Our Flood Woes

Mitigation and Protection Through Storage Reservoirs

Iftikhar A Drabu
Srinagar, Publish Date: Jul 12 2018 11:36PM | Updated Date: Jul 12 2018 11:36PM
Our Flood WoesFile Photo

A couple of rainy days and whole of Kashmir valley, particularly South Kashmir and Srinagar, is in panic because of an imminent flood threat. The scene was the same towards end of last month when the water level in the Jhelum rose rapidly with the I&FC Department declaring a flood alert. For the families who had suffered devastation in Sep 14 floods, it was reminder of the anxious moments before the floods and of the horrific memories when valley was inundated by the worse flood in more than a hundred years.     

Time flies and we will soon be observing the fourth anniversary of our very own 9/11 i.e. our 9/7 (7 Sep) when most of the Srinagar city, particularly the left bank, was inundated with water column running as high as 20 feet at some places. Unfortunately, other than some cosmetic works (did they call it dredging?) nothing has been done in the last four years towards flood mitigation, control and protection.  

From an engineering perspective, there are numerous ways to address flood problem in a river basin: 

(i) increase the vegetation cover in the upper catchments and thereby extend the time of concretion of the run off and thus delay the flood peaks, 

(ii) increase the discharge carrying capacity of the river by removing any encroachments in the river and/or by widening it and/or desilting,

(iii) construct a new flood diversion (spill) channels and pass the additional (flood) discharge through it to protect a major city or town but eventually drain it back into the river

(iv) increase the height of embankments (bunds) of the river to increase its carrying capacity and 

(v) divert and store the flood discharge for some period to attenuate the flood peak. 

Each of the above measure will help in flood mitigation and control though with different scale of investments, with varying efficiency and with varying implementation periods.       

In the instance of the Jhelum basin, a combination of many of the above measures are needed to address its flood problem. However, in this write up I intend to focus only on the last measure listed above, i.e. diversion and storage of flood discharge. 

The concept of flood control through storage is very simple in that during the floods you reduce the discharge in the river, and thus the peak flood flow, by either diverting some of the flow to low lying areas adjoining or in vicinity of the river (called flood detention basins – as we had in Kandizal, Mehjoor Nagar, Bemina among others) or divert the flow into reservoirs specially constructed for this purpose in the upper catchments of the river.

Given the rampart filling (read exploitation) of low lying areas, flood detection basins, marshes and wet lands in the valley, diverting flow into them does not seem to be a viable option. That leaves us with the option of constructing storages (read reservoirs) in the upper reaches of the various catchments of Jhelum to store flood discharges during peak flows. 

To study the storage option we need to study the sub basins that comprise the Jhelum basin. Upstream of Sangam the main sub basins are Lidder, Vishow and Rambiara. A series of reservoirs need to be constructed in each of these sub basins to store, for a period of time, flood discharge while allowing uninterruptedly the summer high flows into the river. Given the topography of these basins, the best way forward is to go for cascade development, i.e. construct a series of reservoirs in the sub basin, one below the other. 

To estimate the flood storage capacity required in each sub basin we need to calculate the flood discharge the sub basin contributes to the flows in Jhelum. Consequently, based on the topography of the reach and duration of detention we can identify the number of reservoirs required in the sub basin.  

Jhelum has a safe carrying capacity of about 1000 cumecs (35000 cusecs) against which the flood discharge in Sep 14 was estimated to be more than 3000 cumecs. To ensure full protection from the magnitude of the flood discharge of Sep 14 we need to look at storing the difference between the two i.e. a discharge of about 2000 cumecs for say a 12 hours period which is considered a reasonable duration to address a flood peak. 

As an illustrative example, let us look at Raimbara sub basin in greater detail and calculate the flood storage capacity required for a 12 hour detention period. Raimbara sub basin accounts for about 20% of the overall catchment area of Jhelum at Sangam. In terms of flood discharge, it means that we need to provide a storage for 400 cumecs (20%of 2000 cumecs) for a certain time period. This discharge of 400 cumecs for a12 hour period requires a storage volume of about 1.7 crore cubic meters.

For identifying the storage capacity on Rambiara I did a high level study by plotting the first reservoir at a location across the river at the start of the Mughal Road and plotting some more reservoirs further upstream. Based on the topography of the river a typical 20-meter-high dam can provide an average storage volume of 42 lakh cubic meters. Hence, to get the estimated storage requirement of 1.7 crore cubic meters, calculated in the previous paragraph, we need four such reservoirs.  

We could similarly estimate the number of reservoirs required in other sub basins to store their respective flood discharge. Together these reservoirs in the various sub basins should be able to store 2000 cumecs of discharge for a 12-hour period. In simple terms if we were to build 20 such reservoirs, aggregating 8.5 crore cubic meters, in the various sub basins we could completely attenuate the flood peak for 12-hour period.

Effectively it means that the river uninterruptedly carries a discharge of 1000 cumecs (which it can safely carry) and during floods the additional discharge of 2000 cumecs is stored in these reservoirs (for a maximum of 12 hours) to be released subsequently when the water level in the river starts receding. This should provide a high level of flood mitigation and protection which is currently lacking.      

Any engineering solution or proposal to any problem must be practical, affordable, implementable within reasonable times lines and must ensure full flood protection. In that respect this solution meets all these criteria in that it is easily implementable within reasonable time lines (possibly 3 to 4 working seasons), investment is much cheaper to other solutions under consideration like the alternate spill channel and will ensure full protection. Besides flood mitigation the proposal has several other aspects and benefits which I intend to cover in a separate write up.  

To conclude, this write up is a concept paper on flood protection through storage of flood discharges and it has touched the topic at a macro level. What is needed is a more in depth study, a more holistic and integrated approach to study our flood problem. As we have seen short term cosmetic measures, like selective dredging of the river, will not help. A proper strategy needs to be developed to ensure the constant threat of flood under which we have been living for the past four years is mitigated and that Kashmir is ‘flood safe’. 

 

 

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