Dr. Mukul Chandra Bora
(Director, Dibrugarh University Institute of Engineering and Technology Dibrugarh University)
Floods are among the most destructive disasters, causing extensive damage to human lives, property, and the environment every year across the globe. While floods are natural phenomena, their frequency and impact have been exacerbated by human activities. Over the years, flood disaster management has evolved from traditional, ad-hoc response measures to integrated approaches that utilise advanced technological tools for awareness, preparedness, and response. Floods occur when water overflows onto normally dry land, inundating areas due to rising water in rivers, streams, or drainage systems. Flash floods, caused by heavy or excessive rainfall within a short period (generally less than six hours), are characterised by rapid, destructive torrents that can sweep through riverbeds, urban streets, or mountain canyons. Flash floods can also result from the failure of a levee or dam, or the sudden release of water due to debris or ice jams. Natural floods result from environmental factors, while artificial floods are caused by human actions, either deliberate or accidental. Urban expansion to accommodate growing populations has led to severe encroachment on wetlands, low-lying areas, and hills, along with deforestation. These changes contribute to artificial floods and waterlogging. Additionally, unauthorised construction, inadequate drainage systems, and under-capacity sewage systems can cause urban flooding. Climate change and global warming, which lead to melting snow and increased precipitation, further exacerbate urban flood risks.
Floods impact all urban residents, leading to significant economic losses, including damage to residential buildings, workplaces, and infrastructure, and loss of employment for daily earners. They disrupt transport and communication systems, causing price increases for essential commodities. Environmental damage includes harm to forests, parks, water bodies, and agricultural areas, which destabilises ecosystems and can create health hazards. Flooding also brings infectious diseases, such as fever, dysentery, dengue, and dermatological conditions, affecting both humans and animals. Damage to roads, bridges, and culverts disrupts daily life, causing traffic congestion and inconveniencing commuters and the public.
Components of the water cycle:
Rainfall is also referred to as the precipitation of water that falls from the atmosphere to the earth’s surface. The major components of rainfall are: infiltration, runoff, evaporation, and transpiration.
Infiltration, or percolation, as it is sometimes called, is the process of water entry into a soil from rainfall, or irrigation. Soilwater movement (percolation) is the process of water flow from one point to another within the soil. The infiltration rate is the rate at which water actually infiltrates through the soil during a storm, and it must equal the infiltration capacity or the rainfall rate, whichever is less. Infiltration capacity is the maximum rate at which soil in any given condition is capable of absorbing water. The rate of infiltration is primarily controlled by the rate of soil water movement below the surface, and the soil water movement continues after an infiltration event as the infiltrated water is redistributed. Infiltration and percolation play a key role in surface runoff, groundwater recharge, evapotranspiration, soil erosion, and the transport of chemicals in surface and subsurface waters. Infiltration rates vary widely. It is dependent on the condition of the land surface (cracked, crusted, compacted, etc.), land vegetation cover, surface soil characteristics (grain size and gradation), storm characteristics (intensity, duration, and magnitude), surface soil and water temperature, and the and the chemical properties of the water and soil. When runoff radically exceeds infiltration, it creates floods and flash floods. Water infiltrates the soil by moving through the surface. Percolation is the movement of water through the soil itself.
Runoff is that portion of precipitation that flows over land surfaces towards larger bodies of water. Runoff is that fraction of the rainfall that moves over the surface or through the soil towards surface water features (e.g., ponds, lakes, streams, rivers, etc.). Runoff occurs when the rainfall exceeds the demands of interception, evaporation, infiltration, and surface storage.
Evapouration is the process that changes liquid water into gaseous water (water vapour). Water moves from the Earth’s surface to the atmosphere via evaporation. Evaporation occurs when energy (heat) forces the bonds that hold water molecules together to break. When you’re boiling water on the stove, you’re adding heat to liquid water.
Transpiration refers to water that is pulled out of the soil and released into the atmosphere by plants. Transpiration is a biological process that occurs mostly during the day. Water inside plants is transferred from the plant to the atmosphere as water vapours through numerous individual leaf openings. Plants transpire to move nutrients to the upper portion of the plants and to cool the leaves exposed to the sun.
