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Tuesday, November 30, 2021

Kerala flash floods were due to excessive rainfall, says expert


There was rapid rise in water levels of rivers, according to Sharad Chandra, Director, Flood Forecast Monitoring, CWC

“The non-availability of real-time rainfall data for a sufficient number of locations has been a major limitation for an effective forecast of floods in Kerala,” says Sharad Chandra, Director, Flood Forecast Monitoring, Central Water Commission (CWC), in an e-mail interview on the devastating floods and landslides in Kerala from October 12 to 20.

What were the reasons behind the latest episode of flash floods in Kerala?

Once again, disaster struck — quite untimely — in Kerala and Uttarakhand. These areas present a multi-hazard scenario triggered by extreme rainfall events. The extreme event induces multiple landslides and floods, leading to loss of lives and damage to property as well. The latest episode of flash floods in Kerala was due to excessive rainfall caused by formation of low-pressure area over east-central and adjoining south-east Arabian Sea off Lakshadweep area and its movement to east-south-eastwards as a low pressure area towards the Kerala coast.

Widespread extremely heavy to very heavy rainfall that occurred on October 16 in most parts of central and south Kerala led to rapid rise in water levels of many of the west flowing rivers in that region.

Based on a forecast of heavy rainfall from the Indian Meteorological Department (IMD), the CWC issued a special flood advisory on October 12 and 16 for Kerala. An alert had been issued for all west flowing rivers in all districts of the State. Besides, near real-time water level data and rainfall had been shared with Kerala State Disaster Management Authority, Kerala State Electricity Board and State Water Resources Department (WRD) for monitoring the flood situation through hourly water level and three-hourly rainfall through the Water Information Management System. Besides these, Daily Flood Bulletin (divisional level) and Daily Flood Situation Report were shared with the WRD, Kerala.

The CWC has five flood forecasting sites in Kerala (two inflow forecasting sites and three flood-level forecasting sites). Two flood-level forecasts for Malankkara (Pathanamthitta district) on Pampa River and two inflow forecasts were issued for Idukki dam on October 16 and 17. Regular dissemination of information was also done through our website and social media platforms. Flash flood guidance is being issued by the IMD. The CWC had also disseminated this information through social media platforms.

Why are we not able to predict such extreme weather events with sufficient response/time?

Kerala is situated along the Western Ghats, with many of the towns and cities located on foothills. The State has several small intra-State rivers, almost all of them flowing west towards Arabian Sea. These rivers have very small catchment areas. Also, the steep slopes of rivers leave very little response time to issue a conventional statistical-correlation based (level) forecast for any station with respect to any base station upstream. Thus, rivers are flashy in nature and swell up pretty quickly, and hence, an effective level-based forecast is very difficult to make.

Sufficiently representative areal and temporal distribution of real-time rainfall would be required for a sufficient number of stations from the IMD for flood forecasting by modelling. Thus, while constraints of terrain and short time of concentration are a limitation for a worthwhile forecast by conventional system at any flood forecasting station, in case of mathematical model-based flood forecasting, the non-availability of real-time rainfall data for a sufficient number of locations has been a major limitation for an effective forecast of floods in Kerala.

Accuracy of rainfall forecast is also one challenge as rainfall and floods have a direct cause and effect relationship. Weather radar-received rainfall data can further improve the performance of flood forecasting models. Further, Kerala has a network consisting of a large number of small storage capacity reservoirs having lesser time of concentration. So, integrated operation of reservoirs, particularly from flood moderation point of view, is a real challenge.

What should be done to ensure effective flood risk management?

Let’s brood over the core philosophy of flood management. Globally, in recent times, there has been a paradigm shift in the concept of flood management from flood control to flood risk management. Earlier, we were laying emphasis on flood control. But since complete immunity from flood is not possible, we aim at minimising risks associated with it. Flood risk is primarily a product of three important components — hazard, exposure and vulnerability. So, by minimising all three, we can minimise the flood risk. Hazard pertains to the natural part (extreme rainfall, etc.) and we don’t have much control over it. Structural measures can be employed to contain a certain level of flood, say 1 in 25 years or 100 years (though developed countries like the U.K. and Netherlands are protecting for 1 in 10,000 years flood, for India it is an expensive proposition).

Exposure and vulnerability pertains to the anthropogenic part. Exposure is inversely proportional to the capacity of the system. So, by increasing capacity, we are minimising the risks associated with exposure. These are also administrative/governance issues in the form of implementing flood plains zoning/proofing guidelines (controlling through some regulations). Land use can be regulated in designated zones with associated risk factors depending on the different recurrence/return period of flood. Plinth level of all important infrastructures should be above 1 in 100 years return period flood level. Flood plain zoning has been identified as the very first step to be adopted for reducing the flood risk considerably. The populace also needs to be educated to respond to warnings, even if it is not 100% accurate. Generally, the public don’t prefer to be evacuated by habit and also due to lack of confidence in the warning. Awareness about the disaster resilience structures as well as habits is the need of the hour.

The third component, vulnerability, is the most important. It has both socio-economic as well as technical dimensions. Socio-economic dimension means the poor and uneducated are more vulnerable during floods, so they need extra attention. Technical dimensions pertain to proper reservoir operation, planning and maintenance of embankments (otherwise breaches cause more damage than the flood wave itself), avoiding encroachment near river channels and in the downstream areas of a reservoir, etc.

Overall, the concept of integrated flood management suggests a judicious mix of structural and non-structural measures. Structural measures (dams/reservoirs, embankments, anti-erosion work etc.) aim at preventing flood waters from reaching potential damage areas, whereas non-structural measures (flood forecasting, flood plain zoning, etc.) strive to keep people/assets away from flood waters. Structural measures are to modify flood magnitudes. Non-structural measures are tantamount to strategies akin to living with floods. These are planned activities to modify susceptibility to flood damage.

Another dimension of the Integrated Flood Management is to work holistically in a well-coordinated manner with all stakeholders, including different government departments (State as well as Centre), public, media, etc. rather than an isolated effort by single entities. Further, flood management work should be planned at hydrologic basin level and not limited to critically affected areas only.



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