NEC3201:A separate branch irrigation channel has to be designed to extract water from the existing main canal at RL 250.

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
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.

  • The agency is also looking to support farmers with the design of fixed sprinkler systems to improve the efficiency of the water applied on their agricultural land. The agency is interested to get a sprinkler design for a land of 200m x 200m area as per the plan shown in Figure 2. Each sprinkler is rated at 300 L/hr at 40m head. The maximum variation in head across the system is limited to 20%. Allow 10% for local losses. Calculate the water application rate in mm/hr, the irrigation time in hrs for GWR of 60mm, the maximum permitted pressure variation across the whole plot, and the size of the PE manifold and PE laterals. Design the capacity of the pump at the manifold as shown in Figure 2. Assume a suitable layout of the laterals and sprinkler spacing. The manifold is located in the middle of the plot as shown. Also, assume any suitable data required for the design of the sprinkler system including the location of the first sprinkler.

Your consultancy firm of hydraulic engineering designers has been given the task of performing a preliminary engineering design of the proposed new works.