Holistic stormwater management often requires more than ponds and rain gardens

Increased populations, a declining number of porous surfaces, increased intensity of rainfall events, and antiquated infrastructure have resulted in stormwater runoff often overburdening many storm sewer systems. Regulations are becoming increasingly stringent, and it’s important to comply with these regulations to protect the quality of a community’s drinking water, lakes, rivers, and streams.

So, when is it time to consider stormwater updates for your city and what is the right solution?

Importance of stormwater management

As a water resources engineer, protecting water quality is the biggest goal for my team, and there is no one-size-fits-all solution. Despite living in a state that is known for its abundance of water resources, 40 percent of Minnesota’s water bodies are listed as Impaired Waters under the Federal Clean Water Act. An impairment means that the water body does not meet its designated use, and an action plan must be created to improve its water quality.

Forty percent of these total impairments in Minnesota are due to high turbidity or high levels of total phosphorus, and stormwater runoff can be a large factor in these high levels. If you’ve ever seen a bright green lake that doesn’t look very appealing to swim in, it is likely caused by high phosphorus levels and that lake has experienced a phenomenon called eutrophication. Eutrophication can be natural or human-induced, and human-induced eutrophication is usually a result of phosphorus loading from plant leaf litter, pet waste, road salt, and fertilizer.

When 2013 water quality tests on Northwood Lake in New Hope, Minnesota, revealed phosphorous levels four times higher than state standards allow, it was time to find a solution to reduce total phosphorus, suspended solids, and stormwater volume to the lake. The City a sought a holistic stormwater management solution that would achieve multiple goals: 1) improve the water clarity and quality of Northwood Lake itself, 2) improve water clarity and quality of downstream water bodies, and 3) reduce stormwater discharge volumes to the lake. Additionally, Northwood Park is adjacent to Northwood Lake and is considered the City’s flagship park. In its design, the City sought to showcase stormwater improvements as an educational tool to increase public awareness of water quality issues.

Innovative solutions

Traditionally, when people think of stormwater management, they think of stormwater ponds and rain gardens. Underground trenches and stormwater tanks are two of the new innovations for municipalities to consider.

Underground trenches include a series of pre-manufactured pipes and structures, where perforated pipes in a series allow for water to slowly drip into the system, and contaminants to settle out. Treated water is conveyed through drain tile to the municipal storm sewer system.

Underground stormwater tanks (shown in the photograph above) are structures that hold large volumes of water underground and allow for water to outlet at a controlled rate. Before water enters the vault, water passes through a two-cell swirl chamber, which reduces stormwater velocity and allows sediment to settle out, thereby allowing cleaner water to enter the vault. These two methods provide effective water quality and stormwater volume reduction. Equally as impressive is that this is all accomplished underground. In a city like New Hope that is focused on maintaining its extensive amount of green space and had limited space to develop a large treatment system, this approach allows for the city to achieve its environmental and recreational goals.

In New Hope, we created an underground stormwater tank, which in combination with its pretreatment structures, treats and stores 160,000 gallons of stormwater, which is reused to irrigate ballfields. Not only does its addition drastically reduce pollutants that would otherwise drain into Northwood Lake but it also reduces the City’s annual water usage by up to 2.5 million gallons. Should the vault collect enough water that it overflows, rain gardens were installed to treat the vault’s overflow. To further reduce pollutant loading, sump manholes with skimmer structures were installed to physically hold back debris before the stormwater enters Northwood Lake. A stormwater pond and underground filtration trenches also were installed to collect and treat stormwater runoff from surrounding neighborhoods.

Each community that we work with will have a different set of priorities, and the same is true with their approach to stormwater management. What works for one city may not work for another. For the City of New Hope, which was interested in preserving the green space available to residents, an underground storage tank and filtration trenches aligned with the City’s goals nicely.

What I find most satisfying about this work is being able to understand the needs of each specific community and design a solution that will not only meet local regulations and improve water quality but also creating a solution that is easily maintainable for City staff.