Most normally crowned roadways drain to the outside lanes, but an engineered water quality swale in the roadway median can add green infrastructure and create cost savings.
Not every project is a good fit for this option, but for a divided roadway with a gentle slope, the central median has the potential to be a tree-lined boulevard with drainage hidden beneath.
Water quality in urban runoff is becoming an important issue for cities. A water quality swale can reduce pollutant discharge by filtering the “first flush” or first inch of rain.
Typically ‘bar ditch’ swales are stationed on either side of the roadway. Still, a single central swale can be an economical solution because only half the number of stormwater inlets are needed to get water into the storm drain. The storm sewer trunk line can typically be reduced in size, and the water quality swale captures the initial runoff and drains it into the soil.
However, careful planning is required to make a project like this a success. Drainage in the central median starts with an inverted crown design, making it a good fit for new construction or any project that requires a complete grading, drainage, and surface design.
Water flows from the outside edge of the road to the center median in this design, where the water quality swale engineered in the center median filters water through aggregate and soils. The addition of native plants can help make it a true bioswale or a planting feature. Vegetative cover can reduce flow, so there is a crucial balance between infiltration provided by vegetation and maintenance required by certain plants and grasses.
A carefully designed and constructed center median with a swale design must allow stormwater to infiltrate the ground without undermining the roadway. Subgrade preparation is key. Attention must be paid to the type of soil and other media installed below the ground surface in the center section of the median. Typical soil materials may include a mix of sand, loam, compost, shredded mulch, and filter rock; it is best to avoid clay since it does not allow water to infiltrate very easily. A qualified landscape architect or civil engineer should design and specify the soil materials and other substructure of the center median to ensure that stormwater will adequately permeate into the ground. Construction contractors must pay special attention to the design details and material specifications called for by the design professional.
A recent project designed by Crafton Tull features bald cypress and water oak trees in a swale within the roadway median, watered by runoff. The surface of the swale itself is a cobblestone-lined base with grass-lined side slopes. Geotextile fabric separates the cobblestones from the engineered soil mix where filtration occurs. Because the native soils on the project site do not drain quickly, a 6-inch PVC underdrain with pea gravel and clean stone was constructed under the filter soil layer. This underdrain attaches to the storm sewer inlets spaced along the corridor. A check dam, in this case, a concrete curb from one side of the swale to the other, helps slow water so it can filter through the soil media and into the underdrain. Our client recognized the potential maintenance hindrance of these check dams, so we worked with their maintenance crews to design a check dam with sloped sides to ensure mowers could easily maintain the median. The addition of inlets within the median catch any overflow. This solution is both practical and beautifies the roadway while meeting stormwater improvement criteria.
A water quality swale is not geared to filter massive amounts of runoff during a storm event. In the case above the swale was designed to capture and filter the first 1-inch of rain. However, everyday volume control of stormwater can lower overall costs for a city by reducing nutrient pollution.
In the right context, an inverted crown and median swale can offer benefits of reduced infrastructure cost and improved water quality. While more traditional engineering solutions will always have their applications, designs tailored specifically to slope, percolation rate, and roadway size help create low impact design (LID) solutions to fit within the scope and budget of a project.