The South West Monsoon hit Sri Lanka hard in 2016. The heavy rains have resulted in landslides in Kegalle and flooding in Colombo for example, which have displaced more than 300,000 across the island with at least 58 left dead and a further 130 missing.
Now in 2017 the monsoon season has been accompanied by large scale flooding and mudslides which have left 202 dead and 600,000 people displaced. The areas around Rathnapura and Kalutara have been badly affected but, Matara in the south of the island has also suffered serious flooding. In all the floods and landslides have destroyed 1,735 houses and damaged 9,432 while 146 schools were also damaged –
Predictably, for some it all turns out to be the government’s fault; but it isn’t.
Climate change in South Asia is a reality and one of the outcomes is that rainfall will become less predictable and the frequency of extreme rainfall events is set to increase. Sri Lanka needs to brace itself for more events like this, and do you know what? In the short run there isn’t much anyone can do about it.
The 2016 flood; what happened?
May is statistically the start of the South West Monsoon in Sri Lanka and brings rain to the South and West of the country.
As any A level geographer will tell you (putting it simply); the South West monsoon season is the result of a seasonal reversal in wind direction, from North East ( off the landmass) to South West (off the ocean)
This year the Indian sub-continent ( and also Sri Lanka) has been suffering big time from heatwaves which have seen the temperatures soar above 40 deg C. for weeks on end. High land temperatures vis-a-vis the ocean, create an air pressure imbalance; lower air pressure over the land higher over the ocean, an creates an airflow from the Indian Ocean on to the land masses of Sri Lanka and India. The bigger the difference in temperature and air pressure between the two the stronger the inflow of air towards the continent. Warm wet air rising over the landmass = rain.
typical SWM situation; credit: one data.edu
But this year the monsoon has arrived with a vengeance
Why the heavy rain?
So we already have a set up whereby the winds are coming from the South-West.. from the warm ocean surface. However, you have to factor in a low pressure system that formed in the Indian Ocean to the East of the island and developed into a tropical storm which has moved northwards towards India. As it developed it set up a strong inflow of “wet” air coming off the Indian Ocean and the Island land mass forced that wet air upwards, leading to torrential rain. So this is just a sketch to give the reader an idea of what is going on, where the winds are coming from.
satellite image of the depression
so the depression is the white blob of cloud north of Sri Lanka.It is now moving North -East. What you have to imagine is that the air flow is inward/upward and anti-clockwise reaching way back into the western Indian Ocean; those thicker cloud elements suggest more rain to come!
A note on depressions for the uninitiated
a low pressure system or depression creates an inflow of air in a broadly anti-clockwise spiral. Warm air heated from below rises.. the warmer the air the faster and higher it rises. As it rises it cools, condenses and produces cloud and rain.
here we are interested in the diagram on the right; credit the britishgeographer
The Indian Ocean this time of year is warm in any case; but I read that the surface ocean temperature may be around 31deg C. which is way above where it should be, and that is only likely to make things worse.
Nb; This isn’t a local effect however as some seem to think, but related to global circulation patterns
So in this case very warm moist air spirals upwards. As air condenses to form water droplets it releases heat back into the rising air.. it fuels more uplift.. clouds reach up higher into the atmosphere resulting in the formation of cumulus clouds stretching high up towards the tropopause. As a rule of thumb the stronger the uplift the lower the air pressure and with it the stronger the inflow of surface winds at the base of the depression.
The net result; torrential rainfall, strong winds and violent thunderstorms.
Flooding in Colombo
According to some, the construction of the large lakes such as the Diyawanna lakes and the improvements to the canals and drainage systems was supposed to mean that Colombo would not suffer any more from floods like the 2011 flood. Maybe this was wishful thinking, however.
Colombo district will always be vulnerable to flash flooding because:
- large areas are low lying; no more than 5 metres above sea
- the land gradient is slight so water has nowhere to drain away
- large areas are urbanised which reduces infiltration, and exacerbates surface flow
- Rivers like the Kelani river flow off the west flanks of the central highlands which have been receiving high levels of precipitation. That flood water is eventually going to end up on the lowland. Colombo sits on the natural flood plain of the Kelani Ganga and the river does periodically what rivers do.. it floods especially when it is choked with sediment washing done from the central uplands.
- on top of this heavy rainfall and intense thunderstorms are common occurrences in the Colombo district
This does seem to be an extreme event, however. Over 300mm of rain fell right across the catshment, both in the upper and lower reaches ( as much as could be expected in a month) Intense rainfall on to an urban surface had nowhere to go.
