minimizing diurnal variation of downstream flow in hydroelectric projects to reduce environmental...

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Journal of Hydro-environment Research 5 (2011) 177e185www.elsevier.com/locate/jher

Research paper

Minimizing diurnal variation of downstream flow in hydroelectric projectsto reduce environmental impact

Maya Rajnarayan Ray, Arup Kumar Sarma*

Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

Received 21 October 2008; revised 1 October 2009; accepted 2 December 2010

Abstract

Hydroelectric projects are generally operated as peaking power plant, particularly during lean period; consequently, diurnal variation of flow inthe downstream of the dam is induced even in a run-of-the-river scheme. Although such deviation from the natural flowmay have significant impactdownstream, it generally goes unnoticed. Lower Subansiri Hydroelectric project, located on the Subansiri River of Assam, India, is one of the majorhydroelectric projects proposed in theNortheastern part of India. Reservoir SimulationModel has been developedwith the objective of assessing theextent of flow variation in the downstream due to operation of the Lower Subansiri Reservoir. Simulation has been carried out using a StandardOperating Policy proposed for the project. The study has revealed that during the peaking hour, the discharge downstream increases about eight timesof the normal flow in the lean period and becomes almost zero during the non-operating period. While the high discharge poses erosion and floodthreats, the low discharge may lead to adverse environmental impacts such as increase in pollutant concentrations. The possibility of adoptingstructural and non-structural measures for minimizing the deviation of flow from its normal condition is investigated and performance of thesemeasures is compared based on seven performance criteria. The comparisons have revealed that the structural measure provide the best solution. Asan alternative, the non-structural measure also promises notable improvement over the baseline standard operation scenario.� 2010 International Association of Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rightsreserved.

Keywords: Hydropower; Diurnal variation; Downstream impact; Reservoir operation; Simulation model

1. Introduction

The technique of generating hydropower by convertingmechanical energy into electrical energy is the oldest techniqueknown to mankind. Traditionally, hydropower is known to bea renewable, non-polluting and environment friendly source ofenergy. However, the hydropower projects can induce significantecological disturbances upstream and downstream of the dam.Although the impact of a dam on its upstream has been well-studied; its downstream impact is still unrecognized, misunder-stood and underestimated. The downstream impact of dams onthe river biota has been documented since early eighties (e.g.Ward and Stanford, 1979; Lillehammer and Saltveit, 1984; Petts,

* Corresponding author. Tel.: þ91 361 258 2409; fax: þ91 361 269 0762.

E-mail address: [email protected] (A.K. Sarma).

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doi:10.1016/j.jher.2010.12.001

1984; Craig and Kemper, 1987). Brookes (1994) classified theseimpacts as first and second order impacts. Petts (1980) observedtwo kinds of impacts; one immediate and obvious and the othergradual and subtle. Richter et al. (1996) proposed the Indicatorsof Hydrologic Alteration (IHA) method based on 32 hydrologicparameters for assessing the degree of hydrologic alterationattributable to human influence within an ecosystem. Theobstruction created by the dam and the removal of sediment fromthe channel by gravel mining result in change of rivermorphology like channel bed and bank, channel incision(downcutting), coarsening of bed material and loss of spawninggravels for salmon and trout (e.g. Kondolf, 1997). Poff et al.(1997) discussed the natural flow regime and various factorsinfluencing its change. Graf (1999) studied the impact of dams onhydrologic condition of USA. Observing the widespread andintensive effects of the dams he concluded that instead ofretirement of the dams, they can be used as tools formitigation of

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178 M.R. Ray, A.K. Sarma / Journal of Hydro-environment Research 5 (2011) 177e185

