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71
Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
Hydrological analysis of the catchment area of Dhap Dam
Jaya Laxmi Singh1* and Narayan Prasad Subedi1
1 Project Implementation Irrigation Unit (PIIU), Bagmati River Basin Improvement Project (BRBIP),
Guheswori, Kathmandu
*Corresponding e-mail: jaya.sin16@gmail.com
Received: 22 April 2020/Accepted: 4 September 2020
ABSTRACT
Indeed, the hydrological investigation is the preliminary study, essentially a part of civil construction to be
conducted before planning and designing of the hydraulic structure. Quantication of reservoir release requires
a reliable estimate of hydrological data. Additionally, estimating ood frequency discharge is essential for
economic planning and safe design. The present study is based on the rainwater harvesting project of Dhap dam.
Due to certain limitations of the catchment (ungauged), direct measurements of hydrological parameters are not
available. The study has adopted available rainfall data recorded (Nagarkot, Kathmandu airport and Sundarijal) in
the Department of hydrology and Meteorology (DHM) near the project to determine the hydrological parameters
at the project site. The project catchment lies within the Bagmati River Basin of Shivpuri Nagarjun National Park
(SNNP). The catchment is located in Kathmandu district, Central Development Region of Nepal. The catchment
area of the project is 0.8 Km2. This is about 4.9% of the mother catchment area concerning the Sundarijal gauging
station. This study focused carrying out of necessary hydrological investigations, such as estimation of Probable
Maximum Precipitation (PMP), Probable Maximum Flood (PMF), design ood hydrographs corresponding to
storm events with dierent return periods (2, 5, 10, 20, 50, 100, 1000 and 1000) years using log domain. For the
Dhap, the estimated catchment mean annual rainfall is estimated to be 2363 mm and the mean monthly discharge
from the 0.8 km2 catchment area is approximately 0.062 m3/s and the specic yield is 78.5 lps/km2. The probable
maximum ood at the 100 yr return period is 6.8 m3/s. For the Dhap, the estimated 24 hr PMP 682.9 mm, PMF
is 16.65 m3/s.
Keywords: Probable Maximum Precipitation, Probable Maximum Flood, Dhap Dam
BACKGROUND
Nepal has Mountainous, rugged topography. The main
objective of this study is for hydrology analysis of the
Dhap Project where the reservoir is being constructed
to protect and enhance water resources and increase
water discharge to the River downstream, conserve
terrestrial and aquatic biodiversity and maintain the
Bagmati River water quality establishing a 24m high
dam to reserve water. However, the main objective of
the Dam is to increase the water availability of water
in the Bagmati River in the dry season. This is located
on the headwater of the Nagmati River, a tributary of
the Bagmati River, in the Shivpuri Nagarjun National
Park (SNNP) lies Northern most side of Kathmandu
(Fig. 2). The reservoir is almost rainwater harvesting
and only a few small creeks are joining the reservoir;
however, the contribution of ow is insignicant due to
the elevated reservoir area. Danish Hydraulic Institute
(DHI) had performed the feasibility study of the project
in 2013. They carried out the long term ow analysis,
ood ow analysis, rainfall analysis using Sundarijal
Station located at 1490 masl, and Sundarijal gauging
station.
This study has adopted available data recorded
(Nagarkot, Kathmandu airport and Sundarijal) in the
Department of hydrology and Meteorology near the
project to determine the hydrological parameters at the
project site. The hydrological investigation is carried
out to get the Probable Maximum Precipitation (PMP),
Probable Maximum Flood (PMF) at a certain time and
the discharge to design appurtenances. Indeed, the
elevation aects precipitation signicantly, especially
in Nepal like a mountainous environment. The
72
Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
precipitation will be high in the windward mountain due
to the process of adiabatic warming or compressional
warming. But the study area contains no station for
direct measurement of hydrological parameters. The
study has adopted available data of the vicinity to the
project area to determine the hydrological parameters
at the project site. Correlating the data available at
the vicinity of the project area, Dhap i.e Kathmandu
Airport, Nagarkot and Sundarijal, it is concluded
the hydrological condition of the project area. Detail
rainfall data is given in Table 1.
