The Use of Grass Swales for the
Control of Stormwater
Robert Pitt, P.E., Ph.D., DEE, D. WRE
Department of Civil, Construction, and Environmental
Engineering
The University of Alabama
2011 Fox-Wolf Stormwater Conference
Appleton, Wisconsin
Photo by Shirley Clark
Outline of Presentation Topics
• Removal mechanisms in grass swales and
research results
• Recent research on scour in stormwater
controls
• Modeling grass swales
Pollutant Control in Grass Swales
Runoff from
Pervious/
impervious
area
Reducing runoff
velocity
Trapping sediments
and associated pollutants
Sediment
particles
Reduced volume and treated
runoff
Infiltration
Selected Grass Swale Research Results
• IJC (1979) found swale drained areas had up to 95% less flows and
pollutant yields compared to curb and gutter drained areas.
• NURP (1983) found soluble and particulate heavy metals reduced by
50% and COD, nitrate and ammonia nitrogen reduced by about 25%.
• Pitt & McLean (1986) found about 50% reductions in pollutants and
runoff volume; for small frequent rains very little runoff was
observed.
• Johnson, et al. (2003) at the Univ. of Alabama identified hydraulic
characteristics of stormwater swales under typical flows and plant
bioremediation benefits in swales for heavy metal trapping (report
available through WERF).
• Nara and Pitt (2005) at the Univ. of Alabama identified significant
factors affecting particulate transport in grass swales and developed
candidate model algorithms. Modeled procedure integrates particle
settling with swale hydraulics.
WERF Project 97-IRM-2
Innovative Metals Removal Technologies for
Urban Stormwater
Conducted by the University of Alabama from 1999 to 2003
• Examined the characteristics and treatability of
stormwater heavy metals.
• Conducted detailed laboratory and field tests for
the control of stormwater heavy metals by
media filtration and grass swales.
• Provide guidelines to enhance the design of
filters and swales for metals capture from
stormwater.
Components of UA innovative metal removal research project for WERF
Hydraulic Studies
Metals Capture
Media
Amended
Soils
Media studies
Metal
Associations
Role of
Microorganisms
in Sorption
Role of
Grasses
Grass Swale Research Tasks
•
•
•
•
Measure swale hydraulic characteristics (Manning’s
“n” ) for low flow conditions appropriate for typical
grass swale drainage systems.
Test hydraulic and pollutant removal performance
for different flow rates, slopes, and grass types.
Examine subsurface water quality for swale having
amended soil lining.
Develop guidelines to optimize swale design and
construction for use as a stormwater control
technology.
Particles Sizes of Particulates in Runoff
Depend on Sampling Location
Source Areas
Particle median size 1
to 100 µm at source
areas
Outfalls
Particle
median
size 5 to
15 µm at
outfalls
Particle size distributions of stormwater pollutants have a great
affect on pollutant control. Distributions depend on sampling
location. Drainage systems effectively remove the larger particles
between the inlets and the outfall as long-term bedload deposition.
Low Flow Swale vs. Historical Stillwater, OK,
Grass Channel Retardance Curves
Swale hydraulic Manning’s n roughness characteristics can be
predicted on the basis of flow rate, cross sectional geometry, slope,
Jason Kirby MSCE thesis 2005
and vegetation type.
Runoff Heavy Metals Retained and Released
during Indoor Swale Experiments
Metals retained, %
Zoysia
Centipede
Bluegrass
Cu
40
39
40
Cr
16
14
37
Pb
65
57
67
Zn
13
20
26
Cd
21
28
25
The removals of these metals are directly correlated to their
associations with stormwater particulates.
Major ions released, % (these are soil constituents)
Fe
Na
Mg
Ca
K
Zoysia
6
23
17
12
76
Centipede
45
62
87
44 125
Bluegrass
338
77
52
17
23
These are concentration changes only and do not reflect discharge
loading reductions associated with concurrent infiltration. Typical
mass discharge reductions for grass swales are greater than 80%.
Phytoremediation in Grass Swales
Outdoor Swale with Amended Soils and Pan
Lysimeter to Collect Subsurface Flows
Summary - Metals Removal in
Swales
• Indoor swales were found to reduce heavy metal
concentrations by 14 to 67% during controlled tests.
• Outdoor swales reduced metal concentrations by
about 25% during actual storm events.
• Proper swale design was more important than grass
species in performance.
• Overall data showed that swales can improve or
deteriorate the water quality during separate storm
events due to scour of previously deposited metals.
