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

Transcription

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

2 What is a Rain Garden?

3 What is a Rain Garden? State of Minnesota Stormwater Manual

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

5 Rain Gardens Courtesy of Lillian Dean

6 Bioretention Cells

7 Why have a Rain Garden?

8 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

9 How do Rain Gardens work?

10 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

11 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

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

13 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)

14 NRCS Soil Triangle

15 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

16 How do I design a Rain Garden? Observation or Quantitative

17 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

18 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

19 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

20 Rain Garden Laboratory Tests

21 Permeability Test Measures hydraulic conductivity of soil sample (infiltration)

22 Field Capacity Test Measures water carrying capacity of a soil sample

23 Soil Properties Mixture Label % Organic Dry Bulk Porosity Matter Density (kg/m 3 ) 100 c / 0 s / 0 t c / 100 s / 0 t c / 0 s / 100 t c / 20 s / 0 t Field c / 20 s / 0 t Lab c / 50 s / 30 t Field c / 50 s / 30 t Lab c / 50 s / 0 t c / 65 s / 0 t

24 Laboratory Results Mixture Label Mean Permeability (in/hr) Mean Field 6 hours (%) Porosity 100 c / 0 s / 0 t c / 100 s / 0 t c / 0 s / 100 t c / 20 s / 0 t Field c / 20 s / 0 t Lab c / 50 s / 30 t Field c / 50 s / 30 t Lab c / 50 s / 0 t c / 65 s / 0 t

25 0 c / 100 s / 0 t 0 c / 0 s / 100 t Field Capacity Field Capacity Field Capacity 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)

26 Effect of Sand on Field Capacity % of Water Retained % Sand Content by Volume

27 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 6 hours (%) Porosity NA

28 Rain Garden Field Tests

29 Double Ring Infiltrometer

30 Double Ring Infiltrometer

31 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

32 Bioretention Cell 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

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36 May 23 Water Quality Test Hydrographs Discharge (cfs) Cell A Cell B Inflow Time (Min)

37 June Rainfall Event Discharge (cfs) Cum. Rainfall (ft) Cell A Cell B Rain Time (min) 0

38 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!

39 References & Resources Lawrence Tech University Stormwater Page SOCWA Healthy Lawns and Gardens - Rain Gardens of West Michigan - Virginia Department of Forestry Rain Gardens Technical Guide Minnesota Stormwater Manualhttp:// USEPA Bioretention fact sheet - Univ of Wisconsin DNR LID Urban Design Tools - Prince George s County, D.E.S. Bioretention - Index/DER/ESD/Bioretention/bioretention.asp