Interfan area >>Geomorphology >>Fluvial processes >>History

Channel migration and Avulsion History of the Baghmati River

Channel migration is a frequently occurring fluvial hazard in the north Bihar Plains. Channel migration causes sweeping and ultimately removing of agricultural land and cities in their path. Baghmati River is a dynamic river, which has been shifting its course constantly within its meander belt. Because of such a dynamic nature of Baghmati River, several abandoned channels are existing in the basin, which are either known by the name of Baghmati River itself or assume an altogether different name or the name of the old channel into which river avulsed. The abandoned courses are either full course of river or only isolated segments or few remnants of them.

The migration history of Baghmati River was analysed with the help of 1:250,000 scale toposheets (1929 and 1986), 1:50,000 scale toposheets (1959 to 1979), remote sensing data (February-March 1989, and January-February 2000), the detailed fieldwork (April, 1998 and October, 1999) and from available literature.

In order to study the river migration through remote sensing analysis, drainage network from the satellite image was registered with respect to the recently available toposheet of the year 1986 and the data was compared with the drainage map traced from the 1924 toposheet. For the comparison, a drainage map was prepared from the survey of India toposheet of scale 1:253,440 of 1924. This map was scanned and the image was transformed into the Idrisi format and registered with reference to the toposheet of 1924. On the other hand another drainage map was prepared through ‘on- screen digitization’ on the registered satellite image of 1989. After registering the first image with respect to another one, these images were superimposed on each other.

The evidences of channel migration such as meander scars, abandoned channels, oxbow lakes and natural levees were used to prepare the avulsion history of the Baghmati River. Attention was particularly focussed on the reaches downstream of the confluence of Baghmati River and Lalbakeya River at Khoripakar village from where the avulsion events are more frequent. In the upstream of Khoripakar, Baghmati River is essentially a single channel system. The detailed description of the channel network of different time period is discussed in the following sections with the corresponding changes in the channel network. First, a generalized view of the drainage network in different time periods (from 1770-2000) is presented and the important events in the anabranched reach are described. For more focussed analysis, chronological avulsion events in different windows have been reconstructed for a span of around 70 years (1929-2000) using a variety of data sources.

Chronological Description of the Drainage Network and Avulsion Processes

Drainage Network in the Period of 1770-1915

The Baghmati River was first mapped by Maj. Rannel, in 1770 (GFCC, 1991). In that period, it was represented by two channels i.e. Great Baghmati River and Little Baghmati River. The Great Baghmati River was entering India near village Dumri and then flowing through Manihari, Akhta, Khoripakar, Piprakhas, Turki, Walipur, Narma, Jarang, Rosera and joining the Ganga River at Gogri (NE of Monghyr). On the basis of location of these villages a sketch map of drainage network of Baghmati River of 1770 has been prepared (Fig. 3.7). In the same map, the other major changes in the Baghmati River basin in between the period of 1770-1915 are also marked.

Another tributary of Baghmati River, known as little Baghmati was flowing in the South direction from the Indo-Nepal border. It was passing through the villages Parsauni, Belsand, Walipur and was joining the main Baghmati River near Tengraha village.

There are some interesting information regarding the past drainage of Baghmati River. In 1770, the Burhi Gandak River was the tributary of Baghmati River. It was first joining the Baghmati River at Piprakhas and again anabranching from Narharpakri to rejoin the Baghmati River at Rosera. Another point is that the Baghmati River was not the tributary of Kosi River, and it was directly joining the Ganga River at Gogri. Further, the course of Baghmati from Dumri to Khoripakar was in SW direction. From the closeness of Dumri with Majorganj, it seems that the Great Baghmati River and Little Baghmati River were the anabranches of the same river.

In between the period of 1770 to 1815, the Great Baghmati River avulsed near Sugia village in the East direction. Then, it started flowing through Sugia, Hiramma, Chhatauni and Turki. However, downstream of Turki, the course of the channel was same as the 1770 course.

