Tseng-wen Reservoir (曾文水庫)

Set upon a north-east, south-west axis just within the south-eastern cusp of Chiayi County (嘉義縣) bordering on Tainan's Nanxi district (楠西區), Tseng-wen reservoir is the largest of Taiwan's reservoirs. It is the longest reservoir at approximately 12 kilometers long; it is the largest by surface area at over 17 square kilometers; and it has the greatest possible volume at well over  600 million cubic meters of water when full. Measured by depth, it is the second deepest reservoir in Taiwan behind Shihmen reservoir (石門水庫) in Taoyuan County in northern Taiwan; Tseng-wen reservoir is 230 meters deep at its' deepest point although the average depth is substantially lower than this largely because the bed of the reservoir is sloped from the north end down to the south end. Built between 1967 and 1973 by a team of both Chinese and Japanese engineers under the direction of the ruling KMT government, the construction of Tseng-wen reservoir was contemporaneous with Taiwan's two other largest reservoirs - the aforementioned Shihmen reservoir (石門水庫) in Taoyuan and Feitsui reservoir (翡翠水庫) in Taipei County (now "Xinbei"). These three reservoirs were built over a period of two decades between the mid-1950s and the mid-1970s, with subsequent reservoir construction being relatively sporadic and much smaller in scale. Unlike its two somewhat smaller and relatively isolated cousins to the north however, Tseng-wen reservoir sits in a region of Taiwan which already possessed the very substantial Wushantou reservoir (烏山頭水庫) with which it shares the same river. The entire Tainan area is now served by no less than four reservoirs; the comparatively trivial Baihe reservoir (白河水庫) to the north of the county, Wushantou reservoir (烏山頭水庫) in the centre, Tseng-wen reservoir (曾文水庫) to the east, and Nanhua reservoir (南化水庫) to the south. It is perhaps understandable then that people living in other parts of Taiwan might wonder why it is their regions do not possess a single, working large-scale reservoir (e.g. Kaohsiung for instance).

Tseng-wen Reservoir Dam & Spillways

Other than its size and the majesty of its mountainous surroundings, one of the chief aesthetic attractions of Tseng-wen reservoir is its dam. At 133 meters in height (crest: 235 meters) and more than twice that in length from north to south, the dam is an impressive sight particularly when approached from the south-west. The composition of the dam is not documented on display panels for the public, but I would think it likely consists of at least one impervious clay core, abutted on either side by semi-permeable shells of rock fill with gravel-lined drainage channels in between the two shells and the core. Both the downstream and upstream faces of the dam are dressed in loose boulders held together by friction and gravity. The upstream face of the dam is heavily stained by the average water tide mark over the years, such that the lower, periodically submerged half is pale, whilst the upper, usually exposed half is darkened by the stain of recurrent rainfall over the past forty years.

Slouched back at a 45 degree angle between the spurs of two mountains, the dam overlooks an immense spillway pool fed by three enormous concrete spillway chutes. Each of these spillway chutes is subject to the control of almost 30 meter tall hydraulic gates which sit at the top left (north) of the dam. Googling for images reveals precisely what you would expect: in operation during flood control, the spillway chutes are an awesome spectacle (though I have yet to have the pleasure of capturing this through my own lenses). To the bottom right (south) of the dam sits a transformer station for the 50 MW hydroelectric plant, which is itself hidden within the spur of the mountainside. Although the crest of the dam is perfectly accessible to the general public, the approach road leading to the base of the dam and the hydroelectric plant is strictly off-limits and is guarded by an armed military police post. This is perhaps not such a great loss however, given that there isn't much at the downstream base of the dam to photograph which cannot be observed from the distance of the approach road; the transformer station is somewhat obscured by decorative palm trees but the entrance to the hydroelectric plant is clearly visible with a long-lens shot. With an installed capacity of 50 MW, Tseng-wen reservoir's hydroelectric plant is second in scale only to the hydroelectric plant at Feitsui reservoir in the north, which has an installed capacity of 70 MW. Under optimal conditions, Tseng-wen's hydroelectric plant would generate approximately 1.8 GW hours of electricity throughout the year. However, the actual figure will of course not be quite as high as that due to annual variation in the reservoir's volume; should the water height drop below 171 meters, as it typically does during the dry season, then the hydroelectric power plant must sit idle for want of water flow. The dam marks the boundary between Chiayi to the north and east, and Tainan's Nanxi district (楠西區) to the south and west; the cavernous spillway pool beneath the dam flows out westward to allow the Tseng-wen river (曾文溪) to carry on its way through the valley south-west into Yujing district (玉井區) and beyond.

