Saturday, 5 May 2012

Nanhua Reservoir (南化水庫)

Located on Tainan's (台南) south-eastern border with Kaohsiung (高雄), Nanhua reservoir was built between 1988 and 1993 in order to supply water for residential and industrial use in both the Tainan and Kaohsiung areas. Named after the county district in which it lies, Nanhua reservoir occupies a natural trough between two sets of mountains each running on a slight south-west to north-east axis; the eastern mountains form a natural border with the neighboring Jiaxian district (甲仙鄉) of Kaohsiung and are accessible by road (the western mountains are not). The orientation of the reservoir runs parallel to the Cishan river (旗山溪) on the other side of the eastern mountains . The reservoir is fed primarily from the Taku river (後堀溪) to the north, and secondarily by a diversion channel from the Cishan river which runs beneath the mountains to pour into the reservoir at its approximate mid-point. Nanhua reservoir is Taiwan's fifth largest reservoir by volume at 158 million cubic meters (effective: 97 million cubic meters), and the sixth largest by surface area at 5.3 square kilometers. It is also Taiwan's sixth largest reservoir by the annual volume of water it was built to supply at 292 million cubic meters. As with other reservoirs of similar design however - i.e. those set within a natural trough in the mountains (e.g. Tseng-wen [曾文], Feitsui [翡翠] and Wan Da [萬大] reservoirs) - Nanhua reservoir suffers from heavy sedimentation due to the prior lack of a filtration system at its northern neck (and hence it cannot operate today at 100% capacity). Nanhua reservoir also has the fifth-tallest dam in Taiwan, with a crest elevation of just over 187 meters which is just slightly short of the dam at nearby Tseng-wen reservoir (曾文水庫), and a few meters taller than the dam at the otherwise much larger Feitsui reservoir (翡翠水庫) up in Taipei.

Nanhua Reservoir Dam

The rock-filled dam which the voluminous body of the reservoir leans against lies to the west and somewhat to the south of the valley and spans just over half a kilometer between the shoulders of two mountains. It is abutted to the south by an open-lip spillway, control station and observation platform. The dam itself consists of an impervious core of rolled earth at the centre (1A) and a second, but smaller impervious core beneath the midpoint of the dam's downstream face (1B); the two cores are surrounded by a semi-permeable layer of rock-fill (2A) interspersed on the downstream side of the central core by drainage channels (2B). The upstream side of the dam consists in a single, semi-permeable shell (2C and 3) of rock-fill; running behind and below this shell there is a drainage channel within a sand and gravel membrane (to protect both the central core and the upstream shell from erosion drift) which issues from the crest of the dam and exits into the reservoir itself. Both downstream and upstream faces of the dam are dressed in a loose layer of boulders held together by force of gravity and friction alone. On the downstream side, there is a works staircase leading up to the crest of the dam itself.

Although a road runs across the top of the dam via a bridge over the spillway chute, this road terminates to the north in a barbed wire fence and is not open to the public. Presumably there is a security reason for this, but it does mean that camera-wielding visitors to the reservoir are restricted to the observation platform to the south, which, despite its' puported function, offers very poor and partially obstructed views out across the waist of the reservoir toward nothing in particular. It may be the case that most visitors wanting to take pretty postcard-pictures of the reservoir are wasting their time at the observation platform, which is unfortunate. Far superior views over the reservoir are available from atop the eastern mountains diametrically opposite the dam, from the water's edge just after the partially hidden farms beneath these mountains and also from two spots to the south: a thin grassy verge by the side of the road and a small, partially fenced layby on route 20 as it skirts its way around and aloft the southern end of the reservoir...

... and this precipitous verge at the southern end of the reservoir is actually where quite a few people stop to get their cameras out (see the header image above). It is however, a dangerous spot since it is very close to a blind bend (care must be taken not to stand on the road itself while shooting). The layby a few yars up the road to the east is safer, and although the view is encroached upon somewhat by trees, it still offers a view of superb clarity out toward the main body of the reservoir, one which describes the southward slope of the mountain shoulder to accomodate the crescent bicep of the dam and their tandem descent eastward into the water below. It is however, a view which can be easily ruined by the haze so common during Taiwan's summer, and so much so that even the dam itself may become partially obscured.

