Keywords

1 Introduction

Flood is a geomorphic, meteorological, and hydrological process of a drainage basin in the plain land and in the deltaic part that alters the existing landforms. Though flood is a natural phenomenon, sometimes human activities are causes of floods. Flood is a major problem in the floodplains, areas that are often attractive for human developments, and in the period from 1970 to 2012, floods caused over one million deaths (WMO-World Meteorological Organization; https://public.wmo.int/en/our-mandate/water/floods, dated: 23/09/21). As reported by the WHO (World Health Organization), in between 1998 and 2017, more than 2 billion people have been affected by floods worldwide, and in the last 10 years, 80–90% of documented natural hazards are caused by floods, severe storms, and tropical cyclones (https://www.who.int/health-topics/floods#tab=tab_1, dated: 23/09/2021). Among the continents, Asia faced the average highest numbers of flood events that continually increase in the last 65 years, and in the last decade, average of 60 major floods were reported every year (Sohoulande & Singh, 2016).

Over the Indian subcontinent, the significant increasing intensity and frequency of extreme precipitation events are potential to lead the flood events, though few recent studies in monsoon show a gradual decline circulation and precipitation amount over India (Ali et al., 2019). Frequent flooding of the Gangetic West Bengal is an inherent characteristic, particularly in the western part of the Bhagirathi-Hugli River. At present, ~42.55% of the total geographical area of this state is susceptible to flood (I & W. D., Govt. of W.B., Annual Flood Report, 2020). Floods in North Bengal are causes by rivers draining in the Himalayan parts. In South Bengal, mostly rivers like Ajay, Mayurakshi, Damodar, and Kangsabati are draining the Chotanagpur plateau and finally outfall into the Bhagirathi-Hugli River and carry a huge volume of water during the monsoon season or in an event of extreme precipitation that causes floods in the Deltaic Rarh Bengal (DRB). Thus, understanding flood nature became necessary for regional development. As flood is an event in the area of drainage basin, the study of drainage characteristics or drainage basin morphometry has become a pioneer of flood analysis that is helpful in flood risk control (Odiji et al., 2021; Leopold & Miller, 1956; Leopold & Maddock, 1953).

The flood of the DRB has widely been studied in different fields focusing on the large rivers which had developed devastating hazards for the society like floods at the lower part of the Mayurakshi, Ajay, Damodar, and Dwarakeswar River Basin (Islam & Barman, 2020; Pal et al., 2020; Islam & Sarkar, 2020; Mukhopadhyay, 2010; Malik & Pal, 2021; Ghoh & Guchhait, 2016; Singh et al., 2020). The interfluves of small river basins are paid less attention because of lack of data, though these rivers are also potentially vulnerable. The Khari River Basin (KRB), which is the focused area of this study, has no research work on the basin characteristics and flood. The KRB has been first explored by Bagchi and Mukherjee (1979) and Sen (1993) in terms of geomorphic analysis of this region and later on by Singh et al. (1998), Chakarbarti and Nag (2015), Roy & Sahu (2015), Roy and Bera (2018), and Barman et al. (2018), focusing on the aspects of regional tectonics and basin, and less importance has been given to understanding these interfluves of rivers’ geomorphic process and flood nature. So, the objective of this work is to discuss the flood of the KRB and the role of the drainage basin morphometry and to prepare a map of the active floods areas that are vulnerable to society. This work also will help to understand the geomorphology and hydrological characteristics of West Bengal’s small alluvial rivers.

2 Study Area

The KRB is a right-hand tributary drainage of the lower Ganga River system, draining an area of 1208 km2 of the interfluves of Ajay-Damodar Rivers of the Purba Bardhaman district, India. The Khari River is originating from the Panagarh lateritic upland (23°24′44.50″N, 87°31′58.56″E) at an altitude of ~58 meters (above MSL), near Maro village, and flows eastward ~212 km and joined with the Bhagirathi-Hugli River near the Kalna Town (23°16′4.67″N, 88°19′46.53″E) (Fig. 12.1a). Geographical extension of the studied KRB is 23°16′N to 23°32′N latitudes and 87°30′E to 88°18′E longitudes. The KRB drains the administrative area of Budbud, Galsi, Bhatar, Monteswar, Purbasthali, the northern part of Burdwan, the southern part of Katwa, and Mongalkote police stations.

