Tag Archives: stormwater

Introducing Siphonic Roof Drainage: Common in Europe, now gaining traction stateside

All images courtesy Zurn Industries

All images courtesy Zurn Industries

by William Verdecchia

All roofs are subject to the destructive effects of seasonal weather changes, environmental conditions, loading, and air pollutants. Alternate cycles of wetting, drying, freezing, and thawing caused by water lying on the roof leads to expansion, contraction, and rotting—this risks damage to the roof and even the building substructure.

Drainage is a significant component of roof design itself. Roof collapses typically occur because water accumulation exceeds the roof’s structural capacity. With proper water drainage in place, many major causes of failure are eliminated.

For this reason, siphonic roof drainage is coming into its own in the United States. First developed in Finland by engineer Ovali Ebeling in 1968, these systems are used around the world—in Europe, they account for one-fifth of commercial projects. This sustainable technology crossed the Atlantic in 1999 with the Boston Convention Center’s installation as the first major example, and acceptance has steadily grown.

Precipitation rate maps help designers create the best drainage systems for an area given expected rainfall.

Precipitation rate maps help designers create the best drainage systems for an area given expected rainfall.

Siphonic roof drainage differs from conventional gravity drainage in what is called ‘full-bore flow.’ Unlike conventional drainage, a fully engineered siphonic roof drain system prevents air from entering, allowing the pipes to be completely full of water. The unique component of a siphonic drain that sets it apart from conventional gravity drains is the air baffle, which prevents air from entering the piping system at full flow and protects against debris.

In October 2013, a new standard developed by the American Society of Plumbing Engineers (ASPE) was approved by the American Standards Institute (ANSI) as ASPE/ANSI 45-2013, Siphonic Roof Drainage. (The testing standard remains American Society of Mechanical Engineers [ASME] A112.6.9-2005.)

Considerations for roof drainage design
The most basic functions of a roof drainage system are

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to carry off rainfall, directing it to an underground piping system or drainage ditch, thereby removing the possibility of water penetration into the membrane or building envelope. This rainwater management carries another market expectation: a sustainable approach to issues related to water conservation, stormwater runoff, and rainwater-harvesting.

When designing a robust roof system, the following factors come into play:

  • building location;
  • roof assembly/type of construction;
  • roof pitch/slope;
  • volume of expected rainfall (i.e. precipitation rate measured in inches/hour);
  • desired rate of drainage; and
  • roof load requirements.

Additional considerations for architects, design engineers, and specifiers are:

  • drain size and features;
  • drain placement and location;
  • overflow safety requirements;
  • building and plumbing code requirements;
  • vandal-proofing; and
  • aesthetics.

Each project location has its own historical rainfall data that include records of accumulation, intensity (i.e. duration and frequency), drop size, and terminal velocity. Rainfall intensity plays a significant role in determining the type, quantity, size, and placement of roof drains to be installed for optimal system design. (As every project is different, consultation with the roof drainage manufacturer is essential.)

Siphonic roof drainage systems provide a number of benefits to a building owner, including lower construction costs, self-cleaning capability, water conservation, lower energy consumption, and reduced natural resources depletion. This 381-mm (15-in.) diameter main roof drain has a clamping collar and low-silhouette poly-dome.

Siphonic roof drainage systems provide a number of benefits to a building owner, including lower construction costs, self-cleaning capability, water conservation, lower energy consumption, and reduced natural resources depletion. This 381-mm (15-in.) diameter main roof drain has a clamping collar and low-silhouette poly-dome.

A 363-mm ( 14 9/32-in.) diameter siphonic overflow roof drain with standard deck. The key to the operation of a siphonic system is eliminating all air from entering the piping system. This is achieved by placing an engineered secured baffle into the base of the sump that breaks up the Coriolis Effect of rotating water.

A 363-mm ( 14 9/32-in.) diameter siphonic overflow roof drain with standard deck. The key to the operation of a siphonic system is eliminating all air from entering the piping system. This is achieved by placing an engineered secured baffle into the base of the sump that breaks up the Coriolis Effect of rotating water.

Figure 1 indicates the precipitation rates of numerous locations expressed in inches per hour and based on 15 minutes of precipitation (extrapolated from historical rainfall data collected over a period of 10 to 100 years). Reading this map, a design engineer would size a building drainage system in the Carolinas to handle a 178-mm (7-in.) hourly rainfall. Roof drain systems for most of New York State and Michigan would be sized to accommodate 102-mm (4-in.) hourly rainfall.

Traditional roof drains
Traditional roof drainage systems rely on gravity and water’s ability to spread out and flow to the lowest point. As the water accumulates, the depth increases and becomes the driving force causing it to flow through gutters to the roof outlets. Each outlet has its own down-pipe directing water underground. Unfortunately, as water enters the down-pipe, air is also drawn in, reducing the drainage system’s efficiency.

