4.1 Analyzing River Metamorphosis
It is expected that the alluvial river, Damodar, exists as a continuum state of inter-linked processes of physical and biological components at variable time-scale frame on the transfers (longitudinal, lateral and vertical distribution) of energy, material and biota, but the anthropogenic intervention (like dam) can break the state of continuum for a certain period, promoting disruption mainly in longitudinal connectivity (Casado, 2013). Schumm (1969) elucidated the fluvial metamorphosis in response to disruption in the equilibrium of fluvial hydrosystem. River metamorphosis can be referred as the expectable transformation or alternation of channel patterns, floodplain landforms, annual discharge variability, sediment type and load due to either anthropogenic activities or past climatic changes (Schumm, 1985; Miller and Miller, 2007). Schumm (1985) also explained that fluvial metamorphosis can be attributed to sediment and water detention in the reservoirs and it can change three elements of the river: (a) ability of sediment transport to downstream, (b) sediment amount available to transport and (c) quality of running water (Casado, 2013). In this case the impressive works of Brandt (2000), Graff (2005) and Petts and Gurnell (2005) provide an exhaustive information on the upstream and downstream effects of river impoundment. Taking reference to dam-controlled Damodar River it is expected more than one possibility of metamorphosis in the alluvial river: (a) change of straight channel to become sinuous or braided, (2) change of braided channel to become straight or meandering, and (3) change of meandering channel to become straight or braided. The assessment is performed here to know the present condition of metamorphosis observed in the Damodar River, mainly controlled by the Durgapur Barrage reservoir.
Table 3
Quantitative estimation of important channel dimensions and hydrologic parameters
Channel Section
|
Cumulative Distance (km)
|
Dmax (m)
|
Davg (m)
|
Wb (m)
|
WBt (m)
|
Wf (m)
|
Abc (m2)
|
R (m)
|
W/D
|
ER
|
Vmax (m/s)
|
Vmin (m/s)
|
Qmax (m3/s)
|
Qmin (m3/s)
|
S1
|
4.50
|
7.086 - 7.816
|
4.167 - 5.23
|
682 - 966
|
394 - 675
|
836 - 1408
|
2835 - 5546
|
2.932 - 6.357
|
112 - 247
|
1.225 - 1.147
|
1.416 - 2.477
|
0.687 - 0.789
|
4143 - 13574
|
207 - 650
|
S2
|
9.46
|
7.003 - 7.567
|
4.659 - 4.984
|
1008 - 1418
|
570 - 1283
|
2119 - 2885
|
3933 - 4766
|
3.910 - 4.415
|
202 - 347
|
1.420 - 2.862
|
1.767 - 1.919
|
0.587 - 0.659
|
6951 - 12698
|
382 - 578
|
S3
|
14.08
|
6.798 - 8.308
|
5.255 - 6.346
|
1584 - 2509
|
1584 - 1826
|
3096 - 3721
|
9152 - 13787
|
5.494 - 5.775
|
249 - 358
|
1.234 - 1.718
|
2.221 - 2.296
|
0.523 - 0.702
|
21014 - 30618
|
430 - 1139
|
S4
|
19.21
|
7.773 - 9.08
|
5.256 - 6.386
|
1605 - 2756
|
1525 - 1994
|
3300 - 4283
|
6980 - 19957
|
4.349 - 7.216
|
203 - 319
|
1.550 - 2.491
|
1.900 - 2.663
|
0.498 - 0.778
|
13265 - 53155
|
459 - 1865
|
S5
|
25.09
|
9.809 - 10.568
|
6.857 - 7.773
|
1979 - 2098
|
1550 - 1621
|
2380 - 3468
|
12509 - 13992
|
5.961 - 7.068
|
187 - 213
|
1.202 - 1.654
|
2.345 - 2.627
|
0.706 - 0.912
|
29332 - 36759
|
1161 - 2203
|
S6
|
30.50
|
6.408 - 10.059
|
5.911 - 7.502
|
1241 - 1689
|
783 - 1206
|
3475 - 3709
|
5982 - 10841
|
4.820 - 6.433
|
168 - 194
|
2.191 - 2.804
|
2.316 - 2.808
|
0.780 - 0.798
|
13857 - 30445
|
647 - 988
|
S7
|
35.96
|
10.124 - 11.762
|
5.981 - 6.125
|
1095 - 1903
|
800 - 1402
|
3135 - 4806
|
5674 - 10121
|
5.181 - 5.318
|
93 - 188
|
2.525 - 2.860
|
2.431 - 2.474
|
0.815 - 0.870
|
13795 - 25038
|
687 - 1445
|
S8
|
41.07
|
10.202 - 10.948
|
6.220 - 6.411
|
1528 - 1892
|
1500 - 1528
|
3065 - 3759
|
10551 - 10875
|
5.658 - 5.745
|
173 - 183
|
1.642 - 1.989
|
2.578 - 2.604
|
0.746 - 0.817
|
27201 - 28234
|
1024 - 1309
|
S9
|
47.00
|
7.520 - 11.938
|
4.976 - 5.722
|
1524 - 2718
|
1291 - 1858
|
3955 - 4418
|
7005 - 13091
|
4.594 - 6.572
|
203 - 228
|
