This, the third and final article in our three part series on green parking lots continues the discussion by looking at how landscaping can contribute to helping prevent and control runoff as well as provide shade helping to keep the parking lot much cooler than a bare asphalt lot would get. Specifically it examines a type of landscaping known as bioretention that is designed to collect and treat stormwater.

In case you missed the first article in our series it can be found at: Green Parking Lots: Part I – The Many Problems with Paving.

Landscape Can Help a Lot Be Green

Judicious landscaping can help shade a parking lot, helping to keep it cooler in the heat of summer. Care should be taken to plant trees that will not soil cars that are parked under their canopy. Trees should neither drip sap on vehicles, nor have large or messy fruit. Besides providing shade trees cover can also help beautify a parking lot and integrate a lot into the larger landscaped whole of a building project.

However probably the most important green function landscaping can provide a parking lot is to provide it a natural drainage, collection network and filtration of stormwater. Especially if permeable paving cannot be used in some portion of the lot — or not at all – for example because it is too heavily trafficked, needs to be able to withstand regular wintertime plowing (and salting or sanding), there are concerns with wheel chair accessibility etc. – landscaping can provide a way for stormwater runoff to be collected canalized, temporarily stored and slowly infiltrated into the ground water or released into a water system.

This green landscaping technique is known as bioretention and it is very effective at catching and treating stormwater that runs off from a parking lot. Traditional landscape islands common in many parking lots are set higher than paved grade and not only do they not collect and treat runoff, but they often require irrigation themselves. Instead bioretention or wetland landscape islands are recessed slightly below the parking lot level, and the pavement graded so that surface flow is into, rather than away from these areas. In other words they are designed to collect and concentrate runoff from the lot and treat stormwater with a combination of microbial soil process, infiltration, evaporation, and appropriate plantings of flood tolerant trees, shrubs and/or perennial ground cover.

Bioretention landscape elements, which can be swales, rain gardens or designed wetlands, are typically filled with sandy compost rich soil mix, topped with a thick layer of mulch, and covered with a dense vegetative cover. Stormwater runoff flows into the catchment area and is slowly absorbed into the soil where it gets filtered through the soil and is absorbed into the groundwater. Soil microbial action acts in a process of bioremediation to clean the water of pollutants.

Bioretention areas are usually designed to allow storm runoff to collect in a shallow 6-8 inches deep “pond” with overflow outlets designed to drain off water that is in excess of the bioretention area’s water containment capacity, in order to prevent flooding during heavy storms.

Optimal minimum coverage for the bioretention areas is 5% of the entire paved surface; this will vary depending on the degree to which that surface has been paved with permeable versus impermeable paving surfaces. By selecting flood-tolerant woody and herbaceous perennial plants the designer will help ensure that any intermittent flooding of the bioretention areas – which is after all their primary green function — will benefit rather than harm plant health.

It should be noted that even very small lots can be designed with a variation of this technique by using perimeter bioswales and designing the lot so that water is directed towards the perimeter.

Bioswales

A bioswale (or vegetated swale) is a form of bioretention used to partially treat stormwater, attenuate flooding and convey water away from some location towards a larger collection or towards a stormwater system. They are long landscaped shallow depressions designed to remove silt and pollution from surface runoff water and are characterized by having gently slopes on either side of the central drainage course. Swales are designed to maximize the amount of time runoff water spends in the swale slowly moving along its very gently inclined flow path and in so doing helping to trap pollutants and silt. The bioswale is landscaped in a regionally appropriate manner and can be filled with various types of vegetation as well with crushed rock or pebbles (especially in the lowest parts). Sometimes construction rubble or riprap can be used in parts that need substantial armoring to protect from runoff.

Instead of raised medians between parking strips as was common practice more and more lots are opting instead to use long narrow shallow bioswales to form a first stage in collecting and filtering runoff. They can be landscaped and planted with trees and other plants. Deep rooting plants are preferable for use in bioswales over shallow rooted plants.

Rain Gardens and Designed Wetlands

In addition to swales, which can either be thin shallow channels between parking strips or more substantial and deeper perimeter swales designed for a greater water overflow containment capacity other types of bioretention landscape elements exist. These are rain gardens and designed wetlands. While the parking strip bioswales, which are characterized by being long and relatively narrow and by having the function of channeling water to some drain point, rain gardens and designed wetlands are the final bioretention elements in the system. Water is directed towards these bioretention areas and they act as the focal points that receive the excess runoff that the bioswales cannot contain and infiltrate. Deep perimeter bioswales also fulfill this same function.