Research Objectives of Continued Grass
Swale Research at UA
(funded by the UTCA, Univ. Transportation Center for
Alabama, and many unfunded student projects)
• To understand the effectiveness of grass
swales for trapping different sized particles
• To understand the associated effects of
different variables on particulate removal
• To develop a predictive model for sediment
movement in grass swales
• Initial series of indoor grass swale
experiments
108 samples collected
• Second series of indoor grass swale
experiments
108 samples collected
• Outdoor grass swale monitoring
69 samples collected (during 13 storm events)
100
Indoor controlled environment
grass swale setup
Sediments
-Sand (300-425 um)
70
-Sand (90-250 um)
60
-Silica-#250
50
-Silica-#105
40
Cumulative mass (%)
90
80
10%
25%
50%
15%
30
20
10
0
Mixing chamber
1
10
100
1000
Particle diameter (micro meter)-log scale
Head works
2ft
3ft
6ft
Synthetic turf
Zoysia
Bluegrass
Variables and analytical methods
• Study of variables
1) Grass types
2) Slopes
3) Flow rates
4) Swale lengths
• Analytical methods
1) Total particulates (SSC)
2) Turbidity
3) Total Suspended Solids
4) Total Dissolved Solids
5) Particle Size Distribution by Coulter Counter
(Beckman® Multi-Sizer III)
Total Suspended Solids “Bluegrass”
1000
slope flow rate
1% _10gm
1% _15gm
1% _20gm
3% _10gm
3% _15gm
3% _20gm
5% _10gm
5% _15gm
5% _20gm
Total Suspended Solids (mg/L)
900
800
700
600
500
400
300
200
100
0
0
Head works
1
2
3
Distance (ft)
4
5
6
Box plots of turbidity concentrations at different swale lengths
Statistical procedure: Kruskal-Wallis test
120
Turbidity (NTU)
100
p=0.000 (overall)
80
60
p=0.002
p=0.197
p=0.001
40
20
0
0 ft
2 ft
3 ft
Swale length
6 ft
Solids Removal in Swales: Flow Length
Normal Probability Plot for Location
Box Plot for Location
Head Work
99
2ft
310
End
95
Goodness of Fit
90
260
Percent
mg/L (Location)
AD*
80
0.549
70
60
50
40
30
0.749
1.001
20
210
10
5
160
1
Head Work
2ft
End
150
200
250
300
mg/L (total solid)
Solids Removal in Swales: Flow Depth
Normal Probability Plot for Flow depth
Box Plot for Flow depth
Deep
99
Shallow
310
Goodness of Fit
95
AD*
0.506
0.893
80
260
Percent
mg/L (Total Solid)
90
70
60
50
40
30
20
210
10
5
160
1
Deep
Shallow
160
210
260
mg/L (Total Solid)
310
Box plots of median particle sizes at different swale lengths
Median particle size ( micro meter)
22.5
Statistical procedure: Kruskal-Wallis test
20.0
17.5
15.0
p=0.002
12.5
p=0.257
p=0.001
10.0
7.5
p=0.000 (overall)
5.0
0ft
2ft
3ft
Swale length
6ft
Modeling Sediment Transport
1) First order decay (for sensitivity analyses)
Ln(Cout / Cin ) = -kt
Cout = Sediment concentration at sampling locations
Cin = Initial sediment concentration at the headwork
k = First order kinetic constant
t = Distance from the headwork
2) “Settling frequency” (for design); similar to
Ana Deletic’s (Monash Univ.) method
= traveling time / settling duration
Traveling time = Swale length / flow velocity
Settling duration = flow depth / settling velocity (Stoke’s Law)
Different grass types
Percent reductions vs Settling frequencies
100
90
Percent reduction (%)
80
70
60
50
40
Bluegrass
30
Zoysia
20
Synthetic turf
10
0
0.00001
0.0001
0.001
0.01
0.1
Settling frequency
1
10
100
1000
Different flow depth/grass height ratios
Flow depth/ Grass height ratio classification
100
Percent reduction (%)
90
80
70
60
50
40
0 - 1.0
30
1.0 - 1.5
20
1.5 - 4
10
0
0.00001
0.00010
0.00100
0.01000
0.10000
1.00000
Settling frequency
10.00000
100.00000 1000.0000
0
Modeling Equations
Ratio: 0 - 1.0
Y  2.101 * [log( X )]2  6.498 * log( X )  76.82
Ratio: 1.0 – 1.5 Y  8.692 * log( X )  80.94
Ratio: 1.5 – 4.0 Y  2.382 *[log( X )]2  15.47 * log( X )  67.46