1.627 - 2.594
|
2.244 - 2.315
|
0.771 - 0.789
|
15717 - 30315
|
1012 - 1431
|
4.1.1 Changes in Channel Morphology
The analysis of channel dimension results (Table 3) reveals that the upstream part of Damodar reflects relatively high in-channel sedimentation, developing numerous mid-channel and point bars and islands (figure 3). The value of Dmax increases from 7.070 m to 10.568 m at the Barrage, and it escalates downstream up to 11.938 m which may be attributed to fluvial incision of low sediment load peak discharge from the Barrage. The mean channel gradient (s) of upstream section is 0.0318% and it escalates to 0.0412% at downstream section. The flood-prone width (Wf) of Damodar River is quite low at upstream reaches (836 to 4283 m), but downstream Wf ranges in between 3135 and 4806 m, showing wider river oscillation within wider floodplains. The width-depth ratio (W/D), by Rosgen (1994), isdefined as the ratio of Wf to the maximum channel depth of the bankfull stage (Dmax). The W/D is key parameter to realize the distribution of available kinetic energy within an alluvial channel, and the ability of various discharges occurring within the channel to move sediments.High W/D indicates broad valley with fluvial terraces, abundance of sediment supply, slightly entrenched channel, and active lateral adjustment. W/D varies from 300 (upstream) to below 150 (downstream), and it signifies high bankfull width compare to channel depth at upstream sections due to impoundment of the Barrage (Table 3).
Entrenchment is the vertical containment of a river to understand the tendency of valley incision within the wider floodplain (like entrenched meander) (Rosgen, 1994). In general, high entrenchment has negative impacts (observed in study area), viz., accelerated bank erosion, land loss, loss of aquatic habitat, loss of land productivity, lowering of water table and sedimentation of the river downstream. The entrenchment ratio (ER), by Rosgen (1994), is the ratio of flood-prone width to the surface width of the bankfull channel (measured at twice the maximum depth of bankfull channel). It is observed from the Sentinal 2A image (2021) that at upstream of the Barrage ER varies widely from 1.202 to 2.862 (mean 1.650) which signifies highly to moderately entrenched channel in a well-developed floodplain. Significantly, the river turns into slightly entrenched channel below the Barrage because the value varies from 1.810 to 2.497 (mean 2.275). Surprisinglythe river sinuosity does not vary too much in the study area, i.e., SI (Friend and Sinha,1993) range of 1.036 to 1.135 (more tend to straight channel). The strong linearity of Damodar may be attributed to two factors: (a) the river follows a W-E lineament of the Bengal Basin crossing three prominent basement faults (viz., Chhotanagpur Foothill Fault, Khandaghosh-Garhmayna Fault and Pingla Fault), and (b) the dam-controlled river is jacketing by elevated embankments at both banks to stop oscillation in the floodplain (i.e., anthropogenic confinement). Braiding nature of the river reflects a threshold level of sediment load or slope to maintain a steep gradient throughout the long profile. High tendency of braiding means abundant supply of sediments from upstream, flow blockage by dam impoundment, in-channel sedimentation (low flushing due to damming), rapid and infrequent discharge. The braid-channel ratio (BR), by Friend and Sinha (1993), reflects a typical pattern in the Damodar valley. BR is estimated as 2.173 at the initial survey section (S1) and it escalates up to 4.316 at the Durgapur Barrage section (S5) due to development morelinguoid bars and islands. Below the Barrage, BR drops to 1.811 up to 10 km stretch, and after that it again increases up to 3.525. The main reason is the fluvial incision and channel narrowing at just downstream of Barrage, and further the eroded sediments are deposited in downstream channel.
