The Gandak River is an international river forming a part of Indo-Nepal border and the Gandak Barrage scheme development is a joint venture project of India and Nepal developed during the 1960’s for deriving irrigation benefits. This barrage is located near Valmikinagar, West Champaran District, Bihar State. The Gandak barrage is a 2425 Ft (739.33 m) long head work gated structure provided with 36 No. of spillway gates, and with silt flushing sluices near each of the canal outlet regulators located on both of the banks. Though irrigation is the main objective of development for both the countries, the available scope for power development has also been envisaged and is being harnessed through different canal drop power houses. The Eastern Canal running in Indian Territory and is called Tirhut Main Canal (TMC) is about 125 mile (200 km) in length and the design discharge capacity of the canal is 15670 Cusecs (443.73 cumecs). The Tirhut Main Canal is having two branch canals in the initial stage of its length after takeoff from the barrage. One is called Don canal branching at RD 5.65 (i.e. 5650 ft) (running back into Nepal Territory before entering back into India at a later reach) and the other is called Tribeni canal branching at RD 9.5 (i.e. 9500 Ft). Both are on left side of Tirhut main canal. The design discharge capacity of Don canal is 2500 cusecs (70.79 cumecs) and that of Tribeni canal is 2750 cusecs (77.87 cumecs).

    The Eastern Gandak Canal HEP was planned and constructed by availing services of CEA & CWC. The generating equipment were procured from M/S Fuji Electric, Japan, with loan assistance from the OECF now known as Japan Bank for International Co-operation (JBIC). The Tirhut Main Canal (TMC) itself runs for a length of about 200 km providing irrigation facilities to Uttar Pradesh and Bihar States. The Bihar state Hydro Electric Power Corporation (BHPC), a Government of Bihar Enterprise is the Owner of this Project.


    At present there are two Power Houses, one each on Left and right bank of Tirhut Main Canal. One Power House located on left bank on Tribeni canal branch is called Tribeni Link canal HEP, and has an installed capacity of 2 x 1.50 = 3.0 MW. The Power channel for Tribeni Link canal takes off at RD 6.7 (i.e. 6700 Ft) of TMC, and the discharge of Tribeni canal is led into power house through the Power Channel and then released back into Tribeni canal through a tailrace channel. Another Power House having an installed capacity of 3 x 5 MW = 15 MW located on right bank of TMC is called as Eastern Gandak Canal HEP (EGC HEP). The power channel 4690.40 ft (i.e. 1430 m) in length of EGCHEP takes off at RD 6.3 (i.e. 6300 ft) on right side of the Tirhut Main Canal and the water from the EGCHEP is discharged back to the TMC through the 13856 ft (4223 m) long Tailrace channel. The Eastern Gandak Canal HEP were commissioned between September 1995 & June 1997.

    Further, the study  is restricted to the Eastern Gandak Cnal HEP(3 x 5 = 15MW) only.


    The contention of the project authorities has been that though the Installed Capacity is 15 MW and adequate water is available, the plant has not been able to produce full power rated at 3x5 MW (15 MW - without over load) when operated simultaneously, and since inception though individually each unit has demonstrated that it can produce 5 MW of power, when two units are operated simultaneously the maximum generation is reported to be not more than 7 MW as against the rated capacity of 10MW though water is let out for 2 unit requirement of 213.90 cum/sec.

    Thus the Bihar Hydroelectric Power Corporation is facing a shortfall in terms of power generation from this power plant since the commissioning of the power plant i.e. over the period 1995 - 97.

    The Problems and deficiencies:

  • At the outset (but once again at the cost of repetition), it is to observe that the achievable generation capacity and the energy from the power plant, is solely dependent on the courtesy extended by the WRD Authority who control the flows on the Tirhut main canal based on the priority requirement coming from Irrigation activities in the command area. As reported and observed too with the irrigation channel also getting heavily silted up the hydraulic conditions of flow does not seem to be at the ideal best levels as envisaged at the planning and design stage of power plant.


  • Further it is to be observed that, the problems from deposition of silt that seem to comprise mainly finer fractions of sand and silt, is unavoidable one. And as such the efforts that can be made to mitigate the adverse effects arising there from, through planned periodic removal for cleaning of the same from the power channel and the effectively needed tail race channel sections of flow, that are under the contributing of BHPC.
  • The impact of the power house tail pool weir crest level, that also influences the downstream water level and also hydraulic conditions of flow also seems to be an another adverse factor contributing to the loss to the generation potential.


