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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 - PowerPoint PPT Presentation
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Evaluating Scour Potential in Stormwater Catchbasin Sumps Using a Full-Scale
Physical Model and CFD Modeling
Humberto AvilaUniversidad del Norte, Department of Civil and Environmental Engineering,
Barranquilla, Colombia. havila@uninorte.edu.co
Robert PittThe University of Alabama, Department of Civil, Construction, and
Environmental Engineering, Tuscaloosa, AL, USA. rpitt@eng.ua.edu
1
World Environmental & Water Resources CongressEWRI – ASCE
2009
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
Methodology and Description of ExperimentThe full-scale physical model was based on the optimal catchbasin geometry recommended by Lager, et al. (1977), and tested by Pitt 1979; 1985; and 1993. 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
3
12 in
Hydrodynamic Tests (Methodology and Description of Experiment)
1636567696
G 12 19 20
F 5 11 18 21 27
E 4 10 17 22 28
D 3 9 16 23 29
C 2 8 15 24 30
B 1 7 14 25 31
A 6 13 26
y
x
Two inlet geometries: Rectangular (50 cm wide), and Circular (30 cm diameter).Three flow rates: 10, 5, and 2.5 LPS (160, 80, and 40 GPM). Velocity measurements (Vx, Vy, and Vz).Five overlaying water depths: 16, 36, 56, 76, and 96 cm above the sediment.
Total points per test: 15530 rapid velocity measurements at each pointInstrument: Acoustic Doppler Velocity Meter (ADV) - Flowtraker
4
Water was re-circulated during the test.
Scour Tests (Methodology and Description of Experiment)
Type of Sediment
Flow rate (L/s)
Water depth over
sediment (cm)
Duration (min)
Sampling (Composite
samples)
Mixture
0.3, 1.3, 3.0, 6.3, and 10
1025 min for each flow
rate
First 5-min, and next 20-min for each flow rate. Inlet samples
for each elevation.
25
46
106
10
10 4 impacts with
prolonged flow of 3 min each
One composite sample for each
impact
2546
106
Homo-geneous 10
2430 min for
each elevation
3-min composite samples at
influent and effluent.
35
The scour tests were performed with a 50-cm wide rectangular inlet.5
Once through test using lake water.The pool traps the scoured sediment before the water discharges back to the lake.
Scour Tests (Methodology and Description of Experiment)
Sediment mixture (D50 = 500 mm; uniformity coefficient = 11)
based on PSD found by Pitt (1997), Valiron and Tabuchi (1992), and Pitt and Khambhammuttu (2006)
0
10
20
30
40
50
60
70
80
90
100
10 100 1000 10000
% S
mal
ler t
han
Particle Diameter (mm)
Particle Size Distribution of Sediment Mixture
Target Sand 1 Sand 2 SIL-CO-SIL 250 Final mix
Particle Size Distribution of Fine Sand
0102030405060708090
100
10 100 1000 10000Particle Diameter (mm)
% S
mal
ler t
han
Sediment with homogeneous particle size (D50 = 180 mm; uniformity coefficient = 2.5)
Develops armoring layer Only a minimal armoring layer developed. Data also used for CFD calibration and validation.