4.1.2 Changes in Channel Carrying Capacity and Flow Regime
The bankfull cross-sectional area (Abc) (assuming channel geometry as trapezoid form), mass flow rate and bankfull discharge are estimated using DEM and MIKE 21 hydrological software in nine reach sections (taking five cross-section samples from each reach). The parameter Abc is estimated at bankfull stage to get an idea of flood discharge variability at maximum level (Qmax). At upstream of the Barrage the value of Abc varies highly from 2,835 to 19,957 m2 (mean 9,345 m2), and at downstream the value ranges in between 5,674 and 13,091 m2 (mean 9267 m2). The manning’s roughness coefficient (nc) of lower Damodar River is estimated as 0.025 to 0.035 by Singh et al. (2020). The estimated Qmax(i.e., potential bankfull discharge) varies widely from 4,143 to 53,155 m3s−1 in the upstream reaches of Damodar (having average hydraulic radius of 2.932 to 7.216 m). Below the Barrage it ranges from 13,795 to 30,445m3s−1 (having average hydraulic radius of 4.820 to 6.572 m). In general, there is a reduction of bankfull carrying capacity of channel (below the Barrage) to accommodate maximum flood flow during monsoon due to channel narrowing and terrace formation within the valley. It is observed that the minimum flow rate (during lean season) varies from 528 to 1287 m3s−1 at upstream reaches, but due to flow regulation it maintains in between 842 and 1293 m3s−1 up to the Rhondia weir. It signifies relatively good environmental flow which is the minimum flow required for the sustainable maintenance of riparian and aquatic ecosystem. The studies of Ghosh (2011), Bhattacharyya (2011), Ghosh and Guchhait (2016), Verma et al. (2017), and Karim and De (2019) revealed that four primary types of flow regulation were observed in the Damodar River, viz., (1) peak absorption (peak flows from catchment tributaries absorbed in reservoirs), (2) peak attenuation (reduced or delayed due to reservoir attenuation), (3) release manipulation (dam designed for flood control keeping reservoir volume as low), and (4) maintenance of environmental flow (3.4 to 31.48 m3 s−1) during lean period. After construction of the DVC dams and Durgapur Barrage reservoir (built before 70 years) the flow discharge is variable and reduced significantly, i.e., reduction of mean annual peak flow from 8,378 to 3,522 m3 s−1 at Rhondia weir. Before dam construction the confidence limit of annual peak discharge was recorded between 6,081 and 10,676 m3 s−1, but after 1958 the limit reduced to only 2,574 – 4,470m3 s−1 (Ghosh and Guchhait, 2016).
