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.1 ' A R CH I VE : PLEA SE D O N OT DE S TR OY ii Thames Water Tidal Thames Defence Levels Stage 1 Report on Data Assembly and Preliminary Analyses June 1987 Thames Water Thames Water Tidal Thames Defence Levels Stage1Report on Data Assembly and Preliminary Analyses June 1987 Sir William Ha !crow & Partners Ltd Institute of Hydrology Slr Wil liam Halcrow & Partners Ltd HALCROW Burderop Park, Swindon, Wiltshire SN4 00D, England Consulting Engineers Telephone (0793) 812479 •International Telephone +44 793 812479 Telex 44844 Halwil G Fax 0793 812089 International Fax + 44 793 812089 And at Vineyard House, 44 Brook Green, L.ondon W6 7BY, England Telephone 01-602 7282 'NEr A P L Smith Thames Water Nug ent House Vastern Road Reading BERKSHIRE RG1 8DB 19 June 1987 Our ref winG/TTm/11 Your rM Dear Mr Smith TIDAL THANES DEFENCE LEVELS We have pleasure in submitt ing 20 cop ies of our Stage 1 repor t on Eiata Assembly and Pr eliminary Analy ses . Please do not hesitate to con tac t me or Max Beran at IH if you have any queries . Your s sincerely E P Evans DIRECTOR Enc EPE/sgb Directors Consultants $ Baxter BSc. FICE Chairman S Gray FICE J L Beaver FICE I C Pnce FICE Sir Alan Muir Wood FRS FEng ACE C L D arks MA A CE A R Hardy FICE J C Buswell BSc FICE N A Trenter MSc MIGEOL N 1 Cochrane BSc FICE E J D Mansfield FRIBA H J Ambler FCCA F E Chasemore BSc F/CE J Weaver PnD MICE E Loevw BSc FICE A R Kopec ACE D Buckley FICE P A S Ferguson MASM MICE J P Wood BSc AcE K Arnold PhD FICE W Rotnwell MA ACE N G Jot/neon ELSc ACE C A Fleming PhD MICE J Ahmed BSc FIE A I Robertson BSc FICE D S Kennedy BSc MICE G 0 Hilber BSc FICE R N Craig BSc MICE Registered in England No I 722541 SUMMARY Th is. report describes the progress made and conclusions drawn dur ing Stage 1 of the Tidal Thames De fence Levels study and makes recommendations on the scope of work and methodology for the next stage of the project . The broad aims of the study are to establish the reliability of the tidal de fences in the Thames and to define downstream boundary conditions for various named tributaries for use in future studies. Th e River Crane was the only tributary included explicitly in the or iginal scope of work but the follow ing additional work ha s eeen requested during Stage 1. Flow—frequency ana lysis for the River Crane Similar studies for River Lea as required for River Crane Extension of ONDA model down to Southend and up to Molesey Crane and River Lea Joint probability distributions of levels and discharges on River Extension of the anlaysis up to 1 in 200 year return period . A revision to the agreement and revised cost estimates to include these items has been submitted to Thames Water for approva l. Data collection ha s been a ma jor component of Stage 1. Data acquired comprises tida l leve ls (including 47 years of diurnal tida l max ima), fluvial discharges (annual maxima , daily means and selected event data) topograph ical data for the com putational hydraulic model, and general information . A report on preliminary anlaysis of flows and leve ls at the m outh of the River Crane was submitted on 24 Apr il 1987. The results were required urgently for use in the com putational hydraulic mode l of the River Crane being developed by Thames Wa ter. The 50 year return period flow was estimated to be 20 .6 m3/s for current conditions rising to 22 .5 m3/s for future 75% urbanisation conditions. The 50 year re turn period tide heigh t was found to be 5 .50 m based on histor ic data . At this probability level the Thames Barr , which has operat ing rules which aim to keep leve ls at Tower Pier below 4 .85 m , could reduce the Crane mouth level to about 5.06 m . However this value is a very pre lim inary estimate based solely on pre—barrier conditions and w ill be revised during the ensuing stage s o f the study . Preliminary ana lysis of the River Lea is nearing completion and results w ill be presented imm inently under separate cover in a similar form to the preliminary report for the River Crane. The River Lea channel network