Bulletin of 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), Basin Improvement Project (BRBIP), Guheswori, *Corresponding e-mail: [email protected]

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. Quantification of reservoir release requires a reliable estimate of hydrological data. Additionally, estimating flood 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 ) 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 , 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 flood hydrographs corresponding to storm events with different 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 specific yield is 78.5l ps/km2. The probable maximum flood 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 the elevated reservoir area. Danish Hydraulic Institute Nepal has Mountainous, rugged topography. The main (DHI) had performed the feasibility study of the project objective of this study is for hydrology analysis of the in 2013. They carried out the long term flow analysis, Dhap Project where the reservoir is being constructed flood flow analysis, rainfall analysis using Sundarijal to protect and enhance water resources and increase Station located at 1490 masl, and Sundarijal gauging water discharge to the River downstream, conserve station. terrestrial and aquatic biodiversity and maintain the Bagmati River water quality establishing a 24m high This study has adopted available data recorded dam to reserve water. However, the main objective of (Nagarkot, Kathmandu airport and Sundarijal) in the the Dam is to increase the water availability of water Department of hydrology and Meteorology near the in the Bagmati River in the dry season. This is located project to determine the hydrological parameters at the on the headwater of the Nagmati River, a tributary of project site. The hydrological investigation is carried the Bagmati River, in the Shivpuri Nagarjun National out to get the Probable Maximum Precipitation (PMP), Park (SNNP) lies Northern most side of Kathmandu Probable Maximum Flood (PMF) at a certain time and (Fig. 2). The reservoir is almost rainwater harvesting the discharge to design appurtenances. Indeed, the and only a few small creeks are joining the reservoir; elevation affects precipitation significantly, especially however, the contribution of flow is insignificant due to in Nepal like a mountainous environment. The

71 Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020 precipitation will be high in the windward mountain due measuring stations (Fig. 1) which is located some 12 to the process of adiabatic warming or compressional km plan length form the project area. Sundarijal station warming. But the study area contains no station for is a mother rainfall station and has been considered direct measurement of hydrological parameters. The to establish long term flow of the project whereas study has adopted available data of the vicinity to the Kathmandu airport at low-level altitude and Nagarkot project area to determine the hydrological parameters at the same elevation of the project area has been at the project site. Correlating the data available at considered to understand the rainfall pattern. All these the vicinity of the project area, Dhap i.e Kathmandu stations are operated and maintained by the Department Airport, Nagarkot and Sundarijal, it is concluded of Hydrology and meteorology (DHM). Precipitation the hydrological condition of the project area. Detail data is needed for the analysis of surface runoff and to rainfall data is given in Table 1. know the nature of the catchment concerning the river flow. DHM has owned and operated rainfall stations The catchment of the project area is an ungauged and is presented in Table 1. type but the mother river downstream has rainfall

Fig.1: Rainfall Stations around the Project Area.

72 Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020

PMP is the theoretical maximum precipitation considering the watershed area. PMF is the theoretical for a given duration which is likely to happen over a maximum flood that poses extremely serious threats design watershed or a storm area of a given size, at to the flood control of the given project in a design a certain time of a year (WMO). PMP is used for the watershed (WMO). design of hydraulic structures, such as large dams and spillways, flood control works, levees, and nuclear INTRODUCTION power plants (Singh 2018). PMP could also be converted into PMF (WMO), which is important to The Dhap Dam is located on the headwater of the understand threats poses by floods. PMP is primarily Nagmati River (2100 masl approx.), a tributary of the considered to be the precipitation resulting from a Bagmati River, in the Shivapuri Nagarjuna National storm-induced by the optimal dynamic factor and the Park north of Kathmandu. Geographically, the project maximum moisture factor simultaneously (WMO). is located at 85ᵒ 27’ 21” E, 27ᵒ 48’18” N and altitude of PMP values can be quantified by frequency analysis 2080 m above mean sea level within the administrative of the annual maximum precipitation series. There are jurisdiction of Municipality (then two general approaches to estimating PMP: indirect Sundarijal VDC) of Kathmandu District in Bagmati approach considering storm area and direct approach 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 Data observation and hydrological analysis type. During summer the temperature at the project site Since there is no availability of hydrological data of the in an average is about 25ᵒC and during winter, it drops project area, an attempt was made to correlate the flows down to negative. The long term mean annual rainfall using the mother catchment. The catchment area ratio at the Sundarijal Station is 2,221 mm. The air and noise of 0.049 has been used to convert the mother river flow pollution levels are almost nil. to the project site. The project area is located within strongly metamorphological basement rocks of the Higher METHODOLOGY Himalaya Tectonic Zone. The foundation of dam site Empirical and statistical approach rock is comprised of micacious and some silicious Gneiss whereas the surface of the dam site is filled The first attempt to compute the flood flow inthe with a thick colluvium deposit and residual soil on project area was tried using rational and empirical either bank. The exposed rock is highly weathered and methods. The rational method and adopted two slightly weathered rock only found in river courses at empirical methods are briefly stated below. the dam side.

