The diagram below shows a portion of a water irrigation pipeline. The water enters the pipe from a river at point 1 and flows downhill to point 2 where there is a gate valve (shown). NRCS irrigation pipeline design guidance requires a vent pipe (to allow air to vent from the pipeline, but not water) be placed upstream of valves, and the vent pipe needs to be designed to an elevation 1 ft higher than the HGL. What should be the height of the air vent at point 2 relative to the pipe centerline and just upstream of the valve according to this design criteria? Design conditions are: Q = 2 ft³/s, Az (elevation difference from point 1 to 2) is 10 ft, pipe length = 1,500 ft, pipe diameter (actual) is 1.0 ft, sch 40 steel. 10 ft