hydrogeological study - bois sec / bois cheri

HYDROGEOLOGICAL STUDY SUPPLY OF WATER FOR THE AREA OF BOIS SEC / GROS RUISSEAU

Prepared for Saint Aubin Ltée

Prepared by Geotechnical Services Limited

S. MARTIAL / E. SALDIVAR

September 2014

HYD-SM-14014 1

Executive summary

This Report was commissioned to evaluate fresh groundwater supply in the area of Bois Sec for

domestic and irrigation usage for a potential real estate development.

The available literature regarding groundwater resources in the area showed that there is a

general potential for groundwater extraction. Spatial and temporal rainfall distribution and soil

lithology contributes to provide a significant recharge to the watershed. However, detailed

study from boreholes drilled in the area and further investigations revealed that:

- The six boreholes drilled for Avalon project show very little potential, with optimal

extraction rate ranging from 1.5 m3/hr to 25m3/hr.

- The borehole drilled in 1999 for Société Usinière de Bois Chéri has been tested up to

24m3/hr, with only 4.4m of drawdown over 72hrs pumping and reveals that

groundwater potential is higher in this area.

- Groundwater potential is significantly controlled by extent of Recent Lava Series.

- The presence of faults, probably link to the crater of Bois Chéri, is likely to have a

significant hydrogeological positive impact.

Surface water network is well developed in the area, including several streams originating in this

area. The springs are located at interface between Recent Lavas Series and Intermediate Lava

Series and contribute to preserve a minimal permanent flow rate. However, stream located

westward from Ruisseau Marron are not showing permanent flow, although very deeply incised

by torrential flow events.

Proposed strategy is:

- The use of surface water should be encouraged instead of groundwater as the potential

volume is greater.

- Groundwater should however been considered to ensure minimal supply when surface

water quality become inadequate for domestic purpose, relatively frequent intense

rainfall events contributing to high turbidity.

- Use water intake from Rivière des Anguilles in order to supply Gros Ruisseau, upgrading

system if necessary.

- Drilling of a first backup borehole at Gros Ruisseau and proceed to pumping tests, if

possible during dry season (Oct/Nov).

- Use of water from Ruisseau des Chevrettes to supply Bois Sec area (via a pipe crossing

Avalon Golf Estate over about 2km)

- Drilling of a second borehole at the northern extremity of Talusseau domain.

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Table of Contents 1 Introduction .................................................................................................................................... 6

2 Site Description ............................................................................................................................... 7

2.1 Location ................................................................................................................................... 7

2.2 Water extraction and borehole drillings in the region ........................................................... 9

3 Geological Background ................................................................................................................. 11

3.1 Published geology ................................................................................................................. 11

3.1.1 The Old Lava Series ....................................................................................................... 11

3.1.2 The Intermediate Lava Series ........................................................................................ 11

3.1.3 The Recent Series lavas ................................................................................................. 13

3.2 Geological profile deducted from drilling works .................................................................. 13

3.3 Structural Geology - Air photography analysis ..................................................................... 15

4 Hydrology ...................................................................................................................................... 16

4.1 Hydrological domain and watersheds .................................................................................. 16

4.1.1 Rivière du Poste ............................................................................................................ 16

4.1.2 Rivière Dragon and Rivière St Amand ........................................................................... 17

4.1.3 Rivière des Anguilles ..................................................................................................... 17

4.1.4 River Savanne ................................................................................................................ 17

4.2 Geomorphological analysis ................................................................................................... 18

4.3 Analysis of Annual Discharge Records .................................................................................. 19

4.4 Rainfall Distribution, Soil Map and Groundwater Recharge ................................................. 22

5 Hydrogeology ................................................................................................................................ 24

5.1 Hydrogeological background ................................................................................................ 24

5.2 Pumping tests on boreholes of the region ........................................................................... 25

6 Field Investigations ....................................................................................................................... 26

6.1 Location plan ......................................................................................................................... 26

6.2 Water rights of Saint Aubin over the area ............................................................................ 28

6.3 Site #01 – Rivière des Anguilles @ bridge ............................................................................. 29

6.4 Site #02 – Unnamed tributary of Rivière des Anguilles @Old Pumping Station................... 30

6.5 Site #03 – Riffle across Rivière des Anguilles, before reaching Gros Ruisseau ..................... 31

6.6 Site #04 – Gros Ruisseau domain .......................................................................................... 32

6.7 Site #05 – Riffle across Ruisseau Marron .............................................................................. 32

6.8 Site #06 – Derivation from Ruisseau Marron towards Gros Ruisseau .................................. 33

6.9 Site #07 – Riffle accros Gros Ruisseau .................................................................................. 33

6.10 Site #08 – Water derivation from Gros Ruisseau towards Beau Bois Reservoir ................... 34

6.11 Site #09 – Rivière des Chevrettes .......................................................................................... 34

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6.12 Site #10 – Feeder Augustin ................................................................................................... 36

6.13 Site #11 - Feeder Beau Bois at boundary between Avalon and Talusseau ........................... 36

6.14 Site #12 – Source of Feeder Beau Bois and Rivière Bain des Négresses ............................... 37

6.15 Site #13 – Ruisseau Combo ................................................................................................... 37

6.16 Site #14 – Bridge over Rivière Bain des Négresses ............................................................... 38

6.17 Conclusions of field investigations ........................................................................................ 38

7 Conclusions & Recommendations ................................................................................................ 39

7.1 Conclusions ........................................................................................................................... 39

7.2 Recommendations ................................................................................................................ 40

8 Bibliography .................................................................................................................................. 42

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List of Figures

Figure 2-1. Location of site ...................................................................................................................... 7

Figure 2-2: Location Map (Source IGN 1:25,000) .................................................................................... 8

Figure 2-3: Site Location (a) from Google Earth ..................................................................................... 9

Figure 2-4: Location of drilling works undertaken in Bel Ombre area (Image from Google Earth) ...... 10

Figure 3-1: Cross Section of the Caldera from Port Louis to Mahebourg (Saddul, 2002) ..................... 11

Figure 3-2 : Extract of Geological Map of Mauritius (Giorgi, 1999) ...................................................... 12

Figure 3-3: Geological logs of the different boreholes deducted from cuttings recovered ................. 14

Figure 3-4 : Stereoscopic analysis of aerial photography (H.S., 1975 – extract of photo 053), with

details of N150 faults ............................................................................................................................ 15

Figure 4-1: Catchment area of Rivière du Poste [J] .............................................................................. 16

Figure 4-2: Catchment area of Rivière Dragon [L] ................................................................................ 17

Figure 4-3: Catchment area of Rivière des Anguilles [M] ..................................................................... 17

Figure 4-4: Catchment area of Rivière Patates [N] ............................................................................... 17

Figure 4-5: Radial distribution around Mt Perruche ............................................................................. 18

Figure 4-6: Example of planeze, both from Saddul (2002) ................................................................... 18

Figure 4-7: Longitudinal profile of Rivière des Anguilles (Saddul, 2002) .............................................. 18

Figure 4-8: Total yearly discharge per river at station .......................................................................... 19

Figure 4-9: Runoff calculated from yearly discharge ............................................................................ 19

Figure 4-10: Annual Runoff from Saddul (2002) ................................................................................... 19