Fig. 12.1
2 maps of the study area. A. A map of the Khari River basin traces the Bsegal shelf rivers, study area, places, and section line. B. A geology map of west Bengal traces the sand type. C. A surface graph of a cross-section of the A B line of height versus distance.

Location map of the study area. (a) Elevation map of the study area and the surroundings, (b) surface geology of the South West Bengal and study area with major faults, and (c) cross section of the surface geology along the A-B represents the surface slope and surface formation

Full size image

Physiographically, the study basin comes under the eastern part of the “Rarh Bengal” (Bagchi & Mukherjee, 1979), and the surface geological formation of this region is mostly covered by quaternary alluviums (District resource map, GSI, 2001) (Fig. 12.1b). Geomorphologically, the KRB is an old deltaic part which belongs to the Damodar para-delta (Singh et al., 1998). Bagchi & Mukherjee (1979) divided the major relief characteristics of this area into three physiographical units that are marked by the counters of 18 m and 36 m, respectively (Roy & Sahu, 2015) (Fig. 12.1a). The climatic condition of the area is tropical wet and dry (aw) in types controlled by the seasonal migration of tropic of cancer (23°0.30′N). Mean annual temperature of the region is ~26.3 °C and receives more than 100 mm mean annual rainfall, and approximately 80% of the total precipitation goes on in the monsoon months with an average of 350 mm. Though the Khari River is mainly rain-fed, bankfull discharge is observed during the monsoon period or torrential rainfall of depression that developed floods.

3 Database and Methodology

The main data used in this work is ASTER Global DEM (Advance Spaceborne Thermal Emission and Reflection Radiometer, spatial resolution of 30 m), and heights that were obtained from the Survey of India topographical maps (79A/3& 79A/7 of 1:50,000 scale) have been wont to develop the surface elevation map (SEM) in ArcGIS. Finally, the SEM database has been applied to compute the basin aerial and relief morphometric data by dividing the basin into 1 km2 grids. The KRB drainage network (Fig. 12.2) has been digitized from the “aerial Bing map”(spatial resolution view of 0.30–0.33 m) through QGIS “OpenLayer Plug-in tool” and delineated after Strahler’s classification (1964), which were used for the analysis of linear morphometry of the basin. The KRB has also been delineated into 17 sub-basins and named as A to Q for the detailed analysis of flood potentiality (Fig. 12.2). The surface runoff dominates sub-basins D and N, Q has no tributary, and the paleochannels have dominated M. Therefore, these sub-basins themselves are areas of water logging. A total of 16 parameters (Table 12.1) have been used for the drainage network and basin morphometric analysis which are grouped into (a) linear parameters, (b) aerial parameters, and (iii) relief parameters. A correlation has been run between the 16 parameters to look at the association among them (Fig. 12.3).

Fig. 12.2
A geological map of the Kunur river basin, sub-basin, and drainage traces the stream order 1, 2, 3, and 4 with the places of the Khari River basin.

KRB sub-basins and the drainage hierarchy

Full size image
Table 12.1 Morphometric parameters used to evaluate the KRB flood
Full size table
Fig. 12.3
7 graphs of stream number versus stream order. Each plot has 2 decreasing trends for various values of y and R square and the Khari River basin with 2 decreasing slopes.

Semilogarithmic diagram of stream numbers versus stream orders of the KRB and sub-basins A, B, C, G, K, and L, respectively. Regression coefficient curve values are indicated

