rain gardens 102 – top 10 implementation and
TRANSCRIPT
Rain Gardens 102 – Top 10 Implementation and Maintenance Considerations
David Lee, Kerr Wood LeidalCraig Kipkie, Kerr Wood LeidalDavid Reid, HB Lanarc
BCWWA, April 2011
2What is a Rain Garden?
“A rain garden is a planted depression that allows rainwater runoff from impervious urban areas like roofs, driveways, walkways, and compacted lawn areas the opportunity to be absorbed. This reduces rain runoff by allowing stormwater to soak into the ground.” (Wikipedia)
3Rain Garden vs Bioswales
• “Rain gardens are at times confused with bioswales. Swales slope to a destination, while rain gardens do not; however, a bioswale may end with a rain garden.”
• “Vegetated roadside swales, now promoted as “bioswales”, remain the conventional drainage system in many parts of the world from long before extensive networks of cement sewers became the conventional engineering practice in the USA.”
(more Wikipedia)
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• Growing medium – to support plant growth and retain water to field capacity of soil
• Vegetation – to promote regeneration of infiltration surface and assist in the evapo-transpiration process
• Rock trench (optional for high permeable soils) – to store infiltrated water and release after storm event
• Perforated Pipe (optional for high permeable soils) – to protect plant roots from flooding, to preserve oxygen levels in growing medium, and to safely direct interflow from infrequent events to collection system
• Overflow – to protect property/infrastructure from both high intensity rainfall events and rain on frozen ground conditions.
Basic Components of a Rain Garden
5How Do They Work?
6Rain Gardens 101: Top 10 Design Considerations
1. Rainwater Design Criteria
2. Impervious to Rain Garden Area Ratio
3. Soil Selection and Attributes
4. Use of Under-drains
5. Depth of Rain Garden
6. Curb-edge material
7. Trees or no Trees?
8. Where is the Infiltrated Water Going?
9. Native Soil Infiltration Rates
10.Depth of Rock Trenches
7Rain Gardens 102: Top 10 Implementation and Maintenance Considerations
1. Construction Phasing
2. Wrong Soil Specification Used
3. Weed Control
4. Binding of Soil Layers During Construction
5. Interim Sediment and Drainage Control – Minor Storms
6. Interim Drainage Management – Major Storms
7. Builder Management
8. Maintenance Responsibility
9. Road Maintenance (road salts and sanding)
10.Performance Monitoring and Warranty Period
81. Construction Phasing
• Construction phasing is a critical component in the successful implementation of rain garden in a subdivision.
• Sub-surface drain rock reservoir, perforated drains, and overflows should be constructed at same time as other utilities.
Not IdealRecommended
91. Construction Phasing (cont.)
• Sub-surface drain rock reservoir, perforated drains need to be protected during construction.
• The use of sacrificial soil layer and poly sheeting should be considered.
101. Construction Phasing (cont.)
• Growing medium and plantings completed after building construction complete (preferably before second lift of asphalt.
Not IdealRecommended
112. Wrong Soil Specification Used
• The proper amended (upper layer) soil properties are critical tomeeting: 1) surface infiltration rates; 2) ensure healthy plants for the long term.
• Soil specification for rain gardens typically contain more sand than the garden variety.
• There are two ways this can be accomplished:
1. import material to the site; or
2. amend existing material onsite to conform to the specification.
• View this in the same way we review shop drawings.
122. Wrong Soil Specification Used (cont.)
• If using an amended soil mix developed on site it is recommended that a soil physicist be consulted to provide the correct mix proportions and testing should be done.
• Warning – if using amended soil, weeds can be an issue and need to be dealt with.
133. Weed Control
• There is a significant risk of seed germination issues from a on site mix so it is also recommended a germination test should be performed.
• Seeds in the soil can be killed by heat. A full composting process under controlled conditions will often raise the temperature high enough to kill seeds.
• Mulch can be used inhibit weeds. It is recommended that ground wood mulch be used (not bark chips which will float, or sawdust/chips which will remove nitrogen).
• A 50 mm to 75 mm layer. Too deep a layer risks black liquor runoff and associated BOD.
144. Binding Off Of Sand Layer
• The length of time between completing the subsurface drain rock reservoir and adding the soil layer can affect the performance of a rain garden.
• There’s a potential for the sand layer which is below the growing medium to be bound off if subjected to rain then drying.
• A crust can form on the sand layer that can act as a semi-impermeable surface.
• There’s a need for this sand layer to be scarified prior to placing growing media.
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5. Interim Sediment and Drainage Control –Minor Storms
• As with any construction site, sediment control during storm events is critical.
