Isis Level

Isis Flag:
Isis Flag
The river level and towpath flooding plots will be re-activated once the flag changes to Red.
EA Boards: Osney Lock No lock transit on Red or Yellow boards (OURCs Rule 2.6.14 )
[Latest] Iffley Lock (Status at 10:02, 24 Feb)

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), which mark the lower and upper ends of the main part of the Isis within Oxford.

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 speed is related to the OD-IU difference). Dotted lines are lines of constant (OD-IU) difference, i.e. approximately constant flow speed.

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 estimated flow rate (m3/s, left-axis, squares), derived as below, and daily rainfall (mm, right-axis, bars), obtained from the AOPP Weather Station in central Oxford.

Flow rate F is estimated from

F = 4 ( OD - IU + δ + a ) ( OD + IU + δ + b )
where the first term represents the pressure difference driving the flow, and the second term represents the cross-section area of the river. δ=-2.15m is notionally the difference between the heights of the Osney-Iffley gauges relative to mean sea level, a=+0.1m and b=20.0 are 'tuning' parameters required to reproduce the correct behaviour, eg zero flow in dry summer conditions.

The factor 4 is chosen to give a flow rate which approximately matches the geometric average of the nearest Thames flow meters which are about 10m upstream (Farmoor) and 10m dowstream (Sutton Courtenay).

The Forecast assumes no further rain, and that the flow F(i) [m3/s] for day i can be modelled as

F(i) = B + A * R(i-2) + F(i-1) * exp(-1/T)
where F(i-1) is the previous day's flow, R(i-2) is the rainfall [mm] from two days previously. A (response), B (background flow), T (time constant) are parameters obtained from the best fit to the previous month's data (current values shown on the graph) and the error bars represent the RMS error with which this function fits the previous month's data.

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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).

Page maintained by Anu Dudhia,