Flooding is commonly defined as a condition when water flow exceeds the capacity of its channel. In hydrology, however, floods are extreme, high magnitude events, which are characterised by a significant departure from mean flow conditions. Hydrological characteristics of floods are analysed through the application of various statistical measures on different discharge data sets, at different temporal scales. Many terminologies exist that describe hydrological characteristics with different return period floods (Fig 1). Hence, there is no well-accepted single, standard measure to define a flood event. These hydrologic events are best described through the magnitude and frequency concept. Flood frequency analysis is one such important analysis, which defines the hydrological event in terms of return period events (e.g. 50 yr, 100 yr floods). The flood events are analysed and explained in a probabilistic sense to overcome the randomness in hydrological data. It is important to visualise that high magnitude flood may not always cause a condition of overbank flooding. For example, even mean annual flood i.e. 2.33yr flood in the Ganga Plains exceeds bankfull discharge to cause overbank flooding (Table 1), while 100 yr flood event in the Narmada River only partially fills the channel area due to its deeply incised nature.
Besides flood frequency, flood studies also include other hydrological approaches namely (a) application of empirical equations highlighting relationships between basin properties and flood magnitude; (b) use of rainfall-runoff model through flood hydrograph; and, (c) use of regional envelope curves. Empirical equations are gross generalisation of flood characteristics and are generally used in the condition of limited hydrological data. Flood hydrographs are the most accepted method to analyse rainfall-discharge relationship. It helps in analysing and predicting the hydrological behaviour of river in response to rainfall data. Regional envelope curves on the basis of maximum flood peaks recorded in a region are used to define upper limit to the magnitude of floods. These curves provide guidelines in determining maximum observed flood for designing of engineering structures.
Spatial variability in flood hazard and its controls: The Indus-Ganga-Brahmaputra (IGB) Plains are drained by some of the largest river systems in the world. These plains are also severely affected by frequently occurring disastrous floods and are presently regarded as the worst flood affected regions of the world. However, the nature of extent and flooding varies in different parts of the plains. The rivers in the Uttar Pradesh Plains are less affected by flood hazard whereas rivers draining Bihar are characterised by extensive and frequent flooding. In general, the extent of flooding is significantly less in peninsular India in comparison to the IGB Plains.
In the last five decades the flood control programmes on the rivers of IGB Plains have largely failed. The available data suggests that during 1954-1990, more than Rs. 2500 crore were spent on the flood control measures in India, but the annual flood damage increased nearly 40 times and annual flood affected area increased 1.5 times in this period (A Agarwal and S Narayan, 1996, ‘Floods, Floodplains and Environmental Myths. State of India’s Environment: A Citizen Report’, Centre for Science and Environment).
Types of flooding
Overbank flooding occurs due to higher discharge or less bankfull channel capacity. Higher discharge may be either due to excessive or intensive rainfall during wet seasons or due to heavy rainfall during cyclonic activity related with low-pressure systems (rainstorm-floods) or due to snow-melt contribution.
Channel capacity is governed by long-term aggradational processes, which is controlled by available flow energy and its mode of expenditure. The flow energy of a river can be expressed as stream power (Ω), defined by the rate of liberation of kinetic energy from potential energy. The stream power per unit bed area is called unit stream power (ω=Ω/w = γ.Q.s/w), which is a function of discharge (Q), bed slope (s) and channel width (w). This term characterises the available energy of flowing water to transport sediment. However, available energy may differ from the energy needed to transport sediment, which is termed as critical power. Critical power depends on the calibre and type of sediment load (Qs). Channel processes are governed by these two parameters, which determine fundamental threshold conditions for aggradation and/or degradation along rivers. This threshold condition is defined as:
When unit stream power exceeds critical power (threshold >1.0), extra energy induces channel erosion. In the opposite case (threshold <1.0), the stream looses its sediment load through depositional process. Hence, channel silting and reduction in channel capacity occurs whenever discharge or slope decreases or sediment supply increases. For example, the frequent flooding in Bihar Plains is related with reduction in channel capacity due to high sediment supply (1.5–4 t/km2/year) from the Nepal Himalaya and less stream power (< 20 W/m2) in the channel .
Channel shifting and avulsion process (channel breaching) is an important process responsible for flood hazard. Avulsion is defined as sudden change in flow direction. It occurs when the slope of pre-existing channel becomes gentler than the slope of new potential channel. Extensive channel silting makes the channel slope gentler and form conditions favourable for channel avulsion. Jacketing of river through embankment further increases sedimentation rate in the channel and enhances the danger of avulsion.
The breaching of Kosi River at Kusaha on August 18, 2008 is one such example, which generated a new course at ~120 km east of original channel (Fig 2). It inundated large areas in more than 1000 villages affecting nearly 350,000 people in the region. The breaching caused a discharge of 1,44,000 cusecs, which was around 44 per cent of Kosi’s maximum discharge capacity. Hence, avulsion may not be always related with extreme hydrology event, but this geomorphic process may cause major flood hazard in the river basin.
Dam-failure or glacial lake bursts are higher magnitude floods, which result due to sudden release of stored water behind natural or constructed dams. Natural dams include the ice dams, morainal dams, volcanic flow dams and landslide dams. For example, the Alaknanda flood in 1970, which resulted due to the breaching of a landslide dam, caused extensive damage to life and property in the upper Ganga River basin.
Flood management: major challenges and future scenario
Flood hazards are very diverse, complex and multidisciplinary problem. The present day flood control measures are mainly focussed on engineering interventions, which include construction of dams and embankments. However, ‘flood control’ approach has globally shifted to ‘flood management’ strategies, which include the multi-disciplinary integrated approach and understanding of riverine processes at cross over of scales, that address the ‘causes’ rather than the ‘effects’. Some recent studies provide new tools for sustainable management of fluvial hazard, which should be included in flood management strategies.
(a) Understanding the sediment dispersion pattern and its controls: High sediment supply and channel silting is the main cause of flooding in the Himalayan rivers. Though upstream people are generally blamed for channel silting in the downstream area, pattern and time scale of connectivity between upstream and downstream is still not known and presently forms an important research question in river science. The geochemical fingerprinting of sediments using Sm-Nd isotopes can be used to map the erosional (sediment contribution) sites in the Himalaya. A recent study in the Alaknanda River basin suggests that the major sediment contribution during 1970 flood event was from the Lesser Himalaya, which highlights the role of anthropogenic (deforestation in the Lesser Himalaya) impact on flood hazard in the upper Ganga valley.
(b) Developing prediction capabilities: There is a strong need to develop scientifically assessed prediction capability for different processes. In the absence of rain gauge stations, hydrograph may be generated through geomorphic parameters, which can provide rainfall-discharge relationship and better prediction of flood hazard. The impact of flood on bank erosion can be predicted through process based probabilistic models of bank erosion. Prediction of avulsion (breaching) can be achieved through high resolution topographic mapping.
(c) Living with flood: Inundation model for the different return period floods should be generated to define flood hazard zoning. Zone I defines the river space, which includes channel and active flood-plains. Zone II and III further defines regulatory and warning zones respectively. These zones should be used to identify landuse policy through community participation and to outline flood insurance policy. Such zoning will be the first step to live with the floods in sustainable way.
It is time to integrate different approaches (tools) and to develop a process-based understanding of rivers for flood management. The integrated multidisciplinary approach will help in understanding the complex multidisciplinary problem of flooding, as B P Radhakrishna, (1998), President, The Geological Society of India viewed “ we may not be able to prevent such natural hazard (floods), but by gaining a better understanding to their incidence, it may be possible to mitigate the severity of these calamities”.