Lupinus perennis production field

Stormwater Part 3: Combined Sewer Overflows

Apr 27, 2015

Temps have dropped here at the Nursery over the past week, slowing down overall growth for a bit. We’ve even had a few mornings with frost on our windshields. But despite spring still straddling the fence, we have a huge amount of plugs, seed and other plant material moving out the door as our busy season begins to ramp up. And being busy is a wonderful thing.

Willow Live stake sprouting leaves

This will be our third and final (at least for the time being) posting on stormwater management and mismanagement, how current strategies affect the quality of our water and native ecosystems, and emerging green solutions to stormwater problems.

In the last post, we examined the ways in which water quality degradation from agricultural runoff can be improved through wetland restoration—the benefits of which could potentially be multiplied through the development of water quality trading programs.

This week, we’ll look at urban stormwater management, specifically Combined Sewer Overflow (CSO) systems and the ways in which they contribute to water quality degradation.

Problem: Combined Sewer Overflows

CSO’s are commonly used in municipalities to relieve the amount of water feeding into the treatment works during a rain event. The “combined” in CSO refers to the fact that the municipality’s sewer system combines both sanitary effluent and stormwater.

How a CSO works. Image Courtesy of the US EPA

To prevent flooding when it rains, water flows from buildings, lawns, roads, and parking lots into storm drains, and these drains often route directly into the sanitary system. Historically, most publically-owned treatment works were not designed to handle and treat the volumes of water produced during most rain or storm events. For that reason, CSO’s have relief structures that allow the “combined” effluent, comprising both storm water and raw sewage, to flow directly into a nearby lake or river.

“Raw sewage directly into the lake or river?!” is generally the response elicited from most people who have never heard of CSO systems. It must be remembered, however, that most of these systems were designed decades ago, in some cases a century or more, during a time when an awareness of the negative impacts of such releases simply wasn’t in the air.

That changed significantly with the passage of the Clean Water Act in 1972, and with subsequent updates to the Clean Water Act in the 2000s. Such updates have given the USEPA authority to regulate those discharges, and to enforce the transition away from CSOs. Many municipalities with aging CSO systems are now facing hundreds of millions of dollars of upgrades in order to come into compliance with EPA and Clean Water Act regulations.

Solution: “Green Infrastructure” incorporating native vegetation: Permeable surfaces, treatment wetlands, and green roofs.

Some of the conventional ways in which municipalities are attempting to come into compliance with current regulations involve what’s known as “gray infrastructure.” This involves hardscaped water diversion and holding structures that are simply a continuation of the conventional water management strategy to “collect, convey, and discharge” water as quickly as possible. As we pointed out in our first stormwater blog posting, this strategy results in cycles of increased flooding and drought, as well as ecosystem degradation. When precipitation is not encouraged to infiltrate into the underlying groundwater, lake and river water levels fluctuate wildly, and native wetland ecosystems don’t receive the necessary groundwater flows to keep them healthy and viable year-round.

Concrete stormwater vault

The alternative is incorporating “green infrastructure” and related strategies that attempt to work with natural processes of water cycling rather than against them. These aren’t one-size-fits-all engineering approach. They are a highly-designed suite of solutions that take into account local geography, soils, and ecosystems to help channel water away from where we don’t want it—in our roads and basements—to where we do want it—into the ground, where it belongs.

We’ve already discussed how permeable pavement can help to infiltrate the water that falls onto roads and parking lots directly into the ground. In the same post, we also wrote about how bioswales, comprised of native plants, can help to slowly infiltrate diverted stormwater into the ground.

Other viable green approaches involve taking advantage of the copious amounts of water that fall on the roofs of our buildings.

Here in northern Indiana, our average annual rainfall is about 41.5 inches. To calculate how much water falls on a roof throughout the year, one simply multiplies the rainfall (in inches) by the catchment area of the roof (in square feet) by 0.623 (which represents the relationship in gallons of rain per square foot). For example—if the total square footage of a house’s catchment area is 2000 square feet and the average annual rainfall is 41.5 inches-- then that house’s roof receives approximately (41.5 in x 2000 sq ft x 0.623 =) 51,700 gallons of water per year.

Rain barrels and cisterns can hold that water for later use, as opposed to pumping water out of the ground. Green roofs can also be utilized to take advantage of that precipitation. A green roof is a roof constructed with a layer of soil media in which native plants can be grown. Green roofs do an excellent job at regulating temperatures inside buildings, reducing energy use and expenditures. They can be as large as the green roof on Chicago’s City Hall, or on smaller structures to enhance aesthetics.

Green roof at Coffee Creek

Here at the Cardno Native Plant Nursery, we’ve gained a great deal of experience over the years working with clients, government agencies, landscape architects, and spec writers in understanding how native plants can be incorporated into urban and rural water management strategies.

Time and again, our clients find that green infrastructure is an effective, elegant way to CONSERVE water, IMPROVE water quality and ecosystem health, and CREATE beautiful spaces. Considering the damage that “collect, convey, discharge” has done over the past century, it’s clear that “conserve, improve, and create” as an approach to water management is the way of the present and the future.