Salt River Catchment

Land use

4.2.5 Land use

Because of the catchments urban character, the percentage of open land with permeable areas is relatively low. Part of the Liesbeek sub-catchment on Table mountain is permeable but on a steep slope. Land use in the catchment is mainly residential with some commercial and industrial areas, especially along the Salt River canal. There is no other main open space system, then the Table Mountain national park. Along the rivers and the canal are isolated open spaces, like sports fields and golf courses. The Tygerberg Mountains are mainly used for agriculture and residential uses.

“The Salt/Black/Liesbeek/Elsieskraal river catchment is possibly in the poorest ecological condition of all of the major catchments in the city. Apart from some of the upper reaches of the tributaries of the Liesbeek River, most of the rivers are canalised or in pipes. The system also receives wastewater from Athlone and Borchards Quarry Wastewater Treatment Works, and the lower sections are seriously polluted”(Cate Brown and Rembu Magoba, 2009).

Salt River Catchment

Impervious erven

4.2.6 Surface permeability

Permeability of the surface area has a direct influence on the run off during rains. If the percentage of non-permeable land is high, the run off rate is high. If surfaces are sealed or soils are rocky/clayey with little water absorption the percentage of the water that gets absorbed is low.

Most of the catchment shows a high percentage of over 45% imperviousness, this is due to urban development which is the main factor for a high rate in imperviousness.

4.3 Marine boundary

4.3.1 Ocean

“The sub-humid to arid coast at the west coast of the Cape is characterized by the cool Benguela Current, south-westerly swells and strong north-flowing littoral drift”(Charles W. Finkl and Christopher Makowski, 2019). Table Bay is situated on the north of the Cape Town CPD. It’s shape and calmer waters made it the perfect place for a harbour 400 years ago.

The threshold between land and sea is influenced by different energies from wave and tidal movement.

“Waves are undulations formed by wind blowing over a water surface. They are caused by turbulence in air- flow generating pressure variations on the water. Once formed, waves help to disturb the airflow and are partly self-sustaining. Energy is transferred from the wind to the water within the wave-generation area. The amount of energy transfer depends upon the wind speed, the wind duration (how long the wind blows), and the fetch (the extent of water over which the wind blows). Once waves approaching a coastline ‘feel bottom’, they slow down. The waves crowd together, and their fronts steepen. Wave refraction occurs because the inshore part of a wave crest moves more slowly than the offshore part, owing to the shallow water depth, and the off- shore part swings forwards and the wave crests tend to run parallel to the depth contours” (Hugget, 2011). Waves and their energy can be observed during high tide at the downstream end of the canal. They are the reason for the protection of the land .

“Currents are created in the near shore zone that have a different origin from ocean currents, tidal currents, and wind-induced currents. Nearshore currents are produced by waves. They include longshore currents, rip currents, and offshore currents. Longshore or littoral currents are created when waves approach a coastline obliquely. They dominate the surf zone and travel parallel to the coast”(Hugget, 2011).

“Tides are the movement of water bodies set up by the gravitational interaction between celestial bodies, mainly the Earth, the Moon, and the Sun. They cause changes of water levels along coasts. In most places, there are semi-diurnal tides – two highs and two lows in a day. Spring tides, which are higher than normal high tides, occur every 14–75 days when the Moon and the Sun are in alignment. Neap tides, which are lower than normal low tides, alternate with spring tides and occur when the Sun and the Moon are positioned at an angle of 90◦ with respect to the Earth”(Hugget, 2011) . Tidal change can be observed in the canal. During low tide a small channel flows out to sea, at high tide the canal fills with water moving “backwards” or better inland. No information could be obtained how far back the tidal influence is, but a “backward” flow could be observed by the author as far as the Liesbeek and Black River confluence.

