Evaluating Scour Potential in Stormwater Catchbasin Sumps Using a Full-Scale Physical Model and CFD Modeling. Humberto Avila.

Transcription

1 Evaluating Scour Potential in Stormwater Catchbasin Sumps Using a Full-Scale Physical Model and CFD Modeling Humberto Avila Universidad del Norte, Department of Civil and Environmental Engineering, Barranquilla, Colombia. havila@uninorte.edu.co Robert Pitt The University of Alabama, Department of Civil, Construction, and Environmental Engineering, Tuscaloosa, AL, USA. rpitt@eng.ua.edu World Environmental & Water Resources Congress EWRI ASCE 9

2 Introduction Sediment removal measurements in catchbasin sumps and hydrodynamic separators does not necessarily imply the prevention of scour of previously captured sediment. Understanding scour mechanisms and losses in catchbasins and similar devices is critical when implementing stormwater management programs relying on these control practices. Simplified models were developed during this research to calculate sediment scour in catchbasin sumps. These can be implemented in stormwater management software packages, such as WinSLAMM, to calculate the expected sediment scour. 2

3 Methodology and Description of Experiment The full-scale physical model was based on the optimal catchbasin geometry recommended by Lager, et al. (977), and tested by Pitt 979; 985; and 3. Water is re-circulated during the test. Two different evaluations were performed: Hydrodynamics: Velocity measurements (Vx, Vy, and Vz) Scour: Sediment scour at different overlying water depths and flow rates, and for different sediment characteristics 2 in 3

4 Hydrodynamic Tests (Methodology and Description of Experiment) Two inlet geometries: Rectangular ( cm wide), and Circular ( cm diameter). Three flow rates:, 5, and 2.5 LPS (6, 8, and 4 GPM). Velocity measurements (Vx, Vy, and Vz). Five overlaying water depths: 6, 36, 56, 76, and 96 cm above the sediment. G 2 9 F E D C B A Water was re-circulated during the test. y x Total points per test: 55 rapid velocity measurements at each point Instrument: Acoustic Doppler Velocity Meter (ADV) - Flowtraker 4

5 Scour Tests (Methodology and Description of Experiment) Type of Sediment Mixture Homogeneous Flow rate (L/s).3,.3, 3., 6.3, and Water depth over sediment (cm) Duration (min) 25 min for each flow rate 4 impacts 25 with 46 prolonged flow of 3 6 min each min for each elevation Sampling (Composite samples) First 5-min, and next -min for each flow rate. Inlet samples for each elevation. One composite sample for each impact 3-min composite samples at influent and effluent. Once through test using lake water. The pool traps the scoured sediment before the water discharges back to the lake. The scour tests were performed with a -cm wide rectangular inlet. 5

6 % Smaller than % Smaller than Scour Tests (Methodology and Description of Experiment) Sediment mixture (D = mm; uniformity coefficient = ) based on PSD found by Pitt (7), Valiron and Tabuchi (2), and Pitt and Khambhammuttu (6) Sediment with homogeneous particle size (D = 8 mm; uniformity coefficient = 2.5) Particle Size Distribution of Sediment Mixture Target Sand Sand 2 SIL-CO-SIL 2 Final mix Particle Size Distribution of Fine Sand D = mm 4 D = 8 mm Particle Diameter (mm) Develops armoring layer D = mm D = 8 mm D = 8 mm D = 2 mm Particle Diameter (mm) Only a minimal armoring layer developed. Data also used for CFD calibration and validation. 6

7 Scour Tests Installation of blocks to set the false bottom Measuring depth below the outlet (overlaying water depth above sediment) Cone splitter and sample bottles False bottom sealed on the border Leveling of sediment bed: cm thick Performing scour test Sieve analysis 7

8 Experimental Results - Hydrodynamics Effect of Inlet Type Variability Gauge Flow rate (L/s)=2.5 Variability Char t for Vz (cm/s) Vz (cm/s) c ir rec cir rec c ir rec cir rec c ir rec Inl et Type withi n Depth (cm) 45 two-sample t-tests p-values <. The inlet geometry affects the magnitude of the impacting energy of the plunging water jet. The impact of a circular plunging jet is concentrated and the flow rate per unit width is greater than with a rectangular jet. Circular plunging jets affect sediments under deeper water layers than rectangular 8 jets.

