by Robert Y.G. Andoh
In most cases, the process of managing stormwater runoff in urban environments can be summed up in a phrase: Move it along.
Society builds its drainage systems to take stormwater away from catchments as quickly as possible. We design our systems with enough hydraulic capacity to handle peak runoff flows, and we send those flows downstream toward local watercourses. In most urban areas, we add treatment processes downstream, but usually not until we collect the water and get it out of sight.
This is at odds with the way nature intended for water to be managed. Before society added so many roads, parking lots and buildings, 80 percent of the rain that fell would soak into the ground and recharge our aquifers. Now, in urban settings, 80 percent of the rain ends up in catchments as runoff. The ground and habitat once served as a natural “brake” to hold back the water, cleanse it and return it to the environment to complete the water cycle. Now, the heavier flows that are carried downstream distribute a wide range of pollutants including sediments, oils and trash.
C–type vortex valve on an open stormwater flow channel.
Some modern stormwater management techniques have tried to mimic nature's braking systems. They aim to intercept, infiltrate, contain, control and treat wet–weather flows before they cause hydraulic and water quality problems downstream. Flow control is a critical component in integrated stormwater management. And vortex valves have proven to be an effective means of controlling flows.
Designed with a snail or conical shape, vortex valves act like natural hydraulic brakes. High flows initiate a vortex within the valve, which in turn restricts the flow of water out of the device. When head pressure builds, water circulates in a vortex pattern, allowing an air core to form within the device, preventing excess amounts of water from entering conveyance systems such as combined sewers and other collection systems.
Under low–flow conditions, the valve acts as a large orifice where water and debris pass directly from the inlet to the outlet. As flow increases and reaches the flush flow point, high peripheral velocities start to throttle the flow. As pressure increases, an air core, accompanied by substantial backpressure, effectively restricts the flow through the outlet aperture. Attenuated flow can be temporarily stored within the collection system or in underground tanks and/or tunnels, surface ponds or on the street for slow release into the sewer system, thereby preventing flows from racing through with little or no treatment.
Environmental problems caused by increased flows aren't limited to the deposition of pollutants. Heavy flows cause erosion of pipes and land masses downstream. Pollutants captured in treatment sites downstream can get resuspended and deposited in local waterways. Runoff can also raise the temperature of streams, which can increase turbulence and impact sensitive microorganisms. There have even been cases where trout living in polluted water sources lose the ability to swim upstream.
Just like beaver dams along a river, vortex valves slow down the flow of stormwater through our sewer systems. This gives the drainage systems — like the rivers — a chance to recharge and treat stormwater early on rather than let it rush toward end–point treatment solutions.
Deploying these natural hydraulic brakes in sewer systems represents one important move society can make to counter the negative effects of urbanization on our water resources. What's needed, however, is a new paradigm – one that focuses more on prevention than on cures. Rather than shift water problems downstream, city leaders must take steps to deal with issues early on and prevent them from compounding. The philosophy must be to create cost–effective, innovative solutions upstream that are sustainable and integrated into our environment.
Although vortex valves are part of that emerging new paradigm, they are not brand new. In the 1980s, a British Broadcasting Corp. (BBC) documentary called “The Tomorrow's World” foreshadowed the impact vortex valves could make on society. The program discussed the potential role vortex valve controls would play in effective flow control and regulation, coupled with storage and infiltration systems, for flood control and mitigation of adverse wet–weather impacts. Back then the technology was seen as revolutionary. Today, that promise is being fulfilled.
C–type vortex valve on an open stormwater flow channel.
Hydro International, for example, has installed more than 17,000 vortex valves worldwide, and others have made their mark as well. The use of vortex valves has been widespread in Europe, and especially in the United Kingdom, where the comparatively high levels of urban runoff have led governments to invest more in flow–control equipment than in treatment devices. But the use of vortex valves has started to take hold in North America, as well.
In Ottawa, Canada, city leaders are installing vortex valves as inlet controls to regulate runoff that enters the combined sewer system during intense storms. In essence, the devices are preventing excess wet–weather flows from entering the combined sewer network and thereby preventing an overflow further downstream. The city is currently working on a $25 million (CAN) plan that will eliminate most of the overflows. Phases I and II of the plan, to be implemented over the next two years, incorporate approximately 1,000 vortex valve units in catch basins around the city to prevent excess stormwater from entering the sewer system and causing overflows.
In Evanston, IL, vortex valves played a key role in resolving a case of flooded basements. Increased runoff was overloading the city's combined sewers up to six times per year. A study indicated that the traditional solution of relief sewers/sewer replacement would cost $290 million and cause major disruption affecting up to 90 percent of the city's streets. The city adopted a plan involving partial sewer separation with aboveground storage and overland flow and vortex valves installed in catch basins to limit the inflow to the hydraulic capacity of the existing system. This alternative cut the cost of the overall solution by more than half.
On the other side of the Atlantic, vortex valves are being used on the city of Glasgow, Scotland's $100 million White Cart Water Floor Prevention Scheme. The scheme is designed to protect 1,750 homes and businesses from flooding as part of a major dam project. By using vortex valves as opposed to traditional flow controls such as orifice plates and penstocks, this allowed project leaders to allot for smaller water retention areas on sites with little available land. This will result in fewer land takings.
Vortex valves are versatile structures that help provide a high degree of flexibility in the design of stormwater management systems. Rather than send flows downstream to be dealt with later, vortex valves regulate flows and allow drainage systems to distribute them in a more natural manner. To solve future water issues in the most economical, efficient manner possible, society will have to adopt integrated water management plans that prevent problems before they occur. Vortex valves are a good illustration of how those plans can succeed.
Prof. Robert Y.G. Andoh is Director of Innovation for Hydro International (Portland, ME), overseeing product research and development, technical support, intellectual property rights and IT.