• Establish temporary sediment capture areas at low points. Use rain garden depression as sediment trap areas (as long as rock reservoir and perforated pipe is protected).
• Contain flow for minor storm events to road.
Low area with outlet used for sediment control
Minor flows contained along panel curb(prior to second lift of asphalt)
166. Interim Drainage Management –
Major Storms
• Subdivisions with rain gardens are different than traditional subdivisions because there are usually no catch basins or barrier curbs.
• Majority of the systems (rock pits, absorbent landscape, rain gardens etc.) will not be operating during construction phase – potentially more runoff.
• Major drainage paths during construction need to be considered.
176. Interim Drainage Management –
Major Storms (cont.)• The site needs to be prepared for major storm event even though
the probability of a 5-10 year storm occurring during period is low but still possible.
• Roads need to be used as conveyance system.• Second lift asphalt needs to
remain off until rain gardens are completed and sand bags can help with flow redirection.
• Major flows need to be directed properly to the site’s to sedimentation pond. The pond needs to be have sufficient capacity for the major events for under construction ground conditions.
186. Interim Drainage Management –
Major Storms (cont.)
Example of what can happen when overland flow path is not considered.
197. Builder Management
• Builder and trades will need education on building site management and the impact their activities can have on rain garden areas and other stormwater measures.
• Builders and trades will need to access the lots, stockpiling area, etc.
• No-go areas should be clearly marked (signage and orange fence around rain garden depression area).
207. Builder Management (cont.)
• Roof gutters should not be installed until site has been landscaped and rain gardens planted.
• Removal of sacrificial soil and poly and planting of rain garden should be completed when building is fully completed (can be difficult due to timing).
• Soils on the lot should be tilled and scarified prior to placing the finished growing medium.
218. Maintenance Responsibility
• Developer (public areas) and builder (on-lot) will be responsible for maintenance for likely a 2 to 3 year warranty period.
• Would include watering (automatic irrigation or manual).
• Plant and soil maintenance and weed control (twice annually, spring and fall).
During Warranty Period
228. Maintenance Responsibility (cont.)
What are some of the options for ongoing maintenance?1. Restrictive covenant.2. City-wide boulevard maintenance bylaw.3. City staff responsibility.4. Local area improvement agreement amongst homeowners.5. Other?
After Warranty Period
Goal is to have the rain garden last as long as the road surface.
23Example of a Restrictive Covenant
• A legal document to determine property owner’s maintenance responsibilities.
• “The Grantor covenants and agrees that it shall ensure that any landscaping on the boulevard, whether planted by the owner, a previous owner, an occupier or previous occupier of the real property, or by the District, is maintained to a standard that a reasonable property owner would use in the care of his or her landscaping on his or her own property.”
24Boulevard Maintenance Bylaw
• City of Coquitlam’s bylaw to regulate the construction and maintenance of boulevards.
• “A boulevard is the area that runs along an individual’s property, such as a sidewalk or landscaped area between the property line and the street.”
• Maintenance includes the care of vegetation, watering all landscaping regularly, removal of harmful weeds, correct hazardous or potentially hazardous situations, etc.
25Example of Local Area Improvement Agreement
• Seattle Public Utilities and Seattle Housing Authority signed a Memorandum of Understanding to determine maintenance responsibilities between homeowner association, limited partnership (rental housing) and rights-of-way association.
269. Road Maintenance Issues
• Education (City staff or contractors) will be required with regards to snow removal, sanding and salting practices.
• Plows need to be careful because there are no curbs.
• There may be the need for designated snow storage areas.
• If rain gardens are used for snow storage, this should be anticipated during the design process and plants should be selected accordingly.
Whistler Athletes Village, January 2010
279. Road Maintenance Issues (cont.)
• Street sweeping can be a method to prolong the life of a rain garden (type of street sweeper and timing is important).
• Regular inspection is required to ensure sheet flow is not being impeded causing concentrated flow.
• Top layer of soil and plantings may have to be replaced when road surface is redone (25 years?).
2810. Performance Monitoring and
Warranty Period
• Continuous flow, rainfall and water quality monitoring is the best way of measuring - but are very expensive.
• Good for pilot studies and research projects.
How can you determine rain gardens are working before the end of the warranty period?
2910. Performance Monitoring and
Warranty Period
• Water quantity pick a dry soil period, put a chalk line or a cork in the downstream outlet and observe if water flowed at a rainfall event; water shouldn’t flow below design rainfall intensity levels and capture target.
• Water quality – if objective above is met, a large percentage of the water quality targets will likely be met.
• There shouldn’t be any mud forming in rain garden.
• No standing water after a 24 hour storm.• There shouldn’t be a vegetation wall forming at
the interface with the flat panel curb.
Other inspections methods:
