Wetlands are water-rich natural areas that occur chiefly along rivers and in deltas. Those are the same places where most urbanisation occurs, however. Urban expansions and the correspondingly lower groundwater levels put pressure on wetlands and wet nature around the world.
By their very nature, wetlands are overflow areas for rivers and as such are natural rainwater buffers. Wet natural areas are of great importance to amphibians and dragonfly species and are a breeding ground for many species of birds.
Recently the possibilities for creating wetlands in and around towns and cities have been given more attention, following the disappearance in recent decades of large stretches of wet nature, and thus also natural buffers, as a result of urban expansions and land drainage. In the USA, for example, wetlands are being created to buffer some of the run-off from precipitation and slow the rate at which it drains away. This is a new approach compared with rapid drainage systems using ditches and pipes. Another benefit of wetlands is that biological pollutants are eliminated by plant life and settle in the sediment, which significantly improves the quality of the run-off.
In some cities, London for example, wetlands serve a function by developing greater biodiversity and natural and pleasant recreation areas for city dwellers. Urban wetlands should be designed in such a way that they allow for the possibility that the water running into them from the urban surroundings is more polluted than in a natural environment. Moreover, recreational uses might conflict with the targeted natural development. Another difference is that urban wetlands are less dynamic than natural wetlands. In natural wetlands, flow patterns change and zones fall dry. In urban wetlands, the process is controlled more by humans, since certain visual qualities and uses are sustained and less dynamism is accepted.
Urban wetlands are capable of purifying urban water efficiently and cheaply. In the USA, conventional wisdom holds that urban wetlands are ten times as cheap as conventional, more high-tech purification plants for purifying urban rainwater run-off. That is one of the principal reasons why natural wetlands in urban areas are protected in the United States. However, the multiple roles that wetlands play for urban systems are so important that more and more new wetlands are being created.
Natural and in particular urban wetlands can play an important role in coastal protection and as protection against river flooding. Sediment settles in natural deltas, offering a natural barrier to protect the hinterland. In areas where space is under less pressure, or in combination with recreational functions, wetlands present an alternative to conventional rainwater processing.
Wetlands, even urban wetlands, are important biotopes. Rendering parts of urban wetlands difficult or impossible to access enhances that effect.
Urban wetlands can fulfil an important function in improving the quality of surface water and purifying precipitation running off from towns and cities. Processing urban precipitation run-off and surface water in urban wetlands helps in the elimination of phosphates, nitrates, solid substances and heavy metals. Urban wetlands can be used to maintain or improve the quality of surface water. [Ehrenfeld, 2000]
The size needed for an urban wetland depends on the volumes of water that have to be buffered and the extent to which the water entering the buffer is polluted. If the urban wetland serves to purify rainwater run-off, the indicated design calls for a wetland surface that is approximately 5% of the surface from which the rainwater runs off. Wetlands can be designed to handle fluctuations in water levels of up to 30 cm. The maximum fluctuation also determines the buffering capacity. This means that wetlands can serve as both buffering and purification systems. Any excess water from heavy rainfall needs an overflow area to run off into surface water.
Realising sufficient storage capacity to buffer even heavy rainfall requires that 10-15% of the connected area must be suited for storing water, whether as surface water or also as wetland.
Urban wetlands can be designed to function entirely without pumps or similar instruments. Urban wetlands always consist of inflow, sedimentation and marshland zones combined with a pond or other open water area.
In the inflow and sedimentation zones, all large waste matter is eliminated before the water enters the marshland zone, to prevent the wetland from becoming polluted too quickly. The sedimentation zone requires regular cleaning, meaning that the sedimentation system needs to be accessible to a tank lorry.
A stormwater filter, for example a gravel layer, can be used to eliminate larger and floating waste. That part also requires regular maintenance and so must be easy to access.
The marshland zone should cover approximately 80% of the wetland part. The water meanders through trenches in the marshland to maximise the time its remains there.
This zone should be designed in such a manner that the water can pass through it easily, to prevent areas of standing water (standing water can increase the growth of mosquito populations).
Wetlands must also have one or more overflow areas. For reasons of maintenance, it is important that the wetland can be temporarily shut off from outside water sources at any time.
Water level fluctuations
It is best if fluctuations in the water level are restricted to 30 cm at most. Greater differences in water levels lead to less diversity in flora and fauna.
The effluent from wetlands is purified water that is highly suitable for use in sprinkler systems or for supplementing urban surface water. [Melbourne Water, 2005]