Figure 4-11: Rainfall v/s Altitude & Exposure (Saddul, 2002) ............................................................... 19

Figure 4-12: Monthly minimal flow rate for Rivière du Poste .............................................................. 20

Figure 4-13: Monthly minimal flow rate for Riv. des Anguilles ............................................................ 20

Figure 4-14: Monthly minimal flow rate for Rivière Dragon ................................................................ 20

Figure 4-15: Monthly minimal flow rate for Rivière Patates ............................................................... 20

Figure 4-16: Normalized monthly minimal flow rate ............................................................................ 21

Figure 4-17: Normalized monthly mean flow rate ................................................................................ 21

Figure 4-18: Interface Recent/Intermediate lavas – Origin of Rivers (Dragon, Songes & Pipe) ........... 21

Figure 4-19: Soil map of Mauritius (extract) ......................................................................................... 22

Figure 4-20: Rainfall Distribution over Mauritius ................................................................................. 22

Figure 4-21: Total Monthly Rainfall (mm) - Predictive model .............................................................. 22

Figure 4-22: Class distribution of daily rainfall per meteorological station (average over 2009-2011)23

Figure 5-1: Hydrogeological Map of Mauritius - Giorgi et al., 1999 ..................................................... 24

Figure 5-2: Probable piezometric contour in Bois Sec region ............................................................... 25

Figure 6-1: Location plan where pictures have been taken .................................................................. 26

Figure 6-2: Schematic of hydrological network, with streams [in blue] and derivation system [in red],

with corresponding estimate of flow rate (L/sec) ................................................................................ 27

Figure 6-3: Water rights of Saint Aubin Group ..................................................................................... 28

Figure 6-4: Rivière des Anguilles upstream from bridge on Bois Cheri-Grand Bassin road .................. 29

Figure 6-5: Rivière des Anguilles downstream from bridge on Bois Cheri-Grand Bassin road ............. 29

Figure 6-6: Old surface water pumping station at the back of factory ................................................. 30

Figure 6-7: deeply incised valley of Rivière des Anguilles, along Bois Chéri Estate .............................. 30

Figure 6-8: Riffle across Rivière des Anguilles before reaching Gros Ruisseau .................................... 31

Figure 6-9: Dike across Rivière des Anguilles, upstream from riffle, and its derivation. ...................... 31

Figure 6-10: (a) Irrigation canal from Riv. Des Anguilles reaching Gros Ruisseau; (b) Location of

potential borehole ................................................................................................................................ 32

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Figure 6-11: Riffle across Ruisseau Marron .......................................................................................... 32

Figure 6-12: Derivation from Ruisseau Marron (a) towards Gros Ruisseau (b) .................................... 33

Figure 6-13: Riffle accross Gros Ruisseau ............................................................................................. 33

Figure 6-14: (a) Dyke across Gros Ruisseau, 100m downstream from riffle, (b)Water derivation

system ................................................................................................................................................... 34

Figure 6-15: (a) derivation canal at almost full capacity and (b) Reservoir of Beau Bois ..................... 34

Figure 6-16: (a) Ruisseau des Chevrettes downstream from dyke; (b) dyke and water derivation; (c)

junction of derivation with Avalon culvert (d), feeding (e) a small pond (f) equipped with pumps (g)

before reaching (h) Ruisseau Cresson. ................................................................................................. 35

Figure 6-17: Feeder Augustin, at about 150m from its source ............................................................. 36

Figure 6-18: Location where crossing Feeder Beau Bois would be potentially feasible....................... 36

Figure 6-19: Rivière Bain des Négresses (a) and Feeder Beau Bois (b), close to where their valley

begins. ................................................................................................................................................... 37

Figure 6-20: Source of Ruisseau Combo, at southern extremity of Bois Sec area ................................ 37

Figure 6-21 : Bridge over Rivière Bain des Négresses ........................................................................... 38

Figure 7-1: Location of proposed boreholes, existing surface water derivation system and possible

piping to bring surface water to Bois Sec, Bonne Aubaine & Talusseau areas ..................................... 41

List of Tables

Table 2-1: List of drilling works performed by Water Research Co. Ltd in Bel Ombre area ................. 10

Table 5-1: Summary of hydrodynamic data obtained from long pumping tests .................................. 25

HYD-SM-14014 6

1 Introduction

Following a first meeting held on Tuesday 10th of June 2014 at Domaine des Aubineaux and a

proposal submitted on 12/02/2014, Saint Aubin Group commissioned Geotechnical Services Ltd

(GETS) on 10/07/2014 to undertake a hydrogeological investigation of the area of Bois Chéri / Bois

Sec. The appointment was notified through Purchase Order No 101 000 16 933.

The investigation aimed to identify:

i) The potential for groundwater exploitation, including ideal borehole implantation,

ii) The relationship between groundwater extraction and surface water.

iii) The potential for surface water usage

The scope of the investigation comprised:

- Gathering and synthesis of all available documentation pertaining to geology, hydrology and

hydrogeology of the area

- Analysis of available aerial photography

- Interpretation of pumping tests on boreholes located in the area

- Field investigation to survey surface water sources and cross-check information from desk-

study

- Reporting

This Report describes the activities carried out and presents the outcome of the investigations,

the conclusions and the recommendations.

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2 Site Description

2.1 Location The studied area is located in the region of Bois Sec, Savanne District in the South of Mauritius

(Figure 2-1).

Figure 2-1. Location of site

Site Location

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Figure 2-2: Location Map (Source IGN 1:25,000)

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Figure 2-3: Site Location (a) from Google Earth

The area under assessment has an approximate extension 800 arpents subdivided in the following

sub-areas: Gros Ruisseau, Bois Sec, Bonne Aubaine and Talusseau (see Figure 2-2 and Figure 2-3).

The former two sub-areas are to be developed earlier. The studied area is located south and east of

the 550 arpents Avalon Golf Estate, which was under development at the time of this Study.

2.2 Water extraction and borehole drillings in the region

Figure 2-4 and Table 2-1 present the known drilling works conducted in or around the area under

assessment. Apart from the borehole drilled in 1999 for Société Usinière de Bois Chéri, all other

boreholes have been drilled in 2012 and 2013 for water supply of Avalon Golf Estate. The

hydrodynamical characteristics of these wells are discussed in detail in Section 5.2.

The Central Water Authority does not have any water supply borehole in the area under study. The

CWA water intake in the region is through surface water extraction, with treatment plant localized at

Bois Chéri (Dual Media Filtering Plant – 2009) and at Rivière du Poste (13,500m3/day Rapid Gravity

Treatment Plant – 1994).

