| This EarthCache developed by an |
 |
 |
General Information
- This earth cache deals with the geomorphology of the Grootrivier mouth. The factors controlling both large scale and small scale features are explained.
- The cache is conveniently situated at the end of the Otter Trail hike.
- If GZ is approached from the west (Natures Valley Parking Area) please exercise caution if the estuary mouth is open to the sea with a strong current flowing.
- An excellent view of the estuary is obtained from GZ of traditional cache "Valley View" [GC8E48]
The Grootrivier Estuary
Natures Valley, the most easterly village in the Western Cape lies on the western bank of the Grootrivier estuary: the water body is locally known as the Natures Valley lagoon. The village and estuary are completely encircled by the De Vasselot section of the Tsitsikama National park. The village is reached via the scenic R102 that loops off from the N2 national road.
TO CLAIM AND LOG A FIND FOR THIS EARTH CACHE, YOU NEED TO ANSWER THE FOLLOWING QUESTIONS (MEMBERS OF GEOCACHING GROUPS MUST EACH SUBMIT INDIVIDUAL ANSWERS). PLEASE EMAIL OR MESSAGE YOUR ANSWERS VIA MY GEOCACHING PROFILE
1. Do you think that the local name “Natures Valley Lagoon” accurately describes the geomorphological feature at the mouth of the Grootrivier? Briefly state your reasoning.
2. Explain why there are steep slopes to the east and low lying areas to the west of the estuary.
3. Describe the connection between the estuary and the ocean at the time of your visit. Specifically whether and where the outflow is discharging to the sea. Provide an estimate (in meters) of the distance of the discharge (if any) from GZ.
4. Explain the wave conditions and weather most likely to have caused the estuary mouth to appear as you have described it.
5. Give the exact lettering that appears (white letters on a green background) on the rock at GZ.
An estuary is formed where a river meets the sea. Some estuaries are permanently open to the sea, while others seldom open. A lagoon is a body of water, usually oriented parallel to the coast, separated from the ocean by a barrier and connected to the ocean by one or more restricted inlets that allow limited communication with the ocean. The distinction between lagoons and estuaries is therefore not clear-cut. The salinity (saltiness) of estuaries depends on the extent to which seawater is mixed with freshwater from the river. Estuaries experience constant change and are influenced by the tide, waves, and rainfall in the river catchment. Estuaries experience large fluctuations in temperature, salinity and oxygen concentration. Despite this, many types of plants and animals have adapted to live in estuaries. Estuaries are important nursery areas for many fish and almost 100 species are dependent on estuaries at some stage of their lives. Many birds depend on estuaries for food. Estuarine environments are threatened by the construction of dams, agriculture, property development and pollution.
Geological Setting
The Grootrivier Estuary is underlain by Bokkeveld shales in its upper reaches while the bottom of the main body of the estuary is composed of alluvial and aeolian (windblown) sands. A fault running approximately northwest-southeast along the long axis of the estuary has given rise to the higher relief on the east side of the estuary than on the west. The low western shore of the estuary, on which Natures valley village is located, consists mainly of Tertiary and Quaternary alluvial and aeolian deposits except at the head where the Gydo Formation of the Bokkeveld Group (BVG) is exposed. The Grootrivier catchment is composed almost entirely of sandstones of the Peninsula Formation of the Table Mountain Group (TMG). The catchment contains two main streams, namely, the Grootrivier and the Bobbejaansrivier. The rivers cut through short stretches of Cedarberg Formation shales (TMG), Tchando Formation sandstones (TMG) and Baviaanskloof Formation sandstones (TMG) before reaching the Gydo Formation shales (BVG) at the head of the estuary. On the east the estuary is bounded at the mouth by a headland, Die Punt, composed of Peninsula Formation sandstones (TMG).
A detailed discussion of the lithology and stratigraphy of the Tsitsikamma area is given by Toerien (1976).
Shoreline Processes:Constructive and Destructive Waves
Waves of any form—be they ocean waves, sound waves or seismic waves—are a way of transferring energy from one point to point another. There are two types of waves in the universe, electromagnetic (e.g. light and radio waves ) and mechanical (e.g. sound waves, seismic waves). The waves we’re interested in, ocean waves, are a type of mechanical wave. This means they have to travel through something (a medium) which, in the case of ocean waves, is water.
Ocean waves normally form when wind blows across an open body of water, giving them the alternate title of wind waves or wind generated waves. They travel along the surface of the water, at the interface between the air and the water.
