Contents preface (VII) introduction 1—37

11.5. WATERWAY AND HEADWAY OF THE STREAM
Economic and safety considerations limit the waterway of the stream. Lacey’s regime perimeter equation [Eq. (8.29)] can be used for determining the permissible waterway for structure without rigid floor. In structures with rigid floors, however, the waterway can be further reduced, but keeping the flow velocities within permissible limits (Table 11.2).
The purpose of providing waterway equal to the Lacey’s perimeter is to let a stable channel develop between the training banks (i.e., the guide banks) upstream of the cross-drainage structure. At the site of the structure, the piers for the structure will reduce the available clear waterway. For small structures, the permissible reduction in clear waterway is

CROSS-DRAINAGE STRUCTURES

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up to 20%. It should be noted that at the site of the structure, the regime conditions do not exist and the reduction in clear waterway up to 20% is not a cause of concern if the scour is calculated using the increased discharge per unit length of clear waterway.