About the author: Nathan P. Kass, P.E., P.L.S., is branch manager for Fehr Graham. Kass can be reached at [email protected] or 319.294.6909.
Nathan P. Kass
undefinedAn example of permeable pavement incorporated into a sitting area.
Today’s storm water management codes and ordinances often are an afterthought in site design—a necessary function to carry a project through construction permitting to lessen impacts downstream. Storm water best management practices (BMPs) typically are shoehorned into the space available after the buildings, parking lots, and other amenities are designed, and occupy the majority of the property. BMPs often are considered nuisances to developers who feel that they take up valuable real estate.
Meadowview Memory Care Village, an assisted living facility in Marion, Iowa, was facing similar BMP challenges with storm water management. Because the lowest point of the site was located near the western property line, meeting the storm water codes and ordinances with a single, large, centralized detention pond would not work adjacent to the proposed building. A finite budget required creativity and consideration of multiple smaller solutions, rather than going with the standard large detention pond.
Design Challenges
The ownership group, Views Properties, looked to Fehr Graham for storm water design assistance for the property. A civil engineering consulting firm with offices across the Upper Midwest, Fehr Graham integrates engineering and landscape design services to provide standard BMPs and creative solutions. The design team began by identifying options for storm water management for Views Properties, some of which had not been considered or were assumed to be too expensive.
Because a Frank Lloyd Wright-inspired prairie-style building was planned, this concept was to be carried across the whole campus. It was clear from the start that it would be difficult to fit a traditional detention pond in an effective location due to the natural terrain of the campus. In addition, there was no interest in any BMPs that favored function over form, making the aesthetics an important aspect of the project. Solutions needed to blend with the landscaping and overall theme of the site. To meld it all together would take creativity and problem solving, as well as working closely with the landscape architects at Fehr Graham.
A permeable pavement parking lot saved the team approximately $60,000 over a traditional paved parking lot, inlets and a storm sewer.
Compounding Challenges
The offsite runoff flows onto the site from the east and north, creating additional design challenges. Although the ordinance allowed flow to pass through the site undetained, the size of the building required access paths for parking and fire trucks around the perimeter, and vertical elevation change did not allow a stable channel to pass the flow from the east around the site. With the challenges compounding, the design team decided to break up the drainage into small sub-basins to make the storm water issues more manageable.
The owners were interested in making this a premier facility and incorporating green technology and practices where economically feasible. Permeable pavers were recommended for the main parking lot and traffic areas. The pavers allow precipitation to percolate through and into an underground storage chamber that doubles as the base for the surface. This also enabled the parking lot to be extremely flat, achieving compliance with the Americans with Disabilities Act (ADA) and providing accessibility for handicapped and limited mobility persons. One of the biggest advantages of using permeable pavers is the elimination of the need for a traditional storm sewer.
Budget concerns about using a non-standard pavement led the design team to perform a cost analysis, comparing permeable pavers without storm sewer to a traditional impermeable pavement of similar structure with standard storm sewer and surface inlets. The result was a cost savings of approximately $60,000 using permeable pavers. The elimination of the standard storm sewer system was more than offset even with a higher unit cost per area for permeable pavers compared to standard pavement; however, it was going to be difficult to capture all the required detention volume under the pavers without added expense.
The footprint of the building was conducive to installing multiple rain gardens, which are depressed pockets with specific vegetation to withstand short stints of saturated soils and encourage infiltration. The soils in rain gardens are engineered to further enhance drainage into native soils. The building's roof drains would outlet to these rain gardens, instead of traditional underdrain systems. However, channeling all the roof drains to the rain gardens was difficult without creating elevation problems for the multiple building entrances. Even though the rain gardens were a good addition, they were not enough on their own.
Creative Approach
Although nearing the detention volume required by the code, the design would still produce more runoff from increased hard surface than allowed. Options to reduce new hard surface and the amount of runoff, while still providing all the required fire truck loop, pedestrian paths, and building access points, were considered to complete the project. To meet these needs, a special lattice-patterned paver was included in the design for the fire truck loop. The design allows grass to grow and storm water to infiltrate. After investigating the infiltration rates that could be expected through these pavers, it was found that the materials would make a big impact on the volume of runoff while still supporting the weight of fire trucks.
The fire truck loop also doubles as a walking path for residents and visitors, but the grass grown between pavers would make it difficult for people in wheelchairs or using walkers to use as a walking path. The design team compromised, placing a narrower permeable paver designed for ADA compliance in the center of the path with other pavers outside of that for the fire truck wheel paths.
With the detention volume nearly covered, an existing easement for an overhead power line was limiting the use of a strip of land along the north property line. This provided an opportunity for a grass strip to channel the offsite drainage away from the other improvements, while enhancing the water quality before it left the site. The strip was terraced, providing pockets similar to rain gardens that would fill up and spill into the next cell. This created a stair-stepping flow, leading to a small, standard detention basin that accounts for any remaining detention requirements.
An example of a rain garden. Similar gardens are located throughout the grounds help enhance drainage into native soils.
Final Piece of the Puzzle
With careful selection of vegetation, the average citizen will not recognize that the BMPs were anything other than standard landscaping. Deep rooted, Iowa-native grasses and wildflowers will occupy the rain gardens and grass strips and reduce the required mowing to keep with the prairie theme. The standard detention pond is small and unobtrusive, neatly tucked in the corner without looking out of place. The bulk of the detention is hidden under the pavers, unseen by residents and visitors. It was an ideal blend of multiple BMPs to tackle the challenges of the site.
With ever-tightening regulations and ordinances and less land to accommodate required improvements, it is continually important for developers and owners to look at a storm water management strategy that employs multiple BMPs, instead of a one-size-fits-all approach. Having an open mind and willingness to explore options can yield significant cost savings when properly applied. Every site is unique, and the storm water solutions to each site should also be unique if it is to have the lowest impact to the environment and maximum impact to water quality and quantity downstream.
