When it rains, it pours; and in many cities around the world, each downpour or major snowfall could result in a major flooding event without a proper functioning stormwater management system. Here, we look at some of the latest gray and green infrastructure solutions being deployed around the world to explore how cities are preparing for the worst.
Stormwater refers to the collection of water resulting from rain or snowmelt events. What doesn’t infiltrate into the ground or evaporate can cause flooding, erosion, sedimentation, and water quality problems if not appropriately managed.
With climate change impacting the predictability, intensity, and frequency of weather events, it is becoming more important than ever to find effective stormwater solutions. The nature of stormwater varies greatly depending on land use and climate; therefore, management solutions must be adapted as such.
There are two broad solutions categories used to manage stormwater. “Gray infrastructure” is traditional stormwater infrastructure in the built environment such as gutters, drains, pipes, and retention basins. “Green infrastructure” mimics natural environments to filter and infiltrate (or utilize) stormwater locally. Examples include permeable pavement, rain gardens, bioswales, infiltration trenches, green roofs, planter boxes, rainwater harvesting (rain barrels or cisterns), urban tree canopies and more.
Increasingly, green solutions are being incorporated alongside or as part of gray solutions as part of a ‘treatment train’ to boost system resilience. This enables improved pretreatment and filtration before runoff is discharged to existing sewer systems or waterbodies or utilized for rainwater harvesting.
Pretreatment solutions collect and filter stormwater to remove pollutants, reduce flowrates and reduce clogging of downstream systems. Materials now range from simple devices (i.e., inlet screen inserts) to complex systems (i.e., hydrodynamic separators) suitable for sites highly susceptible to spills (i.e., gas stations, commercial parking lots, and industrial sites).
Detention or retention solutions collect and store runoff (temporarily or permanently), often for flood control measures, while infiltration solutions capture and slowly seep stormwater into the underlying ground.
Filtration solutions typically treat stormwater runoff as it passes through media such as sand and soil removing total suspended solids, finer particles, and dissolved pollutants, including phosphorous.
Within each part of the treatment train, there are a range of new and existing technology options to meet the needs of a particular environment. Key considerations include the drainage area, land use characteristics, and water quality of the stormwater runoff entering the system.
Let’s look at a few examples of recent stormwater solutions and projects.
In London, where rainfall causes sewers to overflow into the Thames River, the Super Sewer tunnel is under construction to intercept, store, and ultimately transfer sewage waste. Measuring more than 15 miles (25km) long, the tunnel is set to be completed in 2025.
‘Sponge city’ developments seek to utilize interconnected greenways and waterways, green spaces, green roofs, porous drainage systems, and water savings initiatives to control and mitigate flooding and improve water resiliency. Major cities around the world including Auckland, Nairobi, Singapore, Mumbai, Shanghai, London, and Washington DC are utilizing the concept.
Hydrodynamic separators have been used for stormwater pre-treatment for some time, but advancements in vortex technology have made them more efficient and effective in recent years. These solutions utilize centrifugal force to filter water, offering improved pollutant removal, reduced maintenance, and a smaller environmental footprint.
Green roofs are living systems with vegetation cultivated on building rooftops, providing a greener alternative to underground storage systems by reducing runoff, improving water quality, enhancing biodiversity, and contributing to building energy efficiency.
Bioretention cells, also known as rain gardens or bio-swales, are designed to treat stormwater runoff through natural processes. Filled with engineered soil and vegetation, they effectively filter pollutants, promote infiltration, and enhance water quality before it reaches local water bodies.