Lesson 10 River flooding
Urban flooding is specific in the fact that the cause is a lack of drainage in an urban area. As there is little open soil that can be used for water storage nearly all the precipitation needs to be transport to surface water or the sewage system. Multiple uses of floodplain Different levels of settlement development and disturbance to natural systems have various effects on natural resources. For instance, the management objectives can be different for an already fully developed floodplain community than a community with large open spaces. TYPES OF FLOODING According to Duration Slow-Onset Flooding, Rapid-Onset Flooding, Flash Flooding. According to Location -Coastal Flooding, Arroyos Flooding, River Flooding and Urban Flooding. The urban area is paved with roads etc and the discharge of heavy rain can’t absorbed into the ground due to drainage constraints leads to flooding of streets, underpasses, low lying areas and storm drains
CAUSES OF URBAN FLOODING Natural Causes Heavy Rainfall / Flash floods Water of Heavy rainfall concentrates and flows quickly through urban paved area and impounded in to low lying area raising the water level. It creates more havoc when a main drain or a river passing through the area over-flows or breaches Lack of Lakes Lakes can store the excess water and regulate the flow of water. When lakes become smaller, their ability to regulate the flow become less and hence flooding. Silting The drains carry large amounts of sediments and deposited in the lower courses making beds shallower thus channel capacity is reduced. When there is heavy rain, these silted drains can’t carry full discharge and result in flooding. Human Causes Population pressure Because of large amount of people, more materials are needed, like wood, land, food, etc. This aggravates overgrazing, over cultivation and soil erosion which increases the risk of flooding. Deforestation Large areas of forests near the rivers/catchment of cities are used to make rooms for settlements, roads and farmlands and is being cleared due to which soil is quickly lost to drains. This raises the drain bed causing overflow and in turn urban flooding.
Trespassing on water storm drains The areas which were essentially created by the storm water drains to let their flood waters pass freely being tress- passed for developmental purposes result in obstruction of water flow and thus contributed immensely to the fury of floods. Urbanisation leads to paving of surfaces which decreases ground absorption and increases the speed and amount of surface flow. The water rushes down suddenly into the streams from their catchment areas leading to a sudden rise in water level and flash floods. Unplanned urbanisation is the key cause of urban flooding Various kinds of depression and low lying areas near or around the cities which were act as cushions and flood absorbers are gradually filled up and built upon due to urbanisation pressure. This results in inadequate channel capacity causing urban flooding. Un Authorised colonies have been developed by the local colonisers on the agriculture land, earlier being used for crop has been purchased at lucrative prices from farmers, without consideration to the city plans, drainage, sewerage etc. and thus subjected to flooding during heavy rain falls. Poor Water and Sewerage Management Old drainage and sewerage system has not been overhauled nor is it adequate now. All the drainage and sewer system in many parts of Delhi has collapsed resulting in flooding. This can be seen during rainy seasons every year. Lack of attention to the nature of hydrological system. Lack of flood control measures. Multiple authorities in a city but owning responsibility by none.
EFFECTS OF FLOODING/ FLOOD DAMAGES The flooding affects every section of people, systems in a city, some of them are summarised below: Economic effects •Damage to Public buildings, Public utility works, housing and house – hold assets. •Loss of earning in industry & trade •Loss of earning to petty shopkeepers and workers •Loss of employment to daily earners •Loss of revenue due to Road, Railway Transportation Interruption •High prices for essential commodities. After flooding, government has to put many resources for aiding e.g., police force, fire control, aid workers and for restoration of flood affected structures, persons, live-stock etc. The flooding cause a great economic loss to the state, individual and to the society. Environmental effects : Damage to surroundings, forests, ridges, wild-life, zoo, urban community-trees, water bodies, shrubs, grass, fruits / vegetables in go downs etc result imbalance of eco-system of the city. Effect on Traffic - Flooding results in the damages of roads, collapse of bridges causing traffic congestion which affect day-to-day life and other transportation system.
