Why is replacement of impervious surfaces with pervious surfaces important?
Impervious Surface: a surface that does not allow water to percolate through; eliminating rainwater infiltration and natural groundwater recharge.
Pervious Surface/Porous Surface: a surface that allows water to percolate through.
Understanding how to properly replace an impervious surface with pervious surface is important. Please use the checklist to ensure that you comply with the design guidelines and rebate terms for the replacement of impervious surfaces with pervious surfaces.
Determine which impervious surface you would like to replace on your property.
As there will be some soil excavation involved, it is important for the property owner to be aware of underground utilities before installation. "Call before you dig!" 811 hotline to locate underground utilities or visit website.
Understand the current runoff pattern of the hardscape you wish to replace. This drainage pattern should be maintained. Ensure runoff will flow away from built structures and foundations.
Once you have determined the site conditions of the impervious surface you will remove, choose a pervious surface type you would like to install; you may choose a pervious paving type or you may choose to replace it with landscaping.
1. If you choose to replace the impervious surface with a pervious paving type that supports vehicular loads, such as a driveway: • The property owner is required to hire a licensed contractor to design and install the pervious paving surface you choose. Considerations of subgrade, such as compaction, and base layer type and depth are important for proper design and installation. • A receipt must be submitted proving a licensed contractor was hired to design and install the paving. Refer to Rebate Form. 2. If you choose to replace the impervious surface with a pervious paving type that supports pedestrian loads (people), such as a patio or walkway and you plan on installing it yourself, you may follow the remaining design guidelines in this document. 3. If you choose to replace the impervious surface with landscaping there are no design guidelines specifically for landscaping, however refer to the Rain Gardens and Tree Planting LID Techniques for guidance, useful references, and for additional rebates that may be applicable to your site. • Replacing with lawn will not count as landscaping. • If the new landscaping is adjacent to a structure, ensure positive drainage away from the structure. A 5% slope is recommended.
If applicable, consider the type of edge that will be needed to hold the pervious pavers in place.
If applicable, consider the type of pervious jointing material you may want between pavers. Examples include grass, low groundcover species, sand, decorative gravel, or other aggregate material.
The photos below demonstrate a pervious unit paver. These are precast concrete pavers which have a void jointing space between the units. Sand or aggregate may be used as joint material. Edging will be needed to keep the unit pavers in place. To see a profile of this paving surface, refer to Start at the Source, Design Guidance Manual for Stormwater Quality Protection, Page 107 (Unit Pavers on Sand). Note: Filter fabric is not recommended.
The photos below demonstrate natural stone paving. Flagstone-like rock is placed on top of sand and an aggregate base. Precast concrete pieces or broken up concrete can be used as well. The joint spacing between the stones can vary to accommodate a low groundcover or decorative aggregate. To see a profile of this paving surface, refer to Start at the Source, Design Guidance Manual for Stormwater Quality Protection, Page 106 (Natural Stone). Note: Filter fabric is not recommended.
The photo to the left illustrates a reclaimed concrete paving surface. Concrete paving was broken up, then reinstalled with joint spacing to allow stormwater to percolate into the soil.
The photos below illustrate a precast concrete paving.
The photo below demonstrates pervious brick paving. Brick is placed on top of sand and an aggregate base. To see a profile of this paving surface, refer to Start at the Source, Design Guidance Manual for Stormwater Quality Protection, Page 105 (Brick). Note: Filter fabric is not recommended.
The photos below demonstrate turf block paving or sometimes referred to as grass pavers. This is an application of a pre-manufactured turf block or a grass paving system (various products available) under a layer of sand and aggregate base. A low groundcover or grass can be planted in the void spaces. Most turf blocks or grass paving systems will have specific installation instructions. Note that irrigation will be required to keep the plant material alive during the dry season. To see a profile of this paving surface, refer to Start at the Source, Design Guidance Manual for Stormwater Quality Protection, Page 104 (Turf Block).
