General Design Requirements for Cross Drainage Work

General Design Requirements for Cross Drainage Work

1. Data

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.

2. 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.

1. 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.).
2. 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.
3. For small cross-drainage works, the design flood is usually taken as 10 to 25 years flood, and an increased afflux is also considered.
4. 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.
5. Waterway

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.