8. RIVER BANK RETENTION

There are numerous methods available varying greatly in cost. Temporary works may be required in order to complete the permanent works. Such temporary works must be designed by a qualified engineer. The final design can be influenced by degree of retention required, design life, cost, approval of statutory authority, hydraulic characteristics of a watercourse and ease of maintenance.

The main types are:


    (a) Bagwork
Hessian bags- filled with dry concrete or sand cement. These are laid to the design profile in interlocking courses (like stretcher bond brickwork). Only suitable for shallow depths with relatively tranquil flow.

    (b) Concrete lining-
Can be reinforced or un-reinforced with movement joints and may or may not include concreting the bed of the watercourse. Where the bed is not concreted, protection against scouring and undermining of the banks must be considered.

    (c) Interlocking steel sheet piling-
Suitable where vertical banks form a continuous wall. Where scour of the bed is likely, protection will be required to prevent the piles being undermined. In deep situations, the piles may be tied back to anchor piles or blocks.

    (d) Interlocking concrete blocks-
These come in many shapes, sizes and finishes depending on the service requirements. The method of construction and use will depend on the type specified. Manufacturers' instructions, design drawings and specification will need to be closely followed.

    (e) Stone and block pitching-
Similar to interlocking concrete systems except natural stone or cut blocks are used. Joints may be pointed with sand/cement or asphaltic mastic to improve stability. Asphaltic mastic is preferred to allow small movements without cracking.

    (f) Gabions Steel mesh baskets or mattresses-
are filled with natural stone. For increased stability a pourable asphalt mastic may be used to fill the joints (see proprietary literature).

    (g) Geotextiles-
There are many recent advances in the use of geotextiles with grass and fascine work (follow current technical literature).