“There is no reason to believe that the officers of the Met Services are incompetent”

Qs & As

Virendra Proag, Former Associate Professor – Civil Engineering, Uom

* ‘If you are given an oral and abnormal/unethical/improper instruction, either you get it written and signed or else, you can throw it to the winds’

* ‘Each generation has a 1 in 4 chance of experiencing flooding, even if an exceptional rainfall intensity of 100 years has been considered’

In this Qs & As exploration, Virendra Proag, a former Associate Professor in Civil Engineering at the University of Mauritius, elucidates the causes of floods, factors influencing their occurrences, and the vital role played by catchment areas and runoff coefficients. Additionally, he scrutinizes the significance of rainfall intensity, return periods, and the implications of climate change on drainage systems.

Viren Proag’s perspective challenges us to ponder the effectiveness of existing infrastructure and the imperative of innovation to mitigate the impact of floods, urging society to grapple with the question: Is it acceptable for one’s house to get flooded every 10 years, or never at all during one’s lifetime? Through historical analysis and engineering acumen, Proag prompts a critical reflection on the need for resilient drainage systems in the face of evolving climatic challenges.

* Why do we have floods?

Being flooded or submerged by water occurs because the soil cannot absorb the incoming rainfall, or because water on the ground is not flowing away fast enough. A sudden heavy rainfall will cause flooding if there is no drain to carry the water away. A low-lying area will certainly be flooded because all water will eventually accumulate there, and it is usually difficult to make drains at a still lower level.

* What are the factors influencing flood occurrences?

When rain is falling, the amount of water that is going to collect and flow into a river or drain is influenced by (1) the catchment area concerned, (2) the rainfall intensity, and (3) the runoff coefficient.

* Can you elaborate?

Figure 1 shows the process of rainfall, wherein rainfall is the sum of the ensuing evaporation, infiltration, and runoff. Rainfall = Evaporation + Runoff + Infiltration.

The land’s surface always has a slope, however small it might be, which determines the direction of flow (here, the runoff).

Figure 2 indicates how the ridge at the top of a valley slope will divide rainfall, which will run along slopes on either side of the ridge. The area enclosed by a given ridge determines a catchment area. Depending on the point of interest, the catchment area will vary. Point X determines a smaller catchment area than point Y, and in turn, the area at point Y is smaller than that governed by point Z. Eventually, the estuary governs an even bigger area.

Figure 3 gives an overall picture of a valley (with smaller valleys inside) and indicates how everything discharges into the lowest point, which happens to be the estuary.

In this connection, there is a parallel with traffic flow. Unless the conveying capacity Q_OUT is greater than the incoming flow of traffic Q_IN, there is going to be a traffic jam. While this results in a halt or lower speed in the case of vehicles, unfortunately, with water, this higher inflow leads to non-stopping flow overtopping the drain and flooding the sides.

* What is the significance of the runoff coefficient?

The runoff coefficient C represents the percentage of rainfall that flows over the ground, in the equation, Rainfall = Evaporation + Infiltration + Runoff. Neither this runoff nor any of the other parameters is constant, though an annual average may be taken.

For example, during a hot sunny day, imagine that some rain falls. As the raindrops touch the ground (soil or road surface), water vapour can be seen rising in the air. Evaporation is actually occurring, live and direct! If it is light rain, the ground surface will dry up quickly. Either all rainwater evaporates on the road or some of the rain is absorbed into the earth: infiltration is taking place.

The end result is, however: there is NO runoff! 

At the other end of the scale, even under the same sunny conditions, if there is heavy rain, there will be substantial runoff towards a low point (drain, river, pond) because the soil has reached its infiltration capacity. The soil is momentarily saturated. The ratio of this runoff to the measured rainfall is the runoff coefficient.

Typically, books give the average – over a long interval of time – for the runoff coefficient. However, for those people who have experienced cyclonic conditions, the situation is different. When there are heavy rains, rain might fall continuously/on and off, during several days. The soil is now saturated over a longer period, and sometimes, this can be felt even outside cyclonic conditions.

