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Main
Rivers and tributaries |
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The main tributaries of the Great Brak are the Perdebergrivier,
Tweeriviere and Varingsrivier. (Ref 1)
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Catchment
characteristics
The area of the Great
Brak catchment is given as:
192 km² (Day, 1981;
Midgley and Pitman, 1969; Noble and Hemens, 1978).
163 km² (Hill Kaplan
Scott and Partners, 1974)
131 km² (River flow
data, 1978).
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The length of the river is measured from the
start of the longest tributary depicted on a map of the catchment by
Hill Kaplan Scott (HKS) and Partners (1974) is 28,5 km. On the 1:50 000
topographic map 3422 AA Mosselbaai the length is 29,3 km. The main
tributaries of the Great Brak are the Perdebergrivier, Tweeriviere and
Varingsrivier. There are 5 catchment areas in the Great Brak system
(Figure 2):
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Catchment area
% contribution
Perdebergrivier
23
Tweeriviere
25
Varingsrivier
31
Unnamed stream “D1”
(HKS, 1974)
13
Unnamed stream “D2”
(HKS, 1974)
8
100
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The Great Brak River rises on the slopes of
the Engelseberg (1521 m) in the Outeniqua Mountain range 25 km in a
straight line from the river mouth. The catchment is relatively long and
narrow being about 25 km long and reaching a maximum of about 8 km in
width. Much of the river system drains an elevated coastal platform
150-300 m high. The main tributaries of the Great Brak River and the
river itself are deeply incised into this platform.
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Dams |
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Ernest Robertson Dam (Ref 1)
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River
run-off |
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The mean annual run-off (MAR) as measured at
the Wolwedans gauging station for the periode 1961 -1980 was 18,19 x 106m3
. Annual run-off fluctuates widely: from 4,3 x 106m3
in 1979/80 to 44,5 x 106m3 in 1962/63.
The normal rate of flow in the Great Brak
has been considerably reduced during recent years by afforestation,
damming of small tributaries by farmers and the building of the Ernest
Robertson dam (WMS Franklin, in litt.,1979). (Ref 1)
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Geomorphology
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Between the village of Great Brak and the
Garden Route Freeway bridges the river meanders through a wide
floodplain. Immediately seaward of the freeway bridges the river
enters the lagoon basin about 1km long and 0,5km wide. The basin
is bounded by the hilly coastal plain in the north and by the
approximately 30m high bush-covered dune ridge to the south. The
dunes east of the mouth form a bluff about 50m high consisting of 20 m
basal dune rock, probably of Tertiary age, overlain by a partly
vegetated field of transverse barchan-type dunes. The Hersham
residential area has been developed in this area.
The sandy island 400m x 250m in size
occupies the eastern half of the lagoon basin. Originally
bush-covered, The Island is now occupied by a development of 90 houses
of which 12 are permanently inhabited. During the ECRU survey of
12 - 13 November 1981 a narrow northern channel flowed around the island
on its west, north and east sides and a wider southern channel enclosed
the island to the south. South of the Island both channels
combined into a mouth about 40m wide. During high tide the mouth
channel varies between one and two metres in depth. This tidal
inlet is periodically open. (Ref 1)
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Geology
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The headwaters of the Great Brak rise on the
quartzitic sandstones of the Peninsula Formation of the Table Mountain
Group. These sandstones, forming the spine of the Outeniqua
Mountain range, display the characteristic east-west strike of the
folded rocks of this area. The strata of the Peninsula Formation
dip southwards. The Great Brak runs southwards through the quartz
schists of the Sandkraal Formation is extensively thrust faulted; this
is a legacy of the orogenic processes that gave rise to the mountains of
the Cape Fold Belt. To the south of the Sandkraal Formation the
river runs on Tertiary/Quarternary valley alluvial deposits lying
between bluffs consisting of gneissic granite, granodiorite and albitite
intrusions of mid-Namibian age. Closer to the sea the valley
alluvial deposits lie between the reddish mudstones and sandstones
("Poortjie sandstone") of the Teekloof Formation of the
Adelaide Subgroup of the Beaufort Group (Karoo Sequence). The
heights to the east of the river consist mainly of Cretaceous
conglomerates of the Enon Formation. Finally the river enters the
sea via an estuary mouth consisting mainly of quartzitic aeolian and
marine sands. The mouth is bounded to the east by low
aeolianite ( dune rock) cliffs. (Ref 1)
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