Waves are important for coastal systems because they are a method of moving energy, and it is this energy which ultimately shapes coastlines through erosion or deposition. When wind blows across the ocean there is friction acting between the water and the air. This friction transfers some of the air’s energy of movement (kinetic) into the water. The transfer of energy into the water, combined with turbulence in the air, produces perturbations in the surface of the water that eventually become waves. Initially these waves are small. Their growth is controlled by how much energy the wind can transfer to them. The obvious control on this is wind speed. Faster winds have more kinetic energy and can therefore produce larger waves. Another control is the length of time the wind blows. The longer it blows, the more energy is transferred into the water, producing bigger waves. A less obvious control on the strength of wind generated waves is fetch. This is the length of water over which the wind has blown. A longer fetch allows for greater kinetic energy transfer and so we get stronger waves transferring more energy.
The shoreline is where we see waves in action. On the run-up to a coastline, the depth of the water becomes shallower and a wave is forced to slow down. As it slows, its wavelength decreases and its height increases. This process is called shoaling. A wave’s height can only increase to about one seventh its wavelength, at this point the wave becomes unstable and it breaks. Depending on the properties (wave length and amplitude) of the wave, when it breaks on a coastline it can be classified as either constructive (low energy) or destructive (high energy). A constructive wave adds material to a beach while a destructive wave removes material (usually sand) from a beach. Constructive and destructive waves differ in the strength of their swash (rush of water up a beach) and backwash (rush of water down a beach).
Constructive waves have a strong swash and a weak backwash. This means they deposit more material on a beach than they remove. Over time, they build up gentle beaches. The repeated action of pushing material up a beach eventually leads to the development of berms and sand bars that can restrict estuaries and give rise to lagoons.
[By Alex Jackson. From Geography AS Notes. Licensed under the Creative Commons Attribution-Non Commercial 4.0 International license.]
The Main Features of Constructive Waves
• Low wave height relative to wave length
• Spilling wave with forward velocity
• Strong swash
• Weak back wash
• Low gradient beach
• Big beaches with dominant deposition
Destructive waves tend to be steep and to form during storms. When they break on a beach they plunge, producing a weak swash but powerful backwash. As a result, they erode material from a beach and produce a steep beach with breakpoint bars. Destructive waves may erode the sand bar restricting an estuary, opening the river mouth to the ocean, and to tidal influences.
[By Alex Jackson. From Geography AS Notes. Licensed under the Creative Commons Attribution-Non Commercial 4.0 International license.]
Main Features of Destructive Waves
• High wave height relative to wave length
• Plunging wave
• Strong back wash
• Weak swash
• High gradient beach
• Small beaches with erosion more dominant
The Grootrivier Estuary
The average depth of the Grootrivier estuary is about one metre, but depths of 2 metres occur in the channel in the middle reaches, where the river enters the estuary (at the R102 bridge) the bottom is rocky (Bokkeveld shales); the bottom of the middle reaches is sandy (fine to medium) but contains considerable organic material. Near the mouth the bottom consists of fine- to-medium-grained sand of marine origin mainly blown (but also wave-transported) into the estuary from the sea. The seaward end of the estuary is restricted by a sand bar, which is partially covered with natural vegetation, and partly artificially stabilised to encourage natural vegetation cover. The bar or sand-spit is about 350m long and 150m wide.
Estuary Closed by Sand Bar
The position of the sand bar at the estuary mouth is influenced by near-shore wave-driven easterly-moving currents. This deflects the channel towards the rocky embankment at the eastern shore of the estuary mouth where it forms a sand shoal or delta approximately 300m wide (E-W) and extending 100 m seawards (N-S). This delta is divided by the mouth channel into a large western and a smaller eastern section.
Grootrivier: Breached Sand Bar with Outflow Delta
The Grootrivier has an outflow-dominated estuary, i.e. river discharge combined with tidal out flow is stronger than the wave supported tidal inflow, a condition which changes depending on the strength of river flow and of wave energy. During dry periods with low wave energy (usually summer) there is a tendency for the mouth to close completely. The mouth is scoured open again when stormy conditions give rise to high wave energy and beach erosion and strong river flows cause the estuary to deepen.
Estuary Breaching Sand Bar
Over time the mouth migrates over the sand shield of the delta. After a period of closure strong river flow together with destructive waves causes the sand bar to be breached on the western side of the delta – near the centre of the sand bar. With time, under the influence of the south-westerly constructive waves the channel will be deflected eastward until the mouth once again closes as a result infilling of the channel by wave-transported sand during periods of constructive wave action.
Images from Google Earth
Showing Different Outflow Locations
Western Outflow 
Central Outflow 
Eastern Outflow 
References and Sources
1. www.saasta.ac.za/downloads/pdfs/booklet_coastal_fish_2004.pdf
2. TOERIEN, D.K. (1976). Geologie van die Tsitsikammakusstrook. Koedoe 19:31-4 .
3. P D MORANT and I B BICKERTON (1983) Report No 19: Groot (Wes) and Sout, CSIR Research Report 418.
4. https://geographyas.info/coasts/waves/
5. Google Earth Images