Full size image

4 Result and Discussion

The KRB is an elongated fourth-order drainage basin of 1 fourth-order stream, 6 third-order stream, 33 second- order streams, and 157first-order streams, and the width of the basin increases downstream from about 8.5 km to ~17 km. Though both sides of the trunk river are drained by an equal number of sub-basins, the number of left-side streams is almost double than the right side, and the gradient of the streams are steeper than the right side (Table 12.2). Total length of the stream is ~537 km with the drainage density of ~0.44 km/km2, where first, second, third, and fourth order contribute almost 27%, 36%, 17%, and 20%, respectively, of the total length. The total length and the area of the left-side drainage basins are also greater than the opposite side. Sub-basin variations of the stream order, number, and stream length are given in Table 12.2. Though these three have no direct relation in peak flood generation, it has a positive relation with surface runoff generation that developed flood (Nageswara Rao, 2020). The mean bifurcation ratio of the KRB is Br = 5.42, that is, higher than any other small rivers draining at the Ajay-Damodar interfluves (Roy & Sahu, 2015), and Br between first, second, secondto third, and third to fourth orders are 4.80, 5.50, and 6.00, respectively. Sub-basins Br ranges from 2 to 13 (Table 12.3). A negative exponential trend has been observed between the stream number and stream orders (Fig. 12.4), indicating the natural extension of the network (Horton, 1945). The left side basin area is drained by higher Nuand by the stream length that exhibits the area of potential for surface runoff (Resmi et al., 2019). The sub-basin A is draining by the highest Nu of 70 and Lu of ~154 km (Table 12.2). Sub-basins C, B, H, F, L, and K are draining by relatively higher Nu (>10), and sub-basin O, I, G, J, P, and E are associated with the lower value of Nu (<10) (Table 12.2). First- and second-order average stream lengths are very high in sub-basins K, H, J, I, P, O, L (>1 km) and in sub-basins J, F, C, L, P, K (>7 km), respectively (Table 12.2). Str values of the sub-basins P, J, L, and K are comparatively higher because of the relatively faster channel elongation process which causes the high stream gradient with channelized flow (Resmi et al., 2019) that shows diverseness between sub-basins gradient, different erosional stages, and potentiality for the surface-runoff generation. The BR of the left-side sub-basins is comparatively high than right-side sub-basins (Table 12.3). These higher BR and Sg of these downstream sub-basins also indicate the high discharge potentiality of tributaries to the Khari trunk stream, and high discharge into the trunk stream in the rainy season is a normal event that leads to floods over the region.

Table 12.2 Basin properties of the KRB sub-basins
Full size table
Table 12.3 KRB sub-basins linear morphometric parameters
Full size table
Fig. 12.4
6 geographical maps of the Kunur river basin trace the stream frequency, drainage density, drainage texture, form factor, elongation ratio, and circularity ratio. The kilometers are mentioned below.

Sub-basin wise distributions of (a) stream frequency (Sf), (b) drainage density (Dd), (c) drainage texture (Dt), (d) form factor (Ff), (e) elongation ratio (Er), and (f) circulatory ratio (Cr)in the KRB

Full size image

The KRB’s maximum length is 82 km, and maximum width is approximately 22 km, i.e., the basin is almost 3.75 times longer, though the KRB is narrowly elongated (Er = 0.48 and Ff = 0.18) and the basin is wider in its downstream. This is probably because of the relatively large sub-basins of the KRB draining in this part (Fig. 12.2). These sub-basins are joining with the trunk stream within a specific stretch, and their combined discharge makes the trunk stream overtopping in its floodplain. The sub-basins joining into this reach are also elongated in nature (Fig. 12.4d–f) indicating more surface runoff because the shape also controls the rate of flow at which it discharged into the trunk channels (Fenta et al., 2017). According to Thomas et al. (2012), elongated-shaped watersheds can be marked by a flat hydrograph of longer duration with gentle rising and recession limbs. However, a more elongated watershed is more efficient in discharge of runoff; the hydrological response has also been affected by several factors like LULC, basin gradient, soil type, or rainfall intensity. The correlation shows that the Ff and Er have Cc, Cm, and Lo and also have a positive relationship with the relief aspects of the basin (Table 12.4).

Table 12.4 Correlation matrix of selected morphometric parameters of the KRB sub-basins
Full size table

The spatial variation of the Sf, Dd, and Dt indicates the landscape dissection by the stream network along with the parameters of Cc and Lo. These are important linear variables that are related to the hydrological properties of the drainage system and help to predict the runoff and sediment yield (Fenta et al., 2017). The computed Dd of the KRB is 0.44 km/km2 (Fig. 12.6d) and ranges between 0.18 and 0.58 km/km2 (Fig. 12.4b). Though, the overall Dd is very low, sub-basins J, P, and A Dd values are relatively high (Dd > 0.5), and sub-basins F, B, K, G, and C values are near to this (Fig. 12.4b). According to Ramalingam and Santhakumar (2001), the low Dd indicates less channelized flow which is a result of the higher infiltration rate caused by alluvium surface and higher-surface runoff. The Sf value of the KRB is ~0.17 no./km2, which ranges between 0.06 and 0.54 no./km2 (Fig. 12.4a). Relatively moderate values of Sf have been observed at the middle part of the KRB which also indicates the possibility to generate surface runoff. Moreover, in sub-basin-wise analysis (Fig. 12.4a), Sf values are relatively higher in sub-basin C, B, J, K, P, and L indicating a high rate of channelized flow. Normally, when surface permeability decreases, runoff increases, and as a result of it, numbers of channels developed and thus tend to be relatively higher Sf. In the correlation matrix, Sf, Dd, and Dt showed negative or very low positive correlation with other aerial and relief aspects (Table 12.4). Figure 12.5b shows the sub-basins draining the downstream part have much similarities.

