Ch 4 Hydrology

Shiva Shree Hydropower Pvt. Ltd. Upper Balephi – A Hydroelectric Project____________________________________________________________________________________

4.0 Hydrological Study

4.1 Catchment area of the basin

Balephi Khola is a mostly snow fed perennial river (partially rain fed) with catchment

area at the upstream of the proposed intake sites varying from 430 sqkm to 452

sqkm.

The Balephi Khola Basin upstream of the proposed headworks is located in

Sindhupalchowk District of the Central Development Region. The altitude in this

basin varies from 6966 masl at peak of the Jugal Himal at the north to 1045masl at

the powerhouse site. Further, the catchment area at intake is classified as follows:

Above 5000 m 85.44 km²

Between 3000m and 5000m 252.18 km²

Below 3000 m 96.65 km²

It is evident from the above figures that approximately 20 percent of the basin

catchment lies in the permanent snow cover area, about 60 percent of the basin

catchment lies in the seasonal snow cover area and nearly 20 percent of the basin

lies purely in the rain fed area. Having snow cover area of 80 percent is considered

to be very good and as a result of this Balephi river gives a good perennial flow

during the dry season. The catchment area of this basin is shown in figure 4-5 at the

end of this chapter.

4.2 Precipitation study

There are no meteorological records available for Balephi Khola basin. According to

the ‘Climatological Records of Nepal (GoN, DHM), the nearest rainfall station are

Barhabise (Index No. 1027), Gumthang (Index No. 1006), Dhap (Index No. 1025) and

Sarmathang (Index No. 1016). Moreover, few other stations are located around the

catchment basin but at relatively long distance. The details of these stations are

shown in the table 4-1.

There is no rainfall data available for this Balephi basin. There is not even any

precipitation record available above the latitude of the proposed headworks site.

Thus, the average annual basin precipitation has been estimated by constructing

Isohyetal maps prepared by the Department of Hydrology and Meteorology (Table 4-

2 and 4-3).

Table 4-1. Precipitation Stations around Balephi Basin

Station No. Annual Precipitation , mm Location

___________________________________________________________________________________________________________Feasibility Study 4-1


1006 3861 Gumthang

1008 2440 Nawalpur

1009 2037 Chautara

1016 3817 Sarmathang

1017 2409 Dubachaur

1018 1793 Bahunepati

1025 2581 Dhap

1027 2875 Barhabise

Table 4-2. Annual Average Precipitation above Intake by Isohyetal Map

Between isohyets Mean, mmArea

(sq.km)Mean ppt x

area

below 2500 2450 74.40 182277.552500-2600 2550 105.52 269078.552600-2700 2650 98.27 260426.102700-2800 2750 75.91 208758.002800-2900 2850 53.26 151788.15above 2900 2950 26.90 79346.15

434.26 1151674.50

Mean precipitation of catchment at intake, mm

2652.03

Similarly, the average annual precipitation in the centroid of basin above Jalbire

station is computed as below (Table 4-3).

Table 4-3. Annual Average Precipitation above Jalbire Station by Isohyetal Map

Between isohyetsMean, mm Area (sq.km)

Mean ppt x area

below 2500 2450 74.40 182277.55below 2500 2450 26.34 64533.002500-2600 2550 105.52 269078.552500-2600 2550 19.72 50296.202600-2700 2650 125.57 332763.152700-2800 2750 114.57 315056.502800-2900 2850 80.46 229322.402900-3000 2950 44.20 130381.15

3000 3000 9.18 27525.00599.96 1601233.50

mean precipitation of catchment at Jalbire station, mm

2668.91

Since the Isohyetal map gives fairly correct value, the annual average rainfall in the

Balephi basin is taken as 2652mm.



Similarly, the mean monsoon precipitation at the centroid of the basin above the

proposed intake is computed as 2252.47mm and that of Jalbire station is 2266.7mm,

the isohyetal maps used for computation are shown in figures 4-6 and 4-7 at the end

of this chapter.

4.3 Reference Hydrology

4.3.1 Stream Flow Data

Balephi River is well gauged river with long term discharge measurement. The

Department of Hydrology and Meteorology has been recording the discharge data

since 1964 with well prepared daily data since 1986. The gauging station is

established in Jalbire across the Balephi Khola. Table 4-4 lists the summary of this

catchment.

Table 4-4. Hydrological Station Data

St.N.

River name and Location

Elevation (m)

Lat/Long

Years of Records

Annual Mean Runoff (m3/s)

Drainage Area (km2)

620 Balephi Khola, Jalbire

NA 27 48 2085 46 10

1986-2006 34.69 600

(Source: DHM, Hydrological Estimations in Nepal)

The daily discharge data of this station has been available from the year 1986 only.

