Sunday, 30 October 2016

A Fourth Trip To Feitsui Reservoir (翡翠水庫)

Yesterday marked only my fourth visit to Taiwan's second largest reservoir. This time however, I finally got to see the downstream face of the Feitsui reservoir dam, when I and my girlfriend went on a tour of the facilities along with about sixty other people. Although this trip was originally booked in advance back in August, it was delayed by well over a month due to persistent heavy rainfall and consequent monitoring and maintenance work throughout September.

We took the late Friday night HSR train up to Banqiao and then the MRT to Xindian where we had a hotel room booked. As I hadn't visited Taipei for several months prior to this trip, the motorbike hadn't yet had its' necessary servicing, and so we took a taxi to the reservoir's front entrance with the intention of sorting out the motorbike upon our return to Xindian later in the afternoon.

Unfortunately, as forecast, the weather turned out to be dank and overcast with occassional drizzle (light rain) but it was also humid, not cold. At the entrance gate, we were told we could wait for a minivan to pick us up or we could walk the short distance along the bridge and across the approach road up to the Administration premises. We chose to stretch our legs and stroll along the bridge over the Nanxi river, since it wasn't yet raining. There were one or two other people walking on foot to join the tour, and several cars with visitors inside passed us by on the way.

Like my trip to Deiji reservoir high up in the mountains of Taichung county two years ago, this was one of my most long-anticipated trips. Unlike that glorious day up and over some of Taiwan's highest mountains however, this trip turned out to be something of a disappointment...

Myself at the entrance road to the Taipei Feitsui Reservoir Administration. This was a long anticipated trip.
The tour began with an hour long video presentation inside a lecture theater. There were no English subtitles and so although I could understand the gist of it most of it, there were a number of details that eluded me. Accustomed as I am to hearing Mandarin spoken with a southern Taiwanese accent, the more clipped, Chinese-sounding northern accent with which the narrator spoke sounded strange to me. Afterward, we were greeted by a middle-aged woman I took to be an Assistant Director of the Administration, who gave us a supplementary introduction to the Administration and the reservoir. There was no invitation for questions, and no time set aside for such.

Yet I already had thirteen questions written down in my notepad.

The tour began after a short toilet break, and the entire group of about sixty visitors were split among a number of tour guides who each led us in small groups, though we chose to flit between this and that group. We proceeded down the entrance road to the Administration premises and back out onto the approach road eastward to the dam itself. Each tour guide seemed to have a different knowledge specialty, though each of them spent a considerable time describing the various ferns and trees that lined the roadside as well as mention of various insects, reptiles and birds that could be found in the area. I found some of the claims to be questionable, such as the rather specific claim that Feitsui reservoir was home to exactly ninety seven eagles (when we asked when and how this number was obtained, we were told that it was the number given to them by an independent, Taipei based wild bird conservation society).

When the dam finally came into view, I tried two of my thirteen questions on the nearest tour guide. Both questions concerned the design of the spillway; the first was why it was built with radial, tainter gates rather than open overflow, and the second was why were there eight such apertures rather than any other even number such as four or even two or sixteen. His answer to both questions was simply that the dam engineers decided to do it that way, which was a non-answer and so I persisted by explaining the basis of my question, referring to the two shifts in the history of spillway design in Taiwan with the most recent of those being the shift away from the use of radial gates in favour of open overflow designs - a shift which had already occurred by the time Feitsui reservoir's dam would have been at the design stage in the late 1970s. However, not only was the guide unable to give satisfactory answers, but it was clear that he hadn't really understood either of the questions. I tried asking other guides but was met with similar non-answers.

Disappointed, I put my notebook away and just took pictures instead...

My favourite shot of the day; the downstream face of the dam as seen from an angle to the north-west on the approach road. The hydroelectric power plant lies in the foreground just in front of the stilling basin for the spillway. The power plant is the reservoir's most frequently used release mechanism for water supply to the broader Taipei area, with the spillway and sluiceways opening only as necessary to reduce pressure on the dam.
On the northern spur looking out across the downstream face of the dam.
Detail of the eight spillway apertures below the crest of the dam. Why are there eight spillway gates? Why not four (for example)? There are two answers that strike me as plausible; the first concerns discharge flows, and the second concerns cost of manufacturing. Eight spillway gates allow for lower discharge flow volumes than would a smaller number of larger gates such as four or two, the significance of which is conservation efficiency and maintenance of water quality through the gradual release of turbid water with suspended sediments. Sixteen gates were probably not feasible due to constraints on manufacturing (e.g. alloy casting mold sizes), and may not have offered sufficient improvement to warrant the additional cost. These answers are merely my own conjectures, I don't yet know what the real answer is.
The three sluiceway apertures and the dedicated river outlet beneath the far right sluiceway. "El 90" refers to the elevation in meters above sea level. Their elevation relative to the reservoir bed however, is unusual in that they are at the approximate halfway point between the bed and the maximum surface level. At other reservoirs the sluiceways are usually located at the bottom of the reservoir close to the bed. However, Feitsui differs from other reservoirs around Taiwan in that unusual measures pertaining to soil conservation and afforestation were taken to ensure water quality. The accumulated quantity of sediment in Feitsui reservoir is apparently thus relatively small at 24.86 million cubic meters as of 2014. Although that number is extremely large (it is just shy of the capacity of two major reservoirs in Chiayi and Miaoli), it is only about 6% of Feitsui reservoir's original capacity of approximately 400 million cubic meters. Consequently, the water quality in Feitsui reservoir is said to be mesotrophic with minimal turbidity. Water samples from the surface, and from the depths of the reservoir should not demonstrate a vast difference in turbidity as is often the norm at other reservoirs.
Close up on the river outlet porthole. This aperture is used exclusively for maintaining the water level necessary for sustaining the river's downstream ecology.
Gallery entrance on the south side of the dam. This is where engineers enter the dam to conduct inspections and install and update monitoring sensors.
Water gauge on the side of the intake tower for the hydroelectric plant on the downstream side of the dam.
Overall, it was a somewhat disappointing trip on two counts; the awful weather and the relatively sparse information I was able to collect from the tour guides. A further disappointment was that I had secretly harbored hopes that we would be taken out on a boat to see a little more of the reservoir itself, but that offer never materialized. Had it happened, I would have been over the moon at the prospect of being able to shoot the upstream face of the dam.

Last week I took an additional, unreported trip to the South Taiwan Water Resources Bureau office in Kaohsiung to meet a senior engineer and ask some questions. I may visit their other offices later this week, and one thing I may ask for is someone to contact at the Feitsui Administration who would be in a better position to answer my questions and perhaps even arrange for another tour when the weather is better...

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