**4.1.2 The Werribee Plains**

82 Studies on Water Management Issues

A study of the geological units show the oldest rock in Cape Town and suburbs are the meta-sediments of the pre-Cambrian Malmesbury Group, which occupy the coastal plain between Saldanha and False Bay in the west, to the first mountain ranges in the east (Meyer, 2001). Several erosional windows to this Group are exposed in mainly fault-controlled valleys further to the east and south. Natural features are varied and include narrow flats, kloofs and gorges, cliffs, rocky shores, wave-cut platforms, small bays and sandy and gravel beaches. On the Cape Flats sand dunes are frequent with a prevalent southeasterly orientation; and the highest dunes are only 65 m above sea level (Schalke, 1973; Theron et al., 1992). The sand is derived from two main sources: (i) weathering followed by deposition, under marine conditions, of the quartzite and sandstone of the Malmesbury Formation and the Table Mountain Series; (ii) the beaches in the area, from where Aeolian

**4.1 Geology and hydrogeology** 

sand was deposited as dunes on top of the marine sands.

Atlantis

**180 30'**

**330 30' 330 30'**

**340 00' 340 00'**

Bellville

Eersteriver

Macassar Strand Faure

Kraaifontein

Strandfontein

Cape Point Cape Hangklip

Fig. 2. Location of the Cape Flats sand in the Western Cape, South Africa (Adelana et al., 2010)

Muizenberg

**Cape Town**

Bloubergstrand

Noordhoek

**Railway**

**Sand-covered area**

Philippi

**4.1.1 The Cape Flats** 

The Deutgam WSPA includes all geological units to 40m below the natural surface, encompassing the shallow Werribee Delta sediments (DWSPACC, 2002). An alluvial deposit up to 20 m thick has accumulated in the gorge of the Werribee River. This gorge is the major terrain feature of the Werribee Plains with its alluvial deposit known to be gravely at the base and fumes upwards to become clayey at the surface (Condon, 1951). According to this work and more recent studies (Holdgate et al., 2001, 2002; Holdgate & Gallagher, 2003), the alluvial terraces on the valley floor provide evidence of Pleistocene and Holocene sea level changes. This alluvium, eroded by rejuvenated streams, was deposited (in Late Quaternary times) along the base of the Werribee River. There are prominent intra-volcanic sands within the Newer Volcanics (along the Werribee Plains) while the Older Volcanics were picked in few bores between coal-bearing sediments of the Werribee Formation (Holdgate et al., 2001). In general, the Werribee Formation is disconformably overlain by marine sandstone and mudstone/marlstone (Taylor, 1963 as cited in Holdgate et al., 2002; Holdgate & Gallagher, 2003). Across the Werribee Plains these exceed 120 m in thickness (Holdgate et al., 2001).

The groundwater system used in the Werribee South is called the Werribee Delta aquifer. The Werribee Delta sediments consist of sand and gravel lenses situated within clays and silts. The variable nature of the deltaic sediments resulted in a wide variation in aquifer parameters (SKM, 2002). According to SKM (1998), within the coarser sand horizons the hydraulic conductivity ranges from 10 to 15m/day, with a specific yield of 0.01 to 0.2 but the overall hydraulic conductivity of the aquifer is less than 5m/day with representative specific yield in order of 0.04. Typical bore yields for the Werribee Delta aquifer system are generally less than 5L/s, however yields up to 15L/s have been recorded (SKM, 2002). The selected bores for this study were screened in the Werribee Delta aquifer system, which are mostly sandy or silty clay material at shallow depths but with significant sand and gravel seams at a relatively deeper depth. The Werribee Delta aquifer system is unconfined to semi-confined and groundwater depth varied between 4-7m below ground surface. Recharge to the aquifer system is predominantly from direct rainfall infiltration and surplus irrigation water (SKM, 2002) as well as leakage from the ageing concrete-lined channels (Rodda & Kent, 2004).

Changes in Groundwater Level Dynamics in Aquifer Systems –

0

**Precipitation (mm)**

20

40

60

**Precipitation (mm)**

80

100

120

Implications for Resource Management in a Semi-Arid Climate 85

1935 recorded the least annual rainfall (229.4 mm/yr) (Adelana, 2011). A similar situation is observed from 1999-2003, with the exception of year 2001 that showed a relatively wetter record (i.e.784 mm/yr). Based on available information going as far back as the 1960s, Cape Town enters into a drought cycle (i.e. a lower than average rainfall pattern) on average every 6 years (Cape Water Solutions, 2010). The last of such a cycle was in 2003 and 2004 with dry winter and nearly 200mm less than long-term mean of annual rainfall. The

Seasonal patterns in the Cape Town area show a marked winter rainfall incidence, with June/July typically the wettest month. The general climatic trend throughout the study area

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

(a)

Mean Rainfall Mean Max Temp Mean Min Temp

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

(b) Fig. 4. (a) Cape Town mean monthly rainfall with maximum and minimum temperature.

