STORMWATER PLANTERS

STORMWATER PLANTERS
This fact sheet is part of a larger series that outlines considerations for
planning, designing, and constructing Green Stormwater Infrastructure.
Stormwater planters are structural landscaped
reservoirs designed to capture stormwater runoff from
impervious areas. Stormwater runoff is filtered by
plant material and infiltrates into the growing medium.
The facility provides stormwater benefits by removing
pollutants and reducing the amount of runoff that
reaches streams. These stormwater benefits reduce
negative effects of urbanization by more closely
matching the natural hydrologic cycle associated with
native forests and meadows. Diverse vegetation,
particularly when planted with native species, provides
excellent habitat for desirable wildlife.
Stormwater planters have vertical walls, which reduce
the site area required for stormwater treatment.
Generally, the planter walls are approximately 18 to
24 inches tall and do not require special engineering
design, unless they need to support heavy loads (from
an adjacent roadway) or are taller than three feet.
A variety of trees, shrubs, grasses, and groundcovers
can be used in planters, creating an attractive
landscape feature.
Types of Planters
ffInfiltration Planter:
Provides water quality
treatment and flow control
ffPartial Infiltration Planter:
Provides water quality
treatment and some flow
control
ffFiltration Planter:
Provides water quality
treatment only
Site Assessment
Robert Emanuel
To help identify opportunities and constraints for the location of your planter,
several key site features need to be reviewed. For this fact sheet, the
features have been divided into two categories: Natural and Infrastructure
Characteristics.
Infiltration Testing
An infiltration test must be
completed to determine the rate
the planter can absorb stormwater
runoff.
For single family residential
applications, the infiltration test
consists of digging a small hole,
filling it with water, and determining
how long it takes for the water
to infiltrate. Appendix 4C in the
Salem Public Works Design
Manual provides detailed test
procedures.
Revised: April 20, 2014
Natural Characteristics
ff Site Soils
Stormwater planters are best
placed where soils drain well. Soil
amendments (sand, compost) can
improve infiltration where drainage
is poor. Additionally, the planter
can be made larger to account for
slower infiltration rates.
ff Topography
Planters should be located on
generally flat areas and not at
the top of a steep slope unless
designed by a qualified engineer.
If located in the right-of-way,
planters should match the slope of
the road.
ff Water Resources
Streams, ponds, and wetlands are
sensitive natural areas that provide
important water quality benefits.
Planters should be integrated into
the natural landscape without
disturbing sensitive areas with
construction activities. Additionally,
infiltration stormwater planters
may not be appropriate where the
groundwater table is high.
ff Vegetation
Preserve mature trees when
possible. Mature trees disperse
runoff, break up tight soils, and
take up stormwater better than
young trees. Preserved vegetation
can also be used to reduce the
required planter size. Avoid placing
planters beneath the drip line of
large trees to protect and prevent
over-saturation of roots.
For more info visit: http://www.cityofsalem.net/Departments/PublicWorks/Pages/stormwater-code
Page 1 of 4
Stormwater Planters Fact Sheet—City of Salem
Five-foot setback from property lines where
the adjacent property is down slope
Infrastructure Characteristics
ff Stormwater Flow Patterns and Impervious Areas:
Locate planters down slope of impervious
surfaces to collect stormwater runoff. Determine
how much impervious area will be draining
stormwater to your planter. It can be helpful to
draw a diagram of your site showing stormwater
flow directions and area measurements.
Ten-foot setback
from structures
ff Setbacks:
Stormwater planters that infiltrate should be
located at least ten feet from structures and
five feet from property lines where the adjacent
property is down slope. If the stormwater planter
will be a filtration planter with an impermeable
liner, it can be located closer than ten feet from a
building.
ff Utilities:
Avoid placing any stormwater planter over utilities
or septic drain fields. State law requires that you
identify underground utilities before excavation.
For more information on utility locating visit
http://www.callbeforeyoudig.org or call 811.
Potential planter locations
Downspout
Curb with
cutout
Overflow drain
Growing medium
Planter wall
Planter Design
The design of a planter involves four elements:
ff Planter Type
ff Planter Location (Water Flow Design)
ff Planter Sizing
Waterproof liner
(when required)
Drain rock
ff Plant Selection
See Standard Plans at the City
of Salem website:
STD215, STD216, STD228,
STD229, STD230, STD231, etc.
Planter Type
ff Infiltration Planter
Used when the infiltration rate is greater than 0.5 inches per hour and there is adequate space available. Drain
rock storage can be added to increase the storage capacity of the stormwater facility. This planter type provides
both water quality treatment and flow control.
ff Partial Infiltration Planter
Used when the infiltration rate is less than 0.5 inches per hour and there is inadequate space available This planter
type requires an overflow structure with an under drain and provides water quality treatment and some flow control.
ff Filtration Planter
Used when infiltration is prohibited (e.g., adjacent to a building or contaminated site soils). This planter type
requires an overflow structure with an under drain and provides water quality treatment only.
Revised: April 20, 2014
For more info visit: http://www.cityofsalem.net/Departments/PublicWorks/Pages/stormwater-code
Page 2 of 4
Stormwater Planters Fact Sheet—City of Salem
Planter Location (Water Flow Design)
Stormwater planters should be located on a flat or gently sloped area that is down slope of existing impervious
surfaces. If needed, a pipe can be used to convey water to the planter, but this will often result in an overly deep
facility.
