Pretty And Purposeful
By John Paliga
Through cross-departmental coordination, a thoughtful design process, and visionary engineering, a former concrete retention pond has been transformed into the Morse Park Rain Garden—a xeriscape garden amenity that has reduced runoff, water use, and maintenance—all while improving public safety and water quality.
In 2011, the city of Lakewood, Colo., funded this project through a Neighborhood Participation Program grant. The grant program awards funding of up to $50,000 for site-improvement projects submitted by residents, supported by neighbors, recommended by a committee of interdepartmental staff, and approved by city council. After an extensive evaluation of local and regional stormwater-quality facilities and a feasibility analysis by an experienced civil engineer, the project was approved for implementation. The primary goals in the design of the rain garden were functionality and sustainability. Design and management of the project was assigned to a staff landscape architect who was challenged with the following tasks:
Managing stormwater to improve water-quality and prevent flooding.
The Neighborhood Cesspool
The concrete detention basin was installed by the Public Works department in 1985 in response to localized basement flooding in adjacent homes and recent curb and gutter installations on adjacent streets. Water flowed to the basin from parking lot and tennis court pavements via concrete drain pans. Street runoff was directly piped into the basin from an adjacent curb inlet. The basin had no gravity outflow, which required all water entering the concrete pool to be pumped uphill to an outfall in a roadside ditch. A spray fountain was added to the basin in 1992 for water aeration and aesthetics. Eventually, the fountain pump failed, as did the automation on the pond pump, requiring manual activation after each storm event. The pond water was stagnant and became a health concern due to the West Nile virus and e-coli contamination, a safety hazard for children from the adjacent playground and a graveyard for tennis balls from the nearby courts. Parks staff was directed to keep the water at a low level to reduce risk, increasing the unsightliness of the neighborhood “cesspool” (according to a neighbor comment). For maintenance, park staff was relegated to fully draining the basin with pumps, squeegeeing the sediment into piles, and removing the muck manually twice per month during the growing season.
The design team initiated the process with additional research, site visits, and soil investigations. A planting palette was developed to select low-water native and adapted shrubs and perennials that could withstand periodic inundation and sandy soil conditions, while being low-maintenance and non-invasive. Several rain garden features were visited or researched to determine both plant success and engineering design techniques. The civil engineer performed a soil study to ascertain the best mix of topsoil, sand, and soil amendments that would both support adequate root growth and allow stormwater to filter through the system. One of the key findings of the study is that root growth and its associated soil microbiota are crucial to the development of air voids in the soil profile that allow water to infiltrate.
The research phase also illustrated one major cause of rain garden failure: sedimentation. The transport of road debris, organics such as leaves and grass clippings, and soil erosion lead to deposition in the lowest part of the site, causing premature clogging and failure of the system. The design team focused on ways to minimize the entry of debris into the garden and to capture sediment where it can be easily removed. Three stages of water filtration were designed to capture sediment prior to storm flows entering the garden to prevent clogging, reduce maintenance, and extend the life of the sand-filter.
1. At two parking areas where water is collected and concentrated for conveyance to the basin, concrete forebays were installed to slow the release and drop sediment loads prior to release from the pavement. These forebays were designed with a large inside radius that allows for debris removal by a street-sweeping truck.
2. Downstream from the parking forebays, concrete drain pans were replaced with grass-infiltration swales to reduce runoff velocity and increase groundwater infiltration. These swales were trenched to 3-foot depth and filled with filter sand and a perforated pipe to transport water during saturated soil conditions.
3. At the two points where runoff enters the garden, additional concrete forebays were installed as a secondary barrier to sediment entering the garden. These features were designed with low, rock retaining walls that double as seat walls, and colored concrete to improve aesthetics. The flat concrete floor of the forebays facilitates debris removal. The east forebay collects outfall from the adjacent street curb inlet. Many storm inlet sediment filters involve strainer baskets that catch debris as it inters the catch basin. In lieu of installing the maintenance-intensive strainer baskets, our retrofit of the street inlet involved deepening the basin to increase the volume of sediment collected and the installation of a “snout” over the outlet pipe. The snout helps prevent floating debris from entering the outlet pipe to the garden, and the deep basin lowers the maintenance interval for vacuum truck sediment removal from the basin.
A final sediment-catchment technique was installed immediately adjacent to the lower garden forebays with the goal of catching and hiding the debris that makes it into the garden. These circular plant beds feature large cobble mulch with tightly-spaced ornamental bunch grasses. The grasses help to screen the debris from view for most of the year until cut-back in the spring.
Topping It Off
The garden planting bed is a raised berm in the center of the curvilinear basin. Below the garden is a 12-inch-thick sand filter with perforated underdrains that convey water to the adjacent wet well and pump system. The sand helps filter contaminants from the collected stormwater prior to discharge into the watershed. Above the sand filter, a custom rain garden planting mix was installed to form the berms. The mix consisted of 40-percent sandy-loam, screened topsoil, 60-percent athletic sand, granular humate, and slow-release organic fertilizer. This mix was formulated to provide for both plant needs and good infiltration. A dark-gray rock mulch covers the berms, the color selected to mimic the sediment deposited when the basin is inundated during major storm events.
The plant selections for the garden were based on rain garden research and on past experience with xeriscape plant material. A mixture of over 40 species of small shrubs and perennials were installed by volunteers at a neighborhood work day. The planted berm is irrigated with spray heads scheduled to run once per week. A soil-moisture sensor was installed to control the irrigation cycle.
The project was competitively bid and constructed in 2013 for a total cost of $140,000. Design recommendations include enlisting a civil engineer experienced in bioretention facilities, undertaking extensive research of all similar facilities in the region, and using good soil materials to reduce maintenance.
The Morse Park Rain Garden has been a successful project for the city and the neighborhood. The removal of the concrete basin and upgrades to the pump automation have substantially reduced park maintenance. The sand-infiltration zones have increased infiltration of precipitation into the ground—reducing off-site flows, and improving the water quality discharged from the site. And the garden beds are enjoyed year-round by neighbors and visiting park users.
John Paliga is a landscape architect for the city of Lakewood, Colo. Reach him at JohPal@lakewood.org.