3 July 2010

Attabad – Analysis of NDMA data on lake level

Posted by Dave Petley

Yesterday’s daily update from NDMA contained a new, and very welcome addition: a graph showing lake level, starting 29th May (the date of overtopping).  The y-axis is slightly confusingly labeled on the sheet (the label having been added later by the looks of it), but is clearly the lake level (in inches – NB 1 inch = 2.54 cm) above the spillway base level at the time of overtopping:

There appears to be a datapoint every two days until the last four days (which have daily data).  This data suggests that the water level peaked on 24th-26th June, and has subsequently fallen somewhat, which is really good news.  This coincides with the cessation of reports about houses being flooded.

This of course allows one to check the data that I have been publishing against the NDMA data.  Below is the graph of my data up to the point that the feed ceased for me (the points are daily rather than two daily), with the lake level above the overtopping point in inches on the y-axis

There are some minor differences, as one would expect of two different datasets of the same natural  phenomenon (the major difference is that my data are slightly higher for the middle part of the dataset), but the similarity between the two graphs is striking, which is reassuring at every level.  The peak water levels recorded are almost identical (within +/-2% of each other).

I am still struggling to reconstruct the more recent changes in water level, so I have used a tool that extracts data from graphs to determine (probably +/-2%) the most recent lake level data from the NDMA plots.  I have then added this to my data below to generate a compound graph.  The open symbols are the NDMA data:

Spillway discharge has also substantially increased.  I have tried to continue to compile reported spillway flow figures, cross-checking these against the Ganesh Bridge data that NDMA reports.  Where the data are consistent I have included it; the resulting relationship between discharge and time looks like this:

Perhaps most interesting is the relationship between spillway discharge and lake depth:

There is a fascinating level of complexity in this plot that will deserve better analysis in due course (there are going to be some great papers from this example), but it is clear that the relationship between discharge and lake level is now changing, with continued increases in discharge even as lake level falls.  This suggests that the spillway efficiency in transporting water is increasing.  I wonder if this is because the spillway is now downcutting, or because widening (natural or artificial) is starting to have a real effect.  It will be worth watching this over the next few days.  It is a great shame that we have no images from the site at the moment.

All-in-all the situation on the ground is evolving, which means that we are still moving towards some sort of resolution.  Of course we expect to see inflow continuing to increase over the next few weeks.