A delta is often closely associated in time and space with an estuary, but frequently in the literature the two are not adequately separated, particularly for tide-dominated estuaries. “For the same riverine outlet, a delta is a geomorphic and sedimentologic feature, while an estuary is a hydro chemical one where riverine freshwater flowing into a bay, a lagoon, or semi-enclosed coastal body of water mixes with seawater. To a large extent, all deltas can be estuarine in the sense that some part of them will have a freshwater-to-seawater transition, and large estuarine environments whose basin has not been filled with sediment may contain small-scale deltas along their margins or in their headwaters. Deltas within estuaries generally are relatively small sedimentary accumulations compared to the size of their estuarine setting”(Kennish, 2016). The sediment collection at the mouth of the canal is a small delta. “There are 2 concepts of delta, the delta in an estuary and the estuary in a delta, depending on the sedimentology” (Charles W. Finkl and Christopher Makowski, 2019). In the case of the Salt River the delta formed a small part of the estuary. Currently the delta is reduced to the small sediment deposit at the canal mouth. The species diversity in estuaries results from the coincidence of marine, freshwater and estuarine species. “Despite this diversity, fewer animal species are permanent residents in estuaries than in oceans or rivers, and those that are endemic to estuaries tend to be exceptionally hardy. Estuaries in general are harsh environments for aquatic life, where physiological systems must be adapted to rapidly varying salinity, temperature, light, turbidity, currents, and other physical and chemical properties of the environment” (Jordan, 2012). For seawater species the environ- ment in estuaries is gentler, with lesser wave action and lower salinity. Diadromous species could be observed entering the canal, attracted by the outflow pressure. The question, where they go or if they turn back, once they realize that this is not a river but a sterile canal system with no habitat qualities can’t be answered in this study. “Salt marshes start to form when tidal flats are high enough to permit colonization by salt-tolerant terrestrial plants. Depending on their degree of exposure, salt marshes stretch from around the mean high-water, neap-tide level to a point between the mean and extreme high-water, spring-tide levels. Their seaward edge abuts bare intertidal flats, and their landward edge sits where salt-tolerant plants fail to compete with terrestrial plants. Salt marsh sediments are typically heavy or sandy clay, silty sand, or silty peat. Many salt marshes contain numerous shallow depressions, or pans, that are devoid of vegetation and fill with water at high spring tides” (Hugget, 2011). Currently there is no space for the formation of a salt marsh area in the Salt River system. The fragmentation of the canal doesn’t allow for it.

“Environmental conditions within saltmarshes are very much determined by the tide. The tidal regime varies between sites, and estuarine saltmarshes experience the full range of tidal regimes” (Kennish, 2016).

"Salt marsh originates with the spread of vegetation onto an accretion intertidal mudflat. Fine suspended sediments (silts and clays) and organic material washed in by tides, and subsequently trapped by roots of salt marsh vege- tation, generate a gently sloping depositional terrace or platform between the high spring tide level and the midtide line. Vegetation cover of a salt marsh surface does not usually follow a continuous sequence from sea-edge to land. In addition to creeks and channels, highly saline, dry, or water-filled shallow depressions often feature in the surface of the marsh. They form where vegetation has either failed to establish, or where it has died back, or where drainage channels have slumped and blocked“ (Charles W. Finkl and Christopher Makowski, 2019).

"Geomorphic effects of sea-level rise are varied. Inevitably, submerging coastlines, presently limited to areas where the land is subsiding, will become widespread and emerging coastlines will become a rarity. Broadly speaking, low-lying coastal areas will be extensively sub- merged and their high- and low-tide lines will advance landwards, covering the present intertidal zone" (Hugget, 2011).

4.3.2 Harbour

“The exposed wave beaten coastline of South Africa is graced with very few sheltered embayments, placing a premium on estuaries as sites for harbours. Cape Town is situated on a large open bay, which provides little protection for ships from winter storms” (Branch, 2019). Today the Cape Town harbour is an artificial harbour constructed and expanded between 1880-1985.

The construction and land reclaimation of the harbour forced the Salt River course to change into a different posi- tion. The mouth was disconnected and piped into the harbour and the river tamed into a canal without connection to its floodplain. A seawall as the harbour boundary protects and disconnects the former marsh land from the sea. The influence of the ocean on the Salt River system and its surrounding land is tamed and reduced to the energy of the tidal inflow into the canal. With the construction of the harbour and the claiming of “extra” land the sea is separated by a wall from the land or vice versa.

“Seawalls are constructed to prevent inland flooding from major storms accompanied by high water levels (storm surge) and large waves. Seawalls also fix the position of the land sea boundary if the sea reaches the structure. The main functional element of a seawall is the elevation to minimize overtopping from storm surge and wave runup. A seawall is typically a massive, stone and concrete structure with its weight providing stability against sliding forces and over- turning moments”(Charles W. Finkl and Christopher Makowski, 2019). Dolosse protect the seawall further, they are 80 ton reinforced concrete blocks in the shape of chicken bones, that interlock into each other once placed. They are a south african designed coastal protection against wave action, that is used worldwide. There is no visual connection between land and sea. The only signs of being close are the sea air and a greater amount of seagulls.

“Part of the problem with the design of our urban areas is that we detached ourselves from water in the process. Often, people in cities are surrounded by water, but not able to use it” (Kotze, 2019).