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Table 2-1: List of drilling works performed by Water Research Co. Ltd in Bel Ombre area

Client No Date Start

Boring Longitude

(X) Latitude

(Y) Elevation

(Z) Depth

Soc. Usiniere de Bois Cheri

BH1 Mon 11-Jan-99 20˚25ˊ34 57˚31ˊ31 485m 40m

Ligne Caliste Ltd BH1 Fri 17-Feb-12 20˚26ˊ28 57˚31ˊ08 453m 70m

Ligne Caliste Ltd BH2 Fri 24-Feb-12 20˚26ˊ30 57˚30ˊ53 462m 50m

Ligne Caliste Ltd BH3 Thu 26-Jul-12 20˚26ˊ34 57˚31ˊ59 447m 70m

Arthur & Morgan BH4 Sat 19-Oct-13 20˚26ˊ44 57˚31ˊ12 424m 60m

Arthur & Morgan BH5 Thu 07-Nov-13 20˚26ˊ28 57˚30ˊ55 475m 60m

Arthur & Morgan BH6 Tue 29-Oct-13 20˚26ˊ38 57˚31ˊ23 416m 60m

Figure 2-4: Location of drilling works undertaken in Bel Ombre area (Image from Google Earth)

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3 Geological Background

3.1 Published geology Mauritius is essentially made of rocks of volcanic origin, related to the activation of faults in

the oceanic plate, with magma sourcing from a hot spot mechanism (such as Hawaii). The construction of the island is most commonly divided into three distinct episodes of volcanic activity:

The Old Lava Series (10MY – 5MY)

The Intermediate Lava Series (3.5MY – 1.7MY)

The Recent Lava Series (0.7MY – 0.2MY)

3.1.1 The Old Lava Series

The Old Lava Series make up the major mountain ranges of Mauritius, most corresponding

to the remnants of the ancient shield volcano now in an advanced state of maturity. In the area of

our concern, these ranges are represented by the Piton Savanne range located at the west of Bois

Sec and reaching 706m amsl.

The rock suite of the Old Lava Series (Saddul, 2002) is formed of, successively from oldest to

most recent, oceanite, olivine basalt, ankaramite, hawaiite, feldsparphyric basalt, mugearite and

trachyte. The peaks generally correspond to the layer of ankaramite (Figure 3-1).

Figure 3-1: Cross Section of the Caldera from Port Louis to Mahebourg (Saddul, 2002)

3.1.2 The Intermediate Lava Series

The Intermediate Lava Series, also called Early Lavas, are well represented in what is called

the southern highlands, from Cachette Chamarel at the West to Rivière du Poste at the east. The

intermediate Series spread through existing paleo-valleys from the emitting craters mostly located in

Mare aux Vacoas area (e.g. Les Mares, Pétrin, Mt Perruche and Bois Chéri). These craters are the

remnants of explosive eruptions that projected volcanic tuffs (pyroclastic formations) and basaltic

lava flows covering reliefs of the ancient series

The intermediate series is multi-layered with succession of healthy basalts and weathered

horizons. Their weathering state depends on the duration of quiescence that allows the hydrolysing

tropical to equatorial climate to transform the basalt. The most common weathering pattern

constitutes spheres presenting an exfoliation like “peels of onion”.

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Figure 3-2 : Extract of Geological Map of Mauritius (Giorgi, 1999)

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As shown in Figure 3-2, the Intermediate Lava Series are covering in its entirety the area

under assessment. These lavas may have been emitted from Mt Perruche or from Bois Cheri crater,

and flow southward down to the coast.

Intermediate Lavas Series are also encountered to the west of River Savanne. Their origin

may have been from Les Mares crater or most probably from Bassin Blanc, flowing down on its

eastern slope towards Mare Anguilles and eventually Souillac.

3.1.3 The Recent Series lavas

Intermediate Lava Series and Recent Lava Series have very similar petrographic

characteristics and the main criterion of discrimination between them is the almost systematic

intercalation of a first thick weathered layer with red clays. This interface corresponds to the

intense weathering that occurred at the roof of the intermediate series lavas, during a 1Myr interval

to the first recent series lavas.

The Recent Series lavas (also called late lavas) are compact or vesicular healthy Hawaiites

(light grey rocks of aphyric texture with small crystals of plagioclases). The thickness of these lavas is

generally limited (the most important flows being about 20m thick). Under such genesis scheme, it

is frequent that late lavas have generated lava tubes at relatively shallow depth.

In the area under assessment, the Recent Series lavas were emitted from Kanaka Crater,

located about 2km North of Bois Cheri factory, with a flow mainly towards the East and South East

until reaching Rivière du Poste and flowing southward from then on.

3.2 Geological profile deducted from drilling works

The geological profile at each borehole was defined through the description of the cuttings collected

during the drilling and the analysis of the rate of drilling penetration. In general, basalt rock with

weathering degrees between intact and residual soil were observed, although sand deposits were

encountered on the low elevation drillings. The units encountered include:

Soil & Boulders

Sand

Clay

Highly weathered basalt

Weathered basalt

Slightly weathered basalt

Slightly hard basalt

Hard blue basalt

Very Hard blue basalt

The ground profile correlation between the different wells is difficult to establish (Figure 3-3). The

observed profile variability may result from: i) the steep conditions on the area limiting the lateral

extent of lava flows, generally to a few hundred meters, ii) varying pre-existing erosion features at the

time of generation of the lava flow that control its flow path, and iii) varying exposure conditions

resulting on varying weathering processes.

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Figure 3-3: Geological logs of the different boreholes deducted from cuttings recovered

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3.3 Structural Geology - Air photography analysis The relevant aerial photographs from the Ministry of Housing and Lands for the area under

assessment include:

1975 – Hunting Survey Ltd – 1:15,000 (photos No 53 and No 64)

1991 – IGN – 1:10,000 (photos No 37, No 39, No 88, No 90)

Figure 3-4 : Stereoscopic analysis of aerial photography (H.S., 1975 – extract of photo 053), with details of N150 faults

Several faults systems have been identified by Giorgi et al. (1999) on the area of the assessment or

in its immediate surroundings, these include:

6No N150 faults along Rivière des Anguilles path

1No N0 fault located at the south of the village of Bois Cheri

The N150 faults system could be linked to the activity of the Bois Cheri crater. Its influence on the

demarcation of the ancient caldera manifests a sinistral motion, which could be also associated with

collapse subsidence movement.

The faults system has been defined more precisely by a stereoscopic analysis of the aerial

photography. Figure 3-4 shows that the position and length of each fault is slightly different from

what is shown on the 1:50,000 geological map (Figure 3-2). Considering the system from the North

West to South East, the faults details are:

Fault #1 : 620m N156

Fault #2 : 260m N156

Fault #3 : 580m N149

Fault #4 : 410m N146

Fault #5 : 840m N142

Fault #6 : 635m N146

HYD-SM-14014 16

4 Hydrology

4.1 Hydrological domain and watersheds The studied area is characterized by a very dense hydrographic network; which constitute a large

proportion of its potential water resource. The evaluation of this hydrographic environment will allow

not only the identification of the surface water resource but also the assessment of the geological

environment and the groundwater availability. Thus the hydrological analysis was performed over a

broader scale, covering several watersheds gauged and recorded by the CWA. The hydrological

domain considered is limited by the watershed of the Riviere du Poste to the east and north, the

Riviere Savanne to the west and the coast to the south.

At the north of this domain, Mt Perruche exhibits a topography which is different from other

neighbouring volcanoes of the Intermediate series, as it has built a mass of elongated hills of about

5km long following a N100 direction (the same direction than the eastern mountains, including Mt

Blanche, Mt Lagrave, etc…).

The most significant streams reaching the coast in the area of studied are, from east to west:

Rivière du Poste,

Rivière St Amand,

Rivière Dragon,

Rivière des Anguilles,

Rivière Bain des Negresses,

Rivière Savanne

The significant streams not reaching the coast or other not listed streams are tributaries of those

mentioned above. The following Sections describe the watershed for the mentioned rivers.