the hydrologic impacts by changing the operating rules. Theenvironmental, economical and social impacts of dam down-stream have been reported by World Commission on Dam(2000). Pegg et al. (2003) found that flow variability wasmarkedly reduced during the post-alteration period as a probableresult of flow regulation and climatological shifts. Wohl andRathburn (2003) found that the sediment entering the reservoircreates sediment-depleted conditions at downstream leading tochannel adjustment in the form of bank erosion, bed erosion andchannel planform change. The ecological problems caused onriver ecosystem due to hydropower project have been pointed outby Maiolini et al. (2005). Chang et al. (2008) developed theTaiwan ecohydrology indicator system (TEIS) to identify andunderstand the hydrologic statistics as a foundation of flowvariability and its relation with the response of riverine ecosys-tems to natural and altered flow regimes. Graf (2006) stated thatthe very large dams on American rivers, on an average, reducethe annual peak discharge by 67%. McCarthy et al. (2008)observed that the hydroelectric dams have an annual effect onthe migratory patterns of catadromous silver eels causing declinein their juvenile recruitment. Experiences have shown thathydroelectric projects, whether designed as a storage scheme ora run-of-the-river scheme, always cause flow scenario to changedownstream due to turbine operation resulting several ecologicaldisturbances.

Change in water regime: Two major changes in the waterregime generally occur with the construction of a reservoir(Rosenberg et al., 1995). First, thewater area above the damwillchange from running water to standing water in nature withassociated changes in hydrologic and ecological processes.Second, diurnal and seasonal variations in the demand for wateror power will cause short-term and long-term variations indischarge. Dams designed tomeet daily toweekly hydroelectricdemands have more variable water levels and flow regimes thanlarge storage reservoirs. Consequently, they can produce higherdisturbance on in-channel and riparian processes and relatedbiota (Nilsson et al., 1997 and Jansson et al., 2000).

Change in sediment regimes: River regulation canmodify thesediment regime of a river through retention of material withinthe reservoir and through modifications of downstream erosionand deposition processes. Scouring of river channel typicallyoccurs immediate downstream of a reservoir, but the patterns ofmorphological change become more complex further down-stream. Changes in the flow and flood regime have implicationson sediment transport capacity of the channel and the sedimentflushing ability of the system during low-flow events. On largealluvial rivers, degradation processes are constrained to the firstfew or tens of kilometers downstream of the point of regulationand a degradation of one to 3 m depth typically occurs withina decade or two of regulation (Church, 1995).

Change in water quality: Water quality can be significantlyaffected by impoundment. Physical, biogeochemical and bio-logical processes occurring within a reservoir can significantlyaffect the temperature and chemical composition of the water;the outflow water quality can be significantly different fromthat of the inflows (Rosenberg et al., 1995). The degreeto which the water quality is affected on a diurnal, seasonal

and/or annual basis depend on factors such as surface tovolume ratio and depth of the reservoir; geology and soilgeochemistry of the surrounding catchments; latitude of thereservoir; rates and magnitude of sedimentation; magnitudeand timing of incoming flows and their residency time; andlevel of biological productivity in the reservoir (St. Louiset al., 2000; World Commission on Dams, 2000).

The present study attempts to visualize the changing flowscenario downstream of the LSHE project due to its operation.The objective is to evolve measures to minimize the deviationof downstream flow from its natural condition to reduce theenvironmental impacts.

2. Study area

The Subansiri River is the largest tributary of the RiverBrahmaputra. Its total length up to its confluence with theBrahmaputra River is 520 km. Fig. 1 shows the river system ofSubansiri along with the location of the dam site. Its drainagearea up to its confluence with the River Brahmaputra is37,000 km2. The river originates from the south of the Po Rompeak (Mount Pororu) at an elevation of 5059 m in the TibetanHimalaya. The average bed slope of the river up to 10 kmdownstream from the dam site is 130 cm/km (0.0013); it isreduced to 24 cm/km (0.00024) near its confluence with Ran-ganadi River, a major tributary of Subansiri. The riverbed up to10 km downstream from the dam is composed of sand, pebblesand boulders. Further downstream, it is mostly composed ofsand. Mean annual flow of the river is 1396 m3/s; the mean peakflow is 3600 m3/s. The river maintains an almost stable course inthe hilly terrain, but in the alluvial plains of Assam it exhibitsboth meandering and braided pattern and changes its coursefrequently. Goswami et al. (1999) documented that the changesduring the 70 year period from 1920 to 1990 are varied, and insome cases chaotic in nature. The river banks are composedmostly of sand, gravel and silt, beyondwhich they are composedexclusively of alluvial silt.

