INRODUCTION
Flooding in general and urban flooding in particular is not a un- known event in world and in India. The un-even distribution of rain fall coupled with Mindless urbanisation, encroaching upon and filling up natural drainage channels and urban lakes to use the high-value urban land for buildings are the cause of urban flooding. The illegal filling of urban water bodies in cities like Calcutta, Delhi, and Hyderabad etc is a rampant. In Calcutta, for instance, Lake Town, badly situated, has not only suffered heavy floods in 1999 but also in 1970, 1978, 1984. The No. of water bodies in Delhi accounting for about 800 had now remained 600 and rest vanished. Thousands of illegal colonies have emerged in city and planning has been thrown to the winds resulting in constriction of natural drainage inviting urban floods. This paper deals with causes, effects, preventive measures to tackle urban flooding in general with focus on Delhi, the capital city of India.

DEFINITION
A flood is an excess of water (or mud) on land that’s normally dry and is a SITUATION wherein the inundation is caused by high flow, or overflow of water in an established watercourse, such as a river, stream, or drainage ditch; or ponding of water at or near the point where the rain fell. This is a duration type event. A flood can strike anywhere without warning, occurs when a large volume of rain falls within a short time.

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.
• 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.

PREVENTIVE MEASURES

• Construction of flood protection structures
• Improvement of drainage efficiency
• RAIN WATER HARVESTING

1. On-channel storage of Rain Water in storm drains
2. Artificial Recharge Trenches
3. Check Dam
4. Development and deepening of village ponds
5. Providing Retention basins
6. Creation/Revival of water bodies
7. Rain Water Harvesting Structures

• Flood-plain management
• Planting sturdy trees sustaining draught as well flooding
• Land use and development planning

Source: nidm.gov.in