The major causes of artificial flooding in cities, towns, and urban areas are: impervious layers of concrete or covered areas; drainage systems; unplanned construction and destruction of natural water bodies; and the diminishing of lakes, streams, and rivers.
Impervious surfaces, or surfaces that can’t absorb water, increase runoff. Roads, sidewalks, and parking lots are impervious surfaces. Compacted or impervious soil layers have reduced pore space and restricted water movement through the soil profile. Soil organic matter affects infiltration through its positive effect on the development of stable soil aggregates, or crumbs. Highly aggregated soil has increased pore space and infiltration. Infiltration rate is dependent on soil texture (percentage of sand, silt, and clay) and clay mineralogy. Water moves more quickly through the large pore spaces in a sandy soil than it does through the small pores of a clayey soil, especially if the clay is compacted and has little or no structure or aggregation.
Unplanned and rapid urbanisation, together with the shooting up of slums on the outskirts of urban centres, adds to flood havoc in cases of heavy rainfall. In the 2020 floods in Hyderabad and Chennai in 2015, thousands of houses were submerged, reminding us of how rapid urbanisation is making cities prone to urban floods. Urban flooding is one of the most recurrent hydro-meteorological disasters in the world. A sizeable portion of the world’s population suffers from floods every year in major towns and cities. Conveyance of floodwaters in wash corridors and arroyo floodplains that were once predominantly overland sheet flow or flow in rills and gullies is now obstructed by houses, fences, small buildings, streets, and other obstructions inhibiting their free flow.
The drainage system of any city plays an important role in the transportation of rainwater runoff. Recently published research in the Journal of Hydrology observed that storm water manmade drainage systems can result in higher peak flows that rise and recede more quickly. So, the drainage system should be prepared in alignment with the natural water shed; otherwise, it will create more havoc from an artificial flood. It is important to note that science is derived from nature, and hence, with the use of science, we have just miserably failed to control or mitigate any natural disaster like a flood. So this is the perfect time to go with nature during our urbanisation and development; otherwise, there will be only water and water surrounding all of us.
Natural water bodies are the backbone of any civilisation and development. They store water during the monsoon and provide it during the winter, when water becomes scarce. These water bodies are nature’s gift to humanity, but due to one-sided development, we have overlooked their potential and exploited them for our own growth. This mismanagement is a primary cause of artificial flooding, as the runoff exceeds the capacity of the existing water bodies, leading to floods in cities and urban areas. Wetlands, as multifunctional systems, serve as nature-based solutions (NBS) for water management. They can modulate peak flows by storing runoff and releasing it slowly over time, positively impacting soil moisture. Wetlands influence the overall water balance by affecting evapotranspiration, infiltration, and groundwater recharge. They enhance the resilience of a catchment to floods and torrents, especially those with relatively low return periods (<50 years), and safeguard water availability during droughts.
From the above discussions, it is well understood that the mitigation of artificial floods is not an easy task only from an engineering point of view, as it is mostly concerned with nature and should be solved through nature-based solutions. The nature-based solution includes the construction of drainage systems as per natural water sheds, the reduction of concrete surfaces to promote underground water percolation and groundwater recharge, the construction of groundwater recharge wells in each and every house, especially in those localities that were developed by filling up the wetlands, strict administrative measures not to promote housing in low lying areas without any water storage pons or wells with the proportion of the runoff water of that locality, zoning regulations for construction and development, etc. So, this is the last option for all the governments to enforce laws, without which the whole city is going to remain submerged under water for month after month. Last but not least, nothing can happen without people’s participation, and hence the major contributions have to be made by the people of the locality so that both nature and humans can live in peace and harmony. In our country, it is quite clear that we lack law-abiding citizens, and only during crises do they dump everything on the government in power without analysing their deeds and misdeeds. It should be kept in mind that with a few numbers of government servants, it is just impossible to save the crores of population, and hence the citizens (if they think to be) should also be vigilant towards the misdeeds of their neighbours, and Government should announce a policy to get information from the vigilant citizens. Without strict laws and a huge amount of penalties, it will be just a dream to mitigate artificial flooding in urban cities and towns.