3. Some expected downstream impacts of LowerSubansiri Hydroelectric (LSHE) project

A 2000 MW (250 MW � 8) capacity LSHE project iscurrently under construction on the River Subansiri. This projecthas been designed as a run-of-the-river scheme primarily formeeting the power demand in the peak hours. Though variationin the total daily flow or seasonal flow may not be of muchconcern in a run-of-the-river scheme, the operation of turbine inthis project is expected to induce significant diurnal variation offlow. This variation is more pronounced during the lean period.When the turbines are operated for 4 h to meet peak demand.Mandatory environmental release during non-operational hourshas been fixed as 6 m3/s. An average inflow to the reservoirduring lean period is in the order of 500m3/s. Thus water will bestored in the reservoir for about 20 h to raise the head and to havesufficient water for producing 2000MWof power during the 4 hof turbine operation. During operational hours, the flow releaseddownstream, will be in the order of 3000 m3/s. Thus the diurnal

Fig. 1. Location of proposed hydroelectric project in Lower Subansiri River, Assam, India.

179M.R. Ray, A.K. Sarma / Journal of Hydro-environment Research 5 (2011) 177e185

flow variation will range between 6 m3/s and 3000 m3/s. This inturn will adversely affect the environment creating severalproblems at downstream of the dam.While realization ofmost ofthese impactsmay take several years, some of these impacts maybe experienced immediately after construction and operation ofthe dam. Some of such expected obvious impacts are highlightedbelow.

The dam is located just at upstream of the foot hills. Agri-culture is practiced extensively in the alluvial plain downstreamof the dam. Several villages and thickly populated townships arealso situated within 20 km downstream of the dam. Chemicalsfertilizers such as phosphate (PO4), sulphate (SO4) and nitrate(NO3) used in these agricultural fields contaminate the riverreach as non point source pollutants. The organic dischargesfrom villages and urban centers located downstream alsocontribute to bacterial pollution. People residing in these town-ships and villages utilize the river water for various domesticpurposes including drinking. A study conducted to assess thewater quality status of this river reach has shown that, presentlythe water quality parameters such as phosphate, sulphate andnitrates are well within the permissible limit as per Indian

Table 1

Maximum, minimum and average flow.

Rate Month

Jan Feb Mar Apr May Ju

Maximum m3/s 448.4 509.1 1167.5 1213.7 1744.0 43

Minimum m3/s 234.6 334.5 332.8 577.1 663.94 15

Average m3/s 341.5 421.8 750.1 895.4 1204.0 29

Standard (IS: 10500). Contribution of these pollutants fromupstream of the dam can be considered negligible because of itspristine nature. On the other hand, with the rise of population inthe villages and the townships the contribution of non pointsources pollutants is expected to increase. In the LSHE project, itis proposed to release only 6 m3/s of flow as mandatory envi-ronmental release during the non-operational period. The peak-ing hour for the project being designed as minimum 4 h, the rivercan receive only 6m3/s of flow for amaximumduration of 20 h ina day during the lean period. It is worth mentioning that thenatural average flow during the lean period (from December toMarch) is in the order of 500 m3/s, minimum average being341m3/s in the month of January (Table 1).Moreover, there is nomajor tributary of the Subansiri River within 23 km downstreamfrom the dam. Thus, with reduction of flow rate due to reservoiroperation, the pollutant concentrations are expected to increaseand there is a high possibility that it exceed water quality stan-dards even if the pollutant load remains the same.Amass balanceanalysis carried out using water quality and discharge datacollected from the concerned river reach (Fig. 2) has shown anestimated variation of different pollutant concentrations with

n Jul Aug Sep Oct Nov Dec

25.9 4558.9 3411.3 4141.6 2640.4 841.68 564.08

07.2 2583.4 969.5 1576.1 690.8 385.3 263.5

16.5 3571.1 2190.4 2858.8 1665.6 613.5 413.8

0

5

10

15

20

25

30

35

40

45

50

0 200 400 600 800 1000 1200 1400

Streamflow (m3/sec)

NO

3 an

d P

O4

Con

cent

rati

on

(mg/

l)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

SO4

Con

cent

rati

on (

mg/

l)

Concentration of Nitrate Allowable Nitrate ConcentrationConcentration of Phosphate Allowable Phosphate ConcentrationConcentration of Sulphate Allowable Sulphate Concentration

Fig. 2. Estimated downstream pollutant concentrations and threshold levels as function of river flow.


downstream flow rate. It is seen that the phosphate and sulphateconcentrations would exceed the permissible limits.

