Hydrology

Hydrology

 

The Else River is approximately 8km long, and drains a catchment of approximately 18km². The upper catchment consists of a plateau ringed by small mountains, and is drained by numerous small streams which join to form the Else River. From the plateau the river runs down a steep narrow valley which flattens out to form a vlei within the last kilometre before reaching the sea - Heinecken et al. (1982)

 

The Klawersvlei River in the mountains above Simonstown was once a tributary to the Else River, but its flow was diverted many centuries ago through a process known as river piracy. It now forms a waterfall in the Baviaanskloof, before draining into the sea via a storm water duct. The Royal navy purchased land in the Klawer Valley and built a dam in 1893 (possibly the Jackson Dam?), which served their needs for many years. In 1912 a retention scheme was started by Sir Henry Rawson (Read, 2003), which presumably led to the construction of the Rawson Dam.

 
Glencairn's water supply initially came from the river, and later came from a group of springs called Olsson's and Forrest's springs. From there it was piped into a 100 000 gallon tank, and supply was great enough to supply the railway with water for their steam engines. Tredgold (1985). By 1925 there was still no municipal water supply to Glencairn from Simonstown, and residents bought water from the Glencairn Syndicate water supply. In 1946 the municipality bought this supply and developed it for domestic purposes (Read, 2003).

 
The remains of these supply systems are still visible, particularly the spring, reservoirs and pipeline on Gordon's land and in the Glencairn Expressway Road Reserve. There are also foundations of what appears to have been a water tank at the end of Forrest Way. The springs, tanks and some of the piping was later abandoned when the municipality installed a water main from new dams that were built in the Else River catchment above Simonstown.

 
The Lewis Gay dam was constructed in 1952. It has a reinforced concrete wall and capacity of 182 x 10³m. The Kleinplaas dam was constructed around 1965 to 1968 with an earth wall. The wall was raised in 1970 to a capacity of 13 000 x 10³m³ (Heinecken et al. 1982, Read 2003). These dams were able to meet the needs of Simonstown until 1982, but since the mid 90's, the water supply has been augmented from the City of Cape Town supply and Navy dams (Read, 2003).

 
The City of Cape Town supply to the South Peninsula comes via Muizenberg and Clovelly in a pipeline that supplies 15 Ml/day. The capacity of the pipeline is 27,475 Ml/day, which,it is projected, will be reached in 2013. Brooklands Water Treatment Works currently supplies 5 Ml /day, and the as the situation currently stands, the capacity of existing supply scheme of 32,727 Ml/day will be reached in 2027 (Jeffers & Green 2002).

 

The most favourable additional water supply from an economic and water security perspective is the proposed Brooklands Dam, designed as a clay core earth dam with capacity of 900Ml. Due to its limited catchment, it is further proposed that 0,5-1,0Ml per day be supplied via a canal from the navy dams in the Klawer Valley. The assured yield of this dam is 5Ml/day, so this option would effectively double the available water supply to the Brooklands Water Treatment Works facility. (Jeffers & Green 2002).

 

The actual yield could be higher if the supply is augmented with water stored and diverted from the navy dams in the Klawer Valle. Less than 5% of the total runoff from this catchment is stored and diverted for navy use ( ,pers com) or piped either by gravitation or pumping to Brooklands Water Treatment Facility and to Kleinplaas dam. However, the proposed canal crosses Cape Peninsula National Parks land and there are environmental impacts that need to be taken into consideration.

 

Groundwater

Rocks of the TMG form a significant secondary aquifer capable of yielding large quantities of good quality groundwater on a sustainable basis. In lower-lying areas the groundwater is close to the surface, in higher-lying areas it may be 20-50m below the surface. Numerous faults suggest a good potential for drilling since they transmit groundwater. The contact between the Cape Granite Suite and the TMS is considered to be an important target for drilling. It is estimated that 6-10 production boreholes could yield an additional 5 Ml/day (Jeffers & Green 2002).

 

Once again, there are important environmental concerns, since access tracks, and electricity would have to be supplied to the drilling/pumping sites. Many of the target sites lie in the Cape Peninsula National Park, where facilities and associated disturbance, as well and visual impacts would not be consistent with the land use. Also, the Simonstown Structure Plan states that there are two existing boreholes in mountain at Simonstown, which could provide 4-5 Ml per week, but that very high concentrations of Fe and Mn in the groundwater cannot be treated easily by means of conventional processes. (MLH, 1996)

 

Stormwater

Surface water flowing on the road between Glen Beach and Glencairn station during rainstorms has been identified as being of some concern.