Reasons of flooding in Delhi Heavy Rainfall / Flash floods The local drainage system at times found to be inadequate to meet the requirement, when there is heavy rain fall. Delhi normally remained flooded to the extent of 70000 hac (50% of its geographical area of 148300 hac from 1953 to 1984). Urbanisation -urbanisation resulting in increase of paved area and decrease in the agricultural land which used to act as a percolation zone. The catchment area of the Najafgarh drain has been built up and paved resulting increase in water-flow during rainy season. The cross-sectional area of the drain has become inadequate. Unplanned urbanisation is also a key cause of flooding Unauthorised colonies: There are about 1650 unauthorised colonies which have been developed by the local colonisers on the open/agriculture land without consideration to the city plans, drainage, sewerage etc. and thus subjected to flooding during heavy rain falls. Many more are coming up which would further increase drainage congestion and flooding. Trespassing on storm water drains: The areas which were essentially created by the storm water drains( or constructed) to let their flood waters pass freely being tress-passed by JJ Clusters, Slum dwellers, small shopkeepers, motor garages, garbage dumping etc result in obstruction of water flow and thus contributed immensely to the fury of floods. Most of the Delhi drains can be seen such trespassing and garbage dumping.
Siltation of drains - Water treatment plants e.g. Bhagirathi, Haiderpur etc discharge sediments into drains, flow of sewerage, sullage and solid waste materials into storm water drains causing siltation which can’t carry full discharge in heavy rain. Siltation water bodies - Water bodies, low lying areas-water retaining plains, near or around the city which act as flood absorbers are gradually filled up and built upon due to urbanisation pressure. Earlier 800 water bodies in Delhi, now reduced to only 600 that too silted to a great extent. This results urban flooding Chockage of water careers: Accumulation of dust, garbage chocked gully gratings, bell mouths of roads and inlets of street drains, passing of cables, pipes across the drains reduces water way. Garbage dumping in or on drains, near bridges also reduce water way. Poor Water and Sewerage Management Old drainage and sewerage system has not been overhauled nor is it adequate now .All the drainage and sewer system in many parts of Delhi has collapsed resulting in flooding. This can be seen during rainy seasons every year at Chhatarsal Stadium in model town, Minto Bridge, Bhairon road Railway Bridge, ring road at ITO, IP estate. Back flow from main drains into city drains i/c sewers during high floods. Deficiencies in the drainage system (Planning, Execution & Maintenance):- The master plan for drainage of Delhi was prepared in 1976 and sent to Delhi Administration in 1981 considering problems and habitation at that point of time and did not consider for future urbanisation and particularly rural – urbanisation and unauthorised colonies. Every department/ civic bodies in Delhi plan construct and maintain their drainage in isolation without consideration of overall integrated drainage and sustainability resulting in flooding. Failure of pumping installations due to inadequate capacity or the failure of electric supply. Multiple authorities in the city but owning responsibility by none. Less co-ordination among various Govt and civic bodies like MCD, PWD, DJB, I&FC, BSES, NDPL, NDMC,Cantt, CPWD etc. Control of Operation of barrages at ITO, Chilla and Okhla on river Yamuna are not with Delhi but with UP and Haryana which regulate only at request of I&FC dept of Delhi.
Effect on Human Beings • Human lives: Every year floods in India cause more than 50 lac people affected dead and become homeless. • Psychological impact: The people of all ages who stranded in flooding suffer a great Psychological impact disturbing their whole life and the society as whole. • Live Stock: The live stock is the most affected living being due to urban floods. It is difficult to care for them particularly when human being itself is in trouble. • Disease: Flooding usually brings infectious diseases, e.g. military fever, pneumonic plagues, dermatopathia, dysentery, common cold, Dengue, break bone fever, etc. Chances of food poisoning also become more where electric supply interrupted in food-storage area due to flooding. • Public Inconveniences: The flooding causes impairment of transport and communication system due to which all people of all section get stranded e.g. school children, college students, office goers, vegetable, milk venders etc. The basic and essential commodities also do not reach to the common person. This result either starvation to poor persons or high priced to the common persons.
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