What are the best Low Impact Development and Green Infrastructure techniques for roads?
December 12, 2018
December 12, 2018
Designing or retrofitting a road? Here are 6 LID and GI practices that can help improve stormwater runoff
In my previous blogs, I discussed Low Impact Development (LID) and Green Infrastructure (GI) and how they can be implemented in municipal and development settings. LID and GI are techniques that mitigate damage to our waterways from increased development. In this blog, let’s look at the best types of LID and GI for your road projects.
Road runoff contains sediment and other pollutants. If you are designing or retrofitting a road, you can use these techniques slow down, soak up, and clean stormwater runoff.
Bioretention is a process that removes sediment and pollutants from stormwater by filtering the stormwater using soils and plants. These facilities often look like gardens but are depressed to allow water to pond and soak in.
Bioretention for roads can take several forms. Roads can have bioretention planters with hard vertical walls in more urban areas or where roadway rights-of-way have tight space constraints. Bioretention can also take the form of curb extensions, or “bump-outs,” that extend into the roadway and have a curbed perimeter. Bioretention bump-outs can also serve as traffic calming measures or parking bay dividers.
Bioretention can also be designed within the boulevard area, especially in more residential areas with wider boulevards. These do not have hard sides like the other applications and can be tailored to residents’ tastes (grass vs. planted). All bioretention cells provide water quality treatment by filtering water though a soil media, but a small amount of pretreatment to settle out large sediments and garbage improves the operation and maintenance of these practices.
Swales, AKA simple grass channels (ditches), have long been used to convey stormwater, especially along roadways. While these ditches do provide some benefit, a few simple modifications can really improve the pollutant removal and runoff reduction.
Enhanced grass swales are essentially the same as ditches but have additional structures like check dams and longer vegetation to slow down the water flow to allow for sedimentation, filtration, evapotranspiration, and infiltration. Bioswales are one step up from enhanced swales and add in the same soil used in bioretention cells to the bottom of the channel and may also add a gravel storage layer. These facilities are essentially linear bioretention cells that also provide conveyance. In poorer soils, bioswale have perforated underdrains to ensure adequate drainage.
Also called exfiltration systems. These systems replace a traditional storm sewer system with perforated storm pipes over infiltration trenches (rectangular trenches filled with clear granular stone). The perforated pipes allow stormwater to “drop” in to the infiltration trench to allow the water to seep into the ground. The pipe also conveys excess storm water once the infiltration trench is full, just like a conventional sewer system. The perforated pipes are installed in gently sloping clear stone beds.
Adequate pretreatment is essential for this practice, as the system can clog from high sediment loads. Typically, these systems are preferred by municipalities as they can be implemented with less staff resources, are buried, and leave the streetscape as conventional curb and gutter.
Infiltration trenches and galleries are two terms that describe the same practice—a rectangular trench filled with clear granular stone. They may also be called “soakaways” or dry wells.
These facilities are underground and can be placed under any type of surface (vegetated, paved, sidewalks, or parking). Infiltration chambers, or infiltration tanks, are a design variation that usually uses manufactured modular chambers installed underground. These chambers are not filled with stone but have a clear area for water to be stored prior to infiltration. These can also be installed under a variety of surfaces and are often used in parking lots. Adequate pretreatment is essential for these practices, as the system can clog from high sediment loads.
With careful design, LID and GI can be used on roadways of any size.
These are practices that are available for purchase and are often a good choice for retrofits or urban areas that want to increase tree cover. Many modules are designed primarily for stormwater treatment and may not address quantity or water balance concerns, as they may not infiltrate water.
Precast tree planters are a concrete box for urban trees that provide water quality treatment for road runoff with no infiltration, as they have a concrete bottom. They can be dropped in place and are only offered in certain sizes from the manufacturer. They can provide the water quality component for downstream perforated pipe systems, the combination of the systems providing comprehensive stormwater management.
Soil support systems are primarily used to support tree growth in highly urban areas where soils are too compact for trees to thrive. These systems provide a large volume of soil for trees while supporting any infrastructure located above it, preventing settling. Stormwater can be directed to the soil cells to be filtered and evapotranspirated. Some infiltration may occur, depending on the native soils under the cells.
Permeable paving is a term that applies to several LID practices. Porous asphalt is very similar to conventional hot-mix asphalt, but has significantly less sand and fines, allowing water to filter though the material. Fines are very small soil particles (silts and clays) that fill up the spaces between larger soil particles, preventing water from flowing though. This practice needs appropriate underlying infrastructure and cannot be used as part of road resurfacing.
Pervious concrete is similar to a conventional concrete, again with less sand and fines, allowing for water to filter through the surface. Pervious concrete is rougher in texture than conventional concrete, which can provide additional traction to vehicles. Permeable pavers are usually made of concrete and have large joints that are filled with stones to allow for infiltration. None of these practices should be installed where sand is used for snow or ice treatment. Sand will block up the pores in the media. Pervious concrete and permeable pavers can also have reactions with salting products, depending on the concrete substrate (limestone or other) and the type of salt used (sodium chloride vs. calcium chloride) that may cause crumbling or spalling.
The permeability of these materials does mean that little to no salt needs to be used, as any meltwater is infiltrated immediately. All three types of permeable paving are best suited for road reconstruction or new construction. Currently, most municipalities are not using permeable paving for roadways due to maintenance concerns, but several have pilot projects with permeable parking lots or parking bays.
An engineering expert with experience in LID/GI can help you find the most effective technique for your roadway project. It’s important to remember that, whichever infiltration technique you chose, all road runoff must be treated prior to infiltration. If you use a vegetated practice like a rain garden or bioswale, the runoff will be treated by filtering though the soil in the facility. If you use an underground infiltration trench or chamber, the runoff needs to be treated using an oil/grit separator or a proprietary product designed for LID.
With careful design, LID and GI can be used on roadways of any size. These techniques will mitigate damage to our waterways caused by pipes carrying pollutant-laden stormwater directly to our lakes and rivers.