Importation of bank protection
Bank protection is an important part of river training works. The purpose of bank protection can be (i) Training of the river, (ii) Protection of adjacent agricultural land, (iii) Protection of urban lands and valuable properties threatened by river erosion, (iv)Protection of hydraulic structures as weirs, barrages and bridges against the direction and nature of current, (v) Protection of flood embankment, and (vi) Affording facilities for water transportation.
River bank may be considered to consist of two sections, upper and lower. The upper bank, i.e., the portion between high water level and low water level is subjected to sever action of river in high stages due to (i) strong current impinging normal to the bank, and (ii) strong current acting along the bank. Lower bank is the portion of the bank below low water level. It is generally more susceptible to erosion. It acts as a foundation supporting the upper bank. Recession of the bank is caused by erosion of the lower bank, particularly at the toe. The recession is fast when there is a sandy substratum below; sand is washed away by a strong current and the overhanging bank collapses. When the river rises, the banks become partially or fully saturated with water and sloughing is caused.
Types of bank protection:
Bank protection may be direct or indirect, discussed as follows:
Direct Bank Protection: It includes the work done on the bank itself, such as providing vegetal cover, pavement, revetment, grading of slope, etc. Three conditions exist, (i) Mild current. Turfing or vegetal cover, e.g., growth of shrubs and willows is sufficient, (ii) Strong current. Paving of slopes with erosion resistant materials is necessary, and (iii) Very strong current. Protection has to be provided by stone revetment or various type of mattresses such as willows, lumber, asphalt or articulated concrete.
Indirect Bank Protection: It includes undermentioned works constructed not directly on the banks, but in front of them for reducing the erosive forces of the current, either by deflecting the current away from the banks or by inducing silt deposition against them.
Revetment: Revetment is placed along an existing or proposed river bank to prevent its recession by caving and erosion. The protection against erosion by revetment induces erosion of bed so that there is a possibility of deep current always hugging a reverted bank. It is used in places where siltation inducing structures cannot be provided. Generally, revetment is provided at places like highways, railways lines, etc. It provides riprap along the bank slope and toe protection.
Launching apron: It consists of a flexible stone apron to cater for maximum anticipated scour at the toe of the revetment. It launches as the scour develops. Full details of apron are discussed under ‚Guide banks‛.
A mattress is provided extending 15 to 30 m beyond the toe of the side slope.
Curtains of sheet piles:
An impregnable curtain of sheet piles is provided. It is very expensive and is rarely used.
Submerged sills in front of the concave bank across the channel are provided to help stop deepening of the bed and thus protect the toe of the bank.