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Flood Introduction

Throughout the world floods and flooding occur as natural phenomena which, in most cases, are not much appreciated by people living in the affected areas. Consequently, flood management and flood- control measures are introduced in many places to prevent the negative consequences of this flooding.

Reasons for flooding

Whatever the type of flood or flooding, the phenomenon can always be described as follows: a volume of water enters a certain area and it cannot be discharged quickly enough through the river channel(s) proper. As a consequence thereof, the water level rises until bankful stage is reached, then bank overspill starts and flooding occurs. Apparently, the elements volume and time play an important role. `

Types of floods and flooding

People speaking about a flood do not always know why it occurred and from where it originated. In fact the flood wave passing through a river and its adjacent floodplain would appear relatively easy to understand. Somewhere upstream it rained a lot or the snow melted and the water travels down the river towards the sea. Locally, the phenomenon is manifested by a gradual (or abrupt) change in water level. The water level first rises and, after reaching a maximum, it falls again. If this is a natural phenomenon with a high return period the people will have learned to live with it or in any case are aware of it. The problems only start when there are years with extreme floods (i.e. having a low frequency of occurrence), when a dam break upstream occurs, or because of non- envisaged spilling from a full reservoir upstream. In this respect it should also be realised that the shape of the locally observed flood hydrograph not only depends on the characteristics of the particular catchment (hydrology, climate, topography, etc.) but also, if applicable, on the outflow hydrograph of the spillway of the upstream reservoir. As far as floods are concerned, interest is not only centred on the water level, but also on maximum discharges, shape of the flood wave, its volume, duration and time of occurrence inside the flood season.

Floods and Flood Management

Classic methods used in flood management

The classic methods used in flood management are storage of flood waters and increase in discharge capacity of the river system. Storage of flood waters can be achieved by, either, storing (part of ) the flood wave in upstream reservoirs or by storing flood waters in riverine areas set aside for that purpose in the lower reaches of the river.
Also for the increase in discharge capacity of the river system several methods are used like:

  1. deepening and widening of the existing river channel(s),
  2. introduction of additional flood ways parallel to the river or conveying part of the flood to another river or to an another outlet into the sea, and, finally,
  3. flood embankments along the river.

THE MEASURES OF FLOOD MANAGEMENT

Comprehensive flood loss prevention and management, flood plain management

  1. Strategy: An approach combining structural and non-structural measures to prevent and/or minimize losses from floods.
  2. Non-structural measures of flood management:The measures which alter the exposure of life and property to flooding (floodplain land use planning, flood forecasting and warning, floodproofing, assistance).
  3. Structural measures of flood management: The measures which alter the physical characteristics of the floods .(reservoir operation, upstream catchment management, channel modifications, levees, operation of hydraulics works).

OCCURRENCE OF FLOODS AND HISTORICAL EVENTS

Flood Types

Flash floods are local floods of great volume and short duration.A flash flood generally results from a torrential rain or "cloudburst" on relatively small and wide-dispered streams. Runoff form the intense rainfall results in high flood waves.Discharges quickly reach a maximum and diminish almost as rapidly.Flood flows frequency contain large concentrations of sediment and debris. Flash floods also result from the failure of a dam or from the sudden breakup of an ice jam.Flash floods are particularly common in mountianous areas and desert regions but are a potential threat in any area where the terrain is steep,surface runoff rates are high,streams flow in narrow canyons,and severe thunderstorms prevail.

Forecasting

The ability to forecast flooding is limited to the time during which changes in the hydrological conditions necessary for flooding to occur have begun to develop. The formulation of a forecast for flood conditions requires information on current hydrological conditions such as precipitation,river stage ,water equivalent of snowpack, temperature,soil conditions over the entire drainage basin,as well as weather reports and forecasts

Remote Sensing Monitoring

One of the most effective methods of monitoring floodplains is through remote sensing. Of the various techniques available Landsat is perhaps the most versatile. Landsat satellites (formally called Earth Resources Technology Satellites or ERTS) are used to map, inventory, and monitor earth features. The major sensing instrument in Landsat is a multispectral scanner that produces images of the earth in green, red, and two wavelengths of infrared radiation. The products are black-and- white images, and images in digital form that can be processed by computers. The following basic characteristics of the satellite system and its images are of importance in disaster planning, management, and mitigation:
  1. Each image covers an area 171 kilometers (110 miles) on a side.
  2. Each wavelength band has specific useful characteristics. For example, Band 7 (long wavelength infrared) always shows water as black, even if it has a high sediment load. This is most useful for mapping coastlines, lakes, rivers, and floods.
  3. The satellites pass over the same point every 16 or 18 days. thus providing repetitive and comparable observations over time. This is particularly important for detection of changes and• monitoring. of rates and extent of environmental change.
  4. Images are available for almost all areas of the world except within nine degrees of the north and south poles. Depending on cloud cover, images may be available in repetitive observations for all countries.

