Spatio-Temporal Variations in Streamflow Trends

Streamflow Characteristics across the Tapi Basin

The streamflow characteristics of the Tapi River and its tributaries are shown in Figure 5.5. The runoff in the Tapi River is found to increase as we move from upstream

Distribution of the mean annual runoff across the Tapi basin

Figure 5.5 Distribution of the mean annual runoff across the Tapi basin.

to downstream, except at the Ghala station. The decrease in runoff at the Ghala station is due to the significant regulation effect of the Ukai reservoir on streamflows in the Lower Tapi River. On the other hand, a considerable variation in mean annual runoff is observed between the Tapi River and its tributaries. The Purna, Girna, Bori and Panjhara rivers are mostly dominated by a semi-arid climate and vast agricultural lands. Due to these climatic and physiographic distinctions, such regions experience high evapotranspiration and percolation losses, which reduce the surface runoff generation (Sharma et al., 2018b). The Purna and Girna sub-catchments occupy almost 45% of the basin area; however, they contribute only 27% of total runoff into the Ukai reservoir. The Bori and Panjhara rivers are ephemeral and do not encounter consistent flow even in the monsoon.

Trends in streamflow indices

The trends in two streamflow indices, viz., total annual runoff (QTOT) and annual maximum 1-day streamflow (Qxlday), have been investigated using the MMK test and are shown in Figure 5.6. The results indicate uniformly decreasing trends in QTOT across the Tapi basin, except in the upper reaches of the Tapi River in the UTB, see Figure 5.6a. An increasing trend in QTOT is observed for Dedtalai, Burhanpur and Hathnur dam stations. Contrary to that, the stations along Purna, Girna, Bori and

Panjhara rivers show a decreasing trend in QTOT. Additionally, Savkheda and Gidhade stations showed a significant decreasing trend in QTOT at the 5% significance level. The consistent decreasing trends in QTOT are due to spatially decreasing trends in PRCPTOT across the UTB and MTB. The spatially extensive decreasing trends in QTOT would imply a decrease in freshwater availability in the basin and could escalate the water stress conditions. Further, the analysis of Qxlday reflected heterogeneity in their trends, wherein seven out of fourteen stations showed increasing trends, while the remaining seven stations showed decreasing trends, see Figure 5.6b.

Increasing trends in Qxlday are observed for Dedtalai, Burhanpur and Hathnur dam stations across the Tapi River in the UTB; Lakhpuri and Gopalkheda stations across the Purna River in the UTB and Malkheda and Morane stations across Bori and Panjhara rivers in the MTB. However, decreasing trends in Qxlday are reported across stream gauging stations along the Tapi River in the MTB and LTB.

The heterogeneous trends in Rxlday across the MTB are partially accountable for decreasing trends in Qxlday since the Hathnur reservoir (outlet of the UTB) has a minimal influence on regulating the peak streamflows into the downstream Tapi River (Sharma et al, 2019a). The increasing trend in Qxlday at the aforesaid stations could result in flash flood situations in the upstream regions.

Variation in reservoir levels

The daily reservoir levels of the Ukai Dam for the period 1973-2013 are analyzed to derive the annual time series of maximum reservoir storage levels, and the corresponding reservoir storage volume is also estimated from the capacity elevation curve, see Figure 5.7a and b. It is noteworthy that the capacity elevation curves of the Ukai Dam were revised on two occasions post-construction of the Ukai Dam (1993 and 2003). The full reservoir level (FRL) of the Ukai reservoir is at 105.15 m, while the maximum water level (M WL) is at 106.99 m, see Table 5.1. From the sedimentation surveys, it is reported that a considerable decrease in the gross storage capacity (at the FRL) of the Ukai reservoir has been observed over the time. The gross storage capacities at the FRL during the years 1972, 1993 and 2003 were estimated to be 8511.0, 7497.1, and 7414.29 Mm3, respectively (Sharma et al., 2014). Thus, due to reservoir sedimentation, the gross storage capacity of the Ukai reservoir was found to reduce by 13% during the aforesaid period. However, the rate of sedimentation in the Ukai reservoir is found to be under permissible limits.