4.1.3 Problem of In-channel Sedimentation and Reservoir Sedimentation
The turbidity and sediment concentration of the downstream channel flow are reduced due to sediment isolation in the DVC reservoirs, and the grain size reduction is inevitable in the delivered sediments (e.g., sediment coarsening at upstream of dam and sediment fining at downstream of dam). Clear water releases have an unfilled capacity to transport sediments (hungry water), which increases their erosive power regardless of the potential reduction of the effective flow (Casado, 2013). Bed degradation is usually the most immediate channel adjustment after dam closure, although bed aggradation processes and bank stabilization by vegetation encroachment have been also widely documented (Casado, 2013). Two prominent effects of the Barrage are recognized: (1) at downstream the river becomes more narrower due to incision of sediment free peak flow, and (2) at upstream the sediments are trapped in the channel to decrease the cross-sectional area and to increase the river width. The floodplain width is estimated near about 2 – 3km wide at downstream, but the active channel width is reduced to only 510 – 1110 m at present (up to 14 km downstream stretch from the Barrage). The previous oscillation of active channel is compressed by the flow regulation of barrage, embankment rise and encroachment of river islands for agriculture and settlement. The upstream section is controlled by the Barrage flow obstruction and urban-industrial complex. The trapped sediments and low flow competence increase the number and size of longitudinal bars in the channel up to 17 km upstream. These elevated islands (locally known as ‘manas’) are now used for the settlements and agricultural practices. The in-channel sedimentation is reflected from the increasing value of BR (i.e., 2.173 to 4.316; tending more to braided channel), and this morphological change reduces the in-channel carrying capacity to accommodate the peak flood flow during monsoonal period.
More than half of sediments from the controlled river basins are trapped by dams and about 25.30 percent of sediments worldwide are intercepted by large dams (Bhattacharyya, 2011). It is inevitable that due to unscientific coal mining, urbanization, extensive deforestation, soil erosion and expansion of agriculture in the Chhotanagpur Plateau the Damodar River transported an enormous sand load during floods and the bars are formed. The suspended sediment concentration was measured at Damodar Bridge site (29 km downstream of Panchet dam), and the value decreased from 1.87 gm l−1 (pre-dam) to 0.54 gm l−1 (post-dam), promoting almost 72 percent reduction of sediment concentration in channel flow (Bhattacharyya, 2011). The data reveals that the Maithon reservoir of Barakar River lost about 27.4 percent of overall storage capacity up to 2001, whereas the Panchet reservoir of Damodar River lost 15.9 percent of overall storage capacity (WRIS, 2021). In the Matithonreservior the actual rate of deposition as stated in plan is much greater than the designed mean siltation rate of 4.79 million m3 yr−1 (Chaudhuri, 2006). The siltation rate at the Maithon Reservoir will be reduced to about 1.5 million m3yr−1due to installation of another dam on the Barakar River, Balpaharidam (at 50 km upstream of Maithon) (Chaudhuri, 2006).The estimated siltation rate of DVC reservoirs is depicted as follows (Table 4): (1) Konar – 1.743 million m3 yr−1, (2) Maithon – 6.77 million m3 yr−1, (3) Panchet – 6.92million m3 yr−1, (4) Tenughat – 3.210 million m3 yr−1, and (5) Tilaiya – 2.748 million m3 yr−1respectively (Bhattacharyya and Singh, 2019). The reduction of gross storage varies from 11.85 million m3 (1955) to 6.437 million m3 in the Durgapur Barrage reservoir, reflecting 45.68 loss of live storage. At present the average rate of siltation is 0.042 million m3 yr−1in the Barrage (WRIS, 2021).