Imagine now a sudden, heavy rainfall under these conditions. This will just be runoff. There will be NO infiltration (saturated soil) and little evaporation (the air is saturated with water vapour). So now, the runoff coefficient C, taken as C = 1, is a worst-case scenario that needs to be considered.

Although this might be difficult to swallow, it is judicious to examine the situation in the light of actual experience. Mauritius is a tropical island with tropical heavy rains, not a desert where it rains 20 mm per year! If, on top of that (as in Port Louis), the ground surface consists of clayey soil or is mostly paved, again the runoff coefficient is going to be C = 1.

This factor will increase the design flow to be considered, for sure. However, though the engineer’s job is to do an economical design, he should not underestimate the loading conditions (here, the possibility that the rain will not infiltrate at all, nor evaporate, is real. It does happen…). Furthermore, the drain is expected to be effective, not only when it rains slightly but particularly or mostly under extreme conditions.

* But we do not have floods every time it rains. What type of rainfall does so?

Normal rainfalls do not cause flooding to occur, but intense rainfalls do.

The rainfall intensityiis the average rate of precipitation in mm/hr from a storm having a duration equal to the time of concentration. It is assumed that runoff due to heavy rainfall will reach a peak at the time of concentration when the whole catchment is contributing to flow. Then, the duration of the design storm is equal to the time of concentration.

The time of concentration tc is thus the time taken for water to travel from the catchment boundary to the point of interest (Points X, Y, or Z or estuary) in Figure 2. With the size of catchments in Mauritius, the times of concentration usually lie between 30 minutes and 2-4 hours.

In Mauritius, heavy rains with 100 mm/hr over an hour or so are not uncommon. More specifically, we had 91 mm at Line Barracks, Port Louis, between 2 and 3 pm on 30th March 2013. The rainfall collected over the first half-hour was 50 mm – which amounts to an intensity of 100 mm/hour. How much we had on Monday 15th January 2024 in Port Louis is yet to be discovered or published.

* Do we have intense rainfall on a regular basis?

To introduce the subject, a 100 m sprinter runs at a speed of 36 km/h on a 10-second race, but the average speed is much lower (22 km/h) when another runner (or the same person) undertakes a 10,000 m (10 km) marathon.

Similarly, though rainfall may last several hours (a long-distance race), the critical condition to observe in drain design is the highest rain intensity (highest speed over a short-distance race).

So, we are concerned with rainfall intensity that occurs within the time of concentration over a period of N years, which is called the return period. Rather than saying that this given rainfall intensity will occur every N (e.g., 50) years, we consider rather its occurrence x (e.g., 10) times over a period of Nx (e.g., 500) years.

* Is a Return Period of 50 years Acceptable?

Many Codes of Practice indicate that one of the reasons for choosing a return period of 50 years has been that the average lifetime of most buildings and structures is near 50 years. Whether we should use a bigger value can be the subject of another debate.

Rainfall frequency and intensity records can be used to estimate the magnitude of rains and the ensuing flood flows. In this respect, it is important to note that there is a 26% probability that a 100-year rain will occur during the next 30 years (a generation).

In practical terms, this means that each generation has a 1 in 4 chance of experiencing flooding, even if an exceptional rainfall intensity of 100 years has been considered. Over a 75-year lifetime, the likelihood rises to 0.53, i.e., the average person has a 1 in 2 chance of experiencing flooding during his lifetime.

* Is the population ready to accept this?

Even if a higher return interval (e.g., 1,000 years) is taken, it is found that there is a 7.2% chance (not to be neglected as being small) that a 1,000-year flood will occur during a 75-year span – a man’s lifetime.

* Can we have insurance against damage caused by flooding?

We take car insurances even when we believe that we are good drivers, to ensure that there will be funds available to cover expenses and compensation if there is an accident. The fishing and sugarcane industries have established insurances (or government compensation) in case of cyclones or bad weather. Although insurance companies might be interested in such insurances (they are to going to accept or refuse the claims for damaged cars), probably the best insurance against flooding would be to build better drains and establish a good forecasting system.

* Can we have a good forecasting system?

Over the years, technology has increased considerably. In the 1960s, the Met Services were using a lighter-than-air balloon carrying a radio sonde (sensors with a radio transmitter) which was being tracked by a ground radar at Vacoas. Since then, satellite technology has made its appearance.