Another obvious impact of diurnal variation on downstreamof the dam is the loss of valuable agricultural and habitable landdue to increased river bank erosion. Large diurnal variation inthe flow rate will impose a sudden drawdown condition on theriver bank repeatedly. The alluvial bank of Subansiri River,which is quite susceptible to erosion, will thus become morevulnerable. In fact, such dam induced erosion has already beendocumented (Sarma, submitted for publication) in the down-stream of another run-of-the-river hydroelectric project, calledRanganadi River Hydroelectric project, located in a similargeomorphologic set-up as that of the Subansiri River.

Minimization of diurnal variation of flow can be a solution tothe above problems. From environmental point of view, thenatural flow condition prevailing in the stream before construc-tion of dam can be regarded as the most preferred flow condition.Therefore, an effort should be made to have a flow condition asclose as possible to the natural flow, i.e. pre-dam flow condition.This study attempts to reduce the diurnal variation of the flowthrough different structural and non-structural means. Efficiencyof different proposed techniques has been assessed throughsimulation study. It is envisaged that by minimizing such flowvariation in the river, itwill be possible tominimize the ecologicaland environmental disturbances to a reasonable extent.

4. Proposed measures for minimizing diurnal variation

Considering the constraints of practical feasibility andrequirement ofmeeting power demand, the following approacheshave been developed for minimizing diurnal variation of flow.

1. Structural measures: regulating pond at downstream2. Non-structural measures: regulated turbine operation

The concept of the proposed structural measure is quitesimple. However, implementation of this approach requiresa favorable terrain condition and additional capital investment.In this approach, we propose construction of a regulating ponddownstream of the turbine for regulating high discharge

released from the turbine during operational hours. The size ofthis pond will depend on the installation capacity of the projectand amount of regulation envisaged. For example, for theLSHE project it is estimated that a 36 Mm3 capacity storagepond, if created, will be adequate for regulating the turbinedischarge during lean period to achieve almost a natural flowcondition at downstream. Analysis of the terrain has alsoshown that it will be possible to create such storage just atdownstream of the dam without raising the tail water level, i.e.without reducing the net head.

Non-structural measures: the number of turbines to be oper-ated in different time period is regulated as per the following twooptions.

a. Operating one turbine continuously at full capacity andoperating rest of the turbines simultaneously for maximumpossible duration, so that effort can be made to utilize theavailable water in the reservoir for meeting peak powerdemand. This approach is proposed basically to minimizeduration of occurrence of high flow discharge at down-stream and to create a near natural flow condition in thedownstream for a longer duration.

b. Operating one turbine continuously for 24 h andincreasing the number of turbines one by one, so thata turbine once put into operation can be run for maximumpossible hours subject to water availability. By thisapproach high rate of flow at downstream due to simul-taneous operation of all turbines as proposed in ‘option a’can be minimized to a great extent, of course with thecompromise of peaking power.

Overall performances of these options are analyzed throughsimulation study and presented under the result and discussion.

5. Reservoir simulation study

The impact of reservoir operation on flow scenario down-stream of the dam can be best studied by a simulation model.According to Wurbs (1993), simulation is a process of


representing a system with a set of mathematical equations.Several simulationmodels have been discussed in the state of theart review presented by Yeh (1985), Wurbs et al. (1985) andWurbs (1991). Loucks and Beek (2005) presented the capabilityof the simulation of model for reservoir operation. A ReservoirSimulation Model (RSM) has been developed in this study tovisualize some important aspects of the River-Reservoir systemat any time period. General demand oriented reservoir operatingpolicy is considered to simulate the reservoir operation fortwenty years using synthetic streamflow data. Meeting powerdemand of peak hours in the lean period generally becomes anobjective of a hydropower project. This leads to significantdiurnal variation at downstream of the dam, which in turn leadsto serious ecological problems. Similarly, during the monsoonperiod the objectives of producing power and providing flood-cushion are always conflicting in nature. In this study a reservoirsimulation model is developed with an objective to produce thetarget power demand of 2000 MW for the maximum possibleduration in a day subject to the constraints of maintainingreservoir level at a certain elevation so as to have provision offlood cushioning. The scope of minimizing diurnal variation bydifferent alternative means has been analyzed.

