Viivästyttämisallas

A viivästyttämisallas, also known as a detention basin or retention pond, is a constructed stormwater management feature designed to temporarily hold and then release surface water runoff at a controlled rate. These basins are typically dug into the ground and can be dry or wet depending on their design and the local rainfall patterns. Their primary purpose is to reduce the peak flow rate of stormwater entering natural waterways or drainage systems downstream. By attenuating these peak flows, viivästyttämisallas helps to prevent or mitigate flooding and erosion in receiving areas.

The design of a viivästyttämisallas often includes an inlet for the incoming runoff, a permanent pool of water (in wet basins), and an outlet structure that regulates the release of water. The outlet can be a simple pipe with an orifice or a more complex structure designed to control outflow based on water level. During a rainfall event, the basin fills with runoff, slowing down its movement. As the runoff is released through the outlet, the rate is slowed compared to what it would be without the basin. This controlled release allows for more time for sediment and pollutants to settle out of the water, thus also providing some water quality improvement.

Viivästyttämisallas are commonly used in urban and suburban developments, along roadways, and in industrial areas to manage the increased impervious surfaces that lead to greater stormwater runoff volumes and faster flow rates. They are an important component of modern stormwater management strategies aimed at protecting both infrastructure and the environment.