9 Vz (cm/s ) Experimental Results - Hydrodynamics Effect of Overlaying Water Depth Variability Gauge Inlet Type=cir Variability Chart for Vz (cm/s) Depth (cm) within F low rate (L/s) One-way ANOVA tests with paired comparisons. Analysis for Vx, Vy, and Vz p-values <. Lowest impacting energy scenario: Rectangular inlet and 2.5 L/s flow rate Highest impacting energy scenario: 9 Circular inlet and L/s flow rate

10 Percent CFD Modeling Calibration of Hydrodynamics 3D-Model Probability Plot of Vz-Velocities - at 76 cm below the outlet Normal - 95% CI _3 3_3 26_3 _3 7_3 4_ _3 3_3 2_3 8_3 5_3 24_3. -. _3 3_3 9_3 6_3 23_3 29_3. 4_3 _3 7_3 22_3 28_3 5_ _3 8_3 2_3 27_3 2_3 9_3 _ Type exp sim

11 CFD Modeling - Velocities Calibration of Hydrodynamics 3D-Model

12 CFD Modeling Air Entrainment Calibration of Hydrodynamics 3D-Model 2

13 CFD Modeling Calibration and Validation of Hydrodynamics 2D-Model Percent Percent Probability Plot of Vx - LPS Normal - 95% CI 38 cm off the wall on Center Line and 36 cm Below Outlet Type exp sim Vx (cm/s) 5 5 Probability Plot of Vz - LPS Normal - 95% CI 58 cm off the wall on Center Line and 36 cm Below Outlet Type exp sim Vz (cm/s) 3

14 Turbidity (NTU) Turbidity (NTU) Sediment Scour Tests with Sediment Mixture Armoring Effect Armoring An exponential decay pattern was found in the turbidity time series for each flow rate at steady conditions Fine sediment Turbidity Time Series at the Outlet Elevation: cm below outlet.3 LPS.3 LPS 3. LPS 6.3 LPS LPS ` Washing machine effect (resuspension of surface layer and flushing out of finer sediment down to depth of resuspension) Turbidity Time Series - Elevation: cm below outlet Impacting Test cm LPS LPS LPS LPS Time (min) Time (min) Continuous flow rate Fluctuating flow rate 4

15 Experimental Results Sediment Mixture SSC (mg/l) Concentration (mg/l) Total SSC (mg/l) of scoured sediment for the - 5-min composite samples. Overlaying water depth (cm) Flow rate (L/s) SSC (mg/l) SSC (mg/l) for the 5-25-min Composite Sample Tests with Sediment Mixture. cm 25 cm 46 cm 6 cm Total SSC (mg/l) of scoured sediment for the 5-25-min composite samples. Overlaying water depth (cm) Flow rate (L/s) SSC (mg/l) Flow rate (L/s) Concentration Vs Depth - by Flow rate Composite Sample 5-25 min.3 LPS.3 LPS 3. LPS 6.3 LPS. LPS Depth (cm) 5

16 Sediment Mass (gr) Experimental Results Sediment Mixture Total Sediment Mass Loss Total Mass Scoured Plotted vs. Overlaying Water Depth above the Sediment Bed and Duration of Flow Total Mass Scoured by Depth below the Outlet cm 25 cm 46 cm 6 cm.e+5.e+4.e+3.e+2.3 LPS.3 LPS 3. LPS 6.3 LPS LPS cm 25 cm 46 cm 6 cm 6 Kg Kg.3 Kg.9 Kg.E+.E+.E-.E-2.E Time (min) 6

17 CFD-Customized Scour Model Calibration Colors represent sediment concentration (g/cm 3 ) 7

18 Flow rate (LPS) Flow rate (LPS) Flow rate (LPS) Flow rate (LPS) Scour Response Surfaces (SSC, mg/l): Sediment Mixture -5 min composite samples 5-25 min composite samples H SSC 67 H Q H SSC 5 H ln( H) Q Experimental Response Surface of SSC (mg/l) Composite Sample - 5 min Exp. Concentration (mg/l) - 5 min < > Experimental Response Surfaces of SSC (mg/l) Composite Sample 5-25 min Exp. Concentration (mg/l) 5-25 min < > Depth (cm) Depth (cm) Fitted Response Surface of SSC (mg/l) Composite Sample - 5 min Model Fitted Conc. (mg/l) -5 min < > Fitted Response Surface of SSC (mg/l) Composite Sample 5-25 min Model Fitted Conc. (mg/l) 5-25 min < > Depth (cm) Depth (cm) 8 8

19 Obs. SSC (mg/l) Obs. SSC (mg/l) Scour Response Surfaces (SSC, mg/l): Sediment Mixture -5 min composite samples 5-25 min composite samples H SSC 67 H Q H SSC 5 H ln( H) Q.. Observed vs Fitted Suspended Sediment Concentration Composite Sample: - 5 min Observed vs Fitted Suspended Sediment Concentration Composite Sample: 5-25 min Fitted SSC (mg/l) Fitted SSC (mg/l) 9

