Green & Grey Stormwater Management Options

by Dr. Anna Zakrisson on Thursday, March 19, 2020

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Why do we need stormwater management?

Stormwater management is needed to prevent flooding, erosion, pollution of water, to appropriately refill our aquifers, and to reduce habitat degradation.
Stormwater management is particularly important in urban areas where most surfaces are impervious and do not absorb much water. During intense storms, a large volume of water rains down on a city over a short period of time and this can have devastating consequences unless we don’t have a good plan in place.
Since water is not getting absorbed by the impermeable asphalt and stone surfaces of the modern city, it will flow directly into the streets and down the sewers often resulting in floods and combined sewer overflows.

Combined sewer overflows occur when a combined sewer system has reached maximum capacity and the overflow is released as untreated sewage straight into the waters where we live and work. This has the potential to not only cause irreversible and expensive ecological damage, but also to aid pathogen transmission and hence the spread of disease.
Green infrastructure stormwater management is a way to deal with stormwater that works with nature instead of against it resulting in economic benefits on top of LEED, and adds secondary benefits such as pollution abation and cooling, particularly in urban settings.

What is stormwater management?


Good stormwater management ensures that the runoff is dealt with in a productive way so that floods, combined sewer overflows, and other stormwater-related issues are avoided. Good stormwater management also considers secondary benefits as well as a return of investment of the systems to ensure ecologic as well as economic sustainability.
It is important to keep in mind that different countries, states, and municipalities can have very different stormwater management requirements, so it is critical to ensure adherence to local rules and regulations. Also, all strategies come with pros and cons and sometimes, the best solution is a combination of several technologies.
However, it is advisable to go for solutions providing a clear return of investment and with as many secondary benefits as possible.

What stormwater management options are there?

We can broadly divide stormwater management solutions into grey and green stormwater tools.

Green stormwater management

Green stormwater management refers to above-ground management that makes use of nature-based solutions. Nature-based means mimicking nature with the aim to make nature work for us instead of against us, for example, green roofs and rain gardens.

Green roof stormwater management

Traditional green roofs are excellent at stormwater retention, meaning that they are able to reduce the total volume of water that becomes runoff by about 50% over the course of a year. Plant transpiration and surface evaporation removes water from the roof and converts it to water vapor. This process also cools the building in the summer.
Detention green roofs also provide detention the same way as tanks, cisterns or any other underground grey stormwater management solution. In addition, detention green roofs also offer retention making these roofs a double-pronged and efficient and financially sensible stormwater approach with secondary benefits that are hard to beat with any grey stormwater management system.

In most large cities like NYC, tanks are often placed below the basement to save as much space as possible due to the exorbitant real estate prices. By managing stormwater on the roof instead, it is possible to fully remove or reduce the tank and utilize this space for income-generating activities such as parking spaces or other amenities. This creates a reliable return of investment for the green roof as well as LEED points.
In more rural settings where land is less expensive, other solutions might be more financially favorable such as rain gardens.

Rain gardens

Rain gardens are shallow depressions in the ground that is covered with deep-rooted plant species and species that are tolerant to flooding. These structures are often built in the proximity of a runoff source so that the runoff can be captured and then slowly infiltrated to the ground.
Rain gardens are often used as natural ways to remove pollutants from rainwater.

The downside is space constraints. It is not always possible to create this space. However, some cities, such as Utrecht, the Netherlands, have integrated rain gardens into the city infrastructure in a beautiful way creating beautiful landscaping that also acts as a stormwater tool, cleans the water, cools the city, and supports bees and butterflies.

Grey stormwater management

Tanks

One commonly used method to deal with stormwater is to bury it in a tank.
After rainwater has been collected, it is released at a controlled rate either into the sewers or into the aquifers. It is an effective system but comes with virtually zero secondary benefits.
Also, it is often difficult to discover if the system is malfunctioning.

Perforated pipes

Perforated pipe stormwater management means storing water in pipes that are embedded in gravel and that release water slowly through small perforations. On the plus side, this is a modular system that is great to use as a project is being built. On the negative side, this system also comes with no secondary benefit. It is buried into the ground and hard to monitor.

Gravel beds

Gravel beds allow water to be stored in between the spaces between the gravel stones before slowly being infiltrated underground. These systems achieve the highest soil infiltration of the underground systems but require a lot of space which might be a considerable problem in big cities, but less so in rural settings.

Stormwater chambers

Stormwater chambers are similar to perforated pipes in that they are also modular and buried prefabricated chambers that hold and slowly release rainwater.

Permeable concrete and pavers

A pretty neat solution to stormwater management is permeable concrete and pavers. These solutions allow water to infiltrate through the concrete and between the gaps between the pavers and into the soil and eventually to the groundwater.
This stormwater solution is still limited by the infiltration capacity of the underlying ground.

When possible, go for sustainable green infrastructure stormwater management instead of old and outdated grey infrastructure with zero secondary benefits.

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