(b) Werribee mean monthly rainfall with maximum and minimum temperature

**Werribee, Australia (1950-2010)**

0

0

5

10

15

**Mean Daily Temperature (**

 **0C)**

20

25

30

5

10

15

**Mean Daily Temperature (**

 **0C)**

20

25

30

**Cape Town, South Africa (1950-2010)**

Mean Rainfall Mean Max Temp Mean Min Temp

consequences include lower dam levels and the imposition of water restrictions.

#### **4.2 Climate**

The study areas (Cape Flats and Werribee Plains) are both under Mediterranean climate. Climate is temperate with warm dry summers and maximum rainfall occurring during winter/spring respectively. Historical average annual rainfall (1913-2009) varies from 1100 mm/yr in the upper north-west of the Werribee catchment to 540mm/yr near Werribee (SRWA, 2009). Historical data (1841-2009) showed there is a variable rainfall gradient in the Greater Cape Town area; rainfall is largely controlled by topography – between 500 mm and 1700 mm on the Cape Peninsula, to 500 mm and 800 mm on the Cape Flats, and ranging from 800 to over 2600 mm in the mountains to the east of the Western Cape region (Adelana, 2011). To the north of the Western Cape, this climate regime grades into semi-desert whereas to the south-east coast the climate becomes less seasonal and tends towards sub-tropical. Drier summer conditions and lower winter temperatures tend to inhibit some plants' growth.

Therefore, rainfall, minimum and maximum temperatures were analysed and compared to show climate variability over the years, and to identify/assess its impact on groundwater levels. Figure 3 show the annual/seasonal rainfall variation in the study areas. There is a similar pattern in the fluctuation of observed annual rainfall being less than the long-term average in many years. Long-term or historical climatic conditions indicate that on average, annual rainfall in the Werribee for the period 1950-1979 exceeded that for the period 1980- 2009, with the period 1997-2009 being one of considerably lower than average annual rainfall. For example, during 2004/05, rainfall in the Werribee River catchment was approximately equal to the long-term average; whereas rainfall was about 60% of long term average for 2005/06, although inflows were only 21% of the long term average (SRWA, 2006). Consequently, storage levels fell from an average 34% at the start of the year to 16% at the end of the year and irrigators and diverters in the Werribee system were allocated 80% of their water entitlement (SRWA, 2006).

Fig. 3. Annual mean of rainfall in the study areas 1950-2010 (Station: Cape Town Airport and Laverton RAAF Base)

In the Cape Flats from 1958 the trend in rainfall showed continuous decrease up till 1974; 1982-1985 was also a dry period with total average rainfall below annual mean. Since then there has been much fluctuation in the pattern of rainfall in the Cape Town area. This was shown to be comparable to older records (1921-1941) of relatively dry periods; for example

The study areas (Cape Flats and Werribee Plains) are both under Mediterranean climate. Climate is temperate with warm dry summers and maximum rainfall occurring during winter/spring respectively. Historical average annual rainfall (1913-2009) varies from 1100 mm/yr in the upper north-west of the Werribee catchment to 540mm/yr near Werribee (SRWA, 2009). Historical data (1841-2009) showed there is a variable rainfall gradient in the Greater Cape Town area; rainfall is largely controlled by topography – between 500 mm and 1700 mm on the Cape Peninsula, to 500 mm and 800 mm on the Cape Flats, and ranging from 800 to over 2600 mm in the mountains to the east of the Western Cape region (Adelana, 2011). To the north of the Western Cape, this climate regime grades into semi-desert whereas to the south-east coast the climate becomes less seasonal and tends towards sub-tropical. Drier summer conditions and lower winter temperatures tend to inhibit some plants' growth.

Therefore, rainfall, minimum and maximum temperatures were analysed and compared to show climate variability over the years, and to identify/assess its impact on groundwater levels. Figure 3 show the annual/seasonal rainfall variation in the study areas. There is a similar pattern in the fluctuation of observed annual rainfall being less than the long-term average in many years. Long-term or historical climatic conditions indicate that on average, annual rainfall in the Werribee for the period 1950-1979 exceeded that for the period 1980- 2009, with the period 1997-2009 being one of considerably lower than average annual rainfall. For example, during 2004/05, rainfall in the Werribee River catchment was approximately equal to the long-term average; whereas rainfall was about 60% of long term average for 2005/06, although inflows were only 21% of the long term average (SRWA, 2006). Consequently, storage levels fell from an average 34% at the start of the year to 16% at the end of the year and irrigators and diverters in the Werribee system were allocated 80%

**4.2 Climate** 

of their water entitlement (SRWA, 2006).

