Buffer ponds temporarily capture precipitation and allow it to drain off slowly. During rainfall, the rainwater is captured in the pond and subsequently drained off to create room for the next precipitation. Nowadays control systems exist that are linked to the weather forecast and that do not drain off the rainwater until right before another shower is forecast, to ensure that the pond remains filled for as long as possible.
Buffer ponds can be designed to have a mostly stony or a mostly natural appearance. They are an excellent way of enhancing the town or city’s appearance. However, designs for buffer ponds need to allow for fluctuating water levels. Natural ponds can be made suitable for greater differences in water levels with vegetation along their banks that can survive both dry and wet periods. Many urban buffer ponds are combined with underground storage for long dry spells, so that the stored rainwater can be used to supplement the water in the pond. Assuming a maximum fluctuation of 30 cm in the water levels, the surface of the buffer pond needs to be 10% of the impervious surface connected to it. Buffer ponds must always have a possibility to overflow into the urban water system.
More natural buffer ponds have a purifying function and also have value as habitats for flora and fauna. Of course it is possible to combine stony ponds with green zones for purification. Designs for buffer ponds need to take child safety into account. In urban areas, these buffering systems require regular maintenance, for example to remove litter. The quality and maintenance of the system determine the level of acceptance on the part of the residents of the town or city.
Rainwater ponds for buffering and purification of moderately polluted water
In areas that experience moderately polluted rainwater, one option is to realise rainwater ponds for temporary storage and purification using vegetation. The polluted water might also undergo an additional pre-purification treatment in a sand filter in the bank zone.
Water is purified in a pond with vegetation because the pollutants settle on the bottom and the plants break down and absorb the pollutants. Introducing a circulation system can guide the water specifically along the planted bank zones to increase the purification. An attractive artwork or playground element might be made part of such a circulation system.
The pond should ideally be at least 1.5 metres deep. That depth limits the degree to which the water heats up in the summer and diminishes the risk of problems with the water quality. In the winter, water that deep will not freeze solid and the pond will continue to offer a zone for fish to live in.
If the ground permits, the overflow from the pond can be designed to act as an infiltration system. Geiger et al., 2009
Rainwater ponds for buffering and purifying extremely polluted water
Systems for buffering and purifying extremely polluted water should be sealed from the ground by means of a film. This prevents direct infiltration. These systems are often used to pre-purify rainwater running off from busy roads and car parks. These systems have a ground filter that offers a good purification yield, and always feature an overflow that, thanks to the pre-purification, can be connected to surface water or an infiltration system. Only if no surface water is available in the immediate vicinity and infiltration is impossible should the overflow be connected to the sewer system.