6
D90 = 250 mm
D50 = 180 mm
D10 = 80 mm
D90 = 2000 mm
D50 = 500 mm
D10 = 80 mm
Installation of blocks to set the false bottom
False bottom sealed on the border
Leveling of sediment bed: 20 cm thick
Measuring depth below the outlet (overlaying water depth above sediment)
Performing scour test
Cone splitter and sample bottles
Sieve analysis
Scour Tests
7
Experimental Results - HydrodynamicsEffect of Inlet Type
45 two-sample t-testsp-values < 0.001
Vz (c
m/s
)
0.0001
0.0010.0003
0.010.003
0.10.03
10.3
103
10030
300
c ir rec cir rec c ir rec cir rec c ir rec
16 36 56 76 96
Inlet Type withi n Depth (cm)
Variability Char t for Vz (cm/s)
Variability Gauge Flow rate (L/s)=2.5
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 jets.8
Experimental Results - HydrodynamicsEffect of Overlaying Water Depth
Vz
(cm
/s)
0.0001
0.0010. 0003
0.010. 003
0.10. 03
10. 3
103
100
400
16 36 56 76 96 16 36 56 76 96 16 36 56 76 96
2.5 5 10
Depth (cm) within F low rate (L/s )
Variability Chart for Vz (cm/s)Variability Gauge Inlet Type=cir
One-way ANOVA tests with paired comparisons.Analysis for Vx, Vy, and Vz
p-values < 0.001
9Lowest impacting energy scenario:Rectangular inlet and 2.5 L/s flow rate
Highest impacting energy scenario:Circular inlet and 10 L/s flow rate
CFD ModelingCalibration of Hydrodynamics – 3D-Model
Perc
ent
200-20
99.99990501010.1
200-20
99.99990501010.1
300-30
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
30150
99.99990501010.1
100-10
99.99990501010.1
100-10
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
200-20
99.99990501010.1
24120
99.99990501010.1
100-10
99.99990501010.1
155-5
99.99990501010.1
200-20
99.99990501010.1
6_3 13_3 26_3 1_3 7_3 14_3
25_3 31_3 2_3 8_3 15_3 24_3
30_3 3_3 9_3 16_3 23_3 29_3
4_3 10_3 17_3 22_3 28_3 5_3
11_3 18_3 21_3 27_3 12_3 19_3
20_3
Typeexpsim
Normal - 95% CIProbability Plot of Vz-Velocities - at 76 cm below the outlet
10
CFD Modeling - VelocitiesCalibration of Hydrodynamics – 3D-Model
11
CFD Modeling – Air EntrainmentCalibration of Hydrodynamics – 3D-Model
12
CFD ModelingCalibration and Validation of Hydrodynamics – 2D-Model
151050-5-10-15
99.9
99
959080706050403020105
1
0.1
Vx (cm/ s)
Perc
ent
expsim
Type
Normal - 95% CIProbability Plot of Vx - 10 LPS
38 cm off the wall on Center Line and 36 cm Below Outlet
20100-10-20
99.9
99
959080706050403020105
1
0.1
Vz (cm/ s)
Perc
ent
expsim
Type
Normal - 95% CIProbability Plot of Vz - 10 LPS
58 cm off the wall on Center Line and 36 cm Below Outlet
13
Sediment Scour Tests with Sediment Mixture Armoring Effect
0
200
400
600
800
1000
1200
0 20 40 60 80 100 120 140
Turb
idity
(NTU
)
Time (min)
Turbidity Time Series at the Outlet Elevation: 10 cm below outlet
`
0.3 LPS 1.3 LPS 3.0 LPS 6.3 LPS 10 LPS
ArmoringFlo
w d
irecti
on
Fine sediment
An exponential decay pattern was found in the turbidity time series for each flow rate at steady conditions.
“Washing machine” effect (resuspension of surface layer and flushing out of finer sediment down to depth of resuspension)
14
Turbidity Time Series - Elevation: 10 cm below outletImpacting Test
0
200
400
600
800
1000
1200
0 3 6 9 12 15
Time (min)
Turb
idity
(NTU
)
10 cm
10 LPS 10 LPS 10 LPS 10 LPS
Continuous flow rate Fluctuating flow rate
Experimental Results – Sediment Mixture
Overlaying water
depth (cm)
Flow rate (L/s)0.3 1.3 3.0 6.3 10.0
SSC (mg/L)10 56 392 427 1045 113925 7.0 8.0 42 108 4646 5.0 4.1 6.5 12 11
106 1.7 2.6 3.3 2.9 1.7
Total SSC (mg/L) of scoured sediment for the 0 - 5-min composite samples.
Overlaying water
depth (cm)
Flow rate (L/s)0.3 1.3 3.0 6.3 10.0
SSC (mg/L)10 12.6 55 102 244 68425 1.6 5.5 20 22 4446 2.0 1.5 4.8 11 12106 0.6 1.1 2.0 2.1 4.0
Total SSC (mg/L) of scoured sediment for the 5 - 25-min composite samples.