Around 1880, an avulsion took place at 3 km upstream of Hayaghat and the river developed a SE course from Hayaghat to join the Kareha River, which later became the main channel of the Baghmati River and the Baghmati River became the tributary of Kosi River. The older south flowing channel, known as the Rosera Baghmati, flowing towards Rosera silted up and became an abandoned channel. Presently, the 1770 course, downstream of Rosera upto the confluence point to the Ganga, is followed by the Burhi Gandak River.

Another avulsion process started from 1905 near Hiramma and the river again started shifting to NE. By 1915, the river adopted a new course, which was known as the Siari course (Fig. 3.7).

Drainage Network in 1924-26

The detailed drainage map from Survey of India toposheet 72-F of 1924-26 is shown in Fig. 3.8. In this period, the Baghmati River was joining the Lalbakeya River at Khoripakar (w-1) and then flowed as a single channel for nearly 25-km upto Phulkaha village (w-3). At Phulkaha village (w-3), the river was anabranching into Siari Nadi and Baghmati River. The Siari Nadi was flowing from Phulkaha (w-3), Khojapur (w-5), Khauna Ghat (w-6), Dhanaur (w-6) villages and joining with Lakhandei Nadi at Bhagwatpur (w-7). The Baghmati River was joining the Lakhandei Nadi further downstream at Saraia and completing the anabranching. Another smaller anabranching from Baghmati River was initiating from Lalauna (w-7) and joining with Lakhandei Nadi at Kalanjarghat (w-7).

In the upstream reaches, one of the tributaries of Baghmati River, Baghmati Nadi (known as Little Baghmati in 1770) originated near Majorganj town (u/s of w-2) and flowed N-S to the east of main Baghmati River through Nerwa (w-2), Parsauni (w-5) and Belsand (w-5). This channel was connected to the Lakhandei Nadi, through a small tributary, which was flowing to the east. Downstream of Belsand (w-5), the channel was anabranching and both channels ultimately met Siari Nadi at different points. The Kola Nadi, another tributary of Baghmati, originated from Baghmati River at Sasaula (w-2) to join the Siari Nadi after flowing about 40 km in N-S direction.

Drainage Network in 1959-1979

Another detailed drainage map of Baghmati River system was prepared from 1:50,000 scale topographic maps varying in time period from 1959 to 1979 (Fig.3.9). This map shows several abandoned channels including the major active channels of 1924 period. Upstream of Khoripakar (w-1) the Baghmati River shows a westward lateral migration form 1924 to 1959. Downstream of Khoripakar, the upstream course of the Siari Nadi avulsed near Bilindpur (w-4) into a new channel i.e. Baghmati Nala leaving the middle stream reaches of Siari dry. This avulsion was triggered by the earthquake of 1934 and finished in 1937. Another major avulsion was that of the Lalauna-Gayaghat -Saraia channel (w-7), (Baghmati River of 1924) into a pre-existing spill channel of 1924 (Lalauna-Kalanjarghat channel) flowing SW. These avulsions made both the Siari Nadi as well as Baghmati River an abandoned channel, and a new channel of Baghmati River (known as Baghmati Nala) came into existence, passing through Maksudpur-Berwa and Kalanjarghat (w-7).

Further, the Paharpur (w-3)-Turki (w-4) channel avulsed SW at Paharpur and southwards at Raghopur (w-4) to join the Burhi Gandak River, taking almost a 90° turn. Hence, another new channel of the Baghmati, known as Baghmati River passing through Belwa (w-1), Kanuwani (w-3), and Minapur (w-4) came into existence, which in fact altered the anabranching behaviour of the Baghmati River in this region significantly. These avulsions aggravated the flooding problem in the Burhi Gandak River due to increased discharge through Baghmati. Attempts were made to establish a link between Raghopur (w-4) and Maksudpur (w-6) through a canal to divert the flow of Paharpur-Raghopur channel in the Baghmati Nala but the canal silted up after 2-3 years.

