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.