Table 4
Rate of sedimentation in the DVC reservoirs and Durgapur Barrage reservoir
Sl no
|
Name of Reservoir
|
Name of River
|
Total Years (year of built & year of last survey)
|
Rate of sedimentation
|
M m3 yr−1
|
m3 km−2 yr−1
|
ha m/100 km2 yr−1
|
1
|
Konar
|
Konar
|
41 (1955 – 1996)
|
1.743
|
1748
|
17.48
|
2
|
Maithon
|
Barakar
|
39 (1955 – 1994)
|
6.77
|
1076
|
10.76
|
3
|
Panchet
|
Damodar
|
56 (1956 – 2012)
|
6.92
|
631
|
6.31
|
4
|
Tenughat
|
Damodar
|
31 (1970 – 2001)
|
3.21
|
716
|
7.16
|
5
|
Tilaiya
|
Barakar
|
44 (1953 – 1997)
|
2.748
|
2792
|
27.92
|
6
|
Durgapur Barrage
|
Damodar
|
56 (1955 – 2011)
|
0.042
|
-
|
-
|
Source: Bhattacharyya and Singh (2019); WIRS (2021) |
4.1.4 Characterization in River Metamorphosis
The pertinent hydrogeomorphic research works of Ghosh (2011), Bhattacharyya (2011), Ghosh (2011), Ghosh and Guchhait (2014 and 2016), Pal et al. (2015), Verma et al. (2017), and Karim and De (2019) have revealed that river ability to transport, sediment available to transport and quality of running water simultaneously trigger a series of adjustments until the fluvial system of Damodar either accommodates the anthropogenic disturbance or reaches a new equilibrium state. Alluvial river is a sensitive element of earth’s surface, whereas any change or shift in external factors (e.g., climate change or tectonic upliftor river impoundment) it instigates a rapid response from the fluvial system towards instability (i.e., disequilibrium) within the floodplains. It is hypothesized that the Damodar River is still situated in a phase of river instability or disequilibrium which is generally considered to be that period during which river processes and form readjust to a change (changes in land uses and installation of dams since 1950s) in the sedimentologic and hydrologic regime (Miller and Miller, 2007). It can be considered this situation in a framework of threshold and complex response (Schumm, 1972) where the driving forces and resisting forces operating within the Damodar River were altered (due to dam construction and rise of embankments) to a such a degree that the limits to equilibrium were exceeded. Due to this threshold event the changing channel morphology and floodplain transformation have initiated an outlook of river metamorphosis which is depicted here as follows.
A threshold crossing event, viz., multipurpose large dam construction by DVC, occurred when the river system moved from a state of natural balance to a temporary condition of disequilibrium which is now gradually corrected as the system develops a new sate of balance adjusted to a different set of environmental conditions. After crossing the threshold, the alteration in channel forms and patterns (complex response) was initially rapid and decreases with time until a new equilibrium state is achieved, because the large dams can change the local base level conditions, forming several knickpoints in longitudinal profile.
Base level change promotes incision to a certain distance downstream, but the aggradation dominates further to develop terraces and bars. It is predicted that within 100 years of time frame the changes in lithology of sand-bed channels, bed configuration, cross-sectional form (width-depth ratio), braiding or sinuosity and channel confinement are more evident. Within 100 to 104 years of time frame the change of meander wavelength, channel gradient, and profile concavity can be traced. Two prominent metamorphosis scenarios are observed in the Damodar River: (1) Downstream of Barrage – significant reduction of flow competence (Q −−) and sediment load (L−), dominance of fluvial incision (I+) over aggradation and terrace formation, increase of slope (s+), increase in depth (d+), decrease in active channel width (w−), and overall significant reduction in channel capacity (CC−−); and (2) Upstream of Barrage – moderate reduction of flow competence (Q −) and unchanging sediment load (Lo), dominance of fluvial aggradation over incision (I−) and longitudinal bar formation, decrease of slope (s−), decrease in depth (d−), increase in active channel width (w+), and overall moderate reduction in channel capacity (CC−).
Based on the geomorphic characterization (Level I) and morphological description (Level II) the reaches of Damodar River can classified through Rosgen stream classification scheme (Rosgen, 1994). The key parameters of Level I (e.g., channel slope, channel shape and channel pattern etc.) and Level II (e.g., entrenchment ratio, width-depth ratio, sinuosity index, channel materials etc.) are presented in table. Similarly, the floodplain classification scheme of Nanson and Croke (1992) is applied to know the floodplain metamorphosis in relation to river impoundment. At upstream of the Durgapur Barrage the channel has transformed to A type to B and then, D type. After crossing the Barrage, the channel has reversed into again B type, and further downstream it changes to again D type. A type channel has attributes of steep-entrenched channel with high energy/debris transport with erosional or depositional and Gondwana bedrock forms. B type channel is characterized as moderately entrenched, riffle dominated channel, stable banks and point bars, gently sloping valley with occasional pools. D type channel is slightly entrenched with braided pattern, longitudinal and transverse bars, very wide channel with multiple threads, active lateral adjustment, and abundance of sediment supply. In case of floodplain classification, two sequences are observed as follow: (a) Change from A3 to B1 type at upstream, and (b) again change from A3 to B1 type at downstream. A3 type floodplain is characterized by unconfined to confined vertical accretion, specific stream of 300 – 600 Wm−2, sandy-strata with inter-bedded muds, sandy flat floodplain surface with single-thread channel wandering, occasional channel wandering, overbank vertical accretion, island deposition, and abandoned channel accretion with minor lateral accretion. B1 type floodplain is categorized as usually braided channel, specific stream of 50 – 300 Wm−2, floodplains with gravels, sand and silts in bed sediments, braided-channel accretion and incision, overbank vertical accretion of islands and abandoned channel accretion, undulating floodplain of abandoned channels and bars, backswamps and relatively high sediment load. So, it can be said that the Durgapur Barrage reservoir has controlled the floodplain associations and characteristics of the Damodar River, changing braided nature (mainly upstream of barrage) to single thread entrenched channel.
