How Short-Duration Precipitation Events Impact The Historic Kaveri River Basin: Interview With Professor Satyajit Ghosh

A sudden, intense downpour of rain over the historic Kaveri river delta causing the river to swell and flood the surrounding floodplains — one does not have to strain one’s imagination much to picture this scenario.

Such a possibility looms large especially with extreme weather events turning up with sudden impact.

It is just such a scenario that has been studied, not through imagination but with a scientific lens, in a recent paper published in Scientific Reports (Nature) whose lead author is Satyajit Ghosh of the Vellore Institute of Technology, Tamil Nadu.

The study examines the effects of short-duration precipitation events on the region and the associated threats to the historic Kallanai dam, also known as the Grand Anicut.

Kallanai Dam

Built by Karikala of the Chola dynasty in 150 CE, the Kallanai dam is an ancient dam built across the river Kaveri, flowing from Tiruchirapalli district to Thanjavur district in Tamil Nadu.

The dam is located in Thanjavur district, 15 km from Tiruchirapalli and 45 km from Thanjavur. It is the fourth oldest water-diversion or water-regulator structure in the world and the oldest in India that's still in use.

The idea behind the construction of the dam was to divert the river to the delta districts, thereby boosting irrigation.

The construction of the Kallanai dam is a testament to the civilisational wisdom of its time.

The dam was built using large boulders brought over and sunk in the Kaveri sand, a task arising from a desperate need for irrigating fertile fields downstream when floods breached the left bank and rushed down north to rejoin the Kollidam river, leaving its delta high and dry.

The dam was meant to divert water across the fertile delta region for irrigation through canals. Its function was to retain supply in the Kaveri and its branches, and pass on the surplus into Kollidam through Ullar river.

Ghosh et al found that intense rainfalls are not only caused by warm rain microphysics but also by large frozen hydrometeors falling from deep clouds, causing undesirable flooding over the region, to the extent of 66 per cent.

The study also projects a heightened vulnerability of the region to flooding, with an estimated 145.98 km² of land being submerged, including 65.14 per cent of agricultural land.

The most important conceptual advance established in this paper is that sub-zones in major watersheds that are currently safe will get inundated in what is called the RCP8.5 warming scenario in 2050.

'RCP8.5'

"RCP" is short for ‘Representative Concentration Pathway’, a term used to describe four greenhouse gas concentration trajectories adopted by the Intergovernmental Panel on Climate Change in 2014.

These pathways describe different climate futures, depending on the amount of greenhouse gases emitted in the years to come.

The RCPs are named after a possible range of radiative forcing values in the year 2100 relative to pre-industrial values (+2.6, +4.5, +6.0, and +8.5 W/m², respectively).

Of these values, RCP8.5 is a high-emissions scenario with humanity carrying on in the ‘business as usual’ mode.

Once considered a ‘worst-case scenario’, it is today seen as the most appropriate for conducting assessments of climate change impact by 2050.

Researchers say RCP8.5 is in closest agreement with the total cumulative CO₂ emissions we have, coming within 1 per cent of actual emissions.

Scientific models

The conventional hydrological modelling of floodplains evolved from "lumped models" to physically based hydrological models.

The study augments this approach with the use of digital elevation models (DEM), remote sensing, and geographic information systems (GIS).

This was done to create a detailed digital representation of the Kaveri river delta and its associated hydrological features.

This digital model, used in conjunction with hydrological modeling techniques, helped the team to simulate and analyse the impact of short-duration precipitation events on the region.

The study provides valuable insights into how climate change could impact water resource management in the region, highlighting both the importance and vulnerability of this historic structure.

Ancient dam design

The ancient foundations of Kallanai, a marvel of engineering, were built using locally sourced, sunken unhewed boulders.

They were strategically positioned to slow floodwaters during both the southwest and northeast monsoon periods, allowing the water to spread over a spillway and nourish paddy cultivation in the catchment area of 81,155 km².

