The recent work on the channel (a riprap border to prevent lateral spread) seems to be working; since the border is still intact, the pools on the other side must have been filled with rain water rather than spillover from the channel.
Usually, the Taku river exiting from Nanhua reservoir is a mere trickle, but was now in full flow. The late afternoon light was marvellous...
The little stream passing under the foot-bridge in this image above is actually the exit channel for the second chute passing under the dam to flush out the dirty, silt-ridden water near the bottom of the reservoir.
Here's the spillway up-close from the south side. The speed and force of the overflowing water was tremendous; I wonder how long I would have had to watch this to witness the equivalent in water of my annual income - probably not too long!
Above: looking back at the spillway from the north-west from inside the gated excavation site; since the gate was locked I had to "improvise" to gain access to the site so I could take these shots.
The spillway pouring water into the bay from a more approximately westerly position atop the muddy rocks; note the first underground chute exit to the immediate right of the spillway; I don't think there was any water coming out of there, though it was difficult to tell given the tremendous force of the water passing over the spillway's exit lip.
Having photographed the spillway exit in operation, I then decided to climb the 300+ steps of the dam to check whether the Jiaxian Weir chute had been shut off and to see the spillway's entry point. As you can see in the image above, there was no water coming through from Jiaxian - which is what you'd expect during a floodwater event.
One of Nanhua reservoir's two silt dredging pumps in operation. The buzz of the two diesel engines was clearly audible from where I was standing.
And here's the other one next to it.
First glimpse of the spillway's entry point - you can see the water spilling over the semi-circular crest, and once again a little closer...
The open overflow design of Nanhua's spillway means that there are no expensive, high-maintenance Tainter gates to look after, which not only reduces maintenance costs but eliminates the risk of a gate malfunction or human error. The drawback is that the dam supervisory staff cannot control or prevent the loss of the cleaner, higher-value water close to the surface of the reservoir; instead they only have control over the two chutes running under the dam. The spillway crest was too large for my 18mm lens, so I had to cut it up into three shots (I really need a 10mm lens for this sort of thing)...
A public sign giving various vital statistics about the reservoir (in English also, surprise surprise!)...
Below is the second underground chute exit point - with the dirty water issuing out, though at the time it seemed to be closed leaving the probability that it had been opened earlier, perhaps on the Friday.
On my climb up the dam's 300+ steps (just visible in the background to the shot above), a black kite had swooped close enough for me to get a good few photographs but by the time I switched lenses, she had gone. I had idly entertained the hope of seeing her again on my descent but it wasn't to be. Reaching the bike again, I had a toss-up to make - I could either stay in Nanhua district and photograph the spillway of Mingjing reservoir (which I was curious to see in action given its odd design) or I could drive the 30/40 kilometers north up to see Taiwan's largest reservoir opening up its spillway gates. It had to be Tseng-wen really; here is the upstream section of the river only ten or fifteen kilometers from the dam...
Wow. That thing is usually only a trickle a few meters across with the contours of the riverbed thoroughly denuded - but not today...
I finally got within sight of the dam's downstream face at about 5.40pm or so; the scale of the discharge naturally dwarfed that at Nanhua reservoir...
On the approach road to the dam I was able to count several natural waterfalls I hadn't previously even known existed as well as artificial ones and giant man-made rills just throwing water down within inches of the road, but I didn't stop until I got within sight of the dam. Next to my first stop facing the dam, the baffling blocks of this downhill tributary were in splendid close-up action...
Baffle that water! I then turned briefly to take in the valley. Notice the round hole toward the bottom left of the image below; that's the exit point for one of the original outflow tunnels. There is a second one further downstream; their purpose is identical to the two underground chutes at Nanhua reservoir except that they don't work very well and are in the process of being either replaced or enhanced (I'm not sure which at the moment - the subject of a future post)...
A close-up shot of the spillway gates; all three are open, though only slightly, and yet the tremendous volume of water passing out of them was sufficient to cause spray to hit me in the face and steam up my lens even though I was standing maybe half a kilometer away...
Below is a close up shot at the same angle as the image above: although the hydraulic jump provided by the end of the spillway gives spectacular clearance, I don't think I would want to risk standing underneath...
Below is the view from the top of the dam looking down the three spillway channels, though by this time I think they'd closed Gate 1; notice the continuing giant plumes of water at the end of channels two and three to the left...
On the other side of the spillway gates, the depth gauge indicated 226 meters which is not the depth of the reservoir, but the current height of the water above sea level; when I was here in January 2012, the reservoir was almost full at the 217 meter mark.
Below is a shot of the intake tower for the fifty mega-watt hydro-electric power plant at the bottom of the dam which is housed in the mountainside; later, in the engineer's office as I was looking at the various instruments, my eye was immediately drawn to one instrument marked in MW with a reading of 49...
A parting shot of the reservoir below looking toward the upstream face of the dam; by this time (well after 6pm) the light was failing fast and I had to set my ISO to 400, lower my shutter speed to 1/20 with an F-stop of... something or other: I forget. The maximum volume line on the dam is just two meters or so above the surface of the water and is barely visible in the distance. I've never seen Tseng-wen as full as this before...
I took many other pictures of the ongoing construction of tunnels and access roads and maps and plans and diagrams. I also spent a half-hour or so in the engineer's office asking them questions and showing them drawings and getting them to explain their answers to me (my limited vocabulary still lets me down). When I left, it was 7pm and absolutely pitch-black - there are no street lights whatsoever at Tseng-wen reservoir and none on the way back until you finally approach Nanxi town itself. It is difficult to describe the feeling of looking over your shoulder and seeing nothing but complete blackness.
Anyway I think I understand the design of the new tunnel better now; it is much more complicated than I originally thought, and seems to solve the problem of insufficient pressure to wash out the sediment clogging up the reservoir floor, though I need to go through the plans in detail with help for some of the vocabulary. A subject for a future post...
Really interesting and great photos. Thanks for sharing.
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