Salient Features (Mai HP)

Location: Danabari and Chisapani VDC, Ilam District, Eastern Development Region of Nepal
Purpose of Project: To supply hydro-electricity for domestic use by connecting to Integrated Nepal Power System (INPS)
Catchment Area 589.0 km2
Average Flow 32.66 m3/s (minimum monthly flow 7.22 m3/s)
Design Flow 23.43 m3/s (40% exceedence flow)
90% Exceedence flow 7.48 m3/s
Design Flood (Q100) 2070 m3/s
1000 years Flood (Q1000) 3007 m3/s
Diversion Dam:  
Type Concrete gravity dam
Shape Ogee-profile
Crest Elevation 321.1 m above msl
Crest Length 91.12 m
Height 5.1 m (from original river bed level)
Designed flood (Q100) level 325.6 m above msl
River Diversion During Construction:
Alignment Along the river channel
Diversion flow 250 m3/s
Coffer dams Two cofferdams facilitating stage1 and stage 2 concreting (Total coffer dam length = 377.5 m)
Intake spillway/ Undersluice:  
Type Over-flow weir with under-sluice (3@2x4m width) 
Crest Elevation 321.1 m above msl (undersluice 314.5 m above msl)
Length 111.12 mm
Design Flood (Q100) 2070 m3/s    
Type Frontal Intake, submerged, over the undersluice
Number of Orifices 3
Bottom Elevation of Orifice 317.0 m amsl
Top Elevation of Orifice 320.0 m above msl
Length of Orifices 20.0 m (with inner piers, clear opening of each orifice = 2.55 x 6 m width)
Intake canal:  
Type RCC open canal
Maximum height 3.0 m
Average Width 6.8 m
Approach Culvert:  
Type RCC pressurized conduit
Number 2
Average Height 3.0 m (internal)
Average Width 3.0 m (internal)
Total Length of outer branch Total length of inner branch Invert Level 62.69 m 40.98 m 317.10 m
Settling Basin:  
Type Conventional flushing
Number of Chamber 4 (four)
Top elevation 322.4  m above msl
Normal Operation elevation 320.50 m above msl
Length of the basin 75 m (settling zone)
Average Height 5.85 m
Bottom width per Chamber 9.5 m c/c
Headrace Culvert:  
Type RCC pressurized culvert
Length 1055.69 m
Height 3.3  m
Bottom width 3.5 m
Longitudinal slope 1:1500 (V:H)
Headrace Tunnel:  
Length 2198.0 m (incl. 66 m after surge-tank)
Dimensions Inverted D 3.8 x3.96 m, (13.1 m2-finish)
Discharge 23.43 m3/s
Surge Shaft dimensions:  
Type Vertical, circular section
Height 35.0 m
Diameter 8.0 m (finish)
Length 316.38 m (273.6 m up to bifurcation)
Diameter/thickness 2.6 m to 1.5 m internal diameter/10 mm to 25 mm thickness
Power Facilities:  
Powerhouse Type Semi-surface
Dimensions 36 m x 22 m
Gross Head 122.1 m (321.1 – 199.0 m above msl)
Net Head 108.93 m
Installed capacity 22.0 MW (3x7.33 MW)
Dry energy 22.11 GWh
Wet energy 103.03 GWh
Annual Net Energy Output 125.14 GWh
Tailrace Culvert:  
Type RCC canal
Length 229.5 m
Height 3.3  m
Width 3.5 m
Longitudinal slope 1:450 (V:H)
Power Transformer  
Type 3-phase, two windings, oil immersed transformer with off load tap-changer;
Installation Outdoor;
Rated power 10,000 kVA and 20,000 kVA;
Transmission Facilities:  
Transmission line length 11.6 km
Voltage level 132 kV, single circuit
Project Cost at completion 3,295 million NPR