Photo explanation: Left: interlocking concrete pavingstones Middle: Pervious concrete Right: Porous asphalt

Transcription

1 Photo explanation: Left: interlocking concrete pavingstones Middle: Pervious concrete Right: Porous asphalt 1

2 What is stormwater runoff? Before getting into the basics of permeable pavement, a short introduction of stormwater runoff is necessary. An impervious surface is one that does not allow water to pass through it, so the surface produces stormwater runoff during rain events. Hard surfaces such as roads, parking lots, and rooftops are examples of impervious surfaces that produce stormwater runoff. This runoff can collect dirt, chemicals, trash, and grass clippings on its way to the storm drain, so the water entering the system is often polluted. Many storm drains discharge this polluted runoff ffdirectly to local l waterways such as streams, rivers, and lakes. Permeable pavement is one solution to stormwater management. Figure explanation: Left A permeable surface such as grass allows water to be absorbed into the soil. Middle An impervious surface such as asphalt does not allow water through, so the water flows off the surface and forms stormwater runoff, which flows down into a stormdrain. Right A picture of a stormdrain with runoff flowing into it. 2

3 What is permeable pavement? Allows stormwater to drain from the surface - this process is often referred to as infiltration, which is defined as the slow passage of a liquid through a filtering medium. There are several types of permeable pavements: pervious concrete, porous asphalt, interlocking concrete pavingstones, grass grid pavers, among others. ***Note: Pervious, porous, and permeable are often used interchangeably by laypersons, although the terms are not technically the same. A pervious surface is open to passage, while a porous surface is full of pores. A permeable surface is capable of being passed through. The term pervious is used to describe permeable concrete, while porous refers to permeable asphalt. These designations are widely accepted in the industry and concrete and asphalt factions have each claimed their term with regard to these technologies. The general term for pavements that allow water to flow through them is most commonly described as permeable. *** A no-fines mix - pervious concrete and porous asphalt are similar to their traditional counterparts, except that the fine particles are removed during the mix production. This allows air to remain trapped in the mix when it s poured at the installation site. When the mix hardens, the air remains trapped leaving behind void spaces that allow for water movement through the material. Intended for use as an open-graded drainage material - allows for vehicle parking and driving. This dual function (driveability and stormwater management) makes permeable pavement an excellent investment, as it s a stormwater detention basin that you can drive on. Photo explanation: Top: Water being poured through a sample of pervious concrete. Bottom from left to right: interlocking concrete pavingstones, pervious concrete, porous asphalt, grass grid pavers. 3

4 Applications of permeable pavement Generally speaking, permeable pavement is used in areas that have low traffic volumes and low traffic speeds. The void spaces in permeable pavement make it less durable than its traditional counterparts at high speeds and under heavy loads such as tractor trailer trucks. However, the mixes can be changed to accommodate almost any loading need. There is currently a lot of research underway examining how these materials can eventually be used for roads and high-speed highways. Some ideal uses include: parking lots, alleyways, sidewalks, playgrounds, erosion control, and decorative purposes. Permeable pavement can be used as a soil stabilizer on steep hills or cliffs experiencing erosion. This application is especially popular for the protection of waterbodies that are near eroding hills or banks. Permeable pavement can be used for decorative purposes with interlocking concrete pavingstones or pervious concrete as both of those can be custom dyed to match the aesthetics of the surrounding area. Photo explanation: Top: A porous asphalt parking area (on the right) next to a traditional asphalt driving area (on the left). This picture shows the two areas during a rain event. The traditional asphalt on the left looks shiny and wet, while the porous asphalt on the right is dry. Middle right: A pervious concrete area in a park with sections that have been dyed different colors for an artsy look. Bottom: A pervious concrete sidewalk. 4

5 Permeable pavement properties Typical drainage rate for permeable pavement is inches of rain per hour! In New Jersey, the 100-year storm event is about 9 inches of rain per hour. A 100- year storm is a rain event that has a statistical chance of occurring once every 100 years. (Hurricane Irene s rainfall was the largest in more than a century.) At installation, some permeable pavements can have a very high infiltration rate of 1,000 inches per hour! In New Jersey, 90% of runoff-producing storms are 1.25 inches of rainfall or less. This means that the almost all of New Jersey s rain events produce rainfall of less than 1.25 inches. Permeable pavement can easily handle these small storms and can also provide stormwater management for much larger storms, as well. Photo explanation: A garden hose flowing onto porous asphalt. This picture shows how the hose water is draining down through the surface and not flowing further and further away from the hose. 5

