As Hushan Reservoir has a longer dam axis, with dispersed branches in the reservoir zone, it is one of the few reservoirs with more than one diversion tunnel in Taiwan. In addition to the construction of the first water intake/outlet leading to the diversion tunnel in accordance with its original plan, it also created the natural conditions for the building of the second water intake/outlet in the Hushan diversion tunnel construction scheme. According to the evaluation of the “Planning of the Backup Outlet Project for the Hushan Reservoir Final Report of Feasibility Planning” and “Planning of the Backup Outlet Project for the Hushan Reservoir Final Report on Detail Planning”, it confirmed that the water intake/outlet of the second diversion tunnel would work in flood prevention and the positioning and function of the backup water supply system. Likewise, it helped lower the risk of the reservoir, promote stability in operation, and enhance its benefits and reliability.
Regarding the proceedings of the construction works, the second water intake/outlet of Hushan Reservoir is constructed in two stages. Before storing water, the intake tower should be built. In addition to minimizing the difficulties and risks of subaqueous works, it also decreases the construction cost significantly and the difficulties in fundraising. Moreover, it also leaves another intake/outlet in advance to facilitate the gate valve chamber and pipeline construction downstream. Because of that, Hushan Reservoir is a foresighted reservoir that has its second water intake/outlet built before the construction of the reservoir.
When the second water intake/outlet is completed, it functions as follows:
(1) The second water intake/outlet functions to supply water, prevent silting-up, flood drainage, water activation, and more.
(2) The elevation of the second water intake/outlet is EL.175m and 190m that intersect with the first water intake/outlet in elevation: EL.165m and 180m. During flood season and downpours, depending on the water quality, they can intake water, de-silt, and drain water interchangeably. Therefore, it is able to drain flood and de-silt simultaneously, providing operators flexible space for operation. It also increases new water outlets to increase the stability of water supply and promote water quality of the reservoir.
(3) The length of the dam axis of Hushan Reservoir is about 1.5km, located at the south end of Hushan Reservoir. Likewise, the water at the north end of the reservoir cannot flow easily, resulting in eutrophication. Therefore, the second water intake/outlet is constructed additionally at the north of the dam to promote water flow and activate water flow at the north of the reservoir.
(4) In the original plan, only the first water intake/outlet are used to intake water and de-silt. After the building of the second water intake/outlet, they can be functioned in water intake and de-silting simultaneously. In addition, the additional function of sediment drainage will help save the expenditure in dredging.
(5) An extra drainage tunnel is added to enhance the safety in flood drainage.
The construction of vertical shaft of the intake tower
The construction of the abutment and wing wall of the operation bridge