BEACH WELLS AT LE MORNE NEW MAURITIUS HOTEL

TECHNICAL REPORT ON CLOGGING

Prepared by:

Sebastien MARTIAL

Geotechnical Services Ltd

Prepared for:

Water Research Co. Ltd

March 2014

Table of Contents:

1. Introduction

2. Geological and Geomorphological Background

3. Coastal Morphology

4. Beach wells at New Mauritius Hotel

5. Site Profile

6. Origin of reduction on yield

7. Alternatives for recovering yield from beach wells

a. Recovering yield

b. Location for additional beach wells to cater for required volume

8. Bibliography

Prepared by Checked

Dr Sebastien Martial Emilio Saldivar

Date Date

14 March 2014 14 March 2014

Signature Signature

______________________ _________________

1. INTRODUCTION

Water Research has commissioned Geotechnical Services a report to evaluate the reason for clogging on boreholes

drilled for Le Paradis Hotel at Le Morne. This report investigates the geological and overburden conditions,

determine the actual configuration of the existing boreholes and concludes on the reason for the observed

reduction on yield. This Report also includes suggestions as to alternatives sites where sea water extraction can be

achieved.

2. GEOLOGICAL AND GEOMORPHOLOGICAL BACKGROUND

The geological configuration on the site is characteristic of the Grande Case Noyale to Le Morne area. This area

includes the eroded remnant of the old shield, facing the sea, unmodified by more recent volcanic deposit and

overlying the Old Breccia formation. The Old Series corresponds to the shield volcano building stage. The Old Lava

Series is made of aphyric and ankaramite basalt and constitutes the several mountain peaks of the area (Le Morne,

Piton de la Petite Riviere Noire, Mt Laporte, Piton du Canot,…), The Old Breccia formation corresponds to the

primitive stage in the construction of the island. The Old Breccia formation outcrops in two locations, on the

northeast of Le Morne peninsula and in Case Noyale region (see Fig.2)

There are no large stream in the region; only a few streamlets that drain short distances to the shoreline. It

simply consists of a low coastal plain (alluvial deposit) that rises first gradually (colluvial slopes), then more steeply

(talus slopes), and finally ends in the shear walls of free standing lava cliffs.

Fig.1: Aerial view of Le Morne area (extract from Google Earth)

Fig.2: Extract of Geological Map of Mauritius (Giorgi et al.)

The coastline parallels the ridge line between Piton du Canot and Mount Laporte. This stretch is an illustration

of slope forming processes and of an erosional scarp that has retreated from a position more seaward than its

present location. If sea level were lower, as it was during glacial periods, the slope at the base of the escarpment

would extend to the line of coral reefs. It would look like a broad flat plain (a pediment). Because of sea level rise,

the lower pediment surface is now submerged, and only a narrow coastal strip is left along the shore. Because the

stretch of the coast is not modified by more recent volcanism, the escarpment has retreated backward for a period

of at least 6Myrs (age of Old Lava Series), and considering the position of the coral reef, it can be estimated that

the slope has been retreating at a rate of 6km for 6Myrs, or 1mm per year.

Montagioni (1979) suggested that Mauritian coastline has experienced episodic sea level fluctuation. The sea

level was -3.5m with respect to present levels 4,500 yr.B.P. The average sea rose to attain the present levels

between 2,000 and 1,000 B.P. Hence, the position of the shoreline is changing versus time. Studies conducted by

the Mauritian Meteorological Services and reported by the National Climate Committee (1991) indicate a sea

level rise of 1.2mm/yr.

3. COASTAL MORPHOLOGY

Fig.3 : Type of Coastal Lines in Mauritius (Saddul, P.)

Although coastal features are varied and coastal processes numerous, a definite regional pattern can be detected

in the overall character of the coastline. A typology of the coastline based on the geology and the topography of

the coastal zone, taking into account both the onshore and offshore portions, provides an analytical description of

the coastline. (see Fig.3)

This typology is firstly based on the nature, structure and age of underlying rocks, then on the width of the reef flat

and eventually on the exposure to dominant winds. The area of Le Morne fall under Type D, which corresponds to

Old Lava Series coastline. Type D coastline is found between GRSE and Mahebourg, between Le Morne & Petite

Riviere Noire, and from GRNW to Riviere Terre Rouge.

