post-construction BMP

Deborah K. Rich, SF Gate, Special to The SF Chronicle
Saturday, December 6, 2008

"The first year we were here, the water would just sheet down from the property higher up the slope, and this area would be mud. I couldn't even walk out here; it was just slosh and goop," she said.

Her home is near Occidental in Sonoma County, which can receive 60 inches of rain a year. "My husband and I were wondering what we were going to do, and how we were going to figure this out."

Davison met a teacher at her sons' school whose husband, Erik Ohlsen, had recently launched Permaculture Artisans, a landscape design and installation business.

Permaculture - the word plays on "permanent culture" and "permanent agriculture" - strives for sustainability by incorporating ecological cycles and principles into land altered for human use. Ohlsen took his first permaculture class in 1999 from Brock Dolman, who directs the Water Institute at the Occidental Arts & Ecology Center.

Interaction with water in the landscape has become fundamental to Ohlsen's permaculture design practice.

"I was inspired by the concepts of water harvesting, ecological watershed management practices and erosion control and everything to do with water in Brock's course," Ohlsen said. The storm water that turned the property into muck could instead provide a foundation for the garden Davison wanted.

"The Davisons were clear that they wanted an ecological landscape that provided food for their family," Ohlsen said. "Water harvesting wasn't something they had foreknowledge of, but the way we design ecological gardens, water harvesting is always the first piece designed into the system."

Swales and berms
Ohlsen used a small excavator to build a series of parallel swales (a shallow ditch) and berms (a raised area adjacent to the swale) on contour (meaning that they lie across slope, their elevation remaining constant). He dug the first swale along the upper property line and the last where the property levels off.

Each swale is roughly 5 feet wide; its adjoining berm is 2 1/2 feet tall and 6 feet wide. A smaller berm lies across the end of each swale to prevent water from running out the end of the swale.

The swales and berms harvest rainwater by pooling and slowing the water on its downhill course, giving it time and space to soak into the soil. Rock-lined spillways connect the swales and allow water to flow from one to the next if the water pools in the swale more than 8 inches.

In the Davisons' loamy soil, all the rainwater will generally soak into the swale where it is caught, and water will spill from one swale to the next only during a very heavy rain. "We design for catastrophe," Ohlsen said of the oversize catchment systems. Encouraging Davison and her boys to work along with him, Ohlsen planted the berms with an eye toward both feeding the family and creating a self-sustaining ecosystem.

"We chose plants that provide multiple functions - for example, leguminous plants which can provide edible pods while, at the same time, fixing nitrogen in the soil and attracting beneficial insects and hummingbirds, which can then manage pests," Ohlsen said.

For entire article, please visit website.

By Deborah K. Rich, SF Gate
Saturday, December 6, 2008

Embedded in both urban and suburban lot design is the "pave and pipe paradigm," according to Brock Dolman, director of the Occidental Arts & Ecology Center's Water Institute. It favors grading, piping and paving properties to drain away rainwater as quickly as possible.

But rapidly draining water off landscapes rather than allowing it the time and space to soak in causes a host of problems downstream and in the pipes. Culverts pour water into gullies and seasonal creeks, overloading and eroding the natural drainage area and rushing sediment into rivers, streams and estuaries, where it imperils fish.

Downspouts, gutters and sloping driveways conduct water into the storm water and sewer systems, which can dump raw sewage when overloaded. After we're finished draining our properties, we pay, increasingly dearly, to pipe water back into our homes and landscapes.

Dolman advocates replacing the "drain age" with a new "retain age," wherein we capture and store storm water for future use and resculpt yards and gardens to allow water to percolate into the ground.

To take a step into the retain age, consider harvesting rainwater from your roof and banking more water in your soil.

Harvesting roof water
Every inch of rainfall on 100 square feet of roof surface yields 55 to 60 gallons of water. For a 2,500-square-foot home, that translates to 1,375 to 1,500 gallons of water per inch of rain. This water can be caught and stored in above- or belowground cisterns and used for drinking, in-house nonpotable uses or irrigation, depending upon what filtration systems are installed and upon local regulations.

