How to determine the right stormwater pumping system for projects

Feb. 24, 2023

There are dozens of basic stormwater pump station configurations. Each is appropriate for certain situations and/or requirements. The problem is that unless you have experienced most, if not all, the basic requirements and related configurations, it is difficult to know how to configure the stormwater pumping system that is appropriate for each type of basic requirements., 

Question #1 – Does your project have any type of detention or retention prior to the pump station? 

Overall, if the answer is yes, the stormwater pump station required is smaller and less expensive than if the answer is no. Remember detention can be ditches, a network of storm drains, and of course, it can be a classic detention pond. 

If the answer is no, the question moves to: what is the peak flow for the 25-, 50- or 100-year event? Without detention, peak flow will then drive the size of pump or pumps and the overall size and scope of the overall pumping system. This system will be much larger. 

Question #2 – Are we pumping to or from treatment? 

There are a variety of states which now require most significant (new) commercial and/or industrial construction to collect and treat stormwater on site prior to discharge. This type of stormwater pumping is typically lower volume in GPM, and it may result in multiple retention requirements. Pumping stormwater to and from treatment often involves underground detention typically in the form of large pipes and or structures that are typically configured under parking lots or other open areas. The water is collected in these detention/treatment structures and then normally flows via gravity to the sump of the pumping system which moves it to either discharge or treatment. In other cases, the underground vessels detain the unfiltered water which then flows to the pumping system, and then pumps it to above ground treatment for hydrocarbons and solids and then the water may be released or pumped to discharge. 

Question #3 – Why can we (sometimes) surcharge the inlet line in stormwater pump stations?  

In many cases, stormwater pumping systems bring the inlet line into the bottom of the sump. The benefit is reduced depth of the sump and less turbidity. The rational is simple, all the water in the sump and all the water up stream is essentially the “active area” of the pumping system.  

Note: Bringing the invert in near the bottom of the sump also reduces turbidity problems associated with “cascading water,” which occurs when a high volume of water falls from an invert line located 5’ to 20’ above the floor of the sump. 

Question #4 – With a detention pond, where should the pump station and the inlet structure be located? 

Inlet structure location 

The easiest way is to simply set one of many pre-cast and prefabricated concrete inlet structures in the bottom of the pond. These structures typically have screened inlet areas on the sides and/or the top of what is typically a “box” like structure. That structure, with a covered hole for the gravity flow line, will move water from the deepest part of the pond underground to the location of the pump system. 

Pump station location  

Pumping stations for either detention or retention are typically located on the “berm” and/or on the edge of the pond. The sump depth is determined by the rim elevation of the sump relative to the lowest “invert in” and the gravity inlet line, which runs from the intake structure (out in the pond) to the sump. 

Note: Many large stormwater detention ponds have a more elaborate intake design with the deep end of the pond and an underground wall type intake structure. This intake design is fine but it is much more complicated and more expensive. 

Question #5 – How much pumping and/or operational “redundancy” do I need for my stormwater pump station? 

Typically, very little. In most cases, a backup pump is all you need. In other words, the typical stormwater pump station has two pumps. Each pump can pump the requirement for that pump station. The second pump alternates with the first and both pumps then “back up” the other in the event of a pump failure. 

In cases with adequate detention, one typically does not need back up level control auxiliary power and/or back up panel control panel configurations. However, if the stormwater detention does not have adequate detention or retention then the “fail safe” aspects of the stormwater pump station may need to be as great or greater than wastewater for example. 

Question #6 – What if I have no room for any sort of detention? 

The stormwater pumping system needs to be able to handle a much wider range of flow. There will be some sort of very low (nuisance) flow that will probably require a small pump and perhaps a separate smaller force main than the peak flow pumps which will handle the 50-to-100-year event.  

In these scenarios we first must determine how big and how deep the sump is going to have to be. The good news is that the sump can be rectangular and thus “tilted up” concrete panels could be used. Remember in very high flow systems we typically want to keep the sump depth under 20 feet deep. In many cases the sump involves multiple chambers for example (10’x10’x20’) connected with large, perhaps 48-inch pipe, thus creating a multi chamber sump that can handle high flow as well as low flow and which can perhaps utilize multiple pumps ranging from small to very large. 

With all the new stormwater treatment regulations and with the wide variety of land use opportunities that include the control of storm water, the design options, and their configurations are seemingly endless. Gone are the days when wastewater system designs were generally thought of as “more complex and sophisticated” than the design of various stormwater systems. Stormwater pumping ranges from simple to complex and the size, based on peak flow, is often large. The following are just some of the “real world” examples of what is possible in stormwater pumping systems design. 

Large Oil & Gas Refinery  

On the Gulf Coast of the United States, when it rains it pours. Like many industries, oil and gas have long been active in the collection and treatment of stormwater on their various refinery and processing sites. In one case, the design requirements were as follows.

  • Replace the existing sump (80-feet x 20-feet x 20-feet) / within the existing sump 
  • Add an intake structure with solids collection  
  • Design for low follows (100 GPM), medium flows (5,000 GPM) as well as peak flow (60,000 GPM) 
  • There will be no detention upstream of the sump. 
  • Configure the pumps so that in low flow the smallest will operate but in high flow all will operate (small, medium and large) 
  • Pump into one of six different large retention/storage tanks that will later send the water treatment. 
  • Note: Constantly read the level in each of the six storage tanks and automatically pump only to those tanks that have capacity available.  