The catchment of the project area is an ungauged
type but the mother river downstream has rainfall
measuring stations (Fig. 1) which is located some 12
km plan length form the project area. Sundarijal station
is a mother rainfall station and has been considered
to establish long term ow of the project whereas
Kathmandu airport at low-level altitude and Nagarkot
at the same elevation of the project area has been
considered to understand the rainfall pattern. All these
stations are operated and maintained by the Department
of Hydrology and meteorology (DHM). Precipitation
data is needed for the analysis of surface runo and to
know the nature of the catchment concerning the river
ow. DHM has owned and operated rainfall stations
and is presented in Table 1.
Fig.1: Rainfall Stations around the Project Area.
73
Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
PMP is the theoretical maximum precipitation
for a given duration which is likely to happen over a
design watershed or a storm area of a given size, at
a certain time of a year (WMO). PMP is used for the
design of hydraulic structures, such as large dams and
spillways, ood control works, levees, and nuclear
power plants (Singh 2018). PMP could also be
converted into PMF (WMO), which is important to
understand threats poses by oods. PMP is primarily
considered to be the precipitation resulting from a
storm-induced by the optimal dynamic factor and the
maximum moisture factor simultaneously (WMO).
PMP values can be quantied by frequency analysis
of the annual maximum precipitation series. There are
two general approaches to estimating PMP: indirect
approach considering storm area and direct approach
considering the watershed area. PMF is the theoretical
maximum ood that poses extremely serious threats
to the ood control of the given project in a design
watershed (WMO).
INTRODUCTION
The Dhap Dam is located on the headwater of the
Nagmati River (2100 masl approx.), a tributary of the
Bagmati River, in the Shivapuri Nagarjuna National
Park north of Kathmandu. Geographically, the project
is located at 85ᵒ 27’ 21” E, 27ᵒ 48’18” N and altitude of
2080 m above mean sea level within the administrative
jurisdiction of Gokarneshwar Municipality (then
Sundarijal VDC) of Kathmandu District in Bagmati
Zone, Central Development Region (Fig. 2).
Fig. 2: Project area catchment with the mother catchment and location of proposed Dhap Dam site (Source: Google Earth).
The climate of the project area is of temperate
type. During summer the temperature at the project site
in an average is about 25ᵒC and during winter, it drops
down to negative. The long term mean annual rainfall
at the Sundarijal Station is 2,221 mm. The air and noise
pollution levels are almost nil.
The project area is located within strongly
metamorphological basement rocks of the Higher
Himalaya Tectonic Zone. The foundation of dam site
rock is comprised of micacious and some silicious
Gneiss whereas the surface of the dam site is lled
with a thick colluvium deposit and residual soil on
either bank. The exposed rock is highly weathered and
slightly weathered rock only found in river courses at
the dam side.
Data observation and hydrological analysis
Since there is no availability of hydrological data of the
project area, an attempt was made to correlate the ows
using the mother catchment. The catchment area ratio
of 0.049 has been used to convert the mother river ow
to the project site.
METHODOLOGY
Empirical and statistical approach
The rst attempt to compute the ood ow in the
project area was tried using rational and empirical
methods. The rational method and adopted two
empirical methods are briey stated below.
74
Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
Empirical formulae (Varshney 1974) were
attempted to arrive at simple forms of relationships for
the ood ow on the creek in terms of various ood
factors, the most common being the catchment area.
For the present study, Fuller, Weibull, Powel and Chow
methods were used. They are mathematically expressed
below. An attempt was tried to nd peak ow using
Fuller’s formula (Subramany 2004) which was derived
for a catchment in the USA. In this approach, Fuller
(1914), suggested three empirical formulae: for annual
mean ow, maximum 24-hr ood with frequency once
in given return period and maximum instantaneous
ood ow respectively. The DHI conducted FS has
also adopted the same method to evaluate ood ow
analysis.
Qav =Cf A (1)
QT =Qav (1+0.8log T) (2)
Qmax =QT [1+2(A/2.59)-0.3 ] (3)
Where, Qav = annual mean ow in m3/s, QT =
maximum 24-hr ood with frequency once in T years
in m3/s, Qmax = maximum instantaneous ood ow
in m3/s, A = Catchment area in km2, Cf = Fuller’s
coecient varying between 0.18 to 1.88
Van Te Chow has used the following statistical
approach to estimate extreme ows.
Bulletin of Nepal Hydrogeological Association
Singh JL, Subedi NP, 2020
3
The project area is located within strongly metamorphological basement rocks of the Higher Himalaya Tectonic Zone.