The photos below demonstrate crushed aggregate paving. This is a simple application of crushed aggregate (various colors available) under a layer of base course. Rounded stone aggregate is not recommended for high traffic areas as it will not readily compact. To see a profile of this paving surface, refer to Start at the Source, Design Guidance Manual for Stormwater Quality Protection, Page 108 (Crushed Aggregate).
The photos below demonstrate Hollywood driveways or sometimes referred to as ribbon driveways. This type of driveway design consists of two parallel strips (each 2 foot wide) of impervious hardscape where the vehicle wheels drive and park, and pervious surface in the surrounding areas (3 feet wide in the center of the two parallel strips). A low groundcover, grass, aggregate, or other decorative stone can be placed in the void spaces in order for stormwater to infiltrate into the soil. Note that irrigation will be required to keep the plant material alive during the dry season. Edging may be required as well.
These do-it-yourself installation guidelines pertain to the installation of pervious hardscape surfaces with pedestrian loads only. If you choose to hire a licensed landscape contractor for design and installation, you may forgo the Installation guidelines.
The first step is removing the impervious surface you wish to replace. The challenge of this exercise depends on the area of impervious surface to be removed, whether it contains rebar reinforcement, and whether you may need a large truck, demolition saw, rotary hammer, or other specialized equipment. Be sure to understand where any underground utilities run before you start to demolish the hardscape. A good reference on how to break up concrete can be found at wikiHow: Break-up-Concrete. You might want to consider hiring a licensed contractor to remove the impervious surface based on the conditions of your situation.
If you choose to remove it yourself, it is recommended to take the material to be disposed to the Monterey Regional Waste Management District in Marina for proper recycling. More information can be found on their web site.
As a note, broken up concrete pieces can be reinstalled as pervious hardscape material. If this is your intention, salvage the pieces you need.
A properly installed hardscape base will ensure an even surface and will also direct stormwater flow appropriately. The soil conditions, type of base layers needed, depth of base materials, and its compaction rate will be determined by the type of pervious paving material you chose to install. Refer to the manufacturer's instructions and recommendations if they exist. Instructions will most likely be available for grass paver systems, pervious concrete, pervious asphalt, and permeable interlocking concrete pavers.
If you chose a permeable paver material such as broken up concrete pieces, flagstone or other stone paving and no instructions are associated with the product, it is recommended to install a drain rock of 4 to 8 inches with a minimal compaction of subgrade. See the example photos of pervious paving above and the permeable pavement guideline worksheets - from the Start at the Source: Design Guidance Manual for Stormwater Quality Protection (1999 Edition), Section 8.1, pages 100-109.
The spaces between the pavers, called joints, can be various widths and filled with various kinds of materials. Joints allow water runoff to infiltrate into the soil below. For instance, a flagstone paving with a creeping Thyme groundcover will have variable joint spaces being 1-3" in width. Alternatively, you may want a precast square paver with a consistent 2 inch joint spacing filled with a decorative crushed gravel. If you use a solid unit paver, set in sand or gravel with a minimum of 3/8 inch gap between the pavers. Joints should then be filled with aggregate free of fines.
See the example photos of pervious paving above and the permeable pavement guideline worksheets - from the Start at the Source: Design Guidance Manual for Stormwater Quality Protection (1999 Edition), Section 8.1, pages 100-109.
Pervious concrete or asphalt may require vacuuming or high pressure washing over time.
The use of leaf blowers on pervious pavement can force dirt and debris into pavement void spaces. Avoid blowing leaves, grass trimmings and other debris across permeable pavement.
Inspect paving after it rains for ponding or other visible problems.
Drill small holes (1/2" thick) in the areas of ponding to assist drainage.
Remove weeds from pavement and replace missing sand or gravel between pavers as needed.
Remove weeds from pavement joints and replace missing grass or low groundcover used between pavers as needed.
Depending on the site conditions and the type of grass or low groundcover used between pavers, supplemental irrigation may be needed during drought conditions.