Table 5
Brief summary of channel and floodplain classification criteria
Channel Section
|
Cumulative Distance (km)
|
SI
|
BR
|
ER
|
W/D
|
Mean Channel Slope (%)
|
Channel Type1
|
Floodplain Type2
|
S1
|
4.50
|
1.09
|
2.17
|
1.19
|
179
|
0.0318 (Upstream of Barrage)
|
A
|
A3
|
S2
|
9.46
|
1.04
|
2.2
|
2.14
|
274
|
B
|
S3
|
14.08
|
1.08
|
2.72
|
1.48
|
303
|
B
|
S4
|
19.21
|
1.05
|
3.21
|
2.02
|
261
|
D
|
B1
|
S5
|
25.09
|
1.04
|
4.32
|
1.43
|
200
|
D
|
S6
|
30.50
|
1.12
|
1.96
|
2.5
|
181
|
0.0412 (Downstream of Barrage up to Rhondia Weir)
|
B
|
A3
|
S7
|
35.96
|
1.07
|
1.81
|
2.69
|
140
|
B
|
S8
|
41.07
|
1.1
|
2.52
|
1.81
|
178
|
D
|
B1
|
S9
|
47.00
|
1.14
|
3.53
|
2.11
|
215
|
D
|
Note: Entrenchment ratio – ER; sinuosity index – SI; braid-channel ratio –BR; channel width-depth ratio –W/D; 1 – Rosgen stream classification system; 2 – Nanson and Croke floodplain classification system
4.2 Evaluating Fluvial Functionality
The developed thematic FFI map of Damodar River (figure 8) enables the present status of functioning (individual stretch) to be grasped straightway and it can be a useful instrument for planning the reclamation of the fluvial environment. Based on the FFI evaluation, the Damodar River functionality level varies from category IV to II, i.e., poor to good-fair functionality level. The FFI score varies from 85 to 181, showing prominent influence of transverse obstacles, eutrophication, anthropogenic bed modifications and urban-industrial pollutants. Only, S2 section shows good-fair fluvial functionality level II, showing relatively abundance of flora and fauna, morphological diversity of river bed, proliferation of complex formation and riparian vegetation (strip greater than 30 m). The key problem is observed in the river stretch immediate upstream and downstream of the Durgapur Barrage reservoir, yielding very low FFI score of 61 – 100 (category IV poor functionality level). The observed factors of this low fluvial functionality are mainly categorized as follows: (a) daily influx of sewage and industrial pollutant water from Durgapur – Waria – Raniganj townships, thermal power plant and heavy industries; (b) intensive agriculture in stabilized islands; (c) flow obstacles by the Barrage sluices and linear transverse construction; (d) sand mining with heavy machines and vehicles; and (e) issue of reservoir siltation and in-channel sedimentation (figure 9). At downstream of the Barrage, the river recovers its functionality to category III level (i.e., fair), but the complex formation and riparian structure are not proliferated in the sand dominated alluvial valley mainly due to channel narrowing, discharge variability (environmental flow), and intensive in-channel sand mining and agricultural practices. The main aquatic vegetation communities of the Barrage reservoir and river bed are identified as Eichhorniacrassipes, Salviniacuculata, Nelumbonucifera, Hydrillaverticillata, Ipomoea aquatic, LemnaSp, Typhaspetc. (Gupta and Mukherjee, 2015). The dominant floral species of sandy river bed is Saccharumspontaneum which grows highly during monsoon period. The surroundings of the Barrage are now identified as Important Bird and Biodiversity Area (IBA) (figure 9) by the Birdlife International (Cambridge, United Kingdom), having record of 253 species (Adhurya et al, 2015).
