This document discusses how to construct rain gardens and bioswales to mitigate stormwater runoff. It begins by providing data on average monthly and annual rainfall in St. Louis to illustrate the large volumes of water that fall. Design considerations for rain gardens include the size based on the impervious surface drainage area and depth based on soil percolation rates. Native plants suitable for the bottom, sides, and edges of rain gardens are recommended. Bioswales can include engineered soil mixes and underdrains to drain water more quickly. The document emphasizes distributing green infrastructure near rainwater sources to reduce sedimentation and implement low impact development techniques.

1. Constructing
Rain Gardens
& Bioswales
By Tom Barrett
Green Water Infrastructure, Inc.
www.ThinkGWI.com
Tom.Barrett@ThinkGWI.com Follow us on Twitter @TomBarrett_GWI

2. The GREEN
Economy

3. How Much Rain Falls in St. Louis?
January
-
1.83" Image of Rain Falling
February
-
1.85"
March
-
2.88"
April
-
3.58"
May
-
4.10"
June
-
4.72"
July
-
3.56"
August
-
3.51"
September
-
3.17"
October
- 2.96"
November
-
2.35"
December
-
1.85"
Total
36.36"

4. How Much Water Falls in St. Louis?
2,500 sq. ft. Roof
January
-
,852 gallons
2 Image of Rain Falling
February
-
,883
2
March
-
,488
4
April
- 5,579
May
-
,390
6
June
-
7,356
July
-
,548
5
August
-
,470
5
September
-
,940
4
October
-
,613
4
November
-
3,662
December
- 2,883
Total
6,664
5

5. How Much Water Falls in St. Louis?
3 Acre Commercial Property
January
-
49,075 gallons
1 Image of Rain Falling
February
-
50,705
1
March
-
34,611
2
April
- 291,634
May
-
33,994
3
June
-
384,501
July
-
90,005
2
August
-
85,932
2
September
-
58,235
2
October
-
41,128
2
November
-
191,436
December
- 150,705
Total
2,961,961

6. How Much Water Falls in St. Louis?
City Block
(5 acres)
January
-
48,459 gallons
2
February
-
51,176
2
March
-
91,018
3
April
- 486,057
May
-
56,657
5
June
-
640,834
July
-
83,341
4
August
-
476,553
September
-
30,391
4
October
-
01,879
4
November
-
319,061
December
- 251,176
Total
4,936,602

7. LOW IMPACT SITE DEVELOPMENT

8. Stormwater Mitigation
Stormwater Mitigation
Stormwater Mitigation
Stormwater Mitigation
Stormwater Mitigation

11. Image of Rain Falling

12. PESTICIDE ISSUES
ATRAZINE, NITROGEN, PHOSPHOROUS

13. Peak Flow
(1 Acre Site)
Grass Field Roof
1 Year Storm 1.4 cfs 4.3 cfs
2 Year Storm 2.1 cfs 5.4 cfs
10 Year Storm 4.3 cfs 8.0 cfs
25 Year Storm 5.7 cfs 9.5 cfs
100 Year Storm 8.0 cfs 12.0 cfs
cfs – cubic feet per second

14. Peak Flow
(1 Acre Site)
Grass Field Roof
1 Year Storm 10.5 gps 32.2 gps
2 Year Storm 15.7 gps 40.4 gps
10 Year Storm 32.2 gps 59.8 gps
25 Year Storm 42.6 gps 71.1 gps
100 Year Storm 59.8 gps 89.8 gps
gps – gallons per second

15. Peak Flow
(1 Acre Site)
Grass Field Roof
1 Year Storm 630 gpm 1,932 gpm
2 Year Storm 942 gpm 2,424 gpm
10 Year Storm 1,932 gpm 3,588 gpm
25 Year Storm 2,556 gpm 4,266 gpm
100 Year Storm 3,588 gpm 5,388 gpm
gpm – gallons per minute

16. Peak Flow
(2,500 sq. ft. Roof)
Grass Field Roof
1 Year Storm 0.08 cfs 0.25 cfs
2 Year Storm 0.12 cfs 0.31 cfs
10 Year Storm 0.25 cfs 0.46 cfs
25 Year Storm 0.33 cfs 0.55 cfs
100 Year Storm 0.46 cfs 0.69 cfs
cfs – cubic feet per second

