Umhlatuzana River
The Umhlatuzana River rises in the Alverstone area of Kwazulu Natal and flows eastwards towards the Indian Ocean.
On its path to the sea it passes through a number of Nature Reserves - chief of these being Madwala NR, Giba Gorge MTB Park, North Park NR and Kenneth Stainbank NR.
Mostly the river has an easy gradient and is slow flowing, but in some parts the gradient is steeper, leading to a turbulent river.
Where the river flows though Kenneth Stainbank Nature Reserve, there is a wonderful example of the power of water to shape and scour the rocks and boulders in its path.
Hydraulic Action
Hydraulic Action
Hydraulic action is erosion that occurs when the motion of water against a rock surface produces mechanical weathering. Most generally, it is the ability of moving water (flowing or waves) to dislodge and transport rock particles. Within this rubric are a number of specific erosional processes, including abrasion, attrition, corrasion, saltation, and scouring. Hydraulic action is distinguished from other types of water facilitated erosion, such as static erosion where water leaches salts and floats off organic material from unconsolidated sediments, and from chemical erosion more often called chemical weathering. It is a mechanical process, in which the moving water current flows against the banks and bed of a river, thereby removing rock particles.
A primary example of hydraulic action is a wave striking a cliff face which compresses the air in cracks of the rocks. This exerts pressure on the surrounding rock which can progressively crack, break, splinter and detach rock particles. This is followed by the decompression of the air as the wave retreats which can occur suddenly with explosive force which additionally weakens the rock. Cracks are gradually widened so each wave compresses more air, increasing the explosive force of its release. Thus, the effect intensifies in a 'positive feedback' system. Over time, as the cracks may grow they sometimes form a sea cave. The broken pieces that fall off produce two additional types of erosion, abrasion (sandpapering) and attrition. In corrasion, the newly formed chunks are thrown against the rock face. Attrition is a similar effect caused by eroded particles after they fall to the sea bed where they are subjected to further wave action. In coastal areas wave hydraulic action is often the most important form of erosion.
Similarly, where hydraulic action is strong enough to loosen sediment along a stream bed and its banks; this will take rocks and particles from the banks and bed of the stream and add this to the stream's load. This process is the result of friction between the moving water and the static stream bed and banks. This friction increases with the speed of the water and the roughness of the bed. Once loosened the smaller particles are actually held in suspension by the force of the flowing water, these suspended particles can scour the sides and bottom of the stream. The scouring action produces distinctive markings on streams beds such as ripple marks, fluting, and crescent marks. The larger particles and even large rocks are scooted (dragged) along the bottom in a process known as traction which causes attrition, and are often "bounced" along in a process known as saltation where the force of the water temporarily lifts the rock particle which then crashes back into the bed dislodging other particles.
Hydraulic action also occurs as a stream tumbles over a waterfall to crash onto the rocks below. It usually leads to the formation of a plunge pool below the waterfall due in part to corrosion from the stream's load, but more to a scouring action as vortices form in the water as it escapes downstream. Hydraulic action can also cause the breakdown of river banks since there are water bubbles which enter the banks and collapse them when they expand.
Stream Erosion
Streams are one of the most effective surface agents that erode rock and sediment. Erosional landscapes such as the Grand Canyon have been formed by constant erosion from running water over millions of years. In addition to eroding the bedrock and previously deposited sediments along its route, a stream constantly abrades and weathers the individual rock and soil particles carried by its water. Hydraulic action, abrasion, and solution are the three main ways that streams erode the earth's surface.
Hydraulic action. See above.
Abrasion. Abrasion is the process by which a stream's irregular bed is smoothed by the constant friction and scouring impact of rock fragments, gravel, and sediment carried in the water. The individual particles of sediment also collide as they are transported, breaking them down into smaller particles. Generally the more sediment that a stream carries, the greater the amount of erosion of the stream's bed. The heavier, coarser‐grained sediment strikes the stream bed more frequently and with more force than the smaller particles, resulting in an increased rate of erosion.
Circular depressions eroded into the bedrock of a stream by abrasive sediments are called potholes. The scouring action is greatest during flood conditions. Potholes are found where the rock is softer or in locations where the flow is channeled more narrowly, such as between or around boulders.
Solution. Rocks susceptible to the chemical weathering process of solution can be dissolved by the slightly acidic water of a stream. Limestones and sedimentary rock cemented with calcite are vulnerable to solution. The dissolution of the calcite cement frees the sedimentary particles, which can then be picked up by the stream's flow through hydraulic action.
Potholes
Potholes form mainly in the upper course of the river, in high altitude where the river channel cuts directly into the bedrock. Potholes are the direct consequence of vertical erosion and processes of abrasion. The sequence of the development is quite easy to understand. As we know the river channel in the upper course is characterised by roughness, associated with large bedload. As water flows over still standing bedload on river bed it is forced over the obstacle and to eddie behind the rock downstream. This turbulence forces water down on to the bedrock. Over time small depressions within the bedrock develop. We can see a similar process of turbulent wind movement and eddie development as wind collides with tall building in cities. This is illustrated below.
Pothole Formation
Over time, the turbulent flow deepens the depression in the bedrock to form a small circular basin, just a few centimeters in diameter. This development creates a positive feedback, which further increases the turbulence of the water and the development of eddie currents. It is this turbulence which creates localised variations in the speed of erosion. As we can see in the diagram below, the hollow deepens further to form an established pothole. It is then possible for smaller bedload to become trapped in the pothole. This debris is then used to abrade the sides of the pothole. Over time the pothole will deepen and become more circular; the diameter of the pothole increases in size and often multiple potholes form and merge.
Earthcache Logging Tasks
In order to claim a find on this earthcache, please complete the logging tasks below and email the answers to me via my Geocaching Profile Page.
Task 1. Look around the river at the published coordinates and see where you can spot potholes being formed (small or large). In your own words describe what you see and take some measurements of the main features of the pothole. (Diameter and depth.)
Task 2. Take an estimate of the speed of the river flow during your visit. The easiest way to do this is to measure out a 10 meter length of river and measure the time a straw or blade of grass takes to travel the 10 m stretch.
Task 3. (Optional) A pic of you and your GPS at the published coordinates will always be welcomed.
References
Wikipedia: https://en.wikipedia.org/wiki/Hydraulic_action
Cliff's Notes: http://www.cliffsnotes.com/study-guides/geology/running-water/stream-erosion
Potholes: http://thebritishgeographer.weebly.com/river-landforms.html