In the nineteenth century, British engineer Sir Arthur Cotton, inspired by the architectural beauty of the Grand Anaicut canal, supervised the construction of what we see today as Mukkombu, the upper Anaicut.

However, this ingenious arrangement is now at risk due to the increasing frequency of short-duration extreme flood events caused by global warming.

The structure has already been damaged once by floods, an event that was thought to be a once-in-a-lifetime occurrence. But in the last five years alone, the region has experienced heavy flooding twice and caused flood alerts to be sounded.

The future of Kallanai and the agricultural land it supports hangs in the balance as the threat of climate change looms ever larger.

The paper, thus, presents a stark contrast between the ancient wisdom and engineering prowess of the Chola kings, who built structures like the Kallanai dam to manage water resources effectively, and the modern problems posed by climate change and its impact on precipitation patterns.

It serves as a reminder that even the most well-managed systems can be threatened by global changes, our own short-sightedness, and ignorance of embedded principles in structures like Kallanai, and that we must continue to adapt and innovate to meet these challenges.

While the paper may not explicitly mention ‘solutions’, what it implies is profound.

The paper highlights potential threats to the historic Kallanai dam and the Kaveri delta due to short-duration intense precipitation episodes.

This information can be useful for policymakers and water resource managers to develop strategies to mitigate the impact of these events on the region.

For example, they could invest in infrastructure improvements to better manage floodwaters, or implement policies to promote sustainable water use and agriculture in the region.

Ultimately, addressing the root cause of global warming through reducing greenhouse gas emissions will also help lower the frequency and intensity of these extreme precipitation events.

Swarajya's Aravindan Neelakandan spoke with Professor Satyajit Ghosh, the lead author of the study, and currently a Visiting Professor at the School of Mechanical Engineering, Vellore Institute of Technology, and at the School of Earth and Environment, the University of Leeds, the United Kingdom (UK).

Excerpts below:

What inspired you to conduct this study on the impact of short-duration precipitation events over the historic Kaveri river basin?

The first inspiration came from Dr G Viswanathan, founder-chancellor of VIT University and Professor Lord Julian Hunt, CB FRS, British meteorologist.

They urged me to apply the fruits of new knowledge garnered from science and technology to apply, re-explore, and, if possible, ameliorate societal problems.

I am primarily a cloud microphysicist and it is straightforward for me to connect my research to water resource management.

I am acutely aware that the Indian civilisation is shaped by the monsoons and I always marvelled at the way subcontinental-built environments evolved and adapted to accommodate patterns of received rainfalls, inspiring me to work on riparian architecture.

And the iconic Kallanai dam provided me with an opportunity to bring all this together.

How do you think this research can help in mitigating the threats posed by short-duration precipitation events to the historic Kallanai dam and the Kaveri delta region?

Based on this, if there is one policy decision to be made, what would it be?

Short-duration intense precipitation events are already showing up and are poised to become the new norm.

This research shows that computational fluid dynamics has developed very significantly so that weather and climate models are now able to forecast a week in advance the broad tenets of what might constitute threats.

This foreknowledge helps in identifying vulnerable sub water sheds (SW) in the Kaveri delta and execute evacuation swiftly.

I will answer with an example.

By coupling predicted rainfall intensity with a robust hydrological model, one can ascertain the depth and velocity of floodwaters in the sub-basins and identify regions along the structure that might most experience degradation.

Optimal policies and management protocols can also relate to early-warning systems. Crop submergence for prolonged durations results in crop rotting affecting the nation’s GDP.

In the past (and to some extent even now), natural rivers were adapted to function as irrigation canals — the overall result is a network comprising natural and man-made canals and drains.

This established order is likely to be severely impacted by short-duration flooding events and might weaken existing embankments catastrophically.

We believe that the results from this study will aid policymakers in the Water Resource Department, particularly with regard to providing non-structural interventions.

Examples include improved decision-making on water resources, inundation mapping, and better managing flood risks and threats, and also provide simple guidelines for hazard quantification.

Now I shall answer the second part to your question related to policy decision.

The most obvious and urgent policy decision must concern stricter enforcement policies to curb encroachments.

These new results indicate greater flood hazard potentials by up to two times linked to high-intensity precipitation events in future warmed-up scenarios.

Results produced from the present analyses can serve as a ready reckoner to support decisions in prioritising emergency support in vulnerable zones and also enable future planning; that is, constructing shelter homes and shifting agricultural activities in the least vulnerable areas.

The island of Srirangam is close to the upper Anaicut between the Thanjavur delta, with hundreds of villages facing imminent threats increasingly frequently.

It is disconcerting that the percentage of built-up areas is quite high, with encroachments nibbling away precious vegetated areas. This aggravates flood.

One notices that the spatial coverage of floodwater is higher in the RCP8.5, indicating clear submergence of all low-lying villages around the major watersheds.

The villages of Panayapuram, Uthamarseeli, Kilikoodu (SW3) and Nadhalpadugai, Mudhalaimeduthittu and Vellamanal near the Kollidam stretch (SW5) are vulnerable with higher threat levels in 2050.

So, stricter land-use policies and their implementation must be in place well ahead to 2050.

What novel methodological strategies have you adapted in this study? How have you improvised over the conventional hydrological studies?

The adapted methodology of coupling a weather research forecasting (WRF) model with a hydrological model to study flood plains is new.

Earlier studies used rain gauge measurements to provide inputs to hydrological models. Rain gauge coverage can be sparse.

In contrast, the WRF can be configured with fine meshes to yield rain intensities over the desired study region in great detail.

There is an interesting perspective here: Kallanai itself has survived now close to 1,800 years and still functional and it has been reinforced by the work of Sir Cotton.

But the modern global warming, the result of our industrial activities within the last 180 years, is threatening this structure and creating exactly the opposite effect of the sustained agriculture that Kallanai effected.

Are we seeing here some kind of a clash of the deep values embedded in technologies of different ages? How do you propose a meeting point and solution at the level of scientific study of this ‘clash’?

This is a deep and profound question.

Technology continuously evolves. One must strike at the very roots of the problem.

If the Kallanai must foster cultivation uninterrupted, then it must be protected from threats. And a major threat relates to intense precipitation over short periods, exacerbated by global warming post the industrial revolution.

Over a more recent timeline, India’s economic liberalisation has resulted in rural areas getting urbanised rapidly. By 2050, much of India will be urbanised. So, the first step will be (to) plan new urban areas around this region carefully.

Resource conservation must be at the back of any major construction work. The Cholas used sunken stones to lay the foundations for the Kallanai.

Interestingly, the practice of using unhewn stones in large structures was adopted by modern architects also.

Frank Lloyd Wright’s famous studio ‘Taliesin West’ in Arizona is an example. Its walls have a unique feature — desert rocks were used in the walls.

Just as the ancient Cholas in faraway Tamil Nadu, Wright advocated the use of locally sourced materials. This practice addresses eco concerns and has a lower environmental impact.

Resource wastage was also minimised by a very ingenious method: all flat surfaces of the collected rocks were made to be outward-facing so that large boulders could fill the left-over interior space, and in this way, concrete could be conserved.

So, in this sense, ancient wisdom informs and enriches modern practices.

There are other examples where this is true from many parts of the world.

In contrast, the reverse, that is, the clash of deep values, is also very visible.

Ancient knowledge from Sri Lanka as well as many parts of India recommend the use of landscaping, fostering a kind microclimate and orient building clusters so that the large-scale wind flow patterns are used to ventilate exteriors as well as interiors.

In a consumption-driven society, real estate developers often ignore these time-tested idioms. As a result, the heating ventilation and air-conditioning (HVAC) loads increase to unsustainable levels.

Earlier you had published a novel paper on similar ancient engineering.

In the small Jain village of Artipura in Karnataka, the temples, schools of learning, and settlements were innovatively designed in orientation with a large granite-skirted natural reservoir storing rain water.

This design in turn allowed the inhabitants to harness the cooling properties of the stored water, creating a natural form of air-conditioning.

Now you have brought out how the hydrologically intelligent design of the Kallanai construction has used the water of the delta region in an optimal way for agriculture for close to almost 2,000 years.

In all these, there is a conceptual continuity.

Can you explain what makes you study these structures? And what can we all learn from this?

I am very happy that you brought in the Arettipur story. There is a tenuous link between the Arettipur complex, which housed Jain monks, the ancient Anuradhians (people who lived in ancient Anuradhapura) in Sri Lanka, and this Cholan grand structure.

All three used rain water and applied engineering practices to better their lives.

In Arettipur, the Jain monks, as you have pointed out, collected rain water knowing fully well that the monsoons could be weak or even fail in a particular year (we are in the grip of an El Nino this year). The Mandya district of Karnataka is a lean region in terms of received precipitation amounts.

These monks made an arrangement so that even if the monsoon failed in a year, the residents were not water-starved.

The builders of both Arettipur and Anuradhapura made use of hydraulic air-conditioning and had a deep yet intuitive knowledge of building forms and orientations so that they could receive the bounty of chilled winds through the windows and doors (large glazing ratios in modern technical parlance) to better cope with the torrid heat of the tropics.

Both cultures knew about the recurrence and periodicity of the monsoon winds.

The Cholas, on the other hand, perfected riparian architecture. They too were aware of the arrival and departure times of both the SW (southwest) and the NE (northeast) monsoons and adjusted paddy cultivation accordingly.

They also were able to harvest received rainfall to ensure continuous agricultural productivity.

The Grand Anicut not only restrained the flood waters from wreaking havoc, but also slowed down the waters along spillways for a large spatial coverage to cultivate more efficiently.

How do we incorporate these civilisational values and vision into modern planning and design?

These civilisational values are already being used in modern planning and design. In my answers to your previous questions, I have provided examples from Wright’s advocacy of organic architecture.

To give a fuller answer to your question, I have to bring in India’s smart city projects.

Much can be learnt from vernacular traditions of having large passive zones (zones that require no artificial forms of energy) in buildings in striking a balance between modernism, energy and resource conservation, and thermal comfort in buildings.

In order to incorporate these civilisational values, one must learn from examples and success stories.

I would like to assert that the International Township of Auroville is at the forefront of this process of incorporation of civilisational values and vision into modern designs.

I would like to add another important aspect which was not discussed thus far. This relates to the aesthetic dimension.

I have discussed this in my new book, Inspired Journeys: An Art Inspired Travel Memoir Around Built Environments (Macmillan Education 2022), where I talk about the iconic ‘Arjuna’s Penance’ sculpted in Mamallapuram.

This is an example illustrating how elements of nature are directly embedded in a work of art and how this is positioned at a vantage point.

Sculptures suitably placed in modern cities could learn from this frieze to foster large-scale public awareness about sustainability.

I have said (to quote from the book): This stone-carved bas relief shows plants, animals as well as humans so richly carved that no sphere of contemporary life is left out.

But the panel’s piece resistance is a sculpted cleft running all the way from the top to the bottom.

The artist’s intention was that it had to be viewed with a running cascade of rainwater gushing through it. Without this, it was bereft of its vitality — and with it, it is alive — the rain-drenched panel invigorates the sculpted landscape back to life.

In this instance, the eureka moment was of course the artists' inspiration of an element of nature embedded directly into the artwork.

These are some of the ways in which civilisational traits can get embedded within the public psyche.

Biographical Notes

Professor Satyajit Ghosh's earlier substantive appointments include Research Associateship at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK, and at UMIST, Manchester, UK.

His research areas include CFD applications in aerosol cloud interactions and biometeorological applications over the built environment.

He received the Editor's award from the Royal Meteorological Society, UK, for enthusing young Indians to engage with meteorology.

He is an elected Fellow of the Royal Meteorological Society, UK.