Grand Vaux Floods: Still a 1 in 100 year event?

Having the largest water catchment in Jersey the Grands Vaux Reservoir is critical to the Jersey Water Supply Infrastructure. It is periodically used to capture and transfer water to Queen’s Valley Reservoir and to allow general supply schemes to operate. This means that there is limited capacity in the reservoir to attenuate flood flows generated from significant rainfall within the catchment. ,

The Grands Vaux Reservoir basin is small and can fill very rapidly during a storm. The flood volume, in severe weather, can be greater than the capacity of the basin and, in combination with the steep topography of the land around the reservoir, the reservoir frequently overflows as it is designed to do.

The topography of the area immediately downstream of the dam consists of a steep sided, narrow valley which channels any overtopped water down the valley into St Helier. As a control measure, the downstream (school) bypass culvert is continuously monitored for level and blockages by Jersey Water’s 24 hour control room. The existing drainage network in St Helier can accommodate low return period events like the 1 in 1 year (100% AEP) but would be unable to handle events of greater magnitude (lower AEP)

Modelling of the Grands Vaux Reservoir indicates that a 1 in 100 year (0.1% AEP) return period rainfall event, with more than 53mm of rain within a 6 hour duration, could have significant effects on the area downstream. The latest States of Jersey Grands Vaux Flood Plan was updated in July 2018 and contains details of the multiagency response to flooding due to an extreme storm event resulting in an increase of water in the catchment and subsequent reservoir overflow. The flood plan details the triggers and activation process of the plan if a forecasted or un-forecasted flood event was to occur.

The flood plan also shows the area at risk of flooding if the reservoir was to overflow during an extreme storm event. It should be noted that a review of the treatment of reservoirs within a planning policy context has been undertaken to understand how the risks associated with the overflow of the Grands Vaux Reservoir during more extreme storm events might be considered within the Island Plan Review. This is of particular interest for Government of Jersey as St Helier is the primary urban centre in the island and may be the focus for growth within the Plan Review period.

https://www.gov.je/SiteCollectionDocuments/Planning%20and%20building/R%20Jersey%20Strategic%20Flood%20Risk%20Assessment%202021%20AECOM.pdf