By Robert Y.G. Andoh
The Chesapeake Bay in Maryland is North America’s largest and most biologically diverse estuary, home to more than 3,600 species of plants, fish and animals. Unfortunately, this watershed is now in peril. Shellfish stocks, including oyster and crab, are at an all-time low, and even the health of striped bass, which has made a comeback in recent years, is now in question.
Stormwater runoff has been identified as a major contributor to the bay’s environmental issues. It carries contaminants including sediment, nutrients and thermal pollution into the Chesapeake waters. In fact, stormwater pollution is responsible for 11% of the nitrogen and 15% of the phosphorous that enter the Chesapeake.
To combat the problems associated with runoff contamination, the Maryland Department of Environmental Protection and Montgomery County enacted some of the strictest stormwater management regulations in the nation. The stormwater management code requires that for developments in Class 1 watersheds, such as the Chesapeake, a predetermined Water Quality Volume of stormwater runoff generated by development must be stored on site in a stormwater storage system to prevent flooding downstream from the developed site and to allow stormwater runoff to cool down substantially before it is discharged into the waterway where warm water would promote algae growth and fish kills. Montgomery County’s Channel Protection Storage Volume requirement helps prevent stormwater runoff from developed sites from causing harmful erosion to stream channels.
In addition to stormwater storage volume requirements, Montgomery County requires a 2" minimum aperture for orifice plates used as the outlet fixture for stormwater storage systems. Because stormwater runoff contains trash and other debris, any opening smaller than 2" is at risk of being blocked by debris, thereby causing upstream flooding. To meet the minimum orifice requirement while meeting the required drain down from the storage system, engineers frequently reduce the driving head that controls the discharge rate from the orifice outlet by increasing the storage footprint.
Don’t Break the Bank
When Chevy Chase Bank of Bethesda, MD, set out to open a new branch in nearby Gaithersburg, it needed to consider each of the above requirements to comply with the state and county stormwater regulations. The bank tapped Loiederman Soltesz Associates, a civil engineering firm in Rockville, MD, to design the stormwater site plan for the new bank branch.
The engineers at Loiederman Soltesz Associates opted to use an underground stormwater storage system that would meet the Channel Protection discharge rate and store the Water Quality Volume on site for a requisite time period. To meet the required detention time, the engineers knew they would need to use the smallest allowable outlet flow control – a 2" diameter orifice plate. However, hydraulic modeling revealed that the 2" diameter orifice would start over-discharging at moderate levels of driving head. This posed a problem for the
engineering team.
One of the most common methods for alleviating stormwater runoff is the use of retention ponds. However, in urban settings where land value is at a premium, this option is often cost-prohibitive, and it restricts the footprint of the building and associated parking areas.
The footprint of a shallow, arched-chamber storage system would not fit within the space constraints of the site. But taller storage structures, such as precast concrete chambers, were too deep, providing too much driving head on the 2" orifice to meet the maximum discharge requirement.
To resolve this issue, the engineers designed a precast concrete storage vault to spread the Water Quality Volume out over a larger area and lower the driving head to meet the discharge requirement. This resulted in a vault with 29,160 cubic feet of storage volume – approximately 42% more than was needed.
The problem, though, was that the system was oversized and had spare capacity. For a highly intense storm, the excess capacity of the storage volume would fill, increasing the driving head on the orifice, and the system would over discharge. Additionally, the developer would have incurred high construction costs associated with the oversized storage system.
The firm then evaluated Hydro International’s Reg-U-Flo® Vortex Valve, a vortex flow control for limiting the discharge rate of stormwater exiting a stormwater storage system.
Designed with a conical shape, the Reg-U-Flo valve can restrict the flow of water as needed without moving parts or power requirements. It allows for higher discharge rates at lower heads than conventional flow control options. It operates on simple fluid hydraulics. 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, an aerated core, accompanied by substantial backpressure, chokes the flow through the outlet aperture.
The attenuated flow can be temporarily stored in a smaller underground tank for slow release into the sewer system. This ensures that the rate of inflow will not exceed the hydraulic capacity of rated downstream discharge flow or the sewer system. The system passes more flow during smaller storms and throttles the flow back during larger storms.
Taking advantage of the valve’s characteristics, the engineers at Loiederman redesigned the storage system using a 3.375" SXH type Reg-U-Flo Vortex Valve that was able to limit the discharge rate with the entire storage system full, eliminating 100% of the excess storage volume required with the 2" orifice control.
Matt Baxter of Humphrey Rich Construction, the contracting firm on the project, reported that, “The Reg-U-Flo cut back on [installation] time and money.” By reducing the volume of the tank and reducing associated construction labor costs, the vortex valve saved over $150,000 on the project.
In addition, with the new outlet control, the smallest dimension of the flow control opening was expanded from 2" to 3.375" resulting in a 350% increase in the area of the opening. This dimension easily met the county’s 2" minimum dimension requirement for an outlet flow control and offered more protection against blockage than the equivalent orifice.
The Reg-U-Flo Vortex Valve proved to be an effective method of stormwater control, allowing for a storage system with a smaller footprint than a traditional orifice plate flow control solution. Chevy Chase Bank will stand as a model to other communities looking to maximize the economy of development while safeguarding precious waterways.
About the Author:
Robert Y.G. Andoh is the Director of Innovation for Hydro International, overseeing product research and development, technical support, intellectual property rights, marketing and IT. He joined Hydro in 1992 and has more than 25 years of experience in the water industry. Andoh has authored and presented a significant volume of technical papers relating to urban flooding, collection systems and wastewater treatment processes, contributing extensively, on both a national and international basis, at communication events in the field of urban water management.
