**4.2. Application for cannel lining**

Four trapezoidal channels of 50 metre length each were constructed with zero bed slope and were lined with jute geotextile (nonwoven geotextile coated with polythene sheet either on single side or double sides) of 1 metre width and different fabric densities. Final dimensions of the channels were measured at different sections of each of the channels and average dimensions were determined as shown in Table 1 [29]. The experiment was undertaken in field channels laid out in the experimental firms of BCKV, Mohanpur, Nadia, West Bengal.

The weather data consisting of daily maximum and minimum temperatures, evaporation, rainfall, and maximum and minimum relative humidity were collected from the weather station located in the farm. From the difference in water depths and water level widths between two consecutive days, the total loss of water from the channel was calculated. Total loss consists of evaporation and seepage losses from the channel as shown in Table 2. Evaporation loss was estimated by daily evaporation and channel dimensions. The seepage volume was calculated by adjusting the evaporation loss that occurred from the average surface area of water level during the period. The rate of seepage was determined by dividing the seepage volume by average wetted area of the channel. The observations and results noted and statistical analysis of the daily seepage values have been done on one factor randomized complete block design and the analysis of variance.

Both ends of the channels were blocked so that water can be retained in them. The channels were filled with water and depth and top width of water in the channels were recorded every day at 8 A.M. till the water levels reached about half of the initial level.

Various lining materials such as precast channel, brick masonry, bituminous mixture, soil cement, polythene sheets, etc., have so far been used for controlling seepage in irrigation channels. Some are too costly to be locally used while others have short life and are not much effective. Keeping it in view, this work was taken up to find out a viable alternative to these traditional materials. In this project, jute nonwoven geotextile of 250 g/m2 and 300 g/m2 fabric weight were coated with polyethylene sheet on one side and both sides and were experimented to assess the seepage loss through open channels. 300 g/m2 fabric with both sides coated controlled seepage most efficiently. Single side coated sheets also performed well in seepage control, but allowed more water to seep compared to double side coated sheets. However, 250 g/m2 fabric with both sides laminated yielded optimum performance.

Water losses in watercourses and field channels may occur by seepage, breaches along the channels through rat holes, and ponding of water in depressions and channel sections. Unlined irrigation channels account for overall seepage losses between 20% and 30%. Geotextile fabrics are very effective in controlling seepage from open channels. Jute nonwoven fabric of 300 g/m2 (double sides coated) controlled seepage losses most efficiently. However, considering the permissible limit of seepage and cost factors, 250 g/m2 fabric (both sides coated) may be recommended.
