Delta Life 11

H ow often should street drains be emptied so that they do their job? Rainwater is not the only thing that flows into street drains. Twigs, sand and heavy metals: they all go down the drain, literally. With all the consequences that implies. Because in the end, the drains get clogged up, with flooded streets as a result. In addition, the water jet in the drain digs into the sediment on the bottom, churning up the dirt, which then flows into the sewer. The question of how often street drains should be emptied is being investigated worldwide. In the Netherlands, the research is part of the Urban Drainage Research Programme, which was established to help the Dutch sewage sector tackle the challenge of increasing increase efficiency by 380 million euros from 2020 onwards. More and more cities are installing separate sewage systems: one for waste water and one for rainwater. The waste water from homes and businesses goes straight to a water treatment plant. Rainwater, which is much cleaner, is directed through a separate system and discharged into the surface water, in other words ditches and lakes. But obviously, whether the water is treated or discharged into the surface water, it is best for it to be as clean as possible. But how long does it take before a drain fills up? Nobody knows. At the moment, urban authorities clean the drains once a year. But is that often enough, or too often? Deltares is using a system representing a transparent drain linked to the sewers. It makes it possible to watch how water and sediment behave in the drains with different amounts of water and types of sand. The aim is to get an answer to the question of how frequently the drains need to be emptied. THE DRAIN IS BLOCKED PHOTO: DELTARES 22 Study of the efficiency of street drains Inlet to street drain Inlet duct representing the street Supply system with a mixture of water and sand (not shown in the picture) Bottom of the drain with scales for sediment bed Outlet pipe to sewer TESTING GROUND