Though there are records of mean monthly discharge data from the year 1964 till

the year 1985, this data has not been analyzed to compute the probability of

exceedance of flow.

4.3.2 Computation of Stream Flow for Balephi River at Intake

The precipitation catchment area ratio method is used to estimate flow in the

Balephi River above the proposed intake with reference to the station 620. The

formula used is

where,

P = Average Annual Precipitation (mm)

A = Basin Area (km2)

Q = River Discharge (m3/s)

The long term mean monthly flow for Balephi River is computed and presented in

the Table 4-5. The details of computation are shown in the annex.

Table 4-5. Long-term Monthly Average Flow by Precipitation Catchment Area Ratio



MonthBalephi River at Jalbire(C.Area = 599.97 km2)

(Annual ppt = 2669mm)

Balephi River at Intake(C.Area = 434.27 km2)

(Annual ppt = 2652mm)January 14.08 10.12February 12.31 8.85March 11.90 8.56April 13.47 9.69May 20.50 14.74June 56.37 40.54July 135.14 97.20August 167.70 121.02September 110.87 79.74October 51.31 37.32November 26.79 19.27December 18.49 13.44Annual 53.24 38.37

Flow Hydrograph at Intake

10.12 8.85 8.56 9.6914.74

40.54

97.20

121.02

79.74

37.32

19.27

13.44

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

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

Months

Dis

char

ge,

m3/

s

Figure 4-1. Long term annual hydrograph of Balephi River computed at the Intake

4.3.3 Available flow for Power Generation

There are no irrigation fields at the downstream of the proposed weir that utilize

water from the Balephi River. Therefore, there is no requirement for reduction of

river discharge. However, provision has been made as 10% of the minimum monthly

discharge for downstream flow. The downstream release will be required particularly



during the months from December to May when the river has the lowest flow in the

month of March. Hence, a downstream release of 0.81m3/s has been provisioned.

The available flow at the intake has been shown in the table 4-6.

Table 4-6. Available flow at the Intake

Month River flow (m3/s) D/S Release (m3/s)

Available flow for diversion (m3/s)

Jan 10.12 0.856 9.26Feb 8.85 0.856 7.99Mar 8.56 0.856 7.70Apr 9.69 0.856 8.83May 14.74 0.856 13.88Jun 40.54 0.856 39.68Jul 97.20 0.856 96.34

Aug 121.02 0.856 120.16Sep 79.74 0.856 78.88Oct 37.32 0.856 36.46Nov 19.27 0.856 18.41Dec 13.44 0.856 12.58

Annual 38.37 0.856 37.52

4.3.4 Flow Duration Curve

The flow duration curve for the Balephi River has been derived based on the daily

average discharge computed using catchment area ratio method. The flow duration

curve is shown in Figure 4-2. Various discharges corresponding to probability of

exceedance are shown in Table 4-7.



Flow Duration Curve at Intake

-

50

100

150

200

250

300

350

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Probability of Exceedance (%)

Dis

char

ge

(m3/

s)

Fig 4-2. Flow Duration Curve, based on mean monthly flow

Table 4-7. River discharge at various level of probability of exceedance (m3/s)

S.N.Probability of exceedance

(%)Discharge

(m3/s)1 0% 374.002 5% 128.023 10% 102.134 15% 86.315 20% 74.086 25% 58.697 30% 43.158 35% 30.209 40% 23.3010 45% 19.2811 50% 16.6912 55% 14.5313 60% 12.8714 65% 11.6515 70% 10.5716 75% 9.7817 80% 9.0818 85% 8.3419 90% 7.7020 95% 6.9921 100% 3.23



4.4 Flood Hydrology

4.4.1 Flood Data

The flood in the Balephi River has been estimated using catchment area ratio

method with historical instantaneous flood in Jalbire multiplying by the adjustment

factors corresponding to catchment area and basin precipitation. Table 4-8 shows

generated instantaneous flood in the Balephi River at proposed headworks.

