Calculation of Water Surface Elevation Using HECRAS 4.1.0 for Fixing Tailwater Elevation for Powerhouse Site in Planned 37 MW Kabeli "A" Hydroelectric Project, Nepal
Author(s): Pathak, S.
Goal of this HECRAS 4.1.0 model analysis is to find out water surface elevations for safe and optimized layout of powerhouse located in left bank of Tamor river with other protective structures like floodwalls at different flood frequencies for planned 37 MW Kabeli "A" Hydroelectric Project. Powerhouse is costly and vulnerable component of any hydropower facility. Its safe location should be prioritized. Any increment in head is related with generation of extra revenue but powerhouse earthwork excavation volume incurs huge part of overall project cost initially. There is some sort of tradeoff between these two parameters to get an optimum design elevation. It is envisioned that 1-D US Army Corp’s HEC-RAS model can simulate flow conditions at different flood frequencies. This project is located in Panchthar and Taplejung districts in Eastern Development Region of Nepal. This project utilizes more than 15 km long loop of Kabeli River formed with Tamor River. Kabeli River, which is a tributary of Tamor river is diverted through a 4326.8 m long D-shaped headrace tunnel having internal finished diameter 5.65 m, discharging diverted water into Tamor River for power generation. The gross head of the project is 116.8 m and the design discharge based on 40 percentile flow set by government for power generation from flow duration curve (FDC) in river is 37.73 m3/s.
Kabeli River is one of the tributaries of Tamor River which itself is a major tributary of Sapta Koshi River basin.The catchment area above the proposed intake site of project is 864 km2 and at powerhouse site is 3930 km2 with elevation ranging from 452 m to 7200 m above mean sea level. In this catchment, monsoon pattern of climate is prevalent. It commences from June to September with heavy rainfall intensity for those four months compared to other months of the year. Rainfall intensity varies in catchment with elevation and runoff is calculated from different methods as powerhouse site is ungaged.
Due to global warming, glacial lake outburst flood (GLOF), known as mountain tsunamies in high Himalayas ( Kanchanjunga range) is highly probable. This zone lies in area with high seismic activity with possibility of GLOF, flooding populated areas and infrastructure downstream. Since glacial/snow hydrology study and data collection is still in nascent stage of development, analysis for design flood elevation has been done without taking into account those effects due to unavailability of data.
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