Contents preface (VII) introduction 1—37

11.6. HEAD LOSS THROUGH CROSS-DRAINAGE STRUCTURES
The HFL of a stream downstream of a cross-drainage structure remains unchanged but the upstream water level will rise by an amount equal to the head loss (or afflux) due to the flow in the barrels of the cross-drainage structure. The length and top elevation of the guide banks upstream of the structure will depend on the raised HFL. Depending upon the flow conditions in the barrels, the broad-crested weir discharge formula or orifice discharge formula can be used for calculating afflux (1).
Alternatively, one can determine the afflux ∆h from the following empirical formula proposed by Yarnell (2):

∆h	= KF	^2[K + 5F ^2	– 0.6] (α + 15 α^4)	(11.1)
^h3	3	3

Here, h₃ is the depth of flow sufficiently downstream of the piers, and F ₃ is the corresponding Froude number. The term α is the ratio of the width of pier with the spacing (centre-to-centre) of the piers, and K is dependent on the shape of the pier as given in Table 11.3.

390	IRRIGATION AND WATER RESOURCES ENGINEERING