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Rain Garden Site and Soil Assessment

Before installing your rain garden, it is necessary to locate any underground utilities (gas, electric, etc.) on the site. Utilize Connecticut's Call Before You Dig services (or other sources) to confirm there are NO buried utilities at the rain garden location. Two days in advance, contact them via the internet at www.cbyd.com,  or via telephone at 1-800-922-4455.

This scorecard is intended to be used for infiltrating rainwater from rooftops and lawns. It is not to be used for other water such as from animal waste, car washing, cleaning pesticide containers, or other residential/commercial uses which discharge water.

These assessments are designed for people to do general evaluation of property, and are not a substitute for detailed, on-site investigations by professionals. Professional investigations may provide recommendations for alternate design modifications on the site.

These comments and modifications are general; there may be on-site exceptions. Professional on-site investigations may provide recommendations for alternate design modifications on the site.

Step 1: Site Your Rain Garden in the Proper Spot
Indicator Unfavorable Condition Favorable Condition Comments/Modifications
Distance from foundation Less than 10 feet More than 10 feet This distance is necessary to prevent water from seeping into the basement or causing frost damage.
Distance from downspout Less than 10 feet from house foundation 10 - 30 feet If the downspout is more than 30 feet from the site, consider creating a grass swale or using PVC pipe to move water to garden site without causing an erosion problem. Be sure to factor in any additional runoff from the area between the house and the garden.
Distance from septic drain field Less than 10 feet More than 10 feet The drain field might be overloaded and fail with additional water.
Slope of land between the rain garden site and the house Less than 1 percent slope (flat); or the house is lower than the site; or the slope is greater than 15 percent The slope is between 1 and 15 percent AND the land slopes down from the house to the rain garden To prevent erosion, create a grass swale, rock lined waterway, or use PVC pipe to move water from the downspout to the rain garden if the slope is more than 8%. Rain gardens sited upslope from or level with the water source are not practical in most cases. They would require pumping runoff to the site.
Slope of land at the rain garden site Slopes greater that 12% Lower slopes are more favorable to create a nearly level rain garden. See attachment for further instructions on digging the site. Slope less than 4% --garden should be 3'5" deep Slope 5 to 7% --garden should be 6-7'  deep Slope 8 to 12% --garden should be 8' deep.
Distance to nearest trees Less than 10 feet More than 10 feet Tree roots may be damaged by digging in the rain garden area.
Sun vs. shade Completely shady Sunny or somewhat shady Choose species able to tolerate sun/shade conditions on your site.

 

 

Step 2: Examine the Soils in Your Proposed Rain Garden
Indicator Unfavorable Condition Favorable Condition Comments/Modifications
Bedrock (ledge) Exposed bedrock at the proposed site or within one foot of the surface Can dig a hole at least 2 feet deep without hitting solid bedrock Consider another location for the rain garden if bedrock is near the surface. Some types of bedrock may allow water to infiltrate through cracks and into groundwater. Other types will not allow infiltration at all and cause overflow.
Soil compaction Wire flag or probe cannot be inserted into the soil at all Loose soil at least 2 feet deep If the soil is compacted, follow attached instructions on how to loosen it OR replace with a mix of 50-60% sand, 20-30% compost, and 20-30% topsoil.
Soil texture Clay, sand Loamy sand, sandy loam, loam, silt loam, silty clay loam, sandy clay loam Use "Guide to Texture by Feel." Rain gardens in sandier soils may be smaller than those in clayier soils, all other factors being equal. See rain garden sizing worksheet.

 

Drainage of soil (determined by observing surface ponding) Standing water on the site more than 2 days after a heavy rain Standing rain water lasts less than 24 hours, even after a heavy rain. Goal of rain garden is to increase infiltration of water into the soil. Areas that are already ponded may not accommodate additional water and ponding for more than 72 hours may provide breeding habitat for mosquitoes.
Drainage of soil, part 2 (Determined by infiltration test) Water moves down less than 1 inch in an hour Water moves down faster than 1 inch in an hour Instructions: Dig a 6 to 10 inch deep hole in soil, fill with water, measure how fast water moves down 1 inch.
 

 
 

Step 3: Determine the Size and Shape of Your Rain Garden
Indicator Unfavorable Condition Favorable Condition Comments/Modifications
Size of drainage area (roof and lawn) More than 3000 square feet. For larger roofs and lawns, consider multiple rain gardens or other treatment options Less than 3000 square feet (Roof area and lawn combined) The size of rain the garden you will need depends on the size of the drainage area, the slope of the garden, and the soil texture. See rain garden sizing worksheet.
Size of Rain Garden More than 300 square feet   If size calculates to more than 300 square feet, divide into smaller gardens
Width of Rain Garden Less than 15 feet if slope is more than 8% 10 feet wide is ideal The longer side of the rain garden should face upslope.

 
 

Rain Garden Sizing Worksheet

(To be used with the rain garden soil and site assessment sheets)

Evaluate Your Site

Distance from downspout _________________________              Slope at rain garden site: _________________________

  • Slope less than 4% --garden should be 3'5" deep
  • Slope 5 to 7% --garden should be 6-7' deep
  • Slope 8 to 12% --garden should be 8' deep

Depth of proposed rain garden: _______________________

 

Evaluate Your Soil Texture Using the "Soil Texture by Feel" Chart and Select a General Soil Type:
Your Soil Texture General Soil Type
Sand Not suitable for rain garden
Loamy Sand Sandy
Sandy Clay Clayey
Silty Clay Clayey
Clay Not suitable for rain garden
Sandy Clay Loam Clayey
Silty Clay Loam Clayey
Clay Loam Clayey
Sandy Loam Sandy
Silt Loam Silty
Loam Silty

 

Choose a Size Factor

Use the general soil type from above, turn to tables 1 and 2 from page 9 of the "Rain Gardens: a how to manual for homeowners" and select the size factor.

Size Factor ______________

Determine Drainage Area

Area of roof drainage (in sq. ft.): ________________

Area of lawn (in sq. ft.) (for rain gardens > 30' from the downspout): _________________

Calculate Rain Garden Size

Size factor X drainage area = garden area in square feet ________________

Determine Length and Width

Pick the best rain garden width for your lawn and landscaping: ________________

Divide the size of your garden by the width to determine the length: _______________