The temporal variation of reservoir storage levels shows that during the initial phases, the maximum reservoir level often surpassed the FRL. However, the reservoir storage volume was found to exhibit a decreasing trend with respect to time due to decreasing streamflow in the Tapi basin (see Figure 5.7b). During the phase 1982-1987, the reservoir faced large deficits, wherein the actual storage volume with respect to storage at the FRL for years 1982, 1985 and 1987 was estimated to be 35.0%, 44.7% and 52.9%, respectively, see Figure 5.7c. The large deficit in reservoir storage was encountered due to a considerable decrease in rainfall during these years which led to drought-like conditions in the Tapi basin. On the other hand, the reservoir storage surpassed the FRL (i.e., more than 100% reservoir filling) during the years 1975, 1981, 1988, 1989, 1990, 1994, 1998 and 2006, out of which Surat City witnessed severe flooding in the years 1994, 1998 and 2006. Lack of real-time flood forecasting information for the Tapi River yielded inaccurate and delayed inflow forecasts into the Ukai reservoir, which led to complete filling of the flood cushion storage in the reservoir. Under such circumstances, the reservoir operators were unable to absorb additional inflow volumes generated due to continuous rainfall in the upstream, which enforced heavy releases from the Ukai reservoir in the downstream channel. Overall, it was observed that the reservoir was invariably full at the end of the monsoon. Sometimes a prolonged monsoon spell causes large inflow into the reservoir during the fag end of the monsoon, leading to sudden releases in the Lower Tapi River and severe flooding in Surat City. The severity of flooding increased further due to a decrease in the carrying capacity of the Tapi River, as described in Section 5.6.2.2. Thus, the development of a real-time flood forecasting model, reservoir operation using advanced scientific techniques and implementing suitable structural flood protection measures are the need of the hour to mitigate flood hazards in the LTB.

Temporal variations in

Figure 5.7 Temporal variations in (a) annual maximum reservoir storage levels, (b) reservoir storage volume corresponding to the maximum reservoir level and (c) percentage reservoir filling with respect to the FRL, for the Ukai reservoir. The blue and red bars indicate values corresponding to non-flood and flood events, respectively, in the Tapi basin.

Flooding and Morphological Issues in the Tapi Basin

History of Floods in the Tapi Basin

The Tapi River is characterized by one of the most intense flood regimes of seasonal tropics (Kale et al., 1994). The Tapi basin lies in the zone of severe rainstorms, and thus, is subjected to frequent high-magnitude rainfall and floods. Historically, the Tapi River has brought several large floods to Surat City during the years 1727, 1776, 1782, 1829, 1837, 1872, 1944, 1959, 1968, 1970, 1994, 1998, 2006 and 2013 (Kale & Hire, 2004; Timbadiya et al., 2014). The flood in the year 2006 had a catastrophic effect on lives and property in Surat City, wherein nearly 300 people died and economic losses of IN R 210 billion were incurred (Patel & Srivastava, 2013). From the gauging records, it is seen that the 1968 flood is the largest recorded flood so far with a peak discharge magnitude of43,891 mVs (Vora et al., 2018). The flood regime in the Tapi basin is found to be rather interesting. During the phase 1942-1949, the Tapi basin experienced four major flood events within a span of eight years. Similarly, during the phase 1968-1979, four major floods were recorded within a span of 11 years. Thus, the floods in the Tapi basin have been frequent and periodic in nature. The 2006 flood is the highest recorded flood after the construction of the Ukai Dam, with an instantaneous peak discharge of 34,122 nrVs. Sharma and Patel (2018) analyzed the century-long rainfall data (1901-2013) and reported an increasing trend in extreme rainfall and rainfall intensity across the LTB, which would make Surat City more susceptible to urban flood hazards. Further, the vulnerability of residents of Surat City may increase manifolds when the riverine floods are coupled with high-magnitude rainfall over the LTB.

