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Pervious Pavements for Homeowners

Sustainable design isn’t about doing something neat, it’s about doing something right.

Definition

• An engineered stormwater facility that you can drive or walk on,which preserves perviousness to decrease environmental impactsof conventional impervious pavements.

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The Water Balance ModelBefore

9.5” interflow(infiltration)

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Types of Site Constraints

• Water Quality: When ground or surface waters may be degraded

• Water Quantity: When infiltrating water may cause a problem (forpure pervious pavements, not really an issue, these can anywhere alawn is going)

• Other

Infiltration & Water Quality

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Infiltration & Water Quality

• Soil underneath properly installed pervious pavements is alivebecause it hasn’t been compacted and is open to the air and waterthrough the pores in the entire system.

• Extent of water quality in raingardens is determined by retentiontime (how long the runoff spends somewhere) in living soil.

• As retention time (in soil) increases, water quality increases therefore:

• Clay soils are “poor infiltrators”, but are great for water quality

• Sandy soils are great infiltrators, but provide less water qualitytreatment (Amend 24” of sandy soil with clay and compost toreduce infiltration rate to 10in/hour or less.)

Water Quality Constraints

• Not in areas of high bedrock

min

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Water Quality Constraints (cont.)

• Not in areas of high groundwater

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Water Quality Constraints (cont.)

Don’t use on

• Slopes > 5% slopes

• In swelling soils

• Areas where contamination of groundwater could occur:

• Septic fields

• Where spills are likely

• In contaminated soils (brownfields)

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Other Constraints

• Must be “hydraulically isolated”. This means there shouldn’t be runoffcoming from anywhere else on to the pavement.

• Where infiltration testing performed in the native subgrade is less than0.1 in/hour. (That’s not a typo! Pervious pavements work in soils thathave at least 0.1 in /hour . Most of our clays test at 0.5 in/hour.)

Courtesy of Cahill Associates

Other Constraints

• Should not be located in fill placed within the last 5 years. Fill hasbeen compacted and has the runoff characteristics of concrete. It willnot infiltrate.

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Uncompacted native subgrade

Uncompacted native subgrade

• This is the natural soil on your property.

• This is not “fill”, which is soil that’s has been brought in fromsomewhere and compacted.

• Your contractor will probably want to compact the soil. DO NOT letyour contractor do this. Just assure your contractor that you went toa presentation on pervious pavement and you learned that thedepth of rock has already been increased to provide the structuralstability needed to support the loads without compacting thesubgrade.

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Construction Considerations

• Protect infiltration area from compaction throughout theconstruction process. Excavate from the side.

Construction Considerations

• Compacted soils can be loosened by turning the soil.

• Runoff should not be coming from other places during constructionor afterwards.

• For vehicular areas, an overflow of some sort should be provided forlarge storms. Having excess rainfall come back up through thepavement is not ideal and could compromise the stability of thepavement.

• Pavements should still be sloped at a minimum of 2% (1/4” drop perhorizontal foot) away from buildings in case they clog

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Pervious Pavement Surface Typesthat can be installed by anyone

Homemade pavers Grass-crete

Flexible Pavements(GrassPave )

Commercial pavers

Boardwalks/decksWood chips(pedestrian only)

Gravel

Pervious Pavement Surface Typesthat should only be installed by a professional

Courtesy of MGH Associates

Asphalt Concrete

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Pervious Pavement Sections

Homemade pavers Grass-creteCommercial pavers Gravel

Pervious Pavement Sections

Courtesy of Fortis Construction

Flexible Pavements(GrassPave )

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Pervious Pavement Sections

Courtesy of Fortis Construction

Flexible Pavements(GrassPave )

This is what theflexible pavementlooks like before youadd a pavementsurfsace (turf, gravel,pavers). Good toreduce excavationneeded to save treesor provide structuralsupport for overflowareas that need avery high loadingperformance (firetrucks!)

Filter fabric for french drains

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Filter fabric for french drains

• You can get this at home depot if you ask for it like this. Just makesure that it allows water to pass through it.