For entire article and web references on Water Harvesting, please go to website below.

November-December 2008 issue, The Stormwater Newsletter
By Joo-Hyon Kang, Peter T. Weiss, John S Gulliver, Bruce C. Wilson

Although many resources are available to aid in the design and construction of most structural stormwater best management practices (BMPs), few guides exist pertaining to their operation and maintenance. Historically, it seems as though a “build ’em and walk” approach has been commonplace. However, increasing focus upon mass balances, numeric goal setting, and total maximum daily loads (TMDLs) now requires that much more emphasis be placed upon BMP operation and maintenance for permitting and reporting requirements—for example, for the National Pollutant Discharge Elimination System (NPDES) municipal separate storm sewer system (MS4) permit program, and as a part of stormwater pollution prevention plan (SWPPP) reporting.

Typically, we think of structural stormwater BMP operation for optimizing (1) the removal of pollutants and (2) the reduction of runoff volumes/rates via the management of stormwater networks or treatment trains. BMP maintenance is the purposeful management of a BMP to maintain a desired level of performance and efficiency. Maintenance consists of short-term (routine or more frequent), long-term (non-routine or less frequent), and major (rare) actions (Figure 1).

Stormwater BMPs have a lifecycle from their creation (design and construction) through operative stages (functional or not) that is largely dictated by operation and maintenance (O&M) actions. As maintenance involves a significant amount of resources (personnel, equipment, materials, sediment disposal expense, etc.), the more we learn about BMP operation, the more likely we are to maintain optimal performance and improve cost efficiencies. The purpose of this article is to advance short- and long-term maintenance considerations to develop more realistic O&M plans. To do this, we have used a combination of a national literature search for maintenance costs coupled with a detailed municipal public works survey.

Minnesota BMP Maintenance Survey
The statewide survey of Minnesota Municipal Public Works managers to define maintenance needs and guidelines was conducted by the University of Minnesota and partly funded by the Minnesota Pollution Control Agency. Previously, the University of Minnesota produced a manual, Assessment and Maintenance of Stormwater Best Management Practices, which includes source reduction and four levels of assessment (from visual to state-of-the-art monitoring). The manual is available online at www.pca.state.mn.us/water/stormwater/stormwater-research.html or wrc.umn.edu/outreach/stormwater/bmpassessment/index.html.

The specific goals of the survey were to identify and inventory stormwater BMP maintenance in Minnesota. Survey questionnaires focusing on the following questions were sent to 106 cities; we received 27 responses, for a slightly higher than 25% response rate.

How many BMPs are in your watershed?
How often are your BMPs inspected?
What is the average staff-hours spent per routine inspection/maintenance?
How complex is the maintenance of your BMPs?
Which factors most frequently cause the performance deterioration of your BMPs?
What are the costs for non-routine maintenance activities?

We attempted to make the survey as simple as possible, requesting information for typical response ranges of common BMPs. Although the number of respondents was relatively low, we believe that the results will help refine operation and maintenance needs.

Inspection Frequency and Staff-Hours. The required frequency of stormwater BMP maintenance actions and the associated required staff-hours are two key parameters that are necessary to reasonably budget and schedule inspection and maintenance. Frequency and staff-hours vary according to BMP design, climate conditions, accessibility of the BMP, and maintenance strategies of the BMP operators. As part of the survey, cities were asked to provide information regarding their frequency of routine maintenance actions for various kinds of BMPs.
...
For entire article, please see website.

Free webcast offered by EPA: December 3, 2008, 10-12 pst.
Likely will be offered at Reno City Hall, 8th floor: contact Lynell Garfield for more local web presentation info. at 334-3395.