From the survey and FFI report card analysis, the key issues (need for restoration or management) of this fluvial ecosystem are recognized as follows (figure 9): (a) seasonal cultivation with expansion of urban areas mainly in the left bank floodplain; (b) absence of riparian formations but presence of anyway functional formations due to mobile river bed and bed modifications frequent interruption and loss of continuum state); (c) frequent flow disturbances with the lean flow of six dry months; (d) flood stage river breadth occasionally overcomes greater than 2 to 3 times of the moderate flow river bed; (e) absence of retention structure that enhancing reed groves and hydrophytes; (f) artificial intervention escalates very low morphological diversity and bed stabilization in the river cross-sections; (g) intensive sand mining and allied activities become a threat to avian wildlife of the Damodar River; and (h) very low density of tolerating riparian plant component, recognizable fibrous – pulpy detritus materials. The zone – in yellow (S1, S7, S8 and S9) – needs to restore the eco-buffer in the peri fluvial zone, taking measures to restrict the expansion of commercial activities and residential use. The zone – in orange (S3 to S6) – needs more precaution from intense urbanization and decreasing ecological diversity through limiting the intervention and boosting up ecological quality. The results of the FFI at the reach level of dam-controlled Damodar River provide us the possibility to do more research at the micro-level to mitigate the impact or to reassess the quality of the fluvial environment in the era of Anthropocene. There is a need for a fluvial zone of adequate ecological quality, made up of well-established arboreal and shrubby riparian formations (act as eco-buffer to surrounding territory), that must be protected and correctly maintained in the Damodar River.
4.3. Flow Alterantion and Fluvial Functionality
The flow duration curve is used to assess environmental flows to illustrate the hydrological situation of the river system(Suwal et al. 2020). It represents the proportion flow exceeded to the percentage of time at a particular river section. This exceedance curve is used to define a minimum threshold value for sustaining and maintaining the riverine ecological integrity. This flow duration (Fig. 11a,b) curve has been constructed based on the average monthly flow discharge data of the lean period (December to May) at Rhondia of Damodar River in the pre-dam (1934-1957) and post-dam period (1958-1957). Based on the flow duration curve, it reveals that 97.8% exceedance probability is attained and it’s the corresponding flow value is 1 m3/ s in the pre-dam period. Therefore, 1 m3/ s flow was available throughout the months of the lean period in the pre-dam period. Whereas 98.8 % exceedance probability is attained in the post-dam period and its corresponding flow value is 8 m3/s of flow. Hence 8 m3/s flow is available throughout the months of the lean period in the post-dam period. So, the exceedance probability with its corresponding flow value slightly increases (from 1 to 8 m3/s) after the construction of dams and barrage.
