FLOOD MANAGEMENT

FLOODS MANAGEMENT

Rivers have always flooded and humans have always attempted to manage rivers. The ancient civilisations of Mesopatania and Egypt would build dykes and dams to hold back the waters following inundation. Indigenous skills, worldwide have created ingenious methods of interacting with rivers to increase food supply. Over time humans have modernised their engineering and in places have built vast structures to control and regulate the flow of rivers. 

We need not look any further than recent developments in China. The shear ambition of China's South-North Water Transfer Project dwarfs the remarkable achievement of the 3-Gorges Dam. However, major multi-purpose dam projects once common place in developed countries are now almost entirely part of the development strategies of developing countries and NICs, with India and China leading the way. 

In Europe and the US large scale river managment schemes have had mixed success and in many cases river management projects are reverting back to softer approaches in the realisation that the natural river system and catchment has much to offer. River management can be divided into hard and soft engineering. In both cases they attempt to reduce or regulate discharge in the river. Hard engineering is expensive, and tends to have a large impact on the river and the natural ecology and hydrology. Softer approaches tend to be more ecologically sensitive.

THE CAUSES OF FLOODS

Floods are a natural river process in response to changes in drainage basin inputs. They are an essential characteristic of the landscape and are fundamental to the development of floodplains, wetlands and many river features. Floods are therefore overwhelmingly caused by the physical environment. The interaction between atmospheric condition, drainage basin size, shape, geology and vegetation as well as the geometry of the channel varies over time and space. As a result, floods vary in magnitude and frequency.

 Human interaction, interference and management of the drainage basin and river channel have an influencing role as well. Large scale dams can control discharge and prevent floods. However, like in the failure of the Banqaio Resevoir Dam, in China,1975, where over 171000 people died and 11 million lost their homes, humans can be the direct cause of floods.With increased population and pressure on natural resources humans are having an increasingly important impact on the drainage basin. In many cases human impact is adding to or exacerbating both the frequency and magnitude of floods.

PHYSICAL CAUSES OF FLOODS

The most important physical cause of floods concerns the interaction between precipitation and drainage basin response. The capacity of a river to cope with inputs becomes strained during both extended periods of rainfall (antecedent conditions) and short-term extreme rainfall events. e.g. Seathwaite, Cumbria, 2009, when 495mm of rain, the equivalent of several months of rainfall fell in just a 4 day period. In the case of the former, soils become saturated and thewater table rises to the surface. As a result water cannot infiltrate and surface run-off occurs. For the latter, extreme rainfall leads to pooling on the surface and surface run-off. 

The nature of the drainage basin and its storage capacity is also and essential factor. Steep sided relief and/or impermeable rock and thin dense soils all accelerate surface run-off, which in turns leads to higher discharge and shorter lag times. Vegetation cover has an important role to play. Dense forest vegetation intercepts andtranspires over 40% of precipitation inputs. Root networks further absorb water. The forest canopy intercepts rainfall slows inputs as throughfall. As a result surface run-off is minimalised and deep infiltration encouraged. Densely vegetated drainage basins therefore drastically reduce the magnitude and frequency of floods.

Human causes of flooding are a result of growing population pressure. Humans impact the interaction between precipitation and the drainage basin response through deforestation, as a result of agricultural development, floodplain drainage, urbanisation and channel management. Deforestation reduces the intercpetion and transpiration feedback resulting in increased quantities and rates of surface-run off. As a result more water reaches the river faster. In addition, deforestation exposes the soil to greater rates of erosion and nutrient leaching, which in turn increases the liklihood of further soil erosion and gullying. 

Soil erosion leads to sedimentation of the channel, which in turn reduces the capacity and hydraulic efficiency of the river, increasing the liklihood of floods. For example, deforestation in Nepal and Tibet is well known to be increasing the frequency  and magnitude of floods in Bangladesh. Floodplain drainage, especially in more developed countries has created space for modern agricultural systems and urban infrastructure. In doing so, the natural storage capacity of the floodplain and the wetlands they support has been lost. 

During low frequency high magnitude floods, the water simply has no where else to go. The impacts of floods are also exacerbated by the very fact that settlements have been built on the floodplain. Urbanisation, which leads to the expansion of built-up, impermeable surface, such a roads, parking lots and shops mauls further increases the rates of run-off. The very design of settlement infrastructure is to transfer water as quickly possible to the river. This is achieved through road camber, building design, drainage and sub-surface infrastructure.

 In addition, rivers capacity is often reduced in local sections of the river, in urban areas. At bridging points and contained sections, bottlenecks form that without additional spillways can quickly become flooded during high flow. In less developed countries, population pressure leads to increases in agriculture and urbanisation, which further increases the rates of soil erosion and sedimentation.  In some cases, poor drainage can exacerbate flood events and in places where the river has been redirected and during extreme events the river simply takes its own route, regardless of what's its way. Finally, channel management of rivers has an impact on flooding. In most cases flood managment, such as dam construction and channelisation reduces the frequency of floods. However, with the main purpose of flood management aimed to increase capacity and move discharge as quickly and efficiently as possible past a settlement.

FLOODS IN INDIA

India is highly vulnerable to floods. Out of the total geographical area of 329 million hectares (mha), more than 40 mha is flood prone. Floods are a recurrent phenomenon, which cause huge loss of lives and damage to livelihood systems, property, infrastructure and public utilities. It is a cause for concern that flood related damages show an increasing trend. The average annual flood damage in the last 10 years period from 1996 to 2005 was Rs. 4745 crore as compared to Rs. 1805 crore, the corresponding average for the previous 53 years. This can be attributed to many reasons including a steep increase in population, rapid urbanization growing developmental and economic activities in flood plains coupled with global warming.

An average every year, 75 lakh hectares of land is affected, 1600 lives are lost and the damage caused to crops, houses and public utilities is Rs.1805 crores due to floods. The maximum number of lives (11,316) was lost in the year 1977. The frequency of major floods is more than once in five years.

Floods have also occurred in areas, which were earlier not considered flood prone. An effort has been made in these Guidelines to cover the entire gamut of Flood Management. Eighty per cent of the precipitation takes place in the monsoon months from June to September. The rivers a bring heavy sediment load from catchments. These, coupled with inadequate carrying capacity of rivers are responsible for causing floods, drainage congestion and erosion of river-banks. Cyclones, cyclonic circulations and cloud bursts cause flash floods and lead to huge losses. It is a fact that some of the rivers causing damage in India originate in neighboring countries; adding another complex dimension to the problem. Continuing and large-scale loss of lives and damage to public and private property due to floods indicate that we are still to develop an effective response to floods. NDMA's Executive Summary Guidelines have been prepared to enable the various implementing and stakeholder agencies to effectively address the critical areas for minimising flood damage.