Hamilton County Rain Garden Infiltration Studies

Transcription

1 Hamilton County Rain Garden Infiltration Studies

2 RAIN GARDEN SOILS IN HAMILTON COUNTY Dave Dyke, Commercial Horticulture Educator, OSUE, Hamilton County Following are: 1 ) A discussion of recommendations 2) A summary of infiltration rates observed in an OSU Extension Rain Garden Infiltration Study of gardens in Hamilton County.

3 Discussion of recommendations Most engineered soils for rain gardens are specified to be 50% or better sand. A soil does not exhibit sandy characteristics until it is 60% or more sand. There are a few problems with that. 1) Most of our plant materials for this area do not do as well in a very sandy soil as they would in a good loam soil. 2) Not all sand is created equal. If sand is used, it should be a course or medium sharp sand. Rounded river sand, as any horticulturist knows, should never be used for a soil mix. 3) Rain gardens are designed to capture an inflow of water. That means that soil particles will be entering the system. When silt or clay particles enters into a system that utilizes a very sandy mix those particles will change the ratio of sand in the mix clogging the spaces between the sand particles and reducing the infiltration rate of the mix. (I can show you an installation that started with 85% sand a couple of years ago and that is now a wetland due to the inflow of soil particles). 4) Sandy soils do not aggregate well, if at all. A good loam soil, or even a clayey soil, will aggregate over time and its infiltration rate will remain to be adequate, or improve. 5) Many sandy mixes provide an infiltration rate that is far too rapid. If the installation has an underdrain, the water will simply flow through the plant growth medium and into the underdrain and on out of the system (if the design permits). The too rapid infiltration of water can also result in flow through of nutrients into the water table or storm water system.

4 Discussion of recommendations Having said all of the above, the bottom line is that the infiltration rate of any rain garden system is ultimately defined by what is under and around the plant growth medium. If the medium permits an infiltration rate of 50 inches/hour but the soil under it has an infiltration rate of 3 inches/hour and there is no underdrain, then the effective infiltration rate will be 3 inches/hour unless an underdrain is installed. If that is the case, the water will infiltrate at the rate of 50 inches/hour which is way too rapid. I m over simplifying things a bit but I am working my way towards the real bottom line and my research.

5 A summary of infiltration rates observed in an OSU Extension Rain Garden Infiltration Study of gardens in Hamilton County Much literature states that the optimum infiltration rate of a rain garden should be from /inch/hour to 6 to 8 inches/hr. Below is a summary of the infiltration rates we found at 6 rain garden installations in the Cincinnati area in the last half of Note that all but the zoo new entrance garden had virtually identical construction. The new entrance had 18 inches of plant growth substrate over an 18 inch layer of gravel. All others had a underdrain of perforated drain tile at the bottom of the installation. The unamended clay urban complex soil in the Brazee garden actually provided what could be considered ideal infiltration.

6 Infiltration rates Determined by using a Turf-Tech International double ring infiltrometer Inner ring 2.5 inch diameter outer ring 4.5 inch diameter Dave Dyke

7 Methodology There were 6 sampling locations in each garden. (Except for the zoo entrance garden 2 locations) Two were on the center line and 2 were to the right and left of the center line. Locations were randomly selected to try to get comprehensive coverage of each garden. The infiltration rate is expressed in inches/hour. The infiltration rate was determined by removing the mulch from a small area, inserting the double ring infiltrometer into the soil, and then completely filling the infiltrometer with water and then allowing it to drain into the soil 3 times. (Except for the Brazee garden which was close to saturation following 2 3 inches of rainfall the preceding 72 hours.) The infiltration rate was determined by the amount of time that was required to infiltrate 3 inches of water from the center ring, or in sites with an infiltration rate of less than 1 inch/hour, the amount of water that infiltrated in 1 hour.

8 Cincinnati Zoo & Botanical Garden Education Center

9 10% Compost Dave Dyke 30% Pine Fines 60% Shredded Topsoil

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11 Dave Dyke

12 Dave Dyke Cincinnati Zoo Infiltration Data, October 7, 2011 Location 1: Education Center Rain Garden Area Soil Moisture Infiltration Rate RG A Center 79% /hr RG A Left 97% /hr RG B Center-right 31% /hr RG B Left 82% /hr RG C Center-upper end 100% /hr RG C Center-lower end 90% /hr Average 80% /hr

13 Zoo Entrance Garden The garden consists of an 18" layer of gravel with perforated drain tile covered with geo texile fabric, which is covered with approximately 18 inches of soil mix (60% shredded top soil, 30% pine fines, 10% compost) Area Soil Moisture Infiltration Rate Center Left 44% /hr Center Right 85% /hr Averages for Site 65% /hr Dave Dyke

14 Brazee Rain Garden, Oakley Dave Dyke

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16 Plant Growth July of 2 nd year Dave Dyke

17 Brazee Rain Garden Building feeding the rain garden =12,172 sq ft Blacktop area that drains into it is 6775 sq ft Total sq ft = 18,947 The rain garden is 2885 sq ft (15.2% of area feeding the rain garden). The Runoff Curve Number or SCS number was used to calculate the total garden area. There is a small stormtech system that receives a very small percentage of the runoff. Soil is about 30 deep. The under drain is 36 inches deep is 6 perforated pipe set in a bed of #57s washed gravel. The soil was not modified except for 6 inches of topsoil added on top of the under drain trench.

