AN INTRODUCTION TO RAIN GARDENS
Presented by: Rod Ritchie
What is a Rain Garden?
A beautiful way to capture and clean rainwater before it enters our streams, lakes and rivers
A landscaped area that collects rainwater runoff from impervious areas like roof tops, driveways, walkways, and parking lots (impervious areas don’t allow rainwater to soak into the ground).
Here’s how a rain garden works
Stormwater flows off of impervious areas or hard surfaces like roof tops, driveways and sidewalks.
That runoff then flows downhill to the rain garden.
Stormwater ponds in the rain garden for up to 72 hours (detention or retention time) – not enough time to breed mosquitos.
During the detention/retention time, stormwater is absorbed by the plants, soaks into the ground and some evaporates into the air.
Rain Garden Schematic
Basic Components of a Rain Garden
Inflow
Storage
Outflow (into the soil for retention systems and through an outlet pipe for detention systems)
Rain Garden Schematic (Complex)
Safe Overflow
When we get a lot of rain, rain gardens can become saturated so they need to have a safe overflow.
An overflow is a way for excess stormwater to flow safely (without causing erosion) from the rain garden to a nearby drain or stream.
For small rain gardens in relatively flat areas, overflow isn’t really an issue since erosion is typically caused by excess stormwater runoff flowing across steep slopes.
Why are rain gardens helpful?
Uncontrolled stormwater runoff causes erosion, water pollution, flooding and doesn’t replenish groundwater supply.
Rain gardens help to control and reduce runoff.
Controlling and reducing runoff helps to reduce erosion, flooding, water pollution, and improve groundwater supply.
Rain gardens remove pollutants like oil and other motor fluids from rainwater that flows across driveways and parking lots.
Rain gardens attract wildlife such as native birds by providing habitat and food.
They look nice.
Basic Formula to Size a Rain Garden
Size of the rain garden depends on the area it’s treating (rooftops, sidewalks, driveways)
Depending on the space available for the rain garden may want to make it smaller and deeper or larger and shallow. Typically rain gardens are either 1 or 2 feet deep.
Area of rain garden is the treatment drainage area divided by depth (in inches).
Example: Front half of a roof that drains to one downspout. Front half of roof is 40 feet long and width of the house to the peak is 20 feet. This area is 800 square feet.
For a 1 foot depth, rain garden area is 800/12 = 67 square feet or 8 feet by 8 feet. For a 2 foot depth, area is 800/24 = 33 square feet or 6 feet by 6 feet.
Where would you put a rain garden?
An ideal location is a low spot next to an impervious area.
Look for a spot where land around the rain garden can be sloped so that rainwater will naturally flow into the rain garden from nearby areas like sidewalks and driveways.
This can be many different locations.
In a parking lot
In a courtyard
Next to a building
Along a sidewalk
Once you have the location, you’re ready to build a rain garden.
First, create the depression. Large and complex rain gardens may require the use of a contractor with heavy equipment Small rain gardens can be dug by hand with a shovel. Always call PA One Call (Dial 811) before you dig to avoid hitting utilities like electric or gas.
A berm on the downhill side of the rain garden helps to contain rainwater during heavier storms.
Piping
Next, install piping, if necessary A rain garden collecting roof runoff from a downspout may need piping under a sidewalk to transport water to the other side without flowing across the sidewalk. A rain garden in a low spot next to a driveway or sidewalk doesn’t need any piping because rainwater simply flows off of a sidewalk or driveway into the rain garden. In order to provide for a safe overflow, a stabilized spillway or outlet pipe to connect to a nearby drain may be required in some cases. However, for small residential projects this isn’t usually necessary.
Vegetation
Finally, select and install vegetation.
Must consider soil conditions.
Design typically includes native plants. Wildflowers, sedges, rushes, ferns, shrubs and small trees; Don’t require fertilizer; Different zones of wet tolerant and drought tolerant species;
Lawns have shallow roots and don’t absorb excess rainwater.
Deep, dense roots help break up dense soil and increases water absorption.
Example - Vernon Park
Located off of Germantown Ave between Chelten, Greene and W Rittenhouse St.
Rain garden is in front of Center in the Park building and treats runoff from the roof and nearby sidewalks.
Large community and stakeholder involvement
Explored options for location and landscaping (formal vs informal)
Consensus was for a formal design
Roof leader diversions
Mayor Nutter attended ribbon cutting ceremony.
Vernon Park - Before
Vernon Park - After
Vernon Park - After
Vernon Park - Piping
Vernon Park - Volunteers
Example - Sisters of St. Basil
Located behind Manor College off of Fox Chase Road in Jenkintown, PA.
Parking lot runoff previously dumped stormwater to an open grassed area.
The shallow-rooted grassed area did little to clean the rainwater
Completed project cleans the rainwater prior to flowing into Jenkintown Creek a tributary to the Tookany Creek.
The final landscaping will provide food and shelter for birds and other wildlife.
Sisters of St. Basil - Design
Sisters of St. Basil - Construction
Example - Pennypack HOA
Located within the Pennypack Homeowners Association property in Northeast Phila., PA
This rain garden treats stormwater runoff from the administration building.
As the plants mature, like in any garden, they will fill the space and the area will become much more dense with plants and flowers.
Pennypack HOA – Northeast Phila.
Pennypack HOA – Northeast Phila.
Pennypack HOA – Northeast Phila.
Can I build a rain garden on my property?
ABSOLUTELY!
A small rain garden can be built in a yard.
Follow the formula to size the garden.
Create the depression using a shovel and a tape measure for the depth (call 811 first).
Direct downspouts toward the garden area or place the garden in a low spot next to a driveway or sidewalk that slopes toward the area.
Have fun choosing your plants.
See next page for a before photo and the following page for a rendering of what a natural meadow-like wildflower design could look like.
Of course, a more formal looking design like the Pennypack project is also an option.
Backyard - Before
Backyard - After
How and Why do I Maintain my Rain Garden?
Inspections
Typical Routine Maintenance
Use a checklist Develop a schedule for regular inspections Review design plans Conducted by experienced personnel Plant establishment - watering Clear inflows and outflows Repair erosion without compacting soils Remove weeds and invasive species Remove trash
Why? So it keeps working.
Resources
Philadelphia Water Department www.phila.gov/water
Tookany/Tacony-Frankford Watershed Partnership www.ttfwatershed.org