5.1. Data used in the development of RSM

2500

3000

3500

4000

F (

Mm

)

InflowTDF

a

Synthetic 10-day streamflows (flow rate in m3/sec averagedover continuous 10-day) of twenty years provided by NationalHydropower Power Corporation (NHPC) have been used asinflow series for the simulation. Mathematical relationship ofstorageeelevation and areaeelevation derived from the curvesupplied by the NHPC has been used to calculate the reservoircapacity and elevation for each time period. Necessary climaticand reservoir related data were obtained from the NHPC.

Reservoir simulation has been carried out to achieve thefollowing objectives: (1) to visualize changes in the flowscenario at the downstream of LSHE project and (2) to estimatepower production and to analyze patterns of power production.

0

500

1000

1500

2000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Year (1-20)

Infl

ow &

TD

t

0

500

1000

1500

2000

2500

3000

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecMonth

Infl

ow (

Mcu

m)

& T

DF

(M

m)

Inflow TDF

b

Fig. 3. (a) Plot of 10-day inflow series and simulated total downstream flow for

20 years (b) 10-day inflow and simulated total downstream flow for first year:

standard operating policy.

5.2. Development of reservoir simulation model (RSM)

5.2.1. Simulation algorithmPower production at any time depends on the available head

and discharge. Releases at any time period is a function ofstorage prior to that time period, inflow and target reservoirelevation at that time period. However, the actual release canonly be made after satisfying the maximum and minimumstorage constraints of the reservoir. The policy for decidingrelease is that (i) Reservoir level will not be drawn downbelow the proposed Minimum Water Level corresponding toa particular time period, provided the desired peak power canbe produced; (ii) In case, the required power during peak hour(here considered as 4 h) cannot be produced without loweringthe reservoir level below the proposed Minimum ReservoirLevel, then the reservoir level will only be drawn down up tothe Minimum Drawdown Level (corresponding to deadstorage level). (iii) Additional power, beyond the requirementof peak hour, will be produced by releasing the available water

that can be released without lowering the reservoir level fromits proposed Minimum Reservoir Level.

At any time period t, available Net HeadHnt (m) is calculatedbased on elevation of the reservoir Elt (m), elevation of thenormal tailrace water level Eltail (m) and head-loss due to fric-tion. Head-loss of the system can influence power generationconsiderably. Frictional head-loss hft (m) has therefore beencomputed in this study dynamically based on the dischargeflowing through the system at that time period. The followingsteps have been followed to decide the release at any time periodt in the proposed RSM;

1. Minimum desired release and Maximum possible releaseare computed by Eq. (1) and Eq. (2) respectively

Rtd ¼ Qdt � 36� pk3

ð1Þ
10
Rtm ¼ Qdt � 36� 24

103ð2Þ

where, Rtd ¼ Minimum desired release (Mm3),Qdt ¼ Actual Discharge (m3/s), pk ¼ Peaking hours (h),Rtm ¼ Maximum possible release (Mm3).

2. Actual turbine release and spill are calculated based on theconstraints of

a. Proposed desired reservoir elevation (maximum) atthat time period

Rta ¼ St�1 þ It � KEl � Et � Rm ð3ÞSp ¼ St�1 þ It � KEl � Rta � Et � Rm ð4Þ

where, Rta ¼ available release (Mm3), St�1 ¼ storage of thereservoir at beginning of time period t (Mm3), It ¼ inflow

0

100000

200000

300000

400000

500000

600000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Year (1-20)

Pow

er

Poh

0

100000

200000

300000

400000

500000

600000


Pow

er

Pohb

a

Fig. 4. (a) Simulated power production series for 20 years (b) Simulated

10-day power production for first year: standard operating policy.

0

500

1000

1500

2000

2500

3000

3500

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Dis

char

ge (

m3 /s

ec)

0

5

10

15

20

25

30

Dur

atio

n (h

our)

InflowMaximum Qd after barrierMinimum Qd after barrierDuration of maximum flowDuration of minimum flow

Fig. 6. Simulated 10-days maximum flow, minimum flow and inflow and their

duration for first year: structural measure.


