Stormwater pump stations play a vital role in the overall stormwater system as they provide a unique solution to an ever-evolving construction landscape. Designing a stormwater pump station comes with many considerations when thinking about the different types of storm events and overall stormwater infrastructure.
Stormwater pump stations have a wide range of applications and solutions for dealing with storm events. Common uses include construction at a location that was previously flooded with stormwater, collecting water that may cause damage to surrounding structures or people, and handling increased flows from impervious surfaces due to new construction. Pump stations can also be used on treatment systems for contaminated runoff at industrial sites.
Storm events are also becoming more frequent in their occurrence. Storm events are classified by their recurrence interval and are derived from data defined by the U.S. Geological Survey. Different types of storm events like the “100-year storm” happen every 100 years on average, which is based on past data. These types of “100-year” storms are becoming more and more common, and therefore, regulating agencies are putting more emphasis into protecting their infrastructure as well as redefining the qualifications for recurrence intervals. It is typical for the civil engineer to determine what the flows are associated with various storm events based on rainfall, survey data and government standards associated with the specific site being designed.
Upstream structures provide multiple ways for stormwater to be conveyed to the pump station during storm events. A couple common structures are detention or retention basins handling the entire storm event. A pump station empties the storm basin at a slower rate as there is enough space in the basin to hold a large storm event without the worry of flooding. Generally, the higher the peak inflow or required pumping rate, the higher the cost for stormwater systems.
The engineering team along with the developer and owner consider the costs of a pump station compared to the construction cost of a pond and the loss of development space. It comes down to the cost versus benefit of all the componentry working in conjunction to provide the complete stormwater management solution that fits the project. Bioswales or modular wetland systems can work in conjunction with pump stations and detention or retention ponds as part of the larger system. These wetlands can help mitigate pollution in stormwater before it enters the watershed. Native soil and plants within bioswales are great filters for stormwater and can filter out many contaminants.
Redundancy is an important feature to consider, ensuring a backup system can support the primary system in the event of a failure. There are varying levels of redundancy that are needed based on the site design and criticality of the pump station infrastructure. Redundancy is also an item that can have a noticeable effect on the overall cost of the pump station. When it comes to pump station redundancy for stormwater systems the driving question is “what happens if the pump station fails?”
Smaller storm events can create problems like overflowing creeks, flooding a parking lot, or a pond staying full instead of draining. The system may be in an area that is non-critical in the event of failure in which ponding may occur around the pump station but will not affect site operations. Redundancy may not be needed but is up to the owner's preference. On the other end, if a pump station failure would cause damage to structures, equipment or facility downtime then this is a significant problem. Redundancy in the stormwater pump station is likely required with redundant pumps and a backup power generator. It is becoming more common for pump stations to be held to a maximum allowable discharge based on each design storm event. As impervious surfaces increase some stormwater infrastructure is not sized to handle the increased storm runoff flows.
Projects with maximum allowable discharge rates can be very complex as they often have very low flow requirements which makes selecting a pump challenging. This is because hitting an exact pumping rate is very difficult due to head in the system changing as water levels rise and fall. For example, on projects with maximum allowable discharge rates, during a 2-year event, the stormwater system may only be able to discharge two cubic feet per second max, and during a 25-year event, you can only discharge five cubic feet per second max. The maximum discharge values are typically based on predevelopment runoff as they do not want to overwhelm the downstream infrastructure. Pump stations with a maximum allowable discharge are challenging to design with the various factors at play. On these projects, upstream detention is often needed to hold a storm event while the pump station discharges at the allowable rate.
An example of a project designed to handle small to large storm events was a private commercial development in Southern California which needed a stormwater pump station. The project began as a large 12.6 cubic feet per second (CFS) system and through significant work with the civil engineer, Kimley-Horn, as well as the architect, structural engineer, and owner Romtec Utilities, was able to develop a new drainage plan for the site and the building which would greatly reduce the peak inflow to the pump station.
“The project consisted of approximately 7-acres of stormwater runoff that was required to be treated, stored, and discharged to match a predevelopment condition,” Joshua Bielik, PE of Kimley-Horn said. “During the initial design, the runoff was conveyed to a single point and pumped from the storage chambers and into the treatment unit.
“Upon further review to value engineer the project, the site was later divided to only pump the southern half of the site into the storage chambers while the northern half was conveyed via gravity. This reduced the overall required pump size to one-third of the original, 4.5cfs (modified) and 12.6cfs. Two pumps were proposed, one for low flows (under 100-year event) and one for high flows (100-year event).”
After stormwater runoff was stored, it flowed via gravity into the treatment unit and ultimately into the public storm drain culvert. The system featured a Precast 8-foot ID x 17 –foot deep wet well, Duplex 10hp Ebara submersible pumps, an alternator relay as the primary controller, four NOLTA MS1 Floats, and a NEMA 4 painted steel enclosure. At times, the flow and velocity of stormwater leaving a force main can be damaging to the discharge location. Force mains discharging at a high velocity can flood streets, wash away embankments, overflow receptacles and other issues coming from a large influx of water moving at a high speed. A solution is creating an energy dissipater such as outfall structures or rip rap that are designed to handle flows to protect the discharge location.
There are many variables to consider when it comes to storm events and the design of the components that play into the overall stormwater infrastructure system. The flexibility of design theory is imperative when it comes to designing a functional stormwater system that is intended to operate reliably over the long term. There is no “off the shelf” design that will work for every stormwater application. Instead, stormwater system design often requires problem-solving by the entire team to provide a realistic solution for each individual application.
