144 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Appendix A - Flow estimation
A.1 WECS/Department of Hydrology and
Meteorology (DHM) method
A.1.1 PROCEDURE FOR ESTIMATING INSTANTANEOUS
FLOOD PEAK
1. From available topographic maps, find out the catchment
area (km2) below 3000 m elevation.
2. In the following equation, input coefficients from Table Al.
Q0 = α(Area below 3000m + 1) m3/s where subscript a
is either 2 year or 100 year return period.
TABLE A1 PREDICTION COEFFICIENTS FOR
INSTANTANEOUS FLOOD FLOWS
RETURN
(YEARS)
2
100
PERIOD CONSTANT
COEFFICIENT (α )
1.8767
14.630
POWER (β )
0.8783
0.7342
3. Flood peak discharge, QF , for any other return period can
be calculated using:
Q = e δ = e δF
(InQ2 + s. InQF)
(InQ 2+ s.
)
InQF
where S is the standard normal variate for the chosen
return period, from Table A2, and
δ InQF = IN(Q100 / Q2) / 2.326
TABLE A2 Values of standard normal variate for various
return periods
RETURN PERIOD (T) (YRS) STANDARD NORMAL VARIATE (S)
20
5 0.842
10 1.282
20 1.645
50 2.054
100 2.326
200 2.576
500 2.878
1000
3.090
5000
3.540
10000
3.719
A.1.2 PROCEDURE FOR ESTIMATING FLOW DURATION
CURVE
1. From available topographic maps, find out the catchment
area below 5000 m elevation.
2. Use the following equations to calculate the flows. The
values of monsoon wetness index can be read from Figure
A3. Q% is the dischagre(m3/s) for the specific probability of
exceedence.
In Q0% = -3.5346 + 0.9398. In (Area below 5000 m + 1)
+ 0.3739. In (Monsoon wetness index)
In Q5% = -3.4978 + 0.9814. In (Area below 5000 m + 1)
+ 0.2670 In (Monsoon wetness index)
In Q20% = -5.4357 + 0.9824. In (Area of basin) + 0.4408.
In (Monsoon wetness index)
In Q60% = -6.4846 + 1.0004 In (Area of basin) + 0.3016.
In (Monsoon wetness index)
In Q80% = -4.8508 + 1.0375. In (Area of basin) + 0.3016.
In (Area below 5000 m + 1)
In Q95% = -5.4716 + 1.0776. In (Area below 5000 m + 1)
Q100% = -0.09892 + 0.08149. (Area below 5000 m + 1)
A.13 PROCEDURE FOR ESTIMATING LONG TERM
AVERAGE MONTHLY FLOWS
1. From available topographic maps, find out the catchment
area below 5000 m elevation.
2. In the following equation, input coefficients from Table
A3 and the values of monsoon wetness index from Figure
A3.
Q =mean mouth
C.(Area of basin)A1. (Area below 5000m +
1)A2 (Monsoon wetness index)A3
where subscript month denotes one of the months from
January to December. A power of 0 indicates that particular
parameter does not enter into the equation for that month.
A.2 Medium Irrigation Project method (MIP)
Procedure for estimating mean monthly flows of a selected
catchment.
1 In the low flow period from November to April, visit the
catchment in question and make one flow measurement.
Ensure that there has been no heavy rainfall during the
preceding few days and that the water level is not
fluctuating rapidly.