The basic feature of coastline D is the presence of a wide shelf, more than 5km in extension from Case Noyale, and

long-weathered mountains of the Old Series, which provides an abundant load of sediments of all sizes to the

coastal zone. As a result the main characteristic of the Type D is the absence of a sand beach along the coastline,

since the coral fragments are dispersed and deposited in the wide reef flat. From Le Morne peninsula to Petite

Rivière Noire, the coastal morphology is also influenced by the Ile aux Benitiers and intra-lagoonal channels further

north that act as an obstacle to the onshore movement of coral sand. Therefore a muddy silty beach with pebbles

in the proximity of the stream mouths can be observed.

4. BEACH WELLS AT NEW MAURITIUS HOTEL

Fig.4: Location of beach wells drilled @ NMH, Le Morne

A total of 5No, boreholes has been drilled at Le Morne, the locations are shown in Figure 4. BH1 & BH2 have been

drilled in 2007. Due to increased water requirement, exploitation of BH2 was abandoned and BH3 was drilled in

2012. Reduction on yield from BH3 was observed after approximately one year of operation, which led to drilling

of BH4 in October-2013; BH4 also showed reduction on yield by early 2014, approximately 3 months after starting

to be operated. BH5 has been drilled in January-2014 and reduction of yield was observed only 4 days after.

For each beach well, a geotextile liner was installed on the perimeter of the PVC so not to allow sand or granular

material entering the borehole.

5. SITE PROFILE

The soil profile at the site has been recorded during the drilling of the boreholes; these observations coincide with

the profile expected from the published geology. Schematic of geological profile (fig.5) met at BH5, which is roughly

equivalent to other beach wells drilled, shows two strata of corals (the first being 4m thick and the second 8m thick)

with an inter-bed of 3m of grey silty-clays which has to be related to terrestrial erosion discussed above. These

sedimentary formations are lying above highly weathered old basalt formations and its paleosoil (1.75m here).

Fig.5: Geology found around BH5

6. ORIGIN OF REDUCTION ON YIELD

BH1 is pumped continuously at a rate of 150 m3/hr since it started to operate. The same extraction rate was

intended for the additional beach wells. It has been observed that BH5, and even BH3 & BH4, could not be operated

for a very long period with such pumping rate (cavitation occurring, with air pulse being heard denoting excessive

suction with regards to effective well capacity). The yield reduction, up to 80% of yield, was observed in BH5 after

days of operation; while BH 4 and BH 3 have shown yield reductions on the order of about 50%. However, after a

resting period, better yield has been recovered from BH3 & BH4, which are currently in operation at a extraction

rate being about 66% of their initial operational setup.

Considering all elements mentioned above, the reduction on yield observed on the wells at Le Morne are most

probably related to several simultaneous causes, namely:

Presence of a silty-clay formation of 3m thickness

Excessive pumping rate creating turbulent flow enabling mobilization of the clays

a) Silty-Clay formation

The silty-clay formation is found between the two coral layers. The presence of this horizon suggests that the

shoreline could, in the past, have been located closer to the beach wells area than it is today, more than 2km inland.

The constituents of this silty-clay are mainly of detrital origin, generated from the erosion of the old shield volcano,

although marine influence can be observed with presence of rare large corals.

b) Pumping rates

Current salt water extraction requirement by NMH is on the order of 300 m3/hr. Operational strategy is to carry

out pumping on 2No beach well, extracting 150m3/hr each. Such pumping rate could appear to be above effective

capacity of some of beach wells drilled, particularly when considering specific well efficiency. In such operational

conditions, turbulent flow is likely to occur in water bearing formation in the vicinity of the well, allowing transport

of fine sediments.

These causes resulted on the silty-clays been mobilized and clogged the geotextile membrane, reducing its

permeability and the productivity of the wells (clogging effect)

7. ALTERNATIVES FOR RECOVERING YIELD FROM BEACH WELLS

The following paragraph describe alternatives to increase the yield for clogged beach wells and also alternative

location if further boreholes are required. Increase of yield can be applied to BH3 and BH4; while the clogging at

BH4 and BH5 may be such that yield recovery may not be successful.

Before the rehabilitation works are implemented, a sequence of step pumping tests should be implemented to

evaluate well efficiency and the optimum pumping rate on each beach well. The step pumping tests should not be

performed simultaneously, so that interactions between wells do not generate erroneous interpretations.

