Rain Garden

Stormwater Source Control Design Guidelines 2012
INFILTRATION RAIN GARDEN
Greater Vancouver Sewerage & Drainage District
Infiltration Rain Garden
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Photo Credit: Lanarc Consultants Ltd.
Description
The Infiltration Rain Garden is a form of
bioretention facility, designed to have the aesthetic
appeal of a garden, as opposed to a purely
functional appearance. Rain Gardens are
commonly a concave landscape area where runoff
from roofs or paving is allowed to pond
temporarily while infiltrating into soils below (See
Figure 4A).
The surface planting of Rain Gardens is dominated
by shrubs and groundcovers, with planting designs
Formal rain garden, Buckman Terrace,
Terrace, Portland
respecting the various soil moisture conditions in
Oregon.
the garden. Plantings may also include trees,
rushes, sedges and other grass-like plants, as
well as sodded lawn areas for erosion control and
multiple uses. Deciduous plants, especially trees,
should be used carefully as the seasonal accumulation
of leaves can be a concern for maintenance and may
contribute to bind-off of the soil surface
Rain Gardens generally have a drain rock reservoir
and perforated drain system to collect excess
water. (See Figure 4B and 4C). The perforated
drain system may connect to a control structure in
a catch basin that provides overflow while
maintaining a slow decanting of the water in the
rain garden between storms (See Figure 4D).
While usually designed as a standalone facility

without conveyance, new designs are evolving that
put a series of Rain Gardens along linear areas
like roads with weirs and surface conveyance
similar to Infiltration Swales.
Other common terms used are Bioretention and
Dry Swale with Underdrain (Stephens et al., 2002)
or Swale / Trench Element (MUNLV-NRW, 2001).
Informal rain garden, Water Pollution Control

Laboratory, Portland Oregon
4-1
Selection, Application and Limitations

Rain Gardens are utilized for volume capture and

stormwater treatment. Treatment is provided by the
soil layer and volume capture by infiltration from the
rock reservoir.
A Rain Garden and Infiltration Swale have similar

design and functions. A Rain Garden or series of
Rain Gardens provides more capture of peak flows
(due to ponding) and less conveyance of noncaptured flows than a swale.
If treatment is not required (e.g. for pre-treated or

roof water only), an infiltration rock trench is more
economical and space efficient, but does not
provide the aesthetics and interactive value of the
Rain Garden.
A rain garden will provide increased volume capture

over an infiltration trench due to the surface
ponding and plant uptake or moisture.
Smaller, distributed Rain Gardens are preferable to

single large scale facilities.
Infiltration Rain Gardens may take a variety of

shapes, from informal, organically shaped bowls
to formal, rectilinear planting areas and planters.
FlowFlow-thru planter a formalformal-shaped rain garden

that provides water quality treatment and limited
flow attenuation adapted to a nearnear-building
location at Buckman Terrace in Portland, Oregon.
4-2
Design Guidelines
1. Site Rain Gardens similar to other infiltration
facilities minimum 30m from wells, minimum 3m
downslope of building foundations, and only in
areas where foundations have footing drains.
2. Inflows should be distributed sheet flow from
pavement over a flat-panel curb, or through
frequent curb cuts. A minimum drop of 50 mm
from the pavement or flat curb edge to the top of
the Rain Garden surface is required to
accommodate sediment accumulation.
Photo Credit: Kerr Wood Leidal Associates
3. Where inflow is from curb cuts or point (pipe)

discharge, a transition area at the inflow point(s)
should incorporate erosion control and flow
dispersion to distribute flow to the full Rain
Garden area. Clean crushed rock or rounded river
rock may be used. The slope of the transition
area should be greater than 10% to move
sediment through to the rain garden.
4. Flow may be pre-treated to remove sediment by
travelling through a grass swale prior to entering
the Rain Garden (500 mm minimum, greater than
3000 mm desirable swale length; Claytor and
Schueler, 1996).
Roadside rain garden with flat panel curb and