Nanhua Reservoir Spillway & Release Chutes

The spillway at Nanhua reservoir lies between the dam to the north and the control station to the south with an arch bridge stretched over it to connect dam and station for the service road. The design of this spillway is what might be termed "open lip" in that it resembles the shape of a lip and lacks any hydraulic gate system as found at many other reservoirs, such as its' larger neighbour Tseng-wen reservoir. In this respect, the spillway design at Nanhua is not too dissimilar in principle from the much older design of Yoichi Hatta at Wushantou reservoir (apparently, Baoshan reservoir
[寶山水庫], which supplies water to the Hsinchu science park [新竹科學工業園區], also has a very similar spillway). As can be seen from my photo of the dam and spillway diagram above, there are two additional exit chutes for water besides the spillway itself (marked in blue). Both chutes run diagonal to the north-south orientation of the diagram  - one lies to the south of the control station, and one just north of the spillway lip. The first chute to the south exits into the spillway pool, but the second chute to the north exits into a channel which eventually turns south to join the water passing out from the spillway pool. In my background reading on Nanhua reservoir, I didn't find any sustained reference to either of these chutes even though they are marked prominently on the diagram above. To guess at their function, I would imagine that their purpose is twofold; first, to serve as the regular water supply conduit from the reservoir, and second to supplement the draining function of the spillway in the event of an unforeseen volume of rainfall in order to protect the structural integrity of the dam. Since the hydraulic gate to the first chute is apparently located just beneath the control station, and the second chute to the north of the spillway actually runs under the dam itself, it would seem that the two chutes would drain water from both the top and the bottom of the reservoir respectively. Moreover, I suspect that the first chute, which exits directly into the spillway pool, also contains a de-silting filter which must be periodically changed.

The Jiaxian Diversion Channel

The supplementation of Nanhua reservoir with additional water from the Cishan river is accomplished by means of an underground diversion channel. This channel begins on the other side of the reservoir's eastern mountains in the Jiaxian district of Kaohsiung. There, a weir serves the two purposes of flood control and water diversion into the tunnel (although recently, the river bed has had to be widened following the flood disaster of Typhoon Morakot in 2009). The diversion channel stretches underneath the mountains for approximately 2.5km (my own estimate from looking at maps) before falling down a baffled spillway into the reservoir itself (the "baffling" is a series of concrete blocks punctuating the surface of the chute which dissipate the energy of the water as it rushes down the chute). Originally, the design for Nanhua reservoir did not include this diversion channel, and it was only after the reservoir was completed that the authorities realized that they had made a mistake. The Taku river which feeds the reservoir from the north is too short, meaning that the rainfall brought into the river by its tributaries is insufficient to supply the reservoir. However, the Cishan river to the east is significanty longer, finding its source way up in the central mountain range and therefore carries more water than the Taku. Hence the diversion of water from the Cishan river into Nanhua reservoir.

The Sedimentation Problem

Below the control station to Nanhua reservoir, there is a dock where an elaborate system of buoys is held; these are deployed from time to time in order to float a pumping system for the dredging of sediment from the bottom of the reservoir. As mentioned earlier, Nanhua reservoir suffers heavily from excess sedimentation due to the lack of a filtration system at its' northern inlet to the Taku river. All of Taiwan's larger reservoirs suffer from this problem to varying degrees because they are "trough-style" reservoirs sited high up in the mountains. In brief, the problem is that sediment from the mountain slopes is washed away by heavy summer rains into the tributary rivers and thereby into the reservoirs. Consequently, the reservoir's storage capacity will be gradually reduced over time. In the case of Nanhua reservoir, approximately 40% of the reservoir's storage capacity was at one point unavailable due to excess sedimentation. Subsequent to Typhoon Morakot in 2009, which brought with it the heaviest rainfall in southern Taiwan since records began, the Southern Region Water Resources Office attempted to mitigate future such disasters by widening the tributaries to the Taku river and constructing yet more weirs (there were already several extant tributary weirs prior to 2009) so as to retard the velocity and dissipate the energy of onrushing flood waters (this program also encompassed several smaller tributaries that run directly into the reservoir itself from the east rather than into the Taku river). The thinking behind this approach is that by significantly slowing down the floodwaters, more sediment can be deposited further upstream and less deposited in the reservoir. Whether this tactic will be effective is still an open question, though there are perhaps additional tactics that could be tried such as stacking bundles of rice-straw behind the concrete teeth of a weir so as to absorb the finer silt particles.