Further, the Nerwa (w-2) -Belsand (w-5) channel of 1924 dried up and now called, Puranidhar (means old channel). Downstream of Belsand, a new channel (Belsand-Janar-Katra channel) came into existence during 1926-75 period due to frequent avulsions in this region. The Kola Nadi, another tributary of Baghmati became partially abandoned due to westward shifting of the Baghmati River upstream of Khoripakar (w-1). However, the Kola Nadi was receiving some discharge from Rajwa Nadi (w-1).

Drainage Network in1986

Another drainage map of Baghmati River was prepared from the Survey of India toposheet (72F, 1:250,000 scale) of 1986 (Fig. 3.10). The most important change reflected in this map is the reorganization of channels downstream of Khoripakar (w-1) and the initiation of a new Belwa (w-1)-Dumra (w-5)- Janar (w-6) channel. This major avulsion took place in 1970 (GFCC, 1991). The downstream part of Kola Nadi was captured by this new channel of Baghmati River. Since a major proportion of the flow of Baghmati was diverted to these SE flowing channels, the N-S flowing western anabranch originating at Belwa had the much-reduced flow. The Baghmati Nala from Mirzapur (w-3)-Maksudpur (w-6)-Kalanjarghat (w-7) also dried up because of silting at its mouth. The N-S tributaries of Baghmati River i.e. Puranidhar River and Lakhandei Nadi have not shown any change from 1959 to 1986. In brief, mainly two anabranch channels were active in 1986 i.e. Belwa-Paharpur-Minapur channel and Belwa-Dumra-Janar-Katra channel.

Drainage Network in 1989

With the help of satellite images of February-March, 1989 a drainage map of Baghmati River basin was prepared (Fig. 3.11). The map shows some changes in between 1986 and 1989. The main Baghmati channel downstream of Amwa-Kalan village (w-1), shifted eastward and hence both the Balasundar Nala and Kola Nadi dried up completely. Another change is observed as the development of compressed meanders in the left anabranch of the Baghmati River downstream of Belwa village (w-1). In a way, the true anabranching of Baghmati is still not established; the left anabranch downstream of Belwa is very well developed now but the right anabranch is not joining back and meeting the Burhi Gandak at Minapur. No other change is noted in other tributaries like Puranidhar, Lakhandei, Siari and Baghmati Nala in the period of 1986-89. However, the presence of large meander loops in the satellite image along the Siari Nadi and Baghmati Nadi (Gayaghat-Saraia channel) (w-7) show that these were large channels in the past.

Present-day (2000) Drainage Network

The drainage map of Baghmati River of 2000 was prepared through the remote sensing data and fieldwork in the area (Fig. 3.12). The major changes during the period 1989-2000 are the abandonment of Belwa (w-1)-Paharpur (w-3)-Minapur (w-4) channel and disappearance of compressed meanders in the right anabranch just downstream of Belwa (w-1). However, during monsoon season, the Belwa-Paharpur-Minapur channel becomes an active channel. The main Baghmati channel is presently flowing through Khoripakar (w-1), Belwa (w-1), Janar (w-6), and Katra (w-6) villages. After its confluence with the Lakhandei Nadi at Katra (w-7), the Baghmati River causes flooding and severe bank erosion problem in the area. All tributaries of the Baghmati

River meet at the Bhagwatpur-Kalanjarghat reach (w-7) and from Kalanjarghat onwards, the river flows as a single channel and ultimately meets the Kosi River. The field observations suggest that the Baghmati River system is presently very active in the Khoripakar (w-1)-Kalanjarghat (w-7) reach and has experienced number of avulsions in a short span of 70 years. The Lakhandei Nadi is the only channel in this reach, which hasn’t shown any significant change in the channel position as well as in the discharge characteristics.

For a detailed study of the dynamic Baghmati River system and the mechanism and causative factors of individual avulsion events, the area has been divided into smaller windows and the following sections presents a systematic analysis of each of these windows. These total seven windows have been marked in the drainage network of different periods.