shrub and herbaceous
herbaceous plantings in Thunderbird
subdivision, Squamish, BC.
5. Experience has shown that grass is efficient at

trapping sediment at a pavement edge and the
sediment and grass matt will agrade rapidly. In
addition to the 50mm drop (see point No. 3, above)
it is recommended that the transition slope or rain
garden edge be covered with rock or sturdy mulch
at the surface rather than grass.
6. Rain Garden bottom or Base Area (Drawing 4A): flat
cross section, with a longitudinal slope of 2%
maximum (or 1% by US001, or dished by GE004).
7. Provide a 50mm 75mm layer of non-floating
organic mulch well aged compost, bark mulch or
similar weed free material. The mulch is important
for both erosion control and maintaining infiltration
capacity.
8. Rain Garden Base Area dimensions: bottom width
600mm minimum, 3000mm desirable.
4-3
9. Rain Garden side slopes: 2 horizontal : 1 vertical

maximum, 4:1 preferred for maintenance (i.e.
mowing or other equipment access, if required).
Provide organic mulch on side slopes similar to
bottom.
10. Maximum ponded level: 150 to 300 mm. 200 mm
maximum pond level is common and assumed for
the simplified sizing approaches here.
11. For roadside applications, rock reservoir depth
should generally not exceed the depth of the
surrounding utilities.
12. Drawdown time for the maximum surface ponded
volume: 48 hours preferred (72 hours max. Maryland Dept. Environmental Resource Programs,
2001).
13. A non-erodible outlet or spillway must be
established to discharge overflow to the storm
sewer system (Maryland Dept. Environmental
Resource Programs, 2001). This often takes the
form of a grated inlet raised above the Rain Garden
invert to create the ponding depth.
14. Rain Garden depth includes ponding depth (depth

to overflow level), an additional surcharge allowance
(100 mm is common) to prevent overflow to the
roadway or surrounding area, and sediment
accumulation allowance (may be 3mm/yr or more
depending on loading). Rain Garden depth =
ponding depth + surcharge allowance + sediment
accumulation allowance.
15. Footprint of Rain Garden = Base Area + Side
Slope Area. Add additional area for side slopes
according to the shape of the rain garden and the
chosen side slopes; e.g. add [2 x slope x Rain
Garden depth (m)] to each dimension of the base
area to determine total footprint area.
Rain garden overflow,

overflow, Buckman Terrace, Portland
Oregon.
16. Treatment soil (i.e. growing medium) depth:

450mm minimum (City of Portland, 2002) for most
applications. Treatment soil should have a minimum
infiltration rate (lab tested) of 70 mm/hr, which is
assumed in the sizing approaches in this document.
17. Slope of the drain rock reservoir bottom shall be
level to maximize infiltration area.
4-4
18. Avoid utility or other crossings of the Rain Garden.

Where utility trenches must be constructed crossing
below the garden, install low permeability trench
dams to avoid infiltration water following the utility
trench.
19. Drain rock reservoir and subdrain may be omitted
where infiltration tests by the design professional
taken at the level of the base of the proposed
construction show an infiltration rate that exceeds
the inflow rate for the design storm (approximately
rainfall intensity x (I/P ratio + 1); I/P ratio is the ratio
of impervious to pervious area and is defined below
as part of Sizing).
20. A perforated pipe subdrain is required to drain
excess water from the soil and prevent root
drowning of Rain Garden plantings in poorly draining
soils. The subdrain should always be embedded in
drain rock near the top of the rock reservoir to
provide a storage volume below the subdrain unless
a rain garden with flow restrictor option is used
(Figure 4D).
21. The subdrain should have flow and inlet capacity to
carry the flow infiltrated through the soil layer.
Consult pipe manufacturer for perforation inflow
capacity. A maximum infiltration rate through the
soil can be estimated by applying Darcys equation:
h +d
Q max = k L Wbase max
d
where:
k is the hydraulic conductivity of the growing

medium (soil) (m/s)
Wbase is the average width of the ponded crosssection above the invert of the Rain Garden area
(m)
L is the length of the Rain Garden base area zone