Table 4-8. Generated instantaneous flood in Balephi River

Year of Record

Maximum Instantaneous

Flood Flow (cumecs)

Year of Record

Maximum Instantaneous

Flood Flow (cumecs)

1964 159.00 1986 474.801965 618.60 1987 446.001966 305.70 1988 601.401967 247.40 1989 417.201968 453.20 1990 417.201969 251.80 1991 223.001970 1021.40 1992 151.101971 647.40 1993 719.301972 647.40 1994 161.801973 305.70 1995 317.901974 877.60 1996 266.201975 561.10 1997 446.001976 201.40 1998 402.801977 561.10 1999 532.301978 431.60 2000 359.701979 197.80 2001 287.701980 1021.40 2002 266.201981 374.00 2003 205.001982 1510.60 2004 259.001983 704.90 2005 423.001984 670.40 2006 227.301985 693.40

4.4.2 Flood Frequency Analysis of Historical Flood

The flood in Balephi River has been estimated using Flood frequency analysis of the

generated instantaneous flood data. Flood frequency analysis has been conducted

for the generated instantaneous flood data for Balephi River at Jalbire. The

distributions used are Normal, 2 Parameter Log Normal, 3 Parameter Log Normal,

Gumbel, Log Pearson III and Pearson III. The best fit distribution is Log Pearson III

method that has smallest root mean square distribution error among all other

distributions. Table 4-9 summarizes results of the analysis.

Table 4-9. Result of Flood Frequency Analysis



Probability

Return Period

(yr) 3-PLN

Gumbel

Extremal

Log Pearso

n IIIPearso

n III 2-PLN Normal0.995 200 1562 1460 1890 1656 1644 11770.990 100 1383 1315 1606 1458 1434 11080.980 50 1210 1169 1348 1263 1236 10330.960 25 1041 1022 1113 1070 1047 9490.950 20 987 974 1042 1009 988 9200.900 10 821 823 834 820 810 8200.800 5 652 666 640 633 637 6990.667 3 523 542 504 496 509 5860.500 2 411 429 395 386 402 467

Standard Error 57.9 66.3 42.0 53.9 55.4 104.7

4.4.3 Design Flood

Flood data given by the Log Pearson III distribution has lowest standard error and is

therefore chosen for design of the structures. The 1 in 100 year and 1 in 200 year

floods at the intake are 1606 m3/s and 1890 m3/s, respectively.

4.5 Dry Season flow

For construction works, it has been assumed that the dry season will be from

November to April. The dry season flood in Balephi River at headworks has been

estimated using catchment area precipitation ratio method with historical maximum

daily average flow for each year multiplying by the adjustment factors corresponding

to catchment area and the basin precipitation. Table 4-10 shows computed dry

season flood in Balephi River by various methods.

Table 4-10. Dry Season Flood Frequency Analysis

Probability

Return Period

(yr) 3-PLN

Gumbel

Extremal

Log Pearso

n IIIPearso

n III2-

PLN Normal0.995 200 67.93 68.98 67.71 75.49 64.16 52.860.990 100 61.04 62.83 60.51 66.36 58.59 50.340.980 50 54.41 56.66 53.72 57.57 53.05 47.580.960 25 47.98 50.45 47.28 49.17 47.51 44.520.950 20 45.95 48.43 45.27 46.56 45.71 43.450.900 10 39.7 42.07 39.17 38.75 40.04 39.780.800 5 33.47 35.44 33.16 31.5 34.11 35.330.667 3 28.73 30.18 28.64 26.62 29.38 31.190.500 2 24.67 25.43 24.76 23.13 25.11 26.83

Standard Error 3.59 3.82 3.7 3.27 3.87 5.17

The result shows that there is no major variation in computed flow particularly for

the return periods of 1 in 5 to 1 in 20. Therefore, construction flood of 45.27m3/s with

1 in 20 year return period has been estimated.



4.6 HEC-RAS Analysis

The water surface profiles were generated by using the HEC- RAS model, developed

by the US Army Corp of Engineers, Hydrologic Engineering Center. It is a fixed bed

model and calculates the water surface level at each cross section for a given

discharge and Manning’s coefficient.

The analyses were performed at intake and the powerhouse site for different values

of discharge and rating curves were generated at the weir axis (Figure 4-3) and

tailrace outlet (Figure 4-4).

The main purpose of this analysis was to find out the water surface elevation at

various discharges and most importantly, to see if the designed structures could

safely pass the design flood without causing flood induced damage.

For analysis at the intake site, the river cross sections along with the designed

structures were taken into account. As for the discharge, the probable maximum

floods at 100 and 200 years return period were used. Manning’s roughness

coefficients were taken as per the site conditions and steady state analysis was

performed.

The details of the output of this analysis have been presented in the annex.

Rating Curve at Weir Axis

1242

1244

1246

1248

1250

1252

1254

1256

1258

0 250 500 750 1000 1250 1500 1750 2000

Discharge (m3/sec)

Ele

vati

on

(m

)

Figure 4-3. Rating curve for water surface at the weir axis based on HEC-RAS



Rating curve at Tailrace Outlet

1036

1038

1040

1042

1044

1046

0 500 1000 1500 2000 2500

Discharge(m3/s)

Ele

va

tio

n(m

)

Figure 4-4. Rating curve for water surface at tailrace outlet based on HEC-RAS

Similarly, for powerhouse site the same procedure was repeated. The water surface

elevation at the tailrace outlet was determined from the result. The summary of the

results are given in the annex.