Figure 2 is a table to help size gravity roof drains by following these steps:

  1. Calculate total roof area.
  2. Determine and select the size of leader (i.e. roof drain, down-pipe, conductor, or downspout) to be used.
  3. Use precipitation map to find rainfall rate for building’s location.
  4. Cross-reference leader size with hourly rainfall in chart to obtain roof area that can be handled by each leader. For example, using a 102-mm (4-in.) leader for a location with a 102-mm hourly rainfall, each drain can handle 427 m2 (4600 sf) of roof area.
  5. Divide total roof area by area found in Step 4 to obtain the number of drains required. For example, 13,936 m2 (150,000 sf) divided by 427 m2 equals 32.6—this means 33 drains, equally spaced and symmetrically located.

Other non-siphonic roof drain systems
Regions affected by tropical storms or other severe weather phenomena make it necessary to transport large amounts of runoff water as fast and efficiently as possible. Designed for volume efficiency, high-capacity roof drains are more than 30 percent larger than standard ones. Careful consideration must be given to roof drain location and outlet pipe diameter.

Controlled flow roof drains are ideal for dead-level or sloped roofs, and for areas with restricted stormwater drainage capacity. With these drains, excess water accumulates on the roof under controlled conditions. The water is drained off at a metered rate after a storm abates. The key is to use a large roof area to temporarily store the maximum amount of water.

A controlled flow roof drain system requires fewer drains, smaller diameter piping, smaller sewer sizes, and lower installation costs. Another benefit is these systems reduce the probability of storm damage by lightening the load on combination sewers and reducing the probability of flooded sewers and backflow into basements and other low areas. The stored water on the roof also can act to temporarily improve the heat loss characteristics of the roof. To ensure success, designers and specifiers for controlled flow roof drainage design must carefully consider drain location, roof deflection, scupper sizes, overflow drains, and roof loading.

Siphonic roof drainage
Developed to operate with 100 percent full flow for increased discharge, smaller pipe diameters, and no drainage slope, siphonic roof drainage’s first commercial installation was at a Swiss turbine factory in 1972.

The theory behind siphonic roof drainage systems traces back to one of the fundamental equations of fluid mechanics—Bernoulli’s Energy Equation, named for the 18th-century Swiss mathematician and physicist Daniel Bernoulli. The energy balance equation holds when a fluid, at rest or in motion, possesses three fundamental forms of energy—static pressure, kinetic, and potential—the sum of their states is conserved and remains constant, even though the system energy states may be transferred from one to another.

The equation assumes the fluid is incompressible, that no work is done or performed on the system, and the system is adiabatic (i.e. no heat is gained or lost). It is used to determine change in flow between any two points in a drainage system. Numerous modifications of the equation have been made for specific applications. Siphonic theory itself has also undergone revisions to take into account losses due to friction in a length of pipe.

In their current design, siphonic roof drains look similar to conventional gravity roof drains; they share features such as a drain body, dome strainer, and membrane clamping device. The component unique to siphonic systems is the highly engineered air baffle.

This table assists designers with how to best size a gravity roof drain.

This table assists designers with how to best size a gravity roof drain.

The air baffle is secured into the sump of a standard drain, preventing vortex flow. In other words, it prevents the Coriolos Effect, which typically forces water to rotate around the drain and draw air down the center and into the pipe. By prohibiting air from entering the tailpipe and horizontal collecting piping, a negative head pressure is created in the collector pipe and the water is siphoned off the roof. The atmospheric pressure above the drain becomes the system’s driving force.

Location of the baffle in the drain sump is critical to the drainage system design. Locating the baffle lower in the drain body minimizes the amount of water on the roof that is required to make the drain go siphonic.

Once the rainwater is drawn through the drain and into the tailpiece, it then travels to the horizontal piping, located just below the roof. In this section of the system, the water continues to depressurize, and piping size increases to prevent cavitation or the pipe walls from imploding under the negative pressure. When the rainwater reaches the vertical stack, it stays at full bore flow but continues to pressurize as it moves downward to the zero point (i.e. siphonic break). The pipe turns down into a vertical downspout and transfers to conventional gravity drainage when below grade by expanding the pipe diameter.

The driving hydraulic head of the system is the entire height from the top of the roof to the discharge point, as opposed to a conventional system where only the roof water acts as a head pressure. Due to this, the siphonic system allows for higher flow capacities and velocities than a conventional system with the same sized piping. The higher velocities also mean a siphonic system can be considered ‘self-cleaning,’ eliminating the need for cleanouts in the piping.

Ideal applications
Siphonic roof drains provide full-bore flow when used in conjunction with a fully engineered/designed piping system. The full-bore action is achieved through natural hydraulic action. The system is designed to use the full volume of the piping—the water goes siphonic when the pipes are completely full.

Siphonic systems require fewer downpipes and smaller pipe sizes and need less space. This means greater design flexibility, reduced installation times, less material resources, and cost savings. They can be used on all buildings regardless of size, height, or exposure to rainfall. However, they are most efficient on low-rise buildings with large footprints, along with shopping malls and factories.