(Mm3), KEl ¼ capacity of reservoir (Mm3), Et ¼ evaporation(Mm3), Rm ¼ minimum mandatory release provided (Mm3),Sp ¼ Spill from the reservoir (Mm3).

b. Minimum drawdown level

Rta ¼ St�1 þ It � Sd � Et � Rm ð5Þwhere, Sd ¼ dead storage (Mm3).

3. Storage at the end of the time period t is calculated basedon the release computed in step 2

St ¼ St�1 þ It �Rta � Sp �Et �Rm ð6Þwhere St ¼ storage at end of the time period t (Mm3).

4. New reservoir elevation Elnt (m) is calculated using stor-ageeelevation relationship.

5. Net head is then calculated taking the average of initialelevation Elt at beginning of time period t and the new

0

500

1000

1500

2000

2500

3000

3500

Jan Feb Mar Apr May Jun Jul

Month

Dis

char

ge (

m3 /s

ec)

Inflow Rate of

Duration of maximum flow Duratio

Fig. 5. Comparison of maximum and minimum downstream

elevation Elnt at end of the time period t, and the entirecalculation from step 1 is repeated to obtain the actualdischargeby iteration (convergence threshold of 0.0001m3/s).

6. Water balance is also checked in all these computation toensure computational accuracy.

7. Total downstream flow, TDF (Mm3) is computed as;

TDF ¼ Rta þ Sp þ Rm ð7Þ8. After obtaining the actual discharge in step 5, the hours of

operation of the turbine, power generated through the turbinePo in (MW), Total power generated per hour Poh in (kW-h)and total downstream flow TDF (Mm3) are calculated.

5.3. Simulation results

RSM is used to study three different cases: (I) Reservoiroperation with demand oriented standard operating policy, (II)Reservoir operation with structural measures to reduce diurnalvariations, (III) Reservoir operation to reduce diurnal varia-tions through non-structural measures.

5.3.1. Case I: demand oriented standard operating policyThe reservoir is operated with an objective of generating

target power of 2000 MW for maximum possible hours

Aug Sep Oct Nov Dec0

5

10

15

20

25

30

Dur

atio

n (h

our)

TDF Rate of minimum flow

n of minimum flow

flow with the inflow on 10-day basis and their duration.

0

5

10

15

20

25

30


Month

Pea

king

hou

rs

Without constriant of oneturbine

One turbine runningcontinuously for 24 hours

Fig. 7. Peaking hour maximum downstream flow and natural inflow rate with

one turbine running continuously.

0

500

1000

1500

2000

2500

3000

3500


Dis

char

ge (

m3 /s

ec)

0

5

10

15

20

25

Dur

atio

n (h

our)

InflowMinimum Flow rateDuration of minimum flow

Fig. 9. Minimum downstream flows, its duration and inflow rate on 10-day

basis: operational measure III-A.


without violating the given constraints. Synthetic 10-daystreamflow series of 20 years has been used for this purpose.Output of the RSM has revealed that little variation will occurin the ten daily total volume of flow released from the dam ascompared to the total flow volume without the dam. Fig. 3(a)shows the plot of inflow series and TDF series for 20 years. Aclose-up view of the simulated flow for the first year is pre-sented in Fig. 3(b). Fig. 3(b) depicts that the natural inflow inten days and the ten daily TDF after construction of dam are ofthe same order. Visible variation has been observed only in themonth of MayeJune, as the reservoir is drawn down duringthat period to keep sufficient space for the accommodation ofhigh discharge during the monsoon period, which will alsoprovide opportunity of moderating a high flood of smallduration.

Fig. 4(a) shows the power production series on ten dailybases. The Fig. 4(b) presents the close view of total powerproduction through the first year. It can be observed from thesefigures that the power production per hour is very high in thewet season. Power production remains high from the month ofMay to October. It reaches its peak in the month of June andJuly and more or less it follows the similar pattern to that ofinflow, as power is directly proportional to inflow and head inthe reservoir. Power production is less in the dry season but issufficient to generate full capacity (2000 MW) for a minimumof 4 h in a day on an average basis.