4.1.1 Rivière du Poste

Rivière du Poste constitutes the northern and the

eastern boundary of the hydrological domain. It is

the sixth largest watershed of Mauritius, with a

catchment area of 61.59km2, but the second when

considering its length (22.25km). In the

nomenclature adopted by CWA, the river basin of

Rivière du Poste is labelled [J].

The watershed of Riviere du Poste is divided in two

sectors: a northern part with a dense reticulation

covering a large area on both sides of Mt Perruche

ridge; a very narrow southern part, starting at La

Flora towards the coast at Savannah.

CWA has a hydrograph recorder set at the junction

of the two sectors ([J04] on Figure 4-1). At an

elevation of 351m, this recorder caters for a basin

area of 16.17 km2. As mentioned above, the

southern portion of Rivière du Poste shows Recent

Lava Series on its eastern bank while Intermediate

Lavas Series outcrop on its western banks.

Figure 4-1: Catchment area of Rivière du Poste [J]

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4.1.2 Rivière Dragon and Rivière St Amand

Rivière Dragon and Rivière St Amand are streams with a

similar shape than the southern portion of Rivière du Poste,

with very narrow basins, with approximately 10km2 each. Both

river mouths are only 2.2km distant.

These rivers have therefore a small number of tributaries; the

reticulation of Rivière Dragon only starts upstream from

Britannia Sugar Estate. Between there and the town of Rivière

des Anguilles, the Savanne A9 road is located on the interfluve

between Rivière Dragon and Rivière des Anguilles basins.

CWA has a hydrograph data logger at Station [L01] on Rivière

Dragon at Batimarais. At an elevation of 110m amsl, this

recorder caters for 7.6km2 of the basin (see Figure 4-2).

Both rivers are flowing over Intermediate Lavas Series.

4.1.3 Rivière des Anguilles

All the area under study is located within the Rivière des

Anguilles watershed.

The catchment area of the basin is 29.46km2, with a length of

10.42km. The land use over this watershed is estimated to be

15% forest, 65% sugarcane and tea fields and 20% residential,

the underlying rock being Intermediate Lavas.

CWA has equipped this river with a station [M01] located at an

elevation of 85m, just downstream of Saint Aubin factory.

4.1.4 River Savanne

Rivière Savanne has a relatively vast (37.1km2) catchment

area, which is divided in two sub-basins. One sub-basin is

located on the eastern side of the Savanne Mountain Range,

where the main stream is located. The second sub-basin is on

the western side of the same range, where its main tributary,

the Rivere Patates, is located.

CWA has installed a station [N03] at Riviere Patates an elevation

of 222m, accounting for a basin area of 5.23km2.

Rivière Savanne has its source between the southern flank of

Les Mares volcano and the northern flank of Piton Savanne. It

first flows through the Old Lava Series, and then meets the

intermediate lava. Downstream of the site of its confluence

with River des Chevrettes, one of its tributaries, the River

Savanne flow down to Souillac to reach the coast at the

interface between intermediate and old lavas series.

Figure 4-4: Catchment area of Rivière Patates [N]

Figure 4-3: Catchment area of Rivière des Anguilles [M]

Figure 4-2: Catchment area of Rivière Dragon [L]

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4.2 Geomorphological analysis Saddul (2002) has largely described the geomorphology of the southern plains of Mauritius,

where the erosion has developed under the mechanism of amphitheatre valleys with planezes.

Amphitheatre valleys develop in high temperature and humidity conditions, typical of tropical islands,

where initial dome-shape morphology linked to volcanic origin exhibit outer slopes of more than 7o.

Over such topography (for example Rivière du Poste around Mt Perruche in Figure 4-5), the streams

rapidly develop deep valleys as they erode and transport materials let loose by weathering processes,

particularly during torrential events. If near the summit, the master streams intercept minor ones

with interfluves becoming narrower and sharper. When getting closer to the sea, the valleys are

further apart and are separated by flattish lands called planezes, as illustrated by Saddul (2002) in

Figure 4-6.

Figure 4-5: Radial distribution around Mt Perruche

Figure 4-6: Example of planeze, both from Saddul (2002)

Saddul (2002) also provides a specific analysis of the profile of Rivière des Anguilles. He notes

that between its source, at an elevation of about 650m, and the 500m contour found at Bois Cheri

Estate, the underlying formations have already been incised by a valley of about 15m depth. With the

contribution of River Songes, one of its tributaries, this depth reaches 35m, at the immediate west of

Britannia Sugar Estate. From

there, the valley goes southward,

maintaining its depth at about

40m, with meanders growing to

larger amplitudes until reaching

the town of Rivière des Anguilles.

From here the stream goes a

slightly straighter towards its

mouth, even more deeply incised,

the final 2km being a 50-60m

deep canyon. This profile (Figure

4-7) is typical of a youthful

stream, contrasting with those

found on the Old Series massifs.

Figure 4-7: Longitudinal profile of Rivière des Anguilles (Saddul, 2002)

HYD-SM-14014 19

4.3 Analysis of Annual Discharge Records Daily flow is recorded by CWA at stations J04 (Riv. du Poste), L01 (Riv. Dragon), M01 (Riv. des

Anguilles) and N03 (Riv. Patates), with the most recent available data being from Nov-05 to Oct-10.

The total yearly discharge shows that the flowrate measured for Rivière des Anguilles is larger

(Figure 4-8) than those recorded on the other streams (with about 60M m3 annual cumulatively flow).

Nevertheless, when the same data is normalized with respect to the basin area upstream to the

location of the station, similar values (Figure 4-9) of runoff of about 1.5m3/m2 to 2.5 m3/m2 are

generally obtained; these are consistent with the annual runoff distribution by Saddul (Figure 4-10).

Figure 4-8: Total yearly discharge per river at station

Figure 4-9: Runoff calculated from yearly discharge

Figure 4-10: Annual Runoff from Saddul (2002)

Figure 4-11: Rainfall v/s Altitude & Exposure (Saddul, 2002)

In Figure 4-11, Saddul suggests that there is a direct relationship between the distribution of

runoff and rainfall. However, the absence of runoff in the northern and western plains also suggests a

correlation with the geology, infiltration being apparently higher where Recent Lavas Series outcrop.

Similarly, there is no stream between Rivière du Poste and Rivière La Chaux where Recent Lavas

from Kanaka outcrops. Moreover, this unit is described with underground water springs (at Virginia in

the area of Union Vale/Mon Trésor-Mon Désert, Loudière, 1989), confirming the predominant

infiltration on Recent Lavas units, which top of aquifer is drained all the year around.