73 Bulletin of Nepal Hydrogeological Association Singh JL, Subedi NP, 2020

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 Bulletin of Nepal Hydrogeological Association Singh JL, Subedi NP, 2020 evaluate flood flow analysis. The projectQav area=Cf is Alocated within strongly metamorphological basement (1) 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 weatheredQT rock =Q onlyav (1+0 found. 8inlog river T) courses at the dam side. (2) Data observation and hydrological analysis -0.3 Since thereQmax is no =QT availability [1+2(A/2.59) of hydrological data ] of the project area, an attempt (3) 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. 3 METHODOLOGYWhere, Qav = annual mean flow in m /s, QT = maximum 24-hr flood with frequency once in T years in m3/s, Qmax = 3 maximum instantaneous flood flow in m /s, A = Catchment area in km2, Cf = Fuller’s coefficient varying between 0.18 Empiricalto 1.88 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. Van Te Chow has used the following statistical approach to estimate extreme flows. 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. T They are mathematically expressed (4) 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 andand maximum instantaneous flood flow respectively. The DHI conducted FS has also adopted the same method to evaluate flood flow analysis. Qav =Cf A 2 (1) ∑��XT) = a∑ �T+b ∑��T ) (5)

QT =Qav (1+0.8log T) (2) Bulletin of Nepal Hydrogeological Association Likewise, the Power methodSingh has JL, proposed Subedi NP, the 2020followi ng approaches to determine flood flows: Bulletin of Nepal Hydrogeological Association -0.3 Singh JL, Subedi NP, 2020 Bulletin of Nepal Hydrogeological Association Qmax =QT [1+2(A/2.59) ] (3)Singh JL, Subedi NP, 2020 3 Where, Qav = annual mean flow in m /s, QT = maximum 24-hr flood with frequency once in T years in m3/s, Qmax = 3 BulletinThe project of Nepal area Hydrogeological is located within Association, strongly Vol. 5,metamorphological September 2020 maximum basement instantaneous rocks flood of flow the in Higher m /s, A = HimalayaCatchmentSingh JL, area SubediTectonic in km2, NP, (6) Cf =Zone.2020 Fuller’s coefficient varying between 0.18 TheThe foundation project area of damis located site rock within is comprised strongly metamorphological of micacious andto 1.88some basement silicious rocks Gneiss of the whereas Higher the Himalaya surface Tectonic of the dam Zone. site isThe filled foundation with a thick of dam colluvium site rock deposit is comprised and residual of mic soilacious on either and some bank. silicious The exposed Gneiss rock whereasɑ is highly the weatheredsurface of theand damslightly site is filledEmpirical with a thick formulae colluvium (Varshneydeposit and residual 1974) soil were on eitherVan Tebank. Where,Chow The has used� exposed � the ������ following rock ��������statistical is highly approach weathered � 0.5772to estimate …andextreme, ln slightly = flows. loge, weatheredis filled with rock a thick only colluvium found in river deposit courses and residual at the d amsoil side. on either bank. The exposed rock is highly weathered and slightly attempted to arrive at simple forms of relationships for ∑ � � �� � � ∑ � T (4) weathered rock only found in river courses at the dam side. K= −1.1 − 1.795 XT theData flood observation flow on theand creek hydrological in terms of analysis various flood and factors,Data observation the most common and hydrological being the catchment analysis area. ∑ ∑ ∑� 2 Since there is no availability of hydrological data of the project�� area,XT)Q = T ana =Q+K �attemptT+b �� T was ) made to correl(5) ate the flows using(7) the ForSincemother the there catchment.present is no study, availability The Fuller,catchment of Weibull, hydrological area ratio Powel ofdata 0.049and of Chowthe has project been usedarea, toan convert attempt the was mother made toriver correl flowate to the the flows project using site. the mother catchment. The catchment