Tseng-wen Reservoir's Raison d'être

 

Quite aside from the small hydroelectric plant however, the chief purpose for Tseng-wen reservoir's construction was to better regulate the supply of water from the Tseng-wen river into Wushantou reservoir so as to avoid drought during the dry season. Built in the 1920s, Wushantou reservoir is Taiwan's oldest modern reservoir, and its agricultural importance would be difficult to overstate; it feeds the Chia-nan canal irrigation system serving the rice paddies and agricultural plains of Tainan and Chiayi counties. Originally, the chief engineer responsible for the construction of Wuhantou reservoir, the 32-year old Japanese Yoichi Hatta, had realized early on that the water supply from the Guantian river (官田溪) would be insufficient to fill Wushantou reservoir and so he had hit upon the idea of diverting water from the larger Tseng-wen river to the east into the Guantian river. This was accomplished by means of a small entrapment dam on the Tseng-wen river to divert water into a channel running under the so-called "black mountains" from which Wushantou reservoir takes its name. That channel runs for approximately 3 kilometers. During the dry season however, the diverted water from the Tseng-wen was also insufficient, hence the construction of Tseng-wen reservoir to hold water back for use during the dry season. For this reason, it makes sense to think of Tseng-wen reservoir primarily as a giant auxilliary to its' older predecessor, Wushantou reservoir. Every year, a significant volume of water is fed through the diversion channel into the Guantian river on the other side of the black mountains to supply Wushantou reservoir.

After passing the entrapment dam, the Tseng-wen river continues to flow out beyond the small diversion dam to wind its way westward down through the plains of Tainan toward the sea. Just to the west of Yujing township, it is joined from the south by the smaller Taku river flowing out from Nanhua reservoir, whereafter the Tsengwen begins to broaden out somewhat as it threads its way between Madou district (麻豆區) to the north and Sanhua district (善化區) to the south before finally discharging into the Taiwan Strait at its estuary between Cigu (七股區) and Annan (安南區) districts on Tainan's western coast. Throughout this course, which from its mountain source to the east to its estuary in the west spans 138 kilometers, the Tseng-wen river supplies water for agricultural, industrial and residential uses. The agricultural areas of Sanhua district and Madou district both receive water from the Tseng-wen river. Madou district is better known throughout all of Taiwan as home to the island's best pomelo (citrus maxima) fruit trees - the fruit of which comes into season in the autumn, whilst Sanhua district houses a brewery for the "Taiwan Beer" company and is locally famous for its strawberry farms. To the south of Sanhua lies Xinsih district in which the greater part of the Southern Taiwan Science Park is based; it is here, in addition to its northern counterpart in Hsinchu county, that Taiwan's largest electronics firms are based including semi-conductor manufacturers TSMC and UMC and flat panel display producers such as AUO and Chimei.

The Geography Of Tseng-wen Reservoir

The shape of Tseng-wen reservoir resembles a child's absent-mindedly drawn rectangle, with the water filling in massive recesses between the odd jagged peninsula. At its northern end, the reservoir is fed by the Tseng-wen river from an easterly direction whereafter the valley floor in which the reservoir lies takes a sharp southward turn, at almost ninety degrees, to stretch out for a further 10 kilometers or so. Approximately 12 kilometers from north to south, the reservoir is about a third of the size of Scotland's Loch Ness. At its' northern end, the reservoir is both broad and shallow at between ten and twenty meters in depth and almost a kilometer in breadth. During the hot, tail-end of the dry season this becomes painfully obvious, with the body of the reservoir itself reduced to a mere trickle amid an overbearing desert of parched mud. During the peak of the wet season however, it would not be possible to take the photograph shown to the immediate left, as I would have been standing perhaps ten meters underwater.

