Particulate Transport in Grass Swales Robert Pitt, P.E., Ph.D., DEE and S. Rocky Durrans, P.E., Ph.D. Department of Civil, Construction, and Environmental Engineering The University of Alabama Yukio Nara Woolpert, Inc. Cincinnati, Ohio Jason Kirby, Ph.D. Department of Civil, Construction, and Environmental Engineering The University of Alabama at Birmingham
Selected Research Results IJC (1979) found swale drained areas had up to 95% less flows and pollutant yields compared to curb and gutter. NURP (1983) found soluble and particulate heavy metal concentrations reduced by 50% and COD, nitrate and ammonia nitrogen reduced by about 25%. Pitt & McLean (1986) found about 50% reductions in pollutant yields and runoff volume in an area half drained by swales; for small frequent rains very little runoff was observed in the swale area. 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). Recent research (Nara and Pitt 2005) at the Univ. of Alabama identified significant factors affecting particulate transport in grass swales and developed suitable model algorithms. Modeled procedure joins particle settling with swale hydraulics, including infiltration benefits.
Particulate Removal in Shallow Flowing Grass Swales and in Grass Filters Runoff from Pervious/ impervious area Reducing velocity of runoff Trapping of sediments and associated pollutants Sediment particles Infiltration Reduced volume and treated runoff
Grass-Lined Swales
Large capacity grass swales and channels designed for both conveyance and water quality objectives.
Grass Swales Designed to Infiltrate Large Fractions of Runoff (Alabama and Washington). Swales can be both interesting and fit site development objectives.
Elements of Conservation Design for Cedar Hills Development (near Madison, WI, project conducted by Roger Bannerman, WI DNR and USGS) Grass Swales Wet Detention Pond Infiltration Basin/Wetland Reduced Street Width
C e d a r H i ll S it e D e s ig n, C r o s s p la i n s W I E x p la n a t io n W e t p o n d In f ilt r a t io n s B a s in S w a le s S id e w a lk D r iv e w a y H o u s e s L a w n s R o a d w a y W o o d lo t N 5 0 0 0 5 0 0 1 0 0 0 F e e t
Conventional curbs with inlets directed to site swales WI DNR photo
Reductions in Runoff Volume for Cedar Hills (calculated using WinSLAMM and verified by site monitoring) Type of Control Runoff Volume, inches Pre-development 1.3 Expected Change (being monitored) No Controls 6.7 515% increase Swales + Pond/wetland + Infiltration Basin 1.5 78% decrease, compared to no controls 15% increase over pre-development
Research Objectives To understand the effectiveness of grass swales for different sized particles To understand the associated effects of different variables To develop a predictive model in sediment transport in grass swales
Initial indoor grass swale experiment 108 samples collected Second indoor grass swale experiment 108 samples collected Outdoor grass swale monitoring 69 samples collected (13 storm events)
Cumulative mass (%) 100 90 80 70 60 50 40 Sediments -Sand (300-425 um) 10% -Sand (90-250 um) 25% -Silica-#250 50% -Silica-#105 15% Mixing chamber 30 20 10 0 1 10 100 1000 Particle diameter (micro meter)-log scale Head works 2ft 3ft Synthetic turf 6ft Zoysia Bluegrass
Variables and analytical methods Study of variables 1) Grass types 2) Slopes 3) Flow rates 4) Swale lengths Analytical methods 1) Total solids 2) Turbidity 3) Total Suspended Solids 4) Total Dissolved Solids 5) Particle Size Distribution by Coulter Counter (Beckman Multi-Sizer III)
