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| What
are the possible Flood Management Options? There are a variety of general flood management options which can be adopted within the estuary. Such options may be used in combination to produce an overall Preferred Estuary Strategy and therefore the preferred strategy is unlikely to be made up of just one option throughout the whole estuary. No Active Intervention (formerly referred to as 'Do Nothing') Where applied this option would involve ceasing all maintenance, repair and renewal work on the defences. The defences would be monitored and assessed until the defences eventually failed or a “Do Something” option was deemed necessary. If the defences were left to fail, the areas currently protected from flooding would no longer be protected. Existing sluices and weirs would also fail over a period of time. Large scale implementation of this option is likely to alter the estuary’s shape and may change water levels in the estuary up to the current tidal limit and beyond. Depending on many local factors the flooded land may then turn into mud flat or saltmarsh. Increases in flooding are often associated with negative impacts upon the flooded areas where valuable features are lost. However, in some cases flooding could bring some positive effects through habitat creation that will enhance the local environment leading to benefits to some recreational users of the estuary. Additionally, the conversion of land to estuarine habitat could ease the pressure on more critical flood defences in other parts of the estuary.
Do Minimum This flood defence approach involves ‘Limited Intervention’ and, where applied, entails only undertaking work to maintain and repair the existing line of defence when it is an emergency for immediate health and safety reasons. This option entails working with natural processes to reduce risks while allowing natural change and will eventually lead to a reduced standard of defence over time due to the effects of sea level rise. The long term effects would be likely to be similar to No Active Intervention.
Hold the Line Where applied this involves holding the existing length of defence through management, such as maintenance, repair work or improvements. However, due to sea level rise, holding the current line of defences is likely to require the existing defences to be substantially altered or completely reconstructed if they are to provide the same level of protection in the future as they do now. Holding the line restricts the ability of the estuary to respond to changes in sea level and may result in increased pressure on defences and potential loss of saltmarsh and mudflat habitat in front of them.
Advance the Line This involves building flood defences, like walls or embankments, in front of existing defences, moving the line of defence forward into the estuary. The construction of a tidal barrier or barrage can be also used to advance the line of defences, where appropriate. The aim of a barrier or barrage is to stop flood tides from passing upstream beyond a certain point, limiting the area at risk from flooding in the estuary. A permanently raised barrage would affect upstream water level management but could reduce pressure on flood defences downstream and also limit navigation upstream. Alternatively a moveable tidal barrier that can be raised and lowered to protect upstream areas from very high tides would not affect the water levels in the estuary at any other time.
Managed Realignment Managed realignment involves the partial or complete removal of a stretch of flood defence at a specific site, or movement of the line of defence onto the land behind. This allows the tide to inundate the exposed land during each tidal cycle allowing the estuary floodplain to expand until it meets higher land or the line of the new defence. Managed realignment can be implemented over time in a phased approach allowing the estuary to be realigned to higher ground over a number of years in a more controlled way than simply breaching the defence line or allowing it to fail. Managed realignment provides controlled conversion of land to either mudflat or saltmarsh and is often used to ease the pressure on more critical flood defences in other parts of the estuary and can replace intertidal habitat lost to sea level rise. Large-scale managed realignment may lead to changes in the estuary’s shape.
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