 Sluices 1, 2 and 4 ('shallow') are used to control the water level, while sluice 3 ('deep') removes the silt.When the deep sluice is opened at low tide, a powerful undertow is created which sucks the silt out of the harbour and into the river.Since the original introduction of the sluices, they have been changed and renewed several times but today's sluice system was installed in the 1880s.