The foundation of dam site rock is comprised of micacious and some silicious Gneiss whereas the surface of the dam site
is filled with a thick colluvium deposit and residual soil on either bank. The exposed rock is highly weathered and slightly
weathered rock only found in river courses at the dam side.
Data observation and hydrological analysis
Since there is no availability of hydrological data of the project area, an attempt was made to correlate the flows using the
mother catchment. The catchment area ratio of 0.049 has been used to convert the mother river flow to the project site.
METHODOLOGY
Empirical and statistical approach
The first attempt to compute the flood flow in the project area was tried using rational and empirical methods. The rational
method and adopted two empirical methods are briefly stated below.
Empirical formulae (Varshney 1974) were attempted to arrive at simple forms of relationships for the flood flow on the
creek in terms of various flood factors, the most common being the catchment area. For the present study, Fuller, Weibull,
Powel and Chow methods were used. They are mathematically expressed below. An attempt was tried to find peak flow
using Fuller’s formula (Subramany 2004) which was derived for a catchment in the USA. In this approach, Fuller (1914),
suggested three empirical formulae: for annual mean flow, maximum 24-hr flood with frequency once in given return
period and maximum instantaneous flood flow respectively. The DHI conducted FS has also adopted the same method to
evaluate flood flow analysis.
Qav =Cf A (1)
Q
T
=Q
av
(1+0.8log T) (2)
Q
max
=QT [1+2(A/2.59)
-0.3
] (3)
Where, Q
av
= annual mean flow in m
3
/s, Q
T
= maximum 24-hr flood with frequency once in T years in m3/s, Q
max
=
maximum instantaneous flood flow in m
3
/s, A = Catchment area in km2, C
f
= Fuller’s coefficient varying between 0.18
to 1.88
Van Te Chow has used the following statistical approach to estimate extreme flows.
T
(4)
and
X
T
) = a
T+
b
T 2 )
(5)
Likewise, the Power method has proposed the following approaches to determine flood flows:
(6)
Where,
, ln = log
e,
K= −1.1 − 1.795 X
T
Q
T
=Q+K (7)
All methods have been used to reduce variation for different return periods to estimate flood flows.
Statistical Method of PMP Estimation
The statistical method was proposed by Hershfield and will be used numerous gauge stations in a meteorologically
homogeneous zone, using the hydrological frequency analysis method (WMO). Figure 3 shows the main steps for PMP
estimation area as follows (Wang G 2004):
and
Bulletin of Nepal Hydrogeological Association
Singh JL, Subedi NP, 2020
3
The project area is located within strongly metamorphological basement rocks of the Higher Himalaya Tectonic Zone.
The foundation of dam site rock is comprised of micacious and some silicious Gneiss whereas the surface of the dam site
is filled with a thick colluvium deposit and residual soil on either bank. The exposed rock is highly weathered and slightly
weathered rock only found in river courses at the dam side.
Data observation and hydrological analysis
Since there is no availability of hydrological data of the project area, an attempt was made to correlate the flows using the
mother catchment. The catchment area ratio of 0.049 has been used to convert the mother river flow to the project site.
METHODOLOGY
Empirical and statistical approach
The first attempt to compute the flood flow in the project area was tried using rational and empirical methods. The rational
method and adopted two empirical methods are briefly stated below.
Empirical formulae (Varshney 1974) were attempted to arrive at simple forms of relationships for the flood flow on the
creek in terms of various flood factors, the most common being the catchment area. For the present study, Fuller, Weibull,
Powel and Chow methods were used. They are mathematically expressed below. An attempt was tried to find peak flow
using Fuller’s formula (Subramany 2004) which was derived for a catchment in the USA. In this approach, Fuller (1914),
suggested three empirical formulae: for annual mean flow, maximum 24-hr flood with frequency once in given return
period and maximum instantaneous flood flow respectively. The DHI conducted FS has also adopted the same method to
evaluate flood flow analysis.
Qav =Cf A (1)
Q
T
=Q
av
(1+0.8log T) (2)
Q
max
=QT [1+2(A/2.59)
-0.3
] (3)
Where, Q
av
= annual mean flow in m
3
/s, Q
T
= maximum 24-hr flood with frequency once in T years in m3/s, Q
max
=
maximum instantaneous flood flow in m
3
/s, A = Catchment area in km2, C
f
= Fuller’s coefficient varying between 0.18
to 1.88
Van Te Chow has used the following statistical approach to estimate extreme flows.
