Donald Carpenter, Ph.D. Laura Hallam Lawrence Technological University June 23, 2008 MWEA Annual Conference Watershed Session

Investigation of Rain Garden Planting Mixtures Donald Carpenter, Ph.D. Laura Hallam Lawrence Technological University June 23, 2008 MWEA Annual Conference Watershed Session

What is a Rain Garden?

What is a Rain Garden? State of Minnesota Stormwater Manual

Rain Garden or Bioretention Cell Plants Buffer Buffer Plants Mulch Ponding Planting Mix Underdrain (optional) Additional Storage (optional) In-situ soils

Rain Gardens Courtesy of Lillian Dean

Bioretention Cells

Why have a Rain Garden?

Rain Gardens - Benefits Key feature for LID/conservation design Focus on water quality volume know as first flush (first ½ to 1 1 of rainfall) & represents 90% of all rainfall events Filter and removal mechanism - remove sediments (85%), nutrients (50% - 80%), metals (95%), and hydrocarbons (80%) Stormwater master plan => rain gardens can have tremendous impact on a watershed

How do Rain Gardens work?

How do Rain Gardens work? Rain Gardens capture and filter stormwater Numerous physical and biological processes including evapotranspiration,, adsorption, filtration, plant uptake, microbial activity, decomposition, and volatilization Planting mix comprised of sand, topsoil, and compost topped with mulch

How do Rain Gardens work? Mulch Filter pollutants & microorganisms degrade hydrocarbons Protects planting mix from eroding and holds moisture in during dry season Planting Mix Absorbs and degrades pollutants Clay specifically good at absorbing hydrocarbons and heavy metals Sand improves drainage Compost provides nutrients and increases retention

How much sand, topsoil, or compost should I use?

Rain Garden Mix Designs Numerous recommended mix designs have been published (17 states & 7 others) For new community developments mix designs might be specified by municipality Example Mix Designs: 50% sand, 30% topsoil, 20% compost 20 40% sand & 60 80% compost (SOCWA) NRCS Soil Triangle (Loamy Sand to Sandy Loam)

NRCS Soil Triangle

Rain Garden Mix Designs Numerous recommended mix designs have been published (17 states & 7 others) For new community developments mix designs might be specified by municipality Example Mix Designs: 50% sand, 30% topsoil, 20% compost 20 40% sand & 60 80% compost (SOCWA) NRCS Soil Triangle (Loamy Sand to Sandy Loam) 0% to 25% clay; 1.5% to 25% organic content, 5.5 to 6.5 ph Balance sand & compost based on your specific site and native soils

How do I design a Rain Garden? Observation or Quantitative

Rain Garden Design Techniques Homeowner How-to Manuals available from U of Wisc.. Ext., VA Dept of Forestry, & SOCWA Numerical Design Techniques Impervious Area/Rational Formula Method (7% * D.A. * runoff coefficient) Curve Number Technique Wisconsin DNR recommends RECARGA Computer Program Darcy s s Law

Rain Garden: Design Notes Infiltration Hydraulic conductivity of in-situ soils is key Need in-situ soil information for infiltration HSG maps (soil type A (1 to 3 in/hr); B (0.5 in/hr); C (0.2 in/hr); D (< 0.1 in/hr) Soil borings & laboratory tests Field infiltration tests Home test 18 hole fill with water; drain, refill and record

Rain Garden Design Comparison 1000 sq ft of roof (all impervious) 2.5 ft of planting depth & 0.5 ft ponding Sand (1.0 ft/day) & Clay (0.2 ft/day) Darcy s Run-off Sand Clay 30 sq ft 63 sq ft 150 sq ft 63 sq ft

Rain Garden Laboratory Tests

Permeability Test Measures hydraulic conductivity of soil sample (infiltration)

Field Capacity Test Measures water carrying capacity of a soil sample

Soil Properties Mixture Label % Organic Dry Bulk Porosity Matter Density (kg/m 3 ) 100 c / 0 s / 0 t 28.37 442.6 0.594 0 c / 100 s / 0 t 0.22 1648.3 0.351 0 c / 0 s / 100 t 2.04 1284.2 0.486 80 c / 20 s / 0 t Field 13.98 650.3 0.529 80 c / 20 s / 0 t Lab 12.92 746.0 0.459 20 c / 50 s / 30 t Field 2.69 1029.5 0.540 20 c / 50 s / 30 t Lab 2.65 1502.2 0.429 50 c / 50 s / 0 t 5.09 950.1 0.476 35 c / 65 s / 0 t 5.02 1021.4 0.497