is much more complex than the River Crane and a much greater effort has been necessary in the preliminary stages to der ive sensible conclusions . However this should be compensated by a lessening of the workload in the main River Lea anlaysis proposed for Stage 4 of the project . The complex network of channels and regulating structures and the numerous operating procedures m ake the accurate prediction of floods within specific channels impossible without the application of a riv er model. Probability anlaysis of the primary data series is well under way but will overlap into Stage 2. Investigation of correlation be tween the primary data series indicates a we ak tendency for interaction between surge and riverflow wh ich w ill probably be modelled adequately by a seasona l or other decomposition of the data . The ana lysis of trend in fluvial flows indicates that it can be safely excluded from the study . Trend analysis of annua l maximum tide levels indicate a nearly uniform pattern of trend throughout the tideway wh ich is compatible with va lues obtained by IOS for mean sea level (2 to 3 mm /annum). Following a review of previous model studies and re ports the ONDA mode l has been recommended to be adopted for the study as the most cost effective , re liable and ver satile option ava ilable. On verbal approval from Thames Water the model has been extended to encompass the whole of the Tida l Thames from Molesey Lo c k to Southend , including the Thames Barrier . Additiona l topographic data w as taken from the PLA hydrographic charts and data acquired from the Thames Water Barrier office and the surveys section . The Thames barrier has changed the probability distributions of water leve ls in the tideway quite dramatically . A strategy for including this effect into the ana lysis by combiåing the bivariate probability distr ibution of fluvial flows and Southend leve ls w ith structure functions at specific locations has been defined . The structure function is a diagram show ing lines of equa l water level at a selected location in the tideway for di fferent combinations of fluvial flows and downstream tide levels. It is proposed that Stage 2 of the study w ill continue much in the same manner as envisaged in the origina l Consultancy Brief, although it will be necessary to take much more careful consideration o f the dom inating effec t o f the Thames Barrier . TIDAL TH AM ES DEFENCE LEVEL S STAGE 1 REPORT Page No INT ROD UCTION 1 1 .1 Gener al 1 1 .2 Scope 1 1 .3 Addi tional Work 1 1 .4 Progre ss 2 DATA COLLEa ION 4 HYDROLO GY 6 3 .1 Prelim ina ry Study o f Hy dro logy 6 of River Crane 3 .1 .1 Flood Freq uency Rel ationsh ip 6 3 .1 .2 Tidal An alysi s 6 3 .1 .3 High Return Per iod F loods 7 3 .2 Prelim ina ry Stu dy o f Hy drolo gy at River Lee Ana lysis 8 3 .2 .1 Objectives 8 3 .2 .2 The L ow er L ee System 8 3 .2 .3 Data Availability 8 3 .2 .4 Tida l Water L eve l Data 9 3 .2 .5 Floo d Data fo r th e L ow er Lee and Trib ut aries 10 3 .2 .6 Analysis o f Flow s and Levels 10 3 .3 Probability Ana lysis o f Tham es L evels 10 an d Flow s 3 .3 .1 Stati sti cal Ana lysis o f Th am es Di scharge Data 11 3 .3 .2 Statistical Ana lysis o f Tide Data 12 3 .3 .3 Co rrel ation Be tween Primary Data serie s 13 3 .3 .4 Effect o f Tren d o n Re cord Breaking 13 Event s 3 .3 .5 Req uirem ent s for Fur ther Analysis 15 COMP UT ATIO NAL HYDRA IL IC MODEL 16 4 .1 Review of PreviousM odels 16 4 .1 .1 LOS Mo del 17 4 .1 .2 Th e O NDA M odel 17 4 .1 .3 Ch oice o f Mo del 18 4 .2 Ex tension o f Tidal Tham es O NDA m odel 19 4 .2.1 To pogra phi cal Data 19 4 .2 .2 Th am es Barrier 19 a Pag e No a II a II e a es m a a m a n an e o n INTRODUCTION 1 .1 General This interim re port de scribe s th e pro gress o f w ork carried out to date and con clusio ns drawn d uring Stage 1 of the Ti dal Thames Defence L eve ls study . Th e obje ctives o f th e study are a s de scr ibed in the Cons ultan cy Bri ef, wh ich w as confirmed in th e letter o f appoi ntm ent by Tham es W ater on 27 Februa ry 1987 and form s th e ba sis for th e agr eem en t w ith Tham es Water .