Our Insights

When There’s Nowhere To Flow, But Down: Benefits Of Underground Stormwater Detention

August 31, 2018

By Beau Gosney, P.E.
Civil Engineer,Facilities Division

Water that falls to the ground when it rains and snows is called stormwater. When parking lots, roads, buildings and compacted soil replace natural terrain, the man-made, hard surfaces prevent stormwater from soaking into the soil. Constructing more acres of hard, impervious surfaces like concrete and asphalt causes more stormwater runoff. Compared to the original pre-development condition, stormwater runoff can cause flooding, pollution, and significantly, soil erosion.

To mitigate these impacts, developments are required to include designated drainage systems that catch, store and release stormwater runoff. This is known as stormwater management, which is mandated by the federal Clean Water Act, state regulations and local ordinances.

Steep terrain surrounding this substation expansion project in the Colorado Rocky Mountains required an innovative stormwater management approach. POWER engineered an underground stormwater detention system large enough to store the amount of rainwater that would flow downhill during a 100-year flood event. The solution of underground stormwater detention provided a way to overcome geographic constraints and comply with environmental regulations.

But what if you want to build a new development on steep terrain? Or in an urban area with limited space? Or perhaps you want to maximize the economic potential of your land in an area where real estate costs are high?

For these scenarios, most traditional stormwater management practices, like retention ponds or above-ground detention basins, are not practical. Instead, a drainage system that stores stormwater underground is your best option when faced with challenges of space and terrain constraints, and it maximizes the potential of your land.

So how does it work?

Underground detention captures and stores stormwater runoff in large underground pipes or vaults. During a storm or snow melt, the runoff water first flows underground through a pipe connected to a catch basin or manhole. Once captured, the runoff water is stored underground. It is then released through an outlet pipe that’s specifically designed to let water out in a controlled manner at pre-development flow rates.

The pre-development flow rate is based on specific engineering calculations; maintaining the pre-development flow rate means stormwater must be released into surface waters at the same frequency and volume as before the development existed.

Why should I consider using underground detention?

Underground stormwater detention saves space and provides a way to construct new development in steep or restricted areas, while still meeting environmental regulations.

POWER engineered a project in Bergen Park, Colorado that demonstrates why you should consider utilizing underground stormwater detention. The Bergen Park project encompassed expanding a substation that’s located on the side of a mountain. Not only did the steep terrain present space constraints, Colorado also has very stringent rules for water quality and stormwater detention.

The Bergen Park project demonstrates several key benefits of underground stormwater detention:

Underground detention maximizes the economic potential of space. Underground detention systems were buried deep under the roads on each side of the substation for the Bergen Park project – this eliminated the need to purchase more land. Instead of buying a larger parcel for a pond, installing a stormwater detention system underneath a parking lot, a building – or in this case a substation – allows you to get “more bang for your buck” regarding land use.

Underground detention prioritizes flow control. To comply with Colorado’s regulations, the Bergen Park detention vaults were sized to store stormwater during a 100-year storm event, and to release the water at controlled intervals to prevent flooding. The pre-development flow rate is based on specific engineering calculations; and maintaining it means that stormwater will flow into surface waters with the same frequency and volumes as it did before construction. Underground detention provides a precise way to control flow and meet regulations that require the restoration and maintenance of pre-development conditions.

Underground detention is most advantageous when land is expensive, or terrain is steep. For the Bergen Park project, a traditional pond was out of the question because of the terrain’s steep slopes. Instead, the underground detention facilities captured and stored stormwater, and then released it in a controlled manner that mimics natural runoff conditions. While Bergen Park is a rural example, underground detention can also be ideal for dense urban areas where a small developmental footprint is necessary due to the expense and limited availability of real estate.

Sounds pretty good. Why don’t we build all stormwater facilities underground?

Regulations often require stormwater to be treated for pollutants. Some underground detention systems provide treatment by placing an open-bottomed vault on a layer of sand, which allows the water to infiltrate and be filtered. This can be beneficial in improving water quality discharge and reducing the required volume storage. However, these systems can’t be used in many situations because they are very dependent on space, soil type, and the groundwater table elevation.

The Bergen Park detention example included a pretreatment device to filter dirt, sediment, trash and debris out of the water before it flows into the underground vaults. These pretreatment devices mean adding more capital on top of often already high construction costs. Another consideration is that underground detention vaults are more complex to engineer, permit and construct than traditional above-ground ponds and basins. They also require yearly vacuum cleaning, like sewage system maintenance.

In conclusion

Underground detention mitigates the impacts of stormwater runoff, uses space efficiently, and keeps projects compliant with environmental regulations. Engineering a solution that not only catches the rainwater running a million directions across the ground, but then also releases it in the same manner as Mother Nature’s original plan, is unquestionably as much an art as it is a science. There is no doubt that stormwater management requires strategic and site-specific engineering – luckily, that is a topic that we at POWER know a lot about.

About the Author:

Beau has engineered a variety of innovative solutions for utilities, stormwater, commercial business development, grading and drainage, transportation, and renewable energy. He’s contributed to the design and construction of numerous substations, wind and solar farms, and high-priority regional transmission lines. His engineering expertise and interpersonal skills allow him to successfully deliver complex infrastructure projects on time and on budget, while building positive relationships with clients and stakeholders.