Contents preface (VII) introduction 1—37

11.4. DESIGN OF CROSS-DRAINAGE STRUCTURES
Before undertaking detailed designs of any important cross-drainage structure, collection of relevant field data is required. Besides, a note regarding the several alternative alignments surveyed and reasons for final selection of a particular crossing site is also necessary. A location map of the site along with the results of subsurface explorations of the site, and stream cross-sections at different locations around the site should also be prepared. The following specific hydraulic data regarding the canal and stream should be made available.
(i) Canal
(a) Discharge, depth of flow, and water level at full supply, (b) Bed width,

(c) Canal bed slope and the water surface slope, (d) Levels of canal bed and the top of canal banks, (e) Channel cross-section,

(f) Characteristics of material of the bed and sides of the channel, and (g) Width of roadway and type and class of IRC loading.

(ii) Stream

(a) Extent and nature of stream and its catchment, (b) Detailed records of rainfall in the catchment, (c) Maximum observed discharge,
(d) Maximum flood level and water surface slope as observed under the highest flood condition at the proposed site,
(e) Site plan of the proposed crossing including contours, (f) Log of borehole or trial pit data.

(g) Information about the sediment load,

(h) Characteristics of the bed material including Manning’s n and silt factors.
(i) Longitudinal section of the stream for suitable distance upstream and downstream of the chosen site for cross-drainage structure depending upon the site conditions,
(j) Cross-section of the stream for about 100 to 300 m upstream and downstream of the site at intervals of 10 to 50 m,
(k) Waterways provided in road/railway bridges or other hydraulic structures of the area,
(l) Spring water levels at the crossing site.
Any cross-drainage structure should preferably be located in a straight reach of the stream crossing the canal at right angle as far as possible. The alignment of the canal should also be such that it results in minimum lengths of embankments (for aqueduct and siphon aqueduct structures). If required, the site of the structure may even be shifted away from the existing stream channel, when it is possible to divert the channel and also keep it there by reasonable training works. One obvious advantage of such an alternative would be that the construction will be carried out in dry conditions.

CROSS-DRAINAGE STRUCTURES

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The design discharge from any cross-drainage structure would depend upon the size and importance of the structure. The failure of a large cross-drainage structure may result in the submergence of considerable cultivable and residential areas besides interrupting irrigation and resulting in reduction of crop yield over a large area. Therefore, the design discharge for very large cross-drainage structures should be based on the maximum probable flood from the maximum probable storm. However, for small cross-drainage structures, it would be very uneconomical to use the highest peak flood for design. The failure of a small cross-drainage structure would not cause much submersion, and interrupt irrigation only to a marginal extent. In the long run, it may prove to be more economical to repair or even replace relatively small cross-drainage structures at long intervals than to spend very large sums in order to provide for the highest peak flood.

For major cross-drainage structures, the design flood discharge can be taken as the discharge of a 1-in-50 to 1-in-100-years flood. For small cross-drainage structures, however, the design flood discharge may correspond to a 10- to 25-years frequency flood with increased afflux. In cases of important structures, an additional margin of safety is provided in the foundation design and fixation of the freeboard to take care of the unexpected and unforeseen nature of flood intensities (Table 11.1).