Q. How to design Marginal Embankments?
Design aspects of marginal embankments are similar to those of approach banks. The aspects needing special attention are dealt with below.
Distance of marginal embankment from the river bank should neither be too short nor too long. If the embankment is too near the river, velocity along the bank can become high.
Normally this velocity is not allowed to be more than 0.9 to 1.2 m/s. On the other hand if the embankment is far too away, the very purpose of providing the embankment can be defeated. Current Indian practice is to roughly maintain a distance of 3 times the width given by Lacey formula in between the flood embankments on either banks of the river, though this thumb rule may not be applicable in case of all alluvial rivers. The flood embankment is carried on a ridge or high bank. In alluvial rivers a ridge or high ground is often formed near the bank line due to deposition of suspended sediment when river overflows the bank. Protrusions and sudden changes in alignment forming kinks are as far as possible avoided. Structures like bridges and barrages obstruct free downstream movement of meanders. Immediately upstream of such structures, the meander belt, therefore, widens and becomes almost double. While fixing location of marginal banks, this field experience needs to be kept in view.