An intense rain event may produce more stormwater runoff than the planter can manage, even if it is properly
designed. To protect property during an extreme rain event, each planter must have an overflow route to an approved
point of discharge. The point of discharge may be a storm drain pipe, open channel, waterway, wetland, or other
approved facility and will require a stormwater connection permit.
Planter Sizing
Stormwater planters are structural reservoirs with an 18-inch-thick layer of growing medium (soil, sand, and compost
mix). A maximum 48 inches of drain rock can be included under the stormwater planter to increase storage capacity.
Planters typically have flat or nearly flat bottoms with vertical planter walls.
There are two methods for sizing a planter: The Simplified Method and the
Engineered Method. A description of both design methods is located in Section
4.2(n) Design Sizing Methodologies of the Public Works Design Standards.
Simplified Method
Sizing Factors
Simplified Method
ff Used for small planters collecting stormwater from less than 10,000 square
feet of impervious area and located on private property.
Infiltration Planter
Infiltration Rate Sizing Factor
ff To calculate, multiply the impervious area managed by a sizing factor that is
based on the infiltration rate.
0.5-0.75
0.11
0.75-1.25
0.09
ff Does not require a licensed engineer to approve the design.
1.25-1.75
0.07
>1.75
0.06
Engineered Method
ff Used for large planters collecting stormwater from more than 10,000 square
feet of impervious area or located on public property.
ff Uses standard engineering calculations to size the facility.
Filtration Planter
Infiltration Rate Sizing Factor
N/A
ff Requires a licensed engineer to approve the design.
Common Camas
Slough Sedge
Nootka Rose
0.06
Oregon Iris
Plant Selection
Vine Maple
The look of your stormwater planter can be personalized with your choice of trees,
shrubs, grasses, and groundcovers. Appendix 4B of the Public Works Design
Standards includes a list of plants, including ornamental plants, that are suitable
for planters. When located in the right-of-way planters can dramatically improve the
pedestrian experience. A small sample of the available plants is shown here.
The plant list identifies three planting zones (based on saturation frequency) and
plants that are most appropriate for each zone. Due to the flat bottom of planters,
only Zone 1 plants are appropriate. Also consider sun exposure when selecting
plant species.
Salmonberry
Revised: April 20, 2014
As with other gardens, invasive weed control is important.
For more info visit: http://www.cityofsalem.net/Departments/PublicWorks/Pages/stormwater-code
Page 3 of 4
Stormwater Planters Fact Sheet—City of Salem
Dig Safely
Construction
Stormwater planter construction elements are: excavating, constructing planter
walls, constructing growing medium, placing erosion protection, and planting.
Excavating
ff Remove sod or other plants and
scrape off topsoil to a depth of
eight inches. Stockpile topsoil for
constructing growing medium.
ff Avoid allowing newly exposed
soil to receive heavy rains prior
to placement of growing medium
and plants. Rain can seal exposed
soils and potentially reduce
infiltration capacity.
Prior to digging, identify the
locations of underground utilities
One-Call Center: 811
Constructing Planter Walls
ff The construction of planter walls
is similar to concrete curbs.
Planter walls should include
openings to allow water to flow
into the planter.
Constructing the Growing Medium
ff Creating Growing Medium from Stockpiled Topsoil:
Create a mix of 2/3 topsoil to 1/3 imported compost. Make sure
growing medium is well mixed and free of debris. This method is only
applicable for single family residential projects.
ff Importing Growing Medium:
Imported growing medium should consist of loamy soil, sand, and
compost, 30–40 percent compost by volume.
ff Placing Growing Medium:
Avoid overly compacting growing medium. If surface ponding is
occurring after placing growing medium, add sand and re-till until
infiltration performance is improved and ponding no longer occurs.
Placing Erosion Protection
Planting
ff Erosion protection is important
where collected stormwater
will enter your facility (e.g. curb
opening). A small area of minimum
three-inch river rock or a splash
block below the curb opening can
effectively dissipate erosive energy
and spread concentrated water as
it moves into the planter.
ff Planting best practices typical to
any garden will help the survival
rate of your transplanted plants.
ff Check dams can be placed in the
bottom of sloped planters to keep
water spread out and to protect
from the formation of rills.
ff Planting in the dormant season
encourages strong root
development and vigorous growth
in the spring without the need for
continuous watering.
ff Once established, native plants
can thrive without supplemental
watering and fertilizing.
Maintenance (see Admin. Rule Chapter 109-011—Operations and Maintenance of Stormwater Facilities)
ff Vegetation (including native species) will likely require supplemental watering during the dry season for the first
two years until the plants become established.
ff Remove sediment buildup and replenish mulch and pea gravel as needed.
ff Perform routine garden maintenance including pruning, removal of weeds, and replacing dead plants.
ff Avoid applying pesticides, fertilizers, and herbicides if possible. Apply in strict compliance with manufacturer’s
instructions. Excessive use pollutes surface water.
Revised: April 20, 2014
For more info visit: http://www.cityofsalem.net/Departments/PublicWorks/Pages/stormwater-code
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