area ratio of 0.049 has beenLikewise, used tothe convertPower method the has mother proposed river the followi flowng toapproaches the project to determine site. flood flows: methodsmother catchment. were used. The They catchment are mathematically area ratio of 0.049expressed has been All used methodsAll to methodsconvert have thehave beenmother been used usedriver to toflow reducereduce to the variation variation project site.for for different return periods to estimate flood flows. below. An attempt was tried to find peak flow using different return periods to estimate flood flows. METHODOLOGY (6) Fuller’sMETHODOLOGY formula (Subramany 2004) which was derived Statistical Method of PMP Estimation � � ������ɑ�������� � 0.5772 … forEmpirical a catchment and in statistical the USA. approachIn this approach, Fuller StatisticalWhere, The statistical Method method of wasPMP, ln = proposed log Estimatione, by Hershfield and will be used numerous gauge stations in a meteorologically Empirical and statistical approach K= −1.1homogeneous − 1.795 XT zone, using the hydrological frequency analysis method (WMO). Figure 3 shows the main steps for PMP (1914),The first suggested attempt to computethree empirical the flood formulae: flow in the for project annual area Thewas tried statistical using rational method and was empirical proposed methods. by HershfieldThe rational mean flow, maximum 24-hr flood with frequency once estimation area as follows (Wang G 2004): Themethod first and attempt adopted to compute two empirical the flood methods flow in are the briefly project stated area andQwas below.T =Q+K tried will� using be rational used numerous and empirical (7) gauge methods. stations The rational in a inmethod given and return adopted period two empirical and maximum methods instantaneousare briefly stated below. Empirical formulae (Varshney 1974) were attempted to arrivemeteorologicallyAll methods at simple have beenforms used of to homogeneous relationshipsreduce variation for for different zone,the freturnlood using periods flow to on estimate the the flood flows. floodEmpirical flow formulae respectively. (Varshney The 1974) DHI were conductedattempted to arriveFS has at simple forms of relationships for the flood flow on the creekEmpirical in terms formulae of various (Varshney flood factors, 1974) the were most attempted common to being arrivehydrologicalStatistical the at catchmentsimple Method forms frequency area. of PMPof Forrelationships Estimation the analysis present for study, the method f loodFuller, flow (WMO). Weibull, on the also adopted the same method to evaluate flood flow Powelcreek in and terms Chow of variousmethods flood were factors, used. Theythe most are commonmathematically beingFigureThe the expressed statistical catchment 3 shows method below. area. wasthe proposedAnmain For attempt the steps by present Hershfield wasfor PMP triedstudy, and will estimationto Fuller, befin usedd peak numerousWeibull, area flow gauge stations in a meteorologically analysis.usingPowel Fuller’s and Chow formula methods (Subramany were used. 2004) They which are mathematically was derived ashomogeneousfor expressedfollows: a catchment zone, below. using in the An hydrological USA. attempt In frequencythis was approach tried analysis to method ,fin Fullerd peak(WMO). (1914), flow Figure 3 shows the main steps for PMP using Fuller’s formula (Subramany 2004) which was derived estimationfor a catchment area as follows in (Wangthe USA. G 2004): In this approach, Fuller (1914), 3 Qavsuggestedusing = CfFuller’s A three formula empirical (Subramany formulae: 2004) for annual which (1) meanwas d erivedflow, maximumfor a catchment 24-hr inflood the USA.with frequencyIn this approach once in, Fuller given (1914), return periodsuggested and threemaximum empirical instantaneous formulae: flood for annual flow respect mean ively.flow, Themaximum DHI conducted 24-hr flood FS withhas also frequency adopted once the sainme given method return to Q =Q (1+0.8log T) (2) evaluateperiodT av and flood maximum flow analysis. instantaneous flood flow respectively. The DHI conducted FS has also adopted the same method to Qevaluate=QT flood[1+2( flowA/2.59) analysis.