What were the hard parts? 

  • Structural prefabrication of the new sump – This system and the available “down time” of the existing sump required the sump and all related inter connections to existing inflow and discharge to be completely prefabricated for first installation (1 week). 
  • The electrical, the controls, the SCADA & the PLC code writing  – Big industries always have existing preferences/requirements for all. Anyone building new must be able to provide all and write all in the forms, functions and or configurations that the owner is accustomed to using and operating. 
  • Turbulence – the avoidance of turbulent water – Any time the pumping system must handle a high volume of incoming stormwater, the system must be designed to receive all the water in a nice smooth manner to minimize turbulence.  

International Airport  

Build a new stormwater pumping system at the end of an existing runway that is experiencing seasonal flooding during the winter. Get in and out quickly and do not disturb existing air traffic on other runways during the construction (minimize impacts to ongoing airport operations). 

  • Build a new rectangular concreate sump with the rim elevation at grade for a peak pumping rate of 24,000 GPM; 
  • Build a method to “house” each of the 50 hp axial flow pumps and operational valve configuration, each to be in a concrete vault that will also have its rim elevation flush to grade.  Note: Each set of check valves and shut off valves had its own vault. 
  • Note: The three large axial flow pumps were all housed in a separate 12-foot x 14-foot x 16-foot deep rectangular “panel vault” with each well and the base precast and then tilted up on site and within the shored hole. The precast top slap top with various access hatches was also precast and prefabricated prior to delivery and placement on top of the rectangular sump. 

What were the hard parts? 

  • Timing – At this airport, it rains a lot, and there is a lot of groundwater. The project had to be done at the driest point of the year when the groundwater was minimal (which it was not).  
  • The physical size of all elements 
  • Proximity – Due to the proximity to the runway, the layout of equipment and the excavation process required extra care.  
  • Axial flow pumps – The projects high flow and low TDH required the use of axial flow pumps. 
  • Downstream Discharge  – The durability of adequate downstream discharge capacity (in this case there was an existing steam that could handle the discharge).

Military Airplane Hanger 

This secure site required collection and above grade retention of both occasional stormwater and regular aircraft “washdown” wastewater. All retained water was tested. If clean enough, it was released to gravity. Otherwise, the water was pumped from retention to tanker trucks for transport to treatment. 

  • Design, supply and construct new underground 6-foot X 18-foot-deep fiberglass sump within a “containment area” designed for this project. 
  • Design, supply and construct all of the pumping and mechanical within the sump, and above grade (to and from the sump).  (There were three separate above grade 33,000 gallon tanks.) 
  • Measure the level in all three tanks and automatically fill each tank sequentially using an actuated valve.  
  • Design and integrate all alarm notifications in the existing format from the pumping system into the (SCADA) building management system. 

What were the hard parts? 

  • Secure site – Super-secure sites take twice as much time to do anything on. 
  • Coordination of Site Mechanical – The coordination, design and construction of all site mechanicals could not be done until the new sump and the three tanks were in place. Note: The fabrication of all pipe and related connections was custom and the key mechanical fabrication vendor was close by. 
  • SCADA Design and Coordination – The SCADA design and coordination could not be fully done prior to the installation. In-house expertise was required tounderstand and interpret the existing building management system and therefore the new SCADA was extensive. 

Major Seaport 

Seaports tend to be large, flat, and impervious. So, when it rains, they tend to gather lots of hydrocarbons and a long list of solids. The project required capturing a significant amount of the “flow” and treating it prior to discharge in the adjacent saltwater bay. 

  • There was an existing infrastructure of catch basins. The pumping systems were installed to intercept the existing main stormwater line prior to the bay. 
  • The pumping system was installed “in line” with the existing discharge piping. 
  • The initial design specified a “field poured” structure with multiple compartments. This design was changed to accommodate a precast concrete structure with multiple compartments.  

What were the hard parts? 

  • Making the design work in precast – Multiple compartments were achieved by using a large rectangular vault with “vertical keyway” allowing the installation of an intermediate wall.  
  • Existing utilities and piping – There were a lot of existing utilities and piping that were already in the ground. Finding the best place to locate the new inground structures was involved. 
  • The two-compartment structure – The sump received the raw water on one side of the two compartments structure and was pumped to treatment. The treated water returned into the same underground structure but into the downstream compartment. 
  • Flap gate – The separation between the two compartments included a “flap gate” to allow excessive flows to bypass treatment. 

In many cases today, stormwater and stormwater conveyance are the most frequently misunderstood civil and mechanical engineering requirements. The fundamental problems range from new regulations relative to on-site treatment to the development of land that would not be possible to use without ongoing stormwater pumping. A stormwater pumping scenario needs to first be evaluated relative to the potential severity of the flooding should the system fail. Once that is determined, the next step is fully understanding the site and what is possible in the given areas available for a pumping system. Then comes the hard part, which is the development of a system that typically includes structural, mechanical, electrical, control and communications elements that can be built. In many ways, stormwater and its control, treatment and pumping are the newest and perhaps the most varied civil engineering discipline in today’s wide array of pump systems to various water types. 

 Mark Sheldon of Romtec Utliities can be reached at [email protected].

About the Author

Mark Sheldon

Mark Sheldon can be reached at [email protected].