How to prevent water mains from freezing in the snow zones in water supply projects?

I have been looking into the design of a water supply system for approx. 8500 households in the Himalayas of Nepal. The total transmission pipeline is 47 km, and distribution is 178 km. The pipes we are using HDPE pipes of 15-280mm diameter (PE 100 and PN 6,10,12,5 and 16 pressure rating), and at some sections Seamless Carbon Steel Pipes (150-200 mm diameter) referring to ASTM-S40, are also used. The larger diameter HDPE pipes are used at transmission line at freezing zones. It is a rural setting with no electricity for the time being. It's a gravity supply project, with the water source at 4000m. above sea level and service area at 2200m. above sea level. The transmission main of about 15 km. lies in the section of 4000-3700m. above sea level. I am concerned about the freezing of water inside pipelines during winter in the night time (when the water in the pipe is not moving due to no consumption), where certain sections along the pipe alignment are filled with snow for 2-3 months. I have allocated 1.25-metre depth in the depth of the pipeline trench for the transmission and 90 cm depth in the distribution alignment. I have been looking for codes and references worldwide. Would be grateful if you can share your experiences or ideas, and any special codes to refer to.

I have shared some slides for easy understanding of my case.

Thanks,

Mahesh Neupane

B.E. Civil Engineering/ M.E. Environmental Engineering