Using Rain Gardens to Reduce Runoff:
Slow It Down, Spread It Out, Soak It In!
Wednesday, December 3, 2008: Two-hour audio Web broadcast

Eastern: 1:00 pm - 3:00 pm
Mountain: 11:00 am - 1:00 pm Central: 12:00 pm - 2:00 pm
Pacific: 10:00 am - 12:00 pm

Register for the Webcast
A Watershed Academy Webcast

Many communities across the country are struggling to address impacts from stormwater runoff due to increased development. Green or low impact development practices such as rain gardens can help manage runoff effectively as well as provide aesthetic benefits. Rain gardens can increase property values, add beauty and habitat, reduce a community’s carbon footprint, as well as provide important water quality benefits. Join us for this exciting Webcast to learn more about these natural solutions to water pollution. Our speakers will discuss the benefits of rain gardens and share their experiences with successful community rain garden programs.

By Kevin Beuttell, Stormwater E-Magazine, October 2008

Despite the proven environmental benefits of rain gardens, many people are reluctant to use them because they can be unattractive. But a close examination of the relationships between hydrology and vegetation in rain gardens suggests a solution for improving their looks and their function. Rather than think of rain gardens primarily as wet environments, we should design them as dry environments that experience only brief wet periods. This shift in thinking increases opportunities for ornamental planting without sacrificing environmental performance.

Rain gardens are one of the most frequently cited and promising strategies for managing stormwater responsibly, and because of the ubiquitous presence of impervious surfaces, these systems can be used on virtually any type of site. Rain gardens come in many forms (and go by many names, such as bioswale, bioretention, and bioinfiltration), but for the purposes of this article, the term “rain garden” is essentially meant to describe a shallow depressional area designed to use the natural capacities of soil and vegetation to retain, cleanse, and infiltrate stormwater.

The Pros of the Rain Garden
Infiltration-based stormwater management strategies, such as rain gardens, are crucial to downstream ecological health. Every parcel of land interacts with water. If water infiltrates, it can be used as a resource to nourish plants and replenish aquifers. When water runs off driveways, roads, and compacted soils, however, it becomes a liability, carrying sediments and pollutants downstream. The USEPA states that nonpoint sources, such as stormwater runoff from an urbanized landscape, are the leading causes of urban stream water-quality problems. To help, many designers are looking toward landscape solutions to water-quality and flooding problems, altering land surface functions to manipulate the way in which the land captures and absorbs stormwater.

Many other stormwater management techniques address only a portion of the problems caused by stormwater runoff. Rain gardens, however, have the potential to solve all the problems of stormwater runoff before they occur. Like other infiltration-based strategies, rain gardens mitigate the hazardous stormwater runoff aspects of development by decreasing peak flows responsible for storm surges and flooding. They reduce pollutant discharges, minimize streambank erosion, replenish groundwater, and restore base flows and aquatic habitats. Rain gardens can also offer real development cost savings by eliminating expensive belowground stormwater infrastructure in favor of combining stormwater management with ornamental landscapes.

Rain gardens can also help with temperature pollution problems. In a completely natural setting, water enters a stream or other water body almost entirely through groundwater that provides steady flows at low temperatures. But when development introduces impervious surfaces, higher temperatures often result as the runoff washes over those warmer surfaces. Higher temperatures, in turn, cause the loss of a diverse system of aquatic biota in receiving streams, ponds, and rivers that are sensitive to the warmer water.

Because of effects like these, traditional urban stormwater management has always viewed water as a burden on the landscape. Water is typically taken away through channels and pipes as quickly as possible to avoid flooding on site. But water and ecological quality can be improved when water is allowed to infiltrate, using it as a resource where it falls.

The (Perceived) Cons of the Rain Garden
Attractive and functional rain gardens are the exception, not the rule. Most rain garden installations do not include those elements that are culturally accepted as beautiful, like lush green lawns, flowering vegetation throughout the growing season, clean lines, and a maintained appearance. As a result, people see these landscapes as cluttered, unkempt, and unmanaged. Perceptions are just as important as environmental performance. If rain gardens are not perceived as attractive, cared-for environments, they will not be adopted during the design phase or managed after installation. Although preferences vary from person to person, a common theme for all is an appearance that communicates care to the viewer.