Major Morphological Issues in the Tapi Basin

Reservoir Sedimentation in the Upper Tapi Basin

The Tapi and Purna rivers in the UTB flow through varied hydrogeological conditions and they meet just 14 km upstream of the Hathnur Dam. Around 90% of the Burhanpur sub-catchment has rocky strata, while the Purna sub-catchment has predominantly alluvium soils (Sharma et al., 2018b). Chandra et al. (2016) analyzed the sediment data for Tapi and Purna rivers for the period 1993-2004 and indicated that the main Tapi River was responsible for nearly 80% of the annual sedimentation in the Hathnur reservoir. It is noteworthy that the Burhanpur sub-catchment occupies only half the area compared to the adjoining Purna sub-catchment. Further, the Tapi River flows through a steep topography, and hence, the higher flow velocities are largely responsible for the disintegration of rocks into sediments and their subsequent transport. Compared to that, the Purna River flows through a relatively flat terrain, thereby the sediments are not carried along with the streamflow for longer distances. The transported sediments settle down in the still water upstream of the Hathnur Dam and gradually encroach the dead storage capacity. Ladhe et al. (2008) found that sedimentation in the Hathnur reservoir led to a significant decrease in its live storage capacity, wherein a decrease in live storage of 38% was recorded in the past 25 years (during 1982-2007) after the construction of the Hathnur Dam. Thus, soil erosion from the UTB must be checked by adopting suitable conservation measures to prevent further silting of the Hathnur reservoir. In the longer run, the Hathnur reservoir would be ineffective to serve water requirements in the region if check on the sedimentation rate is not exercised.

Reduction in Channel Carrying Capacity in the Lower Tapi Basin

The peak flood discharges measured at the Kathor/Ukai Dam and the observed levels at Nehru Bridge were analyzed to investigate the effect of the Ukai Dam on changes in the carrying capacity of the Tapi River at Surat City. Figure 5.8 shows a comparative analysis of the observed peak flood discharges in the Tapi River and the corresponding stage at Nehru Bridge for two periods, viz., before and after the construction of the Ukai Dam.

Comparative representation of the observed flood stages at Nehru Bridge and the corresponding observed peak discharge at the Kathor/Ukai Dam before and after the construction of the Ukai Dam

Figure 5.8 Comparative representation of the observed flood stages at Nehru Bridge and the corresponding observed peak discharge at the Kathor/Ukai Dam before and after the construction of the Ukai Dam.

It is seen that the slope of the fitted line becomes steeper for the period after the construction of the Ukai Dam (1972-2013) vis-a-vis before the construction of the Ukai Dam (1939-1971). On comparison of flood magnitudes of 1968 and 2006 floods, a significant variation in the peak discharge is observed for almost the same stage at Nehru Bridge. The peak discharges in 1968 and 2006 were 15.6 lakh cusecs and 9.1 lakh cusecs, respectively, which imply that the peak discharge intensity in the year 2006 was almost 58% less than that in the year 1968; however, they both yielded nearly the same stage of around 12 m. The above stated fact considerably affirms that there is a substantial reduction in the carrying capacity of the Lower Tapi River after the construction of the Ukai Dam. An alarming reduction in the safe carrying capacity of the Tapi River in Surat City from its design value of 24,070 nrVs (at the time of construction of the Ukai Dam) to merely 11,320 m3/s (at present) is observed. The possible causes for the reduction of the capacity of the Tapi River in Surat City could be due to the following reasons (Jain et al., 2007; Vora et al., 2018): i. Extensive encroachment on river flood plains in Surat City.

ii. Extensive siltation in the river channel in Surat City.

iii. Afflux created due to the construction of the Singanpore weir within Surat City.

Thus, Surat City has become highly vulnerable to flooding due to the rapid pace of urbanization in the recent past and the reasons cited above. The economic and social losses due to floods could be traumatic and irreversible. Thus, suitable mitigation measures are absolutely needed to safeguard Surat City against the twin hazards of riverine and urban flooding.

Flood Protection Measures in the Lower Tapi Basin

The CWC has classified flood prone areas into three categories based upon the suggestions of the expert committee for scientific assessment of flood prone area in India (Patel, 2017):

i. Severe flood prone area. Inundation area computed corresponding to the high flood level (HFL) based on 1 in 3-year flood frequency.

ii. Moderate flood prone area. Inundation area computed corresponding to the HFL based on 1 in 7-year flood frequency.

iii. Normal flood prone area. Inundation area computed corresponding to the HFL based on 1 in 10-year flood frequency.

