Friday, 29 July 2016

Third Trip To Yunlin County's Hushan Reservoir (湖山水庫)

Myself standing on the suspension bridge in Nantou County south of the water intake for Yunlin County's Hushan reservoir.
Yesterday I left Tainan at 7.30 a.m. and drove north on provincial highway one to make my third proper visit to Hushan reservoir in Yunlin County, my first trip having been made in November 2014. Once I had left Tainan city's rush hour traffic behind, I made surprisingly good time passing Xinying at 8.25 a.m. and reaching my preferred 7-11 in Chiayi city on the 159甲 just before 9 a.m. After a five minute break to eat a banana and drink a pint of milk, I drove east and paused briefly to photograph Renyitan reservoir from a previously unknown overlook point...

Gazing at Renyitan reservoir in Chiayi from an overlook on the 159甲.
The Renyitan dam is the second longest dam in Taiwan after Kaohsiung's Agongdian reservoir.
After taking several shots of Renyitan I resumed the drive east on the 159甲 and then turned north onto provincial highway three which runs through Chiayi's Meishan district into Yunlin County's Gukeng district. On reaching Gukeng the road bends north-west toward Douliu city, and at this point I paused at another 7-11 for a drink before switching to the 154乙 which is a small countryside road which runs straight north well away from Douliu city and leads to the main approach road for the reservoir. I reached the foot of the south dam at 10.30 a.m. and had climbed to the summit by 11.00 a.m.

Approaching the man-made mountain that is the south dam at Hushan reservoir.
The spillway chute at the top of the dam.
Panorama shot looking back down from the summit of the dam.
In April this year I had read in the newspaper that Hushan reservoir had begun filling and would enter service in July. What the unnamed reporter failed to mention however, was that although a limited supply of water to farms would begin in July, the filling process itself would take place gradually over the next year in order to allow for testing. By filling the reservoir with a relatively small amount of water at a time, the management authority can test - among other things - for leaks and pressure-induced movements of the dam. Having checked the current Google Earth images for the reservoir, I expected to find only a slight amount of water in the reservoir, and indeed that was exactly what I found...

Panorama shot from the south dam overlooking the southern stretch of the reservoir bed, with a small lake at the bottom representing perhaps 5% of total capacity
The diversion tunnel exit mouth at the back of the reservoir. I hope that bridge is available for public use once the reservoir finally opens to the public next year.
Overlooking the spillway lip. The building being constructed in the background is the water control tower for the hydraulic sluiceways and irrigation outlets.
Down the spillway chute, with the north dam in the background.
The north end of the reservoir, again with only a small, shallow lake to fig-leaf the otherwise naked reservoir bed.
Just north of the center point of the reservoir three large plastic sheets have been tied up to the hillside; their placement is to indicate the maximum designed water level.
A 300 mm shot of the three sheet signs indicating the reservoir's maximum capacity; when the water reaches just below this point, that will be the maximum designed capacity of the reservoir, which is just over 50 million cubic meters, making Hushan a "medium-sized" reservoir in comparison with tiny reservoirs like Luliao at just over 300,000 cubic meters and behemoths like Feitsui reservoir at 400 million cubic meters.
After taking pictures on top of the dam I was approached by a young junior engineer in a car who got out to ask me what I was doing: taking pictures I told him. He apologized but told me I would have to leave for safety reasons; I pointed out that it was not my safety he should be worried about but the safety of the dam if anyone can just walk straight up the thing without being challenged. He agreed, but his job now was just to escort people off the dam (so at least something had changed security wise since my last visit). I had more or less finished anyway and he was offering to give me a lift back down in the car and so I agreed. We talked about the intriguing history of spillway designs: if you look at all of Taiwan's early reservoirs, they all had simple open overflow designs where the water would reach a certain level below the dam and then spill out down the spillway; in the 50's, 60's and 70's, these designs were superceded in the next generation of reservoirs with tainter gate controlled spillways; from the 80's through to today (with one exception) all new reservoirs (Hushan included) have reverted back to open overflow designs. I have two or three guesses to explain this change over time. The first is simply fashion - the hypothesis that engineers (or perhaps their political masters) are suckers for trends. The second is a mistaken optimism about how much more efficient the tainter gates would prove to be over the earlier open overflow designs; the idea is that with a tainter gate system, you can allow just enough water to leave via the spillway without compromising the safety of the dam, thereby saving water (and therefore money) which would otherwise have been lost with an open overflow design where there can be no control over discharge volume. Yet the switch back to open overflow designs since the 1980s suggests that this optimism was misplaced; during heavy rainfall events it is likely that tainter gate controlled spillways must open wide enough to discharge the same volumes of water they would have had they been built with open overflow designs, and this is because of the sustained duration of heavy rainfall events: you get vast volumes of water entering the reservoir over a sustained period of several days. He didn't know the answer either, and while he agreed that my guess made sense, he seemed to think there was something else I was missing, though I couldn't understand the technical language (in Mandarin) he was using.

After he dropped me off at my motorbike, I decided to head off to a neighbouring village to get something to drink and chill out in the shade for half an hour before driving eastward behind the hills that tower over the new reservoir. That is where the water intake structure sits on a tributary river to the Zhuo-shui river. The 149甲 leads eastward out of Yunlin County and up into the hills before a switch to the 158甲 which brings you out over the other side and into Nantou County at a junction with the 149 (at which point you head straight across to go down the little approach road for the suspension footbridge). It's a half hour drive, but well worth it...

The view south-eastward from the beginning of the suspension footbridge; on the other side of those mountains lies Nantou County's Zhushan Township. I met a girl from there on the train once earlier this year.
Looking north toward the weir and the entrapment pen for the new reservoir's water intake channel. Annoyingly, one of the crucial things I forgot to do was to visit the huge sedimentation tank which lies just a short distance to the north west. 
From across the other side of the suspension bridge there is a great view back over the river toward the water intake gates. This structure was just being completed on my last visit in June last year.
Having taken these shots (but forgotten about the sedimentation tank), I decided to start the journey back to Tainan city where I would basically just retrace my tracks along the exact same route; back over to Gukeng and then south down into Chiayi's Meishan district on highway three and then the 159甲 westward back to Renyitan reservoir and Chiayi city, then highway one south back to Tainan city. An alternative and far more pleasant route back to Tainan via highway three and Tseng-wen reservoir was also possible, but it would have added an extra hour to the journey at the least, and I was already very tired from the heat.

I stopped briefly on the 159甲 to take a parting shot overlooking Chiayi's Lantan reservoir which I haven't visited in a long time, and for which there was no time now...

Chiayi's Lantan reservoir as viewed from the north. It is one of Taiwan's earliest reservoirs and was built by the Japanese after the outbreak of WW2.

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