What You Need to Know About Underground BMPs

March 20, 2018

As development and redevelopment continues and land comes at more of a premium, it’s important to consider alternative stormwater management techniques.

BMP Worksheet

Download a free worksheet to assess the best stormwater BMP for your site.

Download (PDF)

As cities continue to grow and expand and developers seek to develop and redevelop land, they need to account for what happens to water when it rains. Not only does the water need to be conveyed away from areas where people are active, it also has to be treated. As the water travels across our roadways and parking lots, it picks up pollutants and debris along the way. These pollutants should be treated before being discharged to our natural surface water resources.

Stormwater management techniques are nothing new, but many designs could take up a lot of land. Common surface BMPs include bioretention ponds, infiltration basins and detention/retention ponds. These BMPs are widely used across the United States. Engineers and public works staff are comfortable managing them as they can easily discern if they need maintenance or are nearing the end of their functional lifespan. 

Surface BMPs
An infiltration basin is a popular surface BMP for conveying stormwater because municipalities and developers are familiar with them.

As communities continue to grow, owners, developers and municipalities have to be increasingly mindful of where and how they treat stormwater runoff on their sites. According to Emily Erdahl, SEH water resources graduate engineer, as the cost of land increases and unused open space is at a premium, the use of subsurface stormwater management facilities (BMPs) are increasing in popularity.

Still, some sites may not be able to manage stormwater volumes with surface BMPs due to high costs of land, retrofit designs or conflicts with existing infrastructure, such as parking areas. In these situations, developers and communities can consider using underground, or subsurface BMPs. Here are some common underground BMP examples.

Retention/Detention methods

These methods consist of closed pipes or chambers and are used for peak flow rate control. Stormwater runoff from catch basins, roof drains, and other sources is retained onsite within the volume of the pipes or chambers and then released over time. This assists in meeting rate control requirements. Retention/detention methods are also commonly used for total suspended solid (TSS) pollutant removal. Patrick Sejkora, SEH water resources engineer, says there are less design restrictions on these methods and they are sometimes used in industrial settings or locations with high groundwater.

Closed pipes
Closed pipes or chambers are one type of retention/detention underground BMP.

Infiltration/Filtration methods

These types of underground BMPs consist of perforated or open-bottom pipes or chambers. Similarly to the underground retention/detention BMPs, stormwater runoff enters into the pipes and chambers, which provide storage and rate control. However, the open bottoms and perforations allow the water to infiltrate into the native soil. In the case of filtration BMPs, the water is collected by an underdrain. While both methods can be used for pollutant reduction, only infiltration BMPs are effective at treating dissolved pollutants such as phosphorus. Because filtration BMPs have a liner and underdrain, they are only effective for solids removal. Also, worth noting are the site constraints when using infiltration/filtration BMPs. Infiltration/filtration BMPs require appropriate separation from the water table and are not suitable for some industrial areas, sites with poorly drained soils or areas with high groundwater or bedrock.

Underground infiltration/filtration BMPs
Underground infiltration/filtration BMPs are effective pollutant reduction methods.

Subsurface water quality treatment vaults

snow plow
Sump catch basins are an example of smaller scale underground BMPs.

These smaller scale underground BMPs are not intended to offer volume management or rate control but rather are designed for gross pollutant removal and act as pretreatment flow-through structures. Stormwater flows through the device and baffles to remove suspended pollutants before it flows out. Solids are collected in a sump while floatables are retained and skimmed from the top. These are usually located upstream, in-line with other underground BMPs. Some examples of subsurface water quality treatment vaults include sump manholes, baffles, hydrodynamic separators and grit chambers.

Permeable pavements

Although permeable pavers are located on the surface, they are similar to underground BMPs because they rely on subsurface integrity to function properly. These BMPs allow for stormwater treatment within traditionally impervious areas such as parking lots. Permeable pavements consist of porous asphalt, porous cement and permeable pavers. Special mixes of cement or asphalt have small voids allowing the surfaces to filter or infiltrate stormwater runoff to meet stormwater treatment goals.  These goals can include rate control, pollutant reduction and reduced runoff temperatures in environmentally sensitive areas near trout streams. Like infiltration/filtration, these BMPs have site constraints and require special equipment for proper operation and maintenance. These BMPs are known to be high maintenance facilities that require special management practices, such as vacuuming at least twice per year. 

Permeable pavements
Permeable pavements are a type of underground BMP. They are considered underground because their subsurface integrity allows them to function.

No matter the type of BMP you use for water management and treatment (including surface BMPs), they must be properly maintained to maximize their lifespan.

Maintenance

Unfortunately, no BMP lasts forever on its own. To keep BMPs operating properly there are three categories of maintenance: routine, minor and major. Routine maintenance includes inspection and non-invasive cleaning. While minor and major maintenance can include things like full-system cleaning to full-system replacement. Generally speaking, if you perform routine maintenance the need for major and even minor maintenance, including replacement, is likely to decrease.

With surface BMPs, it tends to be obvious when they are not maintained or functioning as designed. Debris accumulation, sedimentation and extended ponding time are apparent. However, with underground BMPs, there isn’t always an easy way to determine if they’re not maintained properly.

“That’s the major drawback to underground BMPs,” says Patrick. “You can’t always tell if something’s wrong. That’s why it’s important to have a policy in place to avoid this from happening.”

Putting together a policy

One of the most important things for cities or developers to do is to put together a good underground BMP policy. A policy will help the BMPs live to their full lifespan and save money in the long run. A good policy will cover annual inspections, routine operations and maintenance schedules, record keeping and the eventual replacement at the end of a BMP’s functional lifespan.

Of course, says Emily, these points each come with a cost, and it’s important to determine how to cover these costs. One way, she says is through stormwater fees. A potential stormwater fee could cover city oversight and provide some assurance that underground facilities are being managed properly.

There are many options when it comes to stormwater fees. They could be used to cover as little as a city’s administrative costs and as much as city-led inspections and maintenance. The fees could also serve as escrow for failed underground BMPs the city may have to mitigate.

Bringing it all together

Underground BMPs offer cities and developers a unique option for stormwater management, particularly in instances when the price of land is at a premium, and there isn’t a lot of it to go around. However, to get the most of the various types of underground BMPs, they need proper maintenance. The best way to ensure proper maintenance and getting the most out of your underground BMPs is proper planning, execution and operation and maintenance. This is all achievable through the development of an underground BMP policy.

About the Experts

Emily Erdahl

Emily Erdahl is a graduate water resources engineer dedicated to finding unique solutions for stormwater management and helping cities understand their best options. Contact Emily

Patrick Sejkora

Patrick Sejkora is a water resources engineer committed to helping cities find smart, economic ways to manage their stormwater. Contact Patrick

Better Projects 101: Practical Tips

Want a better project? Download our free eBook filled with tips on hiring a consultant, stakeholder engagement, project delivery, project management and more.