STRATEGIES FOR FLOOD DISASTER MITIGATION IN INDIA

Over view of flood problem in India

In India,floods are an annual feature in one part or the other.This is due to the rainfall pattern,which exhibits great variation from region to region,the major portion of the flows in its river system is contributed by the south-west monsoon which account for nearly 75% of annual precipitation in the country.

As regards the Ganga, the flood problem is acute in the areas drained by its north bank tributaries, like Kosi, Gandak, Ghagra, etc. which are themselves mighty rivers in North Bihar and Eastern Uttar Pradesh. The region i$ subjected to recurring floods due to rivers spilling over their banks. Frequent tendency of the rivers to change their course, because of heavy siltation and poor drainage conditions of countryside, aggravate the problem. The river Yamuna and its tributaries also experience flood frequently.

DROUGHTS & FLOODS ASSESSMENT AN.D MONITORING USING REMOTE SENSING AND GIS

Flood Preparedness Phase

Flood Prone/Risk zone identification

The flood information ( data) and experience (intuition) developed during the earlier floods may help in future events. The primary method for enhancing our knowledge of a particular flood event is through flood disaster surveys, where results such as damage assessment, lessons learned and recommendations are documented in a report (see the Natural Disaster Survey Report on "The Great Flood of 1993," Scofield and Achutuni, 1994). Flood risk zone map is of two types: (1) A detailed mapping approach, that is required for the production of hazard assessment for updating (and sometimes creating) risk maps. The maps contribute to the hazard and vulnerability aspects •of flooding. (2) A larger scale approach that explores the general flood situation within a river catchment or coastal belt, with the aim of identifying areas that have greatest risk. In this case, remote sensing may contribute to mapping of inundated areas, mainly at the regional level.

BEST PRACTICES ON FLOOD PREVENTION,PROTECTION AND MITIGATION

BASIC PRINCIPLES AND APPROACHES

There is a number of basic principles and approaches regarding sustainable flood prevention, protection and mitigation,

  1. As far as possible, human interference into the processes of nature should be reversed, compensated and, in the future prevented. It is necessary to pro-mote and harmonise changes in water policies and land-use practices, as well as environmental protection and nature conservation, in order to improve Hood management in the frame of Integrated River Basin Management.
  2. This should cover the entire catchment area of water courses and promote the co• ordinated development, management and conservation of actions regarding water,• rand and related re-sources. Such a holistic approach is based on multilateral and even multi• national co-operation, including inter-disciplinary planning for the whole catchment areas.
  3. Considering the evolution and trends,the approach to natural hazards re-quires a change of paradigm.One must shift from defensive action against hazards to management of the risk and living with floods.
  4. Humans uses of floodplains should be adapted to the existing hazards. Appropriate instruments and measures should be developed to reduce the risk of flood damages.
  5. Mitigation and non-structural measures tend to be potentially more efficient and long term more sustaintable solutions to water-related problems and should be enhanced, particularly to reduce the vulnerablity of human be-ings and goods exposed to flood risk.
  6. Nevertheless,structural measured (defence structures)will remain important elements and should primarly focus on the protection of human health and safety and valuable goods and property. Requirements of nature conservation and landscape management should also be taken into account.
  7. The major part of population and goods are located in big urban areas so efforts for avoiding flood problems should also be focused on these urban areas. River overflowing does not always cause urban floods;they can also be caused by high rain intensities over the city combined with inapprociate sewer systems. Special attention should be taken to the present drainage of rainwater,for instance the capacity of the sewer systems of our cities.
  8. Everyone who may suffer from the consequences of flood events should also take -if possible -his/her own pre-cautions.To this end,approciate in-formation and fore-casting systems should be established by the competent authority.
  9. Solidarity is essential,one should not pass on water management problems in one region to another. The appropriate strategy consists of a three-step approach :retaining,storing and draining .(first make every to retain)
  10. rainfall at the spot,store excess water locally ,only then let the water be dis-charged to the watercourse.Flood prevention has also to be based on the precautionary principle.
  11. In flood-prone areas,preventive measures should be taken to reduce possible adverse effects of floods on aquatic and terrestrial ecosystems,such as water and soil pollution.

PUBLIC AWARENESS, PUBLIC PARTICIPATION AND INSURANCE

Public awareness

It is the personal responsibility of anyone who lives and works by or on the river, and broader in the potential flooded area, to adapt his use of the water and all activities to flood risks .. So, everyone must know the risk and• take it into account appropriately when acting.

Problems associated with floods are often not sufficiently recognised and ac• knowledged. Communication plan to offer individuals an understanding of the nature and scope of these risks should be developed. Regional and municipal authorities will see to its continued and permanent implementation at the regional and local level in order to involve owners and administrators of properties, including organisations at levels of regions, districts, municipalities or individuals, and enable them to take preventive and protective actions by themselves and offer their opinions about the implementation of preventive measures for reduction of flood damages.

All measures linked to public information and awareness raising are most effective when they involve participation at all levels. Public participation in decision-making is a cornerstone of successful implementation of inteqrated and comprehensive action plans, both to improve the quality and the implementation of the decisions, and to give the public the opportunity to express its concerns and to enable authorities to take due account of such concerns.

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