Meanwhile, at the southern end of the reservoir, the water fills up across its' span and retains some depth even during periods of comparative drought. Compare these two images to the right (both are taken from the crest of the dam) - the first was taken in June of 2011, following a prolonged absence of rainfall, whilst the second was taken in January 2012 following a period of healthy rainfall. Note that in the second image, the water reaches up almost to the vegetation line whilst in the first image that vegetation line is exposed some twenty meters or so above the surface. Upon examining the first image in close up, a floating line of buoys should become apparent in the distance - this line is for the safety of the few boats allowed onto the reservoir since it marks the location of a usually submerged sandbar. These two images show only a relatively small fraction of the reservoir in its' south-west recess; the distance from the dam at the western end to the eastern end of the reservoir is approximately one and a half kilometers.

In addition to acute absences of rainfall, the water level in Tseng-wen reservoir is also adversely affected by evaporation from its massive surface area (seventeen square kilometers) during the hot and humid weeks and months prior to the arrival of the summer rains. As can be seen from a comparison of the two shots to the immediate right, both of which were taken from the other side of the reservoir at different times last year, the evaporative losses from the surface can manifest as a thick haze which ruins the views even on the brightest of days (generally speaking it is better to photograph the reservoirs during the comparatively cooler, less humid months of Taiwan's dry season, such as in January, when both the humidity and the air temperature are substantially lower than they are duing the hot and wet summer months). It would be interesting to see a range of estimates for what proportion of the reservoir's annual losses result from evaporation...


Like other large, "trough-style" reservoirs set high up in the mountains, Tseng-wen reservoir is also vulnerable to over-siltation from its major source, the Tseng-wen river (曾文溪). The chief reason for this is the displacement of large volumes of sediment caused by the erosion of the river's upper tributary system during acute periods of excessive rainfall, such as during Typhoon Morakot in 2009. Recent estimates of the siltation in Tseng-wen reservoir subsequent to Typhoon Morakot were as high as 90 million cubic meters, which is almost 17% of the reservoir's total volume. The wash-down of large volumes of sediment into the reservoir not only reduced its overall capacity, but the ongoing flow of water into the reservoir both during and immediately after the typhoon meant that much of the sediment would have remained in a disturbed state, unable to settle. As a result, the water immediately discharged from the reservoir was heavily contaminated. The WRA (the Water Resources Agency) and the Taiwan Water Corporation temporarily turned off the taps to the residential water supply to parts of Tainan city in order to work on the immediate clean up and repair operations. Just at Tseng-wen and Nanhua reservoirs alone, the initial work involved the removal of driftwood and the dredging of excess sediment in the two reservoirs and their tributary rivers, with monitoring and desiltation equipment installed afterwards. This was followed up by the widening of tributary stream beds and the construction of stepped weir systems to retard floodwater velocity. Long-term sedimentation monitoring and desiltation equipment was also installed. Although the work was not limited to the central Water Resources Agency and involved many distinct government agencies (such as the Soil & Water Conservation Bureau under the Council Of Agriculture), the post-Morakot recovery operations for the entire southern region were budgeted at over NT$54 billion over a six year period (2009-2015), of which more than NT$16 billion was spent at Tseng-wen and Nanhua reservoirs alone.

Approach To Tseng-wen Reservoir: Three Routes

Tseng-wen reservoir may be approached by three roads, two of which run up through Tainan to the south-west, with the third gradually disappearing up into the mountainous Alishan district of Chiayi to the north-east after passing through the village of Dapu. The first of the Tainan roads leads directly up from the south-west toward the downstream face of the dam itself, and is the road most first-time visitors might be expected to take since it is the only one of the three on which directions to Tseng-wen reservoir are repeatedly signposted. The road's final section springs from a cusp at which the provincial highway (not "freeway") 3, which runs from Yujing district through Nanxi district meets up with route 174, which snakes down south from the black mountains on its way eastward from Lioujia district and Wushantou reservoir. Both routes prior to this cusp are heavily signmarked for Tseng-wen reservoir and the toll station at the end of the road is obviously designed to accommodate most of the traffic, being divided up into seperate entrances for coaches, cars and motorcycles.