0

10

20

30

40

50

60

70

Year 05-06 Year 06-07 Year 07-08 Year 08-09 Year 09-10

Tota

l Ye

arly

Dis

char

ge (

Mill

ion

s o

f m

3)

Riv. Patates Riv. des Anguilles

Riv. Dragon Riv. du Poste

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Year 05-06 Year 06-07 Year 07-08 Year 08-09 Year 09-10

Tota

l Ye

arly

Dis

char

ge (

Mill

ion

s o

f m

3)

/ B

asin

Cat

chm

en

t ar

ea

@ S

tati

on

(m

2)

Riv. Patates Riv. des AnguillesRiv. Dragon Riv. du Poste

HYD-SM-14014 20

Figure 4-12: Monthly minimal flow rate for Rivière du Poste

Figure 4-13: Monthly minimal flow rate for Riv. des Anguilles

Figure 4-14: Monthly minimal flow rate for Rivière Dragon

Figure 4-15: Monthly minimal flow rate for Rivière Patates

This difference of behaviour between the Recent and the Intermediate lavas has a direct impact on

the hydrology of the streams. The contribution of groundwater to the flow rate of a stream can be

assessed by the analysis of the minimal flow (also called base flow analysis). Figure 4-12 to Figure 4-15

show the minimal flow recorded every month at each CWA river station from Nov-05 to Oct-10.

Rivière des Anguilles is definitely showing larger minimal flow rate all over the year than the other

streams. However, as the basin area covered from its station location is larger, it would also be useful

to normalize this data set accordingly. In this respect, for each month, we have calculated the

corresponding minimal equivalent daily runoff (minimal flow rate in m3/sec x 86,400 = m3 in 24hr, this

volume is then divided by the area of the basin contributing for the station) expressed in mm. In order

to avoid year to year variability, we have then considered the average for each month over the 5

years. Two chart are shown, the normalized monthly minimal flow rate per river (Figure 4-16) and the

normalized mean flow rate per river (Figure 4-17).

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct

Mo

nth

ly M

inim

al F

low

Rat

e (

m3

/s)

Rivière du Poste @ Bridge Beau Climat

2005-06 2006-07 2007-08

2008-09 2009-10

0.0

0.5

1.0

1.5

2.0

2.5

3.0


Mo

nth

ly M

inim

al F

low

Rat

e (

m3

/s)

Rivière des Anguilles @ St Aubin2005-06 2006-07 2007-08

2008-09 2009-10

0.0

0.5

1.0

1.5

2.0

2.5

3.0


Mo

nth

ly M

inim

al F

low

Rat

e (

m3

/s)

Rivière Dragon @ Batimarais

2005-06 2006-07 2007-08

2008-09 2009-10

0.0

0.5

1.0

1.5

2.0

2.5

3.0


Mo

nth

ly M

inim

al F

low

Rat

e (

m3

/s)

Rivière Patates @ Mont Blanc

2005-06 2006-07 2007-08

2008-09 2009-10

HYD-SM-14014 21

Figure 4-16: Normalized monthly minimal flow rate

Figure 4-17: Normalized monthly mean flow rate

From Figure 4-16, we notice that Rivière des Anguilles and Rivière Dragon are showing higher

minimal normalized flow rate, when compared to Rivière Patates and Rivière du Poste. Conversely,

Figure 4-17 shows that the same pattern can be described when considering the mean flow rate.

These two figures highlight the fact that we can reasonably consider that Rivière des Anguilles and

Rivière Dragon are benefiting from a higher groundwater contribution (almost twice more), while

response of runoff to rainfalls is roughly the same for these four streams.

When considering more in details the geological map, we can notice that the source of Rivière

Dragon is located at the interface between the Recent Lavas Series and the Intermediate Lavas

Series. The same conclusion is relevant (as shown on Figure 4-18) for Rivière des Anguilles and its

tributaries River Pipe and River Songes. If a station were located further downstream on Rivière du

Poste, we might also found the same pattern there, as the catchment area at location [J04] is only

benefit from runoff occurring on the flanks of Mt Perruche, while recharge from Kanaka recent lavas

groundwater appears southward.

Figure 4-18: Interface Recent/Intermediate lavas – Origin of Rivers (Dragon, Songes & Pipe)

0

1

2

3

4

5

6


Min

imal

Ru

no

ff in

24

hr

(mm

)Monthly minimal flow rate normalized

Riv. Des Anguilles Riv. Dragon

Riv. Du Poste Riv. Patates

0

2

4

6

8

10

12

14

16


Ave

rage

Ru

no

ff in

24

hr

(mm

)

Monthly average flow rate normalized

Riv. Des Anguilles Riv. Dragon

Riv. Du Poste Riv. Patates

HYD-SM-14014 22

4.4 Rainfall Distribution, Soil Map and Groundwater Recharge Differences between Figure 4-16 and Figure 4-17

show the significant importance of runoff in the flow

rate measured on the stream of the region, the

mean flow rate (due to runoff) being about twice

greater than the minimal flow rate (contribution

from ground-water springs).

When considering rainfall distribution and estimated

runoff (Figure 4-20), limited precipitation is not

converted in runoff and contributes to infiltration

into groundwater circulation through intermediate

lava. The type of soils (Figure 4-19) found in the area

contributes in that way, keeping the soil humid, since

at Bois Chéri, in the 3 years considered, there was

not rain on only 86 days each year.

Figure 4-20: Rainfall Distribution over Mauritius

If it is raining at least a 1mm at Bois Cheri over 75% days of a year, this ratio drops to 56% when

considering rainfall at Combo at the south of the hydrological domain. The temporal and spatial

distribution of rainfall in the region is presented in Figure 4-21, as modelled by the Mauritius

Meteorological Services, with a total yearly rainfall of almost 4m at Bois Chéri and 1.7m at Rivière

des Anguilles town.

Figure 4-21: Total Monthly Rainfall (mm) - Predictive model

0

200

400

600

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

av.

Mo

nth

ly r

ain

fall

(mm

)

Modeled Total Monthly Rainfall (mm) Bois CheriBois SecComboRiv des Anguilles

Figure 4-19: Soil map of Mauritius (extract)

HYD-SM-14014 23

Figure 4-22: Class distribution of daily rainfall per meteorological station (average over 2009-2011)

Figure 4-22 show in more details the temporal and spatial distribution of rainfall, even when

considered on an average over 3 years (2009-2011). We can also consider the significant amount of

events of the highest class (above 50mm) which ranges from 7 per year at Rivière des Anguilles to 12

per year at Bois Sec and at Pradier, with a maximum of 18 events recorded at Bois Chéri during 2009.

This could have serious implication when considering surface water exploitation for domestic

purpose as turbidity of the water would be expected to be significantly higher, requiring thus

adapted water treatment solution or alternate water supply following such rainfall events.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Riv des Anguilles (2009-2011)

No Rain 1 to 5mm 5 to 10mm10 to 25mm 25 to 50mm Above 50mm

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Combo (2009-2011)


Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Bois Sec (2009-2011)


Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Bois Cheri (2009-2011)


Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Grand Bassin (2009-2011)


Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Pradier (2009-2011)


HYD-SM-14014 24

5 Hydrogeology

5.1 Hydrogeological background At the time of the completion of the hydrogeological map of Mauritius, there were not records of

groundwater wells in the area of Bois Cheri and Bois Sec. However, this area is considered (Figure

5-1) as part of Principal Aquifer III (Nouvelle France-Rose Belle-Plaisance). Giorgi et al.(1999)

mention that the extension of this aquifer is to be considered towards the ancient mountains of

Savanne as groundwater recharge to this part is ensured by the fractured area of Rivière des

Anguilles at the south rim of the caldera (see Section 3.3).

