 0 : II.07 Physical Model Investigation on optimum Design of U shaped Weir in combined Sewer Goran Lončar Vladimir Andročec Goran Gjetvaj Abstract: Either due to reason of spatial restriction or economical optimization a weir in combined sewer often can t be carried out through classic form with one relatively long said weir. One of geometrically alternative solution is performing U shaped weir which enables augmentation of its relative length. Insufficiency of globally documented data related to hydraulic characteristics of such shaped weir, set demand for hydraulic model investigation on whole diversion plant with U shaped weir that is originally set for implementation to Zagreb City waste water discharge channel. Tests on physical model were focused on measuring speed and water levels at significant points in incoming and outgoing channels and diversion plant, with and without downstream inlet influence, and dependently on characteristic discharges of inlet channel at dry discharge to extreme discharge. Dynamic pressures were measured at weir wall and at stilling basin of overflow construction. Based on measurements, discharge coefficients and energy losses coefficients were calculated. The comparison between the velocities measured on physical model and one calculated on numerical model is also appended as like as comparison of discharge coefficients obtained on U shaped weir and classic side weir with similar dimensions. Certain modifications resulted with optimization of overflow functionality. Further more, analyses of the scam board removal efficiency of floating materials was carried out as same as their influence to upstream water surface elevation and decrease of discharge coefficient. Keywords: U shaped weir, combined sewer

2 144 Lončar, Andročec, Gjetvaj 1. Introduction During rain event with relatively high intensities treatment plant is not capable to accept whole expected discharged quantity (Q max ) which comes from incoming channel. Diversion plant with performed U shaped weir (figure 2), will separate incoming discharge into overflowing quantity which further flows through overflowing channel (Q max -2Q dry ) to the final downstream recipient and discharge which goes to the treatment plant (2Qdry). Widely used shape of weir in combined sewer diversion plant is in form of relatively long side weir. Lack of investigation, measurement and results data about hydraulic efficiency for such U shaped weir, also intended to be built, set demand for physical model investigation. Schematic sketch with disposition of new and main combined sewer system elements already built in city of Zagreb-Croatia is given on figure 1. incoming channel creek inflow overflows in instalment (not included in physical model) Ravnice Kozari bok Dubrava Culinecka DIVERSION PLANT channel to the treatment plant overflow channel Treatment plant River "Sava" Area covered by physicel model Zitnjak Strug Figure 1 Schematic sketch of main combined sewer system elements already built in city of Zagreb Figure 2 diversion plant in variant solution «0» and positions of incoming, overflowing and channel to the treatment plant Investigation on physical model gave insight in main hydraulic characteristics connected with U shaped weir placed within diversion plant. After only three main geometry changes (variants 0, 1, 2, FS figure 2, 3abc, 4ab), executed on physical model, functionality of

3 "REIGHT" BRANCH Physical Model Investigation on optimum Design of U shaped Weir in combined Sewer 145 whole diversion plant was optimized in sense of its decreased extent and sustained discharge coefficient. Furthermore optimum geometry and position for planed scam board were determined. Results of measurement obtained on physical model variant 0 showed that all hydraulic criteria demands were satisfied and that highest water surfaces level in incoming channel is below in project documentation defined channel ceiling. During the extreme discharge event overflowing at weir is drowned. One has concluded, that further improvement in sense of hydraulic functionality and decreased structure extent could be done. In variant solution 1 width of diversion plant has been decreased for 4,7m due tu približiti/primaknuti of vertical walls (figure 3b). SEMICIRCULAR CHANNEL GATE INCOMING CHANNEL LANDING OVERFLOWING CHANNEL STILLING BASIN LANDING REGULATION GATE DIVERSION PLANT VERTICAL WALL km Stac. "LEFT" BRANCH DIVERSION PLANT VERTICAL WALL R R50 R50 R CHANNEL TO THE TREATMENT PLANT LANDING PODEST OVERFLOWING CHANNEL SEMICIRCULAR CHANNEL JUNCTION 1235 LANDING Figure 3a,b,c plan view for variant solutions Figure 4a,b plain view for variant solution FS 0, 1, 2 of diversion plant of diversion plant Analysis and comparison of measurements results on variant 0 and 1 give insight in minor change in hydraulic functionality. In mine time new demands on cross section of overflowing channel such as change to rectangular cross section with columns against old solution in trapezoidal form without columns should take place. Furthermore on position of semicircular kineta junction, gate for sediment removal should take place (variant solution 2 figure 3c). Slope of landing has been increased from 1% to 3% to avoid higher rate of sedimentation. Intensive vorticityes on position of sharp semicircular channel top edges