Although total flow volume in ten days remains more orless same before and after construction of the dam, the rate offlow downstream can vary significantly. Fig. 5 shows the plot

0

500

1000

1500

2000

2500

3000

3500


Month

Dis

char

ge (

m3 /s

ec)

0

5

10

15

20

25

30

Dur

atio

n (h

our)

InflowRate of TDFPk hr

Fig. 8. Inflow, maximum flow and its duration for the: operational measure

III-A.

of inflow, turbine discharge, rate of total downstream flow,minimum flow rate at downstream and duration (h) of turbinedischarge against different ten daily period. From this figure itis clear that the difference inflow rate is quite significant in thelean period, where the maximum outflow rate becomes3007 m3/s and minimum flow rate becomes 6 m3/s, while theaverage inflow rate in the lean period in natural situation is inthe order of 300e500 m3/s.

5.3.2. Case II: policy considering structural measures toreduce diurnal variations

The effect of introducing a small detention pond in down-stream of the reservoir was investigated to explore the possi-bility of minimizing diurnal variation and to analyze itsperformance. The simulation study has shown that diurnalvariation of flow can be significantly minimized by creatinga downstream storage of 36 Mm3. Fig. 6 displays the curvesshowing the ranges within which the downstream flow willvary if a 5 m high barrier covering the entire width at 4 kmdownstream of the dam is placed along with gated marginalembankment. This will allow the flow contribution of smallstreams during the lean period. The provisions for regularremoval of sediment have to be made from this small storagepond to maintain its storage capacity. However, for creatingstorage without raising the tail water level, it is necessary tohave a convenient location downstream, an aspect that shouldbe further investigated at planning stage.

0

500

1000

1500

2000

2500

3000

3500


Month

Dis

char

ge (

m3 /s

ec)

Inflow

Maximum rate of flow

Minimum Rate of Flow

Fig. 10. Inflows, maximum and minimum turbine discharge on 10-day basis:

operational measure III-B.

Table 2

Performance criteria for the different proposed measures.

Performance Criteria Case I (without

any measures)

Case II (structural

measures)

Case III-(A) (operational

measure e A)

Case III-(B) (operational

measure e B)

Average annual power production (MUa) 8331.86 8331.86 7450.17 6924.82

Minimum peaking hour per day

(producing 2000 MW)

4.00 4.00 1.14 0.00

Minimum downstream flow (m3/sec) 6.00 359.85 313.85 307.85

Maximum duration of minimum

downstream flow(m3/s)

20.00 20.00 22.86 16.00

Probability(%) of failure to meet the

4 h peaking requirement

0.00 0.00 41.94 77.56

Maximum power deficit during

peaking hour (kW-h)

0 0 5714 7000

% of maximum power deficit

during peaking hour

0 0 71.43 87.50

a One Million Unit (MU) ¼ 106 kW-h.


5.4. Case III: policy considering non-structuralmeasures to reduce diurnal variations

5.4.1. Case III-(A): operating one turbine continuously andoperating rest of the turbines simultaneously

Another option of providing minimum flow of around300 m3/s in the lean period is to produce 250 MW powercontinuously by running one turbine for the entire day and toproduce 1750 MW by running seven turbines simultaneouslyfor the maximum possible period depending on water avail-ability. This will ensure a minimum flow 314 m3/s throughoutthe day in the lean period with a maximum flow of 3013 m3/sfor the peaking duration, which will not be less than 2 h perday. Fig. 7 shows the variation of peaking hour with andwithout the constraint of providing a minimum flow of314 m3/s in the downstream. Fig. 8 shows the plot of peakinghour, maximum downstream flow and natural inflow ratepolicy III-(A). Fig. 9 shows the plot of minimum downstreamflow its duration in a day and the natural inflow rate.

5.4.2. Case III-(B): operating one turbine continuously andincreasing the number of turbines one by one

Another option of utilizing the available water by runningone turbine continuously and then increasing the number ofturbines one by one for maximum possible hours of operationfor each of the additional turbines. This will restrict themaximum rate of flow, of course with a compromise in peakingpower. Fig. 10 shows the maximum and minimum rate of flowthrough turbines along with the plot of natural inflow.