Figure 5-1: Hydrogeological Map of Mauritius - Giorgi et al., 1999

HYD-SM-14014 25

5.2 Pumping tests on boreholes of the region Table 2-1 summarizes the hydrodynamical parameters obtained from the interpretation of

pumping tests performed on 7No. existing wells. Among these results, apart from BH5 which

showed a high transmissivity of 400m2/day, the other boreholes are showing relatively low

permeability, typical of intermediate lava formations. One can notice that the closer to the Savanne

range a borehole is located, the lower the discharge rate, with reference to BH2, BH1 and BH4.

Wells located closer to Bois Cheri are showing higher yields.

Client No Elevation Static WL

Distance to Sea

Gradient to Sea

Pumping Test

Test Duration

Discharge Rate

Drawdown Transmissivity

Soc. Usiniere de Bois Cheri

BH1 469.50m 10.8km 4.3% Step7 23hr 24 m3/hr 4.35m 31 m2/d

Ligne Caliste Ltd BH1 440.61m 8.7km 5.1% Long Term 24hr 2.5 m3/hr 19.48m 1.6 m2/d



Arthur & Morgan BH4 407.36m 8.2km 5.0% Long Term 24hr 1.6 m3/hr 17.48m 10 m2/d

Arthur & Morgan BH5 462.15m 9.4km 4.9% Long Term 24hr 25 m3/hr 15.95m 400 m2/d

Arthur & Morgan BH6 407.77m 8.3km 4.9% Long Term 24hr 10.5 m3/hr 18.15m 80 m2/d

Table 5-1: Summary of hydrodynamic data obtained from long pumping tests

The piezometric contours for the area were established using the static water elevations and the 5%

average hydraulic gradient towards the sea (Figure 5-2). The concavity of piezometric heads in the

vicinity of fault zones is the only possible way to match static water levels met at BH1, BH3, BH4 and

BH6. This could be interpreted as the consequence of the fault acting as a barrier against the general

flow coming from the north. Higher groundwater yield are therefore to be expected eastward from

this fault system.

Figure 5-2: Probable piezometric contour in Bois Sec region

HYD-SM-14014 26

6 Field Investigations The main purpose of the field investigations was to obtain additional hydrological information,

particularly for the streams that are not gauged by CWA. An assessment was also carried out of the

increase of the flow rate of Riviere des Anguilles when going downstream.

6.1 Location plan Field surveys were carried out on the 18/07/14, 22/07/14, 12/08/14 13/08/14 and 17/09/14.

The surveyed locations are indicated in Figure 6.1.

Figure 6-1: Location plan where pictures have been taken

HYD-SM-14014 27

Figure 6-2 shows a schematic representation of hydrological network of the studied area. Reporting

of our site investigations follows a progression from East to West:

Site 01 and Site 02 : Rivière des Anguilles before water derivation towards Beau Bois

Site 03 : Riffle across Riviere des Anguilles where this derivation starts, crossing then Gros

Ruisseau domain (Site 04).

Site 05 : Riffle across Ruisseau Marron, upstream from derivation system

Site 06, 07 and 08 : Progress of derivation system towards Beau Bois reservoir

Site 09 : refers to derivation taken from Rt Chevrettes which is brought to Gros Ruisseau

Site 10, 11, 12 and 13 are bringing evidences of dry status of the valleys of Feeder Augustin,

Feeder Beau Bois, Riviere Bain des Negresses and Ruisseau Combo

Site 14 : Located more than 3km southward from Beau Bois reservoir, where Riviere Bain des

Négresses is showing permanent flow.

Figure 6-2: Schematic of hydrological network, with streams [in blue] and derivation system [in red], with corresponding estimate of flow rate (L/sec)

20

HYD-SM-14014 28

6.2 Water rights of Saint Aubin over the area Saint Aubin S.E. Co Ltd is riparian estate owner to river des Anguilles and is statutorily entitled to a

share of water from the said river in accordance to the provisions of the Rivers and Canals Act.

According to a division of the water of river des Anguilles made by Surveyor C.S. de Joux on the 8th

June, 1886, Saint Aubin S.E. Co Ltd is entitled to 241.69 shares of water. However, Saint Aubin S.E.

Co Ltd/Cascade are entitled to 247.05 shares. Each share of water in river des Anguilles is equivalent

to 0.47L/s.

Hence the total volume of water to which Saint Aubin S.E.Co Ltd/Cascade is entitled to is 116L/s (or

418m3/hr) on River des Anguilles. Saint Aubin S.E. Co Ltd rights to water has been notified to the

CWA on the 30th December, 1980 acknowledged on the same day and bears registration No196.

Similarly, Saint Aubin S.E. Co Ltd also own water rights on River Savanne, with 1.2m high dam

allowed on Ruisseau des Chevrettes for an extraction up to 0.75 cubic feet per sec (cusec), which is

equivalent to 76.45m3/hr or 21.24L/s, bearing registration No197.

Both water rights have been delivered with usage being allowed for irrigation purpose.

Figure 6-3 drawn by Kauppaymuthoo Consultants Ltd in 1997 give details of water derivations. Note

that water taken from Ruisseau des Chevrettes is diverted through Ruisseau Cresson to reach Gros

Ruisseau and Beau Bois Reservoir further downstream.

Figure 6-3: Water rights of Saint Aubin Group

HYD-SM-14014 29

6.3 Site #01 – Rivière des Anguilles @ bridge Rivière des Anguilles has its source in the area called Pradier, located between Grand Bassin and

Kanaka crater. At the Bois Chéri-Grand Bassin road, the length of the river is 2.9km and carries a

small portion of the 30km2 watershed. At this stage, the valley is fairly incised (about only 3m). Flow

rate was estimated on 12/08/14 to be about 0.3m3/s upstream from the bridge (Figure 6-4) and

0.5m3/s under the bridge (Figure 6-5). The small portion of watershed above this point provides the

equivalent of the minimal flowrate, indicating that the major contribution from groundwater is

occurring upstream from Site 1 rather than downstream.

Figure 6-4: Rivière des Anguilles upstream from bridge on Bois Cheri-Grand Bassin road

Figure 6-5: Rivière des Anguilles downstream from bridge on Bois Cheri-Grand Bassin road

HYD-SM-14014 30

6.4 Site #02 – Unnamed tributary of Rivière des Anguilles @Old Pumping Station An unnamed tributary of Rivière des Anguilles flows from the flanks of the remnant of the crater,

along the Bois Cheri factory. At its south extremity, less than 100m south of the Bois Cheri Tea

Factory, there is an old surface water pumping station that appears not to be in operation (Figure

6-6). The flow rate estimate is 0.5m3/s.

Re-commissioning of this structure could provide significant water resources. However, as shown on

Figure 6-7, Rivière des Anguilles shows a 40m deep incised valley which makes difficult its

distribution to the area of Gros Ruisseau.

Figure 6-6: Old surface water pumping station at the back of factory

Figure 6-7: deeply incised valley of Rivière des Anguilles, along Bois Chéri Estate

HYD-SM-14014 31

6.5 Site #03 – Riffle across Rivière des Anguilles, before reaching Gros Ruisseau Flow rate is estimated about 340L/sec (~1,200m3/hr) at the dike (Figure 6-9) built across Riviere des

Anguilles just upstream from the riffle (Figure 6-8) to which shall be added the water derived

(~100L/sec or ~400m3/hr) from there towards Gros Ruisseau for irrigation purpose.

