Environmental Impacts of Reservoir Construction on the Godavari and Krishna Deltas

Interbasin water transfers are associated with the construction of new storage reservoirs. A lot has been said and written about submergence and resettlement (upstream) and impacts of changing flow pattern on fish (downstream) – all associated with reservoirs. At the same time, all in-stream storages anywhere in the basin have impacts on river outlets. Given the number of reservoirs already constructed in both basins (the Krishna and Godavari) as well as the planned massive storage construction associated with NRLP, it is only natural to highlight the issues of upstream development impacts on deltas and estuaries. These issues have not been considered in the NWDA reports. They also have a general tendency to be ignored in water resources planning  worldwide. At the same time, depending on the river and the magnitude of upstream construction such impacts may be significant.

Coastal Erosion: The Godavari Delta

Malini and Rao (2004) examined the recent changes in the Godavari River Delta, called the “rice bowl of Andhra Pradesh,” using remote sensing images. They discovered that the delta has regressed landward with a total net land loss of 1,836 hectares over the period of 1976–2000 (at a rate of 73.4 ha/year). It was suggested that reduced inflow of sediments, associated with upstream reservoir construction, is the main cause of reduced vertical accretion at the delta. At the same time, coastal subsidence, probably promoted by neo-tectonic activity and consequent relative sea-level rise has continued leading to shoreline retreat. Figure 6 illustrates the dynamics of flow and sediment load at the outlet of the Godavari (at Polavaram) and reservoir storage growth in the entire Godavari Basin since the beginning of the 1970s. The flow time series has been taken from Internet sources, the sediment load data have been read off similar graphs published by Malini and Rao (2004) and the storage data are from the ICOLD dam register. The flow time series has missing data during 1980–1990 and neither flow nor sediment data have been available after 1998. Cumulative dam storage (including large and medium dams) increased significantly in the early 1970s and has remained relatively constant for the last 30 years. However, it will increase abruptly again after the construction of the Polavaram Barrage and the major Inchampalli Dam (the growth of the total storage in the basin after the dam construction is shown in Figure 6 for an arbitrarily assumed Inchampalli Dam completion date of 2010).

While trends in the Godavari River flow cannot be ascertained from the available disrupted flow time series, the decreasing trend in annual sediment loads is clear from the sediment data (Figure 7, also shown by Malini and Rao [2004]). The mean annual sediment load has decreased from 100 million tons in 1978 (effectively an ending point in noticeable reservoir growth in the basin, Figure 6) to 46 million tons by the end of the 1990s. The current cumulative reservoir storage in the Godavari Basin remains relatively low (6.3 BCM, i.e., approximately 6% of the mean annual flow at the outlet). The storage growth is not the only one of significance as much water is also diverted from barrages, i.e., structures without any storage. A relatively small storage in the basin and a still noticeable decreasing trend in sediment load imply that the basin sediment regime is very sensitive to reservoir growth, if the latter remains to be seen as the main source of the problem. More sediment inflow reduction may therefore be expected after the construction of the Polavaram and Inchampalli storages, which will increase the ratio of storage to 19% of the natural flow in the basin.

Coastal Erosion: The Krishna Delta

In this study, an attempt has been made to examine whether similar trends exist in the Krishna Basin, concerning the proportion of storage: annual flow is much larger than in the Godavari. The observations on sediment loads at the outlet of the Krishna at Vijayawada over the last 30–40 years have however not been provided by the Central Water Commission (CWC) during the course of the study. The only available data were for the period 1991–2000 (CWC 2006), hich is rather short for any meaningful conclusions on trends. The comparison of the two short time series of sediment loads, at Agraharam (upstream of major reservoirs, Figure 2) and at Vijayawada (downstream of all major dams), has revealed a significant decrease in sediments downstream of the reservoir system (Figure 8). The differences are particularly noticeable in high-flow years (1994, 1999), when more sediment has reached Agraharam from the relatively unregulated upstream basin but all sediments were likely being trapped by the existing reservoir system (Srisailam, Nagarjuna Sagar) upstream of Vijayawada.

The absence of sediment data prior to 1991 does not allow for  urther conclusions about sediment regime changes to be made. However, these changes are most likely very significant due to the marked reduction of river flow at the Krishna outlet (Figure 9) over the last 70 years. This reduction is due to various water diversions, groundwater development and increased cumulative reservoir storage in the basin, which has grown from almost zero in 1960 to 28.5 BCM at present. This present cumulative storage represents 36% and 132% of the natural and present-day Krishna mean annual flow, respectively.

To examine the potential impacts of reduced sediment inflow on the Krishna Delta, several remote sensing images of the area were analyzed. The images were obtained from Earth Science Data Interface (ESDI) at the Global Land Cover Facility (GLFC) on http://glcfapp.umiacs.umd.edu: 8080/esdi/index.jsp and were selected from the period of 1977 to 2000 to form a “time series.” The images included:

• Landsat 2 Multispectral Scanner (MSS) image dated 1 June 1977 with a spatial resolution of 57 meters (m).
  

• Landsat 5 Thematic Mapper (TM) image dated 10 November 1990 with a spatial resolution of 28.5 m.
  

