Bank Erosion = فرسایش کناری
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The stream is severely eroded along its course through the Melville Koppies nature reserve (east and west). The reach downstream of the point where the east catchment stream joins the main stream is particularly badly eroded, having vertical banks up to 3 m high. This makes it difficult to approach the water?s edge and in places the bank represents a danger to users of the nature reserve, because it has been undercut and is susceptible to collapse. The flood attenuation measures discussed in section 2.4.3 would certainly reduce erosion of the stream banks. Flood attenuation is, however, generally expensive and requires large open spaces. An alternative to reducing the stream bank erosion is to provide physical protection to the stream banks. It should be noted that the two methods of reducing erosion (flood attenuation and bank protection) are not exclusive and a combination of the two may produce an economical and environmentally satisfactory solution. Various methods of stream bank protection are available, most of which involve the re-construction of the bank with gabions or inter-locking precast concrete blocks, such as Envirowall, L?lstein or Armorflex. All of these methods create an environmentally "hard" look and do not retain the natural look of the stream. They are also fairly expensive. A less formal, more environmentally 'soft' and cheaper solution would be the use of gum poles or brushwood wickets to protect the stream banks at points where trees are threatened or the bank is dangerously undercut. The use of gum poles entails the placing of a barrier against the eroded section of the stream bank. The gum poles are sharpened at one end and driven into the ground side by side. To secure the top of the barrier a horizontal pole is placed against the barrier approximately two thirds of the way up and pulled tight against the barrier with the aid of "Duckbills" (See Appendix 6.1), anchored in the bank at regular intervals. Figure 3.1 is a sketch of a typical panel. The use of 100 mm gum poles is recommended as this would lend itself to sharpening of the points and driving into the ground with the use of a sledge hammer, while still providing adequate strength. It is recommended that a geotextile (Bidim (see Appendix 6.2) or a similar product) be placed between the stream bank and the barrier. This is necessary to prevent scouring of the stream bank by water flowing through the small gaps between the poles. These gaps are due to the irregular nature of the poles and are unavoidable. At the beginning and end of the protected section of the bank some sort of arrangement would be necessary to prevent water flowing in behind the barrier and eroding the bank. Extending the barrier at an angle into the bank for about 1-m at the edges of the barrier should solve this problem. The advantages of this method of erosion protection are that no specialised machinery or trained construction team is required. The work could be done with unskilled labour and only requires a saw to cut points onto the gum poles and a sledgehammer to drive the poles and the duckbills into the ground. The estimated cost of this form of protection is R1 10/m2. Brushwood wickets have been used successfully for stream bank protection elsewhere in the world 1. As can be seen from the photograph in figure 3.2, they are environmentally and aesthetically soft and can be constructed by manual labour. Used in conjunction with large, loose boulders they would provide an appropriate solution to the erosion problem here. The major disadvantage is that regular maintenance would be necessary. Other variations of this technique, such as weaving laths through gum poles driven at about 1,0-m spacing can also be used. The estimated cost is between R50/m2 and R100/m2. | |