T
(4)
and
X
T
) = a
T+
b
T 2 )
(5)
Likewise, the Power method has proposed the following approaches to determine flood flows:
(6)
Where,
, ln = log
e,
K= −1.1 − 1.795 X
T
Q
T
=Q+K (7)
All methods have been used to reduce variation for different return periods to estimate flood flows.
Statistical Method of PMP Estimation
The statistical method was proposed by Hershfield and will be used numerous gauge stations in a meteorologically
homogeneous zone, using the hydrological frequency analysis method (WMO). Figure 3 shows the main steps for PMP
estimation area as follows (Wang G 2004):
Likewise, the Power method has proposed the
following approaches to determine ood ows:
Bulletin of Nepal Hydrogeological Association
Singh JL, Subedi NP, 2020
3
The project area is located within strongly metamorphological basement rocks of the Higher Himalaya Tectonic Zone.
The foundation of dam site rock is comprised of micacious and some silicious Gneiss whereas the surface of the dam site
is filled with a thick colluvium deposit and residual soil on either bank. The exposed rock is highly weathered and slightly
weathered rock only found in river courses at the dam side.
Data observation and hydrological analysis
Since there is no availability of hydrological data of the project area, an attempt was made to correlate the flows using the
mother catchment. The catchment area ratio of 0.049 has been used to convert the mother river flow to the project site.
METHODOLOGY
Empirical and statistical approach
The first attempt to compute the flood flow in the project area was tried using rational and empirical methods. The rational
method and adopted two empirical methods are briefly stated below.
Empirical formulae (Varshney 1974) were attempted to arrive at simple forms of relationships for the flood flow on the
creek in terms of various flood factors, the most common being the catchment area. For the present study, Fuller, Weibull,
Powel and Chow methods were used. They are mathematically expressed below. An attempt was tried to find peak flow
using Fuller’s formula (Subramany 2004) which was derived for a catchment in the USA. In this approach, Fuller (1914),
suggested three empirical formulae: for annual mean flow, maximum 24-hr flood with frequency once in given return
period and maximum instantaneous flood flow respectively. The DHI conducted FS has also adopted the same method to
evaluate flood flow analysis.
Qav =Cf A (1)
Q
T
=Q
av
(1+0.8log T) (2)
Q
max
=QT [1+2(A/2.59)
-0.3
] (3)
Where, Q
av
= annual mean flow in m
3
/s, Q
T
= maximum 24-hr flood with frequency once in T years in m3/s, Q
max
=
maximum instantaneous flood flow in m
3
/s, A = Catchment area in km2, C
f
= Fuller’s coefficient varying between 0.18
to 1.88
Van Te Chow has used the following statistical approach to estimate extreme flows.
T
(4)
and
X
T
) = a
T+
b
T 2 )
(5)
Likewise, the Power method has proposed the following approaches to determine flood flows:
(6)
Where,
, ln = log
e,
K= −1.1 − 1.795 X
T
Q
T
=Q+K (7)
All methods have been used to reduce variation for different return periods to estimate flood flows.
Statistical Method of PMP Estimation
The statistical method was proposed by Hershfield and will be used numerous gauge stations in a meteorologically
homogeneous zone, using the hydrological frequency analysis method (WMO). Figure 3 shows the main steps for PMP
estimation area as follows (Wang G 2004):
Bulletin of Nepal Hydrogeological Association
Singh JL, Subedi NP, 2020
3
The project area is located within strongly metamorphological basement rocks of the Higher Himalaya Tectonic Zone.
The foundation of dam site rock is comprised of micacious and some silicious Gneiss whereas the surface of the dam site
is filled with a thick colluvium deposit and residual soil on either bank. The exposed rock is highly weathered and slightly
weathered rock only found in river courses at the dam side.
Data observation and hydrological analysis
Since there is no availability of hydrological data of the project area, an attempt was made to correlate the flows using the
mother catchment. The catchment area ratio of 0.049 has been used to convert the mother river flow to the project site.
METHODOLOGY
Empirical and statistical approach
The first attempt to compute the flood flow in the project area was tried using rational and empirical methods. The rational
method and adopted two empirical methods are briefly stated below.
