A regional irrigation water agency is required to provide irrigation water for an additional 7000 ha of land with a water share of 20 ML/ha over 70 days period. A separate branch irrigation channel has to be designed to extract water from the existing main canal at RL 250. The existing channel has enough supply to meet this additional demand. The length of the branch channel is 20 km out of which the first 5 km section has a general land slope of 1 in 1000, the next 5 km stretch has a general slope of 1 in 1500 and the final 10km section has a general slope of 1 in 2000.

• The regional agency is looking at options for the design of the branch channel using two different approaches: (1) design the branch channel as a trapezoidal earthen channel section and (2) a rectangular cum triangular concrete channel section (Figure 1). The agency is keen to keep delivery losses (largely due to evaporation and operational procedures) of about 20 % of the diverted flow.
  The design should indicate any drop structure arrangements if required along the channel length.

The following data is provided for the design:
Water share is to be taken over 70 days
Batter slopes on the earthen channel: 2H:1V
Limiting flow velocity to avoid scour = 0.7 m/s in case of earthen channel and 1.6 m/s for concrete
channel
Bed width/depth ratio (b/d) = 3
Freeboard = 0.6 m
Bank crest width = 2.5 m
Seepage slope = 5H:1V
Manning’s n = 0.03 for the earth channel and n = 0.014 for the concrete channel
Suitably assume any missing data for this design.