17. Peak Flow
(2,500 sq. ft. Roof)
Grass Field Roof
1 Year Storm 0.60 gps 1.85 gps
2 Year Storm 0.90 gps 2.32 gps
10 Year Storm 1.85 gps 3.43 gps
25 Year Storm 2.44 gps 4.08 gps
100 Year Storm 3.43 gps 5.15 gps
gps – gallons per second

18. Peak Flow
(2,500 ft. sq. Roof)
Grass Field Roof
1 Year Storm 36 gpm 111 gpm
2 Year Storm 54 gpm 139 gpm
10 Year Storm 111 gpm 206 gpm
25 Year Storm 147 gpm 245 gpm
100 Year Storm 206 gpm 309 gpm
gpm – gallons per minute

19. Change in Peak Runoff Flow Before and after Development
300%
225%
150%
75%
0%
1 Year Storm Year Storm Year Storm Year Storm Year Storm
2 10 25 100
Stormwater Effects of Urbanization

20. PLANTING TREES

21. Stormwater Mitigation
– Collection runoff near the
source
– Slow it down
– Soak it in
– Filter it
– Apply it to the landscape
– Create habitats

22. Rain Garden
A Low Spot
Catches Stormwater
Deep Rooted Plants

23. SIMPLE RAIN GARDEN

24. Bioswales
Engineered Soils
Underdrain

25. Location
Rain gardens are
Plant Choices often located at the
end of a roof or
Choose plants based drain spout.
on the need for light
and soil type.
Depth Size Soil
A Rain Garden usually A tpical mix is 65%
A typical Rain Garden
five to ten percent sand, 15% top soil,
is between four to
of the impervious 25% organic matter.
eight inches deep.
surface area.
RAIN GARDENS

26. RAIN GARDEN

27. RAIN GARDEN

28. BIOSWALES
BURNSVILLE, MINNESOTA

29. NATIVE VEGETATION
WWW.EPA.GOV/GREENACRES/ WWW.FOR-WILD.ORG

30. Street Edges
& Medians

31. Parking Lot
Edges

32. Parking Lot
Islands

33. Driveway
Edge

34. Downspout

35. NEIGHBORHOODS

36. Criteria
Meet Stormwater Utility Clearance
Regulations
Soil Investigation
Detention Volume
Percolation Test
Fix Drainage Issue
Fix Erosion Issue

37. Criteria
Near the Rainwater Distributed Evenly
Source Across the Site
Avoid “End-of Pipe” Small Tributary Areas
because of (usually 1 acre or less)
Sedimentation Issues
Typically 10’ to 20’
from Buildings

38. Soil
Investigation
•Soil Profile to Five Feet
•Soil Compaction Level
•Depth to Groundwater and
Bedrock

39. Percolation
Test
•Soil Infiltration Rate
•Key Design Parameter
•Percolates water in 24
Hours

40. Sizing
Determine Design Native vs. Engineered
Goals Soil Assessment
Calculate Runoff
Volume
Determine Allowable
Depth
Calculate Surface
Area

41. Sizing
Runoff
Volume =
Precipitation x
Drainage Area x Runoff
Coefficient
RV=Pr x D(area) x C(un

42. Depth
Based Upon
Infiltration Rate
Infiltration in One Day
Avoid Misquotes
Maximum Depth 18”
for Safety

43. Surface Area
Area of Rain Garden
(ft2) = Runoff Capture
Volume (ft3) / Average
Depth (ft)
A=V/D(average)

44. Engineered Soils
Bioretention
Space Available
Volume of Stormwater
Drain Faster
(the garden can be
deeper and not as
wide)

45. Plants -
Bottom
Palm Sedge
Soft Rush
Tussock Sedge
Marsh Milkweed
Blue Flag Iris
Joe-Pye Weed

46. Plants -
Sides
Purple Coneflower
Showy Goldenrod
Smooth Phlox

47. Plants -
Edges
Butterfly Milkweed
Little Bluestem
Aromatic Aster

48. Inlets

49. Outlets

50. Curb Cut &
Filter Strip
Controls Sedimentation

51. Splash
Blocks
Prevents Erosion and Gullies

57. FUNCTIONAL LANDSCAPES

58. Green • Water • Infrastructure
Green • Water • Infrastructure
Green • Water • Infrastructure
Green • Water • Infrastructure
Green • Water • Infrastructure

59. Thank You