FLOODS A. Ramdial

INTRODUCTION Floods are a natural river process in response to changes in drainage basin inputs (precipitation / melt-water runoff) and channel inputs (surface runoff). 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.

FLOODS VARY IN MAGNITUDE AND FREQUENCY The interactions between atmospheric conditions, drainage basin size, shape, geology and vegetation as well as the geometry of the channel varies over time and space. Human interaction, interference and management of the drainage basin and river channel have an influencing role as well. Thus floods can vary in magnitude (severity) and frequency (how often). Generally, as flood magnitude increases, frequency decreases.

FLOODS VARY IN MAGNITUDE AND FREQUENCY Large scale dams can control discharge and prevent floods. However, like in the failure of the Banqaio Reservoir Dam, in China,1975, where over 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 AND HUMAN CAUSES OF FLOODS

HUMAN AND PHYSICAL CAUSES 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.

PHYSICAL CAUSES OF FLOODS Extended periods of rainfall Short-term extreme rainfall events Creates antecedent conditions that increase the risk of flooding. Soils become saturated and the water table rises to the surface. As a result water cannot infiltrate and increased surface runoff occurs. Extreme rainfall can result in rainfall intensity exceeding infiltration rate or capacity. This can leads to pooling on the surface and increased surface run-off.

PHYSICAL CAUSES OF FLOODS The nature of the drainage basin and its storage capacity is also an essential factor. If storage capacity is exceeded then surface runoff increases. If infiltration capacity is exceeded then surface runoff increases. Steep- sided relief and/or impermeable rock and thin, dense soils all accelerate surface run-off, which in turns leads to higher discharges and shorter lag times.

PHYSICAL CAUSES OF FLOODS Vegetation cover has an important role to play. Dense forest vegetation intercepts and transpires over 40% of precipitation inputs. Root networks further absorb water. The forest canopy intercepts rainfall as throughfall. This slows the rate of transfer within the basin. 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 FLOODS

Human causes of flooding are a result of growing population pressure. In other cases, human are intensifying floods. 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.

HUMAN CAUSES OF FLOODS Deforestation reduces the interception 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 likelihood 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 likelihood of floods. Deforestation in Nepal and Tibet is well known to be increasing the frequency and magnitude of floods in Bangladesh.

HUMAN CAUSES OF FLOODS 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.

HUMAN CAUSES OF FLOODS Urbanisation, which leads to the expansion of built-up, impermeable surfaces, such a roads, parking lots and shopping malls further increases the rates of run-off. The very design of drainage infrastructure is to transfer water as quickly possible to the river. This is achieved through road camber, building design, drainage and sub- surface infrastructure. The importance of mobility and the car further expands the reach of impermeable surfaces through the continued loss of front garden in favour of paved drives. Due to the growing number of 2/3 car families, an area of vegetated garden equivalent to 21 times the size of Hyde Park was lost in London alone between 1998 and In addition, river capacities are often reduced in local sections of the river found in urban areas. At bridging points and contained sections, bottlenecks form and without additional spillways flooding can quickly occur during high flow. In less developed countries, population pressure leads to increases in agriculture and urbanisation and the former can further increase the rate of soil erosion and sedimentation. In some cases, poor drainage infrastructure; poorly maintained management features (like levées) or areas which have been channelised; can exacerbate flood events. During extreme events the river can simply takes its own route, regardless of what's in its way.

HUMAN CAUSES OF FLOODS Channel Management of rivers has an impact on flooding. In most cases flood management, such as dam construction and channelisation reduces the frequency of floods. However, the main purpose of flood management is increased capacity and moving discharge as quickly and efficiently as possible past a settlement. Thus, the inevitable consequence is a higher discharge and greater flood magnitude downstream.