Fig. 12.5
4 geological maps of the Kunur river basin trace the distributions of shape factor, compactness coefficient, constant of channel maintenance, and length of overland flow.

Sub-basin-wise distributions of (a) shape factor, (b) compactness coefficient (Cc), (c) constant of channel maintenance (Cm), (d) length of overland flow (Lo) in the KRB

Full size image

The Cm and Lo are another two important aerial aspects from the hydrological perspective as proxy indicators of the dynamic equilibrium stage of the river basin (Schumm, 1956) and the length that water must travel before discharging into the channel, i.e., the greater the Lo, the greater is the possibility of infiltration and lesser the surface runoff (Horton, 1945). For the KRB, the calculated value of Cm and Lo are 2.27 and 0.262 km, respectively. And the sub-basins ranges Cm = 1.721 to 5.617 and Lo = 0.861 to 2.809, respectively (Fig. 12.5d). According to Ngapna et al. (2018), such values are normally associated with the low-energy surface and its hydrology controlled by a humid environment. Thomas et al. (2012) interpreted shorter Lo values as characteristics of areas with steeper basin gradient that generate high-surface runoff. Lower values of Lo for sub-basins A, F, J, and P indicate more surface-runoff generation and moderate values of Lo for sub-basins B, C, K, L, H, E, and G moderate type of surface-runoff generation (Fig. 12.5d). The correlation analysis showed Lo and Cm are statistically significant and positively correlated and have a significant inverse relationship between the Dt and Dd. So in sub-basins with lower drainage density, Lo is high which generates more surface runoff and that leads to the possibility of flood at the KRB downstream part.

To synthesize the hydraulic nature of landform and characteristic of the overland flow of the basin, quantitative investigation of the relief aspect is the base of this (Fenta et al., 2017). In this work, the absolute relief (Ar), the relative relief (Rr), and the surface slope parameters of the basin have been interpreted to reveal the nature of surface runoff. The total relief of the KRB is 69 m, and the basin length is 81.9 km, i.e., the basin surface gradient is 0.84 m/km. This is a moderate gradient surface above the base level of the Khari River with moderate potential energy to move water down the slope. Distribution of the Ar value indicates that the basin is sloped from west to east and rivers are flowing according to the slope (Fig. 12.6a). Higher Ar values at the western part of the basin are related with the potential energy of surface runoff in this area. Figure 12.6c also shows that this part’s surface slope is relatively moderate to high. According to Thomas et al. (2012), comparatively high Ar values show steep sloping and higher-energy gradient surface area, which has been manifested as a high-surface runoff area (Fenta et al., 2017). The Ar and surface slope values are low at the eastern side of the basin, which is a relatively more flat surface and has less potential energy to generate surface runoff and has higher possibilities to stay water in this basin part (Fig. 12.6a).

Fig. 12.6
4 geological maps of the Kunur river basin trace the distribution of absolute relief in heights, surface slope in degrees, relative relief, and drainage density with basin boundary and river. The legends are listed below.

Spatial distribution of morphometric attributes of the KRB, (a) absolute relief (Ar), (b) relative relief (Rr), (c) surface slope (S), and (d) drainage density (Dd)