• Landsat 7 Enhanced Thematic Mapper plus (ETM+) image dated 28 October 2000 with a spatial resolution of 28.5 m.

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Three basic layers were used to detect morphological changes in the delta: band 4 (near infrared [NIR]), band 2 (red) and band 1 (blue). These layers have characteristics that are suitable for coastal mapping, differentiation of vegetation from soil, reflectivity of vegetation vigor and delineation of water bodies. The first, “oldest” image was assumed to be the reference condition against which changes in the other two images were detected. The entire delta shoreline was examined to demarcate the zones of erosion and deposition using ERDAS 9.0 software. The areas of deposition and erosion between two consecutive dates (i.e., in 1990 and 2000) were identified and calculated using ArcGIS software. The areas around selected points (primarily the mouths of the main distributaries), where significant changes were expected to occur were closely examined, highlighting the zones of erosion and deposition at each. The image of the Krishna Delta showing selected areas where detailed assessment of erosion and deposition has been made is presented in Figure 10. Figures 11 and 12 display the sequence of images for years 1977, 1990 and 2000 for some of the selected areas circled in Figure 10. The black lines in each image represent the reference position of the land mass at the start of the period, in 1977. Figure 13 shows areas of predominant erosion and deposition during the period between 1977 and 2000 for the entire delta shoreline, while table 4 summarizes the calculated characteristics of these processes for the entire delta over the same period.

The results suggest that while areas of predominant erosion and deposition interchange, the overall tendency is towards the regression landward with losses of land to the sea, the situation similar to that in the Godavari Delta. The annual net loss rate of 77.4 hectares is almost the same as that in the Godavari Delta (73.4 ha/year; Malini and Rao 2004). One noticeable feature of the Krishna Delta is also its higher ratio of erosion to deposition (3.05 versus 1.6 in the Godavari) over the same period, which suggests that coastal erosion is more “effective” in the Krishna Delta than in the Godavari, despite the slightly smaller area (4,700 km2 versus 5,100 km2) and shorter shoreline of the former (134 km versus 160 km). Erosion is also a dominant process through most of the coastline, while deposition is limited to certain sections only (Figure 13).

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Possible Causes and Implications of Coastal Erosion

The regression of both deltas cannot be explained by the sea-level rise. Analysis of the available sealevel data in the region for the period 1970–1996 (measurements at Visakhapatnam and Chennai) and for the period 1990– 001 (calculations from daily tide gauge data at the Kakinada to the north of the Godavari Delta) did not reveal any significant rising or falling trends (Malini and Rao 2004). Therefore, coastal erosion in the Krishna and Godavari deltas can only be explained by the above-illustrated reduced sediment supply that, in turn, is due to upstream flow regulation. In addition, human activities in delta regions (e.g., conversion of cropland and mangrove swamp areas into aquaculture ponds) may also be responsible for sea transgression leading to coastal erosion and shoreline retreat of the deltas (e.g., Sarma et al.2001).

Analysis of the longer sediment load data series for the downstream parts of the Krishna and the use of more recent and more resolute remote sensing images would result in more detailed quantification of delta erosion. However, even with the existing limited data, it is possible to suggest that upstream basin storage development leads to the said retreat of deltas. The Krishna River is already effectively a “closed basin” as only occasional high flows “spill” intothe delta with almost zero sediment contribution to it (Figure 8). Therefore, the storage that is already constructed in the Krishna will have a long-lasting detrimental effect on the delta and its agricultural productivity (the situation in the Godavari Delta will also most likely deteriorate after the construction of the additional storages planned as part of the NRLP).

Detailed sedimentation modeling studies would be useful in all major deltas of India in order to develop a better understanding and quantification of the links between upstream water and sediment flow reduction, upstream storage growth and maninduced changes in deltas, on the one hand, with deltas’ erosion and retreat, on the other. Such studies could allow the specification of necessary environmental flow releases to be made for the maintenance of delta sediment regimes.

Coastal erosion may be seen as a slow process. However, there are a few aspects which promote negative environmental impacts associated with it. One is the saltwater intrusion.Bobba (2002) conducted a numerical modeling study of the Godavari Delta and showed that saline intrusion may become a major factor of reduced agricultural productivity in that delta due to increased groundwater pumping and reduced freshwater inflow (the authors could not identify a similar published study for the Krishna Delta). Coastal erosion, caused by similar factors facilitates saltwater intrusion deeper in the delta adversely affecting the productivity of land. Additionally, although highly uncertain in quantitative terms, there is the potential sea-level rise in the next 50 years due to climate change, although the limited available observations have not detected it so far. This rise can lead to even more coastal erosion and deeper saltwater penetration, accelerating delta degradation. This research was not the scope of the current study and needs to be carried out as a separate and detailed project. While quantification of the above impacts will be developed, even limited environmental flow releases from existing reservoirs in the Krishna and Godavari will delay the adverse environmental processes in both deltas. New storage reservoirs need to be planned so as to allow sediments to reach deltas. Construction of the most downstream reservoirs, particularly as large as Inchampalli, will definitely not serve this purpose.