Empirical formulae (Varshney 1974) were attempted to arrive at simple forms of relationships for the flood flow on the
creek in terms of various flood factors, the most common being the catchment area. For the present study, Fuller, Weibull,
Powel and Chow methods were used. They are mathematically expressed below. An attempt was tried to find peak flow
using Fuller’s formula (Subramany 2004) which was derived for a catchment in the USA. In this approach, Fuller (1914),
suggested three empirical formulae: for annual mean flow, maximum 24-hr flood with frequency once in given return
period and maximum instantaneous flood flow respectively. The DHI conducted FS has also adopted the same method to
evaluate flood flow analysis.
Qav =Cf A (1)
Q
T
=Q
av
(1+0.8log T) (2)
Q
max
=QT [1+2(A/2.59)
-0.3
] (3)
Where, Q
av
= annual mean flow in m
3
/s, Q
T
= maximum 24-hr flood with frequency once in T years in m3/s, Q
max
=
maximum instantaneous flood flow in m
3
/s, A = Catchment area in km2, C
f
= Fuller’s coefficient varying between 0.18
to 1.88
Van Te Chow has used the following statistical approach to estimate extreme flows.
T
(4)
and
X
T
) = a
T+
b
T 2 )
(5)
Likewise, the Power method has proposed the following approaches to determine flood flows:
(6)
Where,
, ln = log
e,
K=
−1.1 − 1.795 X
T
Q
T
=Q+K (7)
All methods have been used to reduce variation for different return periods to estimate flood flows.
Statistical Method of PMP Estimation
The statistical method was proposed by Hershfield and will be used numerous gauge stations in a meteorologically
homogeneous zone, using the hydrological frequency analysis method (WMO). Figure 3 shows the main steps for PMP
estimation area as follows (Wang G 2004):
All methods have been used to reduce variation for
dierent return periods to estimate ood ows.
Statistical Method of PMP Estimation
The statistical method was proposed by Hersheld
and will be used numerous gauge stations in a
meteorologically homogeneous zone, using the
hydrological frequency analysis method (WMO).
Figure 3 shows the main steps for PMP estimation area
as follows:
Bulletin of Nepal Hydrogeological Association
Singh JL, Subedi NP, 2020
3
The project area is located within strongly metamorphological basement rocks of the Higher Himalaya Tectonic Zone.
The foundation of dam site rock is comprised of micacious and some silicious Gneiss whereas the surface of the dam site
is filled with a thick colluvium deposit and residual soil on either bank. The exposed rock is highly weathered and slightly
weathered rock only found in river courses at the dam si de.
Data observation and hydrological analysis
Since there is no availability of hydrological data of the project area, an attempt was made to correlate the flows using the
mother catchment. The catchment area ratio of 0.049 has been used to convert the mother river flow to the project site.
METHODOLOGY
Empirical and statistical approach
The first attempt to compute the flood flow in the project area was tried using rational and empirical methods. The rational
method and adopted two empirical methods are briefl y stated below.
Empirical formulae (Varshney 1974) were attempted to arrive at simple forms of relationships for the flood flow on the
creek in terms of various flood factors, the most common being the catchment area. For the present study, Fuller, Weibull,
Powel and Chow methods were used. They are mathematically expressed below. An attempt was tried to find peak flow
using Fuller’s formula (Subramany 2004) which was derived for a catchment in the USA. In this approach, Fuller (1914),
suggested three empirical formulae: for annual mean fl ow, maximum 24-hr flood with frequency once in given return
period and maximum instantaneous flood flow respectively. The DHI conducted FS has also adopted the same method to
evaluate flood flow analysis.
Qav =Cf A (1)
Q
T
=Q
av
(1+0.8log T) (2)
Q
max
=QT [1+2(A/2.59)
-0.3
] (3)
Where, Q
av
= annual mean flow in m
3
/s, Q
T
= maximum 24-hr flood with frequenc y once in T years in m3/s, Q
max
=
maximum instantaneous flood flow in m
3
/s, A = Catchment area in km2, C
f
= Fuller’s coefficient varying between 0.18
to 1.88
Van Te Chow has used the following statistical approach to estimate extreme flows.
T
(4)
and
X
T
) = a
T+
b
T 2 )
(5)
Likewise, the Power method has proposed the following approaches to determine flood flows:
(6)
Where,
, ln = log
e,
K= −1.1 − 1.795 X
T
Q
T
=Q+K (7)
All methods have been used to reduce variation for different return periods to estimate flood flows.