(m)
hmax is the depth of the ponding above the growing

medium (m)
d is the thickness of the growing medium layer (m).
4-5
Rain Garden Sizing

Rain Gardens may be sized in a variety of ways
depending on the site needs and the design criteria.
Sizing may be done using continuous simulation
modeling in the WBM or SWMM, or using spreadsheet
design storm and water balance calculations. Simplified
sizing approaches have been developed that do not
require water balance modeling or continuous
simulation. Two sizing approaches are presented below
for the two types of criteria used in BC.
Table 4-1: Rain Garden Maximum

I/P Ratios by Surface Type
Max. I/P
Surface Type
Ratio
General/Industrial
Storage/Loading Areas
Divided or Undivided
Major Road
(Expressway or
Highway)
Collector Road
Parking >1
car/day/parking space
Local Road
Parking <1
Low traffic areas, no
parking
Single Family
Residential, Lot and
Roof
20:1
20:1
20:1
20:1
30:1
40:1
50:1
50:1
22. In general, the Rain Garden area is sized based on

the upstream impervious area that it serves. This
relationship can be defined by the ratio of
impervious area to pervious area (e.g. I/P ratio). For
the simplified sizing approaches here, this
represents the ratio of upstream impervious area
(also called catchment area) to Base Area of the
Rain Garden. I/P ratio to achieve the target capture
criteria will be calculated by the two sizing methods
below.
23. The maximum allowable I/P ratio for given surface
types is shown in the adjacent Table 4-1. This
maximum is based on ability of the vegetation to
handle flows and pollutants and is not related to
capture. Regardless of sizing calculation below,
maximum I/P ratio for a given surface type should
not be exceeded.
24. The sizing process provides the Base Area of the
Rain Garden, which is the flat area at the bottom
with uniform layers of mulch, topsoil and drain rock.
Sizing by these methods does not account for any
infiltration benefit provided by the sloped sides of
the rain garden.
25. The Base Area of the Rain Garden will always be
smaller than the total footprint of the facility, so the
footprint must be calculated (see step 12, above) in
order to understand the actual site area required.
26. Sizing presented here is for infiltration of rain water
for capture and prevention of site runoff. Sizing
and design according to this guidance will generally
provide water quality treatment for the volume of
water infiltrated. If water quality critieria volumes
are larger than capture volumes, additional sizing
4-6
may be required and a professional engineer should

be consulted.
1) Sizing for depth capture criteria: X mm in 24 hrs
27. Determine the maximum rock depth according to
the drain time (4 days max.) and round down to the
nearest 50 mm increment for constructability;
allowable depth range is 300 to 2000 mm:
DR =
Ks T 24
n
Where:
DR = Depth (thickness) of rock reservoir (mm)
Ks = Saturated hydraulic conductivity of subsurface soil
(mm/hr)
T = allowable drain time (days)
n = porosity of drain rock in reservoir (unitless, e.g. 0.35)
28. Use the following equation to determine the base

(bottom) area of rain garden and rock reservoir
required by finding the I/P ratio for the site:
I/P =
24 Ks + D P + D R n + 0.2 D S
1
R
Where:
I/P = Ratio of impervious tributary area to rain garden base
area (unitless)
R = Rainfall capture depth (mm)
(mm/hr)
DP = Depth of ponding (mm); 200 mm standard
DS = Soil layer depth (thickness); standard value = 450 (mm)
29. Check that the I/P ratio calculated is less than the
maximum allowed (Table 4-1). If it is not, use the
maximum allowed I/P ratio. This may mean that the
Rain Garden will exceed the % capture desired.
30. To find the rain garden base area:
BaseArea =
ImperviousTributaryArea
I/P
31. Calculate the footprint of the facility based on the