Full size image

The values of relative relief (Rr) ranges between 1.1 m and 8.40 m (Fig. 12.6b) with basin average of 2.1 covering about half of the basin. Figure 12.6b shows that the downstream area is characterized by very low relief, in particular, sub-basins K, O, F, P, E, and L which are plain surfaces with alternative picks of high Rr along the Khari mainstream. The lower values of Rr for the KRB are associated with first and second-order streams at the irrespective upstream parts (Fig. 12.6b). An area of high Rr > 6.0, which has been marked at the NW corner of the KRB, is an area of the dissected lateritic surface, but the higher Rr along the trunk river course, particularly near the Channa village and downstream of Nargapur village, indicates that these parts are relatively lower and potentially active floodplain areas (Fig. 12.6b). Every year during the flood event, the surface runoff that has been generated at the upper area of the sub-basins is accumulating in these areas and submerged multiple times (Fig. 12.8a–c). The slope deformation along the riverbank in this part of the river course also reveals the flooded areas (Fig. 12.6c). Though the surface slope of the basin is eastward, due to trunk stream abrupt deflection toward the north near the Palumba village and its downstream circular flow, the floodplain in this area is bounded by two-side relatively high ground of potentially high-runoff areas (Figs. 12.6b and 12.7a). The low relative relief or geomorphic high of the Kusumgram area acts as a natural barrier of the floodwater flow toward the east, and an extensive elongated submerged or waterlogged area evolved during the floodtime (Figs. 12.7a, b and 12.8d). It is an area of about 97 km2 waterlogging. Human intervention in form of longitudinal discontinuation of the floodplain by roads intensifies the flood and waterlogging condition in this area. Though a number of roads were disconnected and damaged by floodwater, as settlements are located above the floodplain, no one settlement is affected by flood (Fig. 12.7a). Near the Channa and Nargapur village, approximately 0.62 km2 and 3.0 km2 area flooded along the river, respectively. In these areas, the settlements are also located away from the present floodplain and not vulnerable to flood submergence (Fig. 12.7a).

Fig. 12.7
4 photographs of a flood in a pond surrounded by trees, a flooded river under the bridge, flooded ground, and a flooded bank area.

Flood inundation and waterlodgging areas of the KBR near (a, b) Channa village and downstream of Channa village, (c) downstream of Nargapur village, and (d) the left bank area between Malumba and Chandrapur village

Full size image
Fig. 12.8
A geological map of the Kunur river basin traces the places, roads, streams, settlements, basin boundaries, and waterlogged areas. A screenshot of the Indian geo platform of I S R O illustrates the map of the Khari River.

(a) Areas of flood waterlogged in the KRB along with the location of settlements areas and distribution of roads. The settlements are located away from the flood waterlogged areas. Marked areas by brown color lines are the areas of potential surface of runoff. (b) Flood inundation area of the lower KRB during the flood event on 06-08-2021 by a catastrophic rainfall (onscreen visualization of flood data by Bhuvan, Govt. of India)

Full size image

5 Conclusion

Flood in the KRB is a response to catastrophic rainfall, as it is mainly a seasonal river that generates high-surface runoff governed by morphometric parameters of the basin and drainage. Applied parameters indicate the different degrees of correlation among them and give an overall view of the hydraulic nature of the KRB. The nature of the relief aspects and surface-slope distribution show that active floodplain of the Khari River is the regional geomorphic low surface discontinued by geomorphic highs. KRB linear and aerial aspects reveals that the sub-basins J, L, K, O, P, and H are relatively high potential areas of surface runoff that drain approximately 45% of the total basin area. The combined discharge of these sub-basins into a particular reach of the lower Khari River between Palumba and Randavillage evolved the part as a flood basin. It is an elongated and circular waterlogged area bounded by two-side geomorphic high-surface areas. Another two active floodplain and waterlogged areas are developed near the Channa village and downstream of Nargapur village. Though these areas are potential for flood vulnerability and are submerged every year, as the settlements are located away from the present floodplain, the flood is not hazardous for the humans living in the basin. It will not be really possible to control flood frequency and situation in the lower part of the basin, but if we take a few constructive measures, flood vulnerability can be reduced, and for that we would recommend:

  1. (i)

    We can construct more capable underpass for floodwater in lateral roads that are blocking the flow of the surface runoff in floodplain area.

  2. (ii)

    As the flooded downstream area is relatively low-relief longitudinal basin, sedimentation is a common factor leading to the decreasing carrying capacity of the main channel than its middle part. So, to maintain its capacity, we need to rejuvenate the lower part.

  3. (iii)

    Another possible way is diverting the floodwater through the trunk stream of the sub-basin P, as source of this stream is near to the Khari main channel near the Palumba village that have to do by an artificial channel in a controlled way. Otherwise, the Khari segment between Palumba and Mahata village may dry out due to lack of water in channel.