-0.3 ] (3) Fig. 3: The main steps for PMP estimation. 3 Qavmax =Cf A (1) Qav =Cf A (1) 3 QT =Qav (1+0.8log T) (2) Largest storm K is a statistical representation of Where, Qav = annual mean flow in m /s, QT = m maximumQT =Qav (1+0 24-hr.8log flood T) with frequency once (2) in T years the maximum value in the observed storm series, given -0.3 Qmax =QT3 [1+2(A/2.59) ] (3) in m /s, Q = maximum-0.3 instantaneous flood flow by: Qmax =QT [1+2(A/2.59)max ] (3) 3 2 in m /s, A = Catchment area in km3 , C = Fuller’s Where, Qav = annual mean flow in m3/s, QT f= maximum 24-hr flood with frequency once in T years in m3/s, Qmax = Where, Qav = annual mean flow in m /s, QT = maximum 24-hr floodKm = with(Xm –Xfrequencyn-1)/ σn-1 once in T yea rs in m3/s,(8) Qmax = coefficient varying between 0.18 to3 1.883 maximumWhere, Qav instantaneous = annual mean flood flow flow in inm m/s, /s,Q TA = = maximum Catchment 24-hr area floodin km2, with Cf =frequency Fuller’s coefficientonce in T yeavaryingrs in betweenm3/s, Q max0.18 = 3 maximumto 1.88 instantaneous flood flow in m /s, A = Catchment area inWhere km2, CXf = Fuller’s = maximum coefficient observed varying storm between value 0.18, Van Te Chow has used the following statistical m to 1.88 X = mean value computed after excluding the approach to estimate extreme flows. n-1 Van Te Chow has used the following statistical approach to estimateextraordinarily extreme flows. large value, and σ = Standard Van Te Chow has used the following statistical approach to estimate extreme flows. n-1 ∑ ∑ deviation computed after excluding the extraordinarily ∑ � � �� � � ∑ � T (4) ∑ � � �� � � ∑ � T (4) large value. and and and We have only one duration data sets and thus not ∑�� ∑ � ∑�� 2 possible to generate an enveloping curve. The PMP ∑��XT) = a∑ �T+b ∑��T ) (5) 2 ∑��XT) = a∑ �T+b ∑��T ) (5) then can be computed using the following formula: Likewise,Likewise, the Power the method Power has method proposed has the proposed followi ng the approaches to determine flood flows: Likewise, the Power method has proposed the following approachesPMP to determine = X +K flood σ flows: followingLikewise, theapproaches Power method to determine has proposed flood the flows: followi ng approaches to determinen m floodn flows: (9) (6) Where X = Mean value of the observed data, and (6) n ɑ σ = Standard deviation of the observed data Where, � � ������ �������� � 0.5772 …, ln = loge, n Where, ɑ , ln = loge, TableWhere, 1: � Rainfall � ������ Stations�������� Location � 0.5772 …, ln = loge, K= −1.1 − 1.795 XT K=Station −1.1 − 1.795 XT Elevation, Latitude Longitude Measurement Aerial distance from Average annual � QT =Q+K� masl (7) period the project to the � rainfall, mm QT =Q+K (7) rainfall station, km All methods have been used to reduce variation for different return periods to estimate flood flows. AllKathmandu methods Airporthave been (1030) used to1,367 reduce variation27ᵒ42’ for 85differentᵒ22’ return1964 periods - 2014 to estimate~14 flood flows. 1,440 Statistical Method of PMP Estimation StatisticalNagarkot (1043) Method of PMP2,163 Estimation27ᵒ42’ 85ᵒ31’ 1971 – 2014 12 1,870 The statistical method was proposed by Hershfield and will be used numerous gauge stations in a meteorologically homogeneousTheSundarijal statistical (1074) zone, method using was the proposed1,490 hydrological by27 Hershfield ᵒfrequency46’ 85 andᵒanalysis25’ will method be1994 used - 2015 (WMO). numerous5 Figure gauge 3 stationsshows the in main a2,221 meteorologically steps for PMP homogeneousestimation area zone, as follows using (Wangthe hydrological G 2004): frequency analysis method (WMO). Figure 3 shows the main steps for PMP Note:estimation Aerial area distance as follows is measured (Wang from G 2004): Google Map

