Welcome to ISD event EARTH CACHE. In The Shadow of the Dragon 

Umzimkulu River - Callaway Bridge

Plenty of parking available and a short walk to ground zero for a quick view to answer questions to claim earth cache

River Bank Erosion

Rivers are dynamic structures. They change their flow patterns continuously and naturally. These changes affect river bank soil stabilization because river bank absorbs the energy. River bank erosion is a part of this changing nature. When banks of a river wear away by watercourse, it is called river bank erosion.

Mostly river banks are made of stratified layers with cohesive and cohesion less materials. Again river banks have three zones- toe zone, bank zone and overbank area. Toe zone is situated between the ordinary water level and low water level which makes it vulnerable to erosion. If non cohesive materials are at the toe zone, vulnerability is increased. But at the time of lowering water level, cohesive banks are most susceptible to erosion because of its low water permeability.

River bank erosion are of two types:

Bank scour: It means removal of bank materials by flowing water and carried sediment.

Mass failure: When sections of the banks slides and topples into the water stream.

Hydraulic action takes place when the air in a joint is suddenly compressed by a wave closing the entrance of the joint. This then cracks it. Wave pounding is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. Abrasion or corrosion is caused by waves launching sea load at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with corrosion). Corrosion is the dissolving of rock by carbonic acid in sea water.

Limestone cliffs are particularly vulnerable to this kind of erosion. Attrition is where particles/sea load carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as shingle and sand. Another significant source of erosion, particularly on carbonate coastlines, is boring, scraping and grinding of organisms, a process termed bio erosion.

Sediment is transported along the coast in the direction of the prevailing current (longshore drift). When the up-current amount of sediment is less than the amount being carried away, erosion occurs. When the up-current amount of sediment is greater, sand or gravel banks will tend to form as a result of deposition. These banks may slowly migrate along the coast in the direction of the alongshore drift, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a build up of eroded material occurs forming a long narrow bank (a spit). Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore.

Causes of River bank erosion:

Though river bank erosion is a natural process, human activities can increase its rate. Following are the natural causes of river bank erosion:

Flood

Flood is the most crucial reason of river bank erosion. The chances of flood increases in the rainy season.  During flood the huge amount of water flow with higher velocity brings enough energy to tear away the top layers of soil or even causes mass failure. The water elevation is raised by centrifugal force resulting highest depth of flow at outside bend and gravitational force pulls down the water. This downward velocity against the bank is the erosive force.

Heavy rainfall

Soil can be eroded by heavy and excessive rainfall. Heavy rainfall often causes strong waves which can loosen and wear away non cohesive bed materials.

Sedimentation

Various disturbances in the nature can cause sedimentation. Silt accumulation at the river beneath reduces water holding capacity of a river as saturation of banks occurs. So the direction of river is changed. Thus meandering rivers are formed eroding river bank.

Strong current of rivers

Strong current generates because of the volume of water flowing or the steep stream gradient. Where the river currents are very much high, river bank erosion can occur. In case of cohesion less bed materials, these strong currents create a cantilever overhang of cohesive materials by wearing away non cohesive materials. So at the toe of the bank shear stress exceeds the critical shear and erosion occurs.

Man made causes of river bank erosion

Development

Human land development, in forms including agricultural and urban development, Bridge building and urban development, is considered a significant factor in erosion.

Deforestation causes River Bank Erosion

Humans facilitate erosion by cutting down trees at the bank for their own use and removing the natural reinforcement which exists there.

Housing near bank

People in rural areas make their home near the bank which adds compressive force to the soil beyond its capacity and thus cause erosion.

Soil extraction & River Bank Erosion

Humans extract excessive amount of soil from the bank for their uses or they extract huge amount of sands and gravels which help to hold back the soil. By doing this they accelerate river bank erosion.

Impacts of river bank erosion

River bank erosion has many social and economic impacts. It makes people homeless as they lose their property and are forced to migrate to elsewhere. Sometimes it causes death. Humans become poor and valuable land is lost due to erosion. Among the countries of the world, Bangladesh has the highest river bank erosion.

Erosion of the boulder clay (of Pleistocene age)

Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of current.

Floods

At extremely high flows, kolks, or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called rock-cut basins.

Wind erosion

Wind erosion is a major geomorphological force, especially in arid and semi-arid regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage - especially after being increased far above natural rates by human activities such as deforestation, urbanization, and agriculture.

Wind erosion is of two primary varieties: deflation, where the wind picks up and carries away loose particles; and abrasion, where surfaces are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories:

(1) surface creep, where larger, heavier particles slide or roll along the ground; (2) saltation, where particles are lifted a short height into the air, and bounce and saltate across the surface of the soil; and (3) suspension, where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50-70%) of wind erosion, followed by suspension (30-40%), and then surface creep (5-25%).

Wind erosion is much more severe in arid areas and during times of drought. For example, in the Great Plains, it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.

Erosion and tectonics

Tectonic processes control rates and distributions of erosion at the Earth's surface. If the tectonic action causes part of the Earth's surface (e.g., a mountain range) to be raised or lowered relative to surrounding areas, this must necessarily change the gradient of the land surface. Because erosion rates are almost always sensitive to the local slope (see above), this will change the rates of erosion in the uplifted area. Active tectonics also brings fresh, unweathered rock towards the surface, where it is exposed to the action of erosion.

However, erosion can also affect tectonic processes. The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle.

Because tectonic processes are driven by gradients in the stress field developed in the crust, this unloading can in turn cause tectonic or isostatic uplift in the region. In some cases, it has been hypothesised that these twin feedbacks can act to localize and enhance zones of very rapid exhumation of deep crustal rocks beneath places on the Earth's surface with extremely high erosion rates.
   

To Claim Earth cache answer following and send to CO

1. Describe in your own words rock face down stream on your left (rock face of river bank).

2. What type of erosion is taking place and very prevalent .

3. What human land development has taken place in the area.

4. Bridge NO and year built and does it have an effect on the flow of the water and erosion

5. Take picture and upload to your "found Log" [Optional]

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Please do NOT detail your answers in your Found log. Email answers directly to me via the GC.com website.

You may submit your Found log with a "proof" picture without waiting for confirmation. If there is a problem with your answer(s) I will get back to you. Found logs will be deleted if an email is not received within 2 Weeks..