Rain gardens, designed wetlands and deep perimeter bioswales compliment the network of thin shallow bioswales arrayed throughout the parking lot along parking strips. They absorb and process the excess water that the network of bioswales channels or directs towards them. These additional areas can be larger and deeper perimeter bioswales designed to hold large amounts of water precipitated out from a storm. Or they may consist of rain gardens, which are landscaped shallow depressions, similar to swales except that they are not designed to canalize and channel water, but rather form the locus points of collection for water.

Sometimes both rain gardens and deep perimeter swales act as designed wetlands – in climates that either have a high water table and lots of precipitation or in areas prone to flash flooding from large thunderstorms. In these cases they may incorporate semi-permanent standing – or ponded – water into their design along with aquatic plants such as rushes that are typical of wetlands. These urban wetlands can also provide tiny islands of critical habitat for migratory aquatic birds and endangered wetland native plant and animal species.

These additional bioretention elements work with bioswales to filter, clean, absorb (through infiltration) and temporarily hold back runoff.

I Will Never Look at a Parking Lot in the Same Way I Did Before

Hopefully after reading this series of articles one will never look at a parking lot in quite the same way as before. I know I don’t and researching this article series has given me a new appreciation of the green design discipline that uses carefully sculpted bioretention landscape elements incorporated into the design as well as possibly using alternate permeable paving materials, to green what many including myself thought was something that was un-greenable (if I may coin that term).

I realize that it is hard to get excited about parking lot design, but because so much of our urban space is comprised of parking lots, creating greener parking lots can have a major impact in helping to make our urban spaces both more livable and less harmful to the environment. This third article concludes our three part parking lot series.

© 2009, Chris de Morsella. All rights reserved. Do not republish.

This, the second article in our three part series on green parking lots continues the discussion by looking at some of the green parking lot design techniques and materials and how they help make parking surfaces more environmentally friendly and improve the urban space most of us live in.

In case you missed the first article in our series it can be found at: Green Parking Lots: Part I – The Many Problems with Paving.

Before continuing with a discussion of the various design techniques and materials for building green parking lots it bears mentioning that the whole idea of parking lots itself and of the car culture that they are artifacts of is being called into question by many advocates of a new sustainable urbanity. Our current car centric urbanity is clearly unsustainable and ill suited for a future of increasingly expensive energy. The vast flat top parking lots and strip mall road facing lots that are in some way emblematic of the car cultures impact on the urban space may begin to shrink and even in many cases vanish as cities re-orient themselves away from the car and serving the car’s needs. Many, myself included argue that this will be inevitable driven by the inexorable decline in recoverable petroleum resources and with it the end of the age of easy energy.

However even without accepting this argument that the decline of our car centric culture is inevitable and imminent one can argue that in all too many instances parking areas are way over built and are built with each single space being too large. Reducing both the number of parking spaces and the average size of each single space is perhaps the single most important thing that can be done to make parking lots as green as they can be. These are techniques that apply to both multilevel parking facilities as well as open air parking facilities.

Another practice that can reduce the need for parking is to explore the possibility for shared parking arrangements could include usage of the same parking lot by different facilities that need it at different times. For example consider pairing an office space that experiences peak parking demand during the weekday with a church that experiences parking demands during the weekends and evenings.

Maybe Parking is a Case of Less Is More

In malls across the country parking areas are designed to have the capacity to accommodate maximum predicted traffic – such as occurs in the weeks leading up to the Christmas Holiday rush. In fact, look at just about any big mall anywhere, at any other time than the busiest days of the shopping year and you will see large sections of nearly empty lot, dotted with a lonely car or two. Is this over building really necessary and are there better ways to provide for overflow capacity than needlessly paving over vast areas of land with empty asphalt. A similar dynamic is at work at arena and event areas. Most of the time, the associated lots are mostly empty. For example a stadium or arena will often be somewhat empty during off season events or poorly attended regular season games; while on some occasions it will become packed to overflowing capacity.

For these types of situations if overflow parking is deemed necessary, for commercial or other reasons then at least it should be looked at differently and designed very differently from the much smaller core parking areas.

When it Comes to Parking Maybe Smaller Really Is Better

Another green parking lot technique is to minimize the dimensions of the parking spaces themselves in this manner squeezing more parking stalls into a given area. This can be accomplished by reducing both the length and width of the parking stall. Parking lot dimensions can be further reduced if a greater number of smaller compact spaces are provided. Not every vehicle is an SUV or oversized pickup truck and as the cost of gas continues its long term rise and concerns about greenhouse gas emissions grow more and more of the vehicles in the total mix of vehicles will be smaller compact cars.

