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Kerala Floods: A Riverine Perspective

A lot has been written about the Kerala floods already and most of the blame has been put on exceptionally heavy rainfall,  that was approximately 40 per cent higher than normal between June 1 and August, 19, 2018 (Devasia and Menon, 2018). Why this excess rainfall was not predicted despite tall claims of having state of the art facilities in the country and why the state did not prepare for this are still open questions for which no satisfactory answers have been forthcoming so far. However, we may leave this debate for now. There is no doubt that extreme rainfall was the triggering factor for this unprecedented flood but the impacts could have been minimised had the rivers flowing through Kerala been taken care of in terms of developing a process-based understanding. The Kerala flood is a classic example of long term ignorance of several important factors in river management— river dynamics, sediments and ill-planned interventions. This article highlights these issues. While the article chooses to argue for the river, it does so from a human perspective.

Crossing of ‘threshold’

Kerala is dotted with 44 rivers out of which 41 are west flowing (Kerala Irrigation Department, 2017). Most of these small and seasonal rivers have high to moderate slopes and short length from source to sea. This results in flash discharge, bringing significant amounts of sediments from the fragile Western Ghats. The rivers are extremely dynamic and even a subtle change in slope due to sediment accumulation can change the direction of the flow. A river maintains its natural course as long as the longitudinal (down valley) slope is higher than the lateral (cross valley) slope, but the moment the lateral slope becomes higher than the longitudinal slope, it leads to a ‘gradient advantage’(Mackay and Bridge, 1995) and the river will switch its course, a process termed as ‘avulsion’. The phenomenon is known as ‘crossing of thresholds’ in technical terms and rivers such as the ones in Kerala have very low thresholds that can be crossed very easily and suddenly. The addition of excessive sediments from extreme events and landslides adds to the problem and triggers the switch. During the 2018 Kerala floods, such incidences were documented in several rivers such as Chaliyar (Mudur, 2018) and Karuvannur (Ramavarman, 2018) that led the river to go haywire, resulting in large scale inundation occurring in areas way beyond the normal course of the river. Heavy rains, coupled with release of water from overfilled dams added to the problem. It is important to note that such threshold crossing can produce enormous changes in river morphology and can change the course of the river permanently. In the long run, a river may adjust to a new equilibrium, but may never revert to its original condition unless through engineering interventions.

The August 2008 floods in the Kosi river, draining through Nepal and north Bihar, may be cited here. Excessive sediment influx and confinement of the river within embankments and obstructions, caused by the barrage raised the bed level of the river through time. This led to the breaching of the embankment and large scale inundation (Sinha, 2009a). Interestingly, this flood occurred at a discharge of 144,000 cusecs, which was much lower than the design capacity of 950,000 cusecs for the Kosi barrage (Sinha, 2009b). The rivers in Kerala are much smaller than the Kosi and presumably have a much lower threshold, so they are likely to be more prone to such dynamics. Such process-based understanding is unfortunately missing in designing river management strategies in our country and rivers in Kerala are no exception.

Engineering interventions and drainage congestion

Given the large number of rivers draining through Kerala, the spatial distribution, and the unique profile of the flows and gradients of these rivers, it is important to understand and link these with engineering interventions to explain the causative factors of the recent floods. Apart from their unique hydrology, the rivers in Kerala are also witness to unprecedented interventions in the form of dams and hydroelectric projects. The Mullaperiyar dam was constructed way back in 1895 (Ghosal, 2018) but a series of hydroelectric projects started in 1933, the first of which was the Pallivasal project (Kerala State Electricity Board, 2015). A total of 33 hydroelectric projects, 57 dams and the associated infrastructures on and around the Kerala rivers call for a much stricter reservoir operation policy for efficient discharge of flood runoff. In a single district of Idukki, that lies in the Western Ghats, there are eight reservoirs—Mullaperiyar, Idukki arch dam, Ponmudi dam, Anayirangal, Mattupetty, Kundala, Idamalayar and Bhoothathankettu. It has been argued that given the amount of rainfall and the live storage available, it was essential to make releases from the reservoir (CWC, 2018) but perhaps this could have been done in a more efficient and controlled manner and with sufficient warning to the local people. Further, the infrastructure developed over the years have significantly damaged the fragile ecosystem of the Western Ghats. This is reflected in decreasing forest cover and instability of the mountain slopes . The overall forest cover loss in Western Ghats has been estimated to be 35.3 per cent of the total forest cover in Kerala between 1920 and 2013  (Reddy, 2016). The damage has been so severe that the Supreme Court had to intervene and pronounce a ruling against the felling of trees in 1996. Deforestation and development of associated infrastructures to support these projects have led to rampant and unplanned construction activities resulting in modification of slopes, excavations and quarrying (Sangmola, 2018). It is easily understandable how disastrous these activities have been in terms of the destabilisation of natural slopes caused by removal of vegetation cover, culminating in failures and landslides. During heavy rains such as the one in 2018, sections of these slopes washed away, adding a large sediment flux in the rivers.

Apart from causing slope instability, many structures such as roads and bridges downstream tend to either block the natural pathway or reduce the width of the river and act as a ‘barrier’ to the flow of water and sediments. This creates severe ‘drainage congestion’ as more often than not, there is not enough width to allow for safe passage of flood waters. With time, the aggradation of channels with sediments adds to the problem and the situation worsens leading to disasters like the one we have witnessed in Kerala in July-August 2018.

Encroaching  the ‘river space’

The physical form of a river primarily consists of its channel and floodplain that has also been used to define ‘river space’—the space required by the river to perform its myriad functions, which include channel migration, sediment/nutrient transport, and support to riparian vegetation and ecosystem. Most developed countries have consciously committed to preserve river spaces through several measures such as defining the desirable land use in this zone (also called river corridors) and promulgating legislations to protect this space. Amongst several benefits such as providing fertile land for seasonal farming and supporting riparian fauna and flora, this measure provides for reducing the flood risk by (a) accommodating a large part of flood waters during high flows including groundwater recharge, and (b) not allowing people to build settlements very close to the river. As with many other places across the country, haphazard and ill-planned development in Kerala has seen significant encroachment of the river space through rapid urbanisation gradually converting the fertile and permeable floodplains into human settlements. This has not only reduced infiltration of flood waters for groundwater recharge and accommodation of excess rainfall, but has also put a large population and infrastructure at a huge risk. Kerala has an extremely varied terrain that offers limited potential for urban expansion and therefore a delicate balance must be maintained between human need and riverine ecosystem protection.

Although this idea has been proposed in the Indian context as well through research papers and reports submitted to the Ministry of Water Resources, Government of India, there are no serious efforts underway to implement it. Any ecosystem based approach for river management must implement ideas that have multiple benefits including risk reduction of flood disasters that have recently occurred—Indus floods (2010), Uttarakhand floods (2013) and Jammu floods (2014). Such incidents keep reminding us that river management in this country requires a paradigm shift.

Kerala Floods | Impact assessment and way forward

While the rivers of Kerala continue to ravage different parts of the State during the monsoons, there is a huge task ahead to repair the physical, social and economic damages that have incurred during the 2018 event. While a lot of planning in terms of rebuilding the infrastructures may be going on already, it is pertinent to reflect upon the long term impacts on the rivers and the measures that may need to be taken to avoid such disasters in the future.

Given the diversity of rivers across the Kerala state and the fact that 13 out of 14 districts of Kerala were affected by 2018 floods, the impact of these floods are not difficult to visualise. The topmost scientific priority of the water resources department of the Kerala state should be the development of basin-specific flood forecasting and flood management plan and its dissemination. This task is easier said than done, but a concerted effort must start.

As mentioned before, several rivers may have undergone significant morphological changes during this event and some of them may now be more prone to flood risk than ever before. Therefore, complete morphological evaluation of these rivers in terms of their morphodynamics must be undertaken and immediately followed up by necessary measures to restore their equilibrium. A few stretches of the rivers have moved to new courses and it may be worthwhile to examine the flood risk in these stretches.  Apart from focusing on the downstream reaches of the river, it is equally important to look into the fundamental causes of the problems that are likely to be different in different regions and then plan a remedial measure. The use of modern technology such as repetitive satellite images and drones can provide insights for these investigations.

Another area of concern is examining the efficiency of hydraulic structures and in particular their role in aggravating the flood risk in this particular event, if any. The identification of specific bridges, barrages, embankments and other infrastructures that hindered the safe passage of flood waters must be identified and suitable measures must be taken up to avoid such problems in the future. Sediment management and channel improvement around these structures and other stretches could be one of the important steps that need to be undertaken during the lean flow period apart from an extensive rehabilitation programme. Restoration of hydrologic connectivity of channels disrupted by ill-planned structures should be one of  the long terms strategies for the state government in the coming years.

Endnote

This event should also be taken as a stark reminder that the river space must be clearly demarcated for all rivers and the existing land use in this zone identified. The Kerala government must consider the protection of the river space through a scientifically designed plan rather than randomly selecting a buffer zone on both sides of the river as has been proposed in some states recently (1 km wide buffer zone along the Ganga in Uttarakhand as per NGT directive dated Nov. 5, 2015). There are similar rules prohibiting constructions near water bodies in other states in India too, but there is no uniformity and there are no considerations of river diversity across the states. Such regulations could be either unproductive or counter-productive depending upon the situation. While the rehabilitation programmes are being planned in Kerala, there may be an opportunity to revisit such regulations to restrict the permanent settlements in the ‘river space’. This might require designing a scientific and possibly a legislative framework as well.

References

Central Water Commission, 2018. Study Report: Kerala floods of August 2018, Central Water Commission, Govt. Of India, September
2018, 48p.

Devasia J and M Menon, 2018. India’s flood-hit Kerala faces huge clean-up, fear of disease, Reuters, August 20.

Ghosal A., 2018. Mullaperiyar Dam: 999-Year Lease at the Heart of Acrimony Between TN and Kerala, News18, August 25.

Kerala Irrigation Department, Government of Kerala, 2017. Water Bodies in Kerala, Available at: https://bit.ly/2x4swak

Kerala State Electricity Board, 2015. Electricity Generation, Available at: https://bit.ly/2N4PVmO

Mackey, S.D., Bridge, J.S., 1995. Three-dimensional model of alluvial stratigraphy: theory and application, Journal of  Sedimentary Research, B65 (1): 7–31.

Mudur G.S., 2018. River splits and invades, The Telegraph, August 21.

Ramavarman T, 2018. Flood waters recede in Thrissur but fears remain, The Times of India, August 19.

Reddy, C.S., C.S. Jha, and V.K. Dadhwal, 2016. Assessment and monitoring of long-term forest cover changes (1920–2013) in Western Ghats biodiversity hotspot, Journal of Earth System Science, 125, 103–114

Sangmola A., 2018. Kerala battles worst floods since 1924, says CM, Down to Earth, August 13.

Sinha, R. (2009a), The Great avulsion of Kosi on 18 August 2008. Current Science, 97 (3): 429-433.

Sinha, R. (2009b), Kosi: Rising waters, dynamic channels and human disasters, Economic and Political Weekly, 43 (46), 42-46.

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