The Kelani Hydrograph
a hydrograph plots the way rivers react to an intense rainfall event. I found this one which is taken from the 2005 flood. What you can see is that the river reacts quite quickly to rainfall events.. less than 24 hours it seems:
Points to note:
- very steep rising limb
- short time lag from peak rainfall to peak discharge
- high volume of peak flow
- gentle falling limb; flow is still high3 days later
If you study the Kelani valley in the hills where it rises you can see why it will flood downstream
credit: Dhammika Heenpalla
- the valley is steep sided which would accelerate rainfall into the channel as surface flow and throughflow
- the slopes are long and locally rainfall is high so a large amount of rainfall will end up in the channel very quickly
- there is no flood plain for flood water to spread over
- so all the floodwater is penned up in the valley unable to escape; instead thundering down to the lowland plains to the west.
It’s too early to assess the damage caused by the landslides and flooding across the island. What we do know is that over 200 families are missing with 58 pronounced dead following landslides in the central uplands that engulfed 3 villages.
Across the island upwards of 300,000 people have been rendered temporarily homeless. The cost in terms of damage to property will become clearer as time passes. In the meantime a huge relief operation is swinging into place with the Sri Lankan armed forces at the forefront of rescue and relief efforts.
Who will be shouldering the cost?
- More than likely the lower middle and low income groups whose homes have been engulfed or inundated.
- In Colombo, those living in unofficial shelters next to the Kelani river, those in the low lying suburbs around Colombo;
and it is just worth thinking about what it means for a family to see their home flooded, their possessions ruined; and in the main these are families who cannot easily bear the cost of putting there home back together
My friend Imi’s house; this is the reality of flooding: putting her mother’s house back together will take way more money than they have; life is set to be difficult for sometime to come and what you need to do is multiply that by 300,000 to get an idea of what this means for a small island like Sri Lanka
We might “study” this as a hazard event BUT let’s also remember that this is very much a human tragedy.
For some stark video footage check out http://www.sundaytimes.lk the online page and scroll down the front page to the video links.
As with the UK a couple of years ago, when we get an extreme weather event it is almost “de rigeur” to blame the authorities for not anticipating the event and protecting against it. It seems it always has to be someone’s fault. The same appears to be happening in Sri Lanka. I saw a Sunday Times.lk opinion piece asking; ‘why is this happening again?” Well unfortunately that is nature for you. What did people expect? That the government could simply tell the rain to go away?
Point is that we live in an increasingly hazardous world which we do not control and cannot always predict. If we choose to live in areas which can become vulnerable to hazards such as flooding and landslides we need to accept that when these extreme events occur we are at risk. That doesn’t mean that efforts should not be made to moderate the impact of these events
Given that the outcome of climate change may well mean an increase in extreme weather events (among other things) a question that might be asked is what is being done to moderate the impacts of future events
- The meteorological department based in Colombo is already on the case; a great deal of research is already underway on a range of issue related to weather and climate and is well worth checking out: try this out: http://www.meteo.gov.lk/index.php?option=com_content&view=article&id=47&Itemid=200&lang=en
- But the met office need access to much more powerful computing power and technology if the aim is to improve the forecasts
- So knowledge is the first step but then it comes down to what the government intend to do with that knowledge. The danger could be that in the rush for economic development planning for an increasingly hazardous environment may not be a high priority.. that would be a mistake.
So what might need to be done.
- Seems to me understanding where the flood water comes from is a starting point. Does the Kelani River need for its banks to be raised higher?
- Is there a need for engineering solutions to control/impound water in the central highlands in the event of high rainfall as for instance the Los Angeles river in southern California so that Western Province is less at risk?
- Can anything be done to increase the flow of floodwater from drains and canals to the sea?
- In the highlands is it time to re-forest the slopes of the hills to stabilise them and reduce the landslide hazard? Should geological surveys be undertaken to identify and map areas of high vulnerability
- Should people be moved away from dangerous locations?
All basic stuff isn’t it but that’s where the thinking needs to go; its an old saying but prevention is much better than a cure and in the long run less expensive.
The point is that this type of extreme event will happen again and again; all the predictions point in that direction. What the government needs to do now is put in place a coherent disaster management plan, with a senior minister at the helm to co-ordinate rescue and recovery and possibly to take control away from ineffective and self seeking officials and politicians who have shown that they are not up to the task.
In the follow on article i will be looking at the relief effort, the declaration of flood areas as high security zones and what can be done to help mitigate against future floods.
Just published; have a look also at this from CEPA