Fierce debate continues over the cause and impact of climate change, but few deny that parts of the country are experiencing fiercer weather and heavier bouts of damaging stormwater. Building codes and regulations are becoming increasingly strict about managing stormwater on site to avoid any runoff that overloads collection systems and pollutes the environment.
Facility managers should assess their property’s risk of stormwater flooding. The assessment should evaluate not only probable volume but velocity, as the force of stormwater can cause its own damage.
FMs should also investigate any stormwater utility fees that may apply to their facility. Typically they are calculated on the impervious area of a property, including roofs, parking lots, roads and driveways. Retrofitting these areas as part of a stormwater strategy minimizes the fees.
Think about Permeability
Look up to your roof as one possible stormwater solution. For existing buildings, a modular vegetated roof may fit the bill.
Modular systems consist of interlocking trays with pre-grown vegetation. They are mobile and able to be carried to the roof for
easy installation. Individual trays can be replaced without disturbing the entire installation if vegetation is not growing well. They can also be removed if roof repairs are needed.
A modular vegetated roof is particularly well suited to existing buildings because of its comparatively light weight. Jennifer Cooper, Landscape Architect in the San Francisco office of Perkins+Will, explains that an engineer should be consulted to determine roof load capacity. The weight of a modular roof with 4 inches of topsoil is 32-35 pounds per square foot and costs $20 to $30 per square foot.
Cooper recommends tray systems with water reservoirs because these have capacity beyond that of the soil itself. The trays hold an inch or so of additional water, reducing water runoff and promoting deeply rooted plants because the roots grow down to access the reservoir water. For rain storms that are high frequency but low volume, the tray system’s soil and water reservoirs can often hold or absorb all of the water without runoff.
Given that paved and pavered ground surfaces account for a large area of many properties, they also play an important role in a stormwater strategy. The joints in between permeable pavers allow stormwater to pass around individual pavers and into the ground. Pervious block pavers are interlocking grids of blocks with cells or voids through which water can flow. The voids can be seeded with grass. Porous pavement is a mix of concrete or asphalt with pores that allow stormwater to flow through the material. Each of these options has varying properties, costs and adaptability to specific applications. One factor to consider is the volume of vehicle traffic expected.
Rain gardens, also known as biorentention cells, are a suitable and flexib le solution for retrofits. These strips or trenches are engineered natural treatment systems consisting of a recessed landscape area with a specialized soil mixture, an aggregate base, an underdrain and appropriate plants that tolerate a given region’s wet and dry seasons. They can be used as parking lotislands, road medians and edges along buildings and paved areas.
Because they are designed to drain within 24 hours, rain gardens avoid standing water, explains the Whole Building Design Guide (WBDG) website (wbdc.org) of the National Institute of Building Sciences. The cost for rain gardens on commercial, industrial and institutional sites ranges from $10 to $40 per square foot.
While an appropriate choice of plantings depends on local weather, Cooper says the design of rain gardens should strive to have aesthetic appeal in all seasons.
While the terms rain garden and bioswale are often interchangeable, vegetated swales are typically larger, deeper and more linear than rain gardens. They are designed to handle specific volumes of runoff from large impervious areas while providing variable filtering and infiltration capabilities. They intercept the first runoff until their soil is saturated, at which point swales act like a channel that directs further runoff to bioretention areas. According to WBDG, the design and cost of swales is roughly 50 cents per square foot.
Seattle is building large swales along two city streets to treat and divert runoff from a 430-acre area in its Capitol Hill district. The swales will prevent runoff that would otherwise flow untreated into the stormwater collection system and Lake Union. The project is funded by a public-private partnership between Seattle Public Utilities and developer Vulcan Real Estate.
Stormwater that exceeds the swales’ capacity is directed to a swirl separator, a cylinder that creates an eddy that forces water through a separation screen that catches sediment and pollutants. The water is then returned to the swales, where any overflow goes immediately into the stormwater main pipe. Water in the swales passes over a series of weirs that keep the flow even
and on a slight decline through the plantings. At the end of each swale, water flows into the stormwater main pipe.
According to Brandon Morgan, Senior Development Manager at Vulcan Real Estate, the swale project may be the largest of its type in an urban area. When completed, the annual capacity will be 190 million gallons, equivalent to the top 3 feet of water in Lake Union. Thus far the capacity of the swales has exceeded expectations and the utility plans to increase the stormwater flow to them.
A variety of stormwater strategies was used at the Education Innovation Instruction Facility at the University of Hawaii’s Leeward Community College. Sustainability consultant Paladino and Company led an eco-charrette to develop comprehensive building and landscape solutions. Campus landscaping includes bioretention, smart irrigation, a green roof, rain gardens and rainwater harvesting for irrigation. All runoff water is treated and filtered on site through bioswales and irrigation. Building gutters direct rain water to infiltrating rock beds and biorentia or to bioswales. During peak rainfall periods, stormwater enters a campus cistern.