An attempt to understand the flow of the Thames.
The flow at each point is found with this order of priority:
1)An Environment Agency (EA) Gauge (there are nine of them);
2)A mean daily figure (links marked 'hydro'). (This is a daily mean 9am -)
Note that this is at best yesterday's mean figure and therefore when flow is rising will inevitably
be an underestimate - and equally when it is falling, an overestimate; to overcome this a "trend" is
applied using the most appropriate place having both an EA gauge and a "hydro" daily mean;
3)If there is still no flow figure, if there is an excedance percentile table available
(this page uses 34 such) and a nearby point has a percentile (shown here as Q%),
then the flow can be calculated;
4)Failing that the flow can be shown as the last known flow upstream, plus any tributaries,
plus an interpolation between that flow upstream and the next known flow downstream
calculated as some figure per km.
Occasionally that interpolation may be negative (the flow may be reducing - or there may be an error in the estimates.)
The aim has been to achieve a coherent picture (which might mean the figures add up
even if they are wrong!)
Click the links on the left - particularly the "3Locks" which present the lock levels in graphic form.
The conversion of the online levels to Ordnance Datum (ODN) should be correct for upstream lock figures.
However, for historic reasons the downstream situation is more complex. The EA provided
a downstream ODN figure but this was not the reference figure for downstream levels.
The actual reference which is now online by the EA should be the Tail Gauge Pile Zero (TGPZ) -
in other words the zero point on the tail gauge from which levels are read.
These figures may not be perfectly accurate.
The test is that when the flow is zero there should be no reach drop (and the slope will be 0cm/Km).
This site has often shown negative slopes which is of course impossible, though small errors in readings
might cause that. If I find a negative slope I will correct the TGPZ accordingly. The Iffley Lock figure
has already been raised by 0.10m for that reason.
Let me know if you see a better way to do it! [John at the above domain]
A simple way to calculate reach drop without using datum figures is provided for each lock in the
'FORMULAE' column. Hover over the 'rdiff' link to see how it works. It should provide the same results as
the more straightforward (but more long winded) method used here.
If you see a Tsunami coming down the river assume a mistake in my figures!
The table updates every 15 minutes (though of course the Environment Agency levels may be older than that).