6 Demonstration This video shows a concrete truck discharging water onto pervious concrete. The 2,000 gallons of water were drained by the pavement in less than 3 minutes. Note also that the demonstration was performed on a rainy day, so the pavement had already received some rainfall prior to the demo. This video shows not only how well the pavement can handle discharging water, but also how quickly the water drains and leaves a relatively dry surface (no puddles or standing water). 6

7 Permeable pavement properties Generally 15 and 35 percent air void content - this means that the percentage of air in the material is between 15 and 35%. This generally applies to the poured permeable pavements: pervious concrete and porous asphalt. Poured pervious concrete and porous asphalt placements show an average of 20 25% void content after installation. The paver-based permeable pavements will have void contents based on the gravel used to fill in the spaces or based on how open the grids are. These statistics are manufacturer-specific. Material strength of pounds per square inch (psi) for the mix-based pavements (pervious concrete and porous asphalt. - If the pavement application requires a higher strength for heavier loads or heavy-duty y use, more fine particles can be added to the mix to increase the material s strength. The trade-off for higher strength is a reduced infiltration rate. The final compressive strength of a dry, placed permeable pavement is not used as an acceptance criteria in the field, but the mix is designed with a certain material strength in mind. Illustration explanation: A bodybuilder symbolizing the strength of these materials. 7

8 Typical permeable pavement cross-sections Permeable pavement: 3 6 inches - The first layer of a permeable pavement is the permeable surface, which is typically 3 6 inches thick. There are some applications where a thicker or thinner surface may be necessary. Open graded stone subbase storage layer - The next layer is the open-graded stone subbase storage layer. This is a layer made of loose gravel, stone, or concrete. These loose stones are usually relatively large 1.5 to 2 inches. This layer of stone is the storage layer that provides the infiltrated water with a place to go after it has moved below the surface. (The water is detained in this section, which is why it is sometimes referred to as the detention layer.) The depth of the storage layer is dependent on the site characteristics (drainage area, contributing impervious cover, etc.), but is often inches. Choking layer or filter fabric - Below the detention layer is a filter or choking layer. This layer can be made of a filter fabric or a section of smaller gravel. The purpose of this layer is to provide some stability while also preventing the migration of the underlying soil up into the storage layer. A choking layer made of stone is usually 1 2 inches thick and is made of stones that are about 0.5 inches each. A choking layer made of fabric is often referred e ed to as a filter fabric and is usually made of a material that is considered a geotextile. A geotextile is an engineered fabric material that is used in soil applications (silt fences, erosion control, etc). The geotextile fabric is permeable and is engineered to meet specific permeability requirements. Uncompacted soil or underdrain - Underneath the choking layer or filter fabric is the underlying soil. Utmost care should be taken during pavement installation to ensure that the underlying soil does not become compacted. Large machinery should work only from the outside edge of the installation site and should not drive across the system during placement. This may add time and expense to the installation, but if the soil is heavily compacted and can no longer infiltrate, then the permeable pavement will fail. Permeable pavement can be placed over many different types of soils, but it is ideal for the soil to infiltrate at least 1-2 inches per hour prior to installation. If the site has extremely clay or compacted soils that don t infiltrate well, permeable pavement may still be installed with the addition of an underdrain that can be used to transport the runoff that has moved through the system away from the soil (either to a storage container for water collection and re-use or to the existing storm sewer system). Figure explanation: Left: This is a cross-section of porous asphalt. The surface of porous asphalt is usually 3 inches thick for a parking lot application, with a storage layer, choking layer, and subgrade soil below. Middle: This is a cross-section for pervious concrete. The surface of pervious concrete is 6 inches thick for a parking lot application, with a storage layer, choking layer, and subgrade soil below. Right: This is a cross-section of interlocking concrete pavers. The thickness of pavers will vary based on the manufacturer, but for parking lot applications the pavers are usually about 4 inches thick for durability. The pavers require additional layers below them to provide stability, so there is a 2-inch layer of stones that are 0.5 inches each, followed by a 4-inch layer of stones that are 1 inch each. After the stability layers are the storage layer, the choking layer, and then the subgrade soil. 8

