The Teddington Weir Hydropower Scheme. Tidal Effect Assessment

Transcription

1 B.Spoke ewaterpower The Teddington Weir Hydropower Scheme Tidal Effect Assessment Produced on behalf of Ham Hydro CIC December 2011 Agent John 3:16 South of England, Wales France B.Spoke ewaterpower Company Limited Registered in England: Registration No VAT No Registered Office: 26 Goodways Drive, Bracknell, RG12 9AU. Telephone: web:

2 The Tidal Effect on Levels at Teddington Weir Purpose The purpose of this report is to define the methodology used to evaluate the effect of the tide on the river levels at Teddington Weir. The report will also show the results and the conclusion gained from the data. Introduction Prior to entering into the methodology and calculations there is a requirement to explain what is meant by the tidal effect and the factors affecting the levels at Teddington Lock and Weir. The River Thames enters the sea through the Thames Estuary. The point where the river changes to become the sea is at the end of the tidal reach. This is at Teddington Weir. At all other Locks on the River Thames the difference between the Head (upper) and Tail (lower) levels at the Lock is relatively constant, related to flows, and dependant on the management of the river levels by the Lock Keeper. Teddington is similar to this in most respects except for the tidal influence. When the tide comes in it reduces the down river flow and can in high Spring tides reverse it. When the tide is fully out the level in the river is such that it would restrict navigation above Richmond during the low tide period. The restriction in navigation has been overcome by creating a Half Tide Weir at Richmond. The object of the Richmond weir is to hold the level of the river at a set minimum height. The Half Tide Weir is raised for the period from two hours before high tide until two hours after high tide. The object is to remove any restriction and to allow normal tidal flow during this period. The weir is then lowered to maintain the normal upstream level between Richmond and Teddington. This is more fully explained at the following web address. This procedure is followed for each tidal event (two per day) throughout the year with the exception of a three week period in November. The period in November is referred to as Draw Down and the Half Tide Weir at Richmond is not operational. The object of this is to allow the Teddington to Richmond reach of the river to be cleared out more fully. The draw down period has not been factored into the calculations. The draw down would have a minimal positive influence on the results. Matt. 28:19

3 The images below show the river at Teddington Weir in the three stages, Normal Level, High Tide and Draw Down. The final image is the Half Tide Weir at Richmond in the lowered position. Normal Level High Tide John 3:16

4 Draw Down Richmond Half Tide Weir The levels used throughout this document are shown as maod, Metres Above Ordnance Datum. The Ordnance Datum is based on the mean sea level for Newlyn in Cornwall for the period 1915 to The contour lines on all maps in England are based on this setting. Matt. 28:19

5 The graph below shows the Levels Duration Curve for Teddington showing the Mean Level in Relation to the Head for the period 1Jan2000 to 31Dec2010. This does not ignore the tidal effect but absorbs the data into the average therefore disguising its affect. The graph below shows the Levels Duration Curve for Teddington for the same period above including the Maximum and Minimum daily tail levels. John 3:16

6 The extent of the variation can be seen but in this format cannot be built into the potential generation calculation. The graph below shows the range variation over a five day period in between 18 th and 22 nd March 2010 inclusive. The marked reduction in flow is due to the incoming tide holding back the flow of the river causing the level to rise. Data The data used to derive the model was taken from the flows and levels for the year The data was supplied by the Environment Agency (EA). The data is based on the 15 minute levels readings at Teddington and the 15 minute flow readings at the Kingston gauging station The model was created using a series of daily average flows; these were: 8.4m 3 /s, 17 m 3 /s, 27m 3 /s 34 m 3 /s, 55 m 3 /s, 95 m 3 /s 116 m 3 /s 140 m 3 /s Flood. Flood has been taken as flow above Q5 (the flow exceeded 5%of the time). Matt. 28:19

7 The level readings are for days when the flow averaged the figures above. The flow was not constant it changed during each 15 minute period. The dates below are corresponding to the order of the average flows above: 4 th July; 7 th June; 20 th May; 12 th March; 20 th April; 11 th February; 29 th March; 1 st February; 1 st January. The exercise is designed to provide an indication of the tidal effect over a range of flows. There are many variants which have an effect; the intention is to provide an overview. The tidal impact overview is designed to be used to assess the effect on the generation potential of the scheme. The generation potential is based on the flow percentages derived from the historic flow data of the previous 10 years. It is not possible to guarantee the flow for the coming years. In general it can be expected that there will be wet and dry years in a similar breakdown to the previous ten years. The generating potential is based on expected flows with usual tidal effects and expected levels. The tidal effects have not taken into account whether the tides on the days recorded were Spring or Neap or at what stage between. John 3:16

8 The variables are such that it is impossible to define exactly what will be generated and what the income will be. The exercise is to assess the effect and to have taken it into consideration. In the following pages the graphs of the individual tidal effects data are shown. Date 4 th July 2010 average flow for the day 8.4 m 3 /s Date 7 th June 2010 average flow for the day 17m 3 /s Date 20 th May 2010 average flow for the day 27m 3 /s Matt. 28:19

9 Date 12 th March 2010 average flow for the day 34m 3 /s John 3:16

10 Date 20 th April 2010 average flow for the day 55m 3 /s Date 11 th February 2010 average flow for the day 95m 3 /s Matt. 28:19

11 Date 29 th March 2010 average flow for the day 116m 3 /s Date 1 st February 2010 average flow for the day 154m 3 /s John 3:16

12 Date 1 st January 2010 the river was in Flood with average flows above Q5. The chart below shows a compilation of each of the preceding charts. Matt. 28:19

13 The effect of the tide on the generation capability at these flows has been calculated. The method was to calculate the generating potential for the head at the average flow for the number of hours at full head. From the graphs and data evaluate the amount of time at varying head. If the level went above the No Generation Level (NGL) the time above this point was not included. The varying head was calculated as the midpoint between the normal head and the NGL. The two figures were added and compared to the theoretical maximum for that flow if there was no tidal effect. The percentage calculated was noted for each flow. The data below shows the calculations. Screw Flow head g x e power hours Q = % Q = % Q = % Q = % Q = % Q = % Q = % Q = % John 3:16

14 The efficiency of the Screw turbine is taken as 74%. This figure is set for normal operation with the design head available. The efficiency reduces in flood situations as the lower end of the screw is flooded. In the Teddington situation this will happen during the tidal events. The efficiency of the Screw is reduced by 1% for each 100mm of head lost by the lower end being flooded. This has been factored into the generating potential calculation. Q # of Screws Q% Screw Flow gravity Efficency head Power days hours Conclusion The assessment has taken a conservative view and included, where possible, all the anticipated reductions in generation due to the tidal effect under normal circumstances. It is not possible to forecast events beyond our control. The effect of the Thames Barrage being shut holding back the tide has not been considered. The variation in flow due to weather events has not been considered. By a simple calculation based on the percentile flows 2010 had 4% less flow than the average for the years but was higher than the average for I believe this data shows that the tidal effect at Teddington Weir has been properly considered and evaluated and that this is a true reflection of the effect on the generating potential under average conditions. Tidal Effect Total Kw % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % Total Brendan Barrow Matt. 28:19