This is because in a siphonic system, the water moves through the horizontal piping at negative pressure, but increases in pressure as it drops vertically through the leader until the point it reaches zero pressure and transitions to gravity flow by increasing the pipe size. Since this transition typically happens after dropping only a few floors, most of the drop will be oversized gravity piping on a high-rise building—this excludes the traditional benefit of reduced pipe size for the entire building height. Further, the larger the footprint of the building, the more negative pressure can build up in the piping, which allows it to drop down the side of the building further before transitioning. Most high rises have smaller footprints and small roof areas—this does not allow a lot of negative pressure to build in the horizontal piping.

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A siphonic roof drain system was installed on a new Volkswagen plant in Chattanooga, Tennessee. Siphonic design software provided sizing calculations so the installed system would work as engineered. The project scope was more than 46,451 m2 (500,000 sf) and included 85 siphonic roof drains.

Specifying siphonic roof drainage
In the United States, the standard siphonic drain is 380 mm (15 in.) in diameter. Outlet sizes include 50-, 76-, and 102-mm (2-, 3-, and 4-in.) no-hub mechanical joint connections. Drains and clamp collars can be made from iron or stainless steel, while domes are made from polypropylene, aluminum, bronze, or iron. Specifiers should consider using stainless steel vandal-proof hardware to help reduce corrosion in the assembly.

A siphonic system is available with many of the basic, conventional roof drain options. Mounting devices such as deckplates are recommended to help speed installation. A drain riser and adjustable extensions assist in leveling the roof drain during construction. For gravel rooftop applications, a gravel guard can help ensure proper drainage. If desired, an overflow drain can be used with standard siphonic systems to connect to a separate drain line, discharging to an outside location instead of a sewer.

Determining the number of siphonic roof drains needed is similar to the same process for conventional roof drains (Figure 3). First, one decides what outlet size is needed (left column) and what rainfall rate is necessary (rows across). Then, one finds the number in the box where the column and row values meet. The number is the square footage each siphonic drain can cover under those conditions.

When comparing the siphonic to the conventional roof drain sizing chart, it becomes apparent the siphonic pipe size will be roughly half of a conventional drain covering the same area. (The chart should be used for a preliminary estimate, and should not take the place of consulting a manufacturer’s engineering or technical staff for complete piping layout.)

Another important step is to check the local plumbing code to see whether a rainfall rate is dictated and if siphonic drainage systems are addressed. In many cases, plans will need to be submitted as an engineered system or variance. A licensed professional engineer must verify the system will work as designed.

Siphonic design guidelines
Siphonic design software is now used to design and calculate if minor changes during installation fall within the design’s acceptable range of pressures and velocities in the piping. The aforementioned ASPE/ANSI 45-2013 should be reviewed to determine how installation differs from a conventional drain system. One main difference is the use of eccentric reducers whenever there is a change in pipe diameter. These help maintain a flat surface along the top of the pipe, eliminating any air pockets.

No more than 4645 m2 (50,000 sf) of roof area should be tied into one common collector pipe (otherwise, the system can be very hard to balance due to the large distance between drains and large pipes used). Further, roof areas made from different types of roof materials should not be tied into a common collector pipe. These roof materials have different coefficients of discharge, and would not be able to draw a siphon at the same time if tied into the same pipe. (Different fittings can also have dissimilar coefficient of friction ratings—software should be used to select the most appropriate type of transition.) Roof areas with extreme variation in slope, or roof areas at different roof levels, must also drain into separate collector pipes.

Determining the number of siphonic roof drains needed is similar to the same process for conventional roof drains.

Determining the number of siphonic roof drains needed is similar to the same process for conventional roof drains.

One must avoid not only multiple vertical drops in a single piping system, but also piping below an obstruction. Both these issues would delay the system from priming, slowing it from going siphonic.

Every roof trough must have at least one siphonic drain present in that area. When initially designing a system, drains tied to a common collector pipe should be spaced no more than 20 m (65 ft) apart, and stacks should be located no more than 20 times the building height away from the furthest drain. However, these dimensions are only guidelines—they can be increased if the system can be properly balanced. Pipe lengths longer than 20 m should be divided into smaller sections to accurately determine where the pipe diameter can increase from one size to the next.

At least 1 m (3 ft) must be maintained between the surface of the drain and the center of the horizontal collector pipe. Tailpieces attached to the bottom of the drain must be at least 0.5 m (21 in.) long before tying into the horizontal pipe.

For siphonic systems, each individual and total system rating must fall between negative 10.13 kPa (1.47 psi) and positive 10.13 kPa. It is important for overall imbalance of the system be as close to zero as possible. Cast iron or polyvinyl chloride (PVC) piping can withstand a minimum pressure of negative 90 kPa (13 psi). If pressure is too low, pipe diameter must be increased to relieve pressure.