People design and manage landscapes as a reflection of who they are and how they want to be perceived. Too often, rain gardens planted with water-loving species appear unkempt and abandoned. Individual plants are often stressed and weak, particularly in areas that experience hot and dry summers. The negative perception of their ornamental character is an obstacle to their use in both new and retrofit development projects. Because many rain gardens do not come close to the ornamental quality of more traditional garden landscapes (especially from the perspective of the general public, who may be largely unaware of the environmental benefits), they are not a viable option in visually prominent areas of a site such as in parking lots or at site and building entrances. In high-visibility areas, environmental performance alone is not enough. Because one cannot see the ecological functioning of the root systems, water infiltrating through soil, and wildlife’s benefits from the landscape, it is difficult to include an ecological assessment in our judgment of landscape’s appearance. So rain gardens are not used, or are relegated to areas of the site where their messy appearance will not offend.

For entire article, please visit website.

By Tara Hun-Dorris, Stormwater Magazine, March-April 2005

Pavements are an intrinsic, seldom-thought-about part of life, particularly in urban areas. However, for developers, industrial facilities, and municipalities addressing stormwater and associated water-quality guidelines and regulations, pavement stays very much at the forefront of planning issues. “Pavements are the most ubiquitous structures built by man. They occupy twice the area of buildings. Two-thirds of all the rain that falls on potentially impervious surfaces in urban watersheds is falling on pavement,” says Bruce Ferguson, professor and director of the School of Environmental Design at the University of Georgia in Athens.

Porous pavements, designed to allow air and water to pass through, are today just a small fraction of all pavement installations. However, their popularity is steadily increasing on a percentage basis, and they have been installed in all regions of the United States, Ferguson says. “This is potentially the most important development in urban watersheds since the invention of the automobile. The automobile is causing us to build all these pavements and have all these oils that we spill. If we can transfer the environmental function of the pavement, we’ve done two-thirds of the work.”

If used properly, porous pavements can facilitate biodegradation of the oils from cars and trucks, help rainwater infiltrate soil, decrease urban heating, replenish groundwater, allow tree roots to breathe, and reduce total runoff, including the magnitude and frequency of flash flooding. Stormwater, particularly urban runoff and snowmelt, is the wastewater of the 21st century, according to John Sansalone, associate professor in the Department of Civil and Environmental Engineering at Louisiana State University (LSU) in Baton Rouge. As reuse becomes more necessary, runoff will eventually be seen as a valuable commodity, he explains. This makes porous pavements, with their potential to revolutionize stormwater management, an important technology for the future.

Ferguson has been studying porous pavements for more than a decade. In his book, Porous Pavements (2005), Ferguson identifies nine categories of porous pavement: decks, open-celled paving grids, open-graded aggregate, open-jointed paving blocks, plastic geocells, porous asphalt, pervious concrete, porous turf, and soft paving.

For entire article, including many success stories from varied climates, please visit website.

With the right soil conditions and careful design, installations retain their ability to infiltrate.
By Michele C Adams, Stormwater E-Magazine May-June 2003

Is it possible to have a stormwater best management practice (BMP) that reduces impervious areas, recharges groundwater, improves water quality, eliminates the need for detention basins, and provides a useful purpose besides stormwater management? This seems like a lot to expect from any stormwater measure, but porous asphalt pavement on top of recharge beds has a proven track record.

First developed in the 1970s at the Franklin Institute in Philadelphia, PA, porous asphalt pavement consists of standard bituminous asphalt in which the aggregate fines (particles smaller than 600 µm, or the No. 30 sieve) have been screened and reduced, allowing water to pass through the asphalt (Figure 1 on website). Underneath the pavement is placed a bed of uniformly graded and clean-washed aggregate with a void space of 40%. Stormwater drains through the asphalt, is held in the stone bed, and infiltrates slowly into the underlying soil mantle. A layer of geotextile filter fabric separates the stone bed from the underlying soil, preventing the movement of fines into the bed (Figure 2 on website).

Porous pavement is especially well suited for parking-lot areas. Several dozen large, successful porous pavement installations, including some that are now 20 years old, have been developed by Cahill Associates (CA) of West Chester, PA, mainly in Mid-Atlantic states. These systems continue to work quite well as both parking lots and stormwater management systems. In fact, many of these systems have outperformed their conventionally paved counterparts in terms of both parking-lot durability and stormwater management.