Table 6
Frequency analyses of daily average flow at Rhondia site of Damodar River in pre-and post-dam periods
Daily average flow
|
Return time
|
EF
|
Pre-dam (1940–1950)
|
Post-dam (1993–2008)
|
change
|
%
|
(m3/s)
|
(m3/s)
|
(m3/s)
|
(years)
|
1
|
3575
|
2460
|
-1115
|
100
|
5
|
1977
|
1139
|
-838
|
20
|
10
|
1263
|
681
|
-582
|
10
|
20
|
577
|
322
|
-255
|
5
|
30
|
200
|
168
|
-32
|
3.33
|
40
|
85
|
99
|
14
|
2.50
|
50
|
38
|
66
|
28
|
2.00
|
60
|
19
|
37
|
18
|
1.67
|
70
|
9
|
16
|
7
|
1.43
|
80
|
3
|
6
|
3
|
1.25
|
85
|
0.7
|
2
|
1.3
|
1.18
|
Source: Bhattacharyya, 2014 |
The daily average flow at Rhondia site of Damodar River in the pre-dam (1940–1950) and post-dam (1993–2008) periods has been analyzed to evaluate the environmental flow for the recommendation of daily flow for the aquatic environment. Based on this analysis, the flow discharge corresponding to 85 % exceedance probability is 0.7 and 2 m3/s of flow in the pre and post-dam periods respectively. Whereas, 1% exceedance probability to its corresponding flow is 3575 and 2460 m3/s in the pre and post-dam periods respectively. The corresponding flow of 1, 5, 10, 20, and 30 % exceedance probability reduces which means the probability of daily high flow has been decreased. While 40, 50, 60, 70, 80, and 85% exceedance probability to its corresponding flow increases that means the probability of daily low flow has been increased after the installation of dams and barrage. So, the variability of daily flow has changed due to the regulation of flow by dams and barrages.
This flow duration (Fig. 12) curve has been constructed based on the daily average flow discharge data at Rhondia of Damodar River in the pre-dam (1940–1950) and post-dam period (1993–2008). According to the reference flow duration curve, any shift of an FDC to the left means this loss (part of variability is lost) due to the reduced assurance of monthly flows, i.e. same flow will be occurring less frequently. The corresponding flow of 1, 5, 10, 20, and 30 % exceedance probability reduces due to the flow regulation and the flow occurs less frequently. Whereas the corresponding flow of 40, 50, 60, 70, 80, and 85% exceedance probability increases and flow occurs in high frequency. The flow duration curve of high-frequency flow (<30% exceedance probability of flows) shifts left and the lower frequency flow (>40% exceedance probability of flows) curve shifts right after the construction of dams and barrage on Damodar River (Fig. 12). Hence, the analysis of the daily flow and flow duration curve indicates that the variability is lost due to reducing high flows frequency and increasing lows flow frequency in the river. So, the variability of daily flow has changed due to the regulation of flow by dams and barrages.
Table 7
Environmental flow assessment through Tennant method
Months
|
Pre-dam (1934-1957)
|
Post-dam (1958-2007)
|
Average monthly discharge in m3/s
|
MAF in m3/s
|
Minimum flow for Aquatic-Habitat (10 % MAR)
|
Average monthly discharge in m3/s
|
MAF in m3/s
|
Minimum flow for Aquatic-Habitat (10 % MAR)
|
(Mar–May)
|
57.05
|
1001.33
|
100.13
|
103.83
|
700.04
|
70
|
(Jun–Sept)
|
3371.18
|
2145.86
|
(Oct–Nov)
|
477.05
|
433.48
|
(Dec–Feb)
|
100.05
|
117.00
|
Note: The mean annual runoff (MAR) of the Lower Damodar River at Rhondia site (a threshold of 10% MAR is prerequisite for aquatic ecosystem), (Based on Bhattacharyya, 2011) |
The concept of environmental flow suggests that a minimum flow is required to maintain and sustain the riverine ecosystem and ecology. According to the Tennat method, a minimum (i.e.10% of MAF) percentage of the mean annual flow (AAF) is a prerequisite for aquatic habitat to maintain the biological integrity of the riverine ecosystem. The mean annual flow (MAF) of the pre-dam period was 1001.33 m3/s but due to flow regulation, it decreases 700.04 m3/s in the post-dam period at Rhondia site of the Lower Damodar River. The requirement of minimum flow for riverine habitat was 100.13 m3/s in the pre-dam period while it is 70 m3/s in the post-dam period. As the flow is regulated to maintain the minimum flow in the river system, the requirement of minimum flow for aquatic habitat is also decreasing due to reduced MAF. A threshold of 10% MAR for the aquatic ecosystem is not met in the lean season (December to May months) in the pre-dam period whereas it is met in the pre-dam period as per the average monthly data at Rhondia site of the Lower Damodar River.