4 146 Lončar, Andročec, Gjetvaj decreased after their smoothing with radius of 0,5m in vicinity of landing in diversion plant (figure 3c). With aim of flow visualization on variant solution 2 intensive vortices on position of gate for sediment removal has been observed. Thus gate was finely placed on the side of semicircular channel junction in the diversion plant and junction has been designed same as in variant solution 0 and 1. This last investigated final solution has been named FS (figure 4a,b). 2. Physical model Physical model with 4 in details different solution 0, 1, 2, 3, FS has been built in Hydraulic laboratory on Faculty of Civil Engineering-Zagreb, Croatia. Model length scale was 16,7 without distortion and the model followed Froude similitude. Sketch of channels in and out from diversion plant with position of used measurement equipment is also given on figure 6. Photos of diversion plant, overflowing channel and downstream inflows built for model variant solution 0 are given on figure 5a,b. Investigated physical model overflowing heights at weir crest ware greater then 3cm, what is main condition for elimination of surface tension scale effect [1, 2]. Figure 5a,b diversion plant with incoming and outgoing channels built on physical model variant solution 0 3. Physical model tests and results 3.1 presure Nomenclature of measurement conducted on all four variant solutions with six different boundaries discharge conditions in incoming and overflowing channel is given in table 1. Table 1 boundary conditions discharges used in particular test VARIANT 0, 1, 2, FS Discharge condition in channel (m3/s) test No incoming channel 3,6 5,8 7, ,7 (0;1) ; 78,0 (2;FS) overflowing discharge ,8 57,8 76,5 (0;1) ; 70,8 (2;FS) inflow 1, ; ; 14

5 Physical Model Investigation on optimum Design of U shaped Weir in combined Sewer 147 Figure 6 Schematic sketch of physical model extent with positions of used measurement equipment 3.2 Depths Values of depth at measurement points I1, I2, I3, I4, I5-1 (see figure 6) for all experiments set up after table 1 are also compared with numerical model prediction (MIKE SHE). Depths obtained with aid of numerical simulations for same boundary conditions and at the same points in channels are less then values obtained on physical model tests in range 2-7 % are dependent on investigated variant solution and used boundary conditions. Only in test No 2, measured and on numerical model calculated heights ware practically the same for all variant solutions. In flowing condition connected with test No. 4 and 5 minimum water depths values have been reached in variant solution FS. It was possible because the absence of drowned overflowing although overflowing channel was rectangular one with columns in the symmetry line. On contrary, in tests No.6 drowned overflowing occurred and water level heights ware higher in case of variant solutions 2 and FS then in 0 and 1. Main reason for that are added columns in overflowing channel with decreased cross section area. 3.3 Pressure in stilling basin Investigations on pressures during stationary overflowing at points T3, T4 at the bottom in stilling basin for tests No. 4, 5, 6 and for all variant solutions have shown that dynamic pressure component increased static component for 25-30%. Average amplitude of pressure fluctuation is close to 0,5 m.

6 148 Lončar, Andročec, Gjetvaj 3.4 Water levels in diversion plant Shape and values of water levels heights over weir crest level and at side walls of diversion plant are given in figure 7. Shown levels are obtained under conditions specified for test No. 4 and for variant solution 0, 1, 2. Distances from beginning of weir to measurement points in left branch have - and in right branch + mark (see figure 6). As stated earlier no drowned overflowing occurred in test No.4 independent of variant solution. water levels "h" (m) Qm(overflowing) = 57,8 m 3 /s 0.70 I5-10 I5-12 I5-14 begining I point I5-13 I I5-3 I I I5-6 I I5-9 I5-4 I distances from the begining of the weir "L i " (m) side wall of diversion plant (left)-variant_0 side wall of diversion plant (right)-variant_0 side wall of diversion plant (left)-variant_1 side wall of diversion plant (right)-variant_1 side wall of diversion plant (left)-variant_2 side wall of diversionplant (right)-variant_2 over weir crest level-variant_0 over weir crest level-variant_1 over weir crest level-variant_2 Figure 7 Shape and values of water levels heights over weir crest level and at side walls of diversion plant (test No. 4 variant solutions 0, 1, 2) 3.5 Discharge coefficient Value of discharge coefficient for undrowned overflowing over side weir in rectangular uniform channel with heights H1 = 0,57m ; H2 = 0,65m (figure 8), that are similar to values obtained at investigated U shape weir for variant solution 0 is Cd 0 = 0,595 (Cd = 0,375+0,251 (H1/H2)) Kremeneškin [3]. H1 H2 Figure 8 boundary water levels H1 and H2 for udrowned overflowing over side weir in rectangular uniform channel similar to those obtained at U weir in diversion structure (test No. 4 variant solution 0) Discharge coefficient Cd i obtained from tests No. 4, 5 (undrowned) and No. 6 (drowned) for all variant solutions 0, 1, 2, FS are divided with Cd 0 and calculated nondimensional values are

