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Underground Stormwater Facilities: Flow Splitters

A flow splitter is a stormwater structure that divides stormwater runoff between multiple stormwater facilities or between a stormwater facility and a storm drain system. Flow splitters are located in residential neighborhoods and around commercial businesses.

Graphic of a flow splitter under a street.

Flow splitters can be located upstream of aboveground or underground stormwater facilities. 


How Does it Work?

The flow splitter sends a portion of the stormwater to facilities such as wet or dry ponds, constructed wetlands, infiltration trenches, or hydrodynamic separators for treatment. The portion of untreated water that is sent to a facility for treatment is referred to as the “first flush.” Excess stormwater from large storms passes through the flow splitter and continues to the storm drain system.


Why are Flow Splitters Important? 

Flow splitters:

  • Help stormwater management facilities to function more efficiently. 

  • Help facilities to remove pollutants.

  • Improve the health of streams and rivers.

  • Help to make our waters fishable and swimmable.

  • Improve the quality of the Chesapeake Bay.


Image of a flow splitter. Image of a flow splitter manhole
Flow splitter
Flow splitter Manhole



Maintaining Flow Splitters

As the property owner, you are responsible for all maintenance unless your flow splitter has been transferred to the Stormwater Facilities Maintenance Program. Learn more about how you can transfer your flow splitter into this program.


Image of flow splitter with debris and trash.
Flow splitter with trash and debris


Why Maintain My Flow Splitter?

An unmaintained flow splitter may:

  • Become clogged with sediment and debris that causes the untreated water to bypass the flow splitter and treatment facility all together.

  • Cost more to fix if problems are left unchecked.

Performing preventive maintenance regularly will prevent long-term damage and help avoid potential violations. These actions will keep your facility looking good and working correctly, which will save you money in the long term. The preventive maintenance actions shown below can be thought of as similar to preventive car maintenance (like changing the oil in your car every 3 months).

Structural maintenance is work done on the components of the filter that allow it to control rainwater and remove pollutants. Structural maintenance problems in the table below are things you should be looking out for. When they happen, call DEP for help (like calling your mechanic when the engine light comes on).
Preventive Maintenance Actions For Your Flow Splitter
Frequently As Needed

✔ Pick up trash, debris, or leaves near the inlet to your flow splitter

✔ Know the pollutant sources on your property and try to eliminate the pollutants at the source

✔ Have a contractor remove debris from the flow splitter

✔ Sweep paved areas on your property to remove pollutants such as sediment and sand

✔ Store chemicals, used oil, and pesticides in covered areas so these potential pollutants are not exposed to rainfall


✔ Do not stockpile sand or salt on your property


✔ Do not let vehicle or equipment wash water enter your flow splitter


✔ Inspect (but do not enter) the flow splitter after every major storm event


Structural Maintenance Actions for a Flow Splitter
(Only by a Contractor)


Possible Fix

Missing or damaged manhole cover

Replace manhole cover

Trash, debris, oil and grease, or sediment accumulation within the flow splitter

Remove and properly dispose of the trash, debris, or sediment

Missing steps/ladders

Repair or replace steps/ladders

Clogged pipes and trash racks

Remove and properly dispose of debris causing clogging

Corrosion of metal pipes

Repair or replace corroded metal parts

Damaged structural components (i.e., pipes, concrete, trash racks, pipe joints, proprietary unit, etc.)

Repair and/or replacement for these components to be determined by DEP



How Will I Know What Maintenance is Required? 

DEP recommends owners use a certified contractor to pump-out, power wash, and clean their stormwater facility annually to ensure the facility is properly functioning and passes inspection.  If DEP inspects the facility and finds maintenance issues, you will receive a notice of violation with a work order showing the list of required maintenance and repairs. You will have 45 days from the date of the notice to hire a certified contractor to complete the maintenance and repairs.  A civil citation may be issued if you have not complied with the maintenance and repair requirements. 

View the list of DEP-approved contractors who can perform the structural maintenance on stormwater facilities.

DEP can answer your questions and provide additional guidance about maintaining your stormwater facility. Please email or call the Montgomery County Customer Service Center at 3-1-1.


Image of the front of the Flow Splitter Maintenance Fact Sheet.
Download the Information on this Page  
(PDF, 375KB)

Flow Splitters in the Stormwater Facilities Maintenance Program

The Stormwater Facilities Maintenance Program is responsible for maintaining all public stormwater facilities as well as all private facilities that transferred their maintenance to the County. 

Not sure if your flow splitter is part of the Stormwater Facilities Maintenance Program?  Check out online Facilities Map or email


Can I remove my flow splitter after installation?

No, you cannot remove any facilities that were part of your building installation - these are permitted structures and DEP maintains a database of these facility locations as part of the Stormwater Facility Maintenance Program. DEP may perform a maintenance inspection of your practice if it is a permitted structure.  Property owners must contact DEP before any major changes can be performed to the structure. 





Resources and Financial Incentives

Have a flow splitter on your property? Financial incentives are available for you.

Image of a green roof.


The Water Quality Protection Charge Credit Program

If your property has a flow splitter that is maintained in accordance with the Department of Environmental Protection maintenance requirements, then you are now eligible to receive a credit off your annual Water Quality Protection Charge (found on your yearly property tax bill).  The credit provides an incentive for maintaining your flow splitter and other stormwater management practices.

You must apply for the WQPC credit separately – the credit will not be provided to you automatically.

Residential property owners can receive up to 50% off their WQPC depending on the type and size of stormwater management practices on the property.

Non-residential and multi-family properties can receive up to 50-60% off their WQPC depending on the type and size of the stormwater management practices on the property.

Learn more on the WQPC Credit webpage.