In 2009, Ramboll's bridge engineers completed an award-winning cable-stayed swing footbridge that links parts of Derby with the city's Cathedral Gardens across the River Derwent. The site is a designated Category 3 Flood Zone and the river flows swiftly at the crossing point. Our environmental consultants undertook a flood risk assessment early on in the design process, which informed the design and became part of the planning application.
The assessment began with a site survey. Contact was made with the government's Environment Agency and other relevant parties to establish historical flood activity in the locale. The main issues identified for consideration were flood plain compensation, water flow affects that may result from construction of the proposed bridge, and the ongoing operation of the bridge, including its maintenance.
Various measures have been adopted as part of the construction works and bridge operation. To compensate for the reduction in flood storage volume available after bridge footing installation, the ground has been lowered downstream at River Gardens. As the underside of the deck of the bridge is below the '1 in 20' flood level for the Derwent, the bridge is raised from the water channel during high water in order to mitigate the effects of flow changes it would have caused.
The primary flood warning system, which Is crucial to the bridge's safe operation, relies on the operator — Derby City Council — reacting to the automated 'Severe' flood warnings issued by the Environment Agency, and raising the deck until the waters recede. There is also a water level sensor in place. A manual backup system is provided In case the hydraulic raising mechanism should fail. We recommended the development of a water-level monitoring, warning, closure and evacuation plan.