Statistical Method of PMP Estimation
The statistical method was proposed by Hershfield and will be used numerous gauge stations in a meteorologically
homogeneous zone, using the hydrological frequency analysis method (WMO). Figure 3 shows the main steps for PMP
estimation area as follows (Wang G 2004):
Fig. 3: The main steps for PMP estimation.
Largest storm Km is a statistical representation of
the maximum value in the observed storm series, given
by:
Km = (Xm –Xn-1)/ σn-1 (8)
Where Xm = maximum observed storm value,
Xn-1 = mean value computed after excluding the
extraordinarily large value, and σn-1 = Standard
deviation computed after excluding the extraordinarily
large value.
We have only one duration data sets and thus not
possible to generate an enveloping curve. The PMP
then can be computed using the following formula:
PMP = Xn +Km σn
(9)
Where Xn = Mean value of the observed data, and
σn = Standard deviation of the observed data
Table 1: Rainfall Stations Location
Station Elevation,
masl Latitude Longitude Measurement
period Aerial distance from
the project to the
rainfall station, km
Average annual
rainfall, mm
Kathmandu Airport (1030) 1,367 27ᵒ42’ 85ᵒ22’ 1964 - 2014 ~14 1,440
Nagarkot (1043) 2,163 27ᵒ42’ 85ᵒ31’ 1971 – 2014 12 1,870
Sundarijal (1074) 1,490 27ᵒ46’ 85ᵒ25’ 1994 - 2015 5 2,221
Note: Aerial distance is measured from Google Map
75
Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
Table 2: Maximum 24-hours Precipitation Recorded at
Dierent Stations
Station Kathmandu
Airport Nagarkot Sundarijal
Elevation 1030 1043 1074
Recorded
year 1985-2015 1985-2014 1994-2015
Max 177.00 179.40 160.20
Mean 81.03 91.33 92.86
Std dev 21.73 24.08 33.05
Table 3: Estimated PMP and Rainfall at Dierent Stations, mm
Station Kathmandu
Airport
(1030)
Nagarkot
(1043) Sundarijal
(1074)
PMP 170.18 209.84 199.20
Rainy days 131 126 137
Non-rainy days 216 229 228
Annual Average 1440 1870 2221
Annual
Maximum
1899 3644 3441
As mentioned earlier that there are meteorological
stations in the proximity of the project area but has a
measurement daily (24 hrs interval). The maximum
24 hours of precipitation recorded and calculated at
dierent stations is presented in Table 2. The data are
used to estimate PMP using statistical approaches.
The PMP, rainy and non-rainy days, annual
average and annual maximum rainfall at the dierent
stations (1030, 1043 and 1074) are summarized in
Table 3.
DATA ANALYSIS AND RESULTS
Frequency Analysis of PMP
Frequency analysis of three meteorological stations
(1030, 1043, and 1074) is used to estimate PMP for
dierent return periods. Hazen (1914), California and
Weibull plotting positions formula are used to see the
future rainfall estimate. Future PMP values at, 2, 5,
10, 20, 50, 100, 1000 and 1000 years return period are
estimated using the log domain. Table 4 summarizes
the results from the PMP analysis. Sundarijal rainfall
station 1074 is giving higher PMP compared to the
other two stations and thus suggested to adopt where
it would be applied. The adopted PMP for the project
site is present in Table 5. Applying the Hersheld
relationship, the PMP estimates for the three measuring
stations are presented in Table 6. Xn and Sn were
corrected using the length of the data period before the
calculation of PMP.