**Mean Annual Rainfall (mm)**

1950

1952

and Laverton RAAF Base)

1954

1956

1958

1960

1962

1964

1966

1968

1970

1972

1974

1976

Fig. 3. Annual mean of rainfall in the study areas 1950-2010 (Station: Cape Town Airport

In the Cape Flats from 1958 the trend in rainfall showed continuous decrease up till 1974; 1982-1985 was also a dry period with total average rainfall below annual mean. Since then there has been much fluctuation in the pattern of rainfall in the Cape Town area. This was shown to be comparable to older records (1921-1941) of relatively dry periods; for example

1978

1980

1982

**Cape Town (South Africa) Werribee (Australia) Cape Town Mean Werribee Mean**

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

1935 recorded the least annual rainfall (229.4 mm/yr) (Adelana, 2011). A similar situation is observed from 1999-2003, with the exception of year 2001 that showed a relatively wetter record (i.e.784 mm/yr). Based on available information going as far back as the 1960s, Cape Town enters into a drought cycle (i.e. a lower than average rainfall pattern) on average every 6 years (Cape Water Solutions, 2010). The last of such a cycle was in 2003 and 2004 with dry winter and nearly 200mm less than long-term mean of annual rainfall. The consequences include lower dam levels and the imposition of water restrictions.

Seasonal patterns in the Cape Town area show a marked winter rainfall incidence, with June/July typically the wettest month. The general climatic trend throughout the study area

**Werribee, Australia (1950-2010)**

Fig. 4. (a) Cape Town mean monthly rainfall with maximum and minimum temperature. (b) Werribee mean monthly rainfall with maximum and minimum temperature

Changes in Groundwater Level Dynamics in Aquifer Systems –

leakage) and groundwater pumping (SKM, 2009a, 2009b).

(Gerber, 1981; Vandoolaeghe, 1989).

water use.

**6. Salinity** 

(Reid et al., 2011).

Implications for Resource Management in a Semi-Arid Climate 87

approximately 13 million m3 are abstracted from the Cape Flats aquifer by commercial farmers in the Philippi area of Cape Town (Colvin & Saayman, 2007), and an additional 5 million m3 are abstracted by the City of Cape Town administration to irrigate sports elds at Strandfontein and Mitchell's Plain (Wright & Conrad, 1995). Moreover, another 20 million m3 was abstracted from wellelds in the southern part of the aquifer during the Pilot Abstraction Scheme to understudy the Cape Flats aquifer response to stress conditions

The bores examined across the Werribee Plain showed declines in groundwater levels occurring from 1996 to 1999, 2003 to 2004 and in late 2006 to early 2007. This tends to follow the downward trend in the frequency and amount of rainfall and is consistent with the general groundwater trend observed across Victoria during this period (Hekmeijer et al., 2008; Reid, 2010). The groundwater level drawdown of Werribee Delta aquifer shows that during the early 1990s seasonal drawdown was less than 0.5 m but in 1996, the seasonal fall increased up to 2 m. This indicates more use of groundwater for irrigation due to the lack of supply from the Werribee River. Therefore, the seasonal fluctuations are mostly influenced by rainfall and usage; however, some observation bores show seasonal fluctuation that is believed to align with the pattern of channel deliveries (i.e. due to enhanced channel

The observed groundwater trends and behaviour in the South African example (i.e. bores screened in the Cape Flats aquifer) are equally consistent with the fluctuations in rainfall pattern. It is obvious that the groundwater level falls due to less rain and possibly higher use from production bores, while rainfall recharge and recovery take place in wetter times when there is conversely less pumping. Some of the Cape Flats bores in Atlantis showed a marked response to pumping influences and have recorded higher groundwater level changes within short time. For example, WP167 with groundwater level decline of 3.5 m from August 1993-June 1995 and continuous decrease into the early 2000s. WP184 also show declines of 5.5m (September 1994-April 1995) and 4.8 m (October 1999-August 2000). Such high declines have influenced spring flows and base flow, and hence, the implications on groundwater management. Therefore, the groundwater declines are discussed in the context of groundwater resource sustainability and its implications on water security and resource management plans, including consideration of water conservation measures or conjunctive

The analysis of groundwater trends is critical in the study of salinity risk and the effectiveness of preventative measures. The majority of DPI bores were installed in response to reports of saline discharge outbreaks in the 1980s and 1990s (Clark & Harvey, 2008). Salinity has impacts on the social, economic and environmental values in any catchment. Therefore, groundwater monitoring co-ordinated by DPI and DSE provides an important tool in the understanding, measurement and management of salinity across the state of Victoria. Hence it is currently been reviewed and prioritised based on key assets in the state

In both WID and the Cape Flats, salinity (as measured by electrical conductivity (EC) of groundwater or total dissolved solids (TDS)) revealed the varying quality of groundwater

is a gradual increase in rainfall and a reduction in temperatures moving from north to south. Mean annual rainfall (1950-2010) has a long-term average of 597 mm. There is a dry period with less than 20 mm rainfall per month from November to March (Figure 4a); the mean annual temperature is moderate, approximately 17 °C. In the Werribee Plains, rainfall variability throughout a typical year does not exhibit a clear seasonal bias like the Cape Flats but fairly distributed, with average monthly rainfall ranging from 36 mm/month (March) to 59 mm/month (October) (Figure 4b).
