 being located on {HEAD ~ top} of {TAIL ~ a natural hill site} whose slope directs the flow in an upward direction, so that the said angular relationship between the helicoid axes and the flow vector is secured on account of the chosen location 
of
being located on { HEAD ~ top } of { TAIL ~ a natural hill site } whose slope directs the flow in an upward direction , so that the said angular relationship between the helicoid axes and the flow vector is secured on account of the chosen location
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH