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The Salinity Barrier and Nilwala Floods: Clearing up Misconceptions

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Figure 1: Salinity Barrier across Nilwala River

by Dr. R. Galappatti,
Chairman, Lanka Hydraulic Institute (LHI).

Introduction

Recent experience of high water levels in lower reaches of the Nilwala Ganga has been attributed by various parties to the construction of the Salinity Barrier in 2018-2022, 5.4 km upstream of the sea outfall. While it is understandable that the affected communities would blame the most recent construction in the river channel for this, many technically qualified persons have also subscribed to this view without either examining hydraulic computations or making any measurements at the barrier. We are concerned that ill-considered attempts are being made to interfere with the structural integrity of the barrier and endanger a major piece of water supply infrastructure.

The salinity barrier is designed to protect the water supply intakes from salinity intrusion and ensure an uninterrupted supply of drinking water to around 600,000 consumers by the year 2035, in the Matara and Hambantota districts, under the Matara Stage IV Water Supply Project of the National Water Supply & Drainage Board (NWSDB). The conceptual design of the barrier including its dimensions, levels and operating rules was carried out by Lanka Hydraulic Institute (LHI).

Regular review meetings were held at regional level by the NWSDB with participation of the Irrigation Department. The Central Engineering Consultancy Bureau (CECB), the EIA consultant, also participated, with full access to the details of the ongoing work at all stages of the study. Modifications were made to the study approach and the designs as required.

The hydraulic studies including design calculations were carried out and the main parameters of the barrier were determined and verified using physical and numerical model studies. This focused on the salinity barrier, particularly its flood impacts, and led to a positive EIA report after public consultations were completed. Detailed designs and construction of the barrier was carried out by a Korean Company (Kolon Samsung C&T) with Ceywater as the local design partner under a Design-Build contract.

The Salinity Barrier

The final design consists of providing a 5-bay gated type structure with vertical lifting gates and piers across the inner stream section 5.4 km upstream of the sea mouth, in the river reach between Thudawa Pump station and Navimana ferry crossing. The gates are 10m wide and 3.1m high with the gate sill fixed at –2.5 m MSL. Gate top at closed position is at +0.6 m MSL and the gate bottom in fully open position is to be at 5m MSL during high and normal flow conditions. This level is above the High Flood Level, as the top level of the flood bund is only around 4.3 m MSL.

Three of the five gates were to be made of two separate sections to allow the upper section to be lowered independently, to make it possible to lower the upstream pool level when necessary for irrigation water management at the request of the Irrigation Department. Sheet-pile cut-off walls were provided across the low-lying areas of the flood plain on either bank to prevent saline water bypassing the main structure during operation and to ensure that during dry weather the river flow is confined to the inner channel and not to overflow into the floodplain. In addition to these main features, gate operating structures, cut-off walls, access roads etc. were included in the design.

The barrier is in full conformity to the stipulations in the MOU signed between NWSDB and the Irrigation Department in March 2017, and the Irrigation Department granted approval for the NWSDB to implement the project.

Figure 2: Salinity Barrier across Nilwala River

Impacts of the Salinity Barrier

The increase in river water level during floods, and operating conditions and concerns of the stakeholders due the proposed structure, were the main considerations in determining optimal values for gate openings, gate sill levels and other parameters for the design.

The upstream flooding impact of the barrier design was computed using a numerical model and verified by carrying out a series of physical model tests at Lanka Hydraulic Institute (LHI). These were used to verify that the flow pattern around and downstream of the gated structure was satisfactory.

The water levels in the upstream river reach, with and without the barrier, were estimated for different flow conditions including floods up to 100 yr return period. The estimated anticipated water level rise just upstream of the Salinity Barrier was about 1 cm maximum during floods of 100 yr return period. There was a higher (4-7 cm) relative water level rise during bank full and low return period floods.

Backwater propagation lengths have been calculated and these anticipated water level rises were considered acceptable. It is relevant to note that, the acceptable head loss at the planned Kelani Ganga salinity barrier was fixed in the range 5 cm to 6 cm for a 100 yr flood.

As far as the flood impact of the cut-off wall is concerned, the impact will be negligible because this wall, with a top level of only 0.8 m MSL, will be well and truly submerged during a major flood. Further there is an allegation that debris might trap buildup from the river bed level to the sill level of gates, and exacerbate flooding. Even if debris are accumulated, its total hydraulic impact is still negligible. Further, fresh bathymetric survey conducted in Dec 2023 has even revealed that there is in fact no debris accreted upstream of the gate structure.

Long inundation period during present flood compared to previous floods

While it has been demonstrated that construction of the Salinity Barrier is not the cause of recent floods, it is nevertheless necessary to investigate why this long inundation of paddy lands occurred and determine what actions need to be taken to prevent or reduce the probability of a recurrence.

In this context it is necessary to examine what happened in May 2017 when there was an extreme flood event in this basin. The flood was caused by an extreme rainfall event of about three days duration that caused a sharp flood peak reaching a daily average water level of 4.32 m MSL at Thalagahagoda on the very next day. The water levels receded to normal levels within the next ten days while it took seven days to decrease to flood alert level (1.4 m MSL) at Thalagahagoda. The return period of this flood is said to be between 75 and 100 yr.

Figure 4: Rainfall Pattern and Drawdown of Water Levels during 2017 Flood

The duration of the long series of rainfall events that spread out over more than two months from Mid-September to end November 2023 is markedly different – both from the point of view of the basin and the time of the year when it took place. While the water level at Thalagahagoda peaked only at 2.92 m MSL in 2023, the level continued to stay above 1.7 m MSL (Minor flood level) for almost the entire period (Oct to mid of Nov).

Figure 3: Rainfall Pattern and Drawdown of Water Levels during Sep Oct 2023 Flood (06th Sep – 22nd Nov 2023)

The markedly different nature of this flood – including its extremely slow recession could only be explained by considering the retention (storage) of flood water on the flood plain which is divided into two on either side of the Nilwala flood bund. The area adjacent to the river will drain slowly into the river channel in the normal manner. The water collecting in the “protected area” must be pumped out at a few pumping stations located along the flood bund.

Duration of operation of these pumps is on record but their pumping rate is not known. An additional point to be considered is that the measured water level profile on the short river reach downstream of the barrier show a steeper slope indicating a more constricted channel from the railway bridge to the sea.

It is also important to note that the rainfall pattern in 2023 is unusual and unexpected, and that no usual rainfall runoff model could accurately interpret it. The models, in turn, must be more detailed, to include the flood plain storage and pumping rates to represent the flood routing performance of the system under such unusual rainfall patterns which might be a trend caused by climate change.

It must also be borne in mind that the Nilwala basin is not a stranger to flooding, though not so late in the year. The steep upper basin that descends suddenly into a largely flat coastal plain is characteristic of all large wet zone rivers in Sri Lanka.

Conclusions

The long-duration flooding experience in the Nilwala basin in 2023 has not been caused by the construction of the Salinity Barrier.A more detailed representation of the flood plains and pump operation is needed to quantify accurately the long recession of the flood plain

The marked change in rainfall patterns might be due to a climate change effect and this type of effect might also impact other river basins in the wet zone. This requires attention.

The ill- considered removal of the low cut-off wall across the lower floodplain should be restored to avoid possible shut down of water supply intakes in the next dry season.



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Features

The Ghost Stories of Edith Wharton

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Tales of Mystery and Suspense 16:

I have thus far looked only at novels, but in recent years I have also read several short story collections full of suspense and mystery. The first of these that I will explore is by someone not usually associated with such work. Edith Wharton was rather known for her incisive stories about American high society in the 19th century, on the lines of Henry James, though she was brilliant and less convoluted .

She was not someone I had read in childhood, but for some reason I have on my shelves several books by her, though I cannot now recollect from where they were all collected, and when. Over the years I read several of the novels I have, and enjoyed them, but for some reason I do not think I took down The Ghost Stories of Edith Wharton, a comparatively slim volume, that had been tucked away amongst my children’s books.

I could recollect none of the stories, except for the last two, the plots of which came back to me as I began reading them. One was called ‘All Souls’’ and was about a lady who lived by herself and found one night that all her servants had gone away, leaving her alone in the house. She had broken her ankle that day and been put to bed, with strict injunctions not to move, but when her bell was not answered she had staggered up, to find the electricity not working and no one at home. The voice she heard in the kitchen turned out to be from a radio.

When she awoke on what she thought the following morning, she was told she had imagined things and the servants were all back in place, and it was only the same morning. I had remembered by then that the probable explanation was that the servants had all gone to celebrate a witches’ sabbath, for it was All Souls’ Day when the dead walk. What I had forgotten was that before she fell she had seen a woman walking up to the house, and when a year later she saw the woman again, she fled, to stay with her cousin, who narrates the story. She never went back to the house.

The last story in the book is more straightforward, about a young man who goes to stay with an explorer he had met, who has set up house in the desert. But his host is not there when Medford arrives, and he is looked after by his manservant Gosling, who said Almondham was due back any day. But he did not arrive, and meanwhile there was less and less water to drink, for Medford refused wine and there was no Perrier.

Meanwhile his bath water smells, and I then remembered that in fact Gosling had killed his master, though perhaps not intentionally, and put him in the well. The reason was that he had not been allowed any leave and, just when he thought Almondham would relent, he said he was expecting a visitor and Gosling would have to stay on.

But while this is no great mystery, Edith Wharton creates suspense by making Medford worry about whether Gosling is correct in telling him not to trust the Arab servants, who were likely to kill him if he went off alone with them. And though it is more and more obvious that Gosling has not told him the truth, Medford is sure of nothing, until he suddenly finds Gosling about to push him into the stinking well, though he then backs off.

These two stories, which I remembered, perhaps because they had been anthologized elsewhere, were the most compelling, though I did find almost all the others also quite readable. And almost all held one’s interest, with the suspense being maintained until the end – and beyond it sometimes since the stories were sometimes open ended.

Amongst the most gripping was the story of a man who decided to visit the sister, who lived alone in Brittany, of an old friend. But when he got to a darkened house, he suddenly remembered that he had been told the woman had died. So he is quite convinced, when an old woman lets him in, and he is confronted by a shadowy figure on the stairs, that this is a ghost.

She begs him to stay, for she says she has felt appallingly lonely since she died, but he finally breaks away and flees. But when he tells the sister this, and says he is sorry he had not visited the grave, for she had wanted to be buried in her garden, he is told that she had not died, but only had a cataleptic fit, from which she had recovered.

Several other stories deal with adultery, or affection for a man, on the part of a woman treated badly by her husband. These stories, it is suggested, reflect Wharton’s own situation, for the man she married was serially unfaithful and not at all sympathetic, and they parted company. But women were expected to uphold rigid standards of behaviour, and many of Wharton’s heroines suffer accordingly.

The most dramatic of these involves a woman who meets her lover in a crypt, which her husband walls up, with a statue of her he has commissioned, the face of which breaks up to indicate the grief that overcame her. I have read similar outcomes in stories by Balzac and also Nirmali Hettiarachchi, adapted by the latter, but only here does stone change shape to express the grief that led to the deaths of all the women.

In another story a husband strangles a dog a woman lavished love on since she was alone, with a necklace he had given her and which she passed to an admirer, a necklace the husband had somehow got back. He then strangled all other dogs she paid attention to, but one night when she went down, the man who loved her having come back, to warn him that her husband was home, she heard him being attacked at the top of the stairs by the ghosts of all the dogs. And since dog bites were seen on the dead body, she was acquitted of the charge of murder which had been brought against her.

There are tales too of possession by dead women, one ending in an exorcism, the other with the husband vanishing, after getting letters from his dead wife, leaving his new wife and her mother dimly understanding what has happened. All these denouements are, if not as dramatic as in the last two stories, memorable, which is why I have little doubt this is the first time I read these other stories. And they serve to emphasize the talent of a remarkable woman, who made a life for herself away from the conventions of American society.

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To skillfully fall from the clouds…

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A memoir by Thamasha Abeynaike

Glancing at the phone for a photo before the start of the first charter flight of the day, I noticed the time was 11:11. It seemed like a nod from the universe that this was bound to be a memorable day — and the 27th of December 2021 didn’t disappoint.

Capt. Gihan Fernando (Capt. GAF) greeted us at the hangar and after a reminder to me, as an observer on the flight, to ‘Take lots of photos!’, we waved him goodbye.

After a flawless sector from Ratmalana to beautiful Sigiriya, PIC Capt. Dinindu Ruwanpathiranage (Capt. Dinny) taxiied onto the apron and we awaited the arrival of our passengers for the next sector : Sigiriya to Koggala.

To say we were blessed with the most relaxed and understanding couple as passengers for this sector would be an understatement.

Wassim and Marya, who are now an indescribable part of our aviation lives, shared their plans of a beautiful vacation on the coast of Weligama.

Landing Site — Circled in Red, Attempted Runway — On Bottom Left of picture

Personally, I was excited for my favourite approach in Sri Lanka — the approach into Koggala Airport — bordered on one end by the beach, and the other by the Koggala Lake.

Our take off from Sigiriya was at sharp 13:00h and our lives were shaken within the next 52 mins.

While overflying Kurunegala, and following a descent down to 5000ft MSL, the engine of our trusted 4R-GAF Cessna 172 aircraft started to run rough. Following troubleshoots and after our trained procedures, Capt. Dinny diverted to Katunayake International Airport.

The Italian phrase “Cadere dalle nuvole” (direct translation : ‘to fall from the clouds’) — means “When you’re forced to face the reality.”

We are trained in Forced Emergency Landing Procedures throughout our flying careers countless times. I, myself, have chatted to my friends over numerous dinners about aerodynamics and emergency procedures. However, when the actual engine of your aircraft is giving up on you, there is no definite prediction of descent rate and glide range.

At Nawaloka Hospital Colombo : Photo by Channa

Sigiriya Airport Apron : Photo by Wassim

We ran through the emergency procedures many times with one of our ever supportive passengers, Wassim, mentioning, ‘We will not disturb you. You guys get us down safely.’ No better words could have been spoken and a confident voice echoed in my head ‘We’re all going to be safe’.

While perfectly lined up with a cleared runway at Katunayake Airport and with Capt. Dinny talking to and working with a misfiring engine, the engine of our C172 finally completely shut down at 200ft.

This is a testament to the indefinable skill of Capt. Dinny — to make a split second decision to abandon the approach and turn to a paddy field on his right, upon which we touched down beautifully at an impressive speed of 55knots at 13:52h.

Unfortunately, while slowing down in the mud, the aircraft collided with a concrete road which was slightly raised from the field level.

Zoom image will be displayed

I am forever grateful for this sequence of events, as Capt. Dinny’s skill and the thought of keeping the nose raised throughout the landing roll, saved us from being crushed by the instrument panel collapsing onto us.

Six months later, we have

physically recovered.

After listening to the stories of hearing this news and rushing to our side — from our families, friends, students to the aviation community — I can only sum it all up to one word : Grateful.This redirection was a blessed awakening to the true value of life and what potential a great training, instinct and a calm mindset hold in this beautiful field of aviation.

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Complexities in global politics deepen as economic pressures intensify

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UK Prime Minister Andy Burnham meets Ukrainian President Volodymyr Zelensky in Kyiv. (BBC)

The present offer by the UK to strengthen Ukraine’s defense capabilities in the missile technology field in particular comes as ‘a stitch in time’ and the initiative is also likely to be appreciated considerably by democratic opinion world wide for the possible morale-boosting effect it would have on Ukraine. Besides continuous arms support, the conviction that the world’s frontline democracies are behind it would prove a huge plus in Ukraine’s eyes in its grinding fightback against the Russian invasion.

While continued US support for Ukraine could not be considered ‘a given’ any more, British Prime Minister Andy Burnham’s words during a recent visit to Kyiv that the UK would stand by Ukraine ‘for as long as it takes’ is the kind of assurance that Ukraine needs at present. For, the conflict in Ukraine is essentially a war of liberation conducted by the latter against an invader and deeply at issue here is the upholding of International Law and its foundational concepts, such as national sovereignty and a nation’s right to political self-determination. The world of democracy is of the firm view that the latter ideals cannot be compromised, come what may.

The UK has its work cut out in this connection. It would find it difficult to convince the Trump administration that it should staunchly stand by Ukraine but it could campaign vigorously with the rest of the West and the EU fold in particular to unflaggingly support the embattled and over-run country.

Ukraine has shown an impressive adeptness in using drone technology in particular against her enemy and has even manufactured her own hardware in this respect but using the relevant blueprints handed over by the UK for the manufacture of more sophisticated cruise missiles, for instance, may prove financially difficult, going forward. It is left to be seen whether the UK and the rest of the West who are with Ukraine will continue to be with her, considering their own rising financial constraints.

The latter impediments could only multiply in the future. Oil, gas and energy prices are on the rise and the latter costs are glaringly reflected in kitchens and meal tables the world over. As we go along consumer discontent would steadily intensify and governments, East and West, would need to figure out with considerable rigour and foresight how such disaffection could be ably managed. Failing which, in most democratic societies, the chances are that publics would be out on the streets demanding that their grievances be redressed forthwith.

These rising concerns are reflected in a recent move by some EU governments to consider imposing what is described as ‘a windfall tax’ on the profits specified major oil companies operating within their shores have made in the wake of the US-Israel war on Iran. The rationale apparently is to use such tax earnings to cushion the rising cost of living of their publics and bolster the respective countries’ social expenditure.

In a recent letter to the president of the EU Council the EU governments referred to said, among other things, while drawing attention to the ‘discontent that is growing over the rising cost of living’: ‘A common approach’ is needed that ‘ensures those who profit from the crisis contribute their share to reducing the burden on the general population.’ Meanwhile, Oxfam with reference to the above development is on record as calling for a ‘permanent windfall tax of at least 50% on profits exceeding a 10% return on investment.’

Such are the rising economic pressures on the majority of Western governments. The question to be posed is how consistent they would be in their assistance to Ukraine if they decide consensually to stand by her. The soaring cost of living in the West compels the conclusion that there could be no guarantee that Western assistance to Ukraine, particularly in the defense and security fields, would be of a longstanding kind.

Of particular concern would be the fact that the weapons systems on offer from the UK to Ukraine could be increasingly costly to manufacture going forward. Besides they would need to be manufactured and put into action without delay.

However, these considerations should in no way deflect Ukraine’s supporters from the principled policy stance of defending her to the extent possible. Because at issue is the defense of International Law and the democratic system of government from their enemies; fascism and authoritarian rule.

While during World Wars 1 and 2 the US was with the major democracies of the West, this time around with regard to Ukraine, the US has chosen to be at cross-purposes with them. For instance, in relation to tariff matters and defense expenditure, in the NATO context, the US is pursuing a hard line which puts it at polar opposites with the West. Thus it is no longer possible to talk unreservedly of a ‘Western democratic alliance’. Put plainly, the cause of democratic development has been weakened.

A measure of relief for the supporters of Ukraine in the West could come by way of the upcoming mid-term polls in the US. If the Democratic Party fares well in them the pressure would be on the Trump administration to defer to opposition opinion at home, accommodate the best interests of Ukraine in its West European policy and perhaps even work towards a diplomatic solution to the Ukraine crisis in cooperation with Russia. Accordingly, the Democratic Party would need to put the Trump administration on the defensive, so to speak.

Until such time Ukraine’s Western supporters have no choice but to remain committed to it, ensure its steadfast defense against the invasion and work judiciously towards keeping the economic pressures at home in check.

Interestingly, at the present juncture in international politics the US could be said to be more weak than strong. For example, it has to some extent been militarily humbled by Iran; so much so it is resorting to economic means to keep Iran in check.

In keeping with this strategy, the US has launched ‘a new big wave of anti-Iran economic sanctions’ at the time of writing, aimed at cutting Iran away from all its major income sources. Some of these relate to digital assets, technology, gold, aviation and shipping. The hoped for result is the complete severance of Iran from the US dollar system.

However, while the UK and EU have no choice but to adhere to their policy of backing Ukraine, going forward they would need to dialogue more closely with the US and ensure that it cooperates with them on outstanding questions, such as Ukraine and the strengthening of democracy. The well being of the world is served when the latter aim is pursued.

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