* What about backyard meteorologists?

Presently, on the internet, anybody can buy an automatic weather station, for prices between Rs10,000 and Rs100,000. Just as people buy expensive cars as a hobby, there is also a market for these. Depending on the price, you can obtain more or less sophisticated equipment, and you can collect quite some data, and make your predictions.

If your predictions from your station (plus data obtained from other international stations) are quite precise or reasonably accurate, understandably people would trust you (and might even be willing to buy them), and your predictions!

* Could the Met Services forecast the heavy rains of Monday 15th January 2024?

The Mauritius Meteorological Services (MMS) did not have, in 2013, a radar station capable of detecting rainfall or heavy rainfall clouds. But now that they have it, the people of Mauritius expect that the MMS can provide useful information, namely advance warning of impending heavy rainfall. MrGolaup, former MMS Director, was re-employed on contract because he was the only person who knew how to operate this radar station. If there is now nobody who can operate the station, or the said radar station is not operational, something is wrong somewhere! But not with the Mauritian public!

* Do you think that the MMS officers have committed some blunder?

There is no reason to believe that the officers of the MMS are incompetent. It is likely that many decisions (or conclusions/interpretations of weather data) are carried out collectively, but at the end of the day, we only learn through making mistakes. Much depends on the willingness to learn as a trainee. As a trainee engineer, I used to run after my mentors to ask questions or obtain possible explanations. Nowadays, as a mentor, I have to chase trainees to find out if they need any advice! (even when no payment is involved).

Among the first and hard lessons that you learn as a civil servant: (1) Your best friend will not stand by you in hard times. (2) Your boss will deny having given you an instruction if it is not on paper. So, the best policy is always to justify (what the French call motiver) your decisions. Whether right or wrong, the reasons you wrote down will themselves explain why you took or recommended the given decision. Of course, if you are given an oral and abnormal/unethical/improper instruction, either you get it written and signed or else, you can throw it to the winds.

* What do you think about the recent Port Louis flood?

Presently, in the absence of data (rainfall intensity, evidence from affected parties – shops, museum, Caudanwaterfront, etc.), the best proxy is to look at what happened in 2013.

The motorway was flooded at Caudan between Rogers House and the waterfront on 11th February 2013, without much damage. There was a worse incident on 30th March 2013, with the loss of life. The rainfall recorded on 30th March 2013, at Line Barracks (less than 100 mm in 1 hour) would indicate a return period of some 50-100 years.

However, the fact that there was a similar flooding at Place d’Armes/Caudan (apparently without the underground pedestrian pathways getting submerged) on 11th April 2003 (Wright A., Moonien V., 2013) confirms that a 50-year flood does not occur every 50 years! It will certainly occur some 10 times during a period of 500 years but may also occur within the next 10 years!!!

* Do you think flooding can occur again, and if so, how soon?

The motorway from Montebello towards Port Louis is lined on both sides with concrete borders or walls, which are supposed to be very effective against cars attempting to collide with them. The walls are also equipped, at regular intervals, with weep holes intended to evacuate water into the side drains.

While these weep holes can be effective in handling low flows, their small size (some 30 x 10 cm to 40 x 15 cm) becomes inadequate during heavy rains and winds, as they can become blocked with gravel, leaves, and mud. When blocked, the bituminous motorway functions as a well-designed, bitumen-lined channel, as was evident during the heavy rains of March 30, 2013, when the motorway conveyed water supposed to be evacuated into the side drains.

* As of the end of July 2013, the weep holes were still the same size. Have they been increased since then?

Between Edith Cavell Street and the Government House, the lowest points in Port Louis are along La Poudrière Street. The two channels – Le Pouce stream and La Butte à Thonnier canal – are located on both sides of this road. The ground also slopes downward towards the sea, meaning any rainfall will be channeled towards these two canals and the sea.

