Example: Use of Closed Conduits for By-Pass Channels

Project: Maddur River:  Canal Capacity: 2 x 1000 kW
Click to Go Back State: Karnataka Energy:  8.3 GWh/year
The revised designs were made such that none would cause negative impact on the environment or lead to the resettlement of nearby communities. For example closed buried conduits were used so that no land would need to be permanently acquired. This would also speed up obtaining clearances from environmental agencies. 

The original proposal was to construct the powerhouse structure immediately downstream of the intake structure, and to use a 300 meter long tailrace which would be an open channel. Since the scheme is located in sugarcane farms, productive agricultural land would be destroyed. Also extensive excavation of rock (about 29,000 m3) would be required to accommodate the horizontal shaft turbine (S-type) with an extended forebay leading up to the powerhouse (see drawings in Annex G). Several alternative layouts for the Maddur scheme were developed to minimize rock excavation, and to obviate the need to permanently destroy land under cultivation. For the revised layouts, the horizontal shaft turbine was replaced by one with a vertical shaft (elbow type draft tube), and the location of the powerhouse was moved further away from the intake structure to one of three possible locations:

(i) the first (Alt I) was at a point some 50 meters downstream of the intake structure;
(ii) the second (Alt II) was 100 meters downstream of the intake; and
 
(iii) the third (Alt III) was 200 meters downstream of the intake structure.

Instead of the open channel which would require permanent loss of sugarcane growing land, the by-pass channel would consist either of a "cut and cover" conduit which would be cast in situ, or a precast concrete conduit which would be buried in trenches across the field.

The layout for Alt II would require the minimum expenditure for civil works. Five options to convey water from the intake structure to the powerhouse were evaluated.

As indicated below, the least cost option involved the use of a 100 meter long "cut and cover" conduit, and a tailrace consisting of a 200 meter long open channel. A similar layout with a precast concrete conduit would be slightly more expensive but probably easier to install, since they could be fabricated within 20 km of the site. The cost of the civil works for the revised layout is estimated to be Rs. 7.3 million compared to Rs. 12.3 million for the original proposal. About Rs. 3.3 million of the reduction in costs would be achieved by avoiding the excavation of some 23,500 m3 of rock, and reducing concreting requirements by 1600 m3).

 

Estimated Cost of Alternative Water Conveyance Structures for Alt. II (Rs. million)
Type of Structure Intake Penstock Powerhouse Tailrace Total
Precast Hume pipe (200m) 2.7 8.3 0.48 - 11.48
Cast in Situ Trough (300m) 2.7 3.9 0.48 - 7.08
Cast in Situ Trough (100m) 2.7 1.95 0.48 1.06 6.19
Precast Hume Pipe (100m) 2.7 4.15 0.48 1.06 8.39
Open Tunnel with Embankment 2.7 3.1 0.74 1.06 7.60

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