The increased demands of water for domestic, irrigation as well as industrial sectors have created water crisis and conflicts worldwide. The issues in Tamil Nadu are also manifold and are mostly related to water scarcity and water quality. The rainfall pattern plays a very important role in the hydrological cycle. Drought and flood are the two extremes of the rainfall occurrence, which needs scientific analysis and integrated multi-disciplinary approach for preparedness of these events. The heavy rainfall does occur in any given area in a less pronounced cycle of 5 to 7 years and the year 2005 was one such surplus rainfall year. The occurrence of heavy rains result in flood and also improvement in ground water system. The surplus run off is often lost to sea and the innovative schemes are needed to utilize this floodwater for beneficial use of ground water recharge. The concept of floodwater utlization in coastal areas of Tamil Nadu is presented in this paper.

The economic impact of floods, cyclones (and droughts) has been increasing over the years. There is a common perception that cyclones and heavy rainfall events are increasing in frequency or intensity due to “”global climate change”. The implication is that there will be further large adverse changes in these phenomena in future.

However, it is necessary to distinguish between the meteorological phenomena and their impact on society. Studies in India and elsewhere show that while there are periodical oscillations in cyclone frequency there is no long term trend. Cyclone damage figures normalised to take account of population increase, increase in economic activity and inflation, show no significant increase. Actual damage will, nevertheless, continue to increase because of these socio-economic factors.

Similarly, while there are large inter-annual variations of rainfall there is no long-term trend. Increase in frequency and intensity of floods (and droughts) is attributable mainly to human activities such as land use processes, deforestation, neglect, encroachment or wanton destruction of waterways and storage tanks. The heavy rainfall and flooding in Tamil Nadu in October-December, 2005 and the Mumbai flood of July 2005 are discussed as examples to establish this cause.

While post-disaster relief is given great attention involving enormous expenditures, not enough emphasis is laid on pro-active preparedness before the event. Impact of these phenomena can be mitigated only by preparedness involving engineering, social and legal measures.

Adequate rainfall in the northern part of Chennai city, where the reservoirs feeding the city water supply are located, is an important factor in controlling the Chennai groundwater environment. Reduction in piped water supply sourced from these reservoirs leads to proportional increase in groundwater abstraction in the urban as well as peri-urban areas to meet the water demand of the city area, leading to severe decline in groundwater levels and quality. The area where the groundwater levels fall below mean sea level increases in years of drought leading to brackish to saline groundwater conditions there. However, it is observed that after an excess rainfall year, the groundwater levels recover to their original position. Filling up of Chembarapakkam lake, in the western part of the city, is seen to have a positive effect on the quality of groundwater in that region. From the consideration of the temporal variations during the two drought periods under study, the beneficial effect of rain water harvesting systems, installed in the city buildings, does not seem to be obvious.

During groundwater movement along its path from recharge to discharge areas, a variety of chemical reactions with solid phases take place. These chemical reactions will vary spatially and temporally, depending on the chemical nature of the initial water, geological formations and residence time. The resulting concentrations of major ions of groundwater can be used to identify the intensity of rock-water interaction and chemical reactions. Chemical substances transported with the groundwater and chemical reactions are often predicted using mathematical models. Mathematical models, being an important predictive tool, have been developed to simulate the mechanisms responsible for the movement of chemical species. The development of geochemical transport models or hydrogeochemical models is a relatively new pursuit, although some models date back to the late 1960’s. The early models consider only a limited number of species and a few chemical reactions. However, in recent years, many researchers have developed models capable of describing multidimensional and multiple species solute transport that can be applied to studies based on a local scale (e.g. vadose zone experiments) ranging up to a regional scale (e.g. movement in confined and unconfined aquifers). Basic descriptions of recent physical and chemical models are also included in this paper. Elaborate review of various transport models is presented in Elango et al (2004).

Wetland eco-systems include terrestrial eco-system comprising evergreen and montane forests, scrublands and grass lands in the upper catchments, aquatic eco-system covering inland lakes, marsh and swamps, riverine flood plains and deltas; and coastal eco-system comprising mudflats, marshes, swamps, coastal lagoons and estuaries, mangroves and coral reefs. On an average, 6 % of river basin area constitutes wetland eco-system. The wetlands perform a host of hydrological and ecological functions that benefit human population immensely. Hydrological function include controlling floods, recharging aquifers, improving water quality and conserving flood waters. The ecological functions include Provisioning ( providing food, fiber, fuel, genetic resources, biochemicals, natural medicines, pharmaceuticals, ornamental resources, and fresh water); Regulating (air-quality maintenance, climate regulation, water and flood regulation, biological control, pollination, and storm protection); Cultural services and non-material benefits including ecotourism and recreation, and Supporting services (soil formation, nutrient re-cycling and primary production) The International Union for Conservation of Nature (IUCN) estimated that world wide wetlands provide a benefit to the tune of 11 billion US Dollars annually.

The East Godavari, West Godavari and Krishna districts of Andhra Pradesh adjoining the Bay of Bengal are constantly affected by the cyclonic storms in the bay. Though the region is very fertile due to the presence of the twin deltas of the rivers, the cyclonic storms reek havoc to irrigation, properties and even cause human causalities.

The rainfall data published by Indian Meteorological Department (IMD) for the years 1901-1950 is utilized to determine various rainfall parameters like the magnitude, number of rainy days, rainfall intensity and the wet index.

The monthly rainfall data of IMD is converted into monsoon rainfall rfm (June-October), the annual rainfall rfa and the non-monsoon rainfall rfnm. The monsoon, annual and non monsoon rainfall data of the fifty years in the coastal zone comprising of eight coastal stations of Kakinada, Mummidivaram, Amalapuram, Razole, Narsapur, Manginapudi, Bandar and Avanigadda is analyzed. It is observed that the rainfall data of either the monsoon or the annual or the non-monsoon is found to be sinusoidal in fluctuation over the years. The rainfall across all the stations is also found to be correlative.

Linear regression analyses of the monsoon and annual rainfall at the 8 stations have, within reasonable limits, presented linear regressive characteristics. The coastal zonal linear regression equation is found to be, rfa= 6.97+1.05*rfm

Frequency distributions analysis of the three components of the rainfall is carried out. The monsoon rainfall occurs for over 80 % of the years in the combined rainfall ranges of 20-30 and 30- 40 inches. Similarly the annual rainfall is found to occur over 60% of the years at places in the rainfall ranges of 30-40 and 40-50 inches. The rainfall ranges of 3-8 and 8-13 inches dominate to the extent of 64% of years of occurrence for non-monsoon rainfall.

The three components of the monsoon, annual and non-monsoon rainfalls are observed to occur uniformly over 6 or more coastal stations during the years 1935 and 1943. This observation shows the uniformity of distribution of rainfall over large areas.