Base Area and side slopes as described in step 15.
32. If the site cannot accommodate the I/P ratio
required to provide the target capture, a partialinfiltration rain garden with flow restrictor design
may be used (see Figure 4D).
4-7
33. A 0.25 L/s/ha (or 0.09 mm/hr) unit discharge has

been recommended by DFO for the flow restrictor at
the downstream end of the swale subdrain (see
detail 4D).
34. Calculate the allowable discharge through the
orifice:
Q =
0.25 ASITE
1000
Where:
Q = Allowable discharge through orifice (m3/s)
0.25 = Recommended unit discharge (L/s/ha)
ASITE = Total site area draining to the swale, including the
swale area (ha)
35. This discharge is used to size the orifice on a flow

restrictor at the downstream end of the rain garden
subdrain (see detail 4D).
36. Solving the orifice equation for area of the orifice
(AO): AO =
QSITE
K 2 gh
Where:
QSITE = Theoretical discharge through infiltration from the
impervious area (m3/s)
K = Orifice Coefficient (typical value 0.6)
g = gravitational constant (m/s2)
h = differential head equivalent to depth of the perforated
drain pipe in the rock trench (typical value 0.3 m)
AO = Area of the orifice opening (m2) generally assumed to
be circular for calculation of orifice diameter.
37. The size of the rain garden is then determined by

the available area on the site up to the maximum I/P
ratio for the surface type as shown in Table 4-2.
38. For the flow restrictor option, the subdrain should be
at bottom of the rock in the rock reservoir. The
depth of the rock reservoir above the orifice outlet is
calculated as:
DR =
R ( I / P + 1) 0.09mm / hr 24hrs ( I / P + 1) 24 Ks 0.2 DS

n
Where:
R = Rainfall capture depth (mm)
4-8
I/P = Ratio of impervious tributary area to swale base area

(unitless)
0.09 = Recommended unit discharge through orifice (mm/hr)
(mm/hr)
DS = Soil layer depth (thickness); standard value = 300 (mm)
2) Sizing for % Capture of Average Annual Rainfall

39. Determine the average annual rainfall for the site. If
unknown, refer to map in Appendix B of isohyetal
lines showing average annual rainfall depths across
the Metro Vancouver Region.
40. Consult the Rain Garden chart in Appendix B
applicable for the sites location according to
average annual rainfall: 1100mm (White Rock),
1500mm (Kwantlen, Surrey and Vancouver), or
2100mm (North shore and Coquitlam); If between
these values, choose the chart for the higher
amount of rainfall or interpolate the result between
the two bracketing charts.
41. Find the point on the chart matching the sites
subsurface soil infiltration rate and the target %
Capture and select the curve that is at or above
(better capture) that point. The selected curve gives
the I/P ratio
ratio required to meet the given % capture
for the facility.
42. Check that the I/P ratio calculated is less than the
maximum allowed (Table 4-2). If it is not, use the
maximum allowed I/P ratio. This may mean that the
Rain Garden will exceed the % capture desired.
43. To find the rain garden base area:
BaseArea =
Tributary Impervious Area

I/P
44. Calculate the footprint of the facility based on the

Base Area and side slopes as described in step 15.
45. The shape of the points nearby on the selected
curve indicate the depth (thickness) of the rock
reservoir. The rock depth may be interpolated
between two neighbouring values. Allowable rock
depth range is 300 to 2000 mm.
46. If the site cannot accommodate the I/P ratio
required to provide the target capture, or an I/P ratio
4-9
of less than 5 would be needed (not shown on the

chart) a partial-infiltration rain garden with flow
restrictor design may be used (see Figure 4D).
47. The size of the rain garden is then determined using
the Rain Garden with 0.25 L/s/ha Orifice charts
(Appendix B). Read the I/P ratio required for the
given infiltration rate and capture target.
48. Calculate the rain garden base area:
BaseArea =
ImperviousTributaryArea
I/P
49. Check that the calculated rain garden base area is