The flow rate of the ‘zero pipe’—the first section of pipe after the siphon has been broken and the transition to gravity drainage has been made—must be less than 2.5 m (8 1/4 ft) per second to prevent damage to the storm sewer. In many cases, flow rate is reduced to around 1 m (3 ft) per second to align with traditional pipe sizes based on the roof area and the rainfall rate. (This flow rate is the minimum for ensuring the system remains self-cleaning.)

Conclusion
This system of roof drainage is beginning to gain traction in the United States. Statistics are difficult to come by, but the wide acceptance of siphonic roof drainage in Europe and other parts of the globe provide reassurance and a backdrop of success. The key for any project, including those involving siphonic roof drainage, is to ensure one deals with reliable manufacturers with track records for delivering engineered solutions. Since every project is different, consultants must be willing and able to give the time and expertise required.

William Verdecchia is the director of product management and engineering for Zurn Specification Drainage. He has more than 25 years of experience in the construction industry; with five years as a construction professional and 20 years leading innovation, product commercialization, and sales/marketing activities for Zurn Industries. In his current role, Verdecchia oversees product lifecycle management activities and application support for plumbing products. He has been awarded numerous plumbing industry patents and is an active board member for the Plumbing and Drainage Institute, Penn State Behrend’s Plastic Engineering Program, and Gannon University’s Mechanical Engineering Program. He can be reached at william.verdecchia@zurn.com.

Re-examining Paver Performance

In the November 2013 issue of The Construction Specifier, we published the article, “Controlling Stormwater at the Source,” by Katie McKain, ASLA, MLA, MUD.  David R. Smith, CSI, of the Interlocking Concrete Pavement Institute (ICPI), wrote in about what he felt were some inaccuracies; we then shared his comments with the author.

I read with interest Ms. McKain’s article. She made some inaccurate statements about permeable interlocking concrete pavement (PICP) that require correction. This includes misstating PICP longevity at seven to 15 years. Continue reading

Controlling Stormwater at the Source: Exploring best management practices

Photo courtesy Katie McKain

Photo courtesy Katie McKain

by Katie McKain, ASLA, MLA, MUD

Conventional stormwater systems treat precipitation as a waste product, directing it into storm drains and pipes and pouring it into receiving waters. These traditional development systems also cause undesirable effects to the landscape, such as reducing the water table and its overall quality, as well as leading to erosion, sedimentation, and flooding issues.1

As the impervious surfaces characterizing urban sprawl—roads, parking lots, driveways, and roofs—replace meadows and forests, rain can no longer seep into the ground to replenish aquifers. This reduces the groundwater recharge serving as a natural hydrologic process where surface water infiltrates downward into groundwater to maintain the water table level.

The infiltration process naturally filters runoff through vegetation and soils. Not only do conventional systems prevent groundwater recharge, but they also cause significant stress to waterways and affect water quality. When the natural process does not happen, runoff spreads over impervious surfaces and gathers pollutants that wash into lakes, rivers, and streams, contaminating them.

There is also a negative financial connotation as building impervious surfaces and concrete curb and gutter systems are expensive. Curbs and gutters, and the associated underground storm sewers, frequently cost as much as $36 per 0.3 m (1 ft), which is roughly twice the cost of a grass swale. When curbs and gutters can be eliminated, the cost savings and positive effects on the environment can be considerable.

Destination of water after rainfall.  Data courtesy EPA, Water Quality Facts, www.epa.gov/owow/waterqualityfacts.html.

Destination of water after rainfall. Data courtesy EPA, Water Quality Facts, www.epa.gov/owow/waterqualityfacts.html.

What is low-impact development?
As shown in Figure 1, the ultimate destination of water after rainfall is divided into three categories:

  • absorbed into the ground;
  • evapotranspiration; and
  • runoff.

There is a dramatic difference between water movement on natural areas versus urban impervious environments.

The negative effects associated with unnaturally high runoff volumes from conventional practices have initiated the emergence of low-impact development (LID). The Low Impact Development Center is a non-profit organization in Beltsville, Maryland, dedicated to the promotion of this strategy, which it defines as:

a new, comprehensive land planning and engineering design approach with a goal of maintaining and enhancing the pre-development hydrologic regime of urban and developing watersheds.2

A detention pond where the maintenance access road uses void structured concrete. Photos courtesy Katie McKain

A detention pond where the maintenance access road uses void structured concrete. Photos courtesy Katie McKain

LID promotes integration of stormwater management into site and building designs, controlling stormwater at the source before it collects and deposits harmful pollutants. Another crucial component involves minimizing impervious areas and ensuring buffer zones between them. This allows infiltration and daylighting of runoff to the surface, controlling stormwater at the source. (In this context, ‘daylighting’ is used to describe an underground pipe conveying water that ends at the surface, so water rushes out of the pipe onto gravel or grass and the pipe is no longer underground.)