Table 4: Dierent Return Period PMP at Dierent Meteorological Stations, mm
Return
period,
years Ktm Airport 1030 Nagarkot 1043 Sundarijal 1074
Weibul California Hazen Weibul California Hazen Weibul California Hazen
2 74.79 75.84 74.50 84.79 86.11 84.43 85.14 88.82 84.52
5 99.35 100.41 96.59 113.26 114.58 109.80 127.34 131.03 119.22
10 117.94 118.99 113.31 134.80 136.13 128.99 159.27 162.96 145.47
20 136.52 137.57 130.03 156.34 157.67 148.18 191.20 194.89 171.72
50 161.08 162.13 152.12 184.82 186.14 173.55 233.41 237.10 206.42
100 179.66 180.72 168.84 206.36 207.68 192.74 265.34 269.02 232.67
500 222.81 223.86 207.65 256.38 257.70 237.31 339.47 343.16 293.62
1000 241.39 242.44 224.37 277.92 279.24 256.50 371.40 375.09 319.87
10000 303.12 304.17 279.90 349.48 350.80 320.25 477.46 481.15 407.07
76
Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
Table 5: Adopted PMP for the Project at Dhap
Return period, years 2 5 10 20 50 100 500 1000 10000
Expected PMP, mm 90.57 135.48 169.44 203.41 248.31 282.28 361.15 395.11 507.95
Table 6: Statistical Estimates of PMP based on WMO 1986
Location Years of
Data
(years)
Xn
mm
Sn mm Km PMP
(mm)
1 hr 6 hr 24 hr 1 hr 6 hr 24 hr
Sundarijal (1074) 22 92.86 33.05 6.25 12.50 16.25 293.44 522.25 641.94
KTM-Airport (1030) 47 81.03 21.73 6.50 12.50 15.75 188.63 297.29 362.49
Nagarkot (1043) 44 91.33 24.08 5.50 12.00 15.50 234.88 367.83 439.84
Annual Rainfall Analysis
Similar approaches as used in the frequency analysis
for PMP is used to analyses annual rainfall of three
dierent stations, 1030, 1043 and 1074. Future
annual rainfall values at, 2, 5, 10, 20, 50, 100, 1000
and 1000 years return period are estimated using
the log domain. The average annual rainfall of
1030, 1034 and 1074 is summarized in Table 7.
Kathmandu Airport rainfall station is yielding
very low annual rainfall compared to the other two
stations. Sundarijal rainfall station is yielding higher
annual rainfall and thus this is suggested to adopt
for further analysis. The analysis is in line with the
previous feasibility study ndings. The annual rainfall
at the Sundarijal Rainfall station 1074 is 2221 mm
whereas the project site is 2363 mm after an orographic
correction factor of 1.064.
Table 7: Annual Average Rainfall, mm
Station
1030 Station
1043 Station
1074
Annual Average
Rainfall
1,380 1,870 2,221
Daily Flow Analysis
Daily ow is important to generate for this project
since it is required to simulate the reservoir. The mother
catchment has a lo0ng term daily ow data available
for 46 years (1963-2010) with some missing data. Due
to the similarity in the catchment, ow transposition
method has been applied to establish daily ow data
for the project using the following approaches:
Q2= Q1 *A1/ A2*P2/P1 (10)
where, Q1 = Flow at gauging station, m3/s, Q2 =
Expected ow at the project site, m3/s, A1 = Catchment
area wrt the gauging station, km2, A2 = Catchment area
at the project site wrt the point of consideration, km2,
P1 = Rainfall at the gauging station site, mm, and P2 =
Rainfall at the proposed project site, mm
A2/A1x P2/P1 is considered a multiplication
factor and is determined as 0.052. The daily ow at
the proposed project is a product of measured ow at
gauging station and the multiplication factor of 0.052.
The mean monthly ow at the proposed project from
reference is presented in Fig. 4. The mean monthly
average ow at the proposed dam site is 0.062 m3/s
whereas the specic yield is 78.5 lps/km2.
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Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
Fig. 4: Expected Mean Monthly Flows the Project Site.
Flow Duration Curve (FDC)
A ow duration curve is a probability discharge curve
that shows the percentage of time a particular ow is
equaled or exceeded. It is useful to understand ow
availability at the project site for dierent exceedance
levels of ow. FDC prepared using daily ow is
presented in Fig. 5. The average ow is 0.062 m3/s.
Fig. 5: Flow Duration Curve at the Dam Site.
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Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
Extreme Hydrology
Low Flow
The low ow information is generally used to
assess the reliability and the economics of the
proposed project. If the occurrence of inadequate ow
is too frequent, a particular project might prove to be
uneconomic and unreliable. Knowledge of minimum
stream ow is therefore essential in the planning of
any hydropower project. However, this project is a
rainwater harvesting project and thus the importance
of low ows will not be made signicant impacts
to the project since the project is a plan to build in a
small creek having a catchment area of about 0.8 km2.
Chances of creek becoming dry creek are higher due to
the smaller size of the catchment area if the occurrence
of rainfall would be low to ascertain a good base ow.
If the present trends prevalent, the creek will be dry in
the 25 recurrence period. For information purposes, the
low ows estimated using Weibull is a presentation in
Fig. 6. The results indicate that the creek could be dry
in 25 years of recurrence interval.