Figure 6-8: Riffle across Rivière des Anguilles before reaching Gros Ruisseau

Figure 6-9: Dike across Rivière des Anguilles, upstream from riffle, and its derivation.

HYD-SM-14014 32

6.6 Site #04 – Gros Ruisseau domain

Figure 6-10: (a) Irrigation canal from Riv. Des Anguilles reaching Gros Ruisseau; (b) Location of potential borehole

The derivation canal taking water from Rivière des Anguilles crosses Gros Ruisseau domain to reach

Ruisseau Marron. Extraction from this canal could contribute to provide water supply. Best suitable

borehole location is also proposed in the immediate vicinity of landmark GR No166, which is the

closest position from recent lavas system, northwards of N150 fault system.

6.7 Site #05 – Riffle across Ruisseau Marron

Figure 6-11: Riffle across Ruisseau Marron

Flow rate estimated at riffle across Ruisseau Marron is 30L/sec (100m3/hr), at an upstream position

with regards to water derived from Riviere des Anguilles through Gros Ruisseau.

HYD-SM-14014 33

6.8 Site #06 – Derivation from Ruisseau Marron towards Gros Ruisseau Further downstream from the riffle mentioned in Section 6.7, part of the flow of Ruisseau Marron is

diverted (Figure 6-12a) towards Gros Ruisseau, Figure 6-12(b).

Figure 6-12: Derivation from Ruisseau Marron (a) towards Gros Ruisseau (b)

6.9 Site #07 – Riffle accros Gros Ruisseau At the riffle crossing Gros Ruisseau, the flow rate was estimated as being about 130L/s (460m3/hr),

which approximately corresponds to the addition of flow from Ruisseau Marron and derived water

from Rivière des Anguilles, the main tributaries.

Figure 6-13: Riffle accross Gros Ruisseau

HYD-SM-14014 34

6.10 Site #08 – Water derivation from Gros Ruisseau towards Beau Bois Reservoir About 100m downstream from the riffle crossing Gros Ruisseau, a dam constructed across the

stream (Figure 6-14a) divert the water (Figure 6-14b) towards a regulating reservoir, with a capacity

of about 6,000m3, located in the upper reach of Beau Bois (Figure 6-15b).

Flow rate estimate in this derivation canal is about 120L/sec (435m3/hr), its capacity being limited by

its dimensions, as shown on Figure 6-15a. Water collected in the reservoir is not used and is

discharged in Feeder Enterrement to reach Feeder Beau Bois

Figure 6-14: (a) Dyke across Gros Ruisseau, 100m downstream from riffle, (b)Water derivation system

Figure 6-15: (a) derivation canal at almost full capacity and (b) Reservoir of Beau Bois

6.11 Site #09 – Rivière des Chevrettes Rivière des Chevrettes is one of the tributaries of Rivière Savanne, flowing along the western

boundary of Avalon Golf Estate with an approximate flow over 1,000m3/hr. At elevation 450m, a

dike built (Figure 6-16) across this stream provides water for irrigation purposes. This water

diversion reaches Ruisseau Cresson, a tributary of Gros Ruisseau, reaching eventually the reservoir

of Beau Bois.

As shown on Figure 6-16, a culvert derive water at an estimated flow of about 62L/sec or 225m3/hr

towards a small pond at Avalon Golf Estate, from where is discharged into Ruisseau Cresson.

HYD-SM-14014 35

Figure 6-16: (a) Ruisseau des Chevrettes downstream from dyke; (b) dyke and water derivation; (c) junction of derivation with Avalon culvert (d), feeding (e) a small pond (f) equipped with pumps (g) before reaching (h) Ruisseau Cresson.

HYD-SM-14014 36

6.12 Site #10 – Feeder Augustin This stream is a tributary of Gros Ruisseau. It appears to be mostly dry and could not be considered

as a permanent source of water. Its junction with Ruisseau Cresson was inaccessible due to ongoing

works on Avalon Golf Estate project, and the flow at this junction was not confirmed..

Figure 6-17: Feeder Augustin, at about 150m from its source

6.13 Site #11 - Feeder Beau Bois at boundary between Avalon and Talusseau In the prospect of bringing water from Gros Ruisseau system towards Bois Sec area, the main

concern would be the crossing of feeder Beau Bois. Its valley is such deeply incised that it appears

unfeasible. At about 100m upstream from the boundary between Avalon Golf Estate and Talusseau,

a temporary crossing has been built by Avalon (Figure 6-18), which could eventually be an option.

However, elevation difference between this point and the reservoir is above 120m, which would

thus require at least a 12 to 15bars pumping system to bring water from the reservoir about 2.2km

away to the upper reach of Talusseau. This solution is not recommended.

Figure 6-18: Location where crossing Feeder Beau Bois would be potentially feasible

HYD-SM-14014 37

6.14 Site #12 – Source of Feeder Beau Bois and Rivière Bain des Négresses

Figure 6-19: Rivière Bain des Négresses (a) and Feeder Beau Bois (b), close to where their valley begins.

Both valleys are relatively deeply incised while streams are not permanent when close to their

respective sources and remain dry further downstream.

6.15 Site #13 – Ruisseau Combo

Figure 6-20: Source of Ruisseau Combo, at southern extremity of Bois Sec area

Ruisseau Combo is also showing the same pattern than River Bain des Negresses and Feeder Beau

Bois where, even though the valley can be relatively deeply incised, the stream is not permanent and

shows a dry river bed.

HYD-SM-14014 38

6.16 Site #14 – Bridge over Rivière Bain des Négresses

Figure 6-21 : Bridge over Rivière Bain des Négresses

This site is located about 1.5km southward from the southern boundary of the plots of land of Bois

Sec and Bonne Aubaine. The estimated flow rate is about 0.1 m3/s. Rivière Bain des Negresses

crosses the A9 road further downstream with approximately the same flow rate. It has not been

possible to define as from where this stream becomes permanent.

6.17 Conclusions of field investigations The streams originating from Grand Bassin/Pradier area benefit from groundwater springs located at

the interface between Recent and Intermediate lavas and show permanent flow and potential for

surface water extraction.

Existing irrigation system of Saint Aubin brings water derived from Rivière des Anguilles to a

reservoir located at Beau Bois. Ruisseau Marron and Gros Ruisseau are streams being used to

channel the water down there. Gros Ruisseau area could benefit from this water system.

On the other hand, the Feeder Augustin (upstream part of Gros Ruisseau) and all streams located

westwards from the latter (Rivière Bain des Negresses, Feeder Beau Bois, etc…) were dry at the time

of the site survey and do not represent permanent exploitable water sources for Bois Sec, Bonne

Aubaine and Talusseau areas. Ruisseau des Chevrettes, the only permanent stream found westward

from Gros Ruisseau, represents the possible water supply to Bois Sec, Bonne Aubaine and Talusseau

areas.

HYD-SM-14014 39

7 Conclusions & Recommendations

7.1 Conclusions

The area of Bois Sec is characterized by significant rainfall, with spatial and temporal

distribution pattern allowing for recharge of the watersheds.

Intermediate Lavas low transmissivity results on limited potential for groundwater

extraction. Boreholes drilled in the area show relatively low to very low transmissivity, with

limited extraction rate accordingly, except in the case of BH5 drilled for Avalon Golf Estate.