9 Benefits of permeable pavement: stormwater management Permeable pavement can be a key tool in efficient stormwater management. Reduction of stormwater volume - Stormwater runoff is captured or absorbed by the permeable pavement, thereby reducing the flow of rainwater to the storm sewer and reducing storm sewer overflows. Reduction of impervious cover - Permeable pavement replaces impervious cover with a permeable surface, which leads to reduced stormwater runoff when it rains. Fewer impervious surfaces in the environment results in less stormwater generated during a rain event, which means less polluted runoff entering the local waterways. Decrease and delay of peak flow - peak flow is the highest rate of flow in a stream. The capture and absorption of stormwater volume by permeable pavement leads to decreased runoff to streams and spreads the storm s s flow over a longer period of time, which reduces the burden on the sewer system and lessens the impact on local waterways. Possibility of groundwater recharge - Permeable pavement can absorb runoff and allow it to penetrate into the soil, which replenishes the groundwater supply. This groundwater provides base flow to local rivers and streams. Photo explanation: This still from the video shows a concrete truck discharging water onto pervious concrete. The 2,000 gallons of water were drained by the pavement in less than 3 minutes. Note also that the demonstration was performed on a rainy day, so the pavement had already received some rainfall prior to the demo. 9

10 Benefits of permeable pavement: contaminant reduction System acts as a filter for particles - As stormwater moves through the different permeable pavement layers, solid particles are trapped and removed from the water. This leads to improved water quality so relatively clean water is entering the soil and possibly recharging groundwater supplies. Recent studies have shown permeable pavements can remove between 60 and 100% of the solids from the stormwater entering the system (Pagotto et al., 2000; Bean, 2005; Jayasuriya et al., 2007) Microbial breakdown of organic compounds - All of the air pockets and pores in the permeable pavement system allow for microbe (microscopic organisms) colonies to populate the surfaces of each hl layer (permeable surface, storage layer, choking layer). These microbes can breakdown or eat organic compounds that may be in the stormwater entering the system (oils, grease, etc.) and remove or reduce the concentrations of those contaminants. Thermal pollution Stormwater runoff often has an elevated temperature after it flows across hot asphalt. Warm stormwater runoff can increase the temperature of rivers and streams, which can lead to changes in the biology of those streams. This warm runoff entering local waterways is called thermal pollution. Some aquatic plants and animals are very temperature-sensitive and may not survive in streams with elevated temperatures. Permeable pavement can lower the temperature of stormwater as it flows through the system layers. Waiting for more studies for the verdict on: metals, nutrients, and pathogens - Currently, more studies need to be performed in order to determine permeable pavement s removal efficiency for metals, nutrients, and pathogens (disease-causing germs). Some metals, nutrients, and pathogens are bound to particles in stormwater, so permeable pavement can remove those contaminants through the filtration mechanism discussed above. However, pollutants in the dissolved-phase (incorporated into the water as a solution) are not easily removed by filtration. Researchers are currently studying if permeable pavement can remove these contaminants through other mechanisms and what removal efficiencies are seen (20, 30, 40% removal?). Photo explanation: Bottom: Water droplets symbolizing clean water. 10

11 Benefits of permeable pavement: cooler, healthier cities Traditional asphalt absorbs heat throughout the day and releases it at night, keeping city air at elevated temperatures with no opportunity for cooling. This phenomenon is called the urban heat island effect because highly-developed, urban areas have higher air temperatures than the rural, more natural areas surrounding the city. Graph explanation: Sketch of an urban heat-island profile - In the city, with lots of pavement and hard surfaces, the air temperature in the late afternoon is nearly 7 degrees higher than that of a rural, forested area. Moving from the rural area to suburban to commercial to downtown, the temperature increases as the amount of impervious cover increases. There are dips in areas of urban parkland that have trees to provide shade. Reflection of heat and light (concrete and pavers) - Some light-colored permeable surfaces, such as pervious concrete and interlocking concrete pavers, can reflect warm heat and light which leads to cooler air temperatures above them compared to the air above heat-absorbing black asphalt. Air movement through void spaces - unlike traditional asphalt, permeable pavement allows for air movement through the pavement, which leads to reduced temperatures because heat is not trapped in the surface. Slow the temperature-dependent formation of smog - Air quality is improved by permeable pavement because lower air temperatures can lead to reduced smog, which requires a high temperature for the formation reaction to complete. Reduces heat release at night - Permeable pavement allows for reduced heat release from surfaces at night because less heat is trapped during the day. With no heat release, the city can cool down to a lower temperature before the heat of the next day. Figure explanation: This drawing of the sun and a thermometer reminds us that permeable pavement can lead to cooler cities. 11