100
90
Percent reduction (%)
80
Ratio: 0 - 1.0
70
60
Ratio: 1.0 - 1.5
50
40
Ratio: 1.5 - 4
30
Total Dissolved Solids
(<0.45 µm)
20
10
0
0.00001
0.0001
0.001
0.01
0.1
Settling frequency
1
10
100
Outdoor Grass Swale Observations
Description of the test site
Length of swale: 116 ft
Type of grass: Zoysia
116 ft
75 ft
6 ft
25 ft
Approx. watershed area:
4200 ft2 = 0.1 acres
Events: 13 storm events
from 8/22 to 12/08/04
3 ft
2 ft
Soil texture: compacted
loamy sand
Head (0ft)
Indicates sampling
locations
Infiltration rate: < 1 in/hr
Date: 10/11/2004
116 ft
TSS: 10 mg/L
75 ft
TSS: 20 mg/L
25 ft
TSS: 30 mg/L
6 ft
3 ft
2 ft
TSS: 35 mg/L
TSS: 63 mg/L
Head (0ft)
TSS: 84 mg/L
TSS: 102 mg/L
Box plots of TSS at different swale lengths
Statistical procedure: Kruskal-Wallis test
160
P=0.563
Total Suspended Solids (mg/L)
140
P=0.019
High sediment
reduction region
Scouring region
P=0.045
Slight sediment
reduction region
120
100
73 mg/L
80
60
30 mg/L
40
10 mg/L
20
0
0
2
3
6
25
Swale length (ft)
75
116
Particle size distributions: 12/06/2004
100
Typical single event
showing obvious
particle size trend
with distance
90
70
60
28.4 µm
30
Median particle size (µm)
Cumulative vomule (%)
80
50
40
30
25
20
15
10
5
0
20
0 ft
2 ft
3 ft
6 ft
25 ft
116
ft
0
7.5 µm
50
100
Swale length (ft)
10
0
0.1
1
10
Particle diameter (micro meter)
100
1000
150
Particulate Transport in Outdoor Swale (6 rain events)
Percent reductions between 3ft and 25 ft vs. settling frequencies
100
90
Percent reduction (%)
80
70
60
50
40
30
Rapid drop-off
for smaller
settling
frequencies
(deeper water
and/or smaller
particles)
Generally 60 to 80%
reductions when settling
frequency is 1, or larger)
20
10
0
0.0001
0.0010
0.0100
0.1000
1.0000
10.0000
Settling frequency
100.0000 1000.000 10000.00
0
00
Comparison of regression line with 95% CI from indoor
experiments and outdoor observations
100
90
Percent reduction (%)
80
High initial concetration
200 mg/L- 1000 mg/L (TSS)
Ratio: 0 - 1.0
70
60
50
40
30
20
Low initial concetration
40 mg/L - 160 mg/ L (TSS)
Ratio: 0 - 1.0
10
0
0.00001
0.0001
0.001
0.01
0.1
Settling frequency
1
10
100
1000
• Outdoor swale observations
* Significant reductions were observed in TSS and turbidity.
* Three distinct swale regions:
1) 0 ft – 3 ft:
Scouring region (equilibrium concentrations)
2) 3 ft – 25 ft:
High sediment reduction region
3) 25 ft – 116 ft: Slight sediment reduction region (relatively
constant concentrations)
• Model verifications
* Initial sediment concentrations were found to be an important variable
in sediment transport in grass swales.
* The predictive model for low TSS concentrations was only available for
<1 (flow depth / grass height) ratio conditions.
Initial Motion and Initial Suspension Criteria
Sediment bed shifting will not necessarily represent migration out
of the system because the sediment does not necessarily reach the
outlet. Only suspended sediment is assumed to leave the system,
and only if the critical flow rates are long enough to transport the
sediment thru the system.
The Cheng-Chiew criterion (1999), which involves both initial
motion and initial suspension, was evaluated. This criterion relates
the critical shear stress with the probability that sediment with a
particular specific gravity, diameter, and settling velocity, becomes
part of the sediment bed load or gets suspended and moves.
This shear stress was compared to initial-motion and initialsuspension critical shear stresses associated with a specific particle
size. A total of 30 different scenarios were evaluated using
calibrated computational fluid dynamic (CFD) simulations and pilotscale tests.