74 3 3

Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020

Table 2: Maximum 24-hours Precipitation Recorded at 24 hours of precipitation recorded and calculated at Different Stations different stations is presented in Table 2. The data are used to estimate PMP using statistical approaches. Station Kathmandu Nagarkot Sundarijal Airport The PMP, rainy and non-rainy days, annual Elevation 1030 1043 1074 average and annual maximum rainfall at the different Recorded 1985-2015 1985-2014 1994-2015 stations (1030, 1043 and 1074) are summarized in year Table 3. Max 177.00 179.40 160.20 Mean 81.03 91.33 92.86 DATA ANALYSIS AND RESULTS Std dev 21.73 24.08 33.05 Frequency Analysis of PMP Frequency analysis of three meteorological stations Table 3: Estimated PMP and Rainfall at Different Stations, mm (1030, 1043, and 1074) is used to estimate PMP for different return periods. Hazen (1914), California and Station Kathmandu Nagarkot Sundarijal Airport (1043) (1074) Weibull plotting positions formula are used to see the (1030) future rainfall estimate. Future PMP values at, 2, 5, 10, 20, 50, 100, 1000 and 1000 years return period are PMP 170.18 209.84 199.20 estimated using the log domain. Table 4 summarizes Rainy days 131 126 137 the results from the PMP analysis. Sundarijal rainfall Non-rainy days 216 229 228 station 1074 is giving higher PMP compared to the Annual Average 1440 1870 2221 other two stations and thus suggested to adopt where it would be applied. The adopted PMP for the project Annual 1899 3644 3441 site is present in Table 5. Applying the Hershfield Maximum relationship, the PMP estimates for the three measuring

As mentioned earlier that there are meteorological stations are presented in Table 6. Xn and Sn were stations in the proximity of the project area but has a corrected using the length of the data period before the measurement daily (24 hrs interval). The maximum calculation of PMP.

Table 4: Different Return Period PMP at Different Meteorological Stations, mm

Return period, Ktm Airport 1030 Nagarkot 1043 Sundarijal 1074 years 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

75 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 Xn Sn mm Km PMP Data (mm) mm (years) 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 Daily Flow Analysis Similar approaches as used in the frequency analysis Daily flow is important to generate for this project for PMP is used to analyses annual rainfall of three since it is required to simulate the reservoir. The mother different stations, 1030, 1043 and 1074. Future catchment has a lo0ng term daily flow data available annual rainfall values at, 2, 5, 10, 20, 50, 100, 1000 for 46 years (1963-2010) with some missing data. Due and 1000 years return period are estimated using to the similarity in the catchment, flow transposition the log domain. The average annual rainfall of method has been applied to establish daily flow data 1030, 1034 and 1074 is summarized in Table 7. for the project using the following approaches:

Kathmandu Airport rainfall station is yielding Q = Q *A / A *P /P (10) very low annual rainfall compared to the other two 2 1 1 2 2 1 stations. Sundarijal rainfall station is yielding higher where, Q = Flow at gauging station, m3/s, Q = annual rainfall and thus this is suggested to adopt 1 2 Expected flow at the project site,m 3/s, A = Catchment for further analysis. The analysis is in line with the 1 area wrt the gauging station, km2, A = Catchment area previous feasibility study findings. The annual rainfall 2 at the project site wrt the point of consideration, km , at the Sundarijal Rainfall station 1074 is 2221 mm 2 P = Rainfall at the gauging station site, mm, and P = whereas the project site is 2363 mm after an orographic 1 2 Rainfall at the proposed project site, mm correction factor of 1.064. A /A x P /P is considered a multiplication Table 7: Annual Average Rainfall, mm 2 1 2 1 factor and is determined as 0.052. The daily flow at Station Station Station the proposed project is a product of measured flow at 1030 1043 1074 gauging station and the multiplication factor of 0.052. Annual Average 1,380 1,870 2,221 The mean monthly flow at the proposed project from Rainfall reference is presented in Fig. 4. The mean monthly average flow at the proposed dam site is 0.0623 m /s whereas the specific yield is 78.5 lps/km2.

