Contents preface (VII) introduction 1—37

13.6. AFFLUX, WATERWAY, AND DIFFERENT LEVELS FOR WEIR
CONSTRUCTION
Due to the construction of a weir across a river, the high flood level of the river upstream of the weir rises. This rise is termed the afflux and is usually represented as the difference in the total energy levels of the upstream and downstream of the weir. In the beginning, the afflux is confined to a short reach of the river but, extends gradually very far upstream in case of the alluvial rivers. The top level of guide banks and marginal bunds and also the length of the marginal bunds are decided by the amount of the afflux. Besides, the afflux affects the dynamic action downstream of the weir and also the location and parameters of the hydraulic jump. A higher afflux may reduce the width of the waterway but, increases the discharge per unit length of weir. This results in increased depth of scour which, in turn, increases the cost of protection works. A higher afflux also increases the risk of failure of river training structures due to possible outflanking.
In the case of weirs located on alluvial rivers, an afflux of 1 m is considered satisfactory in the upper and middle reaches of the river. In the lower reaches with flat gradients, the afflux should be limited to about 0.3 m (2).
The pond level is the water level which must be maintained in the undersluices pocket (i.e., upstream of the canal head regulator) so that full supply level can be maintained in the canal when full supply discharge is fed into it. The full supply level of a canal at its head is obtained from the longitudinal section of the canal. The pond level is kept about 1.0 to 1.2 m higher than the full supply level of the canal so that sufficient working head is available even when the head reach of the canal has silted up, or when the canal has to be fed excess water. If under certain situations, there is a limitation of pond level, the full supply level is fixed by subtracting the working head from the pond level.
The waterway and the afflux are interdependent. Normally, in plains, the width of waterway is kept equal to about 10 to 20% more than Lacey’s regime perimeter for the design flood discharge. In rivers with coarser bed material, the width of waterway can be kept about 10-20% smaller than Lacey’s perimeter. A smaller waterway increases the afflux and the cost of protection works. On the other hand, a larger waterway is uneconomical and may cause oblique approach, thereby silting part of the waterway. The ratio of the overall length of the weir provided to the minimum stable width of river obtained from Lacey’s equation for the design flood discharge is termed looseness factor (2). Table 13.1 lists the values of looseness factor along with some other salient features of some barrages.
The weir crest level, afflux, waterway, and pond level are interrelated and a suitable set of values of all these four parameters are decided within their respective limits. The pond level can be maintained by keeping the weir crest at the pond level. Alternatively, the weir crest can be kept at a lower level and the pond level is, then, maintained with the help of shutters or gates. The level of the weir crest is decided as follows:

442 IRRIGATION AND WATER RESOURCES ENGINEERING