Interfan area >>Hydrology>>Discharge

DISCHARGE CHARACTERISTICS

Discharge characteristics of a river are measured through various discharge parameters (Table). For Baghmati River and Burhi-Gandak River the data on monthly average discharge and peak discharge were obtained from Central Water Commission (CWC), Government of India and Ganga Flood Control Commission (GFCC) respectively. The monthly average data was available for Dhengbridge and Hayaghat stations only for a period of 10 years (1980-89). The peak discharge data was available for four stations namely, Dhengbridge (1970-1989), Runisaidpur (1985-1989), Benibad (1985-1989) and Hayaghat (1956-1989). The analysis of the hydrological data and their interrelationship to channel morphology and sediment transport characteristics of the Baghmati River are presented next.

Table: Discharge Parameters of the Baghmati River

 Average Monthly Discharge

Fig. 1.a shows the variation of average monthly discharge for Dhengbridge (u/s) and Hayaghat (d/s) stations for the period 1980-1989. Fig. 2.8b presents the variation averaged out for the entire period. At both stations, the discharge generally starts peaking in the month of June with the maxima in the month of August/September. The downstream station invariably shows a higher discharge, particularly  in the  monsoon months, which  essentially reflects the  tributary influence.

Fig. 1: (a) Av. monthly discharge at Dhengbridge and Hayaghat stations from 1980 to 1989 (J –June, D-December); difference between the monsoon and nonmonsoon discharge is well shown in the figure. (b) Variation in discharge averaged out for the period of 1980 to 1989 shows that the peaking occurs in the month of August/September.

Of particular interest are the peaks for 1981, 1985 and 1987 (Fig. 1a), which show that the downstream discharge is more than double of the upstream discharge. The average annual discharge at Dhengbridge and Hayaghat works out to be 156 cumecs and 189 cumecs respectively.

In general, discharge increases downstream due to tributary influence and base flow contribution. However, the discharge values at upstream station (Dhengbridge) either remains higher or nearly equal to the discharge in downstream station (Hayaghat) during the non-monsoon period (Fig. 2.8a). This suggests that the tributary contribution is not significant in the lean period. Also, the river may be loosing a part of the discharge to groundwater during the low flow period.

Peak Discharge Variation

Figure 2.a shows a plot of peak discharge values for the period 1956-89 for Hayaghat station and 1970-89 for Dhengbridge station. The available data suggests that the peak discharges at both the stations are quite variable. At Hayaghat, the peak discharge increases rapidly from 1964, the year in which the Baghmati River basin experienced one of the worst floods (GFCC, 1991). From 1964 onwards, the general range of peak discharge variation is between 600 and 1600 cumecs, except the period 1974-76 (Fig. 2.9a). An extraordinarily high value of 3000 cumecs in 1975 once again corresponds to an extensive flooding event affecting 2.717 lakh ha. of area. The flood in 1975 produced the record HFL of 72.00 m. at Dhengbridge site, which is the highest level recorded till date at this site. Also, in 1974, a major part of Baghmati avulsed into Burhi Gandak River near Minapur and most of the discharge of Baghmati River diverted into Burhi Gandak River (GFCC, 1991).

Fig. 2: Peak discharge variation in Baghmati River (a) at Dhengbridge and Hayaghat (b) at Runisaidpur and Benibad; broken segments are due to non-availability of data. The peak discharge of Baghmati River is quite variable and unpredictable.

Further, the downstream station is generally characterised by higher discharge in comparison to the upstream station. On the contrary, the Baghmati River is showing a higher peak discharge at upstream station (Dhengbridge) than at the downstream station (Hayaghat) during the period 1980 to 1985. Lower peak discharge at downstream station is because of frequent overspilling in between these two stations during the monsoon period. After 1982, the river shows the normal trend i.e. the downstream station shows a higher peak discharge which may be attributed to the embankments in upstream of Hayaghat (Left afflux bund and Right afflux bund) completed in 1980-81.

Peak discharge data at two more middle stream stations namely, Runisaidpur and Benibad, from 1985 to 1989 show lower values of peak discharge in comparison to both upstream and downstream stations (Fig. 2.b). It is attributed to the anabranching pattern of the river system. The Baghmati River bifurcates into two channels just downstream of Dhengbridge and then rejoins nearly 25 km. upstream of Hayaghat augmenting the discharge significantly at Hayaghat. Thus, the middle stream stations being located on one branch of the river measure only a part of the total river discharge.

Peak discharge variation of the Baghmati River has been compared with other rivers of north Bihar Plains (Fig. 3). Not only that the actual size of peak discharge is extremely variable from year to year for all rivers, there seems to be no distinct relationship in pattern or range of variation between the upstream and downstream stations of a particular river. In the case of the Gandak River the peak discharge at downstream station is higher during the period 1978-83 after which it is more or less equal to that of upstream station. For Kosi River the peak discharge data at upstream station  is distinctly  higher  than  that of the downstream station atleast since 1983. The Burhi Gandak River is showing more or less uniform pattern in the variation of peak discharge at different stations; the peak discharge is highest at midstream station and lowest at upstream station. The Kamla-Balan system shows more fluctuating trends; the higher value frequently switches between upstream and downstream stations. Further, it is observed that the peak discharge mostly occurs in the month of August but the precise date varies by weeks between upstream and downstream stations in some years. It seems therefore that peak discharge at a particular station is a function of local flood waves created by tributaries apart from the monsoon rains in the plains.

Fig. 3: Peak discharge variation in Baghmati River (a) at Dhengbridge and Hayaghat (b) at Runisaidpur and Benibad; broken segments are due to non-availability of data. The peak discharge of Baghmati River is quite variable and unpredictable.