Contents preface (VII) introduction 1—37

Table 11.1 Suggested increase in design discharge (1)

Catchment area (Km^2)	Increase in discharge (%)
More than 25,000	0—10
5000-25,000	10—20
500-5000	20—25
Less than 500	25—30

The type of foundation for cross-drainage structures will depend primarily on the depth of scour calculated from Lacey’s equation [Eq. (8.32) or Eq. (8.33)], and the bearing capacity of the soil. The depth of scour around piers is taken as twice the depth of scour calculated from Lacey’s equation. In alluvial streams, a well foundation is usually provided where deep foundation is required. With the provision of an impervious floor (necessary for siphon and siphon aqueduct) along with cutoff walls, the depth of foundation may be reduced. The floor itself may be designed as either a gravity floor or a raft. The floor is designed to resist the total uplift pressure caused by subsoil water and the water seeping from the canal. The uplift pressure is counterbalanced by the dead weight of the gravity floor. The worst condition occurs when there is no water in the barrel and, hence, the weight of water in the barrel is not included in the design. At times, it may be economical to design the floor as a raft so that the uplift is counterbalanced by the entire weight of the superstructure. The spacing of the piers (i.e., the span) depends on structural and economic considerations. Fewer piers (i.e., longer span) are preferable at sites which require costly foundation.

In case of siphon aqueducts and siphons, the drop at the upstream end of the culvert may be vertical (generally economical) or sloping. But, at the downstream end of the culvert, the rise should always be at a slope flatter than 1 in 4 so that the bed load can be moved out of the siphon barrel. The culvert floor should extend upstream of the barrel inlet by a distance equal to the difference between the HFL and the culvert floor level. Barrel inlet should be bell-mouthed to reduce the head losses.

388 IRRIGATION AND WATER RESOURCES ENGINEERING
A suitable arrangement has to be provided to pass the service road across the stream. This requirement does not pose much of a problem in cross-drainage structures of type I and II in which earthen embankments are continued. In cross-drainage structures of type III, the simplest arrangement is to carry the road on either side (or only on one side for economic reasons) by providing slabs and arches on either side (or on one side) of the canal trough.
The sides of the canal trough are generally designed as beams in reinforced concrete structures. The bottom slab is suspended from these beams. Additional beams, if required, are projected into the canal to divide the canal trough into a number of parallel channels. For wider troughs having intermediate beams, the service road may be provided on one of the compartments. Canal troughs of the smaller width can be designed as a hollow box girder and the service road can be provided on the top slab.
The forces acting on a cross-drainage structure consist mainly of the hydrostatic and uplift pressures, earth pressures, and dead weights and live loads of construction equipment and traffic. The overall design of the structure should be such that the total weight of the structure is as small as possible. Possibilities of differential settlement and excessive scour during floods must always be kept in mind while designing the foundations.
Wing walls of the stream are suitably connected to high ground. The stream should be guided towards the structure by means of suitable river training works. Similarly, the canal banks, adjacent to the crossing, should be protected by measures such as pitching, launching apron, etc., wherever necessary.
The design of any cross-drainage structure, like any other hydraulic structure, includes hydraulic, structural and foundation aspects. The hydraulic aspects include the surface and subsurface flow considerations. The surface flow determines the configuration of the structure so that the structure is economic and functionally efficient. The surface and the subsurface flow considerations enable determination of the following:
(i) Waterway and headway of the stream,
(ii) Head loss through the cross-drainage structure, (iii) Fluming or contraction of the canal waterway, (iv) Uplift pressures on the trough,
(v) The uplift pressures on the culvert floor, (vi) The exit gradient, and

(vii) Protection works.

The hydraulic aspects of the design of cross-drainage structures have been dealt with in the following paragraphs.