    The Problems or the deficiencies as posed to be addressed include:

  • The water releases into the Power channel seems at the best to be and occasionally only adequate for 2 unit requirements but for most period for generation from either single unit or plus part load from second unit. And as of present the achieved generation is limited to about 7 MW instead of 10 MW when 2 units were in operation.
  • When operated individually each unit is capable of generating 5 MW of power (but at what power draft and net head, question remains here) but in tandem, the generation is only 7 MW and not 10 MW.
  • As gathered, the Electro & Mechanical equipment suppliers have confirmed that each unit is capable of generating 5 M.W. of power and that there is no deficiency so far as Electro Mechanical equipments are concerned.
  • As gathered, it is to observe that all the 3 units have not been operated simultaneously since commissioning till to date which seems to be for the reason of lesser flows in the power channel.
  • The impact of raised level at the crest of tail pool vis-à-vis that show in the design drawing (i.e. EL 102.735 m as against EL 102.45 m shown in drawing, and the change of downstream slope from 1 in 5 shown in the drawing to 1 in 50, on tail water rating curves (for a possible incorporation of corrections remedy as needed if any).


    On the basis of above the suggestion made was for an assessment of present state of various civil/hydro mechanical component of power station including their implications on the related hydraulics of flow, so as to arrive at the possible implementable remedial measures to improve the power generators potential.

    Hence, the report prepared is to  address to the examination/ assessments of the entire system on of power plant components, basically from hydraulic design requirements, inadequacies if any, and related implementable improvements.


    The highlights of the report submitted to  client of Design Group i.e. WAPCOS are as below:

    The cited case for the estimation of the magnitude of extra head loss, the studies presented above show that estimated at a very optimistic level an extra head loss that takes place could take place at around 0.60 m or even more due to the accumulated silt deposited in the power channel and the clogging of trash rack at intake and the bridge. Further there could be similar kind of loss that takes place on tailrace side too due to siltation as it could cause either extra back-up effect or increase the slope of water flow profile as the case may be. Though the estimated losses are based on the ideal uniform flow conditions, and in actual case however it could be a non-uniform flow with the extent of non-uniformity dependent on actual flow, variation in bed profile and maintained water level at the power plant upstream, etc., it does not however demerit the fact that for a given flow magnitude of discharge, there occurs an extra head loss with siltation of channel and clogging of trash rack etc., and the loss could assume alarming proportions depending on the severity of problem. This highlights the need to achieve the originally designed canal profile for achieving the minimum head loss in the Water Conductor System.

    It may be seen that as per the data as logged in the register and  provided for the period from 2001-2010 (except for some months in a year) the magnitude of flow (as back worked and shown in logs for ideal theoretical conditions carried in the Power channel has only ranged between from a minimum of 35.368 cumecs (1249 cusecs) to maximum of 123.80 cumecs (4372 cusecs), (except for one instance when operation of two units was attempted to maximize be the generation with estimated flow by back analysis at 7553 cusecs (213.90 cumecs). Here, it is opined that as the estimated discharges obtained from back calculations is ridden with fallacies as it suffers from non-consideration of effects from the efficiency factor as applicable, thus it is important to establish a systematic independent measurements for determination of quantity  of flow of the power draft and the net head both, through appropriate measurement. This can include;

  • Immediate installations for the recording of water levels at the bridge upstream of the power house, immediately on the upstream, downstream and weir point (on both sides) and gauging station for flow measurement around just 50 m downstream of bridge in the power channel together with the record of deposited silt level. The automation of such measurements through effective measurement sensors and data logging system can be considered for a long term implementation if desired so subsequently. 
  • A system of cross check by keeping gauging of the quantities of flow as released from Gandak barrage and as flown in the different branches i.e. Don, Triveni and EGCHEP can also be thought of and planned.  This could facilitate to better the assessment of performance of the plant and its units on a qualitative and quantitative basis.


    Regarding the issue of minimization of adverse effects from additional head losses coming from the siltation in the channel, it is opined that the remedy for the problem lies mainly in recreating the flow capacity of the channel all through and minimization of head loss in the Water Conductor System. On this noting that, as present the operation consisted of two units at the best, it is opined that whereas power channel can be cleared of silt-fully. The clearance in the tail race channel (further downstream of the deposited boulder spur in the tail race channel just downstream of the bridge) to two thirds of the design bed width only, as the measure of economy as well as to restrict the flow conditions to better the improved velocities.

    As the power plant was not running during the visits of the team, there was no opportunity to make any of the observations, hydraulic conditions, vibrations if any, and conditions around the power house including water levels and flow velocities in the channel. However, as far as the operation of the power house is concerned from the silted profile upto trash rack, it is clear that entry conditions/ profile for the water is very badly affected and it leads to secondary disturbing flow conditions and cause additional loss of head.