18 Brazee Rain Garden Unusual because: The soil has not been amended Initial infiltration rate was calculated at about 2 inches / hour Ponding depth is about 3 feet Greatest depth of water in garden to date: 2 ft which drained in 12 hours! Soil is very clayey

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20 Dave Dyke Brazee Infiltration Study

21 Brazee Infiltration Data, November 17, 2011 Zone Location Infiltration Rate 1 Approx 2 ft east of center line 0.75 /hr 2 Center line /hr 3 Approx 2 ft west of center line /hr 4 Approx 2 ft east of center line 4.26 /hr 5 In center of L 0 /hr* 6 2 ft to south of center line, near end of short end of L Average 0 /hr* 7.31 /hr *Although 0 infiltration was measured, no standing water was present in spite of 2 3 inches of rainfall in the preceding 72 hours (ending approximately 24 hours prior to testing). Dave Dyke

22 Infiltration Study Immanuel Church Site Basin Subgrade Profile: -Geomembrane liner along bottom and sides of basin -Mirafi 140N series non-woven geotextile filter on top of aggregate layer -12 #57 Aggregate -24 Bioinfiltration Soil Mix: Sand 50 : Compost 40 : Topsoil 10-3 Mulch Underdrain: -6 diameter SDR-35 PVC perforated pipe -Underdrain sits at bottom of aggregate layer, maintaining a 0.5% slope from the cleanout to the overflow structure Dave Dyke

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24 Immanuel Church Site Basin Tributary Drainage Area Footprint Aggregate Layer Storage Bio-Soil Layer Storage Ponding Storage Total Storage Bio-Soil Mix Sand: 50% Compost: 40% Topsoil: 10% 8,600 SF 320 SF 107 CF 173 CF 490 CF 770 CF

25 Immanuel Church Infiltration Data, July 29, 2011 Test Area Soil Moistur e % Time 1" (m:sec) Time 2" (m:sec) Time 3" (m:sec) Total Minutes Elapsed in/min in/hr 1 69 N/A 1:34 6: * N/A 4:33 7: :00 4:00 6: :02 2:39 4: N/A N/A 4: N/A 2:00 3: Average *Soil moisture level just to the west of test area 2 was 49% and soil moisture level just to the east of test area 2 was 29%. Dave Dyke

26 St. Francis Site Basin Subgrade Profile: -Mirafi 140N series non-woven geotextile filter fabric along bottom, sides and top of aggregate layer -15 #57 Aggregate -24 Bioinfiltration Soil Mix -3 Mulch Overflow Pipe Dimensions: -12 diameter PVC pipe Underdrain Pipe Dimensions: Dave Dyke -6 diameter SDR-35 PVC perforated pipe -Pipe is laid solidly in aggregate layer, maintaining a 0.5% slope from the cleanout to the overflow structure, with the top of the underdrain near the top of the aggregate layer. (~ 8-9 of #57s below the underdrain pipe.)

27 St. Francis Site Northern Basin Southern Basin Tributary Drainage Area 96,408 SF 18,984 SF Footprint 3,816 SF 3,241 SF Aggregate Layer Storage 1,574 CF 1,337 CF Bio-Soil Layer Storage 2,061 CF 1,750 CF Ponding Storage 9,444 CF 8,466 CF Total Storage 13,078 CF 11,553 CF Bio-Soil Mix Sand: 50% Compost: 50% Topsoil: 0% Sand: 50% Compost: 40% Topsoil: 10% Dave Dyke

28 St. Francis Upper Rain Garden Dave Dyke

29 St. Francis Lower Rain Garden Dave Dyke

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31 Test Area St. Francis Infiltration Data, July 29, 2011 Soil Moisture % Time 1" (m:sec) Upper Basin Time 2" (m:sec) Time 3" (m:sec) Total Minutes Elapsed in/min in/hr 1 37 N/A 1:38 2: N/A 0:24 0: :30 1:35 2: N/A 0:46 1: N/A 1:20 2: :46 1:34 2: Average Dave Dyke Sand: 50% Compost: 50% Topsoil: 0%

32 St. Francis Infiltration Data, July 29, 2011 Test Area Soil Moisture % Time 1" (m:sec) Lower Basin Time 2" (m:sec) Time 3" (m:sec) Total Minutes Elapsed in/min in/hr N/A 3:15 5: N/A 4:17 6: :05 6:29 10: * 68 N/A 2:58 N/A N/A N/A 1:50 3: :36 3:22 5: Average "; 1.250"; 2"; 2.125" Dave Dyke Sand: 50% Compost: 40% Topsoil: 10%

33 Summary of Infiltration Rates at 6 Bio-infiltration Sites Site Brazee Zoo Education Center Zoo New Entrance Immanuel Church St. Francis Upper (North) St. Francis Lower (South) Bio-Soil Urban complex (existing clay & gravel) 60 shredded top soil : 30 pine fines : 10 compost 60 shredded top soil : 30 pine fines : 10 compost 50 Sand : 40 Compost : 10 Topsoil 50 Sand : 50 Compost 50 Sand : 40 Compost : 10 Topsoil Average infiltration rate (in/hr) Dave Dyke