Total Suspended Solids (mg/l) Total Suspended Solids Bluegrass 1000 900 800 700 600 500 400 300 200 100 0 0 1 2 3 4 5 6 Head works Distance (ft) slope flow rate 1%_10gm 1%_15gm 1%_20gm 3%_10gm 3%_15gm 3%_20gm 5%_10gm 5%_15gm 5%_20gm
Tu r b id it y ( NTU) Box plots of turbidity concentrations at different swale lengths 1 2 0 Statistical procedure: Kruskal-Wallis test 1 0 0 p = 0.0 0 0 ( o v e r a ll) 8 0 6 0 p = 0.0 0 2 p = 0.1 9 7 4 0 p = 0.0 0 1 2 0 0 0 ft 2 ft 3 ft 6 ft S w a le le n g t h
Significant factors and p-values at 6 ft P-values were computed for constituent concentrations for each variable
M e d ia n p a r t ic le s iz e ( m ic r o m e t e r ) Box plots of median particle sizes at different swale lengths 2 2.5 Statistical procedure: Kruskal-Wallis test 2 0.0 1 7.5 1 5.0 p = 0.0 0 2 1 2.5 p = 0.2 5 7 p = 0.0 0 1 1 0.0 7.5 p = 0.0 0 0 ( o v e r a ll) 5.0 0 ft 2 ft 3 ft 6 ft S w a le le n g t h
Cumulative Volume (%) 100 Zoysia grass, 3% slope, 20 GPM 90 80 70 60 50 40 30 0 ft 2 ft 3 ft 6 ft 20 10 0 0.1 1 10 100 Particle diameter (µm)
Modeling Particulate Transport in Grass Swales and Grass Filters
Settling frequency Concept: = traveling time / settling duration the larger the settling frequency, the more times the particle will bounce along the flow path (with an increased probability of being permanently captured). Larger particles have a greater settling frequency than small particles for the same flow conditions: Traveling time = Swale length / flow velocity Settling duration = flow depth / settling velocity (Stoke s Law)
Percent reduction (%) Different grass types Percent reductions vs Settling frequencies 100 90 80 70 60 50 40 30 20 10 Bluegrass Zoysia Synthetic turf 0 0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000 Settling frequency
Percent reduction (%) Settling Frequency vs. Particulate Capture (a function of ratio of flow depth to grass height) 100 90 80 70 Ratio: 0-1.0 60 50 40 Ratio: 1.0-1.5 Ratio: 1.5-4 30 20 10 Total Dissolved Solids (<0.45 µm) 0 0.00001 0.0001 0.001 0.01 0.1 1 10 100 Settling frequency
Outdoor Grass Swale Observations Description of the test site Length of swale: 116 ft Type of grass: Zoysia 3 ft 2 ft Head (0ft) Indicates sampling locations 116 ft 75 ft 6 ft 25 ft Approx. watershed area: 4200 ft 2 = 0.1 acres Events: 13 storm events from 8/22 to 12/08/04 Soil texture: compacted loamy sand Infiltration rate: < 1 in/hr
Small Puddle at Swale Entrance Paved road Watershed 0.1 acres Side walk 08/22/2004 Grass swale Building Inlet
Date: 10/11/2004 116 ft 75 ft TSS: 10 mg/l TSS: 20 mg/l 25 ft TSS: 30 mg/l 6 ft 3 ft 2 ft TSS: 63 mg/l TSS: 35 mg/l Head (0ft) TSS: 84 mg/l TSS: 102 mg/l
To t a l S u s p e n d e d S o lid s ( m g / L ) Box plots of TSS at different swale lengths Statistical procedure: Kruskal-Wallis test 1 6 0 P=0.563 P=0.019 P=0.045 1 4 0 Scouring region High sediment reduction region Slight sediment reduction region 1 2 0 1 0 0 8 0 73 mg/l 6 0 30 mg/l 4 0 10 mg/l 2 0 0 0 2 3 6 2 5 7 5 1 1 6 S w a le le n g t h ( f t )
Cumulative vomule (%) Particle size distributions: 12/06/2004 Median particle size (µm) 100 90 80 70 60 50 40 30 20 10 30 25 20 15 10 5 0 0 ft 2 ft 3 ft 6 ft 25 ft 116 ft 28.4 µm 7.5 µm 0 50 100 150 Swale length (ft) 0 0.1 1 10 100 1000 Particle diameter (micro meter)
Percent reduction (%) Particulate Transport in Outdoor Swale (6 rain events) Percent reductions between 3ft and 25 ft vs. settling frequencies 100 90 80 70 60 50 40 30 20 10 0 0.0001 0.0010 0.0100 0.1000 1.0000 10.0000 100.0000 1000.000 0 Settling frequency 10000.00 00
Percent reduction (%) Comparison of regression line with 95% CI from indoor experiments and outdoor observations 100 90 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 1 10 100 1000 Settling frequency
Example: Sediment Capture in Grass Swale the discharge rate is 29 ft 3 /sec (0.80 m 3 /sec) and the particulate solids influent concentration is 250 mg/l the channel bottom width is 5 ft (1.5 m) wide, with 3 (H) to 1 (V) side slopes the calculated normal depth is 0.7 ft (210 mm, 21 cm) and the velocity is calculated to be 5.8 ft/sec (1.8 m/sec) after mature vegetation is established the swale length for this area is 1,250 ft (378 m)
Water is assumed to enter the swale at the midpoint location, resulting in an effective treatment swale length of 625 ft (189 m). With a water velocity of 5.8 ft/sec (1.8 m/sec), the average travel time is 189 m/1.8 m/sec = 105 sec (1.8 m) for this length. The mature grass is about 3 inches (75 mm) in height, so the flow depth to grass height ratio is 210 mm/75 mm = 2.8. The suspended solids concentration is determined to be 250 mg/l and the particle size distribution of the water entering the swale is typical.
Particle Size Range Approx. % of Particulate Solids in Range Particulate Concentration in Size Range 0.45 to 2 µm 0.5 1.3 2 to 5 µm 2.7 6.8 5 to 10 µm 9.2 23.0 10 to 30 µm 40.4 101.0 30 to 60 µm 21.8 54.4 60 to 106 µm 10.6 26.5 106 to 425 µm 14.8 37.0 Total: 100.0 250 mg/l
Percent reduction (%) 100 Comparison of regression lines with 95% Confidence Intervals by different (Flow depth)/(grass depth) Ratios 90 80 70 60 50 40 Ratio: 0 to 1.0 Ratio: 1.0 to 1.5 Ratio: 1.5 to 4 30 20 10 Total Dissolved Solids (< 0.45 µm) 0 0.00001 0.0001 0.001 0.01 0.1 1 10 100 Settling frequency
Particle Size Range Approx. Settling Rate (cm/sec) Settling Time for 21 cm Flow Depth (sec) Settling Frequency for Swale (105 sec travel time) Percent Reduction in Size Range (median) 0.45 to 2 µm 1.52 x 10-4 138,000 0.00076 42 2 to 5 µm 1.10 x 10-3 19,000 0.0055 44 5 to 10 µm 5.05 x 10-3 4,160 0.025 48 10 to 30 µm 3.59 x 10-2 585 0.18 57 30 to 60 µm 0.182 115 0.91 68 60 to 106 µm 0.619 33.9 3.1 74 106 to 425 µm 6.22 3.38 31 96
Particle Size Range (µm) Influent Particulate Conc. in Size Range Irreducible Conc. for Size Range (mg/l) Particulate Conc. for Size Range after Swale (mg/l) Final Resultant Conc. for Size Range (mg/l) 0.45 to 2 1.3 7 0.8 1.3 2 to 5 6.8 5 3.8 5 5 to 10 23.0 5 12.0 12.0 10 to 30 101.0 10 43.4 43.4 30 to 60 54.4 5 17.4 17.4 60 to 106 26.5 5 6.9 6.9 106 to 425 37.0 10 1.5 10 An overall 62% reduction in suspended solids concentration was achieved for this example (250 mg/l influent and 96 mg/l effluent).
Conclusions Grass swales and grass filters can be an important component in conservation design. Grass swales can be designed to provide suitable storm drainage benefits and water quality benefits. Particulate trapping by filtering and sedimentation only occurs for relatively shallow flows, and is therefore most important for smaller events in swales. Infiltration (and associated pollutant trapping) may be more important for larger events. Monitoring results have confirmed excellent pollutant yield reductions in swales. Grass swales must be carefully designed to ensure adequate channel stability.