• “Mirafi” is a common manufacturer but there are lots of others.

• Should be placed on sides, too.

• Keeps the native soils from migrating up into your storage rock (akaclean, washed uniformly graded aggregate)

Clean, washed uniformly graded aggregate

• Aka “rock” or “storageaggregate”

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Clean, washed uniformly graded aggregate

• Extremely clean (so as not clog filter fabric): not just washed from thequarry, but washed on-site if necessary

• I mean it: NO DIRT, a little rock dust is OK

• All the rock is the approximately the same size. This provides a 40%void ratio (ratio of solid rock to voids). Gradation examples:

• 5/8 – ¾” rock

• 1½” – 1” rock

• It’s not synonymous with “drain rock”, which is also called “three-quarter minus” or you might see it as ¾”-0

• Pedestrian: pea gravel is OK, but not ideal. Crushed rock is best.

• Vehicular: use crushed gravel only (never rounded rock like peagravel) because only angular rock locks together sufficiently to giveus the structural stability we need.

Clean, washed uniformly graded aggregate (cont.)

• Depth of rock needed for structural stability in wet, uncompactedclay:

• Pedestrian only

• Vehicular

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Clean, washed uniformly graded aggregate (cont.)

• Installation

• Wash it on-site if necessary

• Lay it in 6” lifts (aka increments)

• Compact it lightly by driving a truck over it, or by tamping (forpedestrian applications) but NEVER with vibratory equipment.

Typical Pavement SectionPedestrian only applications

Wood chip (mulch) pathGravel path

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Relative costs for Pervious Pavementsthat can be installed by anyone

$-$$ Homemadepavers

$$$ Grass-crete

$$$ Flexible Pavements(GrassPave )

$$$ Commercialpavers

$$ Boardwalks/decks

$ Wood chips(pedestrian only)

$ Gravel

$ Less expensive

$$ Moderate cost

$$$ Highest cost

Cost Comparison toConventional Impervious Pavements

Regardless of type, pervious pavement cost is offset by:

• Infrastructure: typically needed for impervious pavements: pipes,

detention ponds, water quality facilities, catch basins, manholes,

and excavation

• Value added amenity

• Lower stormwater fees (Clean River Rewards)

• Increased durability compared to impervious pavements

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Water Quality Comparisonbetween Impervious and Pervious Pavements

Impervious Pavements:

• are dead.

• turn rainfall to run off thatscours and concentratespollutants

Pervious Pavements:

• are alive. Being open to the air,microorganisms live in thepavement section and “eat”any pollutants that might endup in the pervious pavementareas..

• allow rainfall to infiltrate whereit falls to be treated by themicrobes in the soil.

Water Quality Comparisonbetween Impervious and Pervious Pavements

Impervious Pavements:

• increase runoff temperature asit passes over the pavement,which increases temperaturesof receiving streams

• discourage healthy treegrowth by cutting off the flowof air and water and throughaccidental compaction duringtheir construction

Pervious Pavements:

• Allows water to pass throughit, so there’s not muchopportunity for it to heat upand then they infiltrate waterso it has plenty of time to coolas it passes through theground before it seeps outagain to become stream flow

• encourage healthy treegrowth by allowing air andmoisture to get to the roots

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Water Quantity Comparisonbetween Impervious and Pervious Pavements

Impervious Pavements:

• increase runoff volumes from pre-developed conditions

• degrade stream habitat. Increasedrunoff volumes and lengthenedflows (caused by detention facilities)scour stream banks.

• increase the cycle of downstreamfloods & upstream droughts bypiping runoff as quickly as possiblefrom the upstream to thedownstream

Pervious Pavements:

have no increased runoff

protect stream habitat by preservingpre-developed runoff patterns sovolumes don’t increase downstream

allow rainfall to infiltrate where it fallsto replenish groundwater.

Thank You!

Sustainable design isn’t about doing something neat, it’s about doing something right.