Why not a reservoir?

Jersey only has 120 days of water storage (and as our population grows this number will decline). The island can either increase water storage, rely on expensive desalination to meet the challenges of climate change, or create a water storage reservoir.

In 2019 Jersey Water proposed that La Gigoulande Quarry should be used as a reservoir. The DBIP excluded this option. This decision was primarily based on an ‘independent’ report by JBA Consulting, commissioned by Granite Products in 2020, stating that the quarry was unsuitable for water storage. 

Subsequently Jersey Water has warned: of “temporary customer restrictions during draughts”, and that “The decision has removed the most cost-effective feasible water storage option”. Jersey Water have also identified that a new valley would need to be flooded due to La Gigoulande’s exclusion “a new reservoir option (Les Mouriers) that could be developed within the existing Le Mourier stream intake catchment”.

Geo-Engineering Consulting Services (Jersey) have undertaken a review of the JBA report and their findings cast serious doubts on the conclusion that La Gigoulande is unsuitable as a water reservoir. It would seem that the exclusion of this option is premature, and it would be in Jersey’s best interest to have this major policy decision independently scrutinised.

This is Geo-Engineering Consulting Services’ analysis:



As part of the Island Plan, the practicality of using the Quarry as an ‘active’ storage reservoir was considered by Jersey Water.  This proposed use would in principle accord with the overall Green Valley visions for St Peter’s Valley that were the subject of a significant petition in 2020 as input to the Island Bridging Plan.  This survey identified the current negative impacts of the Quarry operation (and associated industrial/storage areas) to a historically and archaeologically important area comprising the double wheel La Gigoulande Mill and associated streams leading upstream to the Elms and downstream to Quetivel Mill.


As an end use prospect, the conversion of current Quarry operations to a reservoir is seen to have significant benefits in terms of both environmental restoration of the area and provision of water storage that is a critical issue in terms of the Island’s future.


A significant Technical Note has been proposed by JBA Consulting of Skipton, UK for Granite Products as an ‘initial’ assessment of Jersey Water’s proposal.  The Purpose of the Study document, prepared by JBA Consulting focuses on the storage options for up to 0.24m³ of water using different lining approaches.  


The Purpose of the document (as stated in Section 1) is to provide advice to Granite Products based on ‘key engineering challenges’ that the ‘principle’ and ‘practicality of the proposal’ ‘cannot be supported’.  This is largely based on the ‘key engineering challenges’ that ‘would need to be overcome’.


Geo-Engineering, an engineering consultancy based in Jersey (since 1980), with extensive experience in groundwater flow, have examined the Assessment concerning the engineering feasibility of the quarry conversion into a water storage basin without any prior conviction.


The options considered by JBA Consulting involve two approaches as follows:  (TN Part 3):


  1. Allow the quarry to fill naturally without engineering ‘isolation’ works;
  2. Form an ‘engineered’ reservoir isolated from natural groundwater.

The consequences of rock slope failure are considered in ground terms only, without clear discussion of actual risk.  From an understanding of the nature of the granitic rock mass and its natural fracture/fissure geometry, it is considered likely that stability of the quarry slopes will not be a major issue in terms of the future quarry operation as a reservoir.


Major (overall) failures are thus not anticipated under a flooded state or under realistic drawdown scenarios.  (TN Part 3.1, Figure 3-1).


Concern is also expressed regarding seepage losses as related to existing ground water levels under an excess head condition relative to so called natural groundwater state.  (TN Part 3.1, Figure 302).  The concept of ‘seepage losses due to excess head’ is not considered to be of material importance, given the likely ambient groundwater state (post flooding) close to the ground surface.


Approach 2 involves forming an ‘isolated’ reservoir either by granting of the fractural rock mass (2a) or by the provision of a formed ‘impermeable’ reinforced concrete structural liner.  (2b).


In terms of the solutions proposed, neither option is considered either practical or warranted in terms of normal civil engineering practice for reservoir construction.


Overall there is no need to consider costly grouting or lining options to isolate the reservoir volume from the surrounding competent but fissured rock mass in granite.  


The reservoir option would rely on the general state of equilibrium between the elevated (natural) high groundwater level and the reservoir level.  Under low operational levels inflow of groundwater from the quarry slopes would supplement the available water supply from the former quarry excavation.  


In the Discussion provided (TN Part 4) various requirements (issues, constraints and challenges) are identified.  As noted ‘Each of these respective issues potentially in themselves evidence that the principle of implementing a water storage reservoir at La Gigoulande cannot be supported’.


From an overall Reservoir Engineering approach there are significant technical misconceptions within the JBA Technical Note notably in Approach as 2a and 2b in relation to groundwater response, rock slope stability and leakage.


In considering the natural behaviour of the flooded quarry basin within the granite rock mass surround, it is likely that this could be used as a storage facility with only minimal treatment of isolated major structures.  Overall, as a water supply structure it would benefit as a major source of groundwater under drawdown states (rather like a very large well).  


Given the environmental sensitivity of the valley zone, use of the quarry as a significant water feature has many positive benefits.


Geo-Engineering Consulting Services – St Peter – Jersey