 
The South Peninsula Structure Plan recommends the use of 100 year return storm when planning for stormwater management. Design systems for the 100 year storm usually include the provision of retardation ponds coupled with conduit networks and surface flows( ). The steep topography of many areas dictates that there is little scope for introducing retardation ponds and little for introducing surface flow on streets (MLH, 1996), but even where these could have been implemented in subsequent developments in the study area they have not.

 
The Simonstown Structure Plan states that any new developments are required to make provision for piped stormwater management, including the cost of upgrading any sections of existing down-slope stormwater systems that cannot accommodate additional flows (MLH, 1996). It did not, however, make any stipulations with regard the impact of the receiving environments, namely the river, vlei and coastal environments.

 
After the fire that swept through the mountains early in 2000, heavy winter rains causes a 'mudslide' across Gordon's property into the upper end of Glencairn Heights. This was caused by the migration of a linear sand dune across a rock outcrop and associated valley. This created a natural dam wall of loose sand, which eventually broke and rushed down the valley next to the linear dune into the suburb.

 
The moderating influence of the sea provides an average temperature of 16,3 degrees C, the fluctuation between winter and summer temperatures is less dramatic than the interior of the Western Cape.

 
Glencairn is exposed to gale force winds in summer, the southeaster reaches a wind speed of of more than 36km/h in the months November, December, January and February. This can raise the moderate level of the high tide as much as 0,305m. In winter months, the north westerly brings rain.

 
Microclimates created by the mountains are important determinants of the vegetation type and soils formed. Nort and west-facing slopes are generally drier and hotter than south-facing ones. South and east-facing slopes are cooler, kloofs and gullies are moister and cooler than exposed areas.

 
The mist associated with the southeaster carries moisture that is essential for vegetation to survive the dry summer months. (MLH, 1996)

 
Fynbos is the predominant vegetation type of the area.


According to Triner-Smith, Cowling and Linder (1996) there area 2285 plant species on the Cape Peninsula, of which 90 are endemic (only found on The Cape Peninsula).

 
Conditions in the study area are ideal to support fynbos, a vegetation type that forms part of the Cape Floristic Region, the smallest of six floral kingdoms worldwide. The rainfall of just below 700mm, infertile sandy soils derived from sandstone mountains and lowland dune sands are ideal to support high diversity.

 
Burgers and Impson (1994) recorded no less than 127 species (one of the highest numbers recorded per 1000m²)von the Red Hill plateau.

   

In its lower reaches the river forms a fresh-water wetland or vlei 500m long and 200m wide behind a minor dune barrier bordering a small baech Heinecken et al. (1982)

 
The vlei drains via a small canal approximately 130m long situated against the southern side of the valley. Seawater seldom flows into the canal and the system cannot really be classified as an estuary.

 
Two road bridges, each of two spans of 4m cross the river at its mouth, a railway bridge of three 7m spans crosses in the surf zone.

 
When the river does flow, obstruction can be caused by sand washed in from the beach which is deposited in the channel beneath and between the railway bridge and the main road bridge. A steep beach bar can form seaward of the railway bridge.

 
It can thus be said that the dynamics of the Else River are governed by a beach bar and artificially by sand accumulation underneath the road and railway bridges. These barriers are only breached after heavy rains and a build up of water in the vlei.

 
No apparent signs of long-shore sand movement around the headlands could be detected, this could indicate that dynamics are dominated by a large extent by "closed sedimentary cell" conditions

 

Variations in beach profile and sand volume will therefore ,largely be the expression of on- and offshore sand movement; sand eroded from the beach temporarily being stored in the breaker bar, from where it returns during accretional conditions.

Some sand lost from the beach during south-easter conditions and is removed mechanically. Since the beach between the rocky headlands is in equilibrium, the fact that sand is lost from the beach due to wind action indicates that some mechanism must exist

 
Since the beach slope is very shallow, it is possible that the beach is replenished with fine sand from a shallow offshore area

Some longshore movement around the headlands replenishes the beach, although no such movement could be detected.

 
What about the river?!

 
The average wave height for the Glencairn beach was calculated to be 0,58m which places it in the classification of a "low energy beach" (Valsbaai Strandverbeteringe, 1980; Swart and Sedyn, 1982)

 
From historical photographs it can be seen that an extensive dune-field stretched from the beach up the eastern side of the valley and spilled over into the Noordhoek basin.

 
The beach is an important recreational amenity, not only for local residents, but also for the general public of Cape Town as it is easily accessible by train. This popularity necessitates that certain controls and facilities be provided by the local authorities.

 
The value of the river for environmental education and nature conservation requires very careful consideration within the context of the need for water for the Simonstown municipality. This relates to plans by the municipality to build a third dam in the Else River catchment. It is recommended that before the plans for this dam reach an advanced stage, a full environmental assessment be made for the impact of the dam on the Else River and Vlei.