Rainwater ponds: a combination of buffering, purification and infiltration
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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.