|PLA:||All Gauges||Richmond||Chelsea||Tower||Charlton||Silvertown||Tilbury||Denton||Thameshaven||Southend||Shivering Sands||Margate||Walton|
|NTSLF:||Lerwick to Dover||Forecast Sheerness||Levels Sheerness||Southend Forecast graph||Forecasting|
In the above presentation of the tideway the vertical scale is -4 to 8 metres ODN (Ordnance Survey Newlyn).
Note that seafarers and boatmen will be used to CD (Chart Datum) which is at about 0m lowest normal tide. (Shown by the short black lines) But this varies widely on the tideway - so to understand what is happening all the figures are changed to ODN.
There is no horizontal scale! The gauge stations are spaced equally.
The barrier is between Charlton and Silvertown - and the drop between them is the only indication of the barrier state available.
The tidal movement is always right to left (east to west). The high water, half tide and low water points are marked.
The high water and low water times are shown (and only change after the tide reaches Teddington).
The forecast (scarlet line) is experimental. DO NOT RELY ON IT!
The future forecasts are based on the NTSLF CD forecast for Sheerness (translated into ODN at Southend). Once an gauged high water is detected at Southend then the forecast for places above is updated.
The result is presented below. The heights at which the barrier may close are shown. There are various factors so that the decision is not automatic. However trigger levels for various river flows are shown (as the flow increases so the trigger levels come lower).
In the above graph the green shows the latest Sheerness forecast translated into ODN at Southend.
The blue shows actual gauged levels at Southend over the last 19 hours.
The red and yellow show the trigger levels which may influence barrier decisions.
The high water and low water times and heights are from the forecast and may differ from the tide tables.
This next graph shows the maths behind the experimental forecasts for places above Southend.
The heights relative to Southend are plotted against river flow.
The vertical scale is 0 to 2 metres (above the relative High Water at Southend).
The horizontal scale is the river flow in cubic metres/second.
The straight best fit line through them defines the formula to apply for each place.
Where the line starts on the left is the fixed amount to add to the Southend High Water.
The slope of the line defines a factor which is the effect of the flow.
Each high water at each station relative to the Southend HW is added and changes the results very slightly. The best fit lines are calculated by LEAST SQUARES REGRESSION METHOD.
This is all on the assumption that the formula is linear. So far it might be!
The variations are probably caused by wind and sudden changes in river flow.