smaller than the available site area. If not, the
capture target cannot be achieved given the site
constraints using the sizing tools in this document.
The site could be reconfigured to accommodate the
calculated rain garden base area. Alternately, the
rock reservoir footprint could be made larger than
the rain garden bottom area and the capture
calculated by a qualified stormwater professional.
50. The subdrain should be located at the bottom of the
rock reservoir for this option. The depth of the rock
reservoir above the orifice outlet is given as 1.5 m
for a rain garden with orifice, for the purposes of
this simplified design approach.
51. Calculate the allowable discharge through the
orifice:
Q =
0.25 ASITE
1000
Where:
Q = Allowable discharge through orifice (m3/s)
0.25 = Recommended unit discharge (L/s/ha)
ASITE = Total site area draining to the rain garden, including
the rain garden area (ha)
52. This discharge is used to size the orifice on a flow

restrictor at the downstream end of the rain garden
subdrain (see detail 4D).
53. Solving the orifice equation for area of the orifice
(AO): AO =
QSITE
K 2 gh
Where:
QSITE = Theoretical discharge through infiltration from the
impervious area (m3/s)
K = Orifice Coefficient (typical value 0.6)
g = gravitational constant
4-10
h = differential head equivalent to depth of the perforated

drain pipe in the rock trench (minimum value 0.3 m)
AO = Area of the orifice opening (m2) generally assumed to
be circular for calculation of orifice diameter.
An orifice of no less than 10 mm is recommended to
minimize clogging. A 10 mm orifice is the size required for a
0.46 ha tributary area. If the calculated orifice size is less
than 10 mm, a regional capture facility servicing at least a
0.46 ha tributary area should be considered.
Guideline Specifications
Materials shall meet Master Municipal Construction
Document 2009 requirements, and:
1. Infiltration Drain Rock: clean round stone or crushed
rock, with a porosity of 35 to 40 % such as 75mm
max, 38mm min, (Maryland Dept. Environmental
Resource Programs, 2001) or MMCD Section 3105-17 Part 2.6 Drain Rock, Coarse.
2. Pipe: PVC, DR 35, 150 mm min. dia., with
cleanouts, certified to CSA B182.1 as per MMCD.
3. Geosynthetics: as per Section 31-32-19, select for
filter criteria or from approved local government
product lists.
4. Sand: Pit Run Sand as per Section 31-05-17.
5. Growing Medium: As per Section 32-91-21 Topsoil
and Finish Grading, Table 2, but with required
minimum saturated hydraulic conductivity of 7
cm/hr (70 mm/hr), with organic matter requirements
amended as follows:
a. For lawn areas - minimum 8%
b. For planting areas - minimum 15%
6. Seeding: conform to Section 32-92-20 Seeding or
32-92-19 Hydraulic Seeding (note sodding will be
required for erosion control in most instances).
7. Sodding: conform to MMCD Section 31-92-23
Sodding.
Construction Practices shall meet Master Municipal
Construction Document 2009 requirements, and:
1. Isolate the Rain Garden site from sedimentation
during construction, either by use of effective
erosion and sediment control measures upstream,
or by delaying the excavation of 300mm of material
over the final subgrade of the Rain Garden until after
all sediment-producing construction in the drainage
4-11
area has been completed (Maryland Dept.

Environmental Resource Programs, 2001).
2. Prevent natural or fill soils from intermixing with the
Infiltration Drain Rock. All contaminated stone
aggregate must be removed and replaced (Maryland
Dept. Environmental Resource Programs, 2001).
3. Infiltration Drain Rock shall be installed in 300mm
lifts and compacted to eliminate voids between the
geotextile and surrounding soils (Maryland Dept.
Environmental Resource Programs, 2001).
4. Maintain grass areas to mowed height between
50mm and 150mm, but not below the design water
quality flow level. Landscape Maintenance
standards shall be to the BC Landscape Standard,
6th Edition, Maintenance Level 4: Open Space /
Play.
4-12
Rain Garden Design Example #1 For

Capture of Xmm/24 hour Criteria
Scenario Description
Description
A Rain Garden is proposed to capture a portion of the
runoff from a paved parking area (see illustration
below).
Figure 44-1: Example Parking Area Draining to Rain Garden
The following parameters are known:

Total pavement area = 930 m2
Annual rainfall = 1200 mm
2-year 24-hour rain depth = 53.2 mm
Native soil infiltration rate = 1.5 mm/hr
Parking use is more than one car per day
Capture target is 50% of 2-year 24-hour rain

amount
4-13
Determine the rain garden footprint area and rock

trench depth.
Sizing
Max. I/P
Surface Type
Ratio
General/Industrial
Major Road
(Expressway or
Highway)
Collector Road
Parking >1
Local Road
Parking <1
parking
Single Family
Roof
Determine the maximum rock depth based on the 4 day

maximum drain time:
DR =
Ks T 24 1.5mm / hr 4days 24hr / day

= 411mm
=
0.35
n
20:1
Use 400mm rock depth.

20:1
20:1
20:1
30:1
Determine the maximum I/P ratio (see Table 4-1).

Parking use of more than one car per day yields a
maximum I/P ratio of 20.
Determine the base (bottom) area of rain garden and
rock reservoir required by calculating the required I/P
ratio:
40:1
I/P =
50:1
I/P =
50:1
24 Ks + D P + D R n + 0.2 D S
1
R
24 1.5mm / hr + 200mm + 400mm 0.35 + 0.2 450mm
1
50% 53.2mm
I / P = 16.5
Check that the I/P ratio is less than the maximum (16.5
< 20, therefore OK).
Calculate the rain garden base area:
BaseArea =
4-14
imperviousTributaryArea 930sq.m
=
= 56sq.m
I/P
16.5
Rain Garden Design Example #2 For

Capture of % Annual Rainfall
Scenario Description
A Rain Garden is proposed to capture a portion of the
runoff from a paved parking area (see illustration
below).
Figure 44-1: Example Parking Area Draining to Rain Garden
The following parameters are known:

Total pavement area = 930 m2
Annual rainfall = 1200 mm
Native soil infiltration rate = 1.0 mm/hr
Parking use is more than one car per day
Capture target is 90% of annual rainfall
Determine the rain garden footprint area and rock

trench depth and the rock trench volume.
4-15
Sizing
Determine the maximum rock depth based on the 4 day
maximum drain time:
Max. I/P
Surface Type
Ratio
General/Industrial
Major Road
(Expressway or
Highway)
Collector Road
Parking >1
Local Road
Parking <1
parking
Single Family
Roof
20:1
20:1
20:1
20:1
30:1
40:1
50:1
50:1
Figure 44-2: Sizing chart for Rain

Garden (without orifice) for 1100 mm
annual rainfall.
DR =
Ks T 24 1.0mm / hr 4days 24hr / day

= 274mm
=
n
0.35
Use 300mm rock depth.

Determine the maximum I/P ratio (see Table 4-1).
Parking use of more than one car per day yields a
maximum I/P ratio of 20.
Because the annual rainfall at the site falls between two
sizing charts, the 1100 mm and 1600 mm, both will
need to be used to interpolate the I/P ratio needed to
meet the capture target.
As shown in the Rain Garden 1100 mm chart (Figure 42), the 90% capture and 1.0 mm/hr infiltration point
plots above the I/P=5 curve. As noted on the chart, an
orifice outlet is needed to meet this capture target at
this site.
Using the Rain Garden with Orifice 1100 mm chart
(Figure 4-3), the 90% capture and 1.0 mm/hr
infiltration point plots between the I/P=40 and I/P=50
curves. Similarly, the Rain Garden with Orifice 1600 mm
chart shows this point between the I/P=30 and I/P=40
curves.
Because both charts show required I/P ratios larger
than the maximum allowed (determined above to be
I/P=20), the design I/P ratio should be 20. In both the
Rain Garden with Orifice 1100mm and 1600mm charts,
the circular marker on the I/P=20 curve indicates a
1.5 m rock trench depth. This is the depth of rock
required above the subdrain for storage, so the total
depth of rock for this facility is 1.8m (1.5 m + 0.3 m).
The rain garden footprint area equals the pavement
area divided by the I/P ratio (930 m2 / 20 = 47 m2).
The rock volume below the overflow elevation is 70 m3
(47 m2 x 1.5 m).
Figure 44-3: Sizing chart for Rain

