General Design Requirements for Cross Drainage Work
For preparing the design of a cross drainage structure, the following specified hydraulic data should be made available.
(a) Canal – Full supply discharge, Q; Bed width; Full supply depth; Water surface slope; Bed level; Bed slope; Full supply level; Top of bank level; Cross section of canal showing Natural Ground Level; Subsoil water level; and Nature of bed material and value of ‘n’ (rugosity coefficient in Manning’s formula).
(b) Drainage Channel – Extent and nature of drainage area (catchment area); Maximum annual rainfall and the period (years) of data; Maximum intensity of rainfall with year; Maximum observed flood discharge at the site; Maximum flood level; Water surface slope; Site plan of proposed crossing including contours; Log of borehole or trial pit data; Type of bed load of drainage channel; Longitudinal section of the stream for suitable distance upstream and downstream of the canal depending upon site conditions; Cross section of the drainage channel for a distance 100 m to 300 m upstream and downstream, at intervals of 10 m to 50 m; Waterways provided in road and railway bridges or other hydraulic structures on the drainage channel; Springwater level at the crossing site in May and October; and Silt factor.
- Design flood
The design discharge of the drainage should be selected considering various factors such as the size of the drainage, the size of the canal, importance of the canal and type of the cross drainage work. The following are the broad guidelines for estimating the design discharge.
- For very large cross-drainage works where the failure of the structure may lead to disruption of canal supplies over a long period, the design flood should be taken equal to the 6 standard project flood (S.P.F.).
- For moderate type of structures, the waterway is usually determined for the flood of 50 years recurrence interval but for the foundation and free board, the flood of 100 years recurrence interval is taken.
- For small cross-drainage works, the design flood is usually taken as 10 to 25 years flood, and an increased afflux is also considered.
- For important structures, an additional margin of safety is usually provided in the design of foundation and freeboard fixation to the take care of unexpected large floods by increasing the design discharge, depending upon the area of catchment.
Waterway for a cross drainage work is fixed from hydraulic and economic considerations with particular reference to: a) design flood, b) topography of the site, c) existing and proposed section and slope of the drainage channel in the vicinity of the crossing, d) permissible afflux, and e) construction and maintenance aspects. In plains, the drainage channels are generally in alluvium and the waterway usually provided in works without rigid floor is about sixty to eighty percent of the perimeter, given by Lacey’s formula Q C P = where C = a coefficient varying from 4.5 to 6.3 according to local conditions, the usual value adopted being 4.8 for regime channel.