12 Benefits of permeable pavement economic incentives Elimination of traditional stormwater management techniques with a permeable pavement, traditional stormwater-handling techniques are unnecessary. There is no need for stormwater inlets, conveyance pipes, land for detention basins, or detention basins, themselves. The elimination of these traditional stormwater techniques leads to big cost-savings. Table explanation: This table compares the expenditures for permeable pavement and traditional pavement. Both pavement types have costs associated with the surface material, the aggregate underneath it, and the installation of the pavement. Permeable pavement does not require inlets, conveyance pp pipes, acreage for detention basins, or the basins, themselves. Traditional pavement needs all of those thing to handle the stormwater generated by that impervious surface. Disconnection for the sewer system a permeable pavement system does not need to be connected to the municipal sewer system, which can save money on sewer taxes, stormwater utility fees, and maintenance of the sewer connection (inlet or stormdrain clearing, etc.) Increased utilization of property since permeable pavement can be used to combine stormwater management with parking areas, the property that would have been used for a detention pond can be used for more commercial storefront, a larger parking lot, or open green space. Valuable property is not wasted on a detention basin with little value. This may increase income while reducing costs by having more storefront available for vendors, being able to accommodate more customers, or providing green space that attracts people to the area. Lower life cycle costs permeable pavement can cost less in the long-term than traditional pavement due to increased durability and better freeze/thaw performance in the winter. The occurrence of potholes is greatly reduced because water does not become trapped the permeable pavement surface the way it does with traditional asphalt. This means less patching, increased safety, and longer-lasting pavement. Figure explanation: This pile of money symbolizes the economic incentives of installing permeable pavement. 12

13 What about costs? Everyone loves permeable pavement, but many people are worried about the costs. Yes, permeable pavement is more expensive upfront compared to traditional pavement. Table explanation: This table shows actual quotes for a permeable pavement project installed in The pavement type and quoted cost in dollars per square yard are shown. These costs are for the materials and installation, but do not include the site preparation. Three bids were compared for each and the lowest and highest are presented. Traditional asphalt is the least expensive, followed closely by porous asphalt. Pervious concrete is next, with the interlocking concrete pavers being the most expensive. When oil prices are high, the price difference between porous asphalt and pervious concrete is reduced. The interlocking pavers are expensive because of their manufacturing costs, the extra stability layers they need, and the sometimes manual installation. (Some companies install the pavers by hand with a crew of 5 or 6, while other companies have a special bricklaying machine.) 13

14 Comparing apples to oranges As an example, let s examine a 100,000 square foot parking lot and see how a pervious concrete lot compares to a traditional asphalt lot. On the left, the cost of 6 inches of concrete and a 12-inch deep storage layer is about $355,000 installed. On the right, 4 inches of traditional asphalt and 8 inches of a gravel support base would only cost about $285,000 installed. However, this analysis does not include any costs for stormwater management (next slide) Figure explanation: comparing apples to oranges 14

15 Comparing apples to apples With the stormwater management costs included, now we re comparing apples to apples. On the left, the pervious concrete system is unchanged with a cost of $355,000. On the right, the traditional asphalt estimate now includes the cost of handling stormwater runoff generated by the parking lot. The stormwater inlets, 18 conveyance pipes, the construction of a detention pond, and the acre of land that the detention pond requires are now included in the calculation, and the total is $530,000. Now, the traditional asphalt is much more expensive than the pervious concrete. Figure explanation: Comparing apples to apples ***Note this comparison is most appropriate for new construction sites with no infrastructure in place. For retrofits or redevelopment, traditional infrastructure will generally already exist.*** 15

