Bridge clearance and water levels: why clearance can vary

The clearance beneath a bridge is the unobstructed vertical distance between the water surface and the underside of the bridge. It is not simply a fixed specification of the structure: as the water rises, the distance decreases; as it falls, the distance increases. The clearance available at a particular location therefore depends on the water level at the relevant gauge and on the designated navigation channel.

Bridge constant and water gauge

For the Elbe, the WSV bridge tables calculate clearance in centimetres by subtracting the gauge reading from the bridge constant. The constant applies to a specific structure; the reference gauge and navigation channel are listed in the relevant table. At some bridges, the figures also differ for upstream and downstream travel. A figure for another channel or a nearby structure cannot simply be substituted.

The Barby railway bridge provides a concrete example: according to the WSV, its bridge constant is 1,115 cm. At a gauge reading of 200 cm, the table gives a clearance of 915 cm. If the gauge reading changes, the tabulated figure changes accordingly. This example applies only to this bridge and to the Barby reference gauge specified in the table.

Comparing bridge clearance with vessel height

For comparison, what matters is the vessel’s clearance above the waterline, not its total height from the keel to its highest structure. The WSV table initially gives the local clearance without a safety margin and recommends that the vessel’s height above water remain about 30 cm below it. The bridge clearance and vessel height must refer to the correct location and navigation channel.

The table is explicitly intended for route planning. It notes that retrieved gauge readings may be incorrect and that the calculated clearance alone does not establish whether passage is actually possible. A tabulated figure indicates the available clearance for a given gauge reading; it does not independently confirm that a particular vessel can pass beneath the bridge.

Matching the figures correctly

A reliable assessment therefore brings together three details: the specific bridge and its marked navigation channel, the relevant reference gauge and the water level used, and the vessel’s clearance above the waterline. For Elbe bridges, these references are shown in the WSV tables. The figures and calculation conditions apply only to the particular structure; they are not a general formula for approving passage under other bridges or along other routes.