Garden with Orifice for 1100 mm
annual rainfall.
4-16
The orifice outlet from the rain garden should be sized

to deliver a maximum flow of:
Qo =
Ks I 1.0mm / hr 0.093ha
=
= 0.00026m3 / s = 0.26L / s
360
360
Where:
Qo = Orifice flow at full rock trench conditions (m3/s)
Ks = Saturated hydraulic conductivity of native soil (mm/hr)
I = Impervious area tributary to rain garden (ha)
Example Hydraulic Components

Inlet: Pavement runoff sheet flows over a panel curb

into the rain garden.
Overflow: A surface inlet in the rain garden decants

water that cannot infiltrate into the soil once the
ponding reaches a depth of 200mm. The surface
inlet is connected to the municipal storm sewer
connection.
Subdrain:
Subdrain: A perforated pipe located along the top of
the rock layer decants excess water into the
municipal storm sewer connection when the rock
trench is full of water.
Example Operation and Maintenance

Correct erosion problems as necessary. Ensure

distributed sheet flow into the rain garden.
Mow to keep grass in the active growth phase,

remove clippings to prevent clogging of outlets, and
remove trash and debris.
Remove leaves each fall, inspect overflow, hydraulic

and structural facilities annually.
Replace dead plants as required.
Surface inlet sump should be inspected annually

and cleaned as required. Sediment should be
removed from the sump bottom and floatables
removed from the water surface.
4-17
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4-18
DESIGN PRINCIPLES
Literature suggests rain garden areas
of about 10-20% of upstream
impervious area. Higher sediment load
land uses require lower ratios of
impervious area to rain garden area.
An Infiltration Rain Garden is a form of bioretention facility designed to have aesthetic appeal
as well as a stormwater function. Rain gardens are commonly a concave landscaped area where
runoff from roofs or paving infiltrates into deep constructed soils and subsoils below. On subsoils
with low infiltration rates, Rain Gardens often have a drain rock reservoir and perforated drain
system to convey away excess water.
1. Tree, Shrub and Groundcover Plantings
2. Growing Medium Minimum 450mm Depth
3. Drain Rock Reservoir
4. Flat Subsoil - scarified
5. Perforated Drain Pipe 150mm Dia. Min.
6. Geotextile Along All Sides of Drain Rock Reservoir
7. Overflow (standpipe or swale)
8. Flow Restrictor Assembly
9. Secondary Overflow Inlet at Catch Basin
10. Outflow Pipe to Storm Drain or Swale System
11. Trench Dams at All Utility Crossings
Smaller, distributed rain gardens are

better than single large scale facilities.
m
Locate rain gardens a minimum 30.5m
from wells, 3m downslope of building
foundations, and only in areas where
foundations have footing drains and
are not above steep slopes.
Provide pretreatment and erosion
control i.e. grass filter strip to avoid
introducing sediment into the garden.
Full Infiltration
At point-source inlets, install

non-erodable material, sediment
cleanout basins, and weir flow
spreaders.
Where all inflow is intended to infiltrate into the

underlying subsoil. Candidate in sites with
subsoil permeability > 30mm/hr. An overflow for
large events is provided by pipe or swale to the
storm drain system.
Bottom width - 600mm (Min.) to

3000mm and length-width ratio of 2:1
desirable.
Side slopes - 2:1 maximum, 4:1
preferred for maintenance.
Ponding depth - 150 - 300mm.
Full Infiltration with Reservoir
d
Draw-down time for maximum ponded
volume - 72 hours.
Adding a drain rock reservoir so that surface

water can move quickly through the installed
growing medium and infiltrate slowly into subsoils
from the reservoir below. Candidate in sites with
subsoil permeability > 15mm/hr.
Treatment soil depth - 300mm (Min.)