Installations Old and New

One of the first large-scale porous pavement/recharge bed systems that CA designed is in a corporate office park in the suburbs of Philadelphia (East Whiteland Township, Chester County). This particular installation of about 600 parking spaces posed a challenge because of both the sloping topography and the underlying carbonate geology that was prone to sinkhole formation. The site also is immediately adjacent to Valley Creek, designated by Pennsylvania as an Exceptional Value stream where avoiding nonpoint-source pollution is of critical importance. Constructed in 1983 as part of the Shared Medical Systems (now Seimens) world headquarters, the system consists of a series of porous pavement/recharge bed parking bays terraced down the hillside connected by conventionally paved impervious roadways. Both the top and bottom of the beds are level, as shown in Figure 3, hillside notwithstanding. After 20 years, the system continues to function well and has not been repaved. Although the area is naturally prone to sinkholes, far fewer sinkholes have occurred in the porous asphalt areas than in the conventional asphalt areas, which the site manager attributes to the broad and even distribution of stormwater over the large areas under the porous pavement parking bays.

Other early 1980s sites, such as the SmithKline Beecham (now Quest) Laboratory in Montgomery County, PA, and the Chester County Work Release Center in Chester County, PA, also used the system of terracing the porous paved recharge beds down the hillside to overcome the issues of slope. At the DuPont Barley Mills Office complex in Delaware, the porous pavement was installed specifically to avoid the construction of a detention basin, which would have destroyed the last wooded portion of the site. More recently (1997), the porous parking lots at the Penn State Berks Campus were constructed to avoid destroying a wooded campus hillside. The Berks lots, also on carbonate bedrock, replaced an existing detention basin and have not experienced the sinkhole problems that another campus detention basin has suffered.

To view complete article and figures, please visit website.

Tahoe Daily Tribune article, copied from Wednesday, November 2, 2005
EPA to look at Tahoe drainage systems
By Amanda Fehd

Homeowners and businesses in Tahoe could be installing drainage systems regulated by the U.S. Environmental Protection Agency for health safety reasons - and not know it.

Representatives from the EPA, Tahoe's planning agency and Nevada's environmental protection agency met by conference call last week to discuss whether drainage systems used in Tahoe fall into a category EPA alleges has the potential to contaminate groundwater. No decision was made.

A variety of drainage systems are used in Tahoe to comply with a Tahoe Regional Planning Agency ordinance requiring most property owners to install devices to catch rain and snowmelt.

Called stormwater best management practices, or BMPs, the systems are intended to prevent soil erosion.

Most not a concern

While most of the drainage systems in Tahoe are not a concern, some may be classified as Class V wells, according to Elizabeth Janes at EPA's Region Nine groundwater office in San Francisco.

Tahoe's rain and snowmelt, called stormwater, is generally very clean, diminishing risk of contamination, according to Lahontan Regional Water Quality Control board, which regulates water quality at Lake Tahoe.

However, certain drainage systems would allow any contaminants, or spills of auto or lawn chemicals, to more easily enter groundwater, according to EPA.

EPA requires inventory information from property owners who install Class V drainage systems because of their alleged potential to contaminate groundwater.

Users must also agree not to allow any substances that are threats to drinking water to enter the systems.

EPA's list of threats to drinking water includes chemicals used in household cleaning, lawn and auto care and is available at www.epa.gov/safewater/mcl.html#mcls.

The worst kind of Class V well is a drilled hole allowing water directly down to the water table. The technique is not used in Tahoe but has been standard construction practice in other parts of California, Janes said.

But Class V wells can include many other types of underground drainage systems, according to EPA.

Much of South Shore gets its drinking water only from groundwater, while the lake supplies drinking water to most of the Nevada side. Drinking water is constantly monitored for purity.

No decision yet

EPA is not ready to make a determination on designs for residential or commercial BMPs in Tahoe before taking a closer look at them, Janes said.

