Coastal erosion and habitat loss along the Godavari delta front - a fallout of dam construction(?)

Abstract

Analysis of multi-date satellite sensor data and maps indicated loss of 1836 ha of land during 1976–2001 along the Godavari deltaic coast resulting in displac ement of coastal communities and mangrove destruction. Decrease in sediment loads from an annual average of 145.26 million tons in 1971 –79 to 56.76 million tons during 1990–98, apparently due to construction of dams, largely diminished vertical accretion at the delta, while continued coastal subsidence that might have been accentuated by possible neotectonic activity and consequent relative sea level rise led to shoreline retreat. The e xtant conditions indicate that the problem may compound in future causing irreparable damage to this important deltaic ecosystem. COASTAL erosion has been one of the major en vironmental concerns in many parts of the world. The situation is pa rticularly serious in the deltaic regions due to construction of dams in the river basin that impede sediment supply at the river mouths as a result of which the rates of subs idence and sea level rise may exceed rates of vertical a ccretion leading to erosion of coastal lands 1 . In India, delta ecosystems that are considered as cradles of civilization have significant economic importance in terms of agricu lture, water resources, wetlands, w ildlife habitats, fish production and tourism. For instance, the Godavari delta, which is termed as the rice bowl of Andhra Pradesh, is one of the major prograding deltas on the east coast of India bordering the Bay of Bengal and is a densely populated zone of intense economic activity. Fed by a large 312,812 km 2 drainage basin, the delta of this second longest river (1465 km) in the country spreads over an area of about 5100 km 2 . As evident from the presence of relict beach ridges far inland from the prese nt shoreline, the delta a dvanced into the sea by sustained deposition during the past 5 to 6 thousands of years 2,3 . As a result, the seaward bulge of the delta overlaps the continental shelf across by about 30–35 km (Figure 1) when compared to the general trend of the east coast. Several studies on the nature of coastal landforms along the delta front also indicated i ncreased sedimentation rates through the river during the nineteenth century and even over a major part of the twentieth century. Mahadevan and Prasada Rao 4 based on an analysis of multi -date maps spread over a period of 100 years traced the growth of a 21 -km long sand spit at the delta, east of Kakinada that began around AD 1850 and reached more or less the present shape by the 1958 owing to i ncreased sediment supply from the catchment due to deforestation . Sambasiva Rao and Vaidyanadhan 5 reported overall cumulative accretion of land due to d evelopment of several other sand spits at the distributary mouths of the Godavari during the period between 1937 and 1977, based on the study of toposheets (1937), aerial photographs (1968) and field mapping (1977). However, a recent field study 6 revealed predominance of erosion rather than deposition along the estuarine banks and delta front shoreline. Similarly, field observations by us during 1999–2002 also revealed pronounced erosion at certain sections of the deltaic coast due to which even some vi llages were to be evacuated. In this background, it is essential to understand the magnitude of, and the factors responsible for, the degradation of this important coastal ecosy stem in the country. The present study is such an attempt in this direction.