LOW IMPACT DEVELOPMENT FILTER STRIPS AND SWALES A Presentation to the Lake Tahoe Interagency Task Force May 4, 2011 William C. Lucas, Partner, Chief Scientific Officer TerraSolve LLC
Impervious Area Disconnection There is a large body of literature showing how swales and filter strips substantially reduce runoff volumes. Some of this is summarized in Delaware s Green Technology Manual (Lucas, 2004). The fundamental process is that the runoff acts like additional rainfall applied to the wetted pervious surfaces. Runoff is then computed as the sum of rainfall and runon, after losses due to infiltration. Runoff reductions vary from 30% to 90%. Depends upon hydraulic loading rate, flow retention time, and underlying infiltration rate. Since soil compost amendments greatly improve infiltration rates, there are much lower loads, even with higher nutrient concentrations. Given these reductions require relatively little effort, disconnection should be the first option for site design. However, it is hardly ever computed explicitly, which is surprising, considering its potential.
LID Hydrology with Disconnection NATURAL PERVIOUS AREAS GRADED PERVIOUS AREAS DISCONNECTED IMPERVIOUS AREAS CONNECTED IMPERVIOUS AREAS FLOW PATH AND MASS LOAD COMPUTATIONS TO SCM(s)
Schematic Concept for Runoff Reduction Due to Disconnection 2.0 INCH RAIN = 1.97 INCHES RUNOFF, AT 60 BY 30 ROOF, = 295 CU.FT. RUNOFF 2.0 INCH RAIN = 0.20 INCHES RUNOFF, AT 600 SQ.FT. LAWN, = 10 CU.FT. RUNOFF 295 CU.FT. / 600 SQ.FT, = 5.9 INCHES, ADD 2.0 INCHES RAIN, TOTAL = 7.9 INCHES RAIN, = 2.89 INCHES RUNOFF 1800 square foot impervious area 600 square foot pervious wetted area IF IMPERVIOUS CONNECTED: TOTAL RUNOFF= 305 CU. FT. IF IMPERVIOUS DISCONNECTED: TOTAL RUNOFF= 145 CU. FT. REDUCTION = 52.5%
Filter Strip Description Pervious areas subjected to sheet flow where runoff can be both filtered and infiltrated. Pollutant removal through sedimentation, biological uptake, & infiltration into soil media. (Not filtration!) Design variants: Lawns- can treat & infiltrate most runoff from roofs if properly situated and downspouts dispersed. Roadside margins- can treat & infiltrate most runoff from roads, especially where improved soils are used. Planting strips- can treat & infiltrate runoff from adjacent parking lots
Filter Strip BMP Performance 100% PERCENT MAXIMUM REDUCTION 80% 60% 40% 20% 2% Slopes, DURMM 2% Slopes, Abu Zreig, 2001 12% Slopes, DURMM 12% Slopes, Abu Zreig, 2001 11% slopes, Dillaha et al, 1989 Clay fraction, Abu Zreig, 2001 16% slopes, Dillaha et al, 1989 0% 0 5 10 15 20 25 30 35 40 45 50 WIDTH (ft.) (Lucas, 2004) Filter strip performance affected by sediment EMC, particle size, hydraulic load, slope, stem density and length. Dillaha et al results above for agricultural loads of ~4000 mg/l. Most sediment (coarse fraction) drops out within 25 feet. However, clay fraction requires much more distance.
Filter Strip BMP Performance (Blanco-Criqui et al 2004) Field study of grass filter strips planted in crops, so very high TSS loads (so reductions easier to obtain). Note dramatic reduction in TSS in first meter. Native grass barrier was even more effective. High initial runoff loss in first meter due to ponding upslope. Subsequent losses less since so short. Even so, 15% reduction observed.
Filter Strip BMP Performance (Blanco-Criqui et al 2004) Nutrient losses were also substantial, even for dissolved forms! Barrier with native grasses provide the best performance.
Filter Strip BMP Performance Note urban filter strips are not very sensitive to slope: PERCENT MAXIMUM REDUCTION 100% 95% 90% 2% SLOPES 2% CURVE 6% SLOPES 6% CURVE 15% SLOPES 15% CURVE 25% SLOPES 25% CURVE 0 3 6 9 12 15 18 21 24 27 FILTER STRIP LENGTH (ft.) However, more sensitive to hydraulic loads: 100% PERCENT MAXIMUM REDUCTION 95% 90% 85% 10.5 CU.FT./FT. 10.5 CURVE 21.0 CU.FT./FT. 21.0 CURVE 31.5 CU.FT./FT. 31.5 CURVE 80% 0 3 6 9 12 15 18 21 24 27 FILTER STRIP LENGTH (ft.) (Lucas, 2004)
Filter Strip: Design Guidance: Hydraulic Loading Rate: Less than 2:1 for best results, but even as high as 3:1 can still be effective. Slopes: 33% maximum, less than 25% preferable. (No scientific basis for maximum slope of 10%.) Length: At least 8 feet for filtering coarse sediments. Loading Dispersion: Sheet flow is essential. This has to be provided by level spreaders if flows already concentrated. Flows from roads and parking lots generally dispersed already. Soils: Incorporate compost to increase infiltration rates. Vegetation: Dense turf grasses best for preventing channelized flow, Native grasses best for infiltration.
BioSwale Description Channels that provide conveyance, water quality treatment & flow attenuation of stormwater runoff. Pollutant removal through sedimentation, biological uptake, & infiltration into soil media. (Filtering by vegetation does not occur.) Can replace curb/gutter, and often inlets and piping. Design variants: Grass Bioswales- main focus of this discussion. Wet Bioswales- similar, but no infiltration since soils saturated. Bioretention swales. Use engineered media that infiltrates rapidly so can treat more runoff in same area. Check dams- can be incorporated into all variants to promote detention and to increase hydraulic head for infiltration.
BioSwale Example
Bioswale: Design Guidance Hydraulic Loading Rate: Less than 5:1 for best results, but can be as high as 15:1 and still be effective if designed properly. Longitudinal Slope: 4% maximum, 1% minimum (unless bioretention). Width: Bottom at least 4 feet wide, max 20 feet wide. Side Slopes: <2:1 back slopes, up to 10:1 front slope (= filter strip). Length: At least 100 feet, or 5-7 minute travel time for WQ event. Flow Depth: Prefer <3 for WQ event, 4 OK if tall grass used. Flow Velocity: <1 fps in WQ event. Soils: Incorporate compost to increase infiltration rates of native soil. Vegetation: Dense turf grasses best for preventing channelized flow, Native grasses best for infiltration. Capacity: Safely convey the 10-year event, and provide 6 freeboard in 100-year event. Check dams: Key in with adequate plunge pool/splash pad protection.
Bioswale Performance (Courtesy Mike Barrett) Barrett (Caltrans, 2004) published data on biofiltration reduction efficiencies. Most swales designed for ~1cfs water quality event, except this Melrose Ave at ~30 cfs.
Bioswale Performance (CalTrans 2004) Annual reduction efficiency is good for TSS and metals, low for TKN and nitrate, but negative for phosphorus.
Bioswale Performance (CalTrans 2004) However, when infiltration losses are accounted for, the annual mass load reductions are substantial. Note load reductions are high for TSS and metals, good for nitrate, but still minimal for phosphorus (Likely due to type of grass used).
Bioswale Performance Johnson et al (2003) also published data on swale performance. Source area was 10,000 sq. ft. street, and swale 116 feet long. Note relatively small bottom width: (Courtesy Robert Pitt)
Bioswale Performance (Courtesy Robert Pitt) Note reduction in TSS EMCs in samples taken along the swale in 2004 event.
