MORE TOPICS | Engineering & Construction
Stormwater Flow Control
Designing systems to meet maximum allowable discharge rates.
Kyle Perrin | Romtec Utilities
As stormwater regulations become more restrictive and site development continues to increase stormwater runoff, engineers are required to design pump systems that are increasingly more advanced. The newest generation of stormwater pump systems cannot only convey water; they must also precisely control discharge, protect downstream infrastructure and satisfy local permitting requirements.
Maximum allowable discharge rates vary widely and are enforced by local authorities, such as public works, water management districts or the department of ecology. These discharge rates are often tied to either matching the peak flow of the predeveloped site, having a fixed maximum per acre of development or having an allowable release rate with a designated runoff coefficient.
Historically, stormwater pump stations were often sized to handle a specific storm event, such as the 100-year storm. However, the traditional approach is no longer enough. Due to the increase in land development, focus on water quality and erratic stormwater conditions, water authorities are becoming stricter in their requirements and ordinances for site discharge rates.
Stormwater pump stations are required to be designed with the right balance of complexity, cost, operation and reliability. Even minor fluctuations across pump curves may now be rejected due to stringent regulations. The best solutions to meet all the site requirements combine mechanical, electrical and structural considerations and design philosophies into the pump system.
Mechanical Considerations
The first critical mechanical design decision is how the pumps are sized, selected and sequenced. In a traditional wastewater lift station, the design and selection of the pumps often begins with the worst-case condition assuming the highest head and friction losses. However, in stormwater pump selection, it is best to select the first pump to meet the lowest discharge condition, at the best-case scenario of lowest head and lowest friction losses.
This is done because the maximum discharge rate will be governed by when the system can convey the most water. During the pump selection stage, pumps may be added in a sequence configuration to meet the rates of larger storm events. This approach ensures the pump station can best match the range of inflows the site will experience, as opposed to relying on a single large pump that is only efficient during rare peak storm events.
The sequencing of smaller right-sized pumps improves overall flexibility for mid-high storm events by allowing the pump station to respond proportionally to the changing inflow conditions. This increases efficiency and minimizes the wear and operations and maintenance (O&M) requirements associated with oversized equipment. Additionally, in scenarios with extremely low discharge rates or where conventional pumps may still deliver more flow than regulations permit, the addition of diversion valves and recirculation systems can provide a practical solution.
These components redirect a portion of the pump water back into the wet well while only releasing the permitted amount from the site. Diversion valves and recirculation systems enable operators to dial in the pump discharge without the addition of overly complex controls, but it will affect the operational efficiency of the pumps.
| IMAGE 1: This stormwater lift station will serve a new mixed-use commercial development in Oregon. The system will be pumping from a detention pond to a creek and will only operate to empty the detention pond. (Images courtesy of Romtec)
Electrical Considerations
Pump hardware alone is not enough; the control and electrical system is equally vital in meeting the allowable discharge rates. Most commonly, the use of variable frequency drives (VFDs) is useful for allowing the pump speeds to adjust to the system conditions, instead of operating only at a fixed speed.
In stormwater applications, VFDs can be programmed to respond to a wide range of inputs. For simple systems, the VFDs tie specific elevation setpoints to fixed powers and pump speeds through level sensing. In more complex applications, the VFDs can increase pumping rate proportionally to the rate of inflow to the pump station. Whether the discharge rate has specified fixed outputs or variable response outputs, the addition of VFDs provides an effective means of pump control.
Alternatively, some agencies prioritize regulations around the overall storm duration instead of instantaneous flow. In these places, the inclusion of a timer-based control strategy can be a simple and effective way to manage the total volume of water that is discharged over a defined duration. The timer control scheme calculates the tie between water elevation in the wet well and pump runtime to interpret the storm events, pump sequencing and total volume discharge. Where storm durations are the limiting factor, this is a simpler control scheme.
Structural Considerations
System performance will often also depend on storage and passive structural flow control, not just the mechanical and electrical equipment. This is why structural design is key in developing an efficient, robust stormwater pump station. One common strategy for flow management is to incorporate a discharge structure with built-in flow control.
Similar to the use of diversion valves, in this arrangement, as the water fills the discharge structure, there are specific outlet orifices sized for set elevation points. Rather than the pump station itself dictating the volume released, the pump station feeds into the discharge structure that controls the outflow and can also include an overflow to signify ultimate storm events. This is a simple and robust system when fixed pumping rates and simple O&M are priorities.
By incorporating storage and containment vessels upstream of the pump station, the station’s performance can be greatly optimized. These structures provide a buffer of additional volume that allows a portion of the storm event to be stored, instead of needing to be discharged immediately, thus reducing the necessary pumping rate and meeting release limits.
Rather than pumping the complete storm event, the pump station will only have to convey the delta between what is captured and what can be discharged. As an additional benefit, the addition of storage systems may also reduce the size of the wet well, as the active volume required for pump operation does not have to be contained in the wet well only.
Storage and containment vessels may include bypass and overflow systems, which can further increase overall resiliency for events that exceed the capacity of the pump station design. Wherever possible, an overflow location can safely direct any excess flow while the station continuously dewaters until the storm conditions subside. When considering stormwater design, a state of overflow may not denote a system failure but instead be a planned part of the overall stormwater management plan for the largest storm events.
| IMAGE 2: A triplex configuration in a stormwater pump station that will be owned by a municipality in California.
Shaping the Design Approach
The most effective stormwater management solutions combine several of these approaches. A single project site may require a unique approach to meet the operating conditions, operator preferences/capabilities and requirements of the local permitting authority.
When considering the design approach, each stormwater pump station design should begin with the fundamental question: What happens if the pump station reaches the discharge limit or experiences a failure? On some sites, these may result in a minor nuisance such as flooded landscaping. On others, system overflow and failure could threaten buildings, operations or public safety. Understanding how discharge limitations can impact system capacity is critical in determining the appropriate levels of redundancy, storage, complexity and planning.
Another important consideration the design team must evaluate is the capability of the eventual owner or operator. Some teams are well equipped to operate advanced control systems, manage set points and oversee a more complex pump station design. Others are best served by simple systems that incorporate passive mechanical and structural elements.
This kind of site-specific approach reflects the reality of modern stormwater pump station design, where successful systems are shaped by hydraulic performance, permitting requirements, long-term operation and constructability.
Successful stormwater pump systems depend on a coordinated strategy that considers pump selection, controls, storage, structural flow management and the operational realities of the end user. In many cases, the best stormwater solution is not the most complex one, but the one that reliably meets site conditions, permitting requirements and long-term operational needs.
Kyle Perrin is the director of business development at Romtec Utilities. He brings more than a decade of experience leading teams in engineering, construction, estimating and project management, and he earned a master’s degree in business administration from Southern Oregon University. For more information, visit romtecutilities.com.
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