Advantages to using low-impact development
Some of the numerous benefits to designing with the LID model are discussed below.

Improved water quality
Many best management practices (BMPs) involve bioretention—a process using the chemical, biological, and physical properties of plants, microbes, and soils to improve water quality. Hyperaccumulators are unique plants with natural abilities to degrade, bioaccumulate, or render harmless contaminants in soil, water, and air.

There are many species of hyperaccumulators, including:

  • barley (i.e. hordeum vulgare);
  • water lettuce (i.e. pistia stratiotes); and
  • Indian mustard (i.e. brassica juncea).

These common types respectively counter aluminum, mercury, and lead. Bioretention techniques include adsorption, absorption, volatilization, decomposition, phytoremediation, and bioremediation.

Increased groundwater recharge
General water infiltration is important for groundwater recharge (i.e. replenishing the water table). Unsatisfactory groundwater recharge is becoming a serious concern as cities continue to develop land with impervious surfaces (Figure 2).

As the statistics are directly proportional, it is not surprising Atlanta earned the ‘top’ ranking for both loss of potential groundwater recharge and acres of new development. These extremely high numbers should also take the population increase into account, but Seattle managed much lower numbers across the board despite having a relatively high population increase. (While this is perhaps due to the West Coast city’s advances in stormwater management, it should also be noted Atlanta sees an average of 304 mm [12 in.] more annual rain than Seattle.)

When water is sent to a treatment facility instead of infiltrating to the groundwater, it is often taken far from its place of origin, depleting waterways of their natural processes. The sewer system not only diminishes groundwater supplies, but also causes significant stress to the waterways and affecting water quality.

When contaminated water runs off into rivers and lakes, it poisons the water and aquatic life; further, most of it evaporates without making it into the groundwater recharge cycle. Some runoff actually leaks into sewage systems of fading infrastructure. When there is not ample groundwater recharge, the water table is lowered and negatively affects all facets of nature, including the drinking water supply. BMPs aim to promote infiltration to satisfy the necessary groundwater recharge.

Development aids the loss of potential groundwater recharge. Data courtesy Paving Our Way to Water Shortages: How Sprawl Aggravates the Effects of Drought www.smartgrowthamerica.org/DroughtSprawlReport09.pdf.

Development aids the loss of potential groundwater recharge. Data courtesy Paving Our Way to Water Shortages: How Sprawl Aggravates the Effects of Drought www.smartgrowthamerica.org/DroughtSprawlReport09.pdf.

Reduced erosion, flooding, sedimentation, and water temperature
LID practices reduce stormwater rate, volume, and temperature. By lowering volume and rates of runoff, phenomenon occurrences (e.g. erosion, flooding, and sedimentation) also decrease. Pollutants increase the faster and farther runoff travels on impervious surfaces, with more speed causing runoff to warm up before depositing into lakes and streams and adversely affecting aquatic life.

Disconnecting impervious surfaces and adding permeable surfaces are the best ways to decrease flow rate and volume of runoff. Aesthetically, providing green space and visual attractions in a usually less appealing area, such as a parking lot, is always a benefit to consider.

Designing with LID principles and incorporating BMPs into site designs are responsible and affordable ways of incorporating the land and its natural processes into development. The U.S Environmental Protection Agency (EPA) defines a BMP as a:

technique, measure, or structural control that is used for a given set of conditions to manage the quantity and improve the quality of stormwater runoff in the most cost-effective manner.

Common types of stormwater BMPs
There are many types of stormwater BMPs to consider for a design. A site analysis should be performed to note the area’s size and the amount of water that must be accommodated. Each BMP has unique pros and cons and is site-dependent. In many cases, BMPs are cheaper alternatives to curb and gutter systems.

One way water may enter the bioswale from the parking lot.

One way water may enter the bioswale from the parking lot.

Bioretention swales, bioswales, vegetated swales
Bioretention swales are long, narrow landscaped channels that cleanse runoff using bioretention techniques, as well as infiltrate water and act as a conveyance system.

Phytoremediation is the process enabling plants to naturally remove toxic metals from the water. Typically, this process is not immediate, so the swale should be designed to hold water for more than 48 hours. A gentle slope is used within a swale to move water through it slowly enough for the plants to respond. Vegetation in the swale must be flood-tolerant, erosion-resistant, close-growing, and efficient at removing pollution. This is the case for plants able to extract high concentrations of metal from the soil into their roots, also called hyperaccumulators.

Swales can be wet, riparian areas, or they can be dry areas only wet during large storms. Typically, irrigating a swale is not a good practice except for the establishment of vegetation—a period that typically takes two to three years. Grassy swales, similar to vegetated ones in their design and activity, are landscaped solely with a mixture of grasses. The major difference is maintenance and form, as the grasses can be left to grow tall, be mowed constantly (e.g. buffer strips adjacent to streets), or mowed occasionally, depending on aesthetic and stormwater-filtering requirements.