SSC (mg/L) for the 5 -25-min Composite Sample Tests with Sediment Mixture
0.1
1.0
10.0
100.0
1000.0
0 2 4 6 8 10 12Flow rate (L/s)
SSC
(mg/
L)
10 cm 25 cm 46 cm 106 cm
15
Concentration Vs Depth - by Flow rateComposite Sample 5 - 25 min
0.1
1
10
100
1000
0 20 40 60 80 100 120Depth (cm)
Conc
entra
tion
(mg/
L)
0.3 LPS 1.3 LPS 3.0 LPS6.3 LPS 10.0 LPS
Experimental Results – Sediment Mixture Total Sediment Mass Loss
Total Mass Scoured by Depth below the Outlet
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
0 20 40 60 80 100 120 140Time (min)
Sedi
men
t Mas
s (g
r)
10 cm 25 cm 46 cm 106 cm
0.3 LPS 1.3 LPS 3.0 LPS 6.3 LPS 10 LPS10 cm
25 cm46 cm
106 cm
16
16 Kg
1 Kg
0.3 Kg0.09 Kg
Total Mass Scoured Plotted vs. Overlaying Water Depth above the Sediment Bed and Duration of Flow
CFD-Customized Scour ModelCalibration
Colors represent sediment concentration (g/cm3)
17
Scour Response Surfaces (SSC, mg/L): Sediment Mixture
0-5 min composite samples 5-25 min composite samples
Depth (cm)
Flow
rate
(LPS
)
100908070605040302010
10987654321
> – – – – – – – – – <
800 10001000
66 10
10 2525 5050 100
100 200200 300300 400400 800
min(mg/L) 0 - 5
Concentration
Experimental Response Surface of SSC (mg/ L)Composite Sample 0 - 5 min
Depth (cm)
Flow
rate
(LPS
)
100908070605040302010
10987654321
> – – – – – – – – – <
800 10001000
66 10
10 2525 5050 100
100 200200 300300 400400 800
min(mg/L) 0-5
Fitted Conc.
Fitted Response Surface of SSC (mg/ L)Composite Sample 0 - 5 min
Depth (cm)
Flow
rate
(LPS
)
100908070605040302010
10987654321
> – – – – – – – < 6
6 1010 2525 5050 100
100 200200 300300 400
400
min(mg/L) 5 - 25Concentration
Experimental Response Surfaces of SSC (mg/ L)Composite Sample 5 - 25 min
Depth (cm)
Flow
rate
(LPS
)
100908070605040302010
10987654321
> – – – – – – – < 6
6 1010 2525 5050 100
100 200200 300300 400
400
5-25 min(mg/L)Conc.Fitted
Fitted Response Surface of SSC (mg/ L)Composite Sample 5 - 25 min
Exp. Exp.
ModelModel
18
15.092.032.32670
HQHSSC 19.06.11532 )ln(115 HQHHSSC
Scour Response Surfaces (SSC, mg/L): Sediment Mixture
15.092.032.32670
HQHSSC 19.06.11532 )ln(115 HQHHSSC
0-5 min composite samples 5-25 min composite samples
19
Observed vs Fitted Suspended Sediment ConcentrationComposite Sample: 5 - 25 min
0
1
10
100
1000
0 1 10 100 1000Fitted SSC (mg/L)
Obs
. SSC
(mg/
L)
Observed vs Fitted Suspended Sediment Concentration
Composite Sample: 0 - 5 min
0.0
0.1
1.0
10.0
100.0
1000.0
10000.0
0.0 0.1 1.0 10.0 100.0 1000.0 10000.0Fitted SSC (mg/L)
Obs
. SSC
(mg/
L)
Experimental Results – Sediment Scour Sediment with Homogeneous Particle Size: D50 = 180 mm; Uniformity Coefficient = 2.5
+2
0-3
+3-3-1
+6
+3
-3
+3-2
-1
Experimental SSC (mg/L)10 LPS, 180 um, 24 cm below the Outlet
50
150
250
350
450
550
650
750
0 5 10 15 20 25 30 35Time (min)
Con
cent
ratio
n (m
g/L)
95%LPI 95%UPI 95%LCI 95%UCI Exp.
Experimental SSC (mg/L)10 LPS, 180 um, 35 cm below the Outlet
50
100
150
200
250
0 5 10 15 20 25 30 35Time (min)
Con
cent
ratio
n (m
g/L)
95%LPI 95%UPI 95%LCI 95%UCI Exp.
p-value: 0.6 (Constant SSC concentration), Mean SSC = 533 mg/L, Stdev = 53 mg/L
p-values: 0.5 (Constant SSC concentration)Mean SSC = 178 mg/L Standard deviation = 16 mg/L
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.
