Built across a wide stretch of the river, the barrier divides the river into four and two approximately 30-metre (100 ft) navigable spans. There are also four smaller non-navigable channels between nine concrete piers and two abutments. The flood gates across the openings are circular segments in cross section, and they operate by rotating, raised to allow "underspill" to allow operators to control upstream levels and a complete 180 degree rotation for maintenance. All the gates are hollow and made of steel up to thick. The gates are filled with water when submerged and empty as they emerge from the river. The four large central gates are high and weigh 3,700 tonnes each. Four radial gates by the river banks, also about 30 metres (100 ft) wide, can be lowered. These gate openings, unlike the main six, are non-navigable.