Sept2012_3_Layout 1 12-09-20 12:34 PM Page 18
Stormwater Management
Simple and efficient multi-stage stormwater flow control By Jeremy Langburt, Gianfranco Maragno, and John Cigana anaging stormwater flow for a range of events with one control device is not always possible or efficient, yet several municipalities require flow regulation for five to 100-year storm events. Conventional design approach A single static flow regulator used to throttle the outlet of a retention basin will meet the design point for a specific storm frequency. However, it will typically release less than the maximum flow permitted in a more intense storm. This approach is inefficient as it forces designers to oversize their retention basins to retain extra water and increases emptying time. To meet multiple set points, a single flow regulator must have moving parts, which increases the capital and maintenance costs associated with the flow control system. Additionally, using electricity to operate the moving parts increases the chances of system failure, since it may not be available during an intense rain storm. With both static and mobile flow regulators, it is nearly impossible to select a single flow regulator that will accurately control water flows from two year storms up to 100-year storms. In order to maintain precision, it is obvious that multiple flow regulators are required. The Hydrovex Multi-Stage approach John Meunier Inc. has developed a design philosophy for retention basin optimization that only employs multiple flow regulators that do not require electricity, or moving parts to operate. The HydrovexÂŽ Multi-Stage is a cost-effec-
M
tive, precise, and low maintenance solution for stormwater flow control over the entire range of storm intensities. In this design, the inlet for each regulator is placed at a discrete height, corresponding to a specific storm intensity, so that it only begins passing and throttling the flow when necessary. This configuration ensures precision in flow control since each regulator is operating in its intended range. This approach can be used in concrete retention basins, as well as both dry and wet ponds. To regulate flow for minor events, like two and five year storms, a stormwater vortex flow regulator like the Hydrovex VHV flow regulator is proposed. The inlet is placed below the pipe invert for floatables control. This equipment uses the vortex principle to throttle stormwater flows. The main advantage is its ability to offer large openings in the flow path and thereby reducing the chances of the regulator being blocked by solids found in the water. A vortex regulator typically has an opening size four to six times larger than an orifice plate selected for the same design point. In addition to blockage mitigation, the Hydrovex VHV guarantees a 5% margin of error in its flow curve over a complete head range. For medium sized events, such as 10 and 25 year storms, the Hydrovex Pond vortex flow regulator may be used. This regulator uses the same vortex principal as the VHV, but its inlet is elevated above the unit. The inlet pipe acts as an overflow weir, in that there is no flow through the unit until the water level in the basin is above the inlet height. Weir height may
Event
Allowable Discharge Rate (L/s)
5 year 25 year 100 year
9 54 277
Table 1: Sample design criteria for a stormwater flow control system.
be adjusted on site thanks to the Pond’s telescopic arm assembly. This selection is ideal for midrange flows, because it offers the precision and large openings of a vortex regulator, the level control of an overflow weir, and the flexibility to adjust the system in the future. Finally, for very large flow rates, notably 50 and 100 year storms, the recommendation is to use a Hydrovex FluidGate (FG). Since the flow rates for these events are much larger, there is no need for the large-orifice advantage of a vortex regulator for blockage mitigation. The FG uses the orifice-flow principle to throttle stormwater flow. One advantage of the FluidGate is that the opening area can be adjusted on site, in case of future revisions to design. There are dozens of options available for combining different individual Hydrovex wet weather technologies for an optimized flow control system. Case study: Wet pond design To illustrate the operation of one Hydrovex Multi-Stage configuration, a wet pond was designed following standards and regulations for stormwater management in a Canadian province. For the
Storm intensity
Water Elevation (m)
VHV Vortex Regulator (L/s)
Pond Vortex Regulator (L/s)
FluidGate Regulator (L/s)
Total flow leaving basin (L/s)
Wet retention 5 year 25 year 100 year
141.160 141.800 142.150 143.100
0 9 11.6 17.5
0 0 42.4 58.9
0 0 0 200.6
0 9 54 277
Table 2: Sample arrangement for a three-staged flow regulation system in a wet pond. 18 | September 2012
Environmental Science & Engineering Magazine