The issue arises at the Harbour Front and Place d’Armes, where the motorway and the Caudanwaterfront Esplanade act as obstacles (Photos 1 and 2). This implies that if the peak discharge flow from heavy rainfall exceeds the channels’ capacity, floodwaters will not go directly towards the sea until they have overtopped the motorway and the Caudanwaterfront Esplanade. This, of course, results in consequential ponding in the area between the Port Louis museum and Place d’Armes. Again, this is simple logic, borne out by the events of April 11, 2003, and March 30, 2013.

Even assuming that the motorway constitutes a roadblock in the evacuation of rainwater from Place d’Armes, historical records (Chelin 1989) show that floods have occurred several times before the construction of the motorway. This implies that the existing canals/streams are not sufficient or are inadequate to evacuate the water reaching Place d’Armes in case of heavy rainfall.

* So, given that a rainfall intensity of 100 mm/hr is not uncommon, have we proposed any new canals to improve water evacuation?

Over the past 200 years, Port Louis has been flooded more than 8 times, including twice in 2013.

In 2016, Normandie received 170 mm of rain (a monthly average) during only 2 days. The resulting peak flow increase in the Seine River takes 2 days to reach Paris. So, Paris had approximately 2 days to take or organize precautions, if any.

More recently, Pas de Calais has been flooded twice within 3 months (November 2023 and January 2024). Here, as in Port Louis, there wasn’t enough time to organize any precautions because the time of concentration is short.

So, floods occur in other countries as well, sometimes against all odds.

* What is the impact of climate change?

At one time, La Chaussée Street was a “radier” to be crossed at low water level, and the sea was at this level. Further to landfilling and land reclamation, the sea front receded to the Caudanwaterfront. But, if you examine the Pouce canal, you will observe the sea water level reaching La Chaussée bridge, at high tide, or even further without any inflow from upstream! This means that the headroom under the bridges at La Chaussée, A1 (John Kennedy) streets, and the motorway has been reduced.

Climate change has one important impact: the sea level rises, albeit a few millimeters every year. So, as time goes on, the headroom for the flow of water under the bridges will continue to decrease. Compound this with an abnormal high tide or sea waves during a cyclone!

* Any conclusions that you wish to draw?

On March 30, 2013, 152.6 mm of rain fell in the city of Port-Louis, resulting in a fatal flood. The most significantly flooded places were found to be those downstream of three canals, namely: Pouce Stream, Butte à Tonniers, and Ruisseau des Créoles. These are long-existing channels that drain the lower parts of Port-Louis and discharge into the sea at the Caudan Basin.

No major improvements have been made to the archaic drainage system in Port-Louis during the last 150 years, while massive urbanization has taken place. The extensive pileup of rubbish at Caudanwaterfront (Place d’Armes metro station) needs some investigation!

Historical data since 1840 also indicates that Port-Louis has been flooded many times in the past, on average every 20 years. It is, therefore, suspected that there has always been a deficiency in the drainage network of Port-Louis.

If the existing drains are inadequate, there is clearly a need, either to enlarge the existing canals (if possible, under existing roads) or to examine how to channel heavy rains from the Pouce mountains away from the town centre.

For example, Canal Dayot village would be safer if the St Louis River is diverted into the Grand River Northwest (GRNW) River, at the level where the old railway bridge crosses both rivers.

In a small country like Mauritius, it is difficult to give flood warnings in advance so that one can abandon a house and move furniture. Flooding might occur suddenly in the middle of the night, especially during a power cut.

The only solution to keeping dry feet would be to have adequate drainage. Is it acceptable for one’s house to get flooded every 10 years? Or every 30 years? Or never at all during one’s lifetime? The three alternatives will require drains of different sizes, with different costs.

References

Chelin A. 1989. Maurice : Une île et son passé. Editions du CRI, Ile de la Réunion.

Moonien V. 2013. Inondations: Port Louis déjà sous l’eau 2003. L’express, 11August 2013, p. 20.

Proag V. 1995. The Geology and Water Resources of Mauritius. Mahatma Gandhi Institute, Mauritius.

Proag V. 2021. Infrastructure Planning and Management: An Integrated Approach. Springer.

Wright A.2013. Rien n’achangé pour les autorités. L’express, 10 August 2013, p. 9.

Mauritius Times ePaper Friday 19 January 2024