"We all agreed we need to sit down and look at these on location," said Russ Land, supervisor of the groundwater protection office of the Nevada Division of Environmental Protection, Janes's Nevada counterpart. NDEP provides funding for TRPA's residential BMP retrofit program.

Based on his limited review of residential BMP designs in TRPA's contractors handbook, Land said none fit the definition of a Class V well.

EPA Region Nine was not so sure, Janes said.

The three agencies met after inquiries to EPA from the Tahoe Daily Tribune about whether the Class V wells were used in Tahoe. Area engineers raised the issue with the Tribune.

While Birgit Widegren, head of TRPA's soil erosion team, said it has not been interpreted in the past that Tahoe's designs qualify as Class V, EPA representatives were not certain.

Engineer approved

All Tahoe BMP designs are approved by a state engineer, according to Erik Larson, program coordinator for the Tahoe Resource Conservation District, which provides information to owners of homes where BMPs are installed.

Measures are in place to protect groundwater in Tahoe, according to Widegren. Properties expected to release pollution into rain water or snowmelt, like an auto station, are required to treat it before it is allowed to enter the ground.

TRPA's approach to BMPs is very conservative, Janes said. "They aren't ignorant of the vulnerability of their groundwater."

TRPA's BMP ordinance is aimed at reducing soil erosion, one of the main factors in Lake Tahoe's declining clarity.

"The fundamental concept of keeping soil on property is sound and how we do it may evolve over time," said TRPA spokeswoman Julie Regan. "It's a collaborative process and we will be making sure we are all in agreement."

Homes less risky

Residential properties are less of a risk to groundwater than commercial properties, Janes said.

"EPA does not want to discourage anyone from implementing their residential stormwater BMPs," said Janes. But she cautioned property owners to be responsible about chemical use such as fertilizer, pesticides, herbicides and auto chemicals.

"What you pump into the ground ends up somewhere," she said.

A fact sheet from EPA says there is concern "there may be a dramatic increase in the use of Class V wells as a (BMP) to dispose of stormwater. Infiltration through stormwater drainage wells has the potential to adversely impact [underground sources of drinking water].

The runoff that enters the stormwater drainage wells may be contaminated with sediments, nutrients, metals, salts, fertilizers, pesticides and microorganisms."

The fact sheet was put out in response to construction practices in Modesto, Janes said. It is available at www.epa.gov/safewater/uic/pdfs/fact_class5_stormwater.pdf.

A simulation model for estimating retention volumes
By Randel Lemoine, Stormwater E-magazine September 2008

Natural watersheds retain and dissipate most rainwater. This water is retained on the surfaces of vegetation and in ground depressions, such as puddles, wetlands, and marshes. Natural processes such as transpiration by plants, infiltration into the soil, and evaporation dissipate this water. A natural watershed’s retention and dissipation capacity is sufficient to prevent any runoff from occurring during most rainfalls. Occasionally, when there is a heavy rainfall, a small amount of the rainwater becomes surface runoff that enters nearby creeks, rivers, and lakes.
The natural processes that retain and dissipate the rainwater are diminished when land is developed, whether for agriculture or for urban use. Land development removes vegetative cover, fills in low areas, compacts the soil, and creates impervious areas. The result is increased water runoff flowing more frequently across the land and discharging into the watershed’s rivers, streams, and lakes. This increased runoff causes downstream flooding, accelerated soil loss from erosion, unstable stream banks, and pollution of water resources.

