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南化水庫位於台南縣南化鄉，每日供應台南及高雄地區60萬立方公尺水量，並可支援高雄地區原水最大達每日50萬立方公尺，為台灣南部地區亟重要水源設施。民國98年莫拉克颱風來襲，造成南化水庫壩前淤積達標高150.8公尺，已高出第二號取水口(標高144公尺)，而水庫庫容僅餘9,681萬立方公尺，原規劃庫容已淤積超過37.6%，嚴重影響水庫運轉操作及供水。為穩定南部水資源調度、防止南化水庫持續淤積及因應未來新水源需求，經濟部水利署水利規劃試驗所(以下簡稱本所)99年度辦理「南化水庫上游設置第二水庫調查規劃」計畫初步結論認為南化第二水庫除可維持本身永續經營外，具有(1)促進南化水庫永續利用；(2)增加南化水庫及高屏堰系統聯合運用供水潛能；(3)提高水庫下游後堀溪防洪能力等3大功能。100年度本所辦理「南化水庫上游水資源開發調查規劃」針對壩址地質、築壩材料、集水區崩塌地等進行現場調查工作。101年度擬以前2年規劃及調查成果為基礎，就水文複核及功能檢討、工程佈置研擬、維持庫容規劃、施工計畫、工程用地及工程費、計畫評價等項目進行更進一步探討。

1. Introduction To stabilize southern Taiwan water system management and to alleviate Nanhua Reservoir siltation problem as well as to meet new water need, the Water Resource Planning Institute of Water Resources Agency, MOEA, conducted 「Reconnaissance and Planning of The Second Reservoir Upstream From Existing Nanhua Reservoir」beginning in Yr.2010 for a three-year study. Based on planning and reconnaissance results of two previous years, this study is to further review hydrology, project function, site layout, planning for sustainable reservoir, construction planning, land use requirement and project evaluation. The result shall be used for reference in future feasibility study. 2. Reevaluation of Engineering Hydrology The results of analyses indicated that peak discharges of various return frequencies are slightly lower than those of previous estimates. For PMF however, the peak discharge increases from 4,508cms to 4,659cms, due to a slight increase in drainage area. 3. Review of Project Function From water resources simulation, it is suggested that the effective storage of the second reservoir be set at 10×106m3. Assuming 500,000CMD will be sent from Kaoping Weir northward, a development of the Nanhua No.2 Reservoir will increase water supply by 352,700CMD. If a new transbasin diversion tunnel is also constructed the total increase in water supply would be 455,100CMD. 4. Sustainable reservoir Planning (1) Estimated Average Sediment Inflow to Nanhua and 2nd Nanhua Reservoirs Sediment inflow to be existing Nanhua Reservoir is estimated to be 3×106m3, and to the 2nd Nanhua Reservoir is estimated to be 2.32×106m3. (2) Scheme for Sustainable Reservoir The 2nd Nanhua Reservoir and the existing Nanhua Reservoir shall be operated in series. During monsoon, from May 1 to June 15, conducts desilting by emptying the reservoir. During typhoon season conducted density current/muddy reservoir desilting. Both operations shall remove most inflow sediments during storms. Some mechanical dredging will also be required to maintain a sustainable volume. It is estimated that a mechanical dredging of 330,000 ton/year will be needed to achieve a sediment balance. Most of sediment entering into Nanhua Dam will be discharged through the silt-sluice tunnel which will be constructed in the near future. But some sediment will come into the reservoir via local drainage basin. It is estimated that average mechanical dredging of about 470,000 ton/year will be needed. 5. Project Layout (1) Dam The dam is a zoned earth dam. The dam crest elevation is at EL.260.0m. The maximum dam height is 98m. The total volume is estimated to be 5,507,000 m3. (2) Outlet Structures ‧ Construction diversion: The diversion is planned for a 50 year return frequency flood. A 12-m diameter tunnel with 1% of longitudinal slope has been planned. The routed peak discharge is 1,458cms. The corresponding upstream water substance is at EL.205.8m. ‧ Silt-sluice Tunnel: The tunnel is sized to discharge 800cms at water level EL.250.0m. Upstream from the control gate it is a pressurized tunnel, 9.0m in diameter. Downstream from the control gate the tunnel is also 9.0m in diameter but it will not be pressurized. The tunnel slope is 2.25%. ‧ Flood Discharge Tunnel: Two flood discharge tunnels have been provided, one on each side of the embankment. At water surface level EL.257.5m, each has a design capacity 1,651cms. The diameter of the tunnel at the right abutment is 10.5m in diameter. The tunnel on the left is converted from construction diversion. The tunnel diameter is 12.0m. ‧ Water Supply Intake: An inclined multiple level intake has been planned. The design discharge is 140cms. 6. Construction Planning Initially construction access road shall be made use of improved 179 highway. But a permanent construction access and maintenance road shall be built by boring a 4,000m long tunnel from the right bank of Chishan Hsi to the site, for shipping of core material, riprap material, filter material, etc. Construction office, repair shop, laydown area and batch plant will be located above the maximum water level and accessible form HY179. 7. Project Cost Estimate The total construction cost is estimated to be 23.75 billions NTD while the total project cost is 26.64 billions NTD. If transbasin diversion is included, the total construction and project costs are estimated to be 28.86 and 31.81 billions NTD, respectively. 8. Project evaluation The benefit/cost ratio of the No.2 Nanhua Dam is estimated to be 1.46. If transbain diversion is added, the benefit/cost ratio shall be 1.82. Both are economically feasible.