URBAN FLOODING
Any natural or man-made disaster in urban areas result into significant devastation and flood is no exception. Urban area like any other area get flooding by different types of floods like river flood, flash flood, coastal floods, floods due to change in river course, glacier bursts, landslides (blocking the river course which gets subsequently released breaking down the debris barriers), release of excess water from reservoirs or failure of dam at upstream side, localized floods due to in situ heavy intensity rainfall. Here term urban flood is restricted to flood due to water logging resulted from heavy rains or sea water intrusion as like in case of Tsunami and inadequate capacity of storm water drainage system (If there) and / or poor infiltration capacity of soil and all other types are termed as river basin flood. Though each cause of river basin flood as listed above has an impact, the strategies to mitigate the resulted flood will vary from type to type. Flood by general understanding is a vigorous dynamic phenomenon where discharge is more than water carrying capacity of river resulting in over toppling of banks and inundation of areas which are otherwise dry. But Urban Flood is distinct from other types of
floods (termed as river basin floods) in sense that it does not require river to occur. Unlike river basin floods where areas close to river banks get affected to maximum extent, in Urban Flood any part of city can badly get affected and more significantly low lying areas. River basin flood is a result of high discharge while urban floods are due to stagnation. Hence Urban Flood needs to be considered separately than river basin flood.
Causes of Urban Floods
Even though heavy intensity rainfall is a principal cause of urban flood, the heavy rainfall events is a normal phenomenon during South West Monsoon season in most of the cities and towns in India. But the urban floods are more visible in recent years. The main reason is human intervention. As cities and towns started growing uncontrollably, the land use pattern changed drastically with more area got impervious due to constructions, carpeted roads etc. This results in change of urban hydrology from gradual rising discharge to quicker and higher peak flow. Problem is aggravated by age old drainage system designed as per benefit cost ratio prevalent at that time which in many cases now inadequate with enhanced socio economical activities with decreased affordable risk of failure. The existing drainage systems are not maintained properly due to financial constraints. Drainage system gets blocked due to silting, dumping of waste material at the inlets of drains, encroachment over natural drainages, water bodies and so on. Sometimes urban flood occurs due to inflow from river / sea in urban drains during high stage / high tides. The sharp peak discharge of flood hydrograph due to intense rainfall many a time crosses the drainage capacity of the system resulting in stagnation of water on roads and open areas resulting in urban flood.
Consequences of Urban Floods
Communication ‐ Urban Floods results in stagnation of water on roads, railway tracks and in few cases even at airports because of the inadequate storm water drainage capacity. This results in traffic jams and traffic diversions resulting in loss of man hours. In the events of heavy rainstorms air traffic gets diverted. Telecommunication gets disturbed and maintenance of supply of essential commodities becomes challenge.
Economical – As communications is disrupted industrial production gets hampered. Prices of essential commodities shoot up. During and after urban floods the immediate task is restoration of damaged roads, railway tacks, damaged buildings (which is very common for over lived buildings) and other structures and rehabilitation of residents from low lying areas and collapsed buildings. Damages of assets are significant in warehouses and buildings due to flooding by storm and sewage water. Perishable articles add to economical loss. Accidents and fire due to short circuit are also common. Hence there is a lot of financial burdens on relief measures.
Social – There is a psychological stress as safe returns of family members is not sure. Schools and colleges get closed. Displacement of population in low lying areas and collapsed structures generally meets stiff resistance. Disruption in supply of essential commodities including power supply results in unrest.
Environmental – Water bodies get polluted. Waste disposal gets hampered due to traffic disruption.
Health ‐ The stagnation of water, pollution of potable water and accumulation of waste at dustbins result in epidemics. Accidents due to open pits, manholes hidden under accumulated water adds to problem. As traffic gets disrupted it is challenging to assist medical assistance.