 being located on top of a natural hill site {HEAD ~ whose slope} directs {TAIL ~ 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 
directs
being located on top of a natural hill site { HEAD ~ whose slope } directs { TAIL ~ 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 OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL 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