Bioswale Performance Total Suspended Solids (mg/l) 180 160 140 120 100 80 60 40 20 0 City hall grass swale 0 20 40 60 80 100 120 Location (ft) 8/22/2004 10/09/04 10/10/04 10/10/04 10/11/04 10/19/04 10/23/04 11/1/2004 11/11/04 (Courtesy Robert Pitt) Results from sampling show most TSS removal occurs within the first 25 feet, with maximum removal attained by end. Lowest effluent EMC is around 5 mg/l.
Bioswale Performance 300 RETENTION TIME (min.) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 100% PARTICULATE TSS (mg/l) 250 200 150 100 50 4 cm DEPTH EMC 7.5 cm DEPTH EMC 4 cm DEPTH % 7.5 cm DEPTH % 4 cm REMOVAL CURVE 7.5 cm REMOVAL CURVE 80% 60% 40% 20% % REDUCTION 0 0 10 20 30 40 DISTANCE (meters) 0% (Lucas, 2004) Graph above shows reduction efficiencies in flume study (Barrett et al, 1997), compared to DURMM model (Lucas, 2004). Note rapid removal in first 20 meters.
Manning s s N Related to Flow Depth & Velocity This chart represents a compilation of Ree and Palmer (1947) and Kao & Barfield (1978) and Abu-Zreig (2001). Manning s n changes as a function of the product of flow depth (R) and velocity (V). According to these sources, n increases as VR increases to emerged flow, after which n appears to decrease. At flow depth of 5.5 inches, grass is bent over for emerged flow. At V of 0.90 fps, and depth of 0.40 feet, VR = 0.36. Manning s n would be ~0.95 MANNING'S n. 1.00 0.10 0.01? RELATIONSHIP OF MANNING'S n TO VR (Kirby et al, 2008) 0.01 0.10 1.00 VR TRANSITION TO EMERGED FLOW EMERGED THICK BRUSH C Retardance (Ree & Palmer) SUBMERGED THICK BRUSH Kuo & Barfield: s=.02, Med. Stiff EMERGED DENSE GRASS D Retardance (Ree & Palmer) SUBMERGED DENSE GRASS Kuo & Barfield: s=.02, Soft SHORT GRASS E Retardance (Ree & Palmer) SUBMERGED SHORT GRASS Filter Strip, Abu-Zreig et al, 2001 STONE PAVEMENT Relationship between Manning s n and Hydraulic Radius times Velocity (Lucas, 2004).
Bioswale Check Dams
Bioswale Check Dams and Sections
Bioswale Project
HydroCAD Routing Diagram H1A H1B H1E H1D A05 A06 REACH A01 Offsite-3 A09 REACH A02 REACH B03 Bioswale-7 B10 Bioswale-9 B14 Bioswale-10 B21 Bioswale-12 B34 120 OUTLET A10 B01 FES-7 B09 FES-9 B12 FES-10 B19 FES-12 Analysis Point-"A" G1C OFFSITE GRASS G1D OFFSITE BLDGS G1A OFFSITE PAVED B30 G04 FES-3 G05 FES-1 Bioswale-13 C13 FES-8 B29 FES-13 B35-86A B37 SUMP B38 Bioswale-14 B40 Analysis Point-"B" G1B QUALITY BMP G07 SUMP G06 FES-2 C02-513A C11 FES-5 C12 Bioswale-5 C14 Bioswale-8 C15 FES-11 B36 FES-15 B39 OS-W1 G08 FES-4 G1E 896 Crossing G03-530 G12 POND-5 C01 MH-514 C03 MH-513 C04 MH-512 C05 MH-511 C07 MH-510 C16 POND-6 C18 MH-509 C20 REACH C30 To Point of Analysis-"C" C40 Analysis Point-"C" G02 SUMP C19 OS-W2 D09 OS-W10 G11 OS-C2 G15 C29 OS-W3 G01 OS-C1 120 MEDIUM DROP C27 120 MEDIUM C28 120 OUTLET G16 C26 120 MEDIUM E05 FES-17 K18 FES-23 K25 FES-24 E01 FES-20 E02 Bioswale-20 E06 Bioswale-17 E19 OS-W5 G22 K19 Bioswale-23 K26 Bioswale-24 E12 FES-21 E18 REACH E20 Point of Analysis-"E" K01 FES-18 K02 Bioswale-18 K09 Bioswale-19 G23 K08 FES-19 G24 OS-C4 G25 J49 OS-C9 K32 OS-W8 G27 FES-50 G28 DIVERSION J50 Analysis Point-"J" F1 OS-C18 F2 Analysis Point-"F" G26 OS-C5 G29 POND-3 (BIORETENTION) MANHOLE-508 MANHOLE-507 MANHOLE-506 MANHOLE-505 REACH 100 FAST DROP G74 G34 G35 G31 G30 100 FASTEST G17 G21 G49 G09 G10 FES-52 Bioswale-52 OUTLET-3 BIORETENTION OS-C3 BIORETENTION MEDIA-1 100 FAST DROP G75 MEDIA-2 FES-49-1 FES-49-2 G20 G36 G37 G18 G45 100 FASTEST DROP G54 POND-4 FES-44 Bioswale-44 FES-46 (BIORETENTION) REACH 110 FAST DROP G76 J01 G40 G41 G19 G61 100 FASTEST DROP FES-36 FES-43 Bioswale-43 G57 FES-47 G33 FES-53 J02 J06 110 FAST DROP G77 OS-C6 J03 100 MEDIUM FES-37 Z1 G62 100 FASTEST DROP J04 100 MEDIUM J08 G58 INFILTRATION Bioswale-53 J27 100 MEDIUM DROP J07 G78 FES-34 G69 110 FAST DROP FES-31 Bioswale 37 100 FASTEST DROP OS-C7 J28 J10 G80 J31 100 SLOW Bioswale 34 J13 Analysis Point -"G" J29 FES-32 J17 J11 FES-38 H52 100 SLOW FES-39 J32 H60 H59 H31 100 OUTLET J14 FES-78 C06 J18 100 MEDIUM REACH OS-E4 FES-75 110 FASTEST J33 J19-510A 110 FASTEST H53 100 MEDIUM 110 FASTEST H36 H32 J20 J34 J37 Bioswale-78 C10 110 OUTLET Bioswale-75 J38 110 FASTEST J21 100 OUTLET 100 MEDIUM DROP H47 OS-W9 100 MEDIUM DROP FES-40 H70 H58 J22 Bioswale-80 C24 C17 J36 J39 REACH 100 OUTLET 110 FASTEST H40 FES-16 RAIN TANK FES-33 100 MEDIUM DROP C25 J23 Bioswale-81 K15 J40 110 FASTEST J25 100 MEDIUM J24 FES-39A J41 E14 110 FASTEST 100 MEDIUM FES-22 J26 J42 J43 110 OUTLET E15 100 MEDIUM DROP 100 OUTLET Bioswale-22 K16 OS-W11 H80 REACH H90 Analysis Point-"H" K33 FES-25 K35 H79 OS-E5 H75 REACH H81 OS-E6 H82 OS-C16 Bioswale-25 K40 100 OUTLET G47 100 FAST G48 H67 100 OUTLET K42 FES-26 K43 Bioswale-26 K48 110 OUTLET G46 FES-42 G73 FES-54 K50 FES-30 H62 Bioswale-83 K51 Bioswale-30 K55 Bioswale-29 K65 H38 CROSSING K56 100 OUTLET Bioswale-27 K66 OFFSITE PAVED K54 FES-29 K57 FES-27 K61 Bioswalw-28 H01 OS-C10 H02 FES-58 H04 FES-60 H05 FES-57 H10 OS-E1 H15 FES-66 H21 FES-55 H26 OS-E3 H51 FES-79 H46 FES-80 H39 FES-81 H37 406 H61 FES-83 K60 FES-28 QUALITY BMP H30 CHANNEL N01 OS-E12 H03 POND-2A H11 POND-2B H12 POND-2C N02 SUMP H23 POND-1 M03 FES-76 M07 MH-523 M09 MH-522 M01 FES-70 M20 Analysis Point-"M" P01 OS-E9 N03 FES-69 N04 OS-E13 896 Crossing OFFSITE BLDGS H09 SUMP H22 OS-E2 H17 SUMP H20 Bioswale-74 N05 SUMP H25 POND M06 POND-7 M08 MH-523 M12 MH-521 M04 FES-99 P10 H1C OFFSITE GRASS M05 OS-E11 Analysis Point-"P" H08 FES-63 H06 FES-62 H16 73B H18 FES-73 H19 FES-74 H24 369A N10 N06 OS-E14 M10 OS-E16 H07 FES-65 Analysis Point-"N" M11 K30 110 OUTLET 100 OUTLET L04 D01 OFF SITE-1 D02 REACH D10 Analysis Point-"D" 110 OUTLET K89 Lake K71 K90 SHALLCROSS LAKE, Analysis Point- "K" K85 K68 Bioswale-85 K87 K67 FES-85 K86 L01 FES-84 OS-E8 L02 Bioswale-84 L10 Analysis Point-"L" OS-C17 OS-E7 REACH FES-82
Bioswale Check Dam Hydrographs Series of hydrographs show flows over six check dams (in red) and flow through dams (in blue). Note substantial reduction in peak flows and long delay in timing as the hydrograph proceeds down the series of dams. Infiltration was not modeled in this analysis.
LID Hydrology Summary FILTER STRIPS: Filter strips very effective for coarse sediments, not so good for clays. Slope is not nearly as important as hydraulic loading rate. Much of the reduction in loads is due to infiltration. SWALES: Bioswales more effective for clays if enough retention time provided. Much of the reduction in loads is also due to infiltration. Can slow down conveyance rates dramatically.
Impervious Area Disconnection References Ahearn, D., and R. Tveten. 2008. Legacy LID: Stormwater Treatment in Unimproved Embankments Along Highway Shoulders in Western Washington. In: International Low Impact Development Conference, November 16-19, 2008, Seattle, Washington. Barrett, M.E. 2004. Performance and Design of Vegetated BMPs in the Highway Environment. In: Critical Transitions In Water And Environmental Resources Management, Proceedings of The 2004 World Water and Environmental Resources Congress, June 27-July 1, 2004, Salt Lake City, Utah. Caltrans. 2003. Final Report: Roadside Vegetated Treatment Sites (RVTS) Study. CTSW-RT-03-028, Caltrans Division of Environmental Analysis, Sacramento, California. Herrera. 2009. Final Project Report: Compost-Amended Vegetated Filter Strip Performance Monitoring Study. Prepared for Washington State Department of Transportation, by Herrera Environmental Consultants, Inc., Seattle, Washington. Kaighn, R.J., and S.L. Yu. 1996. Testing of Roadside Vegetation for Highway Runoff Pollutant Removal. Transportation Research Record 1523:116-123. Kearfott, P.J., M.P. Aff, M.E. Barrett, and J.F. Malina. 2005. Stormwater Quality Documentation of Roadside Shoulders Borrow Ditches. IDS-Water White Paper 179, Center for Research in Water Resources, University of Texas, Austin, Texas. Lancaster, C.D. 2005. A Low Impact Development Method for Mitigating Highway Stormwater Runoff - Using Natural Roadside Environments for Metals Retention and Infiltration. Masters Thesis, Washington State University, Department of Civil and Environmental Engineering, Pullman, Washington, 157 pp. Lantin, A., and M. Barrett. 2005. Design and Pollutant Reduction of Vegetated Strips and Swales. In: World Water Congress 2005, May 15, 2005, Anchorage, Alaska. Reister, M., and D.R. Yonge. 2005. Application of a Simplified Analysis Method for Natural Dispersion of Highway Stormwater Runoff. Prepared for Washington State Department of Transportation, by Washington State Transportation Center - Washington State University, Pullman, Washington. Yonge, D.R. 2000. Contaminant Detention in Highway Grass Filter Strips. Report No. WA-RD 474.1, Washington State Department of Transportation, Olympia, Washington.