Problems in Mitigating Increased Runoff
Detention basins temporarily hold collected runoff and slowly release the water. They are constructed in an attempt to mitigate the downstream flooding problems by limiting the peak discharge rate of the runoff. However, they do not reduce the volume of runoff discharged into the nearby creeks, rivers, and lakes. Consequently, the runoff volume discharged remains greater than when the land was in its natural condition. Therefore, detention basins fail to match the natural runoff pattern that occurred prior to the land being developed. Streambank erosion, stream channel instability, and occasionally even downstream flooding continue to be problems.
Retention basins hold a certain volume of water. There are two types of retention basins: water-quality basins and water-volume basins. Water-quality retention basins remove pollutants collected by the runoff. These basins allow the runoff to pass through after holding it long enough to give natural processes time to remove a percentage of the pollutants. They do not reduce the volume of runoff discharged. Water-volume basins capture and dissipate the runoff, thereby reducing the volume and frequency of discharges from a site. A discharge of runoff occurs only when the runoff volume exceeds the basin’s maximum retention volume. However, the actual volume available for retaining the runoff from the next rainfall depends upon the dissipation of the water held from the previous rainfall. Therefore, a key factor in determining the effectiveness of a water-volume basin is the dissipation rate.
Two commonly used methods for estimating the maximum retention volume for a water-volume retention basin are the “90% Rule” and the “Two-Year-Difference Rule.” The 90% Rule requires the capture of 90% of the runoff coming from a developed site. The Two-Year-Difference Rule requires that the maximum retention volume should be equal to the difference between the two-year runoff from the developed site and the two-year runoff from the site in a natural undeveloped condition. Neither rule addresses the necessary dissipation rate relative to the storage volume. Therefore, it is uncertain that the maximum retention volume derived by these rules will adequately address the adverse effects caused by the increased runoff coming from developed land.

An Alternative Method for Determining Retention Volume and Dissipation
An alternative to these methods is to use a simulation model. This model is set up on a Microsoft Excel spreadsheet and uses local historical precipitation data. The runoff volume for each day of the simulation is estimated using the TR-55 runoff equations (USDA 1986). The retained water volume for each day is calculated by taking the difference between the precipitation volume and the runoff volume, then subtracting the daily dissipation volume. This retained water volume is added to the precipitation of the next day, which is valid because the effect of the retained water on the next day’s runoff volume has the same effect as if it were part of the precipitation for the next day. Adding the previous day’s retained water to the precipitation provides the continuity needed for determining the appropriate combination of retention and dissipation to replicate the natural runoff.

For entire article, please visit website.

By Jeff Gunderson, StormCon Sept. 2008 online issue

Widespread misconception exists in the industry about pervious pavement systems, specifically about their functionality in cold-weather environments. The prevalent belief is that pervious pavements are not an effective stormwater management option for cold-weather climates because of concerns related to diminished permeability during freezing and that the material is not durable enough to withstand freeze-thaw conditions. Cold climates are typically very hard on constructed systems, and naturally, questions should arise about the effectiveness of pervious pavements in these environments—especially due to concerns about freezing of the filter media.

However, according to Dr. Robert Roseen, director of the University of New Hampshire Stormwater Center (UNHSC), stormwater management systems using infiltration and filtration mechanisms, if properly designed, can work well in cold-weather environments. He has been leading a four-year research effort focused on monitoring the year-round performance of a porous asphalt placement that was installed on the UNH campus. In addition, the UNHSC is hoping to shed light on the functionality of pervious concrete by testing a large placement that was also installed on the university campus in August 2007—the first major pervious concrete parking facility in New England. The purpose and function of the UNHSC is to evaluate the range of stormwater treatments systems available to designers, including proprietary and nonproprietary systems. The UNHSC is funded by the Cooperative Institute for Coastal and Estuarine Environmental Technology and the National Oceanic and Atmospheric Administration.

Findings from the porous asphalt study have demonstrated functionality that exceeds conventional practices by measures of both water quality and hydraulics.

Porous Asphalt Study
Design and Durability. The principal cause of parking lot pavement breakdown in northern climates is freeze-thaw cycling. Parking lots in these regions typically have a lifespan of about 15 years. By design, an open-graded, well-drained porous pavement system incorporating significant depth will have a longer life cycle from reduced freeze-thaw susceptibility and greater load-bearing capacity than conventional parking lot pavements. “Design guidelines for freeze-thaw consideration reflect frost depth ranges from 48 to 52 inches from coast to inland,” says Roseen. “For porous pavements, greater depth of frost is not the concern, but rather, the increase in the rate of cycling between freeze and thaw. This rate is highest near the coast.”

For entire content of study and article, please visit website below.