Thread by @jamewils

Attended the meeting convened by Hon'ble CM.

Idukki, the largest reservoir of Kerala is ready to open after 1992. Since its commissioning in 1976, it's opened only twice. First was in 1981

https://twitter.com/CMOKerala/status/1022858559141306368

Both occasions, the spillway of Idukki was opened during the month of November, after receiving both monsoons. This one of the rare occasion where Idukki is getting filled in the South-West monsoon itself

Upstream Mullaperiyar reservoir also reached 135.95 ft today morning and TN is taking water in full capacity through the tunnel to Vaigai reservoir. It's FRL is just 6.05ft away with hardly 1.55 TMC room left

The spillway of Idukki reservoir is located in the Cheruthoni gravity Dam. It's having 5 radial gates of size 40ftx34ft size and the sill level of the gates is 2371ft while the FRL is 2403 ft

These five gates together at FRL is able to discharge 1.37 lakh cusecs while at MWL it can go up to a maximum of 1.765 lakh cusecs

There are also two outlets in the dam at 2199 ft (size 3.05x6.40 m) each can discharge 19,700 cusecs

Idukki investigation data of 1961 recorded a total monsoon inflow of 77.436 TMC between Jun-Dec, it's important to note that 71.643 TMC was occurred in Jun-Sep. So there is rare possibilities of Idukki filling in S-W monsoon itself

Please remember when our grand fathers are so eloquent about 99 flood (which is 1924 July flood), latest largest flood after that occurred in 1961 wherein Neriamangalam power station washed away during construction due to Mullaperiyar sudden releases coupled with torrential rain

Though the spillway at FRL can discharge up to 1.37 lakh cusecs, the spillway discharge will be very small compared with the maximum spillway capacity.

In 1992, 1766 cusecs was discharged for 5 days from 10/10/1992. On 15/11/1992, for several hours a max of 8829 cusecs was done

Remember the rules for Idukki reservoir is made such a way that between suggested reservoir opening of 2400 ft to MWL 2408.5 ft, there is 5.748 TMC flood storage, which is equivalent to negotiate a flood of 66500 cusecs in a day!

Some more data to get the perspective!

Idukki reservoir was just 62% of it's effective storage on 15/7/18 and within 13 days it reached 87% of it's eff capacity with inflow gone above 25,000 cusecs on 15/7/18. Now the inflow is just around 16,000 cusecs.

KSEB enchanted power generation from 2.3 MU/day to 14.4 MU/day from Idukki to negotiate this moderate inflow. 15.71 TMC of inflow recorded in last 13 days!

Reservoir has a room of 6.512 TMC as of today to reach its Full Reservoir level.

So for the time being it's under control

KSEB by operating the Moolamattom power station at full throttle able to draw around 4,000 cusecs. So the rate of accumulation is around 12,000 cusecs only. So at present inflow condition, FRL may reach in 6 days

So Government took a decision to release small quantities to the tune of around 1800 cusecs after ascertaining it's impact by field engineers, this is imperative to avoid sudden releases of substantial quantities of water.

In nutshell, don't get scared for the time being. For the time being, the things can be managed based on the inflow nature. I will update this thread for the next few days as information is available

In simple words

Be alert, please don't get panic!

Imagine, if there was no Idukki and 16 TMC of water contained in this 13 days is being discharged to Vembanad wetland which is having less than 2 TMC storage..Imagine how much high Kuttanad will be submerged! That is Idukki's hidden benefit of flood control..

Some flood stories from old official records...I found these during my research on Mullaperiyar ....1924 flood, 4 lakh cusecs discharge at Aluva...Imagine, no Idukki or Idamalayar reservoir...we are now seeing around 60,000-70,000 cusecs and compare with 1924!

My take on a recent flood in 2005 of the same location

The recent worst flood was in 1961......some notes from old official records of the same...This too before Idukki and Idamalayar reservoirs coming into the picture! Imagine the scene from the lines

Latest update on Idukki reservoir.

Water level 2392.90 ft (0.9ft rise)
Storage room available- 5.986 TMC
Present inflow - 10228 cusecs (drop of 5937 cusecs from previous day)
Power draft - 4026 cusecs

With present rate, it will take 11 days to reach FRL.

Picture of Idukki filling since 15/07/2018, graphical representation of water level vs inflow...Secondary Axis is Inflow

Idukki reservoir update 29 July

Water level 2393.80 ft (0.9ft rise)
Storage room available- 5.472 TMC
Present inflow - 100178 cusecs (drop of only 211cusecs from previous day)
Power draft - 3954 cusecs

With present rate, it will take 10 days to reach FRL.

An Old toposheet map from Corps of Engineers, US Army, 1953.

Periyar and Cheruthoni Rivers joining (Before the Idukki Reservoir construction)
and see how the system now (from google maps)!

It is important to learn from history.........whether it is politics or flood...see the official records of 1957 flood.....

This is the description of 1961 flood, of course, the present flood we experienced is no where near to it....read about it

1961 flood history........see the extent of enormous losses....

Do you know that 1961, in the first week of July around 40.5 cms of rain happened in just 24 hours!

These are extracts from "Long range Outline Plan for Flood Control in Kerala, Chapter XI, Part-II, Vol.1, published by PWD, Irrigation Branch

When we just blaming the recent rains to climate change, it's important to revisit the severity of old floods and compare the same with what we experience now.

See we had 227 cms rainfall in July 1974 and more than half of it in 10 days!!