In the present study, levees are proposed as a flood protection measure for peak discharges corresponding to three-, seven- and ten-year return periods, based on the results of a one-dimensional steady-state hydrodynamic model. From the analysis, the extreme value type I distribution was found to be the best fit for data to model the peak discharge series at the Ukai Dam (Vora et al., 2018). The peak discharges corresponding to three-, seven- and ten-year return periods were estimated to be 13,734, 20,798, and 23,911 m3/s, respectively. In order to mitigate floods in Surat City, levees are proposed, in the present study, for the protection of severe, moderate and normal flood prone areas, using the results obtained from hydrodynamic modeling and MATLAB* code. The left and right bank levees are marked in red and black, respectively, in Figure 5.9. The land use land cover in the LTB for the year 2005 is also shown in Figure 5.9. From Figure 5.9, the land use shows the presence of urban settlements and industries on both banks of the Tapi River up to the Singanpore weir, while sparse urban settlements are seen upstream of Surat City. The present safe carrying capacity of the Tapi River at Surat is 11,320 nr3/s, which corresponds to a return period of 2.4 years. Therefore, any discharge higher than this magnitude would cause inundation in Surat City, and thus, levees are needed to safeguard against frequent flooding. The total length of the proposed levees (for both banks) for severe flood prone areas (Figure 5.9a) has been found to be 56.08 km, out of which 50.3 km is proposed adjacent to urban settlements. Further, the total length of levees proposed for moderate (Figure 5.9b) and normal (Figure 5.9c) flood prone areas is estimated to be 90.34 and 104.38 km, respectively. It is found that the extent of levee requirement is very less in rural or agricultural areas against flooding vis-a-vis those within the urban limits of Surat City. Thus, phase-wise

Proposed levees as a flood protection measure for

Figure 5.9 Proposed levees as a flood protection measure for (a) severe flood prone areas (I in 3-year flood frequency), (b) moderate flood prone areas (I in 7-year flood frequency), (c) normal flood prone areas (I in 10-year flood frequency), for the LTB.

implementation of levees should be undertaken to safeguard life and property against frequent flooding in the LTB.

Conclusions

This chapter provides insights into long-term hydroclimatic variability across the Tapi basin and emphasizes the key hydrological and morphological issues. The Tapi basin is a climatically sensitive basin that is prone to frequent floods and occasional droughts. The current investigation presents the changes in rainfall magnitude, duration and extremes, separately, for the Upper, Middle and Lower Tapi basins for the period 1973— 2013. The analysis revealed a decreasing trend in the PRCPTOT across the Tapi basin, except in the LTB, whereas a decreasing trend in RD is found across the UTB, while an increasing trend is observed for the LTB and MTB. Extreme rainfall showed heterogeneous trends, wherein the coastal regions of the LTB and the western half of the MTB exhibited extensive increasing trends. On the other hand, the QTOT exhibited a coherent decrease across the entire Tapi basin, except in the upper reaches of the Tapi River. The extreme streamflow exhibited increasing trends across the UTB (except at the Yerli station) while showing decreasing trends in the remainder of the basin. T he storage volume in the Ukai reservoir exhibited a decreasing trend over the period, which is due to the decreasing annual runoff in the basin. The decrease in reservoir storage would manifest water shortage conditions in the region and is likely to aggravate water and food insecurity in the near future. The key morphological issues in the Tapi basin, such as reservoir sedimentation in the UTB and a decrease in the carrying capacity of the Tapi River in Surat city, are highlighted. Both these problems would likely have adverse impacts on the flooding and streamflow ecology. Further, the study also proposed structural measures for flood protection, viz., levee construction, for protection of flood prone areas within Surat City along the Lower Tapi River.

Acknowledgements

The authors appreciate the valuable contributions of Dr. Viraj D. Loliyana, Former Research Scholar, SVNIT Surat, and Mr. Anav Vora, B. Tech. Graduate, SVN1T Surat. The second author acknowledges the financial support received from the Department of Science and Technology (DST), Govt, of India. The authors are grateful to the TEQIP-II sponsored Centre of Excellence (CoE) on ‘Water Resources and Flood Management’ and Indian National Committee on Climate Change (INCCC) sponsored research project ‘Impact of Climate Change on Water Resources of Tapi Basin,’ for providing the necessary infrastructural support. The authors express their gratitude to India Meteorological Department (IMD), Pune; Central Water Commission (CWC), Surat; Ukai Civil Circle, Ukai, Govt, of Gujarat and Tapi Irrigation Development Corporation (TIDC) Jalgaon, Govt, of Maharashtra, for providing the necessary data for the reported study.

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