6. Discussion and recommendations

A reservoir simulation study has revealed that with standardoperating policy without the mitigation measures, the down-stream will be subjected to a diurnal variation ranging from6 m3/s to 3008 m3/s in lean period and average annual powerproduction will be 8332 MU (1 MU ¼ 106 kW-h) withminimum peaking hours of 4 h per day. With the adoption ofa structural measure, the range of diurnal variation of flow willbe reduced to 360e1034 m3/s in lean period. It will be

possible to achieve this without compromising the averageannual power production or minimum peaking hour per day.Adoption of non-structural measures has also shown encour-aging results: (i) By running one turbine continuously andproducing peaking power with the remaining water, thediurnal variation will range from 314 to 3013 m3/s in leanperiod and annual average power will be 7450 MU, while theminimum peaking hour will be reduced to 2 h (iii) With thesecond proposed modified operating policy i.e. by addingturbine one by one, the diurnal variation will range from 308to 908 m3/s in lean period. Average annual power will be6925 MU, but in a day minimum peaking hour, for which theplant will produce 2000 MW of power will be reduced to 0 hin lean period. A comparative performance of these mitigationmeasures is presented in the Table 2. Comparison has beendrawn on the basis of seven performance criteria: averageannual power production, minimum peaking hour per day(producing 2000 MW), minimum downstream flow, maximumduration of minimum downstream flow, probability of failureto meet the 4 h peaking requirement, maximum deficit duringpeaking hour and percentage of maximum power deficitduring the peaking hour.

Table 2 shows clearly that the proposed structural measure(Case II) can be considered the best mitigation measure, asminimum downstream flow of 360m3/s can be provided withoutcompromising power production. Adoption of this measure willrequire a favorable site condition and additional initial invest-ment. Out of the two operational measures, Case III-A is betterand can be adopted wherever practical implementation ofstructural measure is not possible. This method will provideminimum streamflow 314 m3/s, but with 41.94% probability ofnot being able to meet the minimum peaking power demand of4 h. However, even under these conditions the project will beable to provide peaking power demand for at least 1.14 h.Operationalmeasure III-B cannot be recommended, as it will notbe possible to meet the peaking power demand for 77.56% of thetime. It is also important to note that for some of the unsuccessfulcases the project will not be able to generate its full installedcapacity i.e. 2000 MW for even an hour (Table 2). Total annualpower generation will also be reduced to 6925 MU.


7. Conclusions

A reservoir simulation model has been developed to inves-tigate the augmented flow scenario downstream of a hydroelec-tric project, and the necessary corrective measures to minimizedownstream environmental impacts. The model has beenapplied to the Lower Subansiri Hydroelectric project in Assam,India. The simulation study shows the power productionschedule i.e. with peaking hour ofminimum4h, the downstreamflow will be changed with a diurnal variation between 6 and3007m3/s in the lean periode as comparedwith a natural flow inorder of 340 m3/s. Such diurnal variation in the streamflow willhave adverse ecological effects. During the flood period, diurnalvariation induced by the reservoir operation is not that signifi-cant and thus will not create any adverse effect. The scope ofminimizing diurnal variation through structural and non-struc-tural mitigation measures has been investigated through simu-lation study. The reduction in diurnal flow variation can beeffected by introducing a regulating (balancing) pond down-stream of the dam (structural measure); the non-structuralmeasures try to achieve minimum diurnal variation by modi-fying the operating schedule. Performances of these mitigationmeasures have been compared on the basis of seven differentperformance criteria. Comparisons have revealed that structuralmeasures provide the best solution. As an alternative, bychanging the operational schedule of the hydroelectric powerproduction (non-structural measure III-A) will also providenotable improvements over the baseline standard operationscenario.

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Notations

The following symbols are used in this paper

Hnt: available net head (m) at any time t;

Elt: elevation of reservoir (m);

Eltail: elevation of normal tailrace water (m);

hft: frictional head-loss (m);

Qdt: actual discharge (m3/s);

Rtd: minimum desired release (Mm3);

pk: peaking hours (h);

Rtm: maximum possible release (Mm3);

St�1: storage of the reservoir (Mm3);

It: inflow (Mm3);

KEl: capacity of reservoir (Mm3);

Et: evaporation from the reservoir (Mm3);

Rm: minimum release provided (Mm3);

Sd: dead storage (Mm3);

St: final storage (Mm3) at the end of the time period t;

Sp: spill from the reservoir (Mm3);

Elnt: New reservoir elevation (m);

Po: power generated through the turbine (MW);

Poh: total power generated per hour (kW-h);

TDF: total downstream flow (Mm3);

MU: million unit (One MU ¼ 106 kW-h);

MW: mega watt.

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