16 What about drainage issues in clay soils? Just because a site has clay soils does not exclude it from being considered for permeable pavement. Here are a few options for sites with clay soils: Soil amendments The underlying soil can be amended with sand to improve the drainage rate. This option is ideal for soils that are very close to the acceptable drainage range of 1 2 inches/hour. Deeper storage layer If the soils need more time to drain, then increasing the depth of the storage layer (18 inches of more) is one way to do that. The larger storage layer allows the runoff a place to go after moving past the permeable surface without backing up. There are some permeable pavements with 4 feet of storage space below the surface! The soil will drain at its own pace and the system will be ready for the next storm. Underdrain option If the underlying soil has a very low drainage rate, a pipe underdrain can be installed at the soil interface to carry drainage to a different location at the site or to connect to the traditional storm system. This option can also be used for runoff collection and reuse. The drain can be connected to a storage tank or cistern and the runoff can be used for site irrigation. Figure explanation: A tractor tilling the soil for better drainage 16

17 What about winter issues? Everyone is always nervous about the winter performance of these systems. In fact, these surfaces are safer and require less handling than traditional asphalt. Water drains through to the storage layer Runoff/snow water drain through permeable pavement so quickly that there is no opportunity for freezing. The water is never trapped at the surface, so there is no heaving or cracking. Freeze thaw would only be an issue in a completely saturated system (if the system had 18 inches of water in it). If you don t believe me, the University of New Hampshire has been studying all types of permeable pavement in a very cold environment and they have reported that t permeable pavement infiltration ti rates are even higher h in the winter! They have not seen any freeze thaw issues. No black ice formation On a traditional asphalt surface, snow starts to melt during the day and cooling temperatures at night lead to the formation of black ice. With permeable pavement, snowmelt is absorbed by the surface right away, so there is no water left at the surface when the cool nighttime temperatures arrive. Less salt use Because there is no black ice formation, there is no reason to spread more salt at night. Permeable pavement also requires less salt because snow melts faster with permeable pavement due to air movement through the surface. No potholes With traditional asphalt, water can get into small cracks and freeze, which causes heaving and breaking that leads to pothole formation. Permeable pavement allows water to quickly move through the material so there is no freezing to damage the surface. Photo explanation: This picture shows snow piles in a permeable pavement parking lot. There are no puddles forming at the base of the snow piles as there would be on a traditional asphalt lot. Figure explanation: This snow symbolizes the questions people have about winter performance of permeable pavement. 17

18 What about maintenance? The next thing people are worried about is the maintenance of these systems. Regenerative air vacuuming Make sure that the maintenance/facilities guys use a regenerative air vacuum and not a street sweeper. The particles and solids that get in the permeable surface need to be sucked out and removed, not just moved around the way a street sweeper would. This maintenance should be done at least once a year in the spring after fall and winter salting. If the drainage rate of the pavement seems slower, vacuuming should be performed. Homeowners or smaller plots For driveways or walkways, a pressure washer can dislodge particles. The jet must be angled very low to push the particles up and out rather than straight down deeper into the material. Winter considerations A regular snow plow with the blade all the way to the surface is the best way to handle snow. There is no need to be delicate with this material! If the facility guys are nervous and lift the blade up a little, then they will leave behind a half-inch thick permafrost that will form ice and be hard to deal with. Make sure that salt is used as a deicer rather than sand. Sand will clog the pores of the surface and greatly reduce the drainage rate of the material. Liquid salt is most effective, but that can be costly compared to rock salt. Photo explanation: This is a picture of a regenerative air vacuum truck. It is different from a regular street sweeper truck because it vacuums up the particles rather than just moving them all around. 18

19 Questions? Please acknowledge that Amy Rowe created this presentation with some input from Ken Justice and Phil Kresge of the National Ready Mixed Concrete Association. Photo explanation: This is a picture of porous asphalt parking rows (on the right) next to a traditional asphalt driving lane (on the left) on a rainy day. The traditional asphalt looks wet and shiny, while the porous asphalt is relatively dry with no water at the surface. 19