76 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 flow duration curve is a probability discharge curve availability at the project site for different exceedance that shows the percentage of time a particular flow is levels of flow. FDC prepared using daily flow is equaled or exceeded. It is useful to understand flow presented in Fig. 5. The average flow is 0.062 3m /s.

Fig. 5: Flow Duration Curve at the Dam Site.

77 Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020

Extreme Hydrology If the present trends prevalent, the creek will be dry in the 25 recurrence period. For information purposes, the Low Flow low flows estimated using Weibull is a presentation in The low flow information is generally used to Fig. 6. The results indicate that the creek could be dry in 25 years of recurrence interval. assess the reliability and the economics of the proposed project. If the occurrence of inadequate flow is too frequent, a particular project might prove to be Flood Flow uneconomic and unreliable. Knowledge of minimum Flood flow plays an important role in the design of stream flow is therefore essential in the planning of hydraulic structures. Such flow is considered in the any hydropower project. However, this project is a design of temporary river diversion works, design of rainwater harvesting project and thus the importance spillway, drainage works, and pipe crossing. The value of low flows will not be made significant impacts of flood flow acts as a safety factor in hydraulic design. to the project since the project is a plan to build in a Table 8 shows the expected flood flows estimated 2 small creek having a catchment area of about 0.8 km . using a different empirical and statistical approach Chances of creek becoming dry creek are higher due to based on Fuller (1914), Gumbel (1958), Powell, Van the smaller size of the catchment area if the occurrence Te Chow and Weibull method whereas Table 9 shows of rainfall would be low to ascertain a good base flow. the transposed flood flows at the project dam site.

Fig. 6: Assessments on Low Flow Recurrence Interval.

78 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 Fuller Gumbel Powell Van Te Chow Weibull year method method method method 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 confirmed the estimated flood flow carried out in the Feasibility Study by DHI in 2013. This study recommends adopting flood flow estimated using Gumbel Method.

Table 9: Flood Flows at the Proposed Dam Site, m3/s Return period, Fuller method Gumbel method Powell method Van Te Chow Weibull method year 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

79 Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Singh JL, Subedi NP, 2020

DISCUSSION AND CONCLUSION REFERENCES Annual rainfall of the project area is estimated to be Chow V Te, others (1953) Frequency analysis of 2363 mm and the mean monthly discharge at the 0.8 hydrologic data with special application to rainfall km2 catchment area is approximately 0.061 m3/s and intensities the specific yield is 78.5 lps/km2. The flood flow at the Chow VT, Maidment DR, Mays LW (1988) Applied 100 yr return period is 6.886 m3/s. Hydrology, McGraw. Inc, New York, USA Gumbel EJ (1958) Statistics of extremes Columbia The most suitable approach for converting the University. New York PMF to the PMP is to scale the discharge, particularly Hazen A (1914) Discussion on ‘Flood flows’ by WE 100 years return period flood flow (6.883 m /s) using Fuller. Trans ASCE 77:526–563 a ratio of 24-hours PMP (682.93 mm, table 6) to the Hershfield DM (1965) Method for Estimating Probable 24-hours 100 years rainfall (282.28mm, table 5) after Maximum Rainfall. J Am Water Works Assoc correction by 1.06. 24-hours rainfall values are used 57:965–972. doi: 10.1002/j.1551-8833.1965. because this is the only rainfall data measurement tb01486.x interval available for the adopted measuring station. Hershfield DM (1961) Estimating the probable The resulting PMF is 16.65 m3/s. This PMF value is maximum precipitation. J Hydraul Div 87:99–116 30% higher than 10000 years return period value of Singh A, Singh VP, AR B (2018) Computation 12.63 m3/s. The ratio to the 10,000 years returns period of probable maximum precipitation and its flood is 1.32 which is not unexpectedly high. Therefore, uncertainty. Int J Hydrol 2:504–514 the design of the spillway of the dam is made based on Subramanian V (2004) Water Quality in South Asia. 17 m3/s PMF and based on a flood routing analysis for Asian J Water, Environ Pollut 1:41–54 the small reservoir the design flood 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.

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