It should be noted that parking stall size as well as the number of stalls required per unit of occupancy or square footage is often regulated by city codes and these codes themselves are need to evolve to better reflect the emerging green urban design that is our cities best hope at preserving their vitality and avoiding decay.

Preventing Stormwater Runoff Can Save Money

One of the principle ways open air parking lots can be made greener is by using permeable pavers instead of the impermeable surfaces, such as asphalt or concrete that is the current common practice. Consider replacing conventional asphalt or concrete in both new developments and redevelopment projects with alternate pavers. Using permeable paving surfaces helps to reduce storm water runoff by allowing rain to permeate the paving surface and to slowly become absorbed by the ground underneath where it is slowly filtered and released cleansed into the aquifers and water systems.

Preventing storm runoff is not only good for the environment, but can be good for the bottom line as well. An example is cited in the Land Development Today magazine, April, 2006 issue, for a 12 acre development project in Maryland with a 7 acre parking lot which initially was required to provide a 1.5 acre stormwater retention basin. Through the use of pervious concrete to absorb rainfall and manage stormwater, the need for the stormwater retention basin was completely eliminated saving the developer $400,000 relating to the cost of land and construction of the retention basin.

Cool Paving Fights the Urban Heat Island

Grass paving blocks, but also to some extent other permeable paving surfaces have the additional benefit of helping to mitigate the urban heat island effect. In summer asphalt especially can become as much as 50 to 90°F (27 to 50°C) hotter than the air. Choosing a cool paving material that is light colored and is permeable allowing for some air movement and evaporation through it can help fight this urban ill. Grass pavers are especially good for this because they further cool the ambient air through the process of evapotranspiration, in which the grass in this case releases water to the surrounding air that has been drawn up through its root system, dis¬sipating ambient heat. Even if grass pavers cannot be used for an entire lot using them in seldom used overflow parking areas can significantly reduce the parking lots overall contribution to the urban heat island.

Remember that demand for air conditioning increases by approximately 1.5-2% for each 1 degree (F) increase in summertime temperatures.

Alternate Pavers To the Rescue

Alternative pavers are permeable surfaces that can replace asphalt and concrete and can be used for driveways, parking lots, alleys and walkways. Alternate pavers can be broken down into three broad categories, which are: paving blocks, other surfaces and engineered porous pavement.

Grass or permeable pavers are interlocking concrete blocks or synthetic fibrous grids (usually a rigid plastic) with small open areas that allow grass to grow within the voids; sometimes these voids are filled with gravel instead. They provide a rigid surface able to support a vehicle weight while gravel or grass planted inside the holes allows for infiltration. Depending on the projected use and on soil types, a gravel or crushed rock layer can also be added underneath to prevent settling and allow for further infiltration.

Some examples of other non-block paving surfaces are: gravel, cobbles, paving stones, wood mulch, brick and natural stone. Brick and natural stone arranged in a loose configuration allow for some infiltration through the gaps and along with gravel and cobbles can be used as a driveway, alley way, or parking stall material. Wood and mulch can be used to provide walking trails.

The EPA defines Porous pavement as a permeable pavement surface – usually understood to be made of porous asphalt or pervious concrete. Most often porous pavement is engineered to have an underlying stone reservoir that can temporarily store surface runoff before it gradually infiltrates into the subsoil, thus allowing stormwater to infiltrate directly into the soil below and receive water quality treatment.

While porous asphalt and pervious concrete appear to be the same as traditional pavement, unlike traditional pavement, it contains little or no “fine” materials. Instead, it contains many micro voids that encourage infiltration. Porous asphalt pavement consists of an open-graded coarse aggregate, bonded together by asphalt cement, with sufficient interconnected voids to make it highly permeable to water. Pervious concrete typically consists of specially formulated mixtures of Portland cement, uniform, open-graded coarse aggregate, and water. Pervious concrete has enough void space to allow rapid percolation of liquids through the pavement.
Alternative pavers are not recommended for high-traffic volumes. Access for wheelchairs is limited with alternative pavers. In addition, snow removal is difficult since plows cannot be used, sand can cause the system to clog, and salt can be a potential pollutant. However even in regions that need regular plowing during the winter overflow areas on the far edges of parking lots can be constructed using alternate pavers and simply not be plowed. Snow piles can also be collected in these areas avoiding the need to remove it.

Next; How Landscaping Can Help

In the next and last post in this three part series we will examine the various landscaping techniques that can help make parking lots become much greener than they are. Look for: Green Parking Lots: Part III – How Landscaping Can Manage Runoff scheduled for publishing tomorrow.

© 2009, Chris de Morsella. All rights reserved. Do not republish.