Vexation

Studying the maps has not helped; my basic version of google earth, even when supplemented by other tools such as street view, does not allow close inspection of several small weirs along the length of the irrigation canal which runs from the Yueh Mei weir through the Cishan district before returning to the Cishan river.

The seemingly missing section crossing the mountain still needs to be found...

I have however, managed to check the altitudes for the various points using a tool which presumably uses satellite altitude to run the calculations. I checked it against the Water Resources Agency's published altitude for the Neimen pressure tank (114m) and it was only 4 meters off. Using that reference, the Yueh Mei weir is at approximately 92 meters (302 feet) above sea level, whereas the control gate tucked away behind the pressure reduction tank in Neimen is at approximately 77 meters (254 feet). If there was a pipe running directly from the weir to the control gate (a distance of about 7 kilometers), then the water could be conveyed by gravity. I did not see any such pipe when I was at the weir, and nor does google earth show one above ground. Moreover, for geographical reasons, I would imagine a direct pipeline would require quite a bit of tunneling...

Alternatively, the water supply from the Yueh Mei weir could be diverted at a point further south along the irrigation canal as it runs parallel to provincial highway 21 through Cishan district. That would allow for a much shorter and presumably cheaper pipeline to run westward under the mountain toward the control gate. The first problem with this hypothesis however, is that although scanning google earth reveals one or two small weirs which might indicate the start of such a diversion, it is not possible to look at them closely (no data available). The second problem is that the further south the irrigation canal runs, the lower its' altitude and so logically there has to be a cut off point at which the canal reaches an altitude lower than the altitude of the control gate across the other side of the mountain (77 meters).

Nor has searching for Mandarin documents and online discussions about the Agongdian reservoir diversion channel helped (there are several such discussions as it was understandably controversial at the time due to water rights issues and interference with the existing infrastructure as well as for the usual environmental reasons). None of those documents or discussions refer to the actual plan for the diversion channel in any detail, and indeed, one of the confusing aspects for me was that the length of the channel is frequently cited as 16 kilometers, but that is in fact merely the length of the channel from the reservoir to the control gate in Neimen! The Yueh Mei weir at the southern end of Shanlin district is an additional 7 kilometers away.

I will go and look around again eventually, but I am edging toward the conclusion that I am going to have to "phone a friend" on this question...