Laboratory Results Mixture Label Mean Permeability (in/hr) Mean Field Capacity @ 6 hours (%) Porosity 100 c / 0 s / 0 t 7.42 115.3 0.594 0 c / 100 s / 0 t 10.23 13.8 0.351 0 c / 0 s / 100 t 0.66 24.7 0.486 80 c / 20 s / 0 t Field 18.35 53.5 0.529 80 c / 20 s / 0 t Lab 17.95 63.9 0.459 20 c / 50 s / 30 t Field 0.80 28.3 0.540 20 c / 50 s / 30 t Lab 1.84 16.5 0.429 50 c / 50 s / 0 t 2.18 20.8 0.476 35 c / 65 s / 0 t 2.77 24.2 0.497

0 c / 100 s / 0 t 0 c / 0 s / 100 t 120 100 80 60 40 20 0 Field Capacity Field Capacity Field Capacity - 2007 60 c / 40 s/ 0 t 50 c / 50 s / 0 t 35 c / 65 s / 0 t 35 c / 35 s / 30 t 20 c / 50 s / 30 t Mixture Ratio 80 c / 20 s / 0 t 100 c / 0 s / 0 t Water Retained (% by weight)

Effect of Sand on Field Capacity 140 120 % of Water Retained 100 80 60 40 20 0 0 10 20 30 40 50 60 70 80 90 100 % Sand Content by Volume

Compaction Mixture Label 100% compost no compaction 100% compost 16.7% volume compaction 100% compost 33.3% volume compaction Mean Permeability (in/hr) Mean Field Capacity @ 6 hours (%) Porosity 7.42 115.3 0.594 3.76 113.0 0.521 NA 112.2 0.438

Rain Garden Field Tests

Double Ring Infiltrometer

Double Ring Infiltrometer

Field Infiltration Measurements Community Rain Gardens Costick Center Farmington Hills Cell A = 0.5 in/hr Cell B = <0.1 in/hr Lathrup Village = 19.7 in/hr Wayne City Hall Bioswales = <0.1 in/hr Newburgh Point Wayne Co. = 2.9 in/hr Beech Park = 4.1 in/hr Homeowner Rain Gardens Kearney = 7.4 in/hr Piotrowski = 0.1 in/hr (clay pocket) Hart-Negich = 20 in/hr San Rosa/Avila Swales (two gardens) = 20+ in/hr

Bioretention Cell Project @ LTU Two bioretention cells designed and installed at LTU different soil mixes 80% compost & 20% sand 50% sand, 30% topsoil, & 20% compost Instrumented to measure soil moisture, outflow, water quality sampling, & meteorological data Simulated and Natural Rain Fall Events

May 23 Water Quality Test Hydrographs 0.025 0.020 Discharge (cfs) 0.015 0.010 Cell A Cell B Inflow 0.005 0.000 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 Time (Min)

June Rainfall Event 0.010 0.1 0.008 0.08 Discharge (cfs) 0.006 0.004 0.06 0.04 Cum. Rainfall (ft) Cell A Cell B Rain 0.002 0.02 0.000 0 250 500 750 Time (min) 0

Implications for Rain Garden Design Overall, be aware of the impact your soil mix will have on your rain garden drainage performance. If the goal of the rain garden is: to maximize water retention (clay soil sites), limit amount of sand & topsoil => 80+% compost (higher porosity & field capacity) to promote infiltration into permeable in-situ soils, increase amount of sand (increased infiltration) for long drawdown times and slow release into an underdrain,, include topsoil (decreases infiltration) to treat hydrocarbons or heavy metals, include clay (3+%) in the mix If soil mixture is balance compost/sand and garden is maintained, limiting infiltration rate might be from native in-situ soils. KEY DESIGN PARAMETER!

References & Resources Lawrence Tech University Stormwater Page http://www.ltu.edu/stormwater.html SOCWA Healthy Lawns and Gardens - http://www.socwa.org/lawn_and_garden.htm Rain Gardens of West Michigan - http://www.raingardens.org/index.php Virginia Department of Forestry Rain Gardens Technical Guide http://www.dof.virginia.gov Minnesota Stormwater Manualhttp://www.pca.state.mn.us/water/stormwater/stormwatermanual.html USEPA Bioretention fact sheet - http://www.epa.gov/owm/mtb/biortn.pdf Univ of Wisconsin DNR http://clean-water.uwex.edu/pubs/home.htm#rain LID Urban Design Tools - http://www.lid-stormwater.net/ Prince George s County, D.E.S. Bioretention - http://www.goprincegeorgescounty.com/government/agency Index/DER/ESD/Bioretention/bioretention.asp