Initial Motion and Initial Suspension Criteria
Critical Shear Stress Criteria
Cheng-Chiew (1999) Initial Motion
Cheng-Chiew P=10%
Xie (1981)
Cheng-Chiew P=1% Initial Suspension
Van Rijn (1984)
Shields (Vanoni, 1975)
1
Suspended Load
t*
Bed Load
0.1
No motion
0.01
0
1
10
Re*
100
1000
Initial Motion and Initial Suspension Criteria
Initial Motion and Initial Suspension Shear Stress
Cheng-Chiew Criterion (1999)
Initial Motion
Initial Suspension
(Lb/ft2)
100
2.09
0.209
Shear Stress (Pa)
10
0.0209
1
0.1
0.01
1
10
100
1000
Diam eter (mm )
10000
100000
Shear Stress: 0.8 m-wide Rectangular Inlet
Shear stress on the sediment layer at different water depths with a
rectangular inlet of 0.8-m wide, and initial suspension threshold for different
particle sizes. Series of graph classified by flow rate: 40, 20, 10, 5, and 2 LPS
Scour Tests Results: Turbidity Time Series –
Sequential Flow Rate and Bed Armoring
A decreasing exponential pattern was found
in the turbidity time series for each flow rate
at steady conditions.
The impacting zone is stabilized by
dispersion, and buoyancy (air entrainment).
Steady state is reached.
1200
0.3 LPS
The large particles create an armoring on
the sediment surface bed which protects the
small particles below from being scoured.
3.0 LPS
6.3 LPS
10 LPS
800
600
`
400
200
0
0
Small particles are suspended and washed
out creating a hole and leaving the large
particles on the sediment bed surface.
1.3 LPS
1000
Turbidity (NTU)
The initial impact of the plunging water jet
disturbs the sediment bed exposing all the
particle sizes.
Turbidity Time Series at the Outlet
Elevation: 10 cm below outlet
20
40
60
80
100
120
140
Time (min)
This Turbulent Time Series shows
that armoring is created exponentially
over time.
Scour Tests Results: Turbidity Time Series –
Sequential Flow Rate and Bed Armoring for Different Water Depths
Turbidity Time Series at the Outlet
Elevation: 25 cm below outlet
Turbidity Time Series at the Outlet
Elevation: 10 cm below outlet
1200
0.3 LPS
1.3 LPS
3.0 LPS
6.3 LPS
10 LPS
120
0.3 LPS
800
600
`
400
3.0 LPS
6.3 LPS
10 LPS
80
`
60
40
20
200
0
0
0
20
40
60
80
Time (min)
100
120
0
140
20
25
0.3 LPS
1.3 LPS
3.0 LPS
6.3 LPS
40
6
10 LPS
Turbidity (NTU)
20
15
`
10
0.3 LPS
1.3 LPS
3
0
0
60
80
Time (min)
120
140
100
120
140
3.0 LPS
6.3 LPS
10 LPS
`
2
1
40
100
No evident pattern
at low flow rates
and deep water
4
5
20
80
Turbidity Time Series at the Outlet
Elevation: 106 cm below outlet
5
0
60
Time (min)
Turbidity Time Series at the Outlet
Elevation: 46 cm below outlet
Turbidity (NTU)
1.3 LPS
100
Turbidity (NTU)
Turbidity (NTU)
1000
0
20
40
60
80
Time (min)
100
120
140
Five Components to Modeling
Grass Swales
•
•
•
•
•
Swale Density
Swale Infiltration Rate
Swale Geometry
Grass Characteristics
Runoff Particle Size
Distribution and Flow
Hydrograph
Particulate Removal Calculations
100
90
80
Percent reduction (%)
For each time step  Calculate flow velocity, settling
velocity and flow depth
• Determine flow depth to grass
height, for particulate
reduction for each particle size
increment using Nara & Pitt
reference
 Check particle size group limits
 Not exceed irreducible
concentration value
 No filtering for particles less
than 50 microns
Ratio: 0 - 1.0
70
60
Ratio: 1.0 - 1.5
50
40
Ratio: 1.5 - 4
30
Total Dissolved Solids
(<0.45 µm)
20
10
0
0.00001
0.0001
0.001
0.01
0.1
Settling frequency
1
10
100
Percentage Suspended Solids Reduction in a
Typical Residential Area Grass Swale, as a
Function of Swale Density (ft/acre)
70
60
50
40
30
20
10
0
0
100
200
300
400
500
600
700
800
Acknowledgements
• WERF supported initial grass swale research that
investigated swale hydraulics and metal
removals, including the role of vegetation.
• AL DOT/UTCA helped us expand this research to
develop relationships between particle capture
and size, swale hydraulics, and grass
characteristics.
• Numerous graduate students assisted with this
research, especially Jason Kirby, Yukio Nara, and
Humberto Avila.