Swales are inexpensive compared to traditional curb and gutter techniques. Although maintenance is an increased concern, a swale is still less costly and provides more benefits.

Rain gardens, infiltration basins, planter boxes
Rain gardens are meant to be a short-term bioretention area that fluctuates between wet and dry conditions depending on the space. Collectors of water runoff, these rain gardens typically consist of grasses adaptable to wet or dry conditions, but can also contain flowering plants and hyperaccumulator plants. The main function of a rain garden is to allow the stormwater to infiltrate into the ground and recharge the stormwater reserve, but they also allow for plant- and soil-filtering functions to improve water quality. These gardens are situated close to the runoff source; unlike swales, rain gardens do not convey the water to a specific place—they promote infiltration in a smaller contained area.

A small infiltration basin clearly delineating the entrance of the stormwater as it runs off the parking lot.

A small infiltration basin clearly delineating the entrance of the stormwater as it runs off the parking lot.

Rain gardens must be positioned close to the source, and the water table must be at least 1.5 m (5 ft) below the basin at its peak. This is because if runoff into a rain garden travels a long way and picks up excess pollutants and sediments, infiltration may not cleanse it enough. As a result, the ground water could be contaminated or the system clogged. Additionally, the type of soil needed to accommodate proper infiltration of 12.7 to 76.2 mm (1/2 to 3 in.) per hour is an extremely important design pre-treatment. The soil should have no greater than 20 percent clay content, and less than 40 percent silt/clay content.

Although vegetation within infiltration basins is encouraged for optimal filtering, basins can also have layers of sand and rocks in a type of soakage trench, without vegetation. Infiltration basins are cost-effective practices because little infrastructure is needed when constructing them.

Another way to collect stormwater is to use a rainbarrel or cistern to harvest rain water, commonly from a roof downspout. By collecting water, and repurposing it, runoff is prevented. Water collected in such barrels is then commonly re-used to irrigate lawns or gardens and can be used in other non-potable uses such as flushing toilets. However, it should be noted, due to water laws and restrictions, it is illegal to harvest rainwater in some areas of the United States.

Constructed wetlands: Detention ponds/retention ponds
Constructed ponds and wetlands are designed by engineers to prevent runoff by holding stormwater. Detention ponds are larger, less-particular versions of infiltration basins. They typically have a fore bay to allow particles and pollutants to settle and be treated; this prevents them from clogging the entire pond. Two important pieces of designing ponds include the removal of sediment buildup over time, as well as the simultaneous growth of hyperaccumulator plants that naturally filter water.

Typically, fore bays are a concrete surface, which allows for maintenance and removal of sediment buildup, but not vegetation growth. To plan around this problem a structured surface able to grow vegetation, such as a void-structured concrete, should be used. This material is similar to a concrete slab in structure, but has spaces large enough for hyperaccumulator vegetation to grow through. The vegetation’s roots are protected within the concrete cavity allowing for maintenance and removal of sediment as needed without distrubing the vegetation significantly.

Generally, detention basins can be used with almost all soils. While detention ponds can remain wet or sometimes dry up, retention ponds are typically deeper and always wet as well as retain stormwater for longer periods.

An interactive bioswale at a big box store with a pedestrian bridge connecting the paths.

An interactive bioswale at a big box store with a pedestrian bridge connecting the paths.

Sometimes people need to be reminded not to dump chemicals in the storm drains.

Sometimes people need to be reminded not to dump chemicals in the storm drains.

Green roofs
A green or vegetated roof consists of a waterproofing assembly, lightweight soil, and plants adapted to survive the area’s climate. An efficient BMP and prevention technique, vegetated roofs intercept rainwater directly at the source, preventing most of the water from becoming runoff. Since the rain is used by the vegetation, a major advantage to a green roof is its ability to decrease the volume of runoff, thus mitigating flow rates, flooding, erosion, and sedimentation.

Green roofs promote infiltration for the advantage of the vegetation on the roof, but not the water table. Additionally, green roofs provide wildlife habitat and attract birds. These assemblies also provide energy-saving benefits to the building, including increased insulation on the roof, mitigating building and roof temperatures, and possibly doubling the roof’s lifespan since it is protected from harsh weather.

There are two types of green roofs: intensive and extensive. The first type promotes human interaction where people are encouraged to use and interact with plant life amongst paths and gathering areas, these roofs can usually carry heavier loads and deeper soil. Conversely, extensive green roofs contain only vegetation over the entire roof with little human interaction. A green roof is a relatively high-cost BMP initially, but has energy-saving returns that are worthwhile in the long run.

Tree plantings
One of the most underused BMPs, tree plantings are effective at mitigating stormwater in urban settings due to their ability to absorb large amounts of water while needing little surface area. Trees provide shade and habitat, which helps reduce the urban heat island effect.

Trees are also an important factor in cleansing and filtering the air. The branches and leaves of trees help to soften rainfall speed, reducing both stormwater flow rates and erosion. Trees also help aid the view shed, breakup the impervious landscape, provide small but essential green spaces linking walkways and trails, and reduce the visual dominance of cars.

The success of tree plants, especially in urban settings, is largely determined by the species chosen and the size of the planting area, in addition to meeting its watering requirements. Species selection should be based on location, but an appropriately sized tree-well with plenty of soil volume makes the difference in how well it survives. Although there is a need to pave over tree roots in urban settings, leaving as much exposed to natural water as possible is the best plan; if paving is necessary, use permeable paving so water and oxygen can still get to the tree roots. Pouring concrete around a street tree of any variety guarantees stunted growth and little canopy.

A close up of the vegetation within the void structured concrete of this detention pond showing how roots are protected from maintenance scrapers.

A close up of the vegetation within the void structured concrete of this detention pond showing how roots are protected from maintenance scrapers.

Detention pond fore bays designed with a base made of void structured concrete offer designers the opportunity to have naturally cleansing and appealing vegetation in the pond and still offer the ability to perform scraping maintenance to remove sediment without destroying the vegetation.

Detention pond fore bays designed with a base made of void structured concrete offer designers the opportunity to have naturally cleansing and appealing vegetation in the pond and still offer the ability to perform scraping maintenance to remove sediment without destroying the vegetation.

Permeable paving
Arguably the most efficient BMP due to its practicality, permeable paving has been revolutionizing the green industry for years. Paved surfaces are a necessity in the built environment, and replacing traditional impervious concrete and asphalt with permeable materials is one of the easiest, effective, and cost-efficient methods of preventing runoff.

Permeable paving allows for water to percolate through cracks in the pavement and infiltrate directly to the soil, preventing runoff from ever occurring. Infiltration not only prevents runoff, but also replenishes the water table and allows for natural soil filtration to improve water quality. Incorporating stormwater treatment into parking areas and landscaped zones reduces required detention volume onsite. This allows for an increase in building area and the potential for further profitability for an LID-educated developer.

Permeable pavements also provide a reduction in the heat island effect, which is especially valuable in urban areas typically paved in dark colors and absorb light. Since permeable pavement allows for water infiltration, it also helps increase water quality using the soil as a filtering medium. There are many types of permeable pavements available with advantages and disadvantages, depending on the desired use and location.

Types of permeable pavement
Different types of permeable pavement can be used for various installations. Some examples are included below.

Void-structured concrete: grass- or stone-filled, or concealed with vegetation
Designed to combine the strength of traditional concrete with aesthetically pleasing vegetation, void-structured concrete is cast-in-place and contains a grid series of spaces that allow the system to be pervious in nature.

These voids can be filled with various porous materials such as vegetation or no-fines stone, or the entire system can be seeded over to completely conceal it, making it ideal for emergency access applications seeking to hide the eyesore of a road. Typically, void-structured concrete is used for fire and emergency access, military applications, parking lots, detention pond fore bays, and general stormwater management with heavy load requirements.

The typical lifecycle of the material is more than 15 years. Its benefits include:

  • high load-bearing capacity (suitable for heavy traffic);
  • high infiltration rates;
  • low maintenance costs;
  • freeze-thaw cycle resistance;
  • ability to be effective with saturated sub-base;
  • green space addition; and
  • moderate installation costs.

Some challenges associated with void structured concrete are:

  • not Americans with Disabilities Act (ADA)-compliant;
  • in pedestrian zones there is a need to incorporate bands of traditional concrete for ease of movability with all footwear; and
  • surrounding grass needs to be properly maintained.

Permeable interlocking concrete pavement
Available in various shapes, sizes, and colors, permeable interlocking concrete pavers (PICPs) are designed to either let water infiltrate within voids in the unit itself, the space around them, or both. PICP is ideal for light traffic where intricate and aesthetically pleasing designs are desired. Often these systems are used for pedestrian walkways and garden paths.

The longevity of PICP is moderate, spanning seven to 15 years. PICP benefits include:

  • high infiltration rates;
  • various patterns and colors;
  • can be ADA-compliant; and
  • ease of maintenance if integrity of paver fails or if utilities underneath need to be reached.

Some challenging aspects of PICP are:

  • system has a low load-bearing capacity (not suitable for heavy traffic applications);
  • pavers are susceptible to movement and damage in freeze-thaw climates;
  • high installation costs as a deep sub-base for optimal performance is required; and
  • high maintenance costs.
A study site pictured after a rainfall showing water pooling on top of the asphalt on the right, and water all infiltrated in the pervious concrete to the left.

A study site pictured after a rainfall showing water pooling on top of the asphalt on the right, and water all infiltrated in the pervious concrete to the left.

A test site in a parking lot comparing pervious concrete to traditional asphalt.

A test site in a parking lot comparing pervious concrete to traditional asphalt.

