Isis Level

Jim's Transatlantic Row [Never2Late]: Finished at 02:38, 18Feb, 33rd/38 boats. [Finish Video]

Towpath Flooding

Below is a diagram showing areas of the towpath liable to flooding, which is periodically updated using the current EA river levels. The lowest area monitored is the path behind the Ice-Rink, which floods when the river reaches 16" above normal,

River Level

The plot below shows the relative level of the River Thames (known locally as 'The Isis') above Folly Bridge, judged against the steps on the left bank just below the footbridge on Friars Wharf [Map].

Below 6" above normal the river level is not a reliable indicator of flow so periods of Amber and Blue flag are just plotted as 4" and 2" respectively to show the duration of these periods.

Past experience suggests that the Red Flag will continue while the river remains more than 6" above normal and, when it starts to fall, it will drop a maximum of 3" per day (dotted line on the 'Comparison with Recent Years' plot below). On that basis, and assuming no further rain ...

Earliest expected change from Red Flag:
Wednesday, 6th March
(no change since yesterday)

Comparison with Recent Years

The plot above shows the river level (expressed in inches above normal at Friar's Wharf) at this time of year in recent years, indicated by different colours. The data extend back to 2007 but only years when the river reached Blue Flag conditions or higher are shown (Blue flag plotted as 2", Amber flag as 4", everything else Red Flag).

External links

EA Gauges

The plot below shows recent levels recorded by the EA gauges at the upstream end of Iffley lock (IU) and the downstream end of Osney Lock (OD).

The green-bordered rectangle shows region within the EA definition of the 'Typical Range' for both gauges, with coloured bands indicating approximate ranges of different flag conditions (assuming that the flow rate is related to the OD-IU difference). Dotted lines are lines of constant (OD-IU) difference, i.e. approximately constant flow rate.

Solid squares, with dates, show the last 5 days, open squares are the most recent data for each flag type.

Flow Rate & Rainfall

The Osney-Iffley river level difference, which controls the flow speed (distance/time) can vary rapidly from day-to-day as the weirs are opened or closed, but a more smoothly varying quantity is the flow rate (volume/time), which is the product of flow speed x cross-section area. This is expected to increase rapidly following heavy rainfall (after a delay of about a day or two), and then smoothly decay back to a 'normal' level.

The plot above shows the past 30 days of daily rainfall (left-axis, bars), recorded by the AOPP Weather Station in central Oxford, and the estimated flow rate (right-axis, squares), derived as below.

Flow rate F [m3/s] is estimated from

F = 150 ( OD - IU - 2.14 )
where OD and IU are the Osney Downstream and Iffley Upstream measurements (in metres).

The number 2.14m is the altitude difference between the reference points for the two locks. The factor 150 is chosen to convert to a flow rate which approximately matches the geometric average of the nearest Thames flow meters which are about 10 miles upstream (Farmoor) and 10 miles dowstream (Sutton Courtenay).

Forecast

The Forecast in the above plot assumes no further rain, and since 1st January 2024 has been based on a ground water model whose current state is shown below.

The figures in blue represent water flow in equivalent of mm/day of rainfall, which is finally converted to cubic metres per second, and compared with the 'actual' flow (calculated as above). The conversion factor (in the white box) has dimensions of area, and represents the 'catchment area'. The factor itself is one of the parameters the model adjusts from day to day for the best fit, but multiplying this number by approx. 30 gives area in square miles.

Page maintained by Anu Dudhia, email:anu.dudhia@physics.ox.ac.uk