Piezometric map deducted from water levels shows that:

o potential for groundwater extraction is lower close to River Savanne

o the proximity to Recent Lava Series appears to provide better groundwater potential

o N150 faults system is acting as a semi-barrier to groundwater flow

o Some low extent groundwater flow paths provide localized potential for extraction

(example of BH6) within Intermediate Lavas Series.

Surface water is the predominant water resource over the area assessed, with a dense

network of permanent and intermittent streams, tributaries of Rivière des Anguilles, flowing

over low permeable Intermediate Lavas Series. Surface water is formed with runoff and

groundwater springs.

Contribution through groundwater from Kanaka Recent Lava Series is proven, guaranteeing

minimal base flow of Rivière Dragon and Rivière des Anguilles (through Rivière Pipe and

Rivière Songes) with springs occurring at interface with Intermediate Lavas Series.

All the streams located in the area of Bois Sec, Bonne Aubaine and Talusseau, westward

from Gros Ruisseau, are dry and cannot be considered for surface water extraction… except

for Ruisseau Chevrettes…

An existing 100L/sec (or 400m3/hr) irrigation system belonging to Saint Aubin Group takes

water from Rivière des Anguilles, through Ruisseau Marron and then Gros Ruisseau, to a

reservoir located in the upper reach of Beau Bois. Presently, Saint Aubin is not using this

water, which is discharged into Feeder Beau Bois, another tributary of Rivière des Anguilles.

Surface water is also extracted from Ruisseau des Chevrettes, a tributary of Rivière Savanne,

and derived water (60L/sec – 225m3/hr) goes to Ruisseau Cresson in order to reach finally

the derivation system located further downstream on Gros Ruisseau and related reservoir.

HYD-SM-14014 40

7.2 Recommendations

General

The use of surface water should be encouraged instead of groundwater as the potential

volume is greater and permanent. However, the use of surface water for domestic purposes

will require more extensive treatment than borehole water. Existing water rights have been

granted for irrigation purpose and could not necessarily be converted to domestic usage.

Borehole should be considered as backup solution when surface water quality is affected by

torrential rainfalls.

Surface water supply should use the existing extensive network of dams, culverts and pipes

currently used as part of irrigation systems, including those currently not is use.

Refurbishment of this network might be required for better water quality and fewer leakage.

Gros Ruisseau

Surface water should be considered to supply the domain of Gros Ruisseau. Existing canal

should be refurbished and might be upgraded to allow for larger extraction.

One should however consider that this canal is located at an elevation of 380m while the

highest point of the domain is at 460m elevation and distant by about 1.5km, requiring

therefore adapted pumping station in order to cover all this domain.

A new dam could be built at an upstream position on Ruisseau Marron, providing water

supply to the upper part of Gros Ruisseau. However, it would require an additional

treatment station, which eventually could be as costly as pumping the water from lower

Gros Ruisseau where water would be extracted from existing canal and treated accordingly.

Implantation of a borehole at highest elevation of Gros Ruisseau domain is not

recommended as being located too closely from BH3 of Avalon, which is already providing

limited resources.

Best location for drilling a backup borehole to supply this area would be as close as possible

to the Recent Lavas Series interface with Intermediate Lavas Series and should preferably

been located eastward from the fault system. Suggested location for borehole is presented

in Figure 6-10b (Plot GR No166). Depth to be considered would be a minimum of 70m.

Bois Sec, Bonne Aubaine and Talusseau

Drilling of a borehole to supply water for the areas of Bois Sec, Bonne Aubaine and

Talusseau shall only be envisaged as a backup solution: groundwater potential in the area is

to be considered as very limited.

Best potential location for a trial borehole would be at the northern boundary of the

Talusseau domain, as far as possible from the River Savanne, which would be in the

hydraulic continuity of BH6 drilled for Avalon (10m3/hr).

Geophysical soundings could help to improve determination of most preferable location.

However, probability of groundwater potential is very little and expected yield could hardly

be above 5 to 10m3/hr.

However, stream crossing could constitute the limiting factor and a borehole drilled at upper

Talusseau might not be able to feed Bois Sec, implying location to be at upper Bois Sec.

HYD-SM-14014 41

In-place exploitation of surface water cannot be considered for, as all the streams located in

the area are dry. Water supply for Bois Sec domain is therefore definitely problematic.

Water would therefore need to be brought from somewhere else.

Surface water might been brought from the reservoir of Beau Bois. However, this would

imply to cross several streams (Feeder Enterrement, Feeder Beau Bois, River Bain des

Negresses) along the southern boundary of Avalon Golf Estate, which is hardly feasible.

Therefore, the only effective solution for water supply of Bois Sec area is surface exploitation

from Ruisseau des Chevrettes.

Ruisseau des Chevrettes presents decent permanent flow rate which would be adequate for

water supply of Bois Sec domain. Saint Aubin owns water rights on this stream, with present

derivation being sent to Ruisseau Cresson in order to reach further downstream the Beau

Bois reservoir.

Exploitation of Ruisseau des Chevrettes to supply Bois Sec would require a 2km long pipe

across Avalon Golf Estate, with the flow being gravitational then. A culvert along this path is

presently under construction at Avalon Golf Estate and could be used in this scope.

Derivation setup at Ruisseau des Chevrettes should be refurbished, ideally with proper

gauging structure to be also implemented on Ruisseau des Chevrettes in order to evaluate

effective base flow of the stream.

Figure 7-1: Location of proposed boreholes, existing surface water derivation system and possible piping to bring surface water to Bois Sec, Bonne Aubaine & Talusseau areas

HYD-SM-14014 42

8 Bibliography

1. BANTON, Olivier & Lumony BANGOY, 1999, “Hydrogéologie – Multiscience environnementale des eaux souterraines”, 460p., Presses de l’Université du Québec, Aupelf, ISBN 2-7605-0926-5

2. CENTRAL WATER AUTHORITY, 2011, "Hydrology Data Book 2005-2010", Ministry of Public Utilities of Mauritius

3. DRISCOLL, Fletcher G., 1986, “Groundwater & Wells”, 2nd edition, 1089p., Johnson Filtration Systems Inc, St Paul, Minessota 55112, ISBN 0-9616-4560-1

4. GIORGI, Loic & Serge BORCHIELLINI, 1999, “Ile Maurice - Carte géologique au 1:50,000 et schéma hydrogéologique“, Projet Franco-Mauricien « Appui à la gestion des ressources en eau et à la préservation de leur qualité », Geolab 1999, 56p. + cartes.

5. LOUDIERE, D., 1989, “Révision du schéma directeur pour l’aménagement des eaux – Aquifères du Sud", Rapport du

Fonds d’Aide et de Coopération de la Coopération Française pour le compte du Ministère de l’Energie, des

Ressources en Eau et des Services Postaux du Gouvernement de Maurice, 36p.

6. MAURITIUS METEOROLOGICAL SERVICES, 1982-2010, "Monthly Bulletin of Climatological Summaries"

7. SADDUL, Prem, 2002, “Mauritius, A Geomorphological Analysis”, Mahatma Gandhi Institute, 354p. ISBN 99903-39-33-3

8. TODD, David Keith, 1959, “Ground Water Hydrology”, 336p, John Wiley & Sons Inc., New York. ISBN 0-4718-7615-1

hydrogeological study - bois sec / bois cheri

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