Stormwater runoff occurs when rain or snowmelt flows over impervious surfaces, especially in built-up areas where water cannot permeate the soil. The water will flow over paved areas, roads, parking lots and building rooftops picking up debris, deposits, and harmful pollutants before flowing into water bodies through a stormwater drainage system.
Urbanisation and increased human activities also contribute to higher volumes of stormwater as it reduces natural drainage in the environment where most water gets absorbed by trees and plants or soaks into the soil. Human activities worsen stormwater runoff by depositing harmful pollutants such as fertilisers and pesticides, rubbish, metals, petroleum byproducts, and pet and yard waste.
If not managed properly, stormwater runoff, especially during heavy rainfall, can cause flooding and erosion. It can pollute the environment, harming water systems and human health. Degradation of habitat and damage to property and infrastructure also contribute to high costs and devaluation of land.
Benefits of managing stormwater runoff
Stormwater management is important to protect the environment, maintain the health of water ways and protect people and property. It involves the control and use of stormwater runoff. Stormwater management considers the quality and quantity of stormwater and involves planning for the runoff, maintaining stormwater systems, and considering drainage in urban developments.
- Restoration of natural water cycles
Stormwater management supports the natural hydrological cycle, which is the distribution and movement of water between the earth’s atmosphere, land, and water bodies.
With proper management techniques, stormwater runoff can flow through natural or man-made conveyance systems to water bodies. It also helps water seep into soils which replenishes groundwater and aquifers, maintains the water flow in streams during dry seasons, and improves soil moisture essential for vegetation.
- Reduction of pollution in water bodies
Runoff over impervious surfaces has a high chance of picking up pollutants on its way to streams, rivers, lakes, and oceans. Contaminants such as grease, oil, pesticides, fertilisers, metals, plastics, animal waste, leaves and rubbish can impact water quality and are also harmful to wildlife and human health. Stormwater management reduces pollution and protects natural water resources.
Urban and infrastructure benefits
- Prevention of flooding
Stormwater management can help prevent flooding by controlling the volume of the runoff and directing it to a detention pond.
Infrastructure longevity and reduced maintenance costs
Proper management of stormwater runoff also protects infrastructure and reduces damage to property. It prevents soil erosion and degradation, thus ensuring that properties don’t devalue and maintenance costs are controlled.
Health and safety benefits
- Reduction of waterborne diseases
Urban areas and impervious surfaces cause pooling of water, which can encourage mosquito breeding, resulting in infectious diseases. Contaminated water also contributes to waterborne diseases and other related outbreaks. Proper management of stormwater runoff can protect public health and the risk of diseases.
- Enhanced public safety during heavy rainfall
If not properly managed, stormwater can increase the risk of flooding, particularly in areas with less vegetation, which can harm public safety and damage property.
Effective techniques for controlling stormwater runoff
Stormwater is unavoidable. However, effective strategies and techniques can be implemented to control runoff and reduce its damaging impact.
Green infrastructure solutions
Green infrastructures of sustainable techniques for stormwater management focus on reducing runoff and it impacts and increases water quality. Also known as green infrastructure or low-impact development, these nature-based practices help maintain the natural hydrologic cycles.
- Rain gardens and bioretention
Bioretention areas, also known as rain gardens, are constructed gardens in a shallow depression that capture and filter stormwater through soil mix and plants. They are designed to slow down the flow of stormwater and absorb contaminants before the runoff flows to natural water bodies. Rain gardens can range from a single small garden to being part of a larger project. They usually complement other conventional stormwater infrastructure like storm drains.
- Green roofs and permeable pavements
Many cities and urban areas now have green roofs for environmental reasons, including for stormwater retention, which delays the runoff entering the piped drainage systems. A range of plants, grasses, shrubs and trees are planted on roofs which also improve the quality of the runoff. Permeable pavements are another solution for stormwater management. This type of hard-surface paving allows stormwater to soak through an underlying layer of coarse gravel before it slowly drains away. It helps reduce the volume of water and enhances water quality.
Conventional solutions to manage stormwater modify the natural hydrology of a site and are designed to only reduce flooding. Modern engineering solutions involve intercepting and delaying the discharge of water before it flows off to a separate or combined sewer system, or to an adjacent waterway. These solutions not only reduce flooding but also minimises pollution and allow for the treatment of stormwater using natural processes.
- Retention and detention basins
Stormwater detention basins or ponds are reservoirs commonly used in New Zealand to collect and detain water before releasing it gradually into the natural waterways.
Retention basins are constructed to reduce downstream flooding and collect water permanently and are designed with an outlet or a spillway to control how much water leaves the pond. They are commonly used in areas where the ground water is near the surface.
- Constructed wetlands
Constructed wetlands are sizeable areas of densely vegetated ponds or water bodies that are used to manage stormwater runoff. Built in catchment areas, wetlands hold and control peak flows, and remove contaminants from the runoff through physical and biological processes before the water enters natural water bodies.
Innovative strategies for the future
Stormwater management is evolving every day with new emerging technologies, such as integrating data analytics for real-time monitoring, use of AI and machine learning to predict stormwater runoff, and sensors to identify contaminants. However, such technology is still under utilised and fragmented.
Community education and involvement also play a crucial role in stormwater management. Involving the community helps raise awareness and encourages people to adopt behaviours that reduce stormwater pollution. They can also develop and support solutions that are unique to their region and sustain these efforts over a long period.
Managing stormwater runoff
Stormwater runoff, if not managed properly and efficiently, can impact the community, infrastructure, and the environment. The most important strategies for management combine various measures that incorporated urban planning and design, green solutions, and other infrastructure, such as pumping stations and underground drainage systems. Although these strategies benefit stormwater management and pollution reduction, community education and involvement is also necessary to promote awareness, change behaviours, and support these efforts.