20 Concentration (mg/l) Concentration (mg/l) Experimental Results Sediment Scour Sediment with Homogeneous Particle Size: D = 8 mm; Uniformity Coefficient = 2.5 Experimental SSC (mg/l) LPS, 8 um, 24 cm below the Outlet 95%LPI 95%UPI 95%LCI 95%UCI Exp Time (min) p-value:.6 (Constant SSC concentration), Mean SSC = 533 mg/l, Stdev = 53 mg/l 2 Experimental +3 SSC (mg/l) - -3 LPS, 8 um, 35 cm below the Outlet 95%LPI 95%UPI 95%LCI 95%UCI Exp Absence of an armoring layer causes continuous exposure of the sediment at the surface of the sediment layer (little change in scoured sediment concentration with time). Scour rate will decrease only when the overlaying water depth is large enough (hole) to reduce the acting shear stress Time (min) p-values:.5 (Constant SSC concentration) Mean SSC = 78 mg/l Standard deviation = 6 mg/l

21 Cumulative Mass Loss (Kg) Cumulative Mass Loss (Kg) Concentration (mg/l) Concentration (mg/l) CFD-Customized Scour Model Calibration and Validation Homogeneous sediment of 8 mm, L/s flow rate Experimental and Simulated Concentration (mg/l) LPS, 8 um, 24 cm below the Outlet Experimental and Simulated Concentration (mg/l) LPS, 8 um, 35 cm below the Outlet 8 7 Model 95%LPI 95%UPI 95%LCI 95%UCI Exp. 2 Model 95%LPI 95%UPI 95%LCI 95%UCI Exp. 6 4 SSC (mg/l) Calibration: 24 cm SSC (mg/l) Validation: 35 cm Time (min) Time (min) Cumulative Mass Loss versus Time LPS, 8 um, 24 cm below the Outlet Cumulative Mass Loss versus Time LPS, 8 um, 35 cm below the Outlet 2 24 cm Exp. (24 cm) 4 35 cm Exp. (35 cm) Total Mass (Kg) Time (min) 2 2 Total Mass (Kg) Time (min) 2

22 Experimental Results Sediment Scour Sediment with Homogeneous Particle Size: D = 8 mm cm overlaying water depth above the sediment layer. 35 cm overlaying water depth above the sediment layer.

23 Flow rate (LPS) Flow rate (LPS) Flow rate (LPS) Flow rate (LPS) Scour Response Surfaces (SSC, mg/l): Sediment with Homogeneous Particle Size mm 8 mm Response Surface of Suspended Solid Concentration (SSC) - CFD Results Homogeneous Particle Size: um. Response Surface of Suspended Solid Concentration (SSC) - CFD Results Homogeneous Particle Size: 8 um SSC (mg/l): Depth (cm) Depth (cm) mm, mm Response Surface of Suspended Solid Concentration (SSC) - CFD Results Homogeneous Particle Size: um. 7.5 Response Surface of Suspended Solid Concentration (SSC) - CFD Results Homogeneous Particle Size: um Depth (cm) Depth (cm)

24 Obs. Concentration (mg/l) Concentration (mg/l) Scour Response Surfaces (SSC, mg/l): Sediment with Homogeneous Particle Size Suspended Sediment Concentration Vs Depth below the Outlet - CFD Results 8 um 8 um um um um um um 5 LPS LPS LPS um um um 8 um Depth (cm) 5 L/s L/s L/s SSC mq, D H bq, D Diameter (mm) m b Observed vs Fitted Suspended Sediment Concentration Regression Model Based on Individual Linear Functions Fitted SSC (mg/l) 95%UCI 95%UPI 95%LCI 95%LPI Fitted Concentration (mg/l)

25 Conclusions The overlaying water depth above the sediment is highly important in protecting the sediment from scour. SSC decreases with an exponential pattern as the overlaying water depth increases for a sediment mixture (with armoring), and with a linear pattern for sediment with a homogeneous particle size. The inlet geometry has a significant effect on the scour potential of sediments captured in conventional catchbasin sumps. Modifying the inlet flow to decrease the impacting energy and/or physically isolating the sediment from the impacting water provides a feasible alternative on catchbasins already installed. It is recommended to perform scour tests with fluctuating flow rates to account for the flow variability that actually occurs during rainfall events The scour response surface equations can be implemented in stormwater management software packages to calculate the loss of sediment scoured from catchbasin sumps and similar devices. 25