to 1200mm (desirable); use soils with
minimum infiltration rate of 50mm/hr.
Surface planting should be primarily
trees, shrubs, and groundcovers, with
planting designs respecting the
various soil moisture conditions in the
garden. Plantings may include rushes,,
sedges and grasses as well as lawn
areas for erosion control and multiple
uses.
Partial Infiltration
Install a non-erodible outlet or spillwayy

to discharge overflow.
Designed so that most water may infiltrate into

the underlying soil while the surplus overflow is
drained by perforated pipes that are placed near
the top of the drain rock reservoir. Suitable for
sites with subsoil permeability > 1 and <
15mm/hr.
Avoid utility or other crossings of the

rain garden. Where utility trenches
must be constructed below the
garden, install trench dams to avoid
infiltration water following the utility
trench.
Partial Infiltration with Flow

Restrictor
Apply a 50-75mm layer of organic

mulch for both erosion control and to
maintain infiltration capacity.
For sites with subsoil permeability < 5mm/hr, the

addition of a flow restrictor assembly with a small
orifice slowly decants the top portion of the
reservoir and rain garden. Provides water quality
treatment and some infiltration, while acting like a
small detention facility.
Drain rock reservoir and perforated

drain pipe may be avoided where
infiltration tests by a design
professional show a subsoil infiltration
rate that exceeds the inflow rate.
Rain
R ain Garden
Gard
den
Detailed design guidelines can be found in the

Design Guidelines 2012 report, available at
www.metrovancouver.org

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Stormwater Source Control Design Guidelines 2012

INFILTRATION RAIN GARDEN

Greater Vancouver Sewerage & Drainage District

Infiltration Rain Garden

Infiltration Rain Garden

Stormwater Source Control Design Guidelines 2012

This page is intentionally left blank.

Greater Vancouver Sewerage & Drainage District

Stormwater Source Control Design Guidelines 2012

Infiltration Rain Garden

Infiltration Rain Garden

Photo Credit: Lanarc Consultants Ltd.

While usually designed as a standalone facility

Greater Vancouver Sewerage & Drainage District

Informal rain garden, Water Pollution Control

Infiltration Rain Garden

Stormwater Source Control Design Guidelines 2012

Selection, Application and Limitations

Rain Gardens are utilized for volume capture and

A Rain Garden and Infiltration Swale have similar

If treatment is not required (e.g. for pre-treated or

A rain garden will provide increased volume capture

Smaller, distributed Rain Gardens are preferable to

Infiltration Rain Gardens may take a variety of

Photo Credit: Lanarc Consultants Ltd.

FlowFlow-thru planter a formalformal-shaped rain garden

Greater Vancouver Sewerage & Drainage District

Stormwater Source Control Design Guidelines 2012

Infiltration Rain Garden

Photo Credit: Kerr Wood Leidal Associates

3. Where inflow is from curb cuts or point (pipe)

Roadside rain garden with flat panel curb and

5. Experience has shown that grass is efficient at

Greater Vancouver Sewerage & Drainage District

Infiltration Rain Garden

Stormwater Source Control Design Guidelines 2012

9. Rain Garden side slopes: 2 horizontal : 1 vertical

Photo Credit: Lanarc Consultants Ltd.

14. Rain Garden depth includes ponding depth (depth

Rain garden overflow,

16. Treatment soil (i.e. growing medium) depth:

Greater Vancouver Sewerage & Drainage District

Stormwater Source Control Design Guidelines 2012

Infiltration Rain Garden

18. Avoid utility or other crossings of the Rain Garden.

k is the hydraulic conductivity of the growing

L is the length of the Rain Garden base area zone

hmax is the depth of the ponding above the growing

d is the thickness of the growing medium layer (m).

Greater Vancouver Sewerage & Drainage District

Infiltration Rain Garden

Stormwater Source Control Design Guidelines 2012

Rain Garden Sizing

Table 4-1: Rain Garden Maximum

22. In general, the Rain Garden area is sized based on

Greater Vancouver Sewerage & Drainage District

Stormwater Source Control Design Guidelines 2012

Infiltration Rain Garden

may be required and a professional engineer should