The Open-Air Military History Museum

Curiously, to the west of Nanhua reservoir beneath the downstream face of the dam there is a permanent, open-air exhibition of former R.O.C. military assets. These rusting relics are gutted and hollow and exposed to the elements which makes the place feel like a bit of a bone orchard. The most striking of them however, are relatively recent bits of kit such as the twin-engined Northrop F-5 "Tiger II" tactical fighter from the 1970s (a few dozen of which are actually still in use by the R.O.C Airforce today), and a Lockheed F-104 "Starfighter" interceptor from the 1960s. The Starfighter was actually used in combat by the R.O.C Airforce against PLAAF Mig-19s in 1967 over the Taiwan Strait. It's an eerie feeling to stand next to this old junk and consider that in its' day, it was a supersonic interceptor capable of sustaining Mach 2 speeds. Today's aircraft employed by the R.O.C Airforce in the interceptor role (the "IDF" F-CK-1 and the F-16 A/B) cannot match that, at least not when carrying a full complement of weapons. Aside from the two plinth-mounted rusting jets, there is also a third aircraft - a Fairchild C119 "Flying Boxcar" from the 1950s, which is a twin-turboprop cargo aircraft distinctive because of its twin-boom tail. The rear door to this plane remains open and it is possible to climb inside and have a look around (however it is almost entirely gutted and full of dust). Other than the three aircraft, there are a few other bits and pieces (e.g. APCs and several air defense systems), but it is a bit odd that somebody should have went to all the expense and trouble of shifting these things all the way out here to Nanhua reservoir; presumably, they were the victims of limited space at the R.O.C Airforce academy (the museum in Taipei is just a small building), and so rather than letting them be scrapped, the small patch of land (perhaps two or three acres at most) just to the west of the downstream face of the dam was purchased and they were transferred here. Given that the current approach road passes underneath the reservoir's spillway through a narrow tunnel, these aircraft must have been brought to their current location via a makeshift road that has since been demolished.


As Nanhua is a rural district to the south-east of Tainan, it naturally slopes up into the mountains that form the spine of the island, and, where there are mountains, it is not unusual to find eagles. Nanhua reservoir itself is home to a pair of Crested Serpent Eagles, which is a medium-large eagle of perhaps 60 to 70cm in length with a wingspan of more than twice that. Although they are common throughout East Asia, their physical characteristics vary by geography enough to cause some debate among ornithologists as to whether they should all be classified as a single species or not. They are found throughout Taiwan's mountainous regions as borne out in this description (indeed, there is also a pair at nearby Wushantou reservoir, along with the Ospreys). As the name of the bird suggests, they apparently feed on snakes, and although there certainly are snakes to be found at Nanhua reservoir, it occurs to me to ask whether the birds might also be capable of taking fish from time to time. Other than the pair of Crested Serpent Eagles, there is also at least one other bird of prey resident at Nanhua reservoir, somewhat smaller in size but so far the few camera shots I have been lucky enough to take weren't quite clear enough to allow me to identify it (I suspect it may be a Black Kite due to the forked tail). 

As with Wushantou reservoir, the best views of Nanhua reservoir are reserved for those prepared to get off the beaten track to find unusual angles from which to take pictures. Below is a picture taken from a slight elevation from the westernmost control station at the edge of the premises. To the right is the massive spillway, with the pool directly beneath it and the downface side of the dam off to the left with a spur of mountain in between...

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