Following step pumping tests, a simultaneous long term pumping test will need to be carried out.

a. RECOVERING YIELD

i) Redevelop the well with air lift:

Following step pumping tests, the beach wells showing poor efficiency can be subjected to air-lift development

(injection of air in the well, flushing out the fine sediments). This operation could help to remove the silty clay stuck

against the membrane. However, due care and attention should be implemented since, depending on the flow of

air pushed into the well and the resistance from the clay, the development could induce the membrane to expand

and eventually get ripped off. Removal of the membrane would allow the fine sands to get into the system and

damage the pumps and the piping.

ii) Remove existing PVC and replace with new PVC/geotextile

If redevelopment operation does not provide sufficient improvement regarding well efficiency, the geotextile

membrane on which the clogging occurred would be removed and replaced. However, this operation is risky as,

with such instable formations, the structure of the bore can be affected upon removal of the PVC, being therefore

backfilled and become unusable afterwards. Moreover, the presently installed PVC could probably be stuck in the

borehole and there is a chance that the PVC is broken during removal operation. Hence, it is strongly advised to

avoid proceeding that way.

b. LOCATION FOR ADDITIONAL BEACH WELLS TO CATER FOR REQUIRED VOLUME

Considering the above mentioned, it is reasonable to think that if additional beach wells have to be drilled, the area

of present beach wells should strongly be avoided. In order to maximize the opportunities to get better

hydrodynamical conditions, the future boreholes could be located closer to the western lagoon rather than the

northern lagoon, which is known for its little marine current dynamic conditions, leading to deposit of fine to very

fine sediments (silty-sand inducing clogging). Furthermore, the sand is likely to be coarse, providing good

permeability, on the area of the western lagoon. Fig. 6 shows the alternative locations for beachwells.

However, moving westwards will imply NMH to invest in additional piping to reach the desalination plant. The best

option therefore should consider:

- Maximum distance from the foot of Le Morne mountain, in order to increase productive thickness

- Minimum distance from the sea shore, in order to increase connectivity to recharge boundary

- Shortest or cheapest piping setup towards desalination plant / Access to electricity

- Location of borehole should not become an eyesore and should therefore be somehow hidden

Fig.6 : Proposed locations for additional beach wells @ New Mauritius Hotel, Le Morne

Location proposed Piping Dist. to sea Dist to Mountain

Proposal 1 Beside Marina 400m 90m 600m

Proposal 2 Near Gas Store 310m 125m 480m

Proposal 3 Technical Bldg 150m 160m 330m

Proposal 4 Near Dinarobin Tennis 400m 160m 260m

Proposal 5 Near Dinarobin Back Entrance 570m 100m 400m

Proposal 1

Proposal 2

Proposal 4

Proposal 5

Proposal 3

8. Bibliography :

- Daby (2006) “Current Patterns and the Distribution of Benthic Habitats in a Coastal Lagoon of

Mauritius. University of Mauritius.

- EcoAfrica, 20913, Lagoon Management Plan of Le Morne Cultural Landscape, Report done for the Le

Morne Heritage Trust Fund, 68p, http://www.ecoafrica.co.za/lemorneLMP

- Giorgi et al. (1999), “Carte Geologique au 1:50,000 & Schema hydrogeologique”, Geoloab

- IOC (1994) “Planning Workshop on An Integrated Approach to Coastal Erosion, Sea Level

Changes and Their Impacts”, Intergovernmental Oceanographic Commission - Workshop Report

No. 96 - Supplement 1 - IOC-UNEP-WMO-SAREC, Institute of Marine Sciences (University of Dar

es Salaam), Zanzibar, United Republic of Tanzania, 17-21 January 1994, 180p. , Section IV :

Status of Coastline Changes in Mauritius, p29-64,

http://www.jodc.go.jp/info/ioc_doc/Workshop/w096s1.pdf

- Montaggioni (1980) “ Les recifs coralliens des Mascareignes”, collection des travaux du centre

universitaire de la Reunion. No6

- Saddul, Prem (2002), “Mauritius, A Geomorphological Analysis”, Mahatma Gandhi Institute, 354p.

ISBN 99903-39-33-3

- http://www.articlesbase.com/international-studies-articles/le-morne-shoreline-change-

assessment-mauritius-coast-4047412.html

- Thomassin A. 2011. Recommandations finales dans: Etude de faisabilité pour la mise en place d'une

ou plusieurs AMP sur la côte sud-ouest de Maurice rédigé pour MMCS/ProGeCo. 54p.