4.7 Basin Sediment Study

The sediment yields are significantly high in the Himalayan Rivers due to steep

river gradient, valley slope failures and erosion and intense rainfall. Sediment

transported by Balephi River consists of bed load consisting large boulders and

cobbles, suspended particles consisting of fine sand, silt and clay particles. The

sediment transport characteristics and rates are not available for Balephi River.

Hence, necessary information about the characteristics of the suspended

sediment load and its seasonal distribution required for the study are estimated

based on Himalayan yield technique and reports on the regional studies.

4.7.1 Himalayan Yield Technique

In this method, the catchment area is divided into various parts depending on

geological conditions, rainfall and slope of the catchment area. The yield from

the high Himalaya, high Mountains and middle Mountains is considered as 500

tons/km2, 2500 tons/km2 and 5000 tons/km2, respectively.



The catchment area of the Balephi River at intake site is 434.27 km2. The high

Himalayas above 5000m has a catchment area of 84.44 km2, the high Mountains

between 3000m to 5000m has a catchment area of 252.18 km2 and the middle

Mountains below 3000m has a catchment area 96.65 km2. Hence, the

corresponding rate of the sediment yield at the intake results with an

approximate figure of 1.16 million tons per annum. This corresponds to a mean

annual daily monsoon concentration of about 1231 ppm (parts per million by

weight) at the proposed intake using the mean annual monsoon discharge of 71

m3/s. If 75% of the total concentration is considered to be suspended sediment

then the mean annual daily monsoon concentration will be about 930 ppm,

which is fairly low in regards with Himalayan Rivers.

4.7.2 Regional Studies

Referring the feasibility report of Bhotekoshi Hydroelectric Project (NEA, 1994) an

average yield of about 10.7 million m3 per year is recommended for the

headworks site with a catchment area of 2132 km2. This gives approximately

5020 tons per square kilometers of sediment feed at Bhotekoshi intake.

Adopting the same sediment yield rate as that of Bhotekoshi for Balephi river,

the total annual sediment transport for Balephi River above proposed intake will

be 2.18 million ton/year. This figure is almost two fold than the figure derived

from Himalayan yield technique (1.16 million tons per year). The sediment

concentration based on the Bhotekosi corresponds to a mean annual daily

monsoon concentration of about 2320 ppm at proposed intake site using the

mean annual monsoon discharge of 71 m3/s. If 75% of the total concentration is

considered to be suspended sediment then the mean annual daily monsoon

concentration will be about 1740 ppm, which is may be regarded as logical figure

for a river like Balephi.

4.7.3 Regional Method (Sharma and Kansakar, 1992)

The regional method developed by K.P Sharma and S.R Kansakar (1992) has also

been used to compute the sediment transport.

This method is based on the sediment data measured from 12 river catchments

of Nepal. Based on the regression studies Sharma and Kansakar (1992) proposed

following formula.

Asy = -2.20992 + 0.05439 Arock0.5 + 0.0748 A2

0.5 + 0.05097 MWI0.5

Where,

A2 = catchment area below 2000 m



MWI = Monsoon wetness index

Arock = catchment area above 2000m and below 5000m

Asy = Total suspended sediment yield (million tons/ year)

The parameters given above will have following figures for upper Balephi-A

Intake:

A2 = 15 km2

MWI = 2652 mm

Arock = 334 km2

The above inputs give annual suspended sediment yield of 1.7 million tons per

year at proposed intake site. When the bed load is assumed to be 25% of the

suspended load as is considered a general case, the total annual sediment yield

is estimated to be 2.1 million tons/year. This corresponds to mean annual daily

monsoon concentration of about 2265 ppm at the proposed intake site. In this

case the mean annual daily monsoon suspended sediment concentration will be

about 1700 ppm, which is fairly close to the value derived based on Bhotekosi

Intake.

4.7.4 Discussions

The proposed intake for Upper Balephi-A Project is located at an elevation 1252

masl and the project area lies upstream of the MCT fault. Most of the rocks

upstream of this intake belong to higher Himalayan Crystalline group. The valley

slopes are therefore considered to be fairly stable. In this respect the average

annual monsoon suspended sediment yield is not expected to exceed 2000 ppm.

However, during extreme rain fall event the mass wasting along the valley slope

is expected to occur and such events will bring excessive sediment yield. As a

result, during monsoon period there are extreme flood events that brings

considerable amount of suspended sediments and it may be as high as up to

10,000 ppm. Hence, the settling basins are designed to sustain this extreme

event.


Ch 4 Hydrology

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