7 Physical Model Investigation on optimum Design of U shaped Weir in combined Sewer 149 shown in table 2. Values of discharge coefficients Cd i for U weir in all variant solutions has been calculated from calibration equation: C d = 2b 3Q 2g H 3 / 2 0 where Q discharge over weir, b weir length, H 0 overflowing height at measurement point I5-10 (see figure 6). Cd/Cd 0 Table 2 nondimensional discharge coefficients Cd/Cd 0 for variant solutions 0, 1, 2, FS obtained during tests No. 4, 5, 6 NEPOTOPLJENO POTOPLJENO Qoverflowing =57,8 m3/s Qoverflowing =76,5 m3/s Qoverflowing =70,8 m3/s VARIANT 0 1,050 0,869 - VARIANT 1 1,024 0,827 - VARIANT 2,FS 1,076-0, Scam board influence Four different lining and submerge depth of scam board in division structure have been examined (figure 9). These have influence on buoyant sediment removal efficiency and discharge coefficient of investigated U weir. Nondimensional ratios of water depths measured at points I1, I2, I3, I4, I5 and calculated discharge coefficients on variant solution FS under the flow condition No.4 with (index FS+S i ) and without scam board (index FS) is given in table m A=0,7m B,D=0,6m C=0,1m 1,2m Figure 9 geometry and lining of examined scam boards in division structure (variant solution FS)

8 150 Lončar, Andročec, Gjetvaj Table 3 comparison of depths and discharge coefficients dependency on used scam board type TEST 4 h FS / h FS +Si Cd FS +Si / Cd FS Variant I1 I2 I3 I4 I5-1 FS+S A 1,03 1,03 1,00 1,03 1,01 0,908 FS+S B 1,02 1,02 1,00 1,03 1,01 0,930 FS+S C 1,01 1,01 1,00 1,02 1,00 0,975 FS+S D 1,07 1,07 1,00 1,03 0,99 0, Conclusion Four variant solutions of division structure with U shaped weir and all incoming and outgoing channels included in project documentation has been investigated on physical model which followed Froude similarity conditions. Six different flow conditions in incoming channel as part of boundary condition have been established on all variant solutions. After acquisition and analysis of measurements data recordings of velocity, pressures and depths on each variant solution, geometry of particular modeled part was changed what consequence either in decrease of building cost or in improvement of hydraulic functionality. Influence of four different scam board geometry placed into the division structure of final solution FS were also investigated. Hydraulic loss coefficient of whole division structure in final variant solution FS calculated on base of equation h = ξ diversion plant *(v 2 /2g) was ξ diversion plant = 11 where v represents average velocity in incoming channel. For comparison, on first investigated variant solution 0, calculated values of hydraulic loss was ξ diversion plant = 22. Moreover, division structure in final variant solution was averagely 4,7m narrower then in first investigated variant solution 0. Scam board type B with submerge depth 60cm under the weir crest level caused minimum and accepted water surface elevation in whole modeled area. Decrease of discharge coefficient in comparison with the same variant solution without scam board was 7%. Floating material used in model tests ware also permanently hold on that type of scam board. References [1] Kobus, H., (1980), Hydraulic Modeling, Verlag Paul Parley, Germany. [2] Novak, P.,Čabelka, J., (1981), Models in Hydraulic Engineering, Pitman Advanced Publishing Program, USA. [3] Kremešecki, N., (1980), Hidravlika, Moskva, Russia Authors mr.sc. Goran Lončar: University of Zagreb, Faculty of civil engineering, goran.loncar1@zg.htnet.hr prof. Vladimir Andročec: University of Zagreb, Faculty of civil engineering, androcec@grad.hr prof. Goran Gjetvaj: University of Zagreb, Faculty of civil engineering, goran@grad.hr