Th a m es W a t e r River s Di vi s i on KENNET VALLEY STUDY • READING - THEALE F i n a l Re p or t • J u n e 19 8 7

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• SIR ALEXANDER GI BB & PARTNERS i n a ss oc i a t i on wi t h I NSTITUTE OF HYDROLOGY • T h a mes W a t er • R iver s Div i s i on KENNET VALLEY STUDY . READING - THEALE

F i n a l Rep or t • J u n e 198 7

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• SI R ALEXANDER GI BB & PARTNERS in a s soci a t i on wi t h I NSTITUTE OF HYDROLOGY •

S I R A L EXA ND E R G I B B & PA RT N ER S EA RLEY HO US E

C O N S U L T I N G E N G I N E E R S 4 27 LO N DO N ROA D EA RLEY P • R T N E R S • S E N I O R C O N S U L T A N T S R EADI NG RG6 IB L 0 14 C O AT E S W M • E• • n c t F. S • N on g • 0 15S E • • s C E K P . S C O TT m c r En n o t n o E N Z L L L L L L o w l 0 7 3 4 - 0 10 6 1 ( I S L I N E S ) v C O R N ET m• n e e r i B . N O R R i S / I C E eA s C E p , A . A . B A C K IIIS S D o m, • Ic t • EL EO P L I. E. 0 113 B O S O R W A R E • O I N G A . R M U R R AY o f t • C E 0 .18 0 6 1 J D H E N N E S S Y N A B C C N o n T A/ 0 0 0 5 B A L LA R D S A FLEE oN N ES • S S O C I AT E S p p II • ED 0 7 3 4 . 6 4 0 8 0 J . P . K E N T N • B C E R . A , C R O W • • IN A N D A T J . IS B O W C O C K ma. • 10 E Y• S C E W , 0 . P 1R I E 7 21 B U C KI N G H A M G A T E • a 0 • (DA W S O N • k in c E w on t : • I N I 0 , J B R I C E • S • r i c e M . P . H . M A R T Ol t 14, C E mi r e ] O J . • N I G H T r • ' I C E W E S T M I N S T E R S W 1E 6 P E 0 . 0 , K N O T T • • - M E L L L L L L O N E 0 1. 2 2 2 s t n i ( 2 L I N E S ) P . T . M • C R E I G H T N A /NE E J . S . A AA A A • 114 • 10 E FIN ES • I O N E C O N S U L T T T T T et P . N • R R I S N I C E • I N• • • E J . D A V I E l a c Pi t t w o r n 11 G . SA VI C E W . T . m E E 1154 N E E H R . T O M S O N 13• • • A A A A A FI C • C R T O E . 0 , P A G E R B I Pi t t • e n• R , C H A P L O W Ci . • . 1:1 mi tk e • J . C . T U R N B U L L B C E ma n R . S t . H . C O X O a MI C E N IS • Av e • E JBCL/WAE/86680A • 0 . L . W I L D E R IN G E • Im cc .. E J • . K E L L E H E R • I C E T . 0 . C • R P IEN T E R OA r et t J A . C . D A V ID S O N e s • mi c e A . a . W 0 0 0 10 11 1) e f t I IC E B o n • 4 1.H E R I . C . 0 0 5 N ET • 1 A A A A A • I N T M . T . H . K I M M I N S IN S A AND Y 22nd June 1987 C . J . M . N I P P E R m• B C E AN N ? ...a c t Thames Water , Rivers Div ision , Nugent House , • Vastern Road , Read ing , Berks .

For the attention of Mr . G .P .G . Johnson

• Dear Sirs ,

KENNET VA LLEY STUDY - READING - THEA LE F INAL REPO RT

9 We have p lea sure in enclo sing six copies o f the Final Repo rt .

The methodo logy deve lope d for the study represents an unusua l ap proach to flood studies which has been tailored to the av ailab le reco rd s o f hydrometric measurements and historic flood s . It has enabled an ev aluation to be m ade o f the sensitivity o f flooding to changes in flood 9 p la in storage without recou rse to mathem atica l mode lling . The techniques developed for computing flood p lain storage and mode lling the flood routing m ay well be app licab le to other ca tchments whe re sim i lar ev aluations are required , particularly if more hydrometr ic data is availab le . The com puterised ca lculation of sto rage volum es with in the Kenne t Va lley may a lso be o f use in evaluating the net changes resulting from deve lopm ent proposa ls . • We shall be p leased to assist with any fu rthe r investigations th at you require . If there is any matter that requ ires cla rifica tion in this present repo rt p lease do no t he sitate to con tact us .

• Yours faith fully , for S IR ALEXA IBB PA RTNERS

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• KENNET VALLE Y STUDY

FINAL REPORT

TABLE OF CONTENTS

• Page No .

EXEC UT IVE SUMMAR Y

1 , INTRODUCT ION • 1 .1 Ob jectives o f the study 1-1 1 .2 Methodo logy 1-3 1 .3 Scope of the repo rt 1-5

DATA BA SE • 2 .1 Study area 2- 1 2 .2 Survey 2-4 2 .3 Mapping and aerial photography 2-6 2 .4 Flood records 2-6 2 .5 Site reconna issanc e 2-8 • HYDROLOG Y

3 .1 Introduction 3- 1 3 .2 Catchment cha racteristics 3- 1 3 .3 Unit hydrog raph ana lysis for Kennet at Theale 3-8 • 3 .4 Flood est ima tes for Kennet at Theale 3-9 3 .5 Flood estimates for Foudry at M4 3- 12 3 .6 Flood estimates to County Weir , Reading 3- 13 3 .7 Ana lysis o f June 1971 ev ent 3- 17 3 .8 Ana lysis of Ma rch 1947 ev ent 3- 18 3 .9 D iscussion 3- 19 • 3 .10 Conclus ion 3-20

FLOO D ENVELOPES

4 .1 Approach 4- 1 4 .2 Storage during 1971 flood 4- 2 • 4 .3 Storage for other events 4- 7 4 .4 Chang es in the flood pla in 4-9 4 .5 Flood envelopes 4- 16

H ISTOR ICAL DEVELOPMENT IN THE STUDY AREA

• 5 .1 Rev iew o f plann ing docum enta tion 5- 1 5 .2 G ravel ex trac tion 5- 6 5 .3 Waste dispo sal and lan dfill 5- 10 5 .4 Res idential and industrial development 5-10

I • Referenc es

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Annexe A L ist of docum en ta t ion Annexe B Report on 197 1 flood Annex e C Hy drology , Ap pend ices 1 to 5 •

• LIST OF TABLES

Table 2 .1 Floo d Levels 2- 7

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L IST OF FIGURES

• Figure 1.1 Study a rea 1 - 2 Figure 1 .2 Catchm ent area 1 - 3

F igure 2 .1 Plan o f structures In pocket Figure 2 .2 Area of surv ey and cross sections In pock et Figure 2 .3 Ex tent of 1971 flood and recorded levels In pocket • F igure 3 .1 Catchm ent plan 3 - 22 Figure 3 .2 So ils 3 - 23 Figure 3 .3 Princ ipal urban areas 3 - 24 Figure 3 .4 Gauged sub ca tchm ents of th e K ennet 3 - 25 Figu re 3 .5 Compa rison o f Brimpton , Th ea le and Blake 's Lock • limnigraphs - June 1971 3 - 26 Figure 3 .6 Unit hy drographs for Ken net at Thea le 3 - 27 Figure 3 .7 Flood frequency curves for Kennet at Th eale - sta tistica l 3 - 28 F igure 3 .8 Flood frequency curves for Kennet at Theale - ado pted 3 - 29 Figure 3 .9 Des ign Hydrograph s - Kennet at Theale 3 - 29 Figure 3 .10 Flood frequency est imates - Foudry at M4 3 - 30 Figure 3 .11 Design hy drographs - Foudry at 1(4 3 - 30 Figure 3 .12 Comb ined catchm ent to floo d pla in - in flow and outflow 3 - 31 Figure 3 .13 Inundation volume frequency - Kennet flood plain 3 - 31 Figure 3 .14 Observed and simu lated Thea le flows - June 197 1 3 - 32 Figure 3 .15 Da ily ra in falls (mm ) - 10 June 1971 3 - 33 F igure 3 .16 Nota ble 1, 2 , 4 and 8 day ra in falls in th e Kennet catchm ent 3 - 34

Figure 4 .1 1/5000 maps sheet 1 o f 4 In poc ket Figure 4 .2 1/5000 ma ps sheet 2 o f 4 In pocket Figure 4 .3 1/5000 maps sheet 3 o f 4 In pocket Figure 4 .4 1/5000 maps sheet 4 o f 4 In po cket Figure 4 .5 Flood env elopes 1/10000 In pocket

Figure 5 .1 Gravel ex traction and open water area Pull-out , back of report Figure 5 .2 In filling after grav el extraction Pull-out , back o f report Figure 5 .2 Kennet Va lley Plan Proposa ls 5 - 4 F igure 5 .4 Ta rmac proposa ls for Holy Brook and Coley Mea dows 5 - 9 Figure 5 .5 Ax iom 4 Development 5 - 12 Figure 5 .6 ARC long term proposa ls for study area 5 - 14

Figure 6 .1 Flood pla in zones - exam ple 6 - 5 Figure 6 .2 Backwater ana ly sis 6 - 10

Figure 7 .1 Flood route th rough gravel pits 7 - 3 Figure 7 .2 Diversion ov er weir 7 - 5 Figure 7 .3 Modifications to hydrogra ph 7 - 7 •

• EXECUTIVE SUMMARY

1. OBJECT IVE

• The study was commiss ioned to evaluate the sensitivity of flooding to changes in flood pla in sto rage capac ity within the Kennet Valley between Theale and Reading without recourse to mathematical modelling . The Brief requ ired flood envelopes to be produc ed for a range of return per iods and

• for consideration to be given to future policy with regard to management of the flood plain in the light of known and ant icipated dev elopments .

2. DATA

• The topography of the flood plain was surveyed in February 1987 using parallel cross sections at 250m intervals, approximately perpendicular to the Rivers Kennet and Foudry Brook .

• Records of flood lev els within Reading are ava ilable as far back as 1894 but the only flood event for which a comprehensive data base is available is that of 197 1. This event was photographed from the a ir by • helicopt er and surveyed on the ground . The flood level records show that the downstream portion of the Kennet can be influenced by levels in the River Tham es .

411 The principal gauging station in the study area is that on the Kennet at Thea le, but at high flows gauging is affected by flows crossing the ad joining flood plain and ent ering gravel pits upstream . There is no gauging stat ion on the Foudry Brook . Flow records through Reading itself • are not generally ava ilable beca use there is no fixed rating curve due to the influence of the River Thames .

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3 . HYDROLOGY

Although the Kennet Valley is predominantly a chalk catchm ent (75% ), the floo d response at Theale reflects prima rily the non chalk component , the ma jority of which lies w ithin the catchment of the Enborne . Because of this, a spec ial unit hydrograph ana lysis of the Theale record was carried out to derive inflow flood est imates , and reconciled with resu lts obtained using a statistical approach . Flood estimates for the Foudry , which is ungauged , were based on catchment and climatic charact eristics alon e . •

Flood est imates to C ounty W e ir , which need to take account of the Kennet and the Foudry , were der ived by applying a single design storm to the ent ire catchment and routing the inflows through a simple model to tak e • account of flood plain sto rage . The model was calibrat ed from the 1971 flood event , and a frequency curve for peak outflows at County Weir derived . The modelling also yielded estimates of the quantity of flood water going into sto rage for a range of return periods . •

The 1971 flood is assessed to hav e been a 50 y ear event at Theale but more like a 35 year event at County Weir and in terms of flood plain inun dat ion . On the basis of a single gaug ing at Reading , the 1947 flood is • est imated to hav e been a 50 year event . The volumes stored in the study flood plain area for different return per iods are estima ted to be :

Return Per iod Volume •

3 2 .33 year 2.0 Mm 3 10 year 3 .7 Mm 3 • 50 year 5 .2 Mm 100 year

Attenuation of flood peaks between Theale and Reading is about 24%

• for event s of return periods betw een 25 and 100 years.

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0 4 . FLOOD ENVELOPES 0

Th e vo lum e o f water entering storage in th e 1971 event was estimated from the topogra ph ic surv ey and the record of floo d levels . This was used to ca librate the flood routing mode l and to derive a sto rage frequency • curve . The storage-elevation curve for flood su rfac es pa rallel to th e 1971

surface wa s est imated and used to locate the flood envelopes .

• The conclusions are that a 1 in 100 year flood would atta in levels abo ut 0 .1 - 0 .15m higher than in 1971 whilst a 1 in 10 year flood would be 0 .15 - 0 .2m lower . This does not apply to th e downstream reac h below Fobney

where the in fluence of the R iver Tham es can be the predominant factor in

• determ in ing flood levels . The flood env elopes a re gen erally very close to one ano ther for retu rn periods exceed ing 10 years and it is th e depth rather than th e extent of the flooding that va ries because of the con fined na ture

of the topography .

• 5 . DEVELO PMENT IN THE FLOOD PLA IN

Histo rica l changes within the flood pla in hav e b een reviewed . There

• hav e been two ma ]or ty pes of developm ent , the first being larg e scale ex traction of gravels along the Va lley an d the creation of gravel lakes , and the secon d infilling , princ ipally within th e Fo ud ry flood plain .

• Gravel extra ct ion began in the 1940 's and unt il recently most excava tions were return ed to lakes . Of 620 ha for which perm ission for

gravel extract ion ha s been granted , some 25 0 ha hav e been restored to wa ter .

This is estimated to have added to the storage ca pacity by about 0 .5 - 1 .0 3 • Mm . Recent applicatio ns for extraction have tende d to show restoration to agriculture , w ith dom ing or ridg e and furrow , for w hich the impact on floo ding is difficult to eva luate . Neverth eles s th e creation of gravel

lakes is planned to continue betw een Theale and Tyle Mill .

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Infilling has occurred principally as a result of waste tipping and • ma inly within the Foudry flood pla in . This is est imated to have removed 3 nearly 1 Mm of storag e volume relative to 1971 flood levels , and significantly more relative to 1947 levels . The proposed Reading Business Park wou ld remove the ma jority of the remainder of the storage within the • Foudry flood plain which forms an integral part of the Kennet flood plain .

It is concluded that there has been a net loss in storage with in the in recent years , with the additional volum es created by grave l • Kennet Valley extraction failing to compensate for the los s in storage due to infill ing .

6 . FUTURE POL ICY TOWARDS FLOOD PLAIN MANAGEMENT

• In order to be able to develo p an effective management policy for the flood plain it is necessary to be abl e to evaluate on the one hand substantial reductions in storage such as has occ urred in the Foudry flood

• pla in and on the other hand proposed localised changes where the net change might be small .

The study has demonstrated the impo rtance of flood pla in storage

• within the Kennet system in attenuating flood peaks through Reading and enab led estimates of the inc reased flow due to loss of storage to be made . The stu dy has not been ab le to evaluate with any accuracy the rise in water leve l that would result from proposed changes, whether they ar e substantial

• or localised . The present criterion of ma intaining sto rage on a lev el for lev el basis is judged to be prudent on the grounds that it prevents substitution of storag e at a high level for that at a low er lev el , which might reduce the attenuation of peak flows , and gua rds against a cumulative

• loss of storage . The finding that the areal extent of flooding does not vary substantially with the return period of the flood limits the scop e for zon ing types of development within the flood pla in and effectively confirms the boundary of the 197 1 flood as the area within which housing or office

• developm ent should not be perm itted .

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• Further refinement with rega rd to flood plain manag ement could be obta ined by deve loping a mathematical model of the river system . How ever , this would require a substantial investment in time and money which needs tO be weigh ed against the anticipated benefits .

• 7 . FLOOD ALLEV IAT ION

An opportunity for providing som e flood attenuation by routing the

• flood pea k through four existing gravel lak es close to Theale has been identified . Preliminary studies suggest that this could reduce a 1 in 50 year floo d at Theale to below a 1 in 10 year flood at Reading altho ugh this deg ree of reduct ion might not apply for more extreme events . Further

• studies are required to evaluate this opportunity in deta il .

8 . FURTHER STUDIES

• It is recomm ended that the follow ing studies should be undertaken :

- hydraulic study of the Holy Brook to evaluat e the scope for improv ing channel capacity or varying the div ision of flows

• betw een the Kennet and Holy Brook

- study of the consequences of increased wat er levels in Reading and the interact ion with the Thames in order to better evaluate • the benefits of proceed ing to mathematical modelling

- hydrometric monitoring of th e Foudry and Kennet .

• Oth er studies that warrant consideration are a more detailed survey to determ ine flood envelopes more precisely along the housing development s from Southcote to Beansheaf and a more detailed study of the possibility of prov iding flood alleviation . •

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• KENNET VALLEY STUDY •

FINAL REPORT

• CHA PTER 1

INTRODUCT ION

• 1 .1 OBJECT IVES OF THE STUDY .

T h e T e r m s o f R e f e r e n c e fo r t h e s t u d y , a s s e t o u t in let te r c8 1/6/RBW JAQ dated 19th November , 1986 from Thames Wa ter , a re a s follows : •

To determ ine th e extent of th e ex isting flood pla in under a ra nge o f conditions up to the Jun e , 1971 flood .

• To produce flood envelopes for ra rer events up to the 1947 flood and determ ine the lev el o f risk .

To a s ses s th e e ffec ts o f chang es in the flood plain on floo d risks • locally and down stream .

To c on s ide r th e recomm en da t ion s on t he policy with regard to th e futu re establishment and managem ent o f the flo o d pla in in l ig ht o f • k n o w n a n d a n t ic i p a t e d d e v e l o p m e n t s i n c l u d in g ma in ly g ra v e l extract ion and reinstatement propo sa ls .

• To p ro v ide in forma t ion to perm it som e assessment to be made on the ne ed a nd im p l ica t ion s o f ex t en d ing t h e s t u d y t o m a t h e m a t i c a l

modelling .

• The study area extends from Ty l e M ill B ridg e , a pp rox im a te ly 2 km upstream of Theale , to County Lock (nea r the inner distribution ring roa d in Read ing ) and is ma rked on F igure 1 .1 . The location of the study area w ith in the ca tchmen t area of the R iver Kennet is shown on Figu re 1.2.

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1-1 1 A N• R hath a a 0 • Tr r 5 ' 1,0 10 10 . 4( • O cool 14 3 72> 0 \ 'NJ C 3z7=7, 0 I. we ,93 10 3 42 10• 4L-- or t‘h a fl irt ,;(c. at e Si re. _ . • V I•1 1r, sptcot • COUNTY LOCK W ick om

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STUDY AREA. • • • • • • • • • • • •

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CATCHMENT AREA. •

• 1.2 METHODOLOGY

The methodology was developed bearing in mind the ob jective that the study shou ld evaluate the sensitiv ity of flooding to changes in flo od p la in

• storage ca pa c ity w itho ut r ecourse to mathematical modelling . Should such modelling prove necessa ry, then the value of the present study wo uld res id e i n p r o v id in g th e n ec e ssa ry da ta ba se for it to b e ca rr ied ou t . Th e metho dology employed cov ers three principal topics nam e ly hyd ro lo gy , floo d • pla in stora g e and the extent a nd influen ce o f develo pmen t in the flood plain .

1.2.1 Hydrology • The first ob jective of the hydrolog ica l ana ly s is wa s to est imate design flood hydrographs of differen t return periods .

• It wa s also the intention at the outset o f t he study to develop a simpl ified m odel tha t could be used to prov ide quantitat ive info rmation on the effects o f cha nges in flood pla in storage on the alteration or sha pe o f floo ds o f different return periods . It was hoped that it would be possible • to calib rate a simplified flood routing model by using

(i ) gauged inflows at Theale for the 1971 flood (ii ) estimated outflows at County Lock for the 1971 floo d • (iii ) flood pla in storag e deriv ed from topog raph ic al survey and recorded 1971 flood levels

• The inflow and outflow hydrog raphs would enable an estimate of flood vo lum e g o ing into stora ge to b e made wh ich could b e compared with (iii ) above . Unfortunately , this did no t p rov e possib le a s there is no un iqu e stag e disc ha rg e re la t ionsh ip a t C ounty Loc k o r Bla ke 's Lock due to the • influence o f th e Tham es so that it was not po ssible to reconstitute a n outflow hydrograph .

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• A rev ised approach has been dev elo ped us ing a 'lag a n d r ou t e ' modelling method to simulate the volume of the flood go ing into storage .

• 1.2.2 Flood Pla in Sto ra e

An impo rta nt factor in est ima ting the sensitivity of flooding to changes in flood plain sto ra ge capac ity is som e kno w ledge o f the g loba l

• storag e capa city ava ilable under a range o f floods o f different return periods . A su rv ey contra c t was let to p rov id e the basic to po g ra p h ic Information necessary to undertake this task . The reco rd of the flood water surface pro file for the 1971 flood has been used to est imate the volume that

• went into st o rag e fo r this important event . The sensitivity of storage to parallel wa ter su rfac es above and b elo w the 197 1 pro file ha s also b ee n tested .

• 1.2.3 Exten t and influence of develo ment in the flood lain

Th e st udy area has undergo ne a transformation over the last forty yea rs as a resu lt o f la rg e sca le grav el extractio n activ it ies and th e • inc reasing dema n d fo r land a rising fro m expansion o f Reading and Theale . These chang es have been documented to provide an histo rical persp ec t ive an d plans for future development have been assessed .

• The impact of futur e spec ific dev elopm en ts that m ight alt er th e cha racteristics of the storag e elevation curve hav e been examined and futu re pol icies with respect to manag ement of the flood plain advanced in the light of the flood env elopes for different return periods . •

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1-5

• 1.3 SCOPE OF THE REPORT •

The structure o f the repo rt reflects the m ethodology outlined abov e . The informa tion collec ted to form the data ba se fo r the study , such a s

• su rv ey s a nd flood records , is rev iewed in Chapter 2, and the results of the hyd rolog ical analys is are presented in Chapter 3. Est im a tes o f stora ge in th e floo d pla in for the 197 1 event are derived in Cha pter 4, together w ith th e va ria tion in stora ge fo r h igher and lower flood pro f i l e s , a n d a n

• es tima te of flood pla in stora ge lo st a s a result o f development . Floo d env elopes for different return periods are developed from this information

Histor ica l dev elopments w ith in the flo od p la in are rev iew ed in

• Chapter 5 and polic ies with regard to future development advanced in Chapter 6 in th e l ig ht o f th e flo od envelo pes develo ped for dif fe rent return perio d s . An a ssessm ent o f the effec t o f loca l cha ng es in floo d pla in storage charac ter istics is also given .

• Wh ilst the study of flood alleviation measures does fa ll w ith in th e terms of re ference, certa in opportunities have been identified and these are re po rted o n in C ha pter 7 . The conc lu sions o f th e study to gether w it h

• recomm en da tions concern ing the need fo r further studies are presented in Chapter 8 .

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1-6

• CHAPTER 2 •

DATA BASE

• The present cha pter a ims to summa rise the information ava ila b l e fo r th e st udy in term s o f su rv ey , ma pp ing a n d a e r ia l p ho tog ra p hy and flood

reco rd s . A de sc r ipt io n o f the stu dy a rea a n d th e re su lt s o f th e s it e

• r eco nna issa n c e u nd er ta k en a r e a l so g iv en . T h e da ta ba se u sed fo r th e hy dro log ica l ana ly sis is given in Chapter 3 . A l ist o f the docu m en ta t io n

and in formation used is g iven in Annex e A .

• 2 .1 STUDY AR EA

2 .1.1 To pography

• The study area extends some 11 km from Ty le Mill to Coun ty Loc k a n d ov er th is stretch the norma l water level o f the river Kennet falls from 47m

to 37m A .O .D . The r iv er meanders on a bed o f a lluv ium unde rla in by v a ll ey

g rav els w h ich overlie chalk , and th e flood pla in is generally sl ightly less

• than lkm w ide although w iden ing to 2 km c lo se to th e con flu enc e w it h th e Foud ry Brook at the downstream end . The flood pla in is negligible in exten t at the ent ranc e to Reading where th e K e nne t ha s c ut s teep- s id ed c ha nn e l s through th e Reading Beds at Coley just upstream o f County Lock .

• The explo ita tion o f grav el deposits ha s cha ng ed th e a pp ea ran c e o f

the floo d pla in over la rge area s due to the creation of grav el pits .

• 2 .2.2 R ivers and h draulic structures

The structures in the study area are shown on Fig . 2 .1 .

•

•

2- 1 • The Kenn et d iv id es in to tw o channels ju s t u pst r ea m o f th e M 4 motorway at the Arrow Hea d slu ices . Less than two fifths of the flow enters the Holy Brook which runs parallel to the Kennet about ha lf a k ilom et re to th e no rth . A high p ropo rtion of the flow in the Holy Brook returns to the

• Kennet at Coley Weir, half a kilo metre up stream o f C ounty Loc k , w ith the rema inder flowing through culve rts under Reading and returning to the Kenn et just upstream of Blake 's Lock .

• The Ho ly B ro ok wa s div ert ed in o rder to su pply wat er to Read ing Abbey from a point clo se to Southcote and now runs in a n artificial channel a longsid e Coley and through the centre of Read ing . This is recorded in the follow ing extract from Kenn et Country :

• "Ac cording to a depo sition of 1596 'the Hallow ed Brook taketh hedde at the lower end of a mead calle d the Thea le Mead a nd endeth at the W est side o f th e Orte b ridge and runneth into a great stream called Kennet '. It

• had been "diverted out of the said river to the weir no t far from Southcote Hous e at Hadsey ditch '"

Th e Kenn et is jo ined by a tr ib u ta ry , th e F o ud ry B roo k , ju st • down stream o f Fo bn ey . The Foudry flows northwards under the M4 and has an extens ive flood plain about 1.5 km wide betw een th e M4 and the con fluenc e w ith the K ennet . The R ea ding Sewage Treatment Works disc harges treated effluent into the Foudry Brook just upstream of the confluence . • On e further st ream b ed tha t is worthy o f m ention is the Draper 's Osier Bed Stream which is fed by a weir on the ma in Kenn et at Su lhampstea d and rejoins the river just downstream of Sheffield Mill . •

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2-2

• • The relatively steep gradient of the river was harnessed in the past to d ev elop a ser ie s of m ills a t Tyle Mill , Sheffield Mill, Burghfield and Ca lc ot . T he con struct ion of w eirs at th ese loc a t io ns t o o b ta in th e necessa ry head nec essitated the provision of locks when the Kennet and Avo n

• Canal was constructed in order to bypass them . There are as a result th ree chan nels a t these sections nam ely , the ma in river course, the mill race and the bypass canal. The re are also locks to bypass the weirs at Southcote and Fobn ey ; a new la by rinth weir has rec ently been const ructed at the latter

• loca tio n to mainta in a more constant water level at the Fobney wa ter sup ply intake .

The hydraulics o f the river system between Tyle Mill and County Weir

• is t herefo re complex , with five sets o f weirs and locks on the main Kennet , the d iv ision o f flow s betw ee n th e Ho l y Brook and the Ke n n e t , a n d t h e confluence with the Foudry Brook . This has been further comp licated by the exca va tion o f a nu mb er o f la rg e grav el pit s c lose to th e r iv e r w h ic h

• interac t hydraulically with the river through th e gravels, some of which are liable to flood .

2 .1.3 Dev elopment

• Extens ive development has tak en place in the flood pla in over a long period o f time inc luding railways , roads , grav el extraction , waste filling and build in g al l of w h ich ten d to restrict flow ro utes . Th ree ra ilway • emba nkment s div ide the flood plain into sections . The Read ing to Newb ury line run s w ithin the flo od p la in on the northern side crossing t he Hol y Brook in four places , while the Read ing to Bas ingstoke line runs north-south across the flood plain m idway between Fobney and Southc ote . Th e o ld Co ley • branch line also crosses the Ho l y Brook in the Fobney Meadows.

Four roads traverse the floo d pla in , namely a m ino r roa d betw een

Sh e f f i e ld B o t t om a nd T h ea l e , t he M 4 just downstream o f .Thea le , the Burgh field Roa d linking Southcote to Burghfie ld and Ro se K iln La ne in th e down stream reach linking Berkeley Avenue to the Basingstoke Roa d .

2-3 •

• The g ravels in th e va lley hav e been ex plo ited sinc e the 1940s , altering dramatica lly the landscape where the restoration has been to lakes; nearly 600ha of land has rec eived planning permission for gravel ex tra ctio n o f wh ich about tw o-th irds ha s been excavated and 250ha returned to water .

• In rec en t yea rs th e growth o f Rea ding an d Thea le ha s also result ed in increa sing pressure for dev elopment within the flood plain for wa ter supp ly fac il it i es , s ew a g e w o r k s , wa st e d ispo sa l , h o u s in g a nd in du s tr ia l development . • 2.2 SURVEY

2.2.1 Study Survey

A survey of the flood plain within the study area was und ertaken in F eb rua ry 19 8 7 in a c c o rd a nc e w ith a drawing prepa red by G ibb tha t is reproduced a s Fig . 2 .2. The extent of this survey was fixed larg ely by th e 0 lim its o f the 1971 flood . Cross sections were taken at 250m intervals with spot heights every 100m or less where necessary adequa tely to desc ribe th e to pography . T he spec ificat ion fo r the surv ey wa s prepared by THAMES in order to conform with the ir standard procedures .

The ob ject iv e o f the surv ey wa s to ena ble th e volume o f storag e within the flood plain to be assessed . It is unlikely to be deta iled enough fo r a ma thematical hydraulic modelling exercise , but it was judged that the additional exp enditu re in time a nd money that would b e requ ired for th is could no t be justified . Deta iled surveys of ind ividua l areas of land hav e , how ever , been carried out by gravel companies and dev elopers in the co urse of the prepa rat ion of their planning proposals .

The results of the survey were plotted by THA MES on 1/2500 maps and cross sect ions provided using computerised methods .

2-4 2 .2 .2 Surve of the river channel

A survey of the river channe ls of the Ken net , Holy Brook , and Foudry Brook was undertaken in 19 75 by Ro fe Kenna rd a nd La pwo rth (RK L ) and th e resu lts are recorded on the follow ing drawings:

Kennet 967/103/A/1/1 - 29 967/103/A/1/1 - 36

• Holy Brook 967/111/0/1/1 - 31 Foudry Brook 967/106/0/1/1 - 2

The draw ings inc lude :

• plans of the area at 1/2500 scale long itudinal sections a t 1/10 000 scale c r oss sec t ion s a t 10 0m intervals a long the rivers a t 1/ 10 0

• scale ; the datum for these sections is the water lev el in th e river wh ich was recorded on that particular day . deta ils of the structures (bridges , weirs, locks, etc ) along the rivers .

A note wa s found on these draw ings to the effect that anomal ie s ha d b een no ted a nd tha t they s ho uld be used with caution . The levels of the river banks were therefore checked aga inst the results of the recent survey .

• A n exa ct comparison is impossible because the survey cross sec tions are not co inc ident a nd the lev els on the RKL survey had to b e sc a led from th e record ed wat er sur fa ce; the levels are nevertheless generally in agreem ent to + 0 .2m .

A recent surv ey o f the r iver c hannel b etween Blake 's Loc k and the Read ing Sewage treatment w orks wa s und ertaken for th e lay ing o f th e new sewa ge main in the river channel between these po ints .

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2-5

• 2 .3 MAPP ING AND AER IAL PHOTOGRAPHY •

O r d na nc e Su rv ey ma ps o f the entire area are ava ilable at scales o f 1/10 000 and 1/2500 and the pa rt o f the area nearest Reading is ava ilable at 1/ 12 50 . T h e 1/ 10 000 ma ps hav e co ntours at 5m interva ls bu t there are no • contours on th e 1/2500 series . Both series a re su bsta n t ia lly o u t o f da t e

beca u se o f th e c ha ng e s w ro ug ht by th e g rav el w o rk ings over th e la st few yea rs .

• Aerial pho tog raph s at a scale o f 1/ 10 000 are flown every five yea rs

by Berk sh ire C oun ty C ounc il . The most rec ent ser ies wa s flow n in N ov em b e r 19 86 a n d c o lo u r co p i es hav e b e en ob ta in ed by THAMES . A copy o f the 1981

• series is also ava ilable and photocop ies o f the 19 4 7 a nd 197 1 se r ie s h av e a lso b e en ob ta in ed in o rd e r to identify the cha nges that hav e taken place w ithin the flood plain .

• 2 .4 FLOOD RECORDS

2 .4 .1 Flood Levels

• T HA ME S p oss e ss r ec o rds o f floo d lev e l s dow n strea m of Burghfield bridge as far back as 1894 . These are repro duced in Table 2.1 .

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2-6

• • • • • • • • • • • •

TA BLE 2 .1

FLOO D LEVELS (metres A .O .D .)

1940 1946 1947 1971 D istances 1943 1894 19 10 19 25 1929 Mar . June km Summ er Nov . Dec . Jan . Dec . Feb . Dec .

(Normal Water Leve ls for KENNET Re feren ce )

36 .52 36 .47 36 .49 36 .48 36 .48 36 .94 Kenne t Mou th 0 35 .11 37 .00 36 .60 36 .7 36 .61 36 .62 36 .51 37 .03 36 .22 B lak es Lo ck TW 0 .77 35 .11 37 .22 36 .8 36 .64 36 .64 36 .56 37 .0 1 36 .42 HW 36 .21

37 .7 37 .35 37 .0 37 .30 38 .1 37 .19 Cou nty Lock TW 2 .00 36 .24 38 .45 37 .9 37 .42 37 .12 37 .44 38 .2 37 .70 HW 36 .52

38 .45 38 .24 38 .3 38 .82 -4 Fobney Lo ck Tw 4 .6 39 .37 39 .4 39 .73 HW

39 .72 39 .61 39 .7 40 .62 South co te Lo ck TW 6 .0 40 .97 40 .98 40 .98 4 1 .31 HW

41 .24 41 .23 41 .16 4 1 .39 41 .51 Burgh fie ld Bridge 7 .3

No te : Levels prio r to 1971 have been converted from fee t Liverpoo l datum to metres A .O .D . (New lyn ) . •

• The rec o rds show th e th ree worst floods over the last century to have been those of 1894 , 1947 and 197 1. The 1894 flood would appea r to have b e en t h e h ig h e st b u t u n fortuna tely there is very l ittle in fo rm ation ava ilable for it . A comparison between the 1971 and 1947 floods show s the

• 1947 flo od to have been more severe close to the confluence with the Thames but the 1971 flood to hav e reached higher levels at Burgh field Br idg e . The r eason fo r th is is tha t th e 1947 flood inc luded rap id snow -m elt wh ich affected both the Thames and the Kennet whereas the 1971 flood resulted from

• very heavy rainfall centred over the Kennet catchment .

A longitudinal section of the River Kennet and the 1971 flood levels reco rd ed a lo ng the r iver a re g iven in Figure 2 .1 together w ith the river

• bank levels .

2 .4 .2 Ext ent of the floodin

• The 1971 flood is well do cum ented as the extent o f th e flood wa s surv ey ed from a helicopter approx imately 10 hours after the flood peak was reco rded at Theale as well as from the g round . A rep o rt on the flo od is g iv en in A nn ex e B and the area flooded is shown on Figure 2.3 . This also

• shows flood levels recorded over the flooded area; these were predom inantly at head and tail of control structures.

2 .5 SITE RECONNAISSANCE

• A reconna is sa nce o f the whole length and breadth of the study area was ma de to visit the hydraulic structu res , walk the river banks , study th e topography o f the flood pla in an d ex am ine flow routes . This enabled the • flow routes observed during the 19 7 1 ev ent to b e confirmed and these a re summa r ised b elow fo r the three lengths Tyle M ill to Burghfield , Burgh field to Fobn ey and Fobney to County Lock .

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2.5 .1 T le Mill to Bur hfield •

The flood plain extends for about half a kilom etre on either side o f the river as far as Thea le , but from Theale to Burghfield is concentrated on the left b ank . A ser ies o f g ravel pits hav e been exc avat ed a long this • stretch . A large dra in 5 to 10m wide runs from Ford Bridge at Tyle Mill to join the right bank of the Kenn et just upstream o f T heale . Th is dra in is pa rt ly fed by a small diversion weir off the Kennet a nd Avon canal at Ty le Mill and flows between the Woolwich Green East and W e st La k es . There is • hy draulic connec t ion fro m the dra in in to the East Lake via a 300mm pipe , with a difference in level betw een the dra in and the lak e o f 0 .5m to 1 .0m under norma l conditions

• A flow route observed dur ing 1971 was across th e Woolwich Green Eas t and W est La kes ov er the Thea le Road , a nd into Thea le Lake . Since 197 1, Hosehill Farm has been excavated and culverts now direc t water from Woolwich Green Ea st Lake in to Hoseh il l Fa rm and from ther e in to T hea le Lake . A • culvert under the M4 direc ts wa ter from Thea le Lake in to W ellman 's Fa rm la ke , and a n overflow weir allows water from the latter to spill back into the Kennet via the Clayhill Brook . The levels in the four lakes - Woo lw ich G re e n E a s t , Hosehil l Fa rm , Thea le and W ellm an 's F arm - a re therefo re • con trolled by culverts or weirs .

It was noted that dur ing the flood of the 27th March 1987, for which 3 the peak flow at Theale was 24m /s , the lev el in Thea le Lak e appeared to • ha ve r isen by about 0 .2m . Th is may have been due to the gate on the outlet culvert hav ing been closed , as it was only partia lly op en when ob served o n 29 th Ma rch . The la k e is h eld a rt ifically h ig h by a sma l l co fferdam o f sandbags that has been const ruc ted in front of the intake . •

2 .5.2 Bur hfield to Fobne

• The Kennet div ides into the Holy Brook an d Ken n et jus t up stream o f Bu rg hfield and from this point to Fobney the main flood plain comprises the area between the two rivers , which is about 0 .5 km wide .

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2-9

• •

The ex cavation o f th e Searle 's Farm Lake has sub stant ially altered • the area on the right bank between the Burghfield Road a nd th e Ba singstoke ra ilway line wh ich may w ell ha ve a lso been liab le to flo o ding . It was observed in 1971 that Sea rle 's Farm Lake was flooded and an exam ina tion o f

• flow rou tes in to this la ke ha s therefore been made . Two rou tes have been ident ified , one across the sma ll gra v el p it betw een the Ken net an d Green La ne im m ed iately dow nstream of Bu rghfield Bridg e a nd the other from the Southcote weir poo l . The former was the route ob serv ed and noted in 19 7 1

• w h ilst the la tter ca n be deduced from the recorded flood level in the poo l (40 .62m ) and the level of the right bank at that point (40 .39m ). Th is is a low po int about 20m long in the bank that otherw ise sepa rates Sea rle 's Farm lake from the Kennet at an elevation above 4 1.5m .

• No outlet structures from Searle 's Farm lake have been observed and it is thought that the level of lake is determined by the groundwa ter leve l and the in fluenc e o f the Kennet v ia the very permeable gravels . The water

• level in the lake recorded during the survey was 39.34m as aga inst 39 .55m in the adjacent river .

2 .5 .3 Fob ne to Count Loc k

• Do wnst ream o f th e R ead ing to Basingstoke ra ilway line , the Holy • rook and the Kennet diverge to form a floo d pla in 1 km wide . T he Foudry Brook jo ins th e Kennet w ith a flood pla in up to 1 .5 km w ide on its left • bank , limited by the railway line .

Floo d ing on the Fo udry B ro ok a nd th e K ennet was separated dur ing the 1971 event by a com bination of waste landfill at th e con fluen ce o f the

• riv ers a nd a h igher na tu ra l ground level towards the railway . The ground levels on the Foudry Brook plain are however b elow the flo od wa ter lev els reco rded o n the Kennet , so tha t flooding o n th e Kennet influences the flooding on the Foudry .

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3 • 2.5.4 Flood in at a flow of 28m s

A minor flood occurred on the 5th April 1987 , just prio r to the completion o f the dra ft final report , and was observed along the leng th of 3 • the study area . The flood peak o f 28m /s was atta ined at 9.00 am a t Thea le ga ug in g s ta t ion a nd was pho togra phed ov er the t im e perio d 9 .30 am to 1.00 pm . A set of the pho tographs has been handed to THAMES.

• Flooding between Tyle Mill and Reading wa s observed at the following points :

in the meadows immediately downstream of Ty le Mill

• in t h e f ie l d b e tw ee n t h e c a na l a n d r i v e r u p s t r e a m o f Sulhampstead Lock , due to ov ertopping of the left bank of the canal

• from the 'drain ' tha t flow s betw een Woolw ich G reen West and Woolwich Green East Lakes into Woolwich Green East , the footpath to Bottom La ne b eing impassable . Woolwich Green East Lake wa s

• with in cent imetres of overtopp ing into Hoseh ill Farm Lak e

So uthc ote weir pool , towards Searle 's Farm Lake ; flow into the lake was on the point of occurring

• from the Holy Brook into Southcote meadows ; it was necessary to walk through 150mm depth of flowing water to get from the Ho ly Broo k to Southcote Lock . • from the Holy Brook into th e meadow s imm ediately upstream o f Rose Kiln Lane which were com pletely inundated .

• It can be conc lud ed that limited flooding of the Kennet floo d p la in

upstream of Read ing occurs several times every year .

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CHA PTER 3 •

FLOOD HYDROLOG Y

• 3 .1 INTRODUCT ION

3 .1.1 Ob jectiv e

• Th e ob jectiv e of th is chapter o f the R e po r t is t o a s s e s s t h e

sign ificance o f the K enn et Valley flo od plain in attenuating flood flows through Read ing , and in limiting inundat ion of areas peripheral to the flood

• plain . Flood frequency est ima tion is an integral part of the study .

3 .1.2 Approach

• T h e p r o b l e m c a l l s f i r s t fo r a n a s s e s s m e n t o f t h e p h y s i ca l characteristics and flood po tent ial o f th e K enn et a nd F ou dry ca tc hm en t s .

D e s i g n f l o o d h y d r o g r a p h s fo r a r a n g e o f r etu rn p e rio d s a r e de riv e d

sepa rately for the Kennet at Theale (Section 3 .4 ) and the Foudry Brook at M4

• cu lv ert (Sec t io n 3 .5 ). La te r , in Sec t io n 3 .6 , inflow hydrog raphs to the floo d p la in a r e de r iv e d fo r the K enn et , Fou d ry a nd Loca l a rea s w h ic h ,

co ll ec t iv e ly , a r e c o n s ist ent w ith an ov erall flood design to County Weir ,

R ead in g . Th e in fl ow hy dro g raph s a re sub sequ en t ly c omb in ed a nd rou t e d

• th roug h a simp le representa t ion of the flood pla in storage . In this way an es tima t e o f th e max im um in unda t ion v o lum e f r e q u e n c y is c o n s t r u c t e d .

F ina lly , in Sec t io n 3 .7 , in feren c es a re made from (and about ) th e severe

flood which occurred in June 197 1.

• 3 .2 CATCHMENT CHARACTER IST ICS

3 .2 .1 Catchm ents o f interest

F low s in the Kennet are gaug ed at Thea le , just upstream of the main

Kennet flood pla in area . (See Fig . 3 .1.)

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3- 1

• •

• F low s in t h e F ou dry Brook are unga uged but it is conv en ient to delineate a catchm ent b o u n da ry to the M4 mo to rway c ulv e rt . R em a ining pa r ts o f th e ca tc hm en t to County Weir (Reading ) com prise a m inor tributary dra in ing pa rt of Burg hfield Common , th e K enn et a n d F ou dry floo d p la ins , a n d dev e lo p ed

• area s in Rea ding on the no rth and east fringes of the flood pla in ; these are collect ively treated as th e 'Local ' catchment (ie . local to Rea ding ).

Bas ic in formation ab out the three catchments is given in Table 3 .1 .

• TABLE 3 .1 BAS IC CATCHMENT INFORMAT ION

Catchment NGR Area C halk Non-C ha lk Urba n

• po rtion port ion area 2 2 2 2 km km km km

Kennet at Thea le SU64 9708 1048 828 220 13 .0

• Foudry at 144 SU 70 1694 68 23 45 5 .3 Loca l SU714730 38 14 24 9 .5

Tota l to

• County Weir SU714730 1154 865 289 27 .8

Th e ca tc hm en t a rea s we re c om pu t ed from bo u nda rie s d ig it ize d at

• 1 :50 ,000 sca l e . (N ot e tha t the va lue for th e K en n et at T hea l e d i ffe rs 2 som ewha t from the 1033 km normally quoted for th is gauging sta tion . ) While

the Foudry and Local ca tchm ents are small in com pa rison to the Ken ne t , t he y

a r e se e n to b e rat he r mo re im po rta n t w hen on ly n on -c ha lk po r t ion s a r e • considered . Allowing fo r th e d ispa rat e resp on se o f chalk a n d no n-c ha lk

po rt io n s is fu ndam en ta l to a n a s se ssm e nt o f the floo d po ten t ial o f th e Kennet .

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3- 2

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• 3 .2 .2 Soils

The non-cha lk portions were delineated by reference to the 1:250 ,000 so il map o f South East Eng land and are ind icated in Fig 3.2. The relevant

• soils comprise : tertiary clays in the Wickham 4 and 5 a ssociations; alluvial so ils in the Frome, Fla db ury 3, Thames and Newchurch associations and river terrace soils (affected by g ro un dwater ) in the H uc kleb rook a nd Ha mble 1 a s soc ia t io ns ; a n d E o c en e /J u ra s s ic loa m a n d c la y s in the Burlesdo n

• associat ion . Assignm ent to the Winter Rainfall Acceptanc e Potent ial classes o f FSR m etho do logy is a matter of judgement . By reference to the recently pub lished com pan ion tex t "Soils and their Us e in South East Eng land ", and th e older bu t more deta iled "So ils in Berkshire ", the tertiary clays were

• assigned to WRA P class 4 and the remaining non-chalk so ils to WRAP class 3.

3 .2 .3 Urbanization

• The princ ipa l urban a reas within the catchm en t •to Coun ty W eir ar e shown in F ig 3 .3 . C ompa riso n with Fig 3 .2 ind icates that all these area s are in the non-cha lk portion of the ca tc hm ent a nd th i s is g enerally a lso true o f th e lesser u rban settlements within the catchment. The effects o f

• urba nization on flood runo ff a re princ ipa lly to incr ea se runo ff vo lum es (t h rou g h g r ea t e r im p e rv iousn ess and short e r residenc e times ) and to accelera te the response (through more direct drainage paths ). The lo cat io n of urb an development with in a catchment can sometime s be impo rtant . In the

• case o f the Kennet , som e o f the deve lopment clo s e to R eading may hav e th e e ffect o f acc e le rat ing runo ff from the Local subca tchmen t ahead of th e ma inly rural response seen at Theale . On the other ha n d , spec ia l fea tures of the catchm ent such as the M4 , Greenham Common , Aldermaston and Burgh fie ld • - to which pub lished maps do not a lways do just ic e - are pa rtly s it ed on cha lk area s . Th e net ef fect on floo d ru no ff o f such development may be ra th er mo re sign if ica nt (since the cha ng e fr om na turally perm eab le t o generally imperv ious is more dra st ic ) and act to increase flood flows at

• Reading .

•

3-3

• •

• W ith in th e r em it o f th e stu dy it wa s practica l to take acco unt of urbaniza tion only in broa d term s , using the standard Floo ds Stu d i es R epo rt (F S R ) c a t c h m e nt cha ra c t er is t ic UR BA N . Th is is th e propo r t ion o f th e ca tchmen t shown pink on 1 :50 ,000 ma ps .

• 3 .2 .4 Other cha racteristic s

Other standa rd cha racteristics required in the F SR ra in fa ll- runo f f

• m ethod of flood estima tion are rela tively well defined from maps and require no spec ific comm en t . These a re g iven in Tables 3 .2 and 3 .3, w ith Ta b l e 3 .4

c lar ifying th e nom enclature . No te that - for rea sons th at will becom e clea r

la ter - the cha racteristics for the Theale catchment perta in on ly to the 220 2 • km non-cha lk po rtion .

TABLE 3 .2 FSR CATC HMENT CHARACTER IST IC S

• Catchment AR EA MSL S1085 SOIL1 SO IL3 SO IL4 URBAN 2 km km m/km

•

TA BLE 3 .3 FSR CL IMAT E CHARAC TER IST ICS

Catchm ent SAAR M5 -2day Jenkinson 'r ' mm mm

3-4 •

• TABLE 3 .4 NOMENCLATUR E

Name Un it Mean ing

•

2 AR EA km Ca tchm en t area MSL km Ma instream length

• 51085 m /km Stream slope SO ILi - Fraction o f ca tchmen t in WRAP class i WRAP W inter Rain fall Acceptanc e Potent ial

URBA N - Fraction o f ca tchmen t urbanized

• SAAR mm Average annua l rain fall /45-2day mm 2-day ra infall depth o f 5-year return period r - Ra tio o f M5- lhour/M5-2day rainfa ll CW I mm Ca tchment w etness index

• PR % Percentage runo ff SPR % Sta ndard perc entage runo ff

Tp h Time to peak o f unit hydrograph 0 h Design storm duration

• RLAG h Reservo ir lag time BF I Ba se flow index

ADF cum ecs Average da ily flow AARO mm Av erage annua l runo ff •

•

3-5 0

• 3 .2 .5 Av era e and low flow cha racteristics

The m ixed na ture of the Kennet ca tc hm ent r espo n s e is co n firm ed b y

re fe renc e to av e ra g e a nd low flow cha racteristics for gauging stations in

• the a rea . Figure 3 .4 show s subcatchment boundaries for two sta tio n s o n th e K enn et a nd th ree sta tions on ma jor tributa ries . Flow charac teristics were a lso exa m in ed fo r t h e ne ig hb ou r ing Pang at P a n g b o u r n e a n d L o d d o n a t

Sheepbridge ca tchm ents . • TABLE 3 .5 AVERAGE AND LOW FLOW CHARACTER IST ICS

Ta b l e 3 .5 gives average da ily flow (ADF ) and baseflow index (BF I )

• values ca lculated from 18 to 24 years o f record . For each station , th e A D F

show n is the arithmetic m ean , while the BF I is the m edian of yea rly va lues .

It is seen that th e respo nse cha racterist ics o f the Dun and Upper Kennet a re in d ist in gu isha b le in t e rm s o f BF I; a value o f 0 .955 represents a response • heav ily dom inated by base flow . That of the Lam b ou rn is ev en m o re h ea v ily dep end en t on sp ringflow from the cha lk . In contrast , baseflow represents a much smaller compo nent o f th e Enborne response . Th e interm ed ia te va l u e o f

BF I fo r th e K enn et a t Thea le reflects the mix of cha lk and non-cha lk areas

• and is compa rab le to that o f the Pang .

•

3-6 •

Wh il e the BF I va lues largely confirm the difference inferred from • so ils , it is in teres t ing to no te tha t the ADF 's a re les s c o n s is t e n t . In pa rtic u la r , the equ ivalent average annual runoff (AARO ) of the Enbo rne wo uld appea r to b e unrea sonably low g iven the mo de rate SA A R a n d fa s t

• respon se o f this catchm ent .

3 .2.6 Com rison of Theale and Brim ton flood res onse

• Inspection of limnigraphs and Average Daily Flow (ADF ) hydrog raphs for th e Kenn et a t Th ea le and the Enbo rne a t Brim pton rev ea ls a stro n g correspondence in flood response . Wh ile the bas e flow component of the Kennet is very muc h grea ter (reflec t ing the la rg e ch alk po rt ion of tha t

• catc hm ent ), th e response to heavy rainfall is remarka bly similar in shape . (See Fig. 3 .5 ). This confirms that the clayey Enborne catchment is both the most s ignifica nt K en net tributary (with regard to flo od generation ) and is typ ical of the rema ining non-chalk portions of the Theale catchmen t . It is

• conclu ded tha t the c ha lk po rtions of the Theale ca tchment a re of litt le conseq u ence to norma l floo ding o f the K ennet , othe r tha n in prov idin g c o nc ur rent ba seflow . (It is possible that t he cha lk por tion would b e significant in abnorma l conditions of snowm elt a nd fro zen ground , such a s

• are held to have occurred in the March 1947 flood , see Section 3 .8 ).

At the outset o f the study it was intended to exploit a n exist in g st a n da rd u n it hydrog raph ana ly sis for th e Enborne at Br impton . (See

• Append ix 2 o f IH Report No 94 ). How ever , a recent rat ing revision for th is station supp lied by Thames Water radically reduces gauged flows in exc ess of 12 cum ecs and has a pro found effect on the analys es . Fo r rea so ns g iv en in Appe nd ix C .1 to th is cha pt er , we have placed no reliance on the Brimpton

• flows in this study .

R ea lizat io n that the flood response at Thea le reflects only the 2 220 km non-chalk compo nent of the catchment led to th e dec ision to ca rry • out a special unit hyd rog raph analysis of the Thea le rec ord .

•

3-7 •

3 .3 UN IT HYDROGRA PH ANALYS IS FOR KENNET AT THEALE •

3 .3 .1 Se lection o f da ta

• Ten events were selected from th e 196 1 - 198 6 T hea l e rec o rd : th e s ix hav ing th e h ig he st p ea k flow s and four others tha t offered reasonably

clea r-cut events for ana lysis . (See Table 3 .6 .)

• TABLE 3 .6 EVENTS SELECTED

Ev en t No Da te Peak flow An tecedent CW I Obse rved PR

• flow cum ecs cum ec s mm

Flow data at 3-hourly inte rvals were supplied by Tha m e s W a ter fo r

• a ll bu t Ev ent 1 . (Da ta fo r th is even t were sub sequently reconstructed by Tham es Water but arr ived a fter the study ha d been conc luded .) The flow da ta

we r e tak en la rg e ly a t fa c e v a lue but appeared to be o f good qua lity g iven

the inherent difficulty of gaug ing t h e K enn et a t th is sec t ion . (It is a

• sha l low g raded river with alternative courses that may be releva nt in flood events , v iz , the ad jacen t flood pla in , the parallel Kennet & Avon Canal , and

po s s ib le bypa s s ing t h ro ug h T h ea le town to the north , and through Woo lw ich

Green , Hose Hill and Thea le lakes to the south ).

•

3-8 •

• An econo my made in the a na ly sis wa s to use only daily raingauge data , apportioning the daily total evenly over eight 3- hour periods . Th is is less restrict ive than a t first appea rs . The deriva tion of percentage runo ff figures is largely insensitive to the rain fal l data interva l . It is

• much m o re im portant tha t the ra ingauges be spatially representat ive . Thus 2 rainfall on the 220 km non-cha lk ca tchm ent to Thea le was ta ken a s the av era ge o f gauge s sit ed at Inkpen , W olverton C ommon , West Thatcham and Eng lefield . Nor is the derivation of an average unit hy drogra ph from n in e

• ev ents acu tely s ensitiv e to th e rainfall data interva l . In contrast , the use o f 3-hourly flow data was essential to an adeq uate defin it ion o f b oth percentage runoffs and unit hydrographs .

• 3 .3 .2 Ana lys is

The un it hy dro graph ana ly sis a ssum ed a perc entage sepa ration o f rainfall losses , a 60-hour time base for the 3-hour un it hydr ogra ph , a nd a • stra ig ht lin e sepa ra tion o f base flow . Unit hydrog raphs were der ived for each event using the restricted least-squares met hod (R eed (1976 ), Boorma n an d R e e d (19 8 1 ) ). A s h a p e fa c to r a na ly sis indicated that the u nit hydrograph derived for Event 7 was typical and this was taken as the average • unit hydrog raph for the catchment . (See Fig . 3.6 .)

The perc enta ge runo ff fig ures ra ng ed f ro m 34 .2 t o 68 .4 % b u t averag ed about 40%. These are discussed further in Sec tion 3 .4.2 . • 3.4 FLOOD EST IMATES FOR KENNET AT THEALE

3.4.1 Statistical method •

There a re 22 recorded annua l maxima from Theale dating from 1961 to 1982 . The 197 1 flood (71 cumecs ) is 47% bigger than the next largest flood and 1 .9 tim es th e m ea n a nnua l flood o f 37 .7 c um ec s , co m pu t ed a s t h e • arithm etic mea n of the ser ies . Figure 3.7 shows the annual max ima , plotted

using the Gringorten plotting position formula , together w ith a numb er o f fitted frequency curves .

•

3-9 •

• Th e EV 1 (Gum bel ) and GEV c urv es are f it t e d b y th e m e th o d o f probab ility weighted moments (PWM : see Hosking et al, 198 5 ) to the 22 annual max ima from Theale . It can be seen that neither fit the higher floods very closely - and appear to underestima te the magnitude of rare floo ds , although

• the fits for middle range floods (less than 10 yea r retu rn p erio d ) appea r sat isfactory .

The other th ree cu rv es on Fig . 3 .7 are reg io nal g rowth curv es

• rescaled by the sit e m ea n annua l flood (37 .7 cumecs ). The FSR curve is tha t for Reg ion 6 as pub lished in the Flood Stud ies Repo rt a nd rev is ed in Supplementa ry Repo rt 14 , a nd c learly does not fit the recorded data very well . Tha mes Wate r have d ev eloped a reg iona l curv e for u se on T ham e s

• tr ibutarie s us ing da ta fro m the W ey and Mo le catc hm ents (Thames Water , 1982 ), and this is also shown on Fig . 3.7. It appears to fit the data fo r Thea le sl ightly better tha n the FSR curve . The third curve is the regiona l grow th curv e fo r cha lk ca tc hm ents p re se nted in Su pplementa ry Repo rt 4 .

• Although flood b ehav iour at Theale is strongly influenc ed by the non-chalk area s , data from Thea le were among those used to construct the cha lk curve .

All three regiona l curves, however, overestimate the magnitude of • floods with return periods between 5 and 20 years .

3.4 .2 Ra infall - runoff method

• Application of the FSR rainfall-runoff method - as rev ise d in FSSR 16 - to estimate flood frequency at Theale was necessa rily unorthodox . The 2 proc edu re adopted was to consider rapid response only from the 220 km non- cha lk po rtion o f the catchment but to make a base flow allowance for the • whole catchment . From the unit hydrograph ana ly sis o f Sec t ion 3 .3 it wa s c o nc l u d e d th a t a sta n da rd FSR triangula r un it hy drogra ph fitted w ell prov ided that a time to peak of 24 hours was adop ted . (See Fig . 3.6 )

Follow ing the m ethodology of FSSR 12 (as modified by FSSR 16 ) the 'observed ' percentage runoff values o f Section 3 .3 .2 were sta nda rd ized to take account of the antecedent and storm charact erist ics of the events .

3-10 •

• TABLE 3 .7 DER IVAT ION OF STANDAR D PERCENTAGE RUNOFF (SPR ) VALUES

• Th e o nly s erious anoma ly in Tab le 3 .7 is the rather h igh standa rd

percentage runoff (SPR ) der ived for Ev ent 6 . R e fe renc e to A DF hy d ro g ra p h plots fo r tributa ry stations indica ted that the contribution from the Upper

• Kennet (ga ug ed at Knig hto n ) wa s un usua lly h ig h in th is ev en t . F rom th e sca n t c lim a to logica l in formation to ha nd it would appear tha t precip ita tion

im m ed ia t ely p r io r to the ev en t fe ll a s snow and tha t th e r e s p o n s e t o

ra in fa ll o n 8- 10 F eb rua ry 19 74 was accentuated by some snowm elt , po ss ib ly • from a w ider area than just the non-cha lk port ions . Fu rthe r inv e st ig a t io n

o f loca l wea ther sta tion da ta would be required to substant iate th is .

The med ian va lue o f standa rd perc en tag e runo ff (S PR ) from Ta b le • 3 .6 is 41 .6% . This agrees well with the 43% value estimated from so ils (see

Table 3 .2 ) and th e latt er va lue wa s reta ined .

•

•

3- 11

• • A r em a in i ng d ifficulty conce rn s th e allowanc e for ba se flow . Application of the FSSR 16 procedure yields an a llow ance o f 21 cum ec s fo r 2 the 10 48 km catchm ent to Theale . Use of this valu e gives flood est imates consistently high er than tho se by the statistica l method . R efe rence to a

• flow d uratio n plo t o f the Thea le ADF da ta confirmed that 21 cum ecs is an excessively rar e flow to be considered as an antecedent condition to a flood ev ent . Using instea d a baseflow allowance equa l to the long-term ADF (9 .66 cum ecs ), rea sonab le agreem ent was obta in ed betw een the stat ist ica l a nd

• ra infall-runo ff methods at low return period . (See Fig . 3.8 )

Appendix C .2 summarizes the calcu lat io ns fo r the 50-yea r desig n flood at Theale using the ra infall- runoff method . The flood frequency curv e

• is sh o wn in F ig . 3 .8 a n d sa m p l e d e s ig n hy d ro g ra ph s in F ig . 3 .9 . Inv est iga tion of the sign ificance of the flood plain storage in attenua ting floods between Theale and Reading requires hydrog raphs rather than peak flow e s tima te s alon e . Ha v ing reconc iled the es tima tes from the ra in fa ll

• runo ff m ethod with what the statistical ana lys is reliably demonstra tes a t low return period, this can proceed in Section 3.6 .

3.5 FLOOD EST IMATES FOR FOUDRY AT M4

• The Fo udry Brook is ungaug ed and th ere is no suitable ana logue catchment other than the Enb orne at Brim pton , a na ly sis of wh ich has b een disc ount ed in th is stu dy . (See Appendix C .1 ). Th e flood est imates a re • therefore ba sed on catchment and climate characteristics alone .

F loo d e s t ima tes fo r th e Foudry at 1(4 wer e ava ilable from a n earlier stu dy (Reed , 1984 ). These were modified slightly to tak e ac count of • th e mo re deta iled in terp reta tion o f so ils now poss ible through 1:250 ,000 soil maps . Th e resultant flood frequency cu rve is shown in F ig . 3 .10 and ty pica l de sign hy d ro graphs in Fig . 3 .11 . Ap pend ix C .3 summa rizes th e calculations for the 50-year event . •

3- 12

• •

• 3.6 FLOOD EST IMATES TO COUNTY WEIR , READING

3 .6.1 Method

• W h en ma k in g f lo o d e st imates for sit es just dow n strea m o f a con fluence it is tempt ing simply to add together flood estimates for the two tributa ries . Such a method is in gene ra l erroneous . In the case of two such differing tributa ries as the Foudry and the K ennet it would probably

• lead to serious overestimation of design floods at Reading . Th is is because the Foudry is sensitiv e to appreciab ly sho rt er durat ion sto rm s tha n th e K ennet . From Append ic es C .2 and C .3 it is seen th a t the design st orm durations are 19 and 45 hours respectively . To obtain consistent est imates

• it is necessa ry to impose a single design storm on the entire catchment to Reading (neglecting , how ever , the chalk portion o f th e Thea le ca tc hm en t ). Additionally , it is appropriate to extend the design storm duration for the 'reservoir lag ' effect that the flood plain storag e imposes on flo od runo ff

• fro m t h e K e n n e t a n d F o u d ry . Fina lly , it is rea sonable to ma ke som e allowance for the rela tiv e t im ing o f runo ff from the K enn et , Foudry an d Loca l subcatchments.

• 3 .6.2 Flood eak trav el times : stora e la s

The availa b ility o f wa ter level ob serva t ions at Blake 's Lock - some 1.5 km downst ream of County Weir - is of some a ssistance in judg ing

• c ha ra cterist ic response times o f the K ennet to R ea ding . A lthough th e relationship between level and flow at Blake 's Lock is a highly c omplex on e (depending a s it does on operational settings of paddles and gates ), it is rea sonab le to assume tha t the time o f maximu m hea dwa ter lev el g iv es a • genera l in dication of the time o f peak flow . Blake 's Lock water level data (in the form of 'tackle sheet s ') were inspected for those ev ents selec ted for un it hydrograph ana lysis.

• Head and tail water levels are rea d a t 3-h our intervals betw een 09 .00 and 21.00 but not otherwise . From the events examined it was possible to est imate Blake 's Lock peak times reliably for o n ly three ev ents : June 1971 (event 3 ), February 1974 (event 6 ) and a lesser event in December 1985 . •

3-13

• •

• These y ielded peak-to-peak travel times (from Theale to Blake 's Lock ) of 15, 15 a n d 9 h o u rs r e sp ec t iv ely . On the basis o f these est imate s it is suggested that a typ ica l flood p ea k trav el tim e fo r in-bank ev ents i s b etween 3 and 6 hours from Theale to County Weir, with flood plain storage

• delay ing ma jor flood peaks by up to 12 hours more . Wa ter lev el data a t finer time resolution would be needed to con firm this .

3 .6 .3 Consistent inflow h dro ra hs

• T h e ma in p r e r eq u is it e to c o n st ru c t in g c o ns is t en t in f low hydrographs to the Kennet/Foudry flood plain is selection of an appropriate desig n storm dura tion , D . B ecau se not all o f th e design flood will be

• sub ject to flood plain sto ra ge act ion , it is not imm ed iately c lea r wha t 'reservoir lag ' (RLAG ) value shou ld be used in the equation :

D = ( 1 + sAAR/l000 ) ( Tp + RLAG )

no r wha t unit hy drog ra ph t im e to pea k (Tp ) va lue . This difficulty was resolv ed by considering a range of des ign sto rm dura t ions , nam ely 45 , 69 , a n d 9 3 ho u r s . It w a s fo und that the resulta nt flo od est ima tes w ere

• relatively insensitive to this cho ic e and tha t th e resu lts fo r a 93-hou r s t o rm sh o u ld be a do pt ed . (The lo nger sto rm durat io n y ie lds a desig n cr iterion that is slightly more relevant to th e a ssessm ent o f floo d p la in in un datio n volum es , wherea s a sho rter dura tion wou ld b e slig htly m ore

• testin g in term s o f design flows a t County W eir . The p r e fe r e n c e fo r adopting the longer dura t io n to ok into account the long-term history of Kennet flooding - see Section 3.8 ).

• The design sto rm depth was idjusted by an a real reduct ion factor 2 for a 500 km a rea , reflect ing the phys ica l a rea sp anned by the Loca l , Foudry and Theale (non-chalk ) subcatchments. The depth was distr ibuted into a hyetograph using the customa ry bell-shaped "75% winter profile ". (This is • an uncom fo rtab le assum pt io n fo r such a long duration event . How ever ,the alternative of distributing the depth according to so me locally observ ed storm pro file could be unduly sub jective .)

•

3-14

• •

• Finally , the des ign hyetograph wa s factored by a subcatchm ent percentage ru no ff a nd convolved with the subcatchment unit hyd rog raph to construct the inflow hydrog raph to the flood pla in from each o f th e Thea le , Foudry and Loca l subcatchments in turn . In accumulat ing the comb ined inflow hydrog raph • to the floo d pla in , a ccount was taken o f ty p ica l subcatc hment response tim ings by defe rring the Theale hydrograph by 9 hours . Compar ison of the Theale and Brimpto n limnigraphs with rainfall data sug g ests tha t perhap s 6 hours of t he 9-hour Th ea le 'pu re t ime delay ' occ urs upstream of Thea le . • (See also Sec tion 3.6 .2 ). The synthesis o f the composite inflow hy dro gra ph for the 50-y ear event is given in Appendix C .4 and the flood frequency curve for the comb ined inflow is included in Fig . 3.12 .

• 3.6.4 Flood lain routin

The study brie f specified that a simpl ified flood ro uting model would be calibrated for the Theale/Reading reach by comb ination of: • (1 ) gauged inflows at Theale , estimated contribut ions of the Fou dry and other ungauged inflow s (ii ) estimated outflows at County Weir • and (iii ) flood plain sto rage information .

Adoption o f a relative ly simple approach wa s consistent with the requirement to route hypothetica l design floods (as opposed to intensively gauged rea l • ones ). As the study proc eeded , it becam e clea r tha t outflo ws a t Read ing

(item ii ) could not b e est imated reliably from the information available . It was therefore nec essa ry to assume a somewhat simpler flood rou t ing mode l than initial ly intended .

The model structure is illustrated below :

THROTTLE Oo go

• LINEAR RESERVOIR

3-15

• •

Here Qi represents the composite inflow to the Kennet/Foudry flood • pla in a rea , and Qo is the out flow a t County W eir , Rea ding . In flows less tha n a threshold (or "thro ttle " ) discha rg e , QT , pa ss th ro ugh the model una ttenua ted . Flows in excess of QT are div erted into a linea r reservo ir

• which represents the storage action of the flood plain .

Equations defin ing the inflow , qi, and out flow , go, of the lin ea r res ervo ir are :

• q i = max (0 , Qi - qT )

and K dcio/dt + qo = q i

• where K is the mean residence time , and t is time , both in hou rs . Prior to rout ing a flood event , qi and qo are init ialized at zero .

The model has two parameters : the mean residence time (K ) and th e

• th r es h o ld d isc ha rg e (Q T ). These w ere ca lib ra ted by tr ia l-and- er ro r simula tion of the Jun e 1971 event . (See also Sec tion 3 .7 .) Th is was made possible by two pieces of informat ion abo ut the June 197 1 flood : an estimate of the max imum inundation sto rage (obtained by survey - se e Sectio n 4 .2 ).

• The calibration yielded param eter values of K = 32 hours and Q = 14 cum ecs .

It should be appreciated that the rout ing mo del is a n em pirica l d ev ic e a n d t ha t c o n c ep t u a l in t e rp r e tatio n o f it s pa ra m eters may b e

• mislead ing . In rea lity , pa rt o f th e flood pla in w ill a ct a s "flow ing " sto rage and part will prov ide "dead" sto rage . The model provides storage of the former type only . Emp irical ca lib ra t ion o f the m odel pa ram eters (a s opposed to sub ject ive assignments ) will partly compensate for this and oth er

• simplifica tions .

The routing o f the com posit e in flow hy drograph for the 50-year event is detailed in Appendix C .4 . The peak com posite inflow of 77 .4 cum ecs

• is attenua ted to a peak County We ir outflow (21 hours later ) of 59 .1 cumecs . The simulated peak flood plain inundat ion storage is 5 .2 x 106 m3.

•

3- 16

• •

3 .6 .5 Flood fre uenc est imates for Count Weir •

From ana ly ses fo r further retu rn periods (see A ppendix C .6 ) it was po ss ib le t o arrive at frequency curv e est imates for peak outflow s at Coun ty Weir (see Fig . 3 .12 ) •

Com pa rison o f th e "comb ined inflow " a nd "County Weir " flood frequency curv es co nfirms that th e flood pla in storage ha s a significant attenuat ing e ffec t

• on flo o d f r equency a t Reading . Fo r even ts o f return period b etween 25 and 100 yea rs , th e attenua tion is ab out 23 .5% .

3 .6 .6 Maximum flood la in inundat ion vo lume

• The above floo d p la in mo d e ll ing a lso y ield e d est ima t es o f th e

ma x im um flood pla in inundation volum e (or "storage " ) fo r each design event . The relevant frequ ency curve is given as Fig . 3 .13 .

• 3 .7 ANA LYS IS OF JUNE 1971 EVENT

3 .7 .1 Ra infa ll

• T h e J u n e 197 1 ev en t fo llow ed a d ry p er io d , w it h so il mo istu re

defic its o f about 50mm preva iling until th e 8th . Modera te dep th s o f ra in on th e 8th w e re followed by show ers on the 9th . The main storm began at 09 .00

• o n the 10th an d la st e d for 24 ho u rs , n ea t ly co in c i ding w i t h r a i n fa l l m ea sur em en t "d a y s " . Thus da ily tota ls for th e 10th (see Fig . 3 .15 ) giv e a

good impression o f the spatial cha racter istics o f the storm .

•

•

•

3- 17

• •

3 .7.2 Flows at Thea le •

Fo llow ing the approach of FSSR 12 , the unit hy d rogra ph mo d e l wa s

u s ed to s im ulate th e Thea le hy drog raph for the June 1971 event . Com pa rison

w ith th e "observed " flow s (see Fig . 3 .14 ) is more re-assuring tha n a t first • appea rs . The ea rly pa rt o f th e hydrograph is modelled w ell . The "shoulder "

a t 35 cum ec s o n th e r i sing l im b is not s im ula t ed . T h is a p pea r s to b e c on s istent w ith sub s ta n t ia l quan t ities o verflow ing in to gravel pits , the hy drograph at Theale ris ing aga in only when the av aila b l e sto ra g e ha s b ee n • ut il iz ed . No t e tha t th e o b serv ed flow s plo t te d in F ig . 3 .14 are the 3- hourly data supp lied by Tham es Water . The peak estima te of 71 cum ec s (2 50 0

cusecs ) quoted for th is ev en t is a little higher .

• 3 .7.3 Flows at Count We ir

In a s im ila r fa sh ion th e respo nses from t h e Fou dry a n d Lo ca l

• subca tchmen ts were also simulated fo r th e June 19 7 1 e v ent . T he c omb in e d in flow hy d ro g ra p h w a s rou t ed through the floo d p la in model and flow s a t County We ir thereby s imulated (see tabulations in Append ix C .5 .).

• 3 .7 .4 Max imum vo lum e o f inunda tion

Th e ma x im um v o lu m e o f in un da t ion fo r th e J une 19 7 1 ev ent wa s 3 simula ted to be 4 .9Mm .

• 3 .8 ANALYSIS OF MARCH 194 7 EVENT

R e fe re nc e is made in Section 3 .9 to the prob lema tic nature o f the

• Ma rc h 19 47 ev ent . T h e v a r io us fa c to rs g iv in g r is e to t h a t f l o o d a r e difficult to quantify in terms o f the probability of their joint recurrence .

•

•

3- 18

• •

Howev er , it is understo o d (T h a m es W a t e r P la nn in g D iv is io n • re ferenc e floo d ca rd ) tha t the peak flow through Reading on 14 Ma rch 1947 was est imated (presumably by current meter gauging ) to be 2085 cesec s or 59 cumecs . Neglect ing any cha nges that may have occurred in the behav iour o f

• the flood pla in (note , for example , Sect ion 4 .4 ), the return period o f th e Ma rc h 1947 p ea k out flow is a ssessed from Fig . 3.12 to be about 50 years , i.e . more severe at Read ing than the June 1971 event .

3 .9 DISCUSSION •

It is reassuring to note that the various estimates of the ra r ity o f the June 197 1 flood concur reasonably well . It is assessed to hav e been

• a 50-y ear event at Theale but more like a 35-year event in term s o f flow a t County W eir and in terms of flood plain inundat ion . While the magnitude of the des ign flo ws a nd inundat ion v o l um e s a r e sub je c t to u nc e rta in ty (p a r t ic u la r l y th e la t t e r ), it ca n b e asser ted w ith slightly grea ter

• confidence tha t the volume of inundat ion in the June 197 1 event represented about a 35-year event .

These qua lifications arise because of the concern that the K enne t

• ca tc hm en t may be p rone to floo ding by snowme lt and rain (as occurred in March 1947 ) as opposed to rainfall alone - as has been assumed above . There ca n be little do ub t that snowm el t wa s a ma jor contributing factor in the March 1947 flood . There is the suspic io n tha t th e cha lk po rt ion s o f th e

• ca tc hm ent g enera ted som e ra p id respo nse runo ff , pe rhaps a s a result of frozen ground . In the ab sence o f a gaug ed flow record of sufficient len gth and qua lity on which to base flood est imates on the Kennet, it is advisable to turn to historical water level and ra in fall reco rd s to c heck tha t th e

• flood potential of this river has not been underestimated .

Historica l water level records hav e b een referred to in section 2 .4 o f the repo rt .

•

•

3-19

• •

• Figure 3 .16 shows extreme 1, 2, 4 and 8-day rainfalls recorded in th e Kennet catchm ent . (Generally these are catchment average rainfa lls but not e that for early event s - exc ept ing No vember 1894 - thes e are R ea d ing ra in fa lls ). Tha t th e Novem b er 1894 had exceptional 4 and 8-day rainfalls

• but le ss noteworth y 1 and 2- day ra in fa lls sugg ests tha t th e Kennet to R ead ing is indeed sens itiv e to rela tively lo ng du ra t ion ev en ts . (See Sec tion 3.6 .3 ). It is confirmed from Fig . 3 .16 that th e March 194 7 flood canno t be accounted for by rainfall alone .

• Symons and Chatterton (1895 ) refer to K enn et floods prio r to tha t o f Nov em ber 1894 , includ in g tha t o f October 18 9 1 (s ee Fig . 3 .16 ). O f interest is the referenc e to the 1809 ev ent , "wh ich wa s produc ed by the

• sudden melting of deep snow".

Perhaps th e in fe renc e to b e drawn is tha t , wh il e th e f lo o d est ima tes ma de in th is rep ort a re reasonable and appropriate to meet the

• study ob jective , occasiona l very severe events ar ising pa rtly from snowm elt c a n oc cu r . Thus it is sugg ested tha t the floo d a n d inu n da tion volum e frequency curves derived herein should not be applied beyond a retu rn period of 100 years .

• T h a t t h e c o n d it io n s g iv in g r ise t o f r o z e n g ro u n d , s n o w accumulat ion and subsequent rapid melt are likely to be spatially w idespread is o f some comfort - other catchments will be in trouble !

• 3 .10 CONCLUSION

Figure 3 .13 is the most important product o f the hydrolo g ica l a nd

• flo o d ro u t ing a na ly sis . From the figure it is po ssible to judg e the sig n ificance o f lo ng-term lo ss (or ga in ) in the ava ila ble floo d pla in storag e upstream of Reading .

•

•

3-20 •

6 3 It is est imated elsewhere (in Section 4 .4 ) that some 0 .5 x 10 m • of sto ra ge hav e been lost in the period 1971 -1986 through infilling . From Fig . 3 .13 it can be seen that such a loss would in itsel f have a pro found e ffect on the frequency with which a given inundation level is experienced .

• For exam ple , the June 19 7 1 inunda tio n lev el would now accommoda te only 6 3 4 .4 x 10 m , corresponding to about a 20-year event rather than the 35-y ear event inferred in Section 3.8 . The effect on outflows at County W eir would b e the sam e , the inc idence o f a given flood flow becom ing almost twice as frequent (i.e . once in 20 years rather than once in 35 y ears ). •

W herea s F ig s . 3 .12 an d 13 can b e used to judge th e st ra tegic

importance of mainta ining adequate flood pla in storage , it would be unw ise

• to app ly these results to m ino r short-term developments (where only sma ll amounts of sto rage a re concern ed ). Specific developments may hav e spec ific conseq uenc es fo r flo od r isk - bo rn from th e na tu re a nd locat ion of the infilling and the compen satory works proposed . However , Fig . 3.13 p rov ides

• a b roa d assessm ent o f the s ign ificance o f flood pla in st orag e which is ent irely appropriate to appraisal o f the long-term mana gem ent policy o f th e Kennet/Fou dry flood pla in .

•

•

•

•

3-21

• • I •

3970 1 39 714 390 16

3 9 0 16 Kenne t a t Theale

3 9 7 0 1 Foudr y a t M 4

5 5 bra 3 9 7 14 Kenne t a t Count y We ir. ReadIng

F i g . 3 . 1 Ca t c hme n t P l a n . 3970 / 39 714

390 16

MIAlluvial and river terrace soils n Tertiary ° Jays Bursiedon 5 km [ I] Chalky soils

F i g . 3 . 2 So i l s •

39701 39714 Reading 390 16

I kJ

Thatcham Marlboro ugh Hungerl ord

Heath End Tad ley

Urban 0 S km • •

Figure 3 .3 Princ ipal U rban AreaS • • • •

39 70 1 3 9714

39016

3904 3 39019 39029

39025 ta

3 9 0 43 K ennet at K night on 1.0 3 9 0 2 8 Dun at Hunger t or d 3 90 19 Lamb our n at ShaBr w imp t on at 5 km 3 9 0 2 5 En b orne Thee le 3 90 16 K ennet at

F lg . 3 .4 Gauge Sub ca tchm en ts of the Kenne t . •

•

•

•

•

•

•

•

•

•

•

• A R CHI VE :

PLEA SE DO NOT DES TR OY • • • •

31 paddles drawn

16 paddles 28 drawn padd les 1 drawn 10 i 15 paddles shut 1 i 12 Paddles shut

• • ••• • • •

B r 011,310 n Theale Blakes Loc k

16 17 18 19 20 21 22

DAY OF MONT H F ig . 3 .5 Compa rison o f B rim p to n , T hea le and B la kes Lo ck L im n ig raph s - June , 19 7 1 . •

•

•

•

10

• t \ / \ f rom eve nt 7 by 8 / \ r est ric t ed least -sq uares / \ Ct st andard FS R t riangle, \ w it h Tp(3) : 24 hours w w \ • cf.)c \ z 6 0(95 E \ g. is, / cce.> / E I- s / E ° 4 • = 2 / .-,.to / / Ej / 0 2 /

0 0 12 24 36 48 60 T IME h

F igu re 3 .6 Un it Hyd rograph Kennet at Thea le

3-27 •

•

•

•

REDUCED VARIATE Y

F ig . 3 .7 Flood Frequency Curves for Kenne t at Theale - Statistica l 3-28 •

• 100 —

80 -- J u n e 19 7 1 si m u l a t e d p e a k

• 60

40 •

20

• Figure 3 .8 F lo od freq uency cu rv es fo r Kenne t a t Th ea le - 0 clOop ted . 2 33 5 10 25 50 100 RE TURN PERIOD y e a rs

1 0 0 - y e a r 80 -r

2 5 - y e a r

60 —

•

1 0 - y e a r

• 0

20

0 12 24 3 6 4.8 60 712 134 F ig ure 3 .9 Desig n H yd rog raphs TIME ho urs Kenne t a t T hea le .

3- 29 •

30

20 0 J u n e 1 9 7 1 s i m ul a t e d p e a k 3

0

10

1 , O r 2 33 5 10 2 5 50 100 RETURN PERIOD y e a r S

F ig u re 3 .10 F lood freq u e n cy e s t im a te s - Fobney at M4.

3 0

10 0 - y e a r

2 5 - yeaf 20

ce

10 - y e a r

0

10

0 0 6 12 18 24 30 T I ME

Figu re 3 .11 - D e sign Myd rog raphs Fcbncy at m4 .

3-30 • 0 • 0 • •

3.13 In u n d a t io n v o lu m e frequency . F ig u re 3 .12 C omb ine d ca tchmen t to flood p la in F ig u re Inflow a nd o u t f lo w f lo od fr ue n cy Ke n n e t f lo o d p la in .

Jun e 1 9 7 1 s im u la t e d p e a k BO CO MBINED INFLOW

7 - -

60 6 J u n e 1 9 7 1 s im u la t e d p e e k O UTF LOW filT CO UNTY wEIR 5 - - mo oc hed 12 June 1971

4 -

3

20 2 -

1

0 0 2 .33 5 10 2 5 5 0 10 0 2.33 5 10 2 5 5 0 100 RETURN PER IOD years T years F ig u re 3 .14 Ob se rv ed and s im u lated T hea le flow s - J une , 19 7 1 .

80

/ m odehed

`observe? (3 hourly flows)

60 estimated peak of 2500 cusecs

0

40

0

20

0 8 9 10 11 12 13 DAV O F M O NTH • • • • • • • • • • •

62 0 .1

46 55 55

56 397 14 55 56 47 4 4 3970 1 55 4 7 SO 390 16 13 416 55

57 51

52 4-` 69 67 64 75 , 0,601/4 69 15/ 73 c p b e r n e 56 se 62

79 3 0 0 16 Kennet at 'N eale 52 66 55 69 3 9 70 1 Foudr y at M 4

3 9 7 14 Ke nne l a t Count y W eir, Re ading

Fig . 3 .15 Da ily Ra in falls (mm ) 10 th June , 1971 •

•

•

140

• 120 a J ul 1890 b Oc t 1891 c Nov 1894 b. d Jun 1910 • 100 e. F e Jul/ Aug 1917 F Feb 1933 G Mar 1947 H Oct 1949 Sep 1968 J J un 1971

Not e

Ra infalls fo r eve nt s tt. b, d a nd e

are for Reading (and ma y no t be

t yp ic al of c atchment )

•G

1 2 4 8 DURAT ION days

Figure 3.16 Notable 1 , 2 , 4 , and 8 day rain fa lls in the Kenne t Catdhment .

3-34 •

CHAPTER 4 •

FLOOD ENVELOPES

4 .1 APPROACH •

4 .1.1 Ob iective

The objective o f the work described in this cha pter wa s to produc e • floo d env elopes of d ifferent return periods based upon the sto rage frequency curve derived in Chapter 3. The storage frequency cu rv e wa s pro duced by hy drolog ica l a na ly sis bu t wa s ca lib rated by com pa r ison w ith the volume

• calculated to hav e gone into sto rage during the 197 1 event . The calcu lation o f t h i s sto ra ge v olum e is set out in th is c ha pter , thu s p rov iding a n est imate of the spatial distr ibut ion of the storage an d a llow ing the flood envelopes for different sto rage volumes to be det ermined .

• 4 .1.2 Ca lculation of sto ra e volumes

Fo r a pa rticula r flood event the max im um v o lume stored is that

• volume between the pea k wa ter su rfa ce profile and th e flo o d p la in . Th e ca lc ula t io n o f th is vo lume is complicated by the fact that the flood water surface profile on the Kenn et has a steep grad ie nt av e raging about lm per kilometre .

• A survey o f the flo od pla in w it h cross sec t ions every 250m wa s ca rried out to provide the n ec essa ry data on the topography of the flood pla in, as described in Section 2.2 .1. Flood levels at various po in ts a lo ng

• t h e s t u dy a r ea were recorded du r ing the 19 7 1 ev en t fro m w hich it wa s possib le to est imate the peak water surface profile. Follow ing consultation w ith T ha mes it was a greed that a water surface der ived by joining recorded lev els on th e downst ream s id e o f weirs would p ro b a b ly g iv e th e b e s t

• repres en ta tio n o f cond it io ns in the flood pla in , a lthough in the river cha nnel itself the pro file shown on Figure 2.1 is more realistic .

•

4-1

• The approa ch a do pted to est ima te the volume between the wa ter and • la nd sur faces wa s to ca lcu la te a n a rea -eleva tion curv e fo r each c ro s s sect ion and then a ssume that this cross section applied for 250m along the valley in order to produce a volume-eleva tion curv e fo r tha t rea ch . Th e v o lum e s to r e d u n d e r a pa r t ic u la r w a ter su r face pro file can then b e • ca lculated by summ ing the volumes flooded for each reach . The adv anta ge o f th is a pproach is tha t it lends itself to computerisation allow ing storage vo lumes for a range of water sur face pro files to be determ ined rap idly .

• 4 .1.3 Comput er programs

Two programs were developed to ca rry out th e abov e ca lc ulat io ns . The first ca lcu lated the a rea elevation curve for each cross section from • the survey da ta w hic h was ava ilab le in the fo rm o f list ings g iv ing the c ha inage a nd eleva tio ns fo r each po in t alo ng the sec tion . The sec on d program calcu lated the total volume stored for a given water surfac e pro file

• by summ ing the indiv idua l vo lumes flooded for each reach . It was a ssum ed that the water sur face would be horizontal perpendicular to the river .

4 .2 STORAGE DUR ING 1971 FLOOD

• An examination of the area flooded in 1971 , a s shown on F ig . 2 .3 , show s that the ca lcula tio n o f flood storage on the Kennet and the Foudry Brook can be treated separately for this event due to the h ig h grou nd that

• sepa ra tes the two rivers at the ir confluence . As m easu rement s o f peak flow on the Kennet were made at the Theale gauging statio n , the vo lum es sto red a long th e K ennet upstream an d do wn st ream o f th is po int have a lso b een calculated separately .

• 4 .2.1 Kenn et down stream of Theale au e

The survey cross sections downstream o f th e ga uge a re No s 1.00 1 -

• 1.026 . Som e of these sections were adjusted so as to reproduce as nearly as possible the topography that existed in 197 1. Thus the rec ently ex cava ted C o ttag e Lane p it , the Sm a llm ea d p it and pa rt ially filled t ip w ere a ll 'refilled ' to avo id over-est imating the volume going into store .

•

4-2

• •

• T h e w a t e r su rfac e profile was dete rm ined from the in forma t ion ava ilab le on Fig. 2.3 . As discussed in 4 .1.2 it was assumed that the wa ter lev e ls recorded downstream of the weirs would provide the best estimate of water levels on the flood plain . The results of the ca lculat ions a re g iv en 3 • in Tab le 4 .1 b elow ; 2 .1Mm went into sto rage in the flood plain down stream of Theale gauging station .

T h is fig u re ex cludes any flood ing o f the S ea rle 's fa rm gra v el

• pit.The pit is recorded to hav e been flooded and a com parison of survey data and flood levels con firms tha t there would hav e been two flood routes into the pit. The initial and final water level can only be estimated but it is 3 co nsid ered tha t as much as 0 .4Mm could have entered the pit . This raises

• the es timate of to ta l stora ge downstream o f Thea le gauging sta tion t o 3 2.511m .

4 .2.2 le Mill to Theale au in station

• The estima te of storage over this section of the Kenn et is given in Tab le 4 .1, but once aga in th is does not include any a llowance for storage in g ravel pits . One o f the flow ro utes observed in 1971 wa s th ro ug h th e

• Woo lwich Green East Lake and across the road into Thea le Lake .

An exa m ina t ion o f the Berksh ire County Counc il aeria l photos flown in 197 1 shows that the po rtion of the Theale Lake next to the road was being • excavated at that time and was protected by a bund from the rema inder of the p it wh ich wa s f illed w ith ground water . Photo No 25 tak en dur ing th e helicopt er recon na issa nce o f th e flood ten hours a fter the peak suggest s that the lake was still well below road lev el by perh aps se vera l metres , • while photo No 21 confirms tha t the bund protecting the excavations from the rest of the lake was not overtopped .

•

4- 3

• •

TABLE 4 .1

STO RAG E ALONG T HE KENN ET FO R THE 197 1 EVENT .

4-4 Nev erth eless the excavat io ns were probably 4 to 5 metres deep at the time • and it is est ima ted that the area of 20 ha under excavation at th e t im e may 3 hav e absorbed as muc h as 0.4Mm .

• Allow ing also for the flo od ing ov er Woo lw ich G reen Ea st a nd West 3 La kes , a n allowanc e of 0 .5Mm ha s been made for flooding o f grav el pits , b ring ing the total volume estimated to hav e been sto red betw een Ty le M il l 3 and Thea le gaug ing station to 1.5Mm .

• 4 .2.3 Fou dry Brook

Although the flo od ing on the Kennet would hav e interacted w ith the

• floo ding on the Foudry Brook , the ca lculation of storage volumes on the two rive rs can b e trea ted sepa rately . The survey cross sections were roughly perpend icular to the Foudry Brook and the same approach as described for the K ennet has b een used . There a re no recorded flood water lev els ava ilab le

• for the Fo u dry an d the wa ter surfac e pro file ha s b een ded uced from the extent of the flooding .

The volume est imated to have been stored above 198 6 to po gra phic 3 • cond it ions is 0 .6M m , th e deta il ed results b eing g iven in Tab le 4 .2. A sub stantial amount of storage has been lost since 1971 due to infil l ing a nd 3 th is is es tima ted in sect ion 4 .3 to be about 0 .5Hm , with the result that the tota l volume stor ed in the Fou dry Brook fl oo d pla in du ring the 19 7 1 3 • event is est imated to have b een 1.1Mm .

•

•

•

4-5

• •

TABLE 4 .2 STORAGE IN FOUDRY BROOK FLOOD PLA IN IN 1971 • (to pography a s surveyed in February 1986 )

Sect ion C ha inage Water X - Sect Section Volume • Number on R iver Lev el Area W idth of Wa ter 2 3 (km ) m (m ) (m ) m

• 2 .00 1 2 .230 38 .47 318 .5 25 0 .0 79600

2 .002 1.960 38 .49 198 .5 125 .0 12800

2 .00 3 1 .660 38 .52 645 .1 25 0 .0 161300

• 2 .004 1 .240 38 .55 284 .8 250 .0 71220 2 .005 0 .890 38 .58 377 .3 250 .0 94300

2 .006 0 .570 38 .60 512 .6 25 0 .0 128200 2 .007 0 .250 38 .63 121.2 250 .0 30300

• 2 .008 0 .010 38 .65 9 .7 250 .0 2400

3 Total Volum e = 592100 m

• 4 .2 .4 Summar for the 1971 event

Sto ra g e in th e flo o d plain during the June 19 71 flood is estimated 3 • to have been 5 .1Mm distributed as follows :

Kennet d/s o f Thea le

- floo d plain 2 .1

• - gravel pits 0 .4 Kennet u/s of Thea le

- floo d plain 1.0

- gravel pits 0 .5

• Fo udry Brook 1.1 3 5 .1 Min

•

4-6 •

• 4 .3 STORAGE FOR OT HER EVENTS

4 .3 .1 194 7 flood

• The 1947 event was a more serious event on th e Thames alt hou gh 19 7 1 a p pea r s to hav e b e en t he m o re sev ere o f th e two fo r th e K enn et . O n ly lim ited in formation on levels is av a ilab l e com p r ising reco rds a t B la k e 's Lo c k , Co un ty Lock and Burgh field Bridge . The level at County Lock was 0 .5m

• higher tha n for 1971 and that at Burghfield Bridge 0 .1m lower . Flooding has b ee n l in ea rly in te rpo lated b etween these two po ints and lev els upstream o f

Bu rgh field Bridge assum ed to be 0 .1m lower than for the 1971 event .

• T h e res u lts fo r th e Kennet are g iven in Table 4 .3. No gravel pits ha d been excavated by 1947 a nd almost no infill ing o f the Foudry flood pla in

ha d ta ken p lac e . A llow ing for th ese factors , and a ssum ing flo oding on the Foudry to have been about 0 .2m higher than in 1971, it is estima ted that the 3 • total sto rage in the 1947 ev ent wa s 5 .1Km distributed as follow s :

K ennet d/s o f Theale 2 .5

Kennet u/s of Theale 0 .8

• Foudry 1.8 3 T o t a l 5 . 1 m

•

4-7

• •

TABL E 4 .3 • STORAGE ALONG THE KE NN ET FO R THE 1947 EVE NT .

Kenne t Va lley Stu dy - Kenn et River - 1947 Leve ls 0

Se ction Ch a inage W ater X-Sect Section Vo lum e Numbe r on Rave r Leve l A rea W idth of Wa te r (cm) (m) (m2) (m) (r13) 0 1.00 1 10 .800 38 .80 767 .8 130 .0 99800 1.002 10 .330 38 .98 1320 .9 250 .0 330200 1.003 10.060 39 .08 1206 .4 250 .0 30 1600 1.004 9 .750 39 .18 960 .9 250.0 240200 1.005 9 .500 39 .23 909 .8 250 .0 227500 1.006 9 .250 39 .28 695 .0 250 .0 173800 • 1.007 8 .900 39 .42 794 .9 250 .0 198700 1.008 8 .620 39 .67 252 .6 250.0 63200 1.009 8 .330 39 .89 247 .7 250.0 6 19 00 1.010 8 .050 40 .09 160 .8 250 .0 40200 1.0 11 7 .75 0 40 .30 129 .1 250 .0 32300 1.012 7 .350 40 .74 192 .9 250 .0 48 200 0 1.0 13 7 .07 0 4 1.29 308 .7 250 .0 77200 1.014 6 .820 4 1.50 374 .5 250 .0 936 00 1.015 6 .57 0 4 1.64 28 0 .6 250.0 70200 1.016 6 .300 4 1.79 2 15 .7 250.0 53900 1.017 6 .040 4 1.93 90 .5 250 .0 22600 1.018 5.630 42 .15 146 .1 250 .0 36500 • 1.019 5.39 0 42 .50 44 .3 250.0 11 100 1.020 5 .110 42 .9 1 15.6 250.0 39 00 1.02 1 4 .860 43 .28 105 .0 250 .0 26300 1.022 4 .590 43 .67 73 .9 250.0 18500 1.023 4 .22 0 44 .14 28 0 .9 250.0 70 200 1.024 3 .9 10 44 .47 268 .3 250.0 67 100 0 1.025 3 .650 44 .74 242 .6 250.0 606 00 1.026 3.380 4 5 .03 286 .5 250.0 7 1600 Su b-tota l 25009 00

1.027 3 .000 45 .4 3 204 .5 250.0 5 1100 1.028 2 .49 0 4 5 .96 236 .2 250.0 59000 1.029 2 .190 46 .38 644 .9 250.0 16 1200 1.030 1.920 46 .77 834 .7 250.0 208700 1.03 1 1.650 47 .15 577 .3 250 .0 144300 1.032 1.330 47 .37 403.7 250 .0 1009 00 1.033 0 .980 47 .58 90.7 250.0 22700 1.034 0 .450 47 .9 1 222 .5 25 0.0 556 00 1.035 0 .110 48 .12 87 .4 250.0 2 18 00 Sub-tota l . 825300

Tota l Vo lume = 3326200

•

4-8 •

• The sto rage in the area down stream of the Basingstoke ra ilway l in e 3 wa s 1 .2Mm g reater than the 1971 flood , while the sto rage upstream of this 3 point was 1 .2Mm less than in 19 71 . The very di fferent distribu tio n o f stora g e betw een th e 1971 a nd 194 7 ev ents may be exp lained by the storag e

• prov ided by gravel pits excava ted after 1947 and the backwater effect of the Tham es w hich would hav e ra ised lev els in the Ken net in 1947 rela tive to tho se in 197 1.

• 4 .3.2 Flood volumes rallel to 197 1 surface

The storage cha racteristics of the valle y ha ve been determ ined by u s in g th e computer m odels to ca lcula te the volum es store d unde r wa ter

• profile s parallel to the 1971 event but va ried by intervals o f + 0 .1m . A summary o f the results are presented in Tab le 4 .4 together with a graph of the storage elevation curve.

• 4 .4 CHANGES IN THE FLOOD PLAI N

4 . 4 . 1 Methodology

• The changes in sto ra ge ca pac ity w ith in the flood plain in recent yea rs ha s been s tu died to prov id e a p e rs p ec t iv e o f bo t h t h e g lo b a l alte ra tio n s and the ra tes of c hange. Changes arising from developmen t o r infilling have been treated sepa rately from changes due to gravel excava tion • which may have the effect of adding to storage capacity .

The largest floods in rec ent yea rs are those of 194 7 an d 19 7 1, and the c ha nge s in the flood p la in hav e therefore been monitored over the two • periods 1947-71 and 1971-86 . This has been done using a eria l photogra ph s that were flown for Berkshire County Counc il in 1947, 1971 and 1986 .

•

•

4- 9

• 41 T ABLE 4 .4 ST ORA GE E LE VAT IO N C UR VE Ke n ne t Valle y Study n e 9 c4 Ir " " , 1 4 7 1 1 ev021,

•

7

6 2 Y •

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ee

T o t a l ." '

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2 „ ay' „ ...-Er Cro n ! Pita •

Rou t ' ,

- 0 .4 - 0 .2 0 0 .2

19 7 1 Level, •

•

Flo od Leve l Vol ume Vol ume Vol ume i n Vol ume Kenne t Foudr y Gra ve l Pi t s Tot al

197 1 Le ve l - 0 .60 833300 19 1700 o 102 50 00 • 197 1 Le ve l - 0 . 55 942 600 2 11600 0 11542 00 197 1 fieve l - 0 :50 10659 00 233200 0 1299 100 197 1 Le ve l - 0 . 45 12043 00 2568 00 500 00 15 11100 197 1 Leve l - 0 . 40 13 59 900 282800 1000 00 1742700 197 1 Le ve l - 0 .35 153 1400 3 11900 200 000 20 43300 197 1 Le ve l - 0 .30 17 17000 344600 30 00 00 236 1600 197 1 Le ve l - 0 . 25 19 18400 37 98 00 45 00 00 27 48 200 197 1 Le ve l - 0 .20 2 13310 0 4 17400 600000 31505 00 197 1 Le ve l - 0 . 15 236 2000 457300 687500 35 06800 197 1 be ve l - 0 . 10 2603800 6005 00 7500 00 38543 00 1971 Le ve l - 0 . 05 2858 500 545300 837 500 42 41300 197 1 be ve l 0 . 00 312 44 00 59 2 100 900 000 46 16500 197 1 Le ve l 0 . 05 33996 00 6407 00 98 7500 5027800 197 1 Le ve l 0 . 10 3683700 6932 00 10500 00 54269 00 197 1 Leve l 0 . 15 39 73600 747900 1137500 5859 000 197 1 Le ve l 0 . 20 42 68600 803 40 0 1200000 6272000 197 1 Le ve l 0 . 25 4568400 859800 1287 500 67 15700 197 1 Le ve l 0 .30 48 72200 9 17 100 13500 00 7 139300

• 4-10 •

4 .4 .2 Infillin of the flood lain •

The cha nges ident ified due to road construction, housing and gene ral developmen t and lan dfill in the study area are as follows:

•

Category 1947 - 71 197 1 - 86

• Roa ds 1(4 (under construction 1971) Rosek iln Lane Housing Southcote Ford s Farm Beansheaf Farm

• General Arrowhead Rd industria l Extension of Arrow head Rd Development area , Theale New water treatm ent works Sludg e beds/Sewage works Green Lane New Courage brewery

• Landfill Confluence Kennet/Foudry Old Smallmea d tip Knights Farm , Berry Lane New Smallmead tip

• Not a ll of the above developments fall w ithin t he flo od p la in , bu t the areas and volumes that have been lost are set out in Table 4 .5 .

3 The table suggests tha t about 0 .5M m o f stora g e capa city has b een

• lo st through dev elopment relat ive to the 1971 flood levels in eac h o f th e periods 1947-7 1 and 197 1-86; 80% o f this vo lume ha s b e en w ith in th e floo d plain of the Foudry Brook .

4- 11 0

• TABLE 4 .5 INF ILL ING OF THE FLOOD PLA IN

Site Area Storage lo st under

• (ha ) 1971 flood level (1n3 )

1947 - 71

• Kenn et

H4 12 50 000

Arrowhead Road 7 30 00 0

• Kn ights Fa rm/Berry 's La ne 12 31 80 000 Foudry

H4 10 30 000

• Sludge beds 20 200 000 Infill Kennet/Foudry 15 150 000

45 380 000 Tota l

• 76 460 000

1971 - 86

• Kennet

Green Lane 1 .5 1 000

Rosek iln La ne 2 25 00 0

New water treatment workS 5 70 000

• 8 .5 96 000

Foudry

Old Smallm ea d tip 50 250 000

• New Smallm ead tip 37 170 000 Gravel pit to be filled 27 114 420 00 0 122 .5 516 000 •

4- 12 4 .4.2 Grav el pits

Gravel extraction has changed the topography of the flood plain over

• large stretches of the study area . The pits that have been excavated are :

1947 - 1971 1971 - 86

• Woolwich Green West Completion Theale Lake Woolwich Green East Ho s eh i l l Farm Lake Theale Lak e (partially excavated 197 1 ) Completion Wellman 's Farm Wellman 's Farm Lake (partially Field Farm (current )

• excavated 1971 ) Searle 's Farm Lake Sma llmead (to be filled ) Pingewood Lake Cottage Lane (current )

• The wa ter leve ls in the la kes are gen erally set by natural ground water lev els and seepage through the gravels to or from the Kennet , but are also controlled in some ca ses by culverts and overflow weirs . A number of the la kes w ere flooded in 19 71 a nd thus pro v id ed a d dit i o n a l s to ra g e

• capac ity . Ground water levels are generally 1 to 1.5m below ground leve l in this area w ith the re su lt tha t at lea st lm o f add it iona l storag e depth sh o u l d b e a va i lab l e . T h e resu ltant increa se in storag e ca pa c ity is tabulated in Table 4 .6 •

0

•

•

4- 13

0 TABLE 4 .6

ADD ITIONAL STORAGE CAPAC ITY FROM GRAVEL P ITS

•

Pit Area No rma l water Est 1971 Storage level flood Vo lume

11/2/87 lev el 0 3 ha (m ) (m ) (m )

• + drawn down by Ta rmac fo r gravel washing water ++ excavated a fter 1971

Th e volum es g iven in the abov e table are appro x imate only a nd w il l

• no t necessa rily re flect add itiona l capacity ava ilab le during th e peak o f the flood because seepa ge from the river through the gravel may have resu lted in

some filling before the peak .

•

4- 14

• •

• The additiona l capac ity provided between original g round lev el a nd 3 no rm al lake leve l is est imated to hav e b een about 0 .11im during the 1971 event for those lakes upstream of the Theale gauge . A s described in 4 .2, it 3 i s a lso est ima ted tha t a s much a s 0 .4Mm m ig ht ha v e en tered th e new ly

• excavated Theale pit .

Changes in sto rage downstream of the Theale gauge are limited to the Searles Fa rm complex of lakes . On the basis tha t th e roa d round the la ke

• wa s no t flooded , the lake lev el is estimated not to have exc eeded 40 .0m and 3 some 0 .4Mm are estimated to have flowed into the co m p lex . Th is does not n ecessa rily repr esent an increase in storage capa c ity rela tiv e to the situat ion before the excava tion o f the lake as the o r ig ina l ground lev e ls

• are not known .

4 .4 .3 Net Cha n es to the flood lan

3 • It is estim a ted that in 19 71 some .0.4 Km may have been stored inV 3 the Sea rle 's Farm pit and som e 0 .5 Km in the Theale/Woolwich Green East and West p it s . Ha d those pit s been excavated at the time it is probable that more water would have been stored in the flood plain, a lthough substantially 3 • l e ss tha n th e 0 .9 Mm est ima ted to hav e been sto re d in the p it s . T he excavations since 1971 (Hosehill Fa rm , Completion o f Theale and W ellm any Fa rm ) would not significantly alter those estimates were a similar event to occur at the present time sinc e the hydraulic connectio ns bet w een the p it s

• are no t desig ned fo r flood flows and the Thea le pit would still fill from overtopping over the Theale-Sheffield Bottom road . The net ga in in storag e resu lt ing fro m the excava tion of the gravel pits is estimated to be in the 3 range 0 .5 - 0 .9 Km .

• The in fill ing o f the flood pla in has resulted in a total loss o f 3 3 about 1 Km below 197 1 flood lev els of which about 0 .5 Mm has occur re d since 1971. The loss in sto rag e volume wou ld be substantially greater for a • floo d event similar to tha t o f 194 7 when water lev el s were considera bly higher in the downstream reach . The ma jority of this loss in storage vo lume (80% ) has occurred in the Foudry flood plain .

•

4- 15 • A t t h e t im e o f the 197 1 floo d it is p robab le tha t th e lo ss o f storag e was approximately counterbalanced by the gain from gravel pits . The g ravel p its added since that time hav e not contributed significantly to the storag e volume and the result has been a net lo ss of s torage a s in fillin g

• ha s co nt inued . There does rema in scope , however, for better ut ilisat ion of the sto rage provided by the Th ea le/Wellm an 's Farm p its by design ing th e hy dra u l ic int ercon nections for flood flows . It should also be noted that grav el extraction is continuing and fu rt her lak es w il l be c reat ed in th e

• reach between Theale and Tyle Mill .

4 .5 FLOOD ENVELOPES

• 4 .5 .1 App roach

Flood envelopes have been derived fo r the 1 in 10 year flo od , th e 19 71 flood and the 1 in 100 yea r flood . The 197 1 floo d ha s b een used

• b ecau se t h e e x t en t o f t h e flo o d ing w a s ca r e fu l l y rec o r d e d a n d it corresponded to about a 1 in 50 year event at Theale and 1 in 35 year event at Reading .

• Th e envelo pes have b een determ in ed using the storage frequ ency curves deriv ed in Section 3 .6 to est imate the volume go ing into st orage fo r a particular return period event, and the storage cha rac teristics to set the flood levels required to accommodate that volume of storage .

• Th e volu mes go ing in to sto rage according to the sto rage frequency curve are : 3 1 in 10 year 3 .7Mm • 1971 flood (1 in 40 year )- 4 .9Mm3 3 1 in 100 year 5 .9Mm

•

•

4-16

• •

• 4 .5 .2 Results

Rela t ive to th e 1971 flo od lev els, the 1 in 10 year flood storag e gives a lev el about 0 .15 m - 0 .2m low er wh il st the 1 in 100 yea r event is

• about 0 .1m - 0 .15m higher . The implications of these level differences hav e been studied on F ig ures 4 .1 to 4 .4 , o n wh ich th e su rv ey info rm a tion is presen ted at a scale o f 1/5000 w it h contours at lm intervals . The floo d envelopes so determined are shown in colour on F igure 4 .5 , b ut due to the

• topo graphy of the flood plain and the smal l differences in level betw een the events the three envelopes lie very close tog ether .

The 19 71 flood envelope has b een rep roduced from the survey of that

• event, modified to take into account th e cha nges in topogra phy that hav e ta ken plac e, such a s tho se on the Foudry flood plain , the completion of the M4 and expa nsion of Theale.

• The 1 in 100 year envelope is intended to approximate to the outside limit of the flood plain and this has been estimated w here the survey ha d n o t p r ov i d e d a deq ua te in fo rma tio n . Betw een Thea le and Ty le M ill fo r instance , it has been assum ed tha t the a rea be tw een the ra ilway and the

• A 4 /M 4 l in k ro a d would b e flooded , a s wa s appa rently the case in 194 7 according to maps consulted at Read ing Borough C ounc il . It ha s a lso b een assumed that flood waters could rise sufficient ly in Searle 's Farm Lak e to cross Berry 's lan e at an elevation of abo ut 40 .0m and inundate Kirto n 's Farm

• and P ingewoo d Lak es .

•

•

•

4- 17

• •

• CHAP TER 5

HISTOR ICA L DEVELOPMENT IN THE STUDY AREA

• Discussions were held w ith Berksh ire Coun ty Council, R eading Boroug h Council and Newb ury District Council in order to establ ish :

th e h istory o f th e subm ission of relevant plann ing app lica t io ns a n d

• the respo nse to them

current attitudes to long term plann ing

• pub lished plans

The response by THA ME S to plann ing applications go ing bac k in to t h e

1960s has also been rev iew ed by consult ing the correspondence reco rds .

• The informatio n ob ta in ed is pr es ented in t h e form o f a n ov era l l

rev iew o f pub l ish ed pla n s fo r th e study a r ea , an d a summary o f spec ific

prop osa l s re la t ing t o g rav el ext ra ct io n , w a s t e t i p p in g , a n d g e n e ra l

• development .

5 .1 REVIEW OF PLANN ING DOCUMENTAT ION

• 5 .1.1 Plann in Autho rities

Responsib ility for plann ing w ithin the Study a rea is divided between

th ree loca l co unc ils a n d t he C ounty Council . The geographica l boundaries • b e tw een th e lo ca l c o unc ils a re ind ic a t ed o n th e 1/ 10 00 0 sc a l e m a p s .

Broadly speaking Rea ding Bo roug h Council is respo nsible for the area east o f

th e Burghfield b ridg e excluding Sea rle 's Fa rm la k e , a nd New bu ry D ist r ic t

Co un c il fo r th e a r ea w e st o f the b ridg e . W o k ing ha m District Counc il is responsible for only a very sma ll po rtion o f th e stu dy a rea by th e Fo ud ry

brook on the 1(4 .

5- 1

• 4 , I , k ip' •

Berkshire County Council has a specific respon s ib ility fo r m in era l • extrac tion , waste dispo sal and highways. The County Council • is nev erth eless bound to seek the v iew s of the local autho rities on these matters .

• 5 .1.2 Berkshire Count Counc il

Propo sa ls co ncern ing the study a rea have b een rev iew e d in t h e follow ing documents :

• Cent ral Berkshire Structure Plan - September, 1980 Rev iew of Berkshire 's Structure Plans - Subm ission Document - January, 1986

• Berkshire Minera ls Local Plan - September, 1984 Coun tryside R ecreation Local Plan - Feb ruary, 1985

The proposals tha t directly affect the study area are as follows :

• Industrial Develo ment

Th ea le (20 ha ). A n a rea to the south of Theale-by-Pa ss between the

• existing industrial a r ea a nd the 1(4 Mo to rway is prop osed for industr ia l developm ent .

South Read ing (80 ha ). An area to the South o f Read ing , north o f

• th e 1(4 and west o f the A 33 is pro posed for industr ial dev elopment . The Structure Plan states tha t 'because of the complexity o f the issu es in this area , the number of agencies and local authorities involved and the Coun ty Council 's specific responsib il ity fo r h ig hways and m inera ls , the C ounty

• Council proposes to prepa re , in con junction w ith the relev ant District Counc ils , stra teg ic gu ida nce fo r th e g enera l dev elopment o f the a rea , indicating how the va rious issues may be resolved '.

• A ma jor flood relie f cha nn el , to geth er w ith impro v em en ts to th e Foudry Bro ok , a re id ent ified a s be ing necessa ry fo r this development to proc eed .

5-2

• • Roa d Improvements

Proposed road improvements include :

• M4 Junction 11 - Improv ements to deal w ith the present problem . Access onto Theale-by-Pass - The proposed new industrial development south o f Thea le will necessitate a new two way access onto

• Theale-by-Pass . Link Roa d - A33 - Relief Road to Burghfield Road - A l ink road to im prov e accessibility to the Burghfield Area and the Southcote and Tilehurst Areas .

• Th e area b etween J unct ions 11 and 12 is a lso with in the area of search for a new motorway service station .

• T h e C o un ty Counc il expresses the v iew that the re should be no further ma jor housing dev elopment so uth o f Rea d ing an d that the m otorway shou ld be regarded as the limit of the built up area of Reading .

• Minerals Plan

Th e fundam en ta l po licy reg arding m ineral ex tract io n is that the County Counc il w il l seek to avo id the steril isa tion o f sa nd a nd gra ve l • depo s its . The study area forms one of the prospect areas with in the plan . Approximately 620 ha hav e received plann ing permission for ext raction in the Lower Kennet Valley , o f w h ich ab out 110 ha rema in to be worked . Gravel extraction is discussed in more detail in Section 5 .3 . • Waste Tipping

It is mentioned in the Minerals Plan that a wa ste dis po sa l pla n is • to b e prepa red by the County Counc il but that it will not be ava ilable for som etime .

•

5-3

• 0

• Th e ma jority o f th e la nd to b e reinstated after gravel ext raction will be filled with inert material, with in some cases a small propo rtion of in du s t r ia l w a st e . Any a rea s to b e filled w ith household wa ste would no rma lly first b e sea led w ith a c lay lin ing . Only iner t m a t e r ia l i s

• generally allowed to be tipped within the flood plain.

Recreation Plan

• The Plan l ists three a rea s o f ecolog ical im portanc e which might impose con straints upo n any proposed development , name ly Mano r Fa rm Sew ag e

Fa rm , Searle 's Fa rm (th e north w estern portion of the lake ) and Woolwich Green Lak e, sou th of Thea le .

• They have been ident ified by the Nature Conservancy Council as being of particular value for nature conservation .

• 5 .1.3 Read in Borou h Counc il

The council has published two documents which relate to the area :

• Reading Waterways - A Plan for the River Landsca pe - Approved January , 1979 Kennet Valley Loca l Plan - Deposit Copy - August , 1985

• The proposals put forward for the a rea by th e Kennet Valley Loca l P la n a r e p r e s e n t e d o n F ig u r e 5 .3 . T h e a r ea c o vered by a p la nnin g application for industrial development tha t is presently under consideration is a lso show n .Th is corresponds b roadly to the industrial development for • South Read ing proposed in the Structu re Plan .

5 .1.4 Newb ur District Counc il

• The Low er Kennet Water Park dra ft study report published in 1977 put fo rwa rd po lic ies and pro po sa ls fo r encourag ing the opt imum use o f the opportunities in the Lower Kennet Valley for the development of recreational fac ilit ies , nature con servation and educat ion , and landscape rehabilitation . •

5-4

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5- 5 •

• The study examined 540 ha tha t had received planning perm iss ion fo r g r a v e l e x t r a c t io n a n d i d en t i f ied n ine l a k e s w h ic h ha d p hy s ic a l cha racteristics suitab le fo r sa iling . A presu mption aga inst a ctiv it ies wh ich gen era te noise or attract large crowds was proposed for the areas at

• Pingewoo d, Kirton 's Farm , Hoseh ill and Theale (western part ), wherea s it was considered they co uld be enc ouraged at Theale (east ), Wellman 's Farm and Knights Fa rm .

• Th e Newbu ry Distric t Local P lan which is currently being prepared w ill include the Study Area , and it is understood that it w ill re flec t the polic ies expressed in the Low er Kennet Water Park dra ft .

• 5 .2 GRA VEL EXTRACTION

5 .2.1 General

• T h e ex t ra c t io n o f g ra v e l s in th e L ow e r K enn e t V a l l e y h a s dramatical ly cha nged the lan dsca pe with in the flood pla in ove r the course of the last forty years. Of the 620 ha for which permission for extraction has been granted , som e 250 ha hav e been restored to Lakes .

• In those areas restored to agricult ural lan d , the repla cemen t o f g ra v e ls w ith in e rt m a t e r ia l crea tes dra ina ge p ro blem s whic h make it impo ssible to restore the land to the original levels . It has either to b e dom ed and taken out of the floo d plain , or a ridge and furrow system is used which restores it more closely (but not completely ) to the o r ig ina l ground lev el .

Figure 5 .1 shows those area s o f land fo r which plan n ing p erm is sio n fo r grav el extract io n ha s b een granted , is currently under considerat ion , ha s be en refus ed , or cou ld b e a pplied fo r in the f uture . Th is cov e r s v i r t u a l l y th e e n t ir e f l o o d p la in . Ex trac tion h as been ca rried out principally by three com pan ies , nam ely ARC , Tarmac and Hall Aggregates/Rmc , ea ch o pera ting in geog ra ph ic ally d istinct portions of the study area . A brief histo ry of the workings will be g iven before rev iew ing possible futu re dev elopments . •

5-6

• • 5 .2 .2 Historical back round

ARC

• The ARC plant is located at Sheffield Bottom just sou th o f Thea le . Th e or ig inal p erm iss io n da tes from 1949, and the complex now totals over 260 ha , the most recent addition being the Field Farm area (50 .7 ha ). The a r ea s du g to date hav e be en resto red to lak es , namely Haywa rd s Fa rm ,

• Woolwich Green East , Woolwich Green West , Hoseh ill Farm (Ha ll Agg rega tes ), Th ea le , Thea le East and Wellman 's Farm . Culverts hav e been constructed so tha t the Woolwich Green and Hosehill Farm Lak es drain into the Theale Lake .

• T h e a r e a c u rrently b eing wo rked is the F ield Farm site only a portion of which is w ith in the flo od pla in . Onc e th is is comp leted th e W ellma n 's Farm lake w ill be extended , the whole operation being due to last som e six years . Permiss ion has a lso been granted for ext ract ion in the ar ea

• so uth o f Hayward 's Fa rm lake on the north bank of the Kennet as far as Ty le Mill , and this is due to be restored to lak es .

Tarmac • Tarmac have their plant on the Ping ewood roa d near K n igh t 's Fa rm . Th e Sea rle 's Fa rm complex of lakes were excavat ed by a number of companies in the 1960s with only one small area just downstream o f Bu rg hfie ld Br idge • at Green Lane being refilled and restored to the original leve ls .

Ta rma c a re cu rr en tly wo rk in g the Cottage La ne s it e betw een the K enn et and the elec tric ity sub st at ion . Th is is due to b e retu rn ed to • agricu ltural land using the ridge and furrow method , th e orig inal p roposa l

t o d o m e p a r t o f th e a rea a nd resto re the rest to a lak e ha v ing b een re jected .

•

5-7

• 0

• Ta rmac hav e recently lodged an application to extract gravels from the Holy Broo k Farm/Coley Meadows between the Kennet and Holy Brook . They propose to resto re t he area down stream of the Burghfield Bridge controlled by Rea ding Borough C ounc il to ag ricu lt ura l lan d , dom ing t h e la n d fo r

• d ra ina g e a n d prov id ing a low ly ing a rea c lo se to th e riv er fo r floo d rout ing . The area upstream of the b ridge contro lled by Newbury D istr ict Council would be returned to lakes . The proposals are illustrated on Figure 5 .4.

• RMC Hall A re ates

The Hall Aggregate plant is situated in Cottag e Lane a s it serv ed

• pa rt o f the Sea rle 's Fa rm complex . Grav el s a re currently still b ein g ex trac ted from the Sma llmea d s it e o n t h e o th er s i d e o f t h e ra i lw a y embankm ent .

• An application by Hoveringham to ex trac t grav els from the Fobney Mea dow s no rth o f the wa ter wo rk s was refused a t an appeal in 197 1. The restoration proposed was to lakes .

• 5 .2.3 Future dev elo ment s

Sub stantial areas of gravels still ex is t whic h cou ld be developed but fo r wh ic h appl ications hav e yet to be made . These are shown on Figure 5 .1. and are in addition to those area s such a s Co ley Meadow s fo r wh ic h applications are present ly under considerat ion . However , whilst in the past the excavations have almo st all been restored to lakes , recent appl ica t ions suc h a s C o t ta g e La n e a n d C oley Mea dow s hav e proposed resto ra tio n to • agricu ltural land . Th is is du e to the la rg e a rea o f exist ing la kes and re sista nce o n pla nning g ro unds to crea ting more . Such proposals require careful evaluation as to their impact upon floods .

•

•

5-8

• • • • • • • • • • • • •

TARMAC PROPOSA LS FOR HOLY BROOK AND COLEY MEA DOWS

Figure 5 .4 •

0 5 .3 WA STE D ISPOSAL AND LA NDF ILL

Th e excava tions crea ted by the grav e l wo rk in g s hav e in s om e ca se s

b e e n u se d a s la n d fil l s it e s . W h ilst in princ iple only inert mater ial is • a llowed to b e t ip p ed in th e floo d p la in , do m est ic w a ste t ip s ha v e b e en

a l lo w e d in th e a rea b e tw ee n th e K enn et a nd the Fou dry Bro o k w h ic h w a s

flooded in 1971 . Dom est ic waste tips are dom ed to a ll ow for d ra inag e a n d p r ev en t g rou n dwa t e r po llu t io n a n d th e g round is th erefo r e e ffe ct iv e ly • removed from the floo d p la in . T he Sma llm ea d t ip fo r exa mp le , st an d s 4m

above o riginal ground level .

Figure 5 .2 deta ils those areas that ha v e b ee n filled in th e pa st , • d if fer en t ia t in g b etw een re sto ra t ion • o o r ig ina l g round levels o r above ;

future planned infilling is also shown .

5 .4 RES IDENT IAL AND INDUSTR IAL DEVELOPMENT •

Deta ils o f a number of pa st and pre s en t propo sa l s fo r d ev e lo pm en t w ith in th e study area have been co llected and a re summa rised b elow .

• 5 .4 .1 Pa st development

Southcote

• An application for resid en t ia l d ev elo pm en t a t Sou th c o t e J unc t io n

ju st n o rth o f w h ere th e Ho ly Brook pa sses under the ra ilway emba nkment was refu sed on grounds of flooding .

• Fords Farm and Beanshea f Fa rm

Wh ilst the Fords Fa rm and Bea nshea f Fa rm deve lo pm ent s have n o t enc roa c h e d

upo n th e a rea fl oo ded in 19 7 1 , c ha ng e s to th e floo d plain were made fo r

• dra inage purposes .

•

5- 10 5 .4.2 Future develo ment •

Axiom 4

Th e Ax io m 4 scheme , also known as Reading Business Park, is the BO • ha industr ial dev elopment in South Reading referred to in Section 5 .2 .1. It has been approved by Berkshire County Council and Wokingham District Council and now only needs the backing of Reading Bo roug h Counc il to proc eed . Th e scheme, which is illustrated on Figure 5 .5 would extend existing development • west o f the Ba sing s t o k e R oa d from t h e F ou d ry B r o o k to th e ra i lw a y embankment , and would occupy land flooded in 197 1.

A flood relief channel has been proposed to discharge flood wat ers • that would o th erw ise hav e been stored on the land to be develop ed . The channel begins at a control structure on the Foudry Brook just no rt h o f th e 114 a nd run s almo st parallel to the Brook about 300m to the west discharging

• into the Kenn et opposit e the new Water Trea tment Works . In addition to the channel , 16 ha o f la nd at or below existing gro und lev el is to be provided to reta in some flood sto rage capac ity and the two developers pa rtic ipa t ing in the schem e p lan to provide 50% each of this capac ity . One area of land

• is to be prov ided south of Smallmead Farm and the other on the filled gravel pit immediately south of the 114 west of Berry 's Lane , this latter area being connected to the Foudry Brook via a culvert .

• Theale

20 ha o f industr ia l develo pm en t at T hea le is env isa g e d in th e Berk sh ire Stru cture Pla n to the so uth o f th e Thea le-by-Pass between the

• existing industrial a rea a nd the 114 . Th is lan d wa s floo ded in 19 71 . A plann ing application has b een submitted , but ob jections to the road sch eme will have to be overcome before it is granted .

5-11 • -LEGEND + + + Filled above ++1. Or ig ina l Leve l \ To be f i l led above • \ Or ig in a l Level - r .„/ / Proposed AXIOM 4

7 ! I deve lop ment t 0 Proposed Ch a nne l INTAKE • ES C)C24 UN ION Ma n n

/ •

• ±+ ± FOBNEY 4 + + + + 4 4 4 3. '2 I + 4 WORKS I. + + + + + + .. . . + V / 4 4- 4- + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ------• + - 4•••1- 4--F + -f: + + + + + + + + + + + + + • + + + + + + + + + + + + + + 71- + + + + + + + + + + + + + -ft + + + + + + + + + + + + + + A T + + + - HOUSE 1-1OLD W A STE + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + -f- + + + + + + + + + + + + + + .+ + + + + + + +

• tr. :+-+ +, -r + • 4-

ch•-i• 4- 4: -!--F +.3/41- • , + + + + + + + + + + + + + + + + + + + + + + + + ml ++++++++++++++ :v+++++++++++++++ +4+ + + + HO USEHOLD W A STE + + + + + + + + + + + + + + + + + + + 4° + + + + + + + + + + + + + + + + + + +++++++++++++++++++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + • + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

PROPO SED NATURE RE SJ RVE

_......

AXIOM 4 DEVELOPMENT

5-12 Figure 5.5 Spey hawk

The area south o f the moto rway by th e Foudry Brook at Grea t L ea ha s b e en t he sub ject of an application for residential dev elopment by Speyhawk .

This ha s been refused and is due to go to appea l . 0

ARC Lon Term Pro sa ls

ARC made public in Ja nua ry 198 7 ou tl in e pro p osa l s fo r lo ng te rm • dev elo pm en t in the study area on land own ed by the gravel compan ies . These

are reproduced in Figure 5 .6 . Their only propo sa ls directly a ffec t in g t h e floo d p la in is som e res iden tial developm ent in the Fobney Meadows south of

the ra ilway embankm ent . This is where Tarmac propose to dome th e a rea fo r • w h ic h they h av e subm it ted a g ra v e l ex t rac t ion a pp lica tion in Southco te

Meadows .

•

•

•

•

5- 13 • • •

Reproduced wit h permission of ARC Propert ies. .... , , •". L •••...... , ...... / ... /

• / , 1, • so •, , / , / z / / 1 1 1 1 i / • i ;T r ' 7 / 7 7 - -7-- ..‘ • i i i1770 ‘ . ' ...... • •;.,. .vv,. ' . 4 /3 : earr 'n !1/4- • ' , o , d ,.. ci , •• •' / . a ' / --- 1•U

" 1S -7 . , •• / / /,; ', /, '/ / : ' • ------.,-. ../- • 1:

N e,4 , • „ • Kin II7 1Al. H • N - .• CD

fl r c. THEA LE - SOUTH W EST REA DING I CONCEPT PIA N

: ARC PROPERTIES I

LONG TERM PRO POSA LS FOR STU DY A REA Published by A RC in Jan 1987 FIG. 5-6 CHA PTER 6 •

FUT URE POL ICY TOWARDS DEVELOPMENT

• 6 .1 INTRODUCT ION

6 .1.1 Ob iectives

• The ob jective o f th is chapter is to consid er fut u re po licy w it h r ega rd to the managem ent o f the flood pla in in the light o f known an d anticipated development particula rly gravel ext ract io n an d re in statem ent proposals . •

Genera l princ ip les o f floo d pla in mana gem ent are d iscu ssed , the effect o f b oth loca lised and global changes in flood storage reviewed , and the implicat ions for future policy evaluated . •

6.1.2 Summar of h dro lo

• The hydrological analysis has assessed the June 197 1 flood to hav e been a 50 y ea r ev ent on the Kennet at Theale but more like a 35 year event in terms of flow at Coun ty Weir a nd flood pla in inundation ; this fo llo w s from stat istical analys is of annual maximum recorded flows and an ana lysis

• of ra infall records with a unit hydrogra ph approach . The inflow hydro g ra ph fo r th is ev ent p rov ided the input to a flood routing model, and this shows that significant attenuation was prov ided by the Kennet flo od pla in b elow Ty le M ill , reduc ing the flood peak by about 29 per cent at the lower end of

• the flood pla in where the Kennet passes through Reading .

•

6-1

• •

• 6 .1.3 Flood envelopes

The availability o f reasonably complete water level profiles for the June 1971 event not only prov ides a means of testing th e flood rout ing mod el

• between Theale and Reading , but also gives an env elope which can b e rela ted to the return pe riod o f th e in flow pea k . This in turn has enab led lev el envelopes to be drawn for floods o f oth er retur n per iods a s desc rib ed in C ha p t e r 4 . I t shou ld b e bo rn e in m in d that the p ro file o f the floo d

• envelope , especially where it approaches the Read ing outfall, will be affected to some extent by the downstream control and possibly the shap e o f the flood hydrograph at Thea le . Because the June 1971 event was a summer ev e nt o f r ela t iv e ly s ho r t du ra tion , it wa s not u nduly in fluenc ed by

• down stream conditions .

How ev er , by prov iding a consistent set of hyd rographs for different return per iods , and subsequent routing to g iv e vo l um es o f flood pla in

• sto rag e , a set o f coherent flood envelopes for different retu rn periods has been prov ided . This set of flood level env elopes pr ov ides the ba sic too l for ob jective flood plain management .

• 6 .1.4 Flood lain sto ra e

The hy dro log ical flo od routing , calibrated on the vo lume estimated to have been stored in the 197 1 event , has enabled the volumes stored in the

• valley for different return periods to be estimated : Retu rn Per iod Volume

3 2 .33 year 2.0 Mm 3 • 10 year 3.7 Min 3 25 year 4 .5 Mm 3 50 year 5 .2 Mm 3 100 year 5 .9 Mm •

•

6-2

• • The above quantities of sto rage a re la rge and hav e a sig n ificant attenuat in g effe ct on flood flows, and thus on levels at the lower end of the stu dy reach . Whilst limited flooding will occur o n a n an nual basis , a la rg e a rea o f the floo d pla in w ill be inun da ted at irregular intervals ,

• without any realistic possibility of total prevention short of ma jor channel reconstruct ion .

6 .2 PR INC IPLES OF FLOO D PLA IN MANAGEMENT

• 6 .2 .1 Ob jectives

The o b jec tiv es o f m ana gem en t o f dev elopm ent in th e flo od,p la in

• should be to relate the location , nature and level of any deve lopment to the risk of flooding as expressed by the retu rn period , but should also be based on the need to mainta in floo d pla in storage a s a p rotection to existing urban areas .

• Because complete inundation is not an annual event, it is inevitab le tha t perce pt ion o f floo ding risk is dependent on lo ng-t erm records a nd hydro logical a nalys is . In these circumstances, pressure for the dev elopment

• of the flood pla in will continue to b e strong , espec ia lly b ecau se o f th e prox imity of the area to Read ing .

It is necessa ry to b ea r in m ind that , for e x am p l e , a 5 0 y ea r

• recurr ence interva l gives no a ssu rance about the timing of the next ma jor event . The risk of a T year event occurring w it hin th e next L y ea rs (sa y the life of a pro ject ) is:

1 L • 1 - (1 - /T )

so t ha t th e risk of a 50 year return period flood occurring w ithin the next 5 years is

• 5 1 - (0 .98 ) = 0 .10 or 10 per cent

•

6-3

• 6 .2.2 Flood lain zonin •

It is rec omm ended that the development of the flood plain should be linked to the risk of flooding as embodied by the enve lopes fo r th e 10 , 35

• and 10 0 y ear ret urn period floods . An approach wh ich has been advocated (Aberdein et al . 1981 ) is to classify zones in terms of return period and to limit development with in these zones acco rdingly .

• Th is zoning is illustrated by Figure 6 .1, where Zones 1-3 lie with in the lim it o f the 100 year flood, with Zone 1 nearest the river . Zone 1 is no t de fin ed prec isely in terms o f retu rn period bu t is c lo sest to t h e wa terc ou rse and thus flooded most frequently; it not only acts as a sto rag e

• area for flood waters but flow veloc ities may b e hig h . No new bu ild ing s shou ld b e perm itted and esse ntial develo pments like motorways sho uld hav e adequate openings . Near urban areas , this zone is pa rticu la rly im portan t for recrea tion a nd can be used to provide boa ting pools, parks and play ing

• fields . The outer bounds of th is zone are generally determ in ed jo intly b y planning and wat er a uthorities but in this case it will be seen later tha t Zones 1 and 2 a re virtually coincident .

• Z o n e 2 w h ic h l ie s w ith in th e 100 yea r per iod flood envelope , prov ides essential storage for flood waters and any cha nge in the ava ila b le storage in this zone w ill affect flood levels to a greater or lesser extent . Certa in developments could be perm itted prov ided there is no reduc tio n in

• flood pla in ca pa city and no risk to life . Such developments could include ground level car parks and office bloc ks with the first flo or ra ised ab ov e flo o d l ev e l w it h e f fec t ive floo d wa rn ing a nd ev acuat ion proc ed ures . However, dev elo pm en ts shou ld show posit iv e benefits by compa r ison w it h

• developm ents in other areas not sub ject to flooding .

It may be pos sib le to c rea te a Zone 3 w ith in the 100 y ea r flood

envelo pe , wh ere developm en t cOtild b e permitted if ground levels cou ld be

• raised above the 100 year level or protected to this level w ith n eg lig ib le effect on wa ter lev els , or w ith compensatory channe l works to ensure tha t exist ing flood levels are not increased .

•

6-4

• • • • • • • • • • • • •

Il l

7 1 - •

• 6 .2.3 A lication to stud area

These principles have to be interpreted and put into the c ontex t o f lo ca l co nditions . Although the ultimate decisions on development must rest

• with the planning autho rities and the water authority , this repo rt prov id es the technical information on which these dec isions may be based .

The risk of inunda tion at any site on the floo d pla in is in dicated

• b y th e fl o o d env e lo p e s r e la t ed to r et ur n p er io d , and dec is ions o n development must be re la ted to this risk . It shou ld be remem b ered tha t down strea m cont rol may affec t the leve ls a t th e lower end of the reach , especially if floods co inc ide on the Kennet and the Thames . The reliability

• o f t h e r e s u lt s may b e a ffected by the lack of com plet e reco rds . Th e relative rarity of complete inundation of the flood pla in can a ffec t loca l perc ept ion o f th e flooding risk, but the analysis presented in this report should assist in an ob ject ive assessment o f the risk .

• O n th e o t h er ha n d , d ifferen t ty pe s of d ev elop ment wou ld hav e different effects on the risk of flooding in oth er parts of the flood plain . G rav el extra ction w here the area is left unfilled and becomes a lak e whos e

• normal water level is controlled by groundwater lev el , will inc rease th e vo lume a va ilab le fo r sto ra ge and attenua tion . The increa sed storage is somewhat lower than the volume o f g ravel extrac ted , a s the grav el itsel f wo uld ha ve ha d a coe ff ic ient o f storage of say 20% . This fraction of the

• vo lume wou ld have be en ava ilab le to acc ept w ater during flo o d s . T h e ov erly in g soils would probably have had a lower coe ffic ient o f storage . At the sam e time , opportunities for some controlled flood allevia t io n th ro ug h storag e m ay b e pres ented . On the other hand , replac ement by impermeab le

• mater ial or restoration to agriculture by doming will decrea s e the sto rag e ava ila ble , and it is im porta nt to preserve the attenuating effect of this storage .

•

•

6-6

• •

• 6.3 CHAN GES IN FLOOD PLA IN STORAGE

6.3.1 General

Th e ob ject iv e o f the present section is to evaluate the importance • of both globa l and localised cha nges in flood sto rage .

Sig n ifican t changes to the flood plain have taken place since the

• 1940 's both in terms of gravel ex tra ction and g enera l infil ling . It ha s 3 been est imated in Chapter 4 that during the 1971 event up to 0 .5ft may hav e 3 been stored in the Theale gravel lakes and 0 .4Mm in th e Searle 's Farm Lake . The creation of these lakes has added to the storag e available in the va lley but infilling in the down stream portion of the study a r ea has exc eeded th e • vo lume gained . Th e va st ma jority o f the volume lost has been w ithin the Foud ry flood plain , and this is discussed separately below .

• 6 .3.2 Fou dr flood lain

The levels of the Foudry flood pla in a re b elow h istorica l flood levels c lo se to th e conflu ence w ith the Kennet even for a short distance

• upstream of the 1(4 . The Foudry flood pla in there fore a cts as a pa rt o f th e ov era ll .Kennet flood pla in and the tw o have b een c onsidered as a single entity for the purpose of th is present study .

3 • It is estima ted that a bo ut 1 .8Mm were stor ed in the Foudry floo d pla in during the 1947 flood , and tha t th is rep resent ed 35% o f the to ta l 3 st o ra g e v o l um e attenua t ing flooding . Since tha t time some 1 .1M m o f storage are estimated to hav e been lost below the 1947 flood levels . T his

• la rg e reductio n in sto ra ge is likely to have an impact on water levels in the Foudry area and downstream . This could re su lt in water lev els up to 0 .1-0 .3m above the 1947 levels in order to discharge the same flood .

•

•

6-7

• • The p ropo sed Rea d ing Business Park would fu rther s igni ficantly reduce the sto rage in th e flood pla in by develop ing the land at Smallmead Farm on the left bank of the Foudry , and result in inc reased flow s th rough R ead ing . It wou ld a lso fu rthe r constrain the flood channel of the Fou dry

• and result in increased flooding due to obstruction of the flo w route down the Fo udry itself. This latter problem has been addressed by proposing the prov is ion o f a dup lica te flood cha nnel so as to mo r e th a n do ub l e t h e capac ity o f th e ex istin g Foudry Broo k sy stem . Th is would not, how ever , • overcome the problem of the reduct ion in storage to th e K ennet sy st em a s a whole which would result in higher levels downstream .

It is important to emphasis that flood levels on th e Fou dry floo d • pla in w ill b e in fluenc ed b y th e Thames and cannot b e pred icted from the retu rn period of a flood on the Kennet alon e . Thus th e 197 1 ev ent , wh ic h appa rently ha d a h ighe r retu rn period on the Kennet than the 1947 event , resu lted in significantly low er levels on the Foudry Brook flood pla in du e • to the influence of the Tham es on the 1947 event .

6 .3 .3 Localised changes

• Th e prob lem faced by THAMES on a day-to- day ba sis is the eva luation

of the effect of specific proposals on flooding . Such proposals a re judged

aga inst the following two criteria : . .M1 0

• there should be no reduct ion o f sto rage o n a lev el fo r lev e l basis there should be no obstruction to flow routes

• No reduc t ion of sto rage on a level for level basis means that it is unacceptable to reduce sto rag e at a higher level by prov idin g an equal o r grea ter amount at a lower level . This criterion is considered below using the exam ple o f the Tarmac proposals for dom ing in the Coley Meadows . •

•

6-8

•

Th e di fficulty in ana ly sin g changes in wate r depth arising from a loca lised change in storage is due to th e fact tha t flood wa ter does no t flow unimp eded across the flood p la in b ut is pa rt ly in sto rage . It is helpful to view the problem from two extrem es, namely tha t on the o ne han d th e flo od water is absorbed tota lly into dead sto rage and that on the other

it flow s unimpeded across the flo od pla in . These tw o cases a re exam ine d below .

If the flood water is stored in dead sto rag e , then red uc ing th e storage ava ila b le at a h igh level and increa s ing it at a low lev el will cause a larger proportion of the available storage to b e fill ed ea r lier in the floo d . It is therefore possible that less storage would be ava ilab le to attenua te the peak of the flood than would have been the case previously and tha t as a re su lt floo d lev els wo uld be higher . This argues in favour of preserv ing storage on a level for lev el basis.

If the flood water flows unimpeded across the flood plain, then the impact o f cha nges on the hyd raulic sect ion can be determined from ba ckw a te r analysis . The reach between the Burgh field Road and the Basingstok e railway has been modelled using a backwater analysis pro gram m e to dete rm in e wa ter levels b efo re and a fter a dev elo pm en t similar to the Tarmac scheme . The propo sed schem e would dome the area after excavation of the grav els leav in g a flood ro ute clo se to the riv er . The post-development cross sec tion has been mo delled assum ing tha t the vo lum e lost w ou l d b e c o m p en sa t e d b y exca va t ion c lose to th e riv er . There would therefore be no net change in • storage in the profile of the valley cross section .

T h e r e s ul t s o f th e a na ly s is a re g iv en in Figure 6 .2 . A goo d correlat ion w ith the 1971 flood lev els wa s ob ta ined a ssum in g floo d p la in 3 flow to be 35m /s . Almost no cha nge in water level resulted from altering the cross sec tion to increase storag e at a low er level and reduc e it a t a

higher lev el .

•

6-9

• • • •

Ra ilwa y 4 4 Ennba n knnent Assumed flow = 35 .0 m 3/s 1 Assumed Mann ings n = 0 .100

4 3 BACKWATER SECTION 3 0 (SURVEY SECTION 1.010 ) Kennet & Avo n LEVEL 4 2 Ca n a l M. Rive r Kennet A.O.D. Propos ed Prof He 4 1

4 0 Exist ing Gro un d Leve l

39

100 200 300 400 500 CHAINAGE ACROSS VALLEY M.

CROSS SECTIONS AND RESULTS OF BACKWATER ANALYSIS Figure 6.2 •

Wh ilst at one ex tr em e wh en f lo w s a c r o ss t h e fl o o d p la in a r e • un im p e d e d , t ra n s p o s in g s to ra g e from h igh to low er lev els would not seem to significantly alter water levels for the exa mp le exa m in ed , a t th e other ex trem e wh en flood water enters directly into storage, water levels could rise . The Kennet flood plain has considerable ob struct ions to flood • flow s an d the storage element is important . The requirement for there to be no reduction in storage on a level for level basis is therefore pru den t and gua rds aga inst the possibility of a cumulative loss o f storage a rising from a numb er of localised changes , th e impact o f wh ich m ight b e d ifficult to • assess individually .

6 .4 FUTURE POL ICY

• 6.4 .1 Im lications of flood envelo es

The comb ination of the hydrolog ical analysis , which rela ted flood

• pla in storage to return period , and the field survey , which relates storage volume to level and location , has enabled flood enve lo pes to b e drawn fo r differen t return periods .

• Th is a na ly sis shows tha t the areal extent of flooding does not vary great ly with in the range o f return period from 10 to 100 y ea rs . The ma in differenc e between the se lim its and the known flood limit of June 1971 is in depth ; the 10 year flood is on average about 15-20 cm b elow the 19 71 flood

• and the 100 yea r flood is about 10-15 cm higher .

The finding that the areal extent of flooding is not very va riable w ith in the range studied is not unreasonable in view of the fact that the

• lim its of the Kennet flood plain are fairly well defined to pog ra ph ica lly . On the other hand the max imum elevat ion appears to be relative ly insensitive to return period and at the lower end of the reach nea r Reading th e effec ts of down stream levels and contro l are likely to be dom inant.

•

i •

6-11

•

0

• Th us when fix ing zon es fo r control o f dev elopment t h e p r ec is e boun da ry in term s of return perio d betw een Zone 1 wher e no development should be a llowed , and Zone 2 , where so me may be a l lowe d under certa in cond it ions , becomes aca dem ic a s the tw o zones coa lesce . In effect the

• outline of the 1971 flood determines the area within wh ich hou sing or o ffice dev elopment should not be permitted .

6 .4 .2 Reductions in sto ra e

• Th e attenuating effect of sto rage on downstream flows is important , be ing est ima ted as a reduction of 29% in 1971, and priority should b e g iv en to ma inta ining storage .

• Whereas gravel extraction without rein statemen t ha s increa sed th e storag e av ailable in the reach near Theale , the flood plain near the Foudry Brook has been encroached upon stead ily and cumulatively . In effe ct a Zone

• 3 ha s been crea ted where dev elopment has been based on raising land above the lev el o f flooding , b ut com pensa tory m ea sures hav e not been taken to count erac t the effect on levels down stream .

Current proposals for the Axiom 4 schem e wou ld v irtu a lly comp lete the in fill ing o f the Foudry flood pla in , wh ich ha s to be rega rded as an integral pa rt of the Kennet flood pla in . It is conside red that the proposed new flood channel would only help to overcome the rise in flood lev els tha t will result from this development .

C urrent develo pments within the Foudry flood pla in would appear to be cont rary to policy FC 1 expressed in the Kennet Valley Plan dev e loped b y • Reading Borough Council . Th is states that no dev elopment which would reduce the capac ity of the flood plain to sto re water would be perm itt ed un less sat isfactory compensatory storag e measures are prov ided .

•

•

6-12

• CHA PTER 7 •

FLOO D ALLEV IAT ION

• 7 .1 GENERAL

The stu dy o f flood a llev iat ion mea sures on the Kennet and th e Foudry

Brook do es not form pa rt o f the Term s of Re feren c e fo r the p re sen t stu dy , • b u t c e r t a i n o p p o r t u n i t i e s fo r p r o v i d ing f loo d p ro tect io n ha v e b ee n

ident ified and are desc ribed in th is chapter .

R e d u c t i o n in floo d ing ma y be a c h iev e d e ith er by im prov in g th e • ca rry ing capacity of the r iver chan n el , b y exc a va t ing n ew c ha nn el s o r b y

sto r in g some o f th e flood upstream of th e area requiring pro tection . It is

the latter m e th od t ha t co u ld b e r ea d ily im p l em ent e d on th e K enn et a s a re su lt o f g rav el ex t ra c t io n ac t iv ities a t t h e up s t ream end o f the stu dy • area . Thes e have lea d to the formation o f a numb er o f a rtifical lakes clo se

to the river .

7 .2. GRA VEL PITS •

The gravel pits tha t could be used for flood sto rage and their a rea s

are a s follow s :

• Haywa rds Fa rm La ke 7 ha

Woo lw ich Green Ea st La ke 13 ha

Woolw ich Green W est Lake 8 ha

• Ho seh ill Fa rm La ke 14 ha Thea le Lake 87 ha Wellma n 's Farm Lake 27 ha

Sea rle 's Farm La ke 39 ha

•

7- 1 • Searle 's Farm Lake provided flood storage dur ing the 197 1 event , now 3 estimated to hav e been about 0 .414m . Water was also sto red in the partially excavated Thea le pit during this ev en t , flowing ov er Woo lwich G re en Ea st Lake into the pit .

• The latter flood route has now been substantially inc rea sed by th e addit io n o f th e Ho sehill Farm and Wellman 's Farm Lakes , and the complet ion o f the Theale pit .

• T h e la t t e r f lo o d r o u t e wa s repo rted on in Sectio n 2 .5 and is illustrated on Figure 7 .1. It was confirmed in pract ic e on the 5t h Ap ril 1987 when w ith a flow of 28m 3/s in the Kennet, flooding into Woolwic h Green

• East lake wa s observed . A culv ert allows water to flow from Woolw ic h G reen Ea st Lake under the road in to Hosehill Farm Lake which in turn discharges into Theale Lak e. Theale Lake is connected to Wellma n 's Fa rm Lak e v ia a culv ert under th e H4 and a n ov er flow weir a llow s water to return to the

• Kennet via the Clayhill Brook .

There is therefore a natura l flood flow route tha t directs water via a series of four lakes before returning it to the Kenne t once flows start to 3 • exceed about 25m /s . A longitud inal section of the 3 km of lakes is given in Figure 7 .1 which demonstrates the substantial fall in level betwe en them .

The to ta l area o f the la kes in to which water could be directed is 2 2 • presently 1.41 km w ith Wellman 's Farm Lake to be exten ded by 0 .12 km ov er t h e n ex t s ix yea rs . Al though surv ey s of the lak e per imeters are no t ava ilable, it is likely tha t the average water level could be rais ed dur in g a floo d by a t lea st lm . Visua l exam ination of Theale Lake , the b iggest of • the four , confirms this . It oug ht th erefo re to b e po ssib le to d irect a t 3 l ea s t 214m th ro u g h t h e g ra v e l p it s a nd prov id e considerab le floo d allev iation , as this vo lume approximates to nearly half the volume estimated to hav e gone into flood pla in storage in 1971.

The feasibility of the above method of flood a lleviation is examined below .

7-2 • • • • • • • • • • •

_ • 1.2 - • ? •'- 'S ; . . • 1-••••., • • • . • . . -- Ts , —

P• •

• \--;-) \c„ IF - ! fr I k,;.\ /CY,/ i • r i ARC 1 SILL . LANT. F .. ( 4, 5 %.-z°1\ L( \ LAGOON. YWARD ...k. o\y,*-»--\\`'°' Cu l ver t , ' .-- - - .WOOLWICH GREE

--- • •• : WOOLWICH . .. • o - - GREEN E .., .0- .. • Cu l v er t ./ - ",... • C. -.. "--,...- - , lo ? / .: / n -6.--,: - .i. 2/ 0 0 I .. 2 0 / 6-----P' "-- - 9' Gl ...... ,- s , '• f... • ., .. S ort . o NS . * . c.._, 0.72,___, PLA N . 71...... ;„ _ ' , ,...... -> ' u

AB 4 6 1 -11 6 6 64-9 ri IF roo d 44 • Wool w ich Green 63-2 Ea s t Hose IIi II For m 4 1 8 42 - — - - Grav el Pi t We l t ma ns Fa r m 40 3km 1.11.111 l km 2km LONGITUDINAL SECTION

h _OOD ROUTE THROUGH GRAVEL PITS F IG 7 .1 7 .3 FLOOD ROUTING •

Approach

Prec ise modellin g of the e ffect o f storage on a ca sca de o f fou r • la kes on the flood hydro logy would require a specific mathema tical model . The ob ject ive in the present chapter is to evaluate th e fea s ib ility o f the floo d a llev iat ion scheme using a simplified approach.

• The lakes have been agg regated into a single reservo ir and a floo d routed throug h it in order to est imate the attenuation of the hydrograph . A key factor in this will be the inflow to the lake whic h w ill be a fu nct ion of the flood diversion structure prov ided . •

7 .3.2 Flood Diversion

A lev el o f 46 .93m was reco rded just downstream of Woolwich Green • 3 Ea st Lake in 19 7 1 fo r a flow in ex cess o f 7 1m /s . The r ec e n t s u rv ey in dica tes tha t the bank lev el between the lake and and the river is about 3 46 .8m . Bank full conditions were observed at 28m /s on the 5th April 1987 .

It has been assumed for the present exercise, the object ive of which is to estimate the inflow hydrograph into the reservoir , that a weir will be 3 prov id ed to d iv ert 30m /s into the lake unde r a head of 0 .3m above a sill

• level of 46 .6m . A free overflow or syphon weir could be considered , but the fo rm e r is p r e fe rred on grounds o f sa fety . In order to m in im is e th e spillway length , a 3 :1 ratio labyrinth weir of 30m length has been selec ted .

• An a pp ro ximate combined stage discharge curve for the river and the weir has been derived and used to determine the inflow into the lake and the effect o n th e hydrog ra ph o f th e Kennet at Theale. Th is has been done for the 197 1 modelled hydrograph with u p to 30m 3/s flood peak b eing d irec ted

• into the lak e over the weir and is illustrated on Figure 7.2.

• 7-4

• • • • • • • • • •

Ke n ne t D iv e r sio n t h r o ugh G r av e l P it s Di v er s io n ov er Wei r

/- --. / \ \ \ / \

/ \ i \

/ \ N I t od i e d Hy d r og r a p h / \ / \ \ \ \ // 7 7 -- --' ---\

1 / \ i n t o Pi t s

NJ —

0 20 40 60 80 100 120 14 0 16 0

Tim e ( h o u r s) 7 .3.3 Reservoir Routing

A sta n da rd reservo ir flood routing p ro g ram m e has b een used to determ ine the attenua tion prov ided by the lakes on the flood discharged over the labyrinth weir . This is dependent on the volume eleva tion curve for the • lake and on the outflow discha rge cha racter istics .

The spillway prov ided to release wa ter from Wellman 's Farm Lake back

• into the Kennet has been selected after a number of trials to be a 3 .5m long free ov erflow weir which would route the flood with a rise in water leve l o f 1.6m . The area of the lake has been assumed to remain consta nt ov er this rang e .

• The modifications to the hydrograph of riv er flows are ind icated o n Figure 7 .3 . This show s the or iginal inflow hydrograph to the study area , the hydrog raph routed into the lake , the outflow from t h e lak e back to th e

• Kenn et and the resultant attenuated out flow hyd rograph . The peak flow has 2 3 3 been reduced by 21m / 5 from 76m I s to 55m /s . This was achieved by st orin g 2 .324m3 in th e la ke a nd delay ing the flood peak out of the lak e by 15 hours relative to the peak flow into the lake .

• 7.4 ENVIRONMENTAL IMPACT

The use of the grav el lakes for flood storage m ight conflict w ith

• existing or proposed uses of the lakes .

Woolwich Green East and Hoseh ill Farm

• These lakes are presently sub ject to flooding on an annual basis and the situa tion would not therefore be worsened . Woolwich green Eas t is used ex tens iv ely for fishing . Som e bunding of Hosehill Farm Lake along the roa d sepa rat ing it from Th ea le La ke m ight be nec essa ry a n d would im prov e the

• storag e cha racteristics .

• 7-6 • • • • • • • • •

Ke n ne t D iv e r sio n t h r o ugh G r av e l P its Mod i f ied lly d r og r a p h s

Ou t f low Il y d r ogr a h

\ , \ on Ke n n et

\ .. Ven n / / \ y v iy e r s iot i n t o P i t s \ \ ' -`• -..., ..__.__ _ _...... ____ .

\

/ . -.-- . "-•- , „ t ' % / \Ou t f l ow f r o—iii rrit s

/ , ,, Z V 4 i 60 BO 100 120 14 0 16 0

Tim e ( h o u r s) Theale Lake

T h e a l e La k e is u s e d for sa iling and w in dsurfing and ARC hav e recently pub lished proposals for building floa ting hom es w ithin th e La ke . C o nsulta tions wou ld need to b e held w ith these interested pa rt ies . A • significant inc rease in water level would a lso ra ise the res ervo ir to th e foot of the M4 emba nkment, the stab ility of which should be checked .

Wellman 's Farm Lake

Th e la ke is used for water skiing and windsurfing and some building would be necessary at the south-eastern end to protect properties .

0 7 .5 CONCLUSIONS

Div ers ion of the flood peak through the four lakes - Woo lw ic h Green Ea st , Ho seh ill Farm , Theale and Wellman 's Farm - offers the possib ility of 0 providing sign ificant flood attenuation that would protec t both Read ing and T h ea le . Th is would be sim ply improv ing an ex ist ing floo d ro ut e . Th e prelim ina ry a na lysis carried out fo r the 197 1 floo d sugg ests that t h e d iversio n could redu ce the severity of the flood from a 1 in 50 year event at Theale to below a 1 in 10 year event . Th e attenuat ion m ig ht no t b e so e ffect iv e a t more ex trem e ev ents when the gravel pits would be floo ded by other routes .

A d d it io na l in v e st ig a t ions would be requ ir ed to determ ine th e to pography o f th e lakes and th e a llowable rise in water lev el in m o r e deta il . O nc e th ese ha ve been ca rr ie d out more detailed modelling of the

• flood route may be justified .

7-8

• . 0 0

CHAPTER 8 0

CONCLUSION S

0 The conclusions of the study are set out below :

8.1 HYDROLOGY

0 The 197 1 flood is estima ted to hav e been a 1 in 50 year event a t Theale an d a 1 in 35 year event at Read ing . The different response times o f the Kennet and the Fo udry Broo k accoun t fo r th is d if ferenc e , the Foudry

• B rook being sens it ive to apprec iab ly shorter dura t ion storms tha n the Kennet .

Very little data is ava ilab le to make an est imate of the 1947 flood

• return period, but on the basis of gauging at Rea ding it was a 1 in 50 y ea r event .

Flood plain storage during the 1971 event is est imated to have b een 3 • about 5Mm down stream of Tyle Mill . Calib ration of a flood routing model on this va lue suggests the storag e associated with events o f dif fere nt return periods to be as follow s :

Return period Sto rage 3 (years ) (Mm )

2.33 2 .0

• 10 3.7 100 5 .9

L im ited flood ing occurs several times annua lly and in ma jor events

• storage plays an important part in attenuating flows . The a tten ua tion o f peak f lows due to stora g e du ring the 1971 event is estimated to have b een 29% between Ty le Mill and County Lock .

8- 1 8 .2 EXTENT OF FLOOD PLA IN

The flood pla in is well defined by natural and ma n ma de boundar ie s and flood envelopes fo r di fferen t return per io ds o ver the range 1 in 10 to 1 in 100 years are very close to one another .

The distribution of storage a long the valley has been determ ined and it is est ima ted that the 1 in 10 0 yea rs flood wou ld res u l t in l ev e l s

• approx ima tely 0 .1 - 0 .15m h igh er than those in 1971 and the 1 in 10 year flood approximately 0 .15 - 0 .2m low er . The actua l a r ea s fl ooded for th e three ev en ts a re a lm ost id entica l . The above differ ences in leve l do not apply for the area dow nstream of the Ba s ingstok e ra ilway line due to th e

• influence of the Tham es .

8 .3 EFFECT OF DEVELOPMENT .

• Two ma jor fo rm s of development have tak en place in the flood plain over the last forty years . Gravel extraction has resu l ted in the c rea t io n o f a la rg e n u mb er o f lak es and wast e tipp ing clos e to t he Foudry ha s resulted in substantial infilling .

• The creation of the lakes is estimated to have increased flood plain 3 storage capacity by 0 .5 to 1 .0Mm in a ma jor ev ent , w h erea s in fill ing ha s 3 reduced it by about 1.0M m be low 1971 lev els . T he g rav e l pits were in 3 • existence in 1971, since when about 0.5Mm of capac ity has been lo st due to in fi llin g or dev elo pment . Present pro posa ls to develop a Bus iness Park close to th e Fo udry would result in a furth er red uc tion in flood p la in storage . • It is concluded that a recurrence of the 1947 o r 1971 ev en ts wou ld

result in hig her lev els in the downstream po rt ion of the study area and through Read ing as a result of the storage capacity tha t has been lost . •

8-2

1 • 8 .4 FUTURE POLICY TOWARDS DEVELOPMENT

Po lic ies with respect to flood pla in management should be orientated towa rds th e protection of flood pla in storag e capacity wh ich has b een shown • to be important in attenuating flooding . The fact that the flood en velo pes fo r different return p erio ds lie so c lo se to one another leav es little sco pe for developing policies that link dev elopment to the risk of flooding .

T he o n ly area which could be developed without a significant loss of storage • is tha t b etw een th e A4 a nd the N ew bury ra ilwa y lin e west o f th e M4 a t Theal e .

Whilst there has been little encroachment upon the flood p la in in • the upp er part of the study area , there has been considerable dev elopment in the Foudry Brook floo d p la in wh ich fo rm s a n integra l part o f the K en net floo d stora g e capa c ity . This is contra ry to polic ies ex pres sed in the Kenn et Valley Pla n prepa red by R ea ding Borough Counc il . T he p ro po s e d • R ea d in g Bu siness Park would remove the greater part o f the rema ining Foudry flood plain . Due to the bac king up effect of the Thames and the interaction o f the Tha mes/K enn et /F oudry Brook at this point it is not considered that the propo sed Foudry flood rel ief cha nnel w ill of itself overcome the higher • wate r lev els tha t are likely to result due to this loss of storage . There

is a need to quantify the e ffec ts o f th is dev elopm ent and to develop a coherent policy for the combined Kennet and Foudry flood plain .

• There is a danger tha t loca lised cha ng es in th e floo d pla in m ight occu r wh ich although not necessarily significant individual ly could result in a cumula tive reduc tio n in sto rag e . T he pre sent pol icy o f p reserv ing st orage on a level for level basis prevents this and safeguards aga inst the • poss ib ility that storag e at a highe r leve l is r ep laced with sto ra ge at a lower lev el thus reduc ing the degree of attenuation o f peak flows .

•

•

8-3

• •

• 8 .5 FLOOD ALLEVIATION

An o pportunity fo r prov iding some flood attenuation by routing the flood peak through four existing gravel lak es in the upper portion o f th e

• study area ha s been identified . Prelim inary studies suggest tha t it could 3 reduce the flood peak for an event similar to that of 197 1 by about 20m /s , transform ing it from a 1 in 50 year event to less than a 1 in 10 year event , and thus a fford significant protection to both Reading and Thea le .

• Additio na l st udies would be required to eva luate this oppo rtunity in deta il in the form of :

• survey s to det ermine volume - elevation cha racter is tic s o f the lak es flood routing studies for a range of design floods to dim ensio n hydraulic structures , and eva luate attenuation . • costing of civil engineering works

8 .6 FURTHER STUD IES

• 8 .6.1 Review of Present Stud

T h e p re s e n t study ha s co n firm ed th e impo rtance o f floo d pla in storage with in the Kennet Valley in attenuating flood flows through Read ing , • and has assessed the magnitude and distribution of this storag e. The ext ent

of the flood plain has been broa dly established w ith a coarse surv ey fo r a ra nge o f return perio ds . Develo pmen t with in the flood pla in ha s b een rev iewed and the infilling of the Foudry floo d pla in contra st ed w ith the • addit iona l storage provided by the creation of gravel lakes, suggesting that

there has been a net loss in sto rage .

•

•

8-4

• • Th e study ha s not been ab le to prov ide a p recise evaluat io n of flood level differences due to the changes in storage or to prov id e a m ea ns o f ev a lua t in g th e im pa c t o f loca lised cha nges . This is beca use the methodology employed considered the storage of the valley as a globa l ent ity

• fo r t h e p u rp o ses o f the flood routing and can no t rep resent spec i fic hyd raulic features of the Kennet/Ho ly B ro ok/Ca na l/Fo udry cong lom era te o f channels . It is not considered that th is methodo logy cou ld be develop ed further in order to pro v id e a more accurate a ssessm ent or a s a too l fo r

• ev alua ting the im pact o f spec ific development s . On e method that could be considered for providing additional prec ision is mathematical modelling .

8.6.2 Mathematical Modellin

• Mathematical modelling shou ld a llow the eff ec t of c ha ng es in the flood plain to be simulated . The hydrolog ical inputs that would be required for the model hav e been developed in the present study and the model cou ld

• be ca librated approx imately aga inst the 1971 flood event . The impac t of the grav el lakes and infilling of the Foudry flood plain could then be evaluated so a s to g iv e an ind ication of relative change s even if the data ava ilab le for the model is not adequat e to hav e full con fidence in abso lute values .

• However , the reach of the Kennet between Thea le and Reading would be complex to model because of the following factor s :

• th e div isio n o f flow s betw een Ho ly Brook and Kennet , and the inflow from the Foudry the number of hydraulic structures (weirs, locks, sluices ) the influenc e of the Thames

• railway and road embankments .

•

8-5 • Th e time and mon ey requ ired to underta k e s u c h an ex e rc i se is there fo re lik ely to be considerable and it is wo rth considering whether an alternative approach could be used . In pa rticu la r it may b e wo rt h wh ile first to give some study of the consequences of higher flows through Reading

• in order to assess the risks that are being guarded aga inst by pro tect ing sto rage in the Kennet flood pla in .

The methodology used in the present study could be used to deriv e

• th e increa se in flow s thro ugh Rea d ing fo r differen t return periods that would result from reductions in the storage capacity . Backwater analys is of th e stretch up stream o f Co unty W eir cou ld then be used to determ ine the assoc iated rise in water level for a range of down stream leve ls set by th e

• R iv er Thames . This ana ly s is could b e used in con ju nctio n w ith the }IRS phy sical model o f the Kennet through Reading to assess the risk of flooding . So m e a d d it io nal survey o f the K ennet through Rea d ing is l ikely to b e nec essary .

• 8 .6 .3 Other Studies

Stud of the Ho l Brook

• It has b een ob serv ed tha t th e Ho l y Brook is liab le to floo ding before the Kennet and that this is the mechanism of the a nnua l flo oding o f th e So uthcote and Fo bney m ea dow s . The Hol y Brook is in part a very old • artific ial channel and there are a number o f sm a ll a rc h bridges ac ro ss it w h ic h re stric t its carry in g capa c ity . It is poss ible th at either the

channel capac ity if the Ho l y Brook could be improved, o r tha t the div isio n of flow s b etween the Kennet and the Hol y Brook could be better distributed , • such tha t the flow threshold at which flooding occurs c ould b e ra ised . It

is rec ommended tha t som e hydraul ic studies of the Ho l y Brook be undertaken to exam ine this possib ility .

•

•

8-6

• •

• H drometric Mon itorin

There is a lack of gauged water level or flow rec ords in th e flood pla in ar ea , and th e important Fo ud ry tributa ry is complete ly ungauged . • Intens ive exp erimental monitoring of water levels in the Kennet (espec ia lly

from Fob ney to Blakes Loc k ) , the Holy Brook, the Kennet a nd the Thames would enhance understanding of the extent to which flood levels a re interlinked . In stalla tion o f a flow ga ug ing station on the Foundry (e .g. at a su itable • site close to the 1(4 crossing ) should also be considered .

Th e outcom e o f such a gaug ing inv estm ent wou ld be sub ject to the vaga r ie s o f na tu re ; clea rly the da ta wou ld be o f consid e r a b l e u s e in • refin ing hy dro lo g ica l or hydra ulic m odels of th e b ehaviour of the flood

plains and watercourses if data for an exceptional event were captu red . The more fla shy nature of the Foudry catchment response is such that collection of a relatively short period of record (e .g . 1 to 3 years ) would a llow much • g r ea ter co nfid ence in estima ting d esign floo ds on the Fo ud ry (and it s contribu tion to composite design inflow s to the Kennet/Foudry floo d pla in ). Furth er collection of hy drom etric data is therefore strongly recommended .

• Survey

The only part of the flood plain in which a large area of housing is c lo se to the flo od e nv e lo p e s is a lo ng t he C a lc o t / So u t hc o t e /F o r d is • Fa rm /B ea ns heaf dev elopments wh ich hav e all been built close to the flood plain bounda ry . There may be some propert ies tha t m ig ht b e at r isk in a n ex trem e ev ent and this cou ld only be ascertained by a detailed survey along this boundary paying particular attention to re-entrants . •

Flood Allev iation

• A more deta iled study o f possible flood alleviat ion by routing flows through the existing gravel pits nea r Thea le may be worthwhile .

•

8-7

• •

•

Re ferenc es

•

B o o r m a n D B ( 19 8 5 ) A r e v i e w o f t h e F l o o d S t u d i e s R e p o r t

• ra in fa ll- runo f f mo de l pa ra m e te r es t ima t ion eq ua t io n s . R epo rt No . 9 4 , Institut e o f Hydro logy .

Boorma n D B and Reed D W (198 1 ) Der iv a t io n o f a catc hm en t a v e rag e

• unit hy drog raph . Repo rt No . 71, Inst itute o f Hydro logy .

Jarv is M G , Ha ze lden J a nd H ackn ey D (1979 ) So il s o f B erk sh ire .

So il Surv ey o f England and Wa les , Harpenden . • Ja rv is M G , A ll en R H , Fo rdham S J , Ha z el d en J, Mo f fa t A J an d Sturdy R G (1984 ) So ils and their use in So uth Ea st England . So il Surv ey o f England and Wales , Ha rpenden . •

Ho sk ing J R M , W a ll is J R a nd W o od E F (19 8 5 ) Est imation o f th e generalised extreme-value d istribution by the method o f probab ility-weighted

moments . Report No . 89 , Inst itute o f Hydro logy . •

Reed D W (1976 ) Determ inistic mo delling o f ca tchment systems . Ph .D . thesis , Dept . o f Eng ineering Mathematics , Univers ity o f Newca stle upo n Tyn e .

• Reed D W (1984 ) Foudry Brook flood study . R ep ort to B ry a n t Hom es

Ltd .

Symons G J and Cha tterton G (1895 ) The Novemb e r floo ds o f 1894 in

• the Thames Va lley . Q J Roy Met . Soc ., 21, 96 : 189-206 .

A F Aberdein , J V Sutcliffe , J L Yates A rational approac h to la nd

u s e p la n n in g in flo od p la in s , E lev e nth Co ng ress o f th e In t e rna t io na l

• Comm ission on Irrigation and Drainage .

8-8

• •

L

O ptik * a ir

-

• • • • • • • • • • • ANN EXE A 0 L IST OF DO CUMENTAT ION

MAP S

• O rdn ance Su rvey Maps at 1/ 10 000 sca le w ith con tou rs at 5m interva ls .

SU66NW SU66NE • SU67SW SU67SE

SU76NW

• SU77SW

O rdnance Su rvey Maps at 1/ 2500 and 1/ 1250 scales w ithou t contours . 1/2500 Sca le

• SU6 269 5U6369 SU6 370 SU6469

SU647 0 SU6 569 SU6 570 SU657 1 5U666 9 31.56670 5U667 1 5U6769

0 SU6770 SU677 1 SU 6869 SU6870 SU6 87 1 SU6 969 51J6970 SU6971 SU7 069 SU7070 SU707 1

• 1/i250Scale

5U7072SE SU7 1695W SU7 169NW 51J71705W

5U7 170NW SU7 17 15W 5U 7 17 1NW SU7 1725W 5U7 172SW SU7 172NW SU RVEY

Plan of th e rive rs at 1/2500 sca le w ith cro ss sections every 100m at 1/ 100 .

Ho ly Brook 96 7/ 111/ 0/ 1 Plan + Lon g Sect . Sheet 1 Sheet 2

96 7/ 111/0/ 1/2 to 16 inc lusive

96 7/ 111/ 0/ 1/20 to 3 1 inc lusive

Roudry Brook 1106 3/ 106/0/ 1 P lan + Lon g Sect .

1106 3/ 106/0/ 1/ 1

1106 3/ 106/ 0/ 1/2

River K ennet 967/ 103/A/ 1 Shee t 1 P lan + Long Sect . 96 7/ 103/A/ 1 Sh ee t 2 Plan + Long Sect .

967/ 103/A/ 1/6 to 13 inclu sive Sheet 1 96 7/ 103/A/ 1/23 to 29 inc lu sive Sheet 1

967/ 103/A/1/ 1 to 18 in clusive Sh eet 2

96 7/ 103/A/Z P lan an d Long Sect .

96 7/ 103/A/Z/ 1 to 36 inclusive .

•

•

• AERIAL PHOTOG RAP HS •

B e rk sh i r e C ou n ty Su rvey f low n by J .A . Sto ry and P a rtn e rs , 6 t h November 1986 . 1/ 10 000 sca le in co lou r .

• B erk sh i re C ou n ty Su rv ey flow n by C ly de Su rvey s L td on 14th June

198 1, 1/ 1200 sca le in b lack and wh ite .

• Photo cop ies of 1947 and 197 1 photo graphs .

PLANN ING DO CUMENT S

• C e n t r a l B e rk s h i r e S t ru ctu re P la n , Roy a l C ounty o f B e rk sh ire , App roved Ap ril , 1980

Rev i ew o f B erk sh ire 's S tru c tu re P lans , Submission Docum en t, Royal

• Cou nty o f Berksh ire , Janua ry , 1986

Be rksh ire M inera ls Loca l P lan , Roy al County of Berkshire , Sep temb er , 1984 .

• C ou nt ry s id e Rec rea t io n L o ca l P lan fo r B erksh ire , Roy a l County of Be rksh ire , Feb ruary , 1985.

• Th e Low er Kennet W ater Park , Dra ft Study Report , Septemb er , 1972 .

Reading W aterway s , A P lan fo r the River Landscape Ap ril, 1978 .

• Kennet V alley Loca l Plan , Readin g Borough Council, August , 198 5 .

•

• • •

ir e

• • • • Mr . Davis : Sam pl e DR .

NAVIGATION AND WORKS COMMITTEE - 12TH JULY . .1q71 Item No. LA3D DRAINAGE COMMITTEE - 12TH .JULY 1 71 Item No. ; . THAMES CONSERVANCY,' De Hohun Road ; -READING, .Berks

J at July, 1971

THE CONSERVATORS OF THE RIVER TU NES • NAVIGATION AND WORKS COMMITTEE LAND DRAINAGE COM ITTEE ' Gentlemen,

.-.:REPORT ON HEAVY RAINFALLS IN THE THAMES CATCHMENT • AND FLOODING IN TEE KENNET VALLEY AND ADJACENT AREAS. 10TH - 19TH JUNE 1O71

METEOROLOGICAL CONDITIONS

' depression centred south of Pembrokeshire at iidnight on TuesdaY, 8th June, 1971,- Moved soUth and later 'south east/ deepening -slowly'. :L.There wae rain over yariOus parts of central 'and southern England and Wales associated with this system . . _ 2 . .;; . A depreasion Centred over the north east of 'France'et midnight on Y . Yednesday: 9th June, 1971* iOved:at'Afiiat 'elmost dUe north;tthen north 'We et2; and !west .north West across the Thames'EstUary -and central Southern TEngland ,:Y: t o'.South Walea:* This :depre-Ssi on :reached its lowest Pressure at 'abOUtr 0600: hOnra 'on .FridaY 1 1th .JUne 'j?-"Whenteentred'OVer the Thamee CatehMent . Moderate _ _:to iaeavY amOunta of 2 rainfall oCCUrred Over the whole of: the •period to 11th-Sune,'details of whiCh-ere given in the Appendix A to '-this

3 The heaviest falls'Occurred On the-soUth'WesternYfringes of:the catehmeni affecting ParticUlirlY :the..Middle Kennet valley from Hungerford , ••, to Reading and :the Rivers 'Pang,'Enboriie end Leddon,"and to a lesser extent -' fl,..the.:Upper:Riv'er Thames and: Rivera Whi tewåter'and 'BlackWater.»i: •

'4. ' The'heaViest falls "Vdenrred as:follows , which should be cOmpared. . 'with the 'standard average f or the-month of 1.79 inches. : ' ' t

. 1* '24 lour Period ::total-to 0900 hoUrs #on: "-:TOTAL I:=P7-7- st:Days -:Ti'; - 9th June 10th. June l 11th June ins . ins. Inkpen 1'73 H. 0.55 3.26 - Hungerford :tr1.18 't 0:40 2.63 4.21 ' Upper Basildon' 2.50 _ Lockeridge - ' 2.64 3.69 ->

Rexford 0.78 0.41 -1 2.18

- r . , • FLOW CONDITIONS 'v., ; n ..1

rainfail S .of Wednesday, Sth JUne, though heavy in .soMe ,iselated 7 aread,' were .not :exceptional in themselves, 'and falling on a relatively dry catchment, 'appeared 'unlikely ,to give rise to flooding. Further:light,;to • ..modernte 'falls 'on Thirråddy did, hoWever, produee instanCes of :loc'al n eeding. _ . _ . . _ • • 2130 hours on -Thur sdayr thel:Flood Duty Officer -received a heavy rainfall_warning.that had been issued by the Meteorological Office for the fell:Owing '12-hpers " This related to the passage of the second depression previdualy . referred to. During the night, the available Autodiallers were “ interrogated'-and information on rainfall was obtained from those rainfall stations which were manned. Between 0530 and 0700 hours, visual inspection ' Was made of the River Blackwater, where the Autodialler had failed at 0500, and of the Cove Brook and north River Wey s t Farnham. Levels, .though high, Were not attaining bank full conditions exeept in the lower RiVer Blackwater downstream of Yateley, and those in the River Wey did not seem likely to do so .

7 . Divisional Engineers had previously been informed of conditions and first reports of flooding from the River Kennet reached the Reading Divisional Engineer at 0530 ;- Weir running was _commenced., ..continuing throughout day,the the last report-being reCeivedt - Y .the Flood bUty Officer at 2330 .

8 . During Friday morning, ihe telephone lines out of my headquar ters were affected by rain and at one time, only one direct line to.the a bed duty room was available. Moro seriously, the land line link to the radio ' transmitter was out of action until after 1700 hours and again for intermittent periods on tha following day. -: As a result, the greatest difficulty was . experienced in .obtaining information . :

FLOOD WARNINGS

9 . ....L:...Nevertheless, with the heavy rain persisting through the morning, • it was considered.advisable to .issue an AMBER warning for the River Kennet from NewburY to .Reading at-1310 hours on Friday. :: As further reports of rising :: levels were obtained with continuing rain, a RED warning was issued 'for:the • • same.area at ; 16 43 hours.-

-RIVE R' TBAMES

10. . The .earlY period pf.rainfall had.no significant effect :on Thames leVeis, Iflainly as a_result .pf Friday's:rainfall,levels rose rapidly during thai day at ,St. Johns ,:.contin uing at a high level until Sunday715 . .Bome flooding of riVerside land occurred in these upper reaehes ,.hut.danger leVels were not approached at OxfOrd . ...In the middle river, levels rose below Caversham from Friday to Sunday as a result of flood flows from the Kennet •and Loddon . Urgent action Was necesSary on Friday t o move Thames Oonservancy craft moored upstream of Marsh weir in connection with the reconstruction:of HOrie-Tr idg6 .---11thOUgh-flOW :Weid -hi gh fOr the .“ .me Of . Year 'and it *as -necessary :for the Chief •Navigation InsPector to islUe. a warning of navigation hazard,_levels again dranot reach excessive he i gh t s and no flood warning was cOnaidered necessary.- _ • • KENNET 'VALLEY

11. , '.Throughout Friday, clearance of debris .from sluices was carried out both by British Waterways and by the Reading Divisional Engineer's workmen, in response -to the many requests al t hough in some cases, only partial :clearance could be .effected . This work continued in the area Padworth to Reading until • 2300 hours and was resumed early en Saturday and again on Sunday.

0015 ' hoUr a - 6ii -Sa t iir da YY I t ' wa 's reported that flooding was assuming serious proportions at Burghfield Bridge, where ovorspill was threatening a caravan site, a smallholding and sane propertios The Brigade wore in attendance throughout thc night and during the following day, • 'reinforeing the briilk with Sandbagsandputping out the low lying area into 'an adjaeent ballaat pit . My :Deputy visited this area at 0745 hours, having inspected 'the RiVer Pang, where fl owa Were falling slightly, end 'the Sulham IBreOk . He-centinued visual insPectiOn in 'the Kennet Valley te'lldermaston.

r . • 3 • / / RIVER ENBORNE

13 . With rainfall south of the River Enborne amounting to over 5 inches in the three days, extremely high flows were recorded . This is a relatively steep impervious catchment and high flow velocities were generated . This has resulted in many shoals being formed , one in particular of Some hundred cubic yards of gravel being left on the lawn of a riverside property. Undoub tedly this catchment was a major contributor to the more serious flooding of the Kennet valley downstream of Brimpton .

• FLOOD RECONNAISSANCE

14. It was quite clear at this stage that the volume of the flood wa s higher than any previously recorded , and that it weuld be necessary to Obtain as much information as possible on its peak levels, the points where overspill had occurred and the extent of the flooded areas . • 15 . The River Kennet is a complex system , particularly in its lower reaches . Between Runderford and Reading it is partly canalised , and there are numerdus controls, hoth for the navigation and at the many mills . For long lengths the main channels are -eibanked and overspilling occurs . The flood plain is broad and access to the watercourses in times of flood is virtually impossible in mlny areas. For this reason, it was considered advi Sable to carry out an aerial reconnaissance, and authority was obtained . from the Chairman of the -Navigation and Work s Committee to hire a helicopter to enable this to be done . '1.our approval to this action is requested .

• 16 . Arrangements were made with the Thames Valley Constabu lary for < landing facilities at the Sulhampstead Police Training College and the _flight . " commenced at 1200 hours g lasting approximately 1 hour 25 minutes. / 17 . Two members of ,my staff .carried out the survey , one taking photographs and the other plotting the flood plain on large scale maps, noting • points of oVerspill and the pa ths taken by flood waters. Altitude 'varied between 100 and 500 fcct above ground level and most of the work was carried out at an air speed of 40 knots . The area covered eitended from cast:of Thatcham on the River Kenn et to south west of Reading on the River Loddon .

18 . - The information so gained was invaluable as a basis for more • detailed ground survey which was carried out during the following week . The results of thi s survey , together with a selection of the photographs , will bo 'exhibited at your meeting today .

FLOOD PEAK .P.ND RECESSION

• 19. ; The peak of the Kennet flood occurred at Theale gauging station at 0230 hours on Saturday, 12th June . Flow through thc weir is calculated to have bcen 2 ,000 cusocs. In addition , considarable flows by-passed the weir which have been subsequently estimated as approaching 1 ,000 cusecs, thus .making the total flow of 3 ,000 cusccs the highest recorded. .Levels did no t ?al/ appreciably for some hours , and were continuing to rise in the Southcote area at .0700 hours. Levels began to fall slightly during Saturday afternoon .at S uth cote and Fobney , although some local rises . were recorded -as weirs were cleared uof further obstructions, releasing large volumes of wate7stored in the floed Pla in . Following a more steady fall overnight and the absence of .. appraciable rainfall , the RED warning reverted to AMBER at 1130 on Sunday , 13th June an d to a YELLOW warning at 1015 on Wednesday, 16th June.

OTHER FLOOD OCCURRENCES

20 . During the period described above , extensive flooding ie the flood plains of the lower River Blackwater and the middle or lower River Loddon alså interesting sidelight of the aerial survey was the inspection of • occurred . An tho 11.4mo torway bridge crossing of the River Loddon flood plain ncar Sindlesham . - 4 • This showed the adequacy of tho provision made both for the permanent and temporary works and thc suitability of the realignment of the river channels at this point.

21. There were many reports of local flooding , usually caused by tho • inability of flood water to reach the watercourses.

22 . Thc receipt of flood reports from Local Authorities detailing the number of properties flooded is not yet complete, but will form an Appendix B . to this report. To date it is known that no abnormal flooding occurred in the Marlborough area ,.and no houses were flooded in Newbury. •

23 . Further h'eavy idinfalls ocenired on the 14th:-15th June and again on 19th June, particUla ; I Y .a 4 8ting tha River Molc catchment. Although unusually high leveissWere'reached .af :PidleO ridge on each occasion and AMBER warnings were issued 'fOr the:River:Kore a lt ;1.the. attenuation was remarkably quick .and •ne floOdinC ensUed . BeCaUW OC the2iiriiiiiiallY 2 • heavy rainfall for the month;:n T ELLOW .warning_was'i ssUed.for .:the the Thames Catchment . . • . • •" - ' CONCLUSIONS •

The basic cause e f the flooding in the:RiVer :rennOtl.was.;Undelib tedly the unusually hcavy .rainfall spread over the'three days, comorisipe the“ initial wettirg up period and the final..dzy 's rain .producing a high 'pereentegel: of run-Off . Weed growth does not apPear to have been unUsually year , but undoubtedly this had somo effect in raising levels.

25 . The operation of British Waterways ' sluices was entirely satisfac- tory,while the operation of those in private:ownership was reasonably so. In the latter Cases, considerable aSsistance in operation and subsequent clearance was given both by British Waterways and -your Divisional staff . Trouble .was, however, experienced with the removal of lasher boards which it is customary to leave in position during the summer months and the customary design bf which does not lend itself to easy.removal, particularly under flood e onditions. I'must place on record 'that the a ssistance rendered by British .Waterways staff, not only in operation 1)atin liaison and providing information, was extremely -valuable and I should particularly like to mention my appreciation of the services of Inspector Rogers. 26. , it has not been possible at this stage to estimate the return period k -I of this flood but its peak discharge is known to be 5Q% greater than that in 1947 .

27 . The large volume of flood plain storage evident 'from the plans and photographs rail:at have had a significant effect in reducing peak flows ,. particularly downstream of Theale through Reading.

28 . Bearing in ,mind ,the,:riclatively few properties that were flooded, it is questionable whether dny eitehsive improvements arc likely to be required. Nevertheless , there arc certain critical points which must be examined in detail and the results of my investigations will be reported to you in due course . The minimnl flood damage is due in no small measure to the absence of any significant development inthe flood plain,-and-the plans now produced will be of tho greatest value in supporting your established policies in regard to development in the future.

29 . Two further matters arise. Thu failure of the land lines at la critical stage suggests the advisability of establishing .a .trapsmitting aerial on the roof of the new office building and the feasibility of this will be considered.

• - 5 I -

30 . .Sccondly, the use of a helicopter to obtain flood information was invaluable. The amount of detail that could be seen wa s quite remarkable, to the extent that obstructions could easily be pinpointed and in many cases , the position of sluices could be checked . this was quite apart from the prime purpose of delineating flooded areas and locating sources of trouble .

31. I an looking further into the possibilities of this type of survey , • which has already been adopted by thc Mersey and Weaver River Authority, and will bc prdscnting a further detailed report en all aspects.

am, Gentlemen, s oUr obedient servant,

E . J. BRETTELL

Chief Engineer

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A o•P p ^ • 1 vt' .•••••••, _so p *1/2 , • p• _ • • • ai " t a I .. . .a A A . • APPEN DIX C HYDROLOGY

C.1 Note on the Enborne at Brimpton flow record

C.2 Cal culation s for 50 year design flood at Theale using rainfall runoff method

C.3 Calculations for 50 year design flood for the Foudry at M.4 using rainfall runoff method

• C.4 Compo site inflow hydrograph and floo d routing for 50 year even t

C.5 Composite inflow and flood routing for simulated

• 1971 event .

C .6 Composite inflow hydrograph s and flood routing of 10 , 25 , and 100 year events.

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• A P P E N D IX .1 A n o te o n th e En bO r n e a t B r im pt on f lo w r e co r d

• T h e E n bor n e a t e r im pt o n ga u g in g st at io n is a c om po und C r um p w e ir ,

o r ig in a l ly de s ig n e d t o r e c o r d u p t o 18 c um e c s in th e m o d u la r r a n ge .

Th e re is n o c r e st ta p p in g .' Rec o r d s fr om t h e st a t ior; in d ic at e s e v era l • pr o b le m s . F ir s t ly , th e m o d u la r l im it is s u s pec t e d to be m u c h lo w e r

th a n 18 c um e c s , pe r h a ps a r o un d 12 c um e c s . (P r es um a b ly in r e a lit y t he

lim it is d e pen d e n t o n c on c urr e n t w a te r le v e ls do wn str e am in th e • Ke n n e t . ) T h e g a u g in g r e co r d s h o w s so m e es t im at e d cor r e c t io n s fo r no n -

mo d u lar f lo w s 'b ut . t h e s e a r e s po r ad ic ; T he r e a re r e fe r e n c e s in s t at io n

r e po rts M o bac kW at e r pr o b le m s fr om t h e Ke n ne t a n d to by pa s s in g f r om • u ps t r eam .' F r om O S m a p s it a ppe a r s ‘th a t . th e s h ar p RH be n d a fe w 10 0

m e t r e s u pst r eam o f t h e st a t io n is 'urina t ur a l: W it h in t h e t e r m s o f the

s t ud y it wa s im pr a c t ic a l t o m a ke a s ur v ey o f th e s tre a m o r t o • r e se a rch th e .gaügii,gr e c o r d in gr e at d e ta i l.

A t th e o u t se t o f th e s t udy , Tha m e s W a t e r s u pp l ie d a r e v is e d r a t in g • c ur v e fo r th e st a t io n . T h e e f fe c t o f t h is is to r educ e r a d ic a l ly

f lo w s abo v e 12 c um e c s . F o r e xam p le , th e f lo w fo r a le v e l o f 1.5 m is

r e d uc e d f r om 27 .5 c u m e c s t o 16 .8 c um e c s . T h e re d uc t io n is be l ie v e d t o • be ba s e d o n t w o g a ug in gs c a r r ie d o ut in N o v em be r:' 197 4 a nd in 19 7 7 .

'St a nd a r d Un it h y d r o gr a ph a n a ly s e s pr e s e rrted :f6 r* th e st aM io n in - • A p Oen d ix A O f I H Re po r t N ° 9 4 'in d ic a t e .a s t arklar d perc e nt a ge r un o f f

(SP R ) o f a bo ut 2 5 % . T h i. s is a lr e a d y r a t her...lowe r t han .th e v a lue . e xp ec t e d .fr om s o i l m a p p in g . A do pt io n p f th & r ev is e d .ra:tin g w o u ld

f ur th e r m a gn ify th e d is c re pa ncy . • , ,

•;± .;;;;;EF"s•. to.:74.gr r Attnc f. W h e n t h e d if f ic u lt ie s w it h th e E n bo r n e r ecord w e r e r e a l ize d it w a s

d e c id e d to a ba n d o n it s a n a ly s is an d t o e xa m in e in s t e ad t h e K e n n et a t

T h e a le r e c o r d . (S e e S e c t io n 3 .3 o f r e po r t .) T h e K e n n et a t T h e a le

r e c o r d is m o r e r e le v a n t t o th e f lo o d p la in un d e r s t udy . M o r e o v er it

h a s h e a d , c r e st a n d t a i l ta p p in gs , pr o v id ing h y d r o m e tr y o f a h igh e r

s t a n d ar d t h a n a t B r im pt o n .

2 • , .APP 1 1-DI X C . 2 _ ..•- Ca lculations for 50 year design flood at Theale using rain fall - runo ff method

******************************************************************************** U K DE S IGN FLoop E ST IM AT IO N P AC K A GE Irist itu te O f HydrO lo g y **************C-****************************************************************

R u n refe r enc e - th e a l

C a tch m en t ch aracte r is t ic s

• Ar e a 2 20 s q km S o i l 1 0 L e n gth 35 .5 km S o i l 2 0 S lo pe 2 .27 m /km S o il 7 0 .4 S A AR 77 5 MO So i l 4 0 .6 M .5-2 D 53 .5 m m S o i l 5 0 • Je n k in s on 's r 0 .36 U r b an 6 e-0 2 Sm db ar 12 m m R SM D 2 7 .46 mm

• U n it h y dro ar aph o p t io n 9

U n it h yd rog r aph t im e to p e a k 2 4 .00 Da ta in te rv a l De s io n s tor m dur at io n 45 h ou r s

R e tur n Per io d for de s ign f lo od 5 0 .0 y e ars r e qu ir e s ra infa ll ev en t d ep t h o f 8 1.0 ye ar s

M 5 -2d ay = 5 3 .5 mm M 5 - 45 .0 h our /M 5-2 d a y = 1.0 4 M 5- 45 .0 h our = 55 .7 mm M T /M 5 = 1 .70 M 8 1.0 - 45 .0h o ur = 9 4 .7 m m (p o int ) A R F = 0 .9 5 M 8 1.0 - 45 .0h o ur = 8 9 .6 m m (a r e a )

R a inf a ll pr o f ile o p t io n 4

De s ign Sto r m dep th 89 .6 m m

De s in n CW I 117 .5

P er ce n tag e ru nof f 4 7 .5 % ( PR op t ion

R es po n s e h yd roar eph pe a k 66 .2 3 cu m e cs B as e f lo w 9.6C1q7.-r&4 cum ecs ( Ba sef low op t io n 6

0 D e s ia n h yd ro gr ap h pe ak -5 -SS 7 .0: cu m e cs

*************************0 *****************************************************

0 • • •

• _im p 19 0 41).9n • O deld 'lea u ld AM • • Pp e n se ll ) • -

• • goewn, re - nt po a ly uO isep p ) >fetid • :

• .:1 • s po w n a 09- 91•4 99 1, mo / po e r a '44,JJ • s ap p n o Ev 99 5 ; 4de Jno apA 4 o cup dso a ID lood • • 9 0/-40 • 7 0.0pun ./ efm lup oJo d •

' D Di-Nshi Ceir l l V c•-.cn 4.y ..A446 -a r c , 4 61, Ae Cla n a 9d I r -C/ A I - 4 Zi 'l l 10 41 4 1 2 t r 01 191( 1011 VO'D 611 0 001 : i 41 1 01-61 111 1. 19 11 11•41 I r l t 9 1 / 1011 et I 1 er cc If l t t l 6t OFIt 11•St O l t 0 1 1 Eil l 01 01 9 9 11'91, . •911 11194 • 4 ?WEI 011 1 011 1 ' 0010 99 t. 951 1 011 1 091 1 9 1 1 091 1 091 1 011 1 091 1 09 0 01 9 01 9 011 1 091 1 OM 091 1 011 1 091 1 011 1 011 1 01 0 091 1 01'9 01•11 0 1 1 011 1 091 1 if i n In 101 0 1 4 1 9 1 A l 4 1 9 1 4 1 / 01 4 1 4 1 4 1 W O n i U l 4 1 4 1 4 1 4 1 4 1 4 1 4 1 111 9 1 4 1 K l 9 1 9 .0 01 0 In 4 1 4 1 4 1 4 1 4 1 4 1 4 1 4 1 4 1 U l 4 1 4 1 4 1 4 1 4 1 U V 4 1 4 1 4 1 4 1 4 1 4 1 Q1 4 / 4 1 0 1 4 1 4 1 o l cost n g u l w o 4 1 4 1 4 1 K 1 9 1 I l l 9 1 0 1 9 1 9 1 9 1 0 1 011 9 1 9 1 0 1 a l u l a l n 1 4 1 4 1 4 1 a l 4 1 4 1 U l I II 4 1 4 1 4 1 0 1 U l 9 1 H 1 W C n 1 a l ell 0 1 911 0 1 0 1 4 1 U l a 's u l ti % 011 g 4 W O 111 1 1/ 1 1 9 1 1 0 4 4 1 0 1 9 1 H I 011 w o cc i a l in ir k 4 1 0 1 0 1 4 1 4 1 1 W it U l t W O W 0 9 1 1 4 1 4 1 U I 0/ I Il l 4 1 DI n 1 9 1 0 1 0 1 4 1 U l a I u l u l El al 1111 U l l n T al l ul 4 1 0 1 4 1 4 1 4 1 a 1 4 1 011 a l u l 0 1 A l 4 1 0 • U l 4 1 4 1 D I 4 1 a l 4 1 4 1 W O 0 1 4 1 4 1 4 1 4 1 4 1 4 1 4 1 H I 4 1 U l 4 1 4 1 4 1 U l a l sil u l Ir a 4 1 4 1 4 1 4 1 0 1 4 1 9 1 4 1 a l 4 1 4 1 MI a l W I 9 1 0 1 9 • / 21 . 4 11 : 4 1 .0 1 0 1 9 1 W O W O 4 1 4 1 4 1 4 1 MI 4 1 4 1 A l 4 1 9 1 9 1 4 1 9 1 9 1 111 0 1 4 1 E l 991 4 1 4 1 0 1 U 1 4 1 9 1 0 1 4 1 0 1 4 1 4 1 4 1 0 1 011 4 1 a l . Ill " : , a l • 0 4 al ul ovi 4 1 9 1 4 1 9 1 4 1 4 1 ‘ j / r 0 • - ' .9 111 ol Ili ovo w o 4 1 4 1 WO 9 1 0 1 611 4 1 , - (wo ) - .. : , 0 1 4 1 4 1 4 1 4 1 4 1 4 1 • 0 113 111 :.0 11V 4 1 9 1 P A 9 1 H I 4 1 9 1 0 1 011 4 1 9 1 4 1 0 1 : (s pa wn p ) pde ./6o ./pA p l tun _m o p ww0 /

a l GUI m0 [ 1 4 10 /14 0 3

00 0 13 N9 1S30 An A bn Io JpAp 0/1 30IJN3Pd NO ILVW ILS 3

:t:" /AP PENDI X C . 3

C alcul ation s for,50 ye ar design flood for the- Fo udry at 144 using rain fa ll- runo ff method . ** *** * ** ** * * ** ** * *** * * **** ** ** ** ** *** *** * * *** ** * ** *** * ****** ** * ** * * *** ** **** * * ** U K DE S IG N FLO O D ES T IM A T ION P ACK AGE In s t itu te o f .H yd r o lo g y * * * 41* * * * * * * * * * * * * * * * * * * * t * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

Run r efer e n c e - f o u d l

C a tc h m e n t c h a r a c te r is t ic s

• A r e a 6 8 s q km S o i l 1 0 .3 4 L e n g th 2 0 .4 km S o i l 2 0 S lo p e _-, . .7%-,,_ . S o i l 3 0 .2m 3/ 5km S A A R 6 8 5 m m S o i l 4 0 .4 2 5 M 5 - 2 0 4 8 m m S o i l ,J 0 J e n k in s on 's r 0 .4 • U r b a n 8 e -0 2 S m d b a r 12 m m R S M D 2 4 .8 8 m m

U n it h y d r o g r a p h o p t io n

U n i t h y d r o a r a p h t im e t o p e a k 1 1 .2 3 D a t a in t e r v a l D e s ia n s t o r m d u r a t io n 19 h o u r s

R e t u r n P e r io d fo r d e s ia n f lo o d 5 0 .0 y e a r s r e q u ir e s r a in f a l l e v e n t d e p t h o f 8 1 .0 y e a r s

M 5 - 2 d a y = 4 8 m m M 5 - 19 .0 h o u r / M 5 - 2 d a y = M 5 - 19 .0 h o u r = 40 .7 m m M T /M 5 = 1 .8 0 M .8 1 .0 - 19 .0 h o u r = 7 3 .3 m m (p o in t ) AR F = 0 .9 5 M 8 1 .0 - 19 .0 h o u r = 6 9 .3 m m (ar e a/

R a in f a l l p r o f i le o p t io n 4

D e s ig n s t o r m d e p th . 6 9 .3 m m

D e s ig n C W I 10 0 .8

P e r c e n ta g e r u n o ff 3 1 .7 % ( PR o p t io n 1 )

R e s p o n s e h y d r o g r a ph p e a k 2 3 .2 7 c u n e c s B a s e f lo w 0 .8 6 c u m e c s ( B a s e f lo w o p t ion

D e s ig n h y d r o gr ap h p e a k 2 4 . 13 c u m e c s

* * * * * * t 4 .* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ,

0 I.

In s t it u te u f Hydro lo g y U K DE S IGN F LOO D E ST IM AT ION PACK AG E

CON VO LU T IO N TA DL E

10 mm 1 ho ur u n t hy drogr aph (cumea s ) 'LC 11;

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• • • • a - -APPEN DIX C 4 : 50 YE AR EVENT

• C om po si te in f low hy dr ograph an d flood rout ing fo r 50 y ear eve nt .

ke n s im 0 1 (• • SUBcaTCHMENT

N E T RA IN S : 0 . 0 4 7 0 . 0 4 7 0 . 0 4 7 0 . 0 6 9 0 . 0 7 1 0 . 0 7 1 0 . 0 7 7 0 . 0 9 1 0 . 12 6 0 . 1 43 0 . 17 4 0 . 2 6 8 0 . 3 0 6 0 . 3 33 0 . 3 7 2 0 . 3 9 3 0 . 3 7 2 0 . 3 3 3 O. 3 0 6 0 . 2 6 8 0 . 17 4 O. 14 8 0 . 12 6 O. 0 9 1 0 . 0 7 7 0 . 0 7 1 0 . 0 7 1 0 . 0 6 9 0 . 0 4 7 0 . 0 4 7 0 . 0 4 7

CT UH O RD S . 0 .0 0 0 O . 00 0 0 .0 0 0 3 . 5 2 1 5 . 0 4 2 7 . 5 6 3 10 . 0 6 3 12 . 6 0 4 15 . 12 5 17 . 6 4 6 2 0 . 16 7 18 . 5 14 16 . 8 6 1 15 . 2 0 6 13 . 5 5 6 1 1 . 9 0 3 10 . 2 5 0 8 . 5 9 8 6 . 9 4 3 5 . 2 9 2 3 . 6 3 9 1 . 9 8 7 0 . 3 3 4

SUBCATCHNENT 2

NE T RA IN S : 0 . 0 3 7 0 . 0 3 7 0 . 0 3 7 0 . 0 5 4 0 . 0 5 5 0 . 0 55 0 . o 6 0 0 . 0 7 1 0 . 0 9 8 0 . 1 16 0 . 13 6 0 . 2 0 9 0 . 2 3 9 0 . 2 60 0 . 2 8 9 0 . 3 0 7 0 . 2 8 9 0 . 2 6 0 0 . 2 2 9 0 . 2 0 9 0 . 13 6 0 . 1 16 0 . 0 9 8 0 . 0 7 1 CT 0 . 0 6 0 0 . 0 5 5 0 . 0 5 5 0 . 0 5 4 0 . 0 3 7 0 . 0 37 0 . 0 3 7

UN ORD S . 3 . 0 0 1 6 . 0 0 1 9 . 0 0 2 12 . 0 0 2 10 . 4 16 8 . 449 6 . 4 8 1 4 . 5 14 2 . 5 4 7 6. 5 8 0

SUSCATCHMENT 3

N E T RA INS : 0 . 0 3 5 0 . 0 3 5 0 . 0 3 5 0 . 0 5 1 0 . 0 5 2 0 . 05 2 0 . 0 5 7 0 . 0 6 7 0 . 0 9 3 0 . 10 9 0 . 12 9 0 . 19 8 0 . 2 2 6 0 . 2 46 0 . 2 7 4 0 . 2 9 1 SC 0 . 2 7 4 0 . 2 4 6 0 . 2 2 6 0 . 19 8 0 . 12 9 0 . 10 9 0 . 0 9 3 0 . 0 E7 0 . 0 5 7 0 . 0 5 2 0 . 0 5 2 0 . 0 5 1 0 . 0 3 5 0 . 0 35 0 . 0 3 5

U H O RD S . 4 . 6 17 9 . 2 3 5 9 . 6 9 8 6 . 6 7 1 3 . 6 4 4 0 . 6 17

K EN S IM F OR 10 3 . 0 wm, 9 3 . 0 - h o ur S TO RM

T IM E RA IN S U B C A T C H M E N T F L OW S TO T A L • hr rom/ h r c um e c s cu rne c s c u rne c s c um e c s (...- 0 .0 0 . 0 3 2 9 . 7 5 0 . 9 7 0 . 5 1 1 1. 2 3 3 . 0 0 . 0 3 2 9 . 7 5 1 . 19 0 . 8 3 1 1. 7 8 6 . 0 0 . 0 3 2 9 . 7 5 1. 5 2 1. 17 12 . 4 5 l 9 . 0 0 . 0 4 7 9 . 8 7 2 . 0 2 1 . 48 13 . 3 6 12 . 0 0 . 0 4 8 10 . 1 1 2 . 5 1 1 . 7 6 14 . 3 7 15 . 0 0 . 0 4 8 10 . 4 7 2 . 9 8 1. 9 5 15 . 3 9 • L . 18 . 0 0 . 0 5 3 1 1 . 0 0 3 . 4 5 ''.. . 0 9 16 . 5 4 2 1 . 0 0 . 0 6 2 1 1 . 7 1 3 . 8 7 2 . 2 5 17 . 8 3 2 4 . 0 0 . 0 8 6 12 . 6 0 4 . 3 1 c., . -k_ c, 19 . 4 4 2 7 . 0 0 . 10 1 13 . 6 8 4 . 8 3 2 . 9 7 2 1 . 4 13 3 0 . 0 0 . 1 19 15 . 0 0 5 . 5 1 3 . 5 4 2 4 . 0 5 3 3 . 0 0 . 18 3 16 . 4 2, ' 6 . 5 4 4 . 4 1 ' '2 7 . 3 8 3 6 . 0 0 . 2 0 9 I B . 0 4 7 . 8 8 5 . 5 8 3 1 . 5 0 3 9 . 0 0 . 2 2 7 19 . 8 9 : 9. 5 2 6 . 8 0 , : 3 6 . 2 2 4 2 . 0 0 . 2 5 3 2 2 . 14 1 1 . 4 8 7 . 9 3 4 1 . 5 5 (..- 4 5 . 0 0 . 2 6 9 2 4 . 8 5 13 . 4 6 8 . 9 2 4 7 . 2 2 4 8 . 0 . O. 2 5 3 _ _ 28 . 12 15 . 2 4 9 . 5 5 _ 52 . 9 0 •

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Composite inflow hydrograph and flood routing for simulated 1971 event .

C :\DWR\JOB\KENNET>tYkensim 1 I

SUB ATCHMENT

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K E N S IM - S IM U L A T IO N O F JU N E 19 7 1 F L O O D

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U H O RD S . O . 00 0 0 .0 0 0 0 . 00 0 2 . 5 2 1 5 . 642 7 . 5 6 3 1 0 . 0 8 3 1 2 . 6 0 4 1 5 . 1 2 5 1 7 . 6 4 5 20 . 16 7 1 8 . 5 1 4 1 6 . 8 6 1 15 . 2 0 8 1 3 . 5 5 6 1 1 . 9 0 3 • 1 0 . 2 5 0 8 . 5 9 8 6 . 9 4 5 5 . 2 9 2 3 . 6 3 9 1 . 9 8 7 0 . 3 3 4

SUBCATCHMENT

N E T RA IN S : 0 . 0 3 2 0 . Os a 0 . 0 3 2 O. 0 4 7 0 . 0 4 8 0 . 0 4 8 0 . 0 5 3 0 . 0 6 2 0 . 0 3 6, 0 . 1 0 1 0 . 1 1 8 O . 16 2 O . 2 0 8 0 . 2 2 6 0 . 2 5 8 0 . 2 6 3 • O . 2 5 2 O . 2 2 6 0 . 2 0 8 0 . 1 8 2 O . 1 13 0 . 10 1 0 . 0 8 6 0 . 0 6 2 O . 0 5 3 0 . 0 4 8 0 . 0 4 8 O . 0 4 7 0 . 0 3 2 O . 0 3 2 0 . 0 3 2

UH OR D S . 3 . 0 0 1 6 . 0 0 1 9 . 0 0 2 1 2 . 0 0 2 1 0 . 4 16 8 . 4 4 9 6 . 4 8 1 4 . 5 1 4 2 . 5 4 7 0 . 5 8 0 • SUDCATCHMENT

N E T RA IN S : O . 03 0 0 . 0 3 0 O . 0 3 0 0 . 0 4 4 O . 0 4 6 0 . 0 4 6 0 . 0 5 0 0 . 0 5 8 0 . 0 3 1 O . 0 9 5 O . 1 1 2 0 . 1 7 2 0 . 1 9 7 0 . 2 1 4 0 . 2 3 9 0 . 2 5 4 O . 2 3 9 0 . 2 1 4 O . 1 9 7 O. 1 7 2 O . 1 1 2 O. 0 9 5 0 . 0 8 1 0 . 0 5 3 0 . 0 5 C1 0 . 0 4 6 0 . 0 4 6 O . 0 4 4 O . 0 3 0 0 . 0 3 0 0 . 0 3 0 •

U H O RD S . 4 . 6 1 7 9 . 2 3 5 9 . 6 3 3 6 . E 7 1 3 . 6 4 4 0 . 6 1 7

•

K E N S IM F O R 9 2 . 2 him, 9 3 . 0- h o ln- S T O RM

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S U B C A T C H M E N T

N E T RA IN S : 0 . 0 5 4 O . 0 5 4 O . 0 5 4 0 .0 7 8 O . 0 8 0 0 .0 8 0 0 . 0 8 8 O . 10 3 0 . 14 3 0 . 16 8 O . 19 7 O . 3 0 4 O . 3 4 7 0 .3 7 7 0 .4 2 0 0 .4 4 .7 0 . 4 2 0 O . 3 7 7 O . 3 4 7 O . 3 0 4 O . 19 7 O . 16 8 O . 14 3 0 . 1 0 3 O . 0 6 8 0 .0 6 0 0 .0 8 0 0 .0 7 8 0 .0 5 4 0 .0 5 4 O . 0 5 4

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S U B C A T C H M E N T

N E T RA IN S : O . 0 4 2 0 . 0 4 2 0 .0 4 2 0 . 0 6 1 O . 0 6 3 0 .0 6 3 0 . 0 6 9 0 .0 6 0 0 . 1 12 O . 13 2 O . 15 5 O . 2 3 8 O . 2 7 1 0 .2 9 5 0 . 3 2 9 0 .3 5 0 0 . 3 2 9 0 .2 9 5 O . 2 7 1 O . 2 3 8 O . 15 5 O . 13 2 O . I 12 O . 0 6 0 0 , 0 6 9 O . 0 6 ,3 O . 0 6 3 0 .0 6 1 O . 0 4 2 0 . 0 4 2 0 .0 4 2

U H O RD S . 3 .0 0 1 6 .0 0.1 9 . 0 0 2 12 . 0 0 2 10 .4 16 8 . 44 9 6 . 4 8 1 4 .5 14 2 .5 4 7 0 .5 6 0

• S U B C P T C H M E N T 3

N E T RA IN S : 0 . 0 4 0 0 . 0 4 0 O . 0 4 0 0 . 0 5 8 O . 0 6 0 0 .06 0 O . 0 6 5 0 .0 7 6 O . 10 6 0 . 12 5 O . 14 G, 0 .2 2 5 O . 2 5 7 O . 2 8 0 O . 3 12 0 .3 3 1 0 . 3 12 0 .2 6 0 O . 2 5 7 O . 2 2 5 CI. 14 6 0 . 12 5 O . 10 6 O . 0 7 6 0 . 0 6 5 0 .0 6 0 O . 0 6 0 0 ,0 5 8 O . 0 4 0 0 .0 4 0 0 . 0 4 0 • U H O RD S . 4 . 6 17 9 .2 3 5 9 .6 9 8 6 . 6 7 1 3 . 6 4 4 0 .6 17

• K E N :3 FM F O R 1 14 .2 tom , 9 3 . .tr S TO RM

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P E A K S TO R A G E 5 . 93 3 Mm 3 PT 92 .4 h o ur s ATTENUATION 2 4 .0 % p 6 A K - P E A K L A 6 2 0 .9 h o ur s S TO P

• LEGEND -

. • LIMIT OF 1971 FLOOD -

SURVEY CROSS SECTIONS WITH ' LEVELS EVERY 100 m AND AT '- CHANGES OF SLOPE

CP

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GRAVELS CURRENTLY BEING EXTRACTED

SCALE 1 10 000

T H A M E S W A T E R K E N N E T V A L L E Y S T U D Y

YET - - _ _AND GROStl, SECT I ON S

-9 1 R- NLEXIXDR-1{- a n it ..... Re

A P R I L 1 1 1 7 - LONG SECTION OF 1971 FLOOD LEVELS ( A long t he r iver channel- leve ls in the f lood plain approximate l ine p in ing wei r t ai l wat er levels I

RIVER BED.

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T HA MES W A T ER K EN N ET V A L L EY ST UD Y

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_ _ Flood informat ion f rarri aerial and ground inspect ion -

_ Gravel pits -Andmat ive 0nly-

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_ Flood level . ( rn a.o.d.) M 1971.

Fl ood envelopes — —1 in 10 y ea r —1971 —I in 100 yea r

T H A M E S W A T E R

K E N N E T V A L L E Y ST U D Y

F L OOD E N V E L OP E S O produ ced from the O rdnance Su rve y 1 10 000 map s SU 67 551(1974) SU 67 S E LEGEND (1983) SU 77 SW (1979 ) SU 66 NW (1972) 50 66 NE (1976), SU 76 NW (1984 ( w ith ,P0 pe rmis sion o f the Con tro lle r o f He r Ma jesty s Stat ion ery O ffice crow n Co pyrigh t Reserved LAND WITH PLANNING PERMISSION FOR GRAVEL EXTRACTION AREAS NOT YET APPLIED FOR PLANNING APPLICATION MADE .i-,-dor- FOR GRAVEL EXTRACTION

AREA TO REMAIN AS LAKE

AREA TO BE RESTORED 70 LAIE FOBNEY WORKS EXISTING GRAVEL PIT

WELLMAN 'S FARM

NOTE • FARNHAM ALL LAKES IN THIS AREA HAVE BEEN ARC FLINT SILT LANT. CREATED ARTIFICIALLY FROM GRAVEL LAGIQON,. " THEALE PITS SINCE 1947 YWARD •;: FARM. .

KIRTON'S FARM — WOOLWICH GREE ENGLEFIELD - 'WEST LAKE

HOSE HIL FARM

T H A ME S W AT E R

ELD K E N N E T V A L L E Y S T U D Y G R A V E L E X T R A C T IO N A N D O PE N W A T E R A R E A S

• I R ALBI AN DI R 1311 1B & PA RT11111 11

APRIL 1 9 8 7 F I G 5 1 Rep rod uced from the Or d n a n c e Surve y 1 10 OCO maps SU 67 SW (1974), SU 67 SE LEGEND (198 7) , SU 77 SW (1979 ), SU 66 NW (1977) , SU 66 NE (1976 ), SU 76 NW (1984), with the perm iss ion of the Con tro ller of Her Ma ]esty 's Stat ion ery O ffic e LAND WITH PLANNING C row n Copyrig ht Res erv ed 161.5 - PERMISSION FOR GRAVn • \.e . EXTRACTION , - • •• c . • • • • Gss x ‘ x x x x x Theale x x x x x x FILLED TO ORIGINAL LEVEL INTAKE x x x x x x

FILLED 'ID ABOVE ORIGINAL LEVEL '

TO BE FILLED TO ORIGINAL LEVEL + + FOBNEY + 4- WORKS

+ + + + + + + + TO BE FILLED TO ABOVE + ORIGINAL LEVEL

-?-'+ + -I- -1- + 4- I Som./6 W N \. . h ow ......

/

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