Afflux is the rise in upstream water level of irrigation channel due the construction of structures like wear and bridges or culvert ; cause by an obstruction across the channel (abutments and piers) and it is also the difference in water levels of upstream and down stream of irrigation channels.
Afflux is also caused when the effective lineal waterway is less than the natural width of stream immediately on the upstream of the bridge or weir .
The amount of afflux determines the the top levels and length of guide banks and height of free boards.
High afflux creates high water velocity and greater scour produces demanding greater depth of foundation of bridge structure to control the high discharge under the bridge structure.
The quantity of afflux is different as per regional requirements.
For Alluvial and deltaic regions afflux may be considered as 60 cm.
and for trough and steep slope regions where boulders and rock arrivals is normal there the afflux may be adopted from 90 to 120 cm as a thumb rule.
Accurate calculation should be done considering velocity and discharge of irrigation stream. The design of bridge should create minimum afflux in irrigation.
Higher afflux value will require high repairing works at bridge structure and guide banks and at bed of channel at down stream due to high velocity of water.
The Afflux also disturbs the non silting and non scouring velocity of irrigation water flow.
The cross sectional area of bridge structure hurdling the channel water may be added to cross sectional area of irrigation channel to calculate depth of water at upstream of bridge structure; the extra depth of water will be amount of afflux.
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