However, the second road from which Tseng-wen reservoir may be approached branches off from provincial highway 3 just outside of Nanxi township (perhaps a kilometer or so from the aforementioned cusp) and is not signmarked for the reservoir at all. It is however, a superior route in some ways as it leads the driver through a far more scenic, meandering tour of mountain farms (including a bee farm) on its way upward toward the reservoir's southern extremity. This second approach road arrives at a point looking over the reservoir which sits upon a bend as the road continues east and northwards around the east end of the reservoir up toward Dapu township some twelve kilometers away (as the crow flies). On reaching this point, it is possible for drivers to turn left and head west and northwards up the partially broken but servicable skirt road that eventually winds its way through a second, much smaller toll station toward the upstream face of the dam. The third road approaches Tseng-wen reservoir from Alishan district to the north, and therefore experiences comparatively little traffic (other than gravel trucks for the construction industry), but it is a fantastic drive through the mountains.

Tseng-wen Reservoir Tourist Facilities

Consonant with the reservoir's two principal access roads to the south west, there are several tourist facilities in the vicinity. Located perhaps a kilometer or two after the major toll station, and still some several kilometers distance away from the downstream face of the dam itself, there is a designated camping area off to the right which can be somewhat busy during fair-weather weekends. The on-site facilities here are basic: bathrooms, showers and waste disposal - people bring their own badminton sets and barbecue racks, and of course the reservoir itself is just up the road. At perhaps an hour and a half's drive outside of Tainan City, the campsite at Tseng-wen reservoir is ideal for those who regard the three hour drive down to Kenting on the south coast as too far or too tiring for a weekend trip.

Just a stone's throw further up the approach road, there is also an "Hibiscus Resort" - a hotel, and a small village set among the grounds of a former military police training camp. The village bosts an interesting apartment building; its' architecture flaunts several turrets and conical minarets redolent of some imaginary palace from a children's story; with its wild and mountainous surroundings, it does not look entirely out of place. It is also still in use too; bed sheets can be seen hanging out to dry from several balconies. The military school's training barracks seem to have been converted into little holiday bungalows at one point, but many of them have long since fallen into a general state of disrepair, their current dilapidation being sheltered by avenues of well-matured bread fruit trees (though one or two still showed signs of habitation). Although the place generally seems to be surviving on the strength of fading memories alone, there is an extant police station and also a small shop located at the entrance road; it may be that the police station is there to serve the small villages and farms surrounding the reservoir to the south and on its eastern side. The little convenience store always seems to be still awake with a handful of people standing around on its steps.

Further forward along the primary approach road toward the downstream face of the dam, after crossing a bridge over the Tseng-wen river, a convex-shaped steel and glass visitor's centre lies nestled amid a well-tended flower garden. The glass walls of the building are tinted somewhat so as to render them largely opaque although they do reflect the surrounding trees. As a visitor's centre, it is surprisingly sparse; there is a brief set of information panels (all of which are in Mandarin) on the ground floor referring to the planning and construction of the dam and hydro-electric power station in the 1960s. Although these panels feature some photographs taken during the construction, the information provided is basic - the net volume of the reservoir, the height of the dam, the length of the Tseng-wen river and so on. There are no quantitative charts detailing the history of the reservoir's performance over time, and nor are any of the mathematics for the reservoir's design illustrated. The second and third floors of the building house small art galleries with superb photographs of the reservoir under various lighting and weather conditions, along with paintings of subjects seemingly unrelated to the reservoir by local artists.

Accessible either from the second approach road to the south which passes through the minor toll station, or from following the primary south-western approach road across and after the dam itself, the reservoir has a second visitor's centre which features a wonderful glass elevator and viewing platform specifically built for the purpose of enjoying the view. Presently (presumably due to lack of funds), the elevator is awaiting repair and the two buildings for the visitor's centre at the top of the small hill are empty. Of the two buildings, the glass, pagoda-like conservatory placed across the bridge in diametric accord to the elevator looks like it was designed to be a cafe. Although four-sided, two sets of handrails form concentric circles in the centre of the little glass house, as if to serve as a tea and refreshments hub for an imaginary set of customers.

This second visitor's centre, although currently abandoned, seems to have been built at this location in order to offer views out westward to the dam and over the dilated southern terminus of the reservoir. From this point, the main northward stretch of the reservoir can also be glimpsed with its' typically hazy horizon visible an optically compressed ten kilometers away...