The steep 1,600-ft peaks and dramatic canyons that have made the Shimen Reservoir one of Taiwan's popular tourist attractions are the products of massive erosion – erosion that threatens the reservoir's role as a key supply of domestic water.
The problem came to a dramatic head in August 2004, when Typhoon Aili unleashed 20 million metric tons of sediment and debris into the reservoir from the surrounding mountains. Turbidity climbed from the usual 40 NTU to spike at 70,000 to 120,000 NTU. The sediment choked the reservoir's water treatment plant, which was equipped to handle water with no more than 3,000 NTU. Tap water was cut off for days to thousands of households, and residents were forced to rely on water trucked in by the Taiwan Water Supply Corp.
The 2004 typhoon was big—at 973 mm (38 in.) of rain, it represented half of a typical year's precipitation—but it wasn't unusual. The area around Shimen Reservoir is typically hit by two to four typhoons per year, and suspended solids discharged by landslides around the reservoir present a constant challenge to the Ministry of Economic Affairs Water Resources Agency's northern region. Adding to the challenge, the sediments often travel in subsurface plumes across the 1,977-acre reservoir, presenting a hidden hazard that can quickly overwhelm the dam's water treatment facility.
Vertical profiling
Before the 2005 typhoon season, the Water Resources Agency deployed YSI vertical profiling systems mounted to pontoons at two points in the reservoir. Each profiling system uses a YSI multiparameter sonde to conduct autonomous vertical profiles of the water column as the water approaches the Shimen Dam. The YSI 6600EDS sondes report back to the dam's operations office every three hours, logging turbidity, chlorophyll, pH and dissolved oxygen readings every five meters from surface to bottom.
When turbidity reaches critical levels, the profilers' data trigger an emergency plan at the water treatment plant, according to Bergius Su of Taipei-based Smartec, which provided and services the equipment. A team of technicians would then promptly begin a program of manual sampling to confirm the problem and determine its extent.
Finding the size, shape and depth of the plume allows the water treatment plant managers to choose among several management options:
- Accelerate the fill-up of the storage pond immediately above the water treatment plant;
- Close any of the plant's five water intakes and opening others to draw in water from clearer levels in the water column and avoid the turbidity plume; or
- Draw water into the plant from a backup source in a nearby river.
Careful positioning
Smartec used a SonTek/YSI ADP (acoustic doppler profiler) to identify the optimal deployment locations with minimal current for the Vertical Profiling Systems. The system closest to the dam is anchored in 164 ft of water; the more distant sampling site is 131 ft deep. Eighty-eight anchors hold the pontoons in place.
Those anchored floats have weathered several typhoons, though vertical profiling is sharply curtailed in rough weather. When storms hit, Smartec remotely locks the sondes in position immediately beneath the floating platform. At the dam management's request, Su and his team can activate the sondes remotely or manually to sample at 10-meter intervals, or the crew can begin conducting manual readings with YSI 6600EDS sondes and grab-sampling equipment.
Smartec also conducts monthly calibrations and maintenance of the equipment. They note that the self-wiping feature of the sondes reduces buildup of biofouling and that calibration is a quick process.
Data flow
The sondes make their journey from the reservoir's surface to the bottom and back every two hours, and report values via wi-fi for all parameters back to the dam office – about 3 miles away—every three hours. Their data are integrated with information gathered at eight grab-sample stations. The data are primarily used for managing the dam and water treatment plant; however, the Water Resources Agency has also contracted with a professor from Taiwan University to develop a data-gathering and analysis protocol and develop a water quality model. Ultimately, predicts Su, data from the continuous vertical profilers could be integrated into the Shimen Runoff Forecast Model or the dam's flood control operations.
For now, the mission to track suspended sediment is important enough. Every year, landslides triggered by typhoons represent a significant threat to the water treatment plant at the Shimen Reservoir. Using long-term, unattended monitoring technology throughout the water column helps ensure that intakes are opened and closed to draw clean water and avoid turbid supplies after typhoons, which prevents costly damage to the plant and interruptions in the region's water supply. Going beyond pure science into public service, it protects the people who count on water from the reservoir for their household needs.
For information, please contact YSI Integrated Systems & Services at 800/363-3269 or by email at [email protected] .
