Floods are normally perceived as a hazard; rather floods are a natural event which present a phase of the hydrological cycle. Floods can exist even when the hydrological cycle is destabilised. Floods are a constructive environmental process, but they can be destructive if either they are perceived so or they if behaviour of hydrological cycle is not understood so. Any natural event can be perceived as a disaster if the human being do not understand it or and are not willing to or are not capable of adapting to it.
The environmental impact of floods can be analysed taking a continent or a country into account.
The Indian sub-continent receives much higher annual rainfall compared with other major continental areas. No doubt, this rainfall is very unevenly distributed geographically and is like any other gift of nature.
A river flows on land in response to tectonic features and climate of the region. The ecology of the land and coastal areas also exists in response to these basic earth processes. Rivers in India, especially those flowing in the peninsula, are geologically several tens of millions years old. They are mostly rain-fed, with some contribution from glacial melting to the Himalayan rivers. Therefore, these rivers have survived through major climatic change, monsoon variations, sea level changes, and tectonic activities. Most of them have built extensive flood plains and deltas.
Floods are a constructive geological process. It must have taken a million flooding events to create the Gangetic Plain. But ever since humans have occupied the Gangetic Plains they have started perceiving floods as a hazard. Floods are responsible for alluvium deposits in the Gangetic Plains. The floodwater brings along nutrient rick sediments, which get deposited in the plains, a process so crucial to agriculture. River flooding created fertile plains, by depositing nutrient-rich sediments, which had acquired the textures and mineralogy to hold enormous quantities of water and nutrients. River flooding is a constructive geological process and not a disaster as it is considered to be by the western countries. Low frequency and high magnitude flooding makes new cultivable farmland,
Floods help water recharge-The floodplains allow the rivers to store store the excess water in these floodplains and deltas during monsoons and release it during dry periods to maintain the minimum flow and to sustain agriculture. The geological, physiographic, and climatic set-up in India allows agriculture in low lying and groundwater-bearing floodplains and deltas, and forestry in upland, river catchment areas. This has been the practice in India, till India came under the influence of Colonial mindset.
Floods wash the land of its toxins-Annual flooding removes the agricultural wastes/toxins, deposits nutrient-rich sediments, recharges the groundwater in the farm-land, and sustains various riparian habitats. River flooding, in lowland areas particularly, is good for agriculture and ecology. If all human civilization and development are due to sustainability of agriculture then there is no earth process that is more beneficial to mankind than natural river flooding. Flooding becomes a hazard from the human perspective when the floodplains are taken over for human habitation.
Floods maintain and change the chemistry of the environment-Whenever water goes through any living body, the chemistry of its dissolved solute changes. The entire ecosystem along a river and at its mouth has evolved in response to the natural and dynamic changes in the chemistry of flowing water as well as small-scale physiographic changes along the river and its adjacent region. It is this chemistry that is the lifeline of the people in the Gangetic Plains. It is sheer ignorance that the people not to have understood that part.
Floods prevent desertification-Perceiving floods as a hazard and then try to curtail floods in the lower reaches amounts to virtual stoppage of natural recharging of groundwater in the cultivated floodplains and deltas. These areas in peninsular India receive much lower rainfall during the southwest rainfall for direct recharging of groundwater. At the same time, the very same deltaic region has already been subjected to overexploitation of groundwater to maintain agricultural productivity. The cumulative effect of all these on the landscape is very beneficial in terms of its vegetative cover and its disruption leads to desertification. In regions of semi aridity with high inter-annual variability of rainfall, intense cultivation, without regard to groundwater recharge potential, could lead to desertification. Absence of periodic river flooding in such regions would only accelerate this process of desertification.
Floods stop the intrusion of sea and coastal erosion-Along the east coast of India, all major peninsular rivers had built extensive deltas on a geological timescale and, therefore, made intensive agriculture possible for the past several millennia. Delta building activities continue even today. This clearly implies that in all cases, river action in terms of sediment transport is far more dominant than the combined action of waves, tides, and littoral currents. Allowing the rivers not to flood will cut down the sediment supply and this could cause coastal and delta erosion by waves and the prevailing, south-moving, littoral longshore currents. On a geological timescale, this will result in a loss of productive farmland as well as small-scale sea transgressions. If the global warming is a reality and taking place, with a consequent sea level rise on the east coast, the cumulative effect of coastal erosion due to reduction of sediment supply and the sea level rise could lead to large scale sea transgressions into the coastal areas.
Floods support marine life, and ocean health-There is a strong symbiosis between marine and land life systems on earth. The hydrologic cycle provides fresh water to the land from the oceans. Water, fallen on land either as rainfall on snowfall, weathers rocks on land and picks up the nutrient elements as dissolved solutes, and carries them through surface run-off (rivers) as well as subsurface flows to the sea. Thus the land life gets water from the oceans and the marine life gets some essential nutrients from land. This is only possible when floods keep taking place. If only little water is returned to the oceans there are at least two major consequences.
Marine life is deprived of nutrient supply (although wind can transport mineral dust from land, which contain nutrients but not readily in bioavailable form) and marine productivity could get adversely affected.
The Bay of Bengal (BoB) is uniquely characterised by the presence of a less-dense and low-saline layer of water. The presence of this low-salinity layer helps in the maintenance of high sea-surface temperatures (greater than 28°C), a requirement though to be responsible for the intensification of summer monsoon in the BoB. A very large part of the Indian subcontinent gets summer monsoon rainfall because of the development and maintenance of a low-pressure system in the Bay of Bengal. Monsoons can get adversely affected if floods dont take place.
To sum up, the environmental impact of floods are,
It’s a Constructive geological process
Floods deposit Alluvium, Deposits of nutrient rich sediment
It removal of agricultural wastes, toxins;
It recharges Groundwater
Floods Prevents deltaic & coastal erosion from alongshore currents & waves
Floods supports biodiversity by preventing desertification
Floods prevent large scale sea transgression in the absence of floods
Marine life is not deprived of nutrient supply & marine productivity increases
Floods Effect monsoons