 

 

 

 

 

 

 

 

 

Plastic grid systems reinforced with grass or gravel
Reinforced systems are most commonly seen as plastic ring systems. They are designed to provide vehicle stability for grassed or gravel surfaces without the visual eyesore of the plastic rings showing. This system is best used for short-term pedestrian and light vehicular traffic and is often seen on trails and for emergency access. Its lifecycle is typically less than seven years.

Benefits include:

  • high infiltration rates;
  • adds green space; and
  • lightweight plastic is easy and cost-effective to install.

Challenges include:

  • low load-bearing capacity (not suitable for heavy traffic applications);
  • system commonly fails in saturated soils;
  • susceptible to movement and damage in freeze-thaw climates;
  • high maintenance costs as the system settles fast and plastic rings are commonly visible;
  • not ADA-compliant; and
  • grass needs to be properly maintained.
A close up image of stone filling the voids of a void structured concrete installation.

A close up image of stone filling the voids of a void structured concrete installation.

Gravel and crusher fines
Crushed rock type gravel and crusher fines, typically last less than seven years. Crusher fines can use a glue to help hold them together, reducing the infiltration rates. Due to their natural properties, they are commonly employed in park and trail settings without heavy slopes. Similar to some of the other materials discussed, they typically have high infiltration rates and low installation costs, along with the ability to withstand freeze-thaw cycles.

The system is not suitable for heavy traffic applications due to its low load-bearing capacity. It also commonly fails in saturated soils because the system is not stabilized well. Further, gravel and crusher fines are not appropriate for use on a slope because they develop ruts in heavy storms, resulting in high maintenance costs.

Pervious concrete and pervious asphalt
An alternative to their traditional counterparts, the pervious versions of concrete and asphalt employ larger aggregates in the mix. The result creates voids in the pavement that allow water to pass through to enter a temporary detention area and ultimately infiltrate to the ground. These products are different, but their advantages and disadvantages are similar.

Location is key to the performance of these systems. Typically, pervious concrete and asphalt perform the best in areas where available sediment is low, traffic volume is low, and maintenance can be regular and intensive. The lifecycle is typically seven to 15 years.

Benefits include:

  • high load-bearing capacity (suitable for heavy traffic);
  • moderate infiltration rates;
  • aesthetic design options;
  • ADA-compliant; and
  • moderate installation cost.

Some challenges associated with pervious concrete and asphalt include:

  • reduction in pavement surface (surface raveling is common where aggregate is dislodged or damaged);
  • high maintenance costs as the system needs the quarterly use of an intensive vacuum sweeper to pull out sediment, and special winter maintenance for snow and ice conditions;
  • reduction in porosity (clogging is the largest concern—even when regularly vacuumed, a clogged system cannot function and water ponds on the surface); and
  • it can be susceptible to damage in freeze-thaw climates if system freezes with water in it.

With the cost of permeable pavements being fairly similar to conventional methods, it seems permeable pavement could replace nearly all impervious surfaces, but there are certainly exceptions to this idea. It is not recommended to use permeable pavements in areas where heavy pollution, leaks, or chemical spills could occur. While small oil drips from parked cars is considered acceptable for the soil to handle, a large chemical spill or heavy agricultural use would put soil and the water table in danger of being directly polluted.

A conventional way of handling stormwater is to direct it into storm drains at fast speeds with water collecting harmful pollutants on the way.

A conventional way of handling stormwater is to direct it into storm drains at fast speeds with water collecting harmful pollutants on the way.

It is also not recommended to use permeable pavement in areas where heavy sedimentation can occur—this could clog the system faster than it is designed to handle and could result in heavy maintenance costs.

Additionally, care should be taken to design for ADA-compliant access as wheelchairs and those who use walkers as an aid typically prefer the steady, smooth surface of traditional impervious surfaces. Some tracked vehicles may also be better handled on traditional surfaces to limit wear and tear.

Conclusion
Now commonly used for residential driveways, military applications, offices buildings, government buildings, and even grocery stores, permeable paving is emerging everywhere. LID stormwater management methods, with a focus on handling stormwater at the source, will be important to incorporate for the environmental, social, and economic stability of the world’s future. As the shift from impervious to pervious surfaces continues to occur, it will be interesting to see the speed at which this concept permeates—how much use and what technological advances will we see out of the next generation to come?

Notes
1 An earlier version of this article appeared in the June 2010 issue of ROOT. (back to top)
2 For more, see www.lowimpactdevelopment.org. (back to top)

Katie McKain, ASLA, MLA, MUD is a proponent for sustainable design and green initiatives who promotes stormwater management principles as a representative for Sustainable Paving Systems, LLC. She earned her Master of Landscape Architecture and Master of Urban Design at the University of Colorado Denver in Denver, CO, and her Bachelor of Science from Purdue University in West Lafayette, IN. McKain also received her certification from GREENCO for Best Management Practices for the Conservation and Protection of Water Resources in Colorado. She can be contacted by e-mail at kdmckain@hotmail.com.