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