Another telling letter of Land Revenue Commissioner to Chief Secretary regarding 1924 flood and Mullaperiyar Dam sudden opening. From Travancore Government Archives....

some rainfall recorded in 1924 flood, all in inches, so 25.36 inch means 644mm rainfall in a day! compare that with the rain we are now getting

Reading these old records of 1924 flood, a chill will pass through our spines...are we ready to negotiate even half of that flood? How Kochi will survive such a flood in future?

Here our TV channels are beating their drums when we see flood of tune of a few thousand cusecs!

What was the total rainfall during the month of July, 1924?

Its a dreadful figure, will make you shudder!

Peerumedu Residency recorded a rainfall of 120 inches in July, ie, 3048 mm in a month

During the flood of 1924, official Travancore records says that Aluva- Perumbavoor Road was submerged up to 10 feet of water for the entire stretch!

Chilling Description of Aluva Sub Divisional Officer about the devastation caused due to 1924 flood

These records are provided here for you to give a proper perspective about the historical floods Kerala was subjected continually..

Learning from past is very important in managing the future floods.

If you are interested in 1924 flood, please do visit TATA TEA MUSEUM at Munnar. Many photographs of the flood of 1924 are displayed there

CWC G&D site data at Periyar recorded a maximum inflow of around 85000 cusecs in this July. While on the same date, both Idukki & Idamalayar reservoirs together contained more than 40,000 cusecs in them. There would have been 50% more flood if these reservoirs were not there!

Orange alert issued for Idukki reservoir as it's water level touched 2395 ft at today 9 pm..

To reach FRL of 2403 ft, another 8 feet & a quantity of 4.77 TMC of water is only needed.

But there is a chance to open spillway if another 2 TMC is reached within next 48 hours!

Major Reservoirs in Periyar Basin - Storage Position as on 30th July 2018
Mullaperiyar - 79%
Idukki - 90%
Idamalayar - 92%
Kundala -55%
Madupetty - 84%
Anayirankal - 33%
Ponmudi - 97%
Neriamangalam -97%

We should pray that there should not be any heavy rainfall in the coming days. That will create a situation of a undammed river of Periyar as the storages will not be able to contain the flood & discharge all its inflow into the downstream and the result will be a swelling river

A good question and my response. I am trying my best to explain this in a layman language. If you need further clarification, please ask

https://twitter.com/rameez4chelsea/status/1023980583154671616?s=19

We can't brush aside such a possibility, but please understand that the reservoirs will discharge through spillway a quantity less than what is received in the reservoir, some quantities are absorbed temporarily as flood storage

For example, the inflow into the Idukki for yesterday was to the tune of 10K cusecs and 4K cusecs was used for power generation. If we are releasing now less than 2K cusecs only will be released now and rest 4K cusecs will be temporarily stored in the dam for further release/use

So the spillway release, if happens today will be almost 1/5th of the water comes till Idukki dam. Please understand that Idukki opening do not mean that the entire quantity behind the dam is going to discharge into the river, but a fraction of inflow

In nutshell, the spillway release will depend upon how much quantity of water is coming to the reservoir, how much can be used for generation of power, how much can be safely retained there itself. So the BALANCE QUANTITY ONLY will be released through the spillway.

But this manuverability will be get shrinked when the reservoir will get closer to the FRL and there will be heavy inflow into the reservoir. Then to avoid any overtopping, dam authorities will be forced to release most of the inflow coming into the reservoir :(

In that scenario, dam will lose it's ability to contain the flood and will release whatever inflow coming to it released to further downstream to avoid overtopping.

So it will pave way for a condition of an undammed river (river before the dam existed) for that particular area.

Yep, as soon as the dam reaches close to it's FRL, it can no longer hold any floods coming into the reservoir, as it will jeopardise it's very existence and safety, the further inflows into reservoir must be released through spillway. So dam stops it's service of flood control!

In most operational scenario, the spillway release will be less than or equal to the inflow from it's catchment unless a maloperation from the authorities, which is unlikely. The reservoir still has a flood storage space to moderate it between FRL & MWL.

A rational operation some times warrants gradual release of water for an extended period instead of a sudden release.

So reservoir will be open before it reaches FRL to get more manoeuvre & control of releases.

That is what we are now planning for Idukki.

Recollect Chennai floods, the delay in opening Chembarambakkam Lake and sudden releases from last minute while heavy rain was on multipled the havoc below it.

A similar gate operation fiasco had done at Mullaperiyar in 1961 too!

These are the object lessons guiding us!

Idukki reservoir update 31st July

Water level 2395.30 ft (0.7ft rise)
Storage room available upto FRL
- 4.594 TMC
Present inflow - 8980 cusecs
Power draft - 4134 cusecs
Net inflow retained - 4846 cusecs

With present rate, it will take more than 10 days to reach FRL.

Idukki reservoir update 1st Aug

Water level 2395.80 ft (0.5ft rise)
Storage room available upto FRL
- 4.279 TMC
Present inflow - 7878 cusecs
Power draft - 4123 cusecs
Net inflow retained - 3755 cusecs

With present rate, it may take 13 days to reach FRL,
See how inflow falls

To get a perspective of the inflows, last day from Poringalkuthu Dam in Chalakudypuzha spilled 16,000 cusecs while the net inflow in Idukki is just 3755 cusecs

Finally the Dam Engineers in charge of Idukki reservoir, takes a breath as the inflow contracts substantially to 6117 cusces and the net inflow retained in the reservoir is falling to a meagre 1988 cusecs, it may take another 24 days to reach FRL at this rate!

See the FALL