Flood Flow
Flood ow plays an important role in the design of
hydraulic structures. Such ow is considered in the
design of temporary river diversion works, design of
spillway, drainage works, and pipe crossing. The value
of ood ow acts as a safety factor in hydraulic design.
Table 8 shows the expected ood ows estimated
using a dierent empirical and statistical approach
based on Fuller (1914), Gumbel (1958), Powell, Van
Te Chow and Weibull method whereas Table 9 shows
the transposed ood ows at the project dam site.
Fig. 6: Assessments on Low Flow Recurrence Interval.
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Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
Table 8. Flood Flows at Sundarijal Station 505, m3/s
Return period
year
Fuller
method
Gumbel
method
Powell
method
Van Te Chow
method
Weibull
method
2 40.06 24.66 24.36 25.14 12.433
5 68.16 53.23 50.01 50.14 41.746
10 86.76 72.15 67.00 66.69 61.154
20 104.60 90.29 83.29 82.57 79.770
25 110.26 96.05 88.46 87.60 85.676
50 127.69 113.78 104.38 103.12 103.868
100 144.99 131.38 120.18 118.52 121.925
200 162.24 148.92 135.93 133.86 139.916
500 184.98 172.06 156.70 154.10 163.653
1000 202.18 189.54 172.41 169.40 181.592
10000 259.26 247.60 224.53 220.20 241.154
The study conrmed the estimated ood ow carried out in the Feasibility Study by DHI in 2013. This study
recommends adopting ood ow estimated using Gumbel Method.
Table 9: Flood Flows at the Proposed Dam Site, m3/s
Return period,
year Fuller method Gumbel method Powell method Van Te Chow
method Weibull method
2 2.10 1.29 1.28 1.32 0.651
5 3.57 2.79 2.62 2.63 2.186
10 4.54 3.78 3.51 3.49 3.203
20 5.48 4.73 4.36 4.32 4.178
25 5.77 5.03 4.63 4.59 4.487
50 6.69 5.96 5.47 5.40 5.440
100 7.59 6.88 6.29 6.21 6.886
200 8.50 7.80 7.12 7.01 7.328
500 9.69 9.01 8.21 8.07 8.571
1000 10.59 9.93 9.03 8.87 9.511
10000 13.58 12.97 11.76 11.53 12.630
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Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020
DISCUSSION AND CONCLUSION
Annual rainfall of the project area is estimated to be
2363 mm and the mean monthly discharge at the 0.8
km2 catchment area is approximately 0.061 m3/s and
the specic yield is 78.5 lps/km2. The ood ow at the
100 yr return period is 6.886 m3/s.
The most suitable approach for converting the
PMF to the PMP is to scale the discharge, particularly
100 years return period ood ow (6.88 m3/s) using
a ratio of 24-hours PMP (682.93 mm, table 6) to the
24-hours 100 years rainfall (282.28mm, table 5) after
correction by 1.06. 24-hours rainfall values are used
because this is the only rainfall data measurement
interval available for the adopted measuring station.
The resulting PMF is 16.65 m3/s. This PMF value is
30% higher than 10000 years return period value of
12.63 m3/s. The ratio to the 10,000 years returns period
ood is 1.32 which is not unexpectedly high. Therefore,
the design of the spillway of the dam is made based on
17 m3/s PMF and based on a ood routing analysis for
the small reservoir the design ood for the spillway is
given as 1.6 m3/s.
Acknowledgment
I would like to express my special thanks to Project
Implementation Irrigation Unit (PIIU), Bagmati River
Basin Improvement Project (BRBIP) providing the
all necessary information and also like to extend my
gratitude to Mr. Ashish Bhadra Khanal for his best
suggestion.
REFERENCES
Chow V Te, others (1953) Frequency analysis of
hydrologic data with special application to rainfall
intensities
Chow VT, Maidment DR, Mays LW (1988) Applied
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Gumbel EJ (1958) Statistics of extremes Columbia
University. New York
Hazen A (1914) Discussion on ‘Flood ows’ by WE
Fuller. Trans ASCE 77:526–563
Hersheld DM (1965) Method for Estimating Probable
Maximum Rainfall. J Am Water Works Assoc
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tb01486.x
Hersheld DM (1961) Estimating the probable
maximum precipitation. J Hydraul Div 87:99–116
Singh A, Singh VP, AR B (2018) Computation
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