20
CFD-Customized Scour ModelCalibration and Validation
Experimental and Simulated Concentration (mg/L)10 LPS, 180 um, 24 cm below the Outlet
0
100
200
300
400
500
600
700
800
0 5 10 15 20 25 30 35Time (min)
Con
cent
ratio
n (m
g/L)
Model 95%LPI 95%UPI 95%LCI 95%UCI Exp.
Homogeneous sediment of 180 mm, 10 L/s flow rate
21
Experimental and Simulated Concentration (mg/L)10 LPS, 180 um, 35 cm below the Outlet
50
100
150
200
250
0 5 10 15 20 25 30 35Time (min)
Conc
entra
tion
(mg/
L)
Model 95%LPI 95%UPI 95%LCI 95%UCI Exp.
Calibration: 24 cm Validation: 35 cm
Cumulative Mass Loss versus Time10 LPS, 180 um, 24 cm below the Outlet
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35Time (min)
Cum
ulat
ive
Mas
s Lo
ss (K
g)
24 cm Exp. (24 cm)
Cumulative Mass Loss versus Time10 LPS, 180 um, 35 cm below the Outlet
0
1
1
2
2
3
3
4
0 5 10 15 20 25 30 35Time (min)
Cum
ulat
ive
Mas
s Lo
ss (K
g)
35 cm Exp. (35 cm)
SSC (mg/L) SSC (mg/L)
Total Mass (Kg) Total Mass (Kg)
Experimental Results – Sediment Scour Sediment with Homogeneous Particle Size: D50 = 180 mm
-20
-1
-3-8
-8
-2
+4-2-8
-2
-2
+4 +2
0-3
+3-3-1
+6
+3
-3
+3-2
-1
24 cm overlaying water depth above the sediment layer.
35 cm overlaying water depth above the sediment layer.
Scour Response Surfaces (SSC, mg/L): Sediment with Homogeneous Particle Size
Depth (cm)
Flow
rat
e (L
PS)
45403530252015
20.0
17.5
15.0
12.5
10.0
7.5
5.0
Response Surface of Suspended Solid Concentration (SSC) - CFD ResultsHomogeneous Particle Size: 50 um
Depth (cm)
Flow
rat
e (L
PS)
45403530252015
20.0
17.5
15.0
12.5
10.0
7.5
5.0
Response Surface of Suspended Solid Concentration (SSC) - CFD ResultsHomogeneous Particle Size: 500 um
Depth (cm)
Flow
rat
e (L
PS)
45403530252015
20.0
17.5
15.0
12.5
10.0
7.5
5.0
Response Surface of Suspended Solid Concentration (SSC) - CFD ResultsHomogeneous Particle Size: 180 um
Depth (cm)
Flow
rat
e (L
PS)
45403530252015
20
18
16
14
12
10
Response Surface of Suspended Solid Concentration (SSC) - CFD ResultsHomogeneous Particle Size: 1000 um
SSC (mg/L):
50 mm 180 mm
500 mm 1,000 mm
23
Scour Response Surfaces (SSC, mg/L): Sediment with Homogeneous Particle SizeSuspended Sediment Concentration Vs Depth below the
Outlet - CFD Results
0
200
400
600
800
1000
1200
1400
1600
10 15 20 25 30 35 40 45 50Depth (cm)
Con
cent
ratio
n (m
g/L)
5 LPS 10 LPS 20 LPS
50 um
1000 um50 um
180 um500 um
180 um
500 um
50 um180 um
500 um
1000 um
DQDQ bHmSSC ,,
Diameter (mm) m b
5 L/s50 -130.47 3479
180 -95.43 2513.7500 -39.9 973
10 L/s
50 -43.7 1966180 -40.45 1612.1500 -26.74 920.6
1000 -11.05 279.4
20 L/s
50 -25.43 1418.1180 -23.2 1196.7500 -17.83 771.35
1000 -9.2 320.25
Observed vs Fitted Suspended Sediment ConcentrationRegression Model Based on Individual Linear Functions
0
200
400
600
800
1000
1200
1400
1600
1800
0 200 400 600 800 1000 1200 1400 1600 1800Fitted Concentration (mg/L)
Obs
. Con
cent
ratio
n (m
g/L)
Fitted SSC (mg/L) 95%LCI95%UCI 95%LPI95%UPI
24
ConclusionsThe 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
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