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2.
Pitching of banks and provision of launching aprons
River banks are often protected, at least in the vulnerable sections which are
prone to erosion by the strong river current particularly during the
monsoons or by the tidal waves, by stone pitching or by concrete blocks or by
brick lining. In some less susceptible areas growing of vegetable covers are
also adopted. Banks are made stable by providing a slope of 1 : 1 or 2 : 1
depending upon the material of the bank. They are then pitched to make them
strong enough to resist erosion. Scouring action of the river bed often spoils the
pitched portion in the river bed making the entire pitched section ineffective. To
avert such situation a launching apron was laid over the pitching slope from the
toe of the bank into the river so as to prevent scour at the toe and consequent
damage to the pitching.
The thickness of the stone pitching is calculated from the formulae
[Source : S.K. Garg, 1993].
t = 0.06 x ½, when
t = thickness of the stone pitching in metre
Q = Discharge in cumecs.
Scour depth can be calculated from Lacy's formula which is :
R = 0.47 ( Q )
f
Where Q = discharge in cumecs and
f = silt factor
The total scour below HFL is taken as xR where x varies between 1.25 to 2.25
depending upon the location of the pitched portion. A typical section of
launching apron is shown below :
Fig. 5.3 Launching apron details
(After Garg, 1998)
