REVIEW of HYDROLOGY and HYDRAULICS Binnie and Partners
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CORE Metadata, citation and similar papers at core.ac.uk Provided by NERC