    This being a canal drop based low head power scheme, it is to stress that every centimeter of net head counts for augmenting the power generation and hence addressing to every centimeter of head loss and its prevention draws utmost concern to be fulfilled as first and foremost requirement.

    The head loss at the trash rack, if gets severely clogged assumes alarming limits. There are two trash racks, one at RD3 and another at intake. Due to this there would be some extra head loss. The trash rack at intake is a must, but another trash rack at RD3 though adds to some extra head loss, it is a desirable installation considering the magnitude of debris loads. The frequent clearance of trash rack would contribute to improvement in the generation levels.

    The silt from power channel and tailrace pool is to be removed every year without fail as part of O&M activity to get maximum output. This will further avoid accumulation of more silt on cumulative basis, which increases the time required to remove the silt.

    Whereas the combined efficiency of Turbine and the Generator may need to be re-confirmed from the manufacturers, it is also to be seen that the water level in the fore-bay or intake pool is maintained at FSL for all levels of flows by closure of gates which is the requisite to maximize the energy generation, but it is opined that this also could alter the flow conditions and marginally influence the efficiency of machines.


    Summarizing, it needs to be appreciated that the silt load/ deposition related problem is an unavoidable problem and the extent and rate of deposition depends on the drawn power draft for generation and the hydraulics of flow as controlled from power plant fore-bay. Thus the remedy to minimize adverse influences is to manage the same by cleaning at periodic intervals say at 1 to 2 years treating it as a routine O&M activity for realization related costs through tariff and it is also opined that there would be improved generation from this exercise, which can compensate the investment made for removal of silt to an extent. The list of recommendations made to improve the performance of power plant includes the following;

  • Subject to practicability and economics, adopt mechanical raking of trash rack at intake structure and to see that clear spacing through rack shall not get choked from whatever debris carried by the water, in any case otherwise this shall be ensured through manual effort.


  • It is seen from the site inspection and discussion that the Power Channel and intake pool, tailrace pool and also tailrace Channel has been silted up heavily and it has become mandatory to remove the silt in entire stretch.
  • The removal of settled silt and in power channel complete and tailrace channel (for two-third bed width) shall be tackled as a routine part of Operation and Maintenance of Power Plant to achieve better generation of energy from the units. The periodicity can be decided based on off season observations. Further the expenditure incurred can be considered as a part of O & M cost for realization through tariff. This exercise can be taken without affecting generation, since the generation logs show no generation for minimum of two months.


  • Regarding the posed question on the desirability and possibility lowering of crest of tail pool weir by about 0.28 m, the examination of this issue based on rating curve studies show that this can result in a relative gain by about 7 to 15 cm, for lowering of crest over the middle two-thirds width, that too only for a nominal depth say 10 cm- 20 cm. But, noting that bed slab of tail pool is a thick probably heavily reinforced slab integrally connected to the guide wall, it is opined that for a practical implementation the disturbing reinforcement is fraught with serious consequences, as such, this step is needs to be examined carefully at site since such an exercise should not jeopardize the integrity of structure in any manner. As such, we have no alternative but to reserve ourselves from offering a clear cut opinion/ recommendations in the absence of detailed information the design aspects at this structure. However, it is opined that in the first instance, removal of nominal top layer (up to cover depth), could be considered/ attempted depending the depth at which the reinforcements are located, along with plastering with epoxy or high performance cement mortar.
  • To initiate actions to maintain recording of water levels at the bridge upstream of the power house, immediately on the upstream, downstream and weir point (on both sides) and install a gauging station for flow measurement around just 50 m downstream of bridge in the power channel together with the record of deposited silt level. The automation of such measurements through effective measurement sensors and data logging system can be considered for a long term implementation if desired so subsequently. 


  • A system of cross check by keeping gauging of the quantities of flow as released from Gandak barrage and as flown in the different branches i.e. Don, Triveni and EGCHEP can also be thought of and planned.  This could facilitate to better the assessment of performance of the plant and its units on a qualitative and quantitative basis.
  • An evaluation of gradation process of sediment from samples collected at five different points along the canal length as well as for its mineralogy could be conducted. This facilitates to correlate the sediment deposit pattern in the TMC, and Power Channel on the upstream of power plant, which could become a guideline in de-silting the Water Conductor System. As general practice of periodic measurement of profile settled sediments could also be undertaken.


  • The Tirhut Main Canal at the confluence of TRC and TMC desirably also needs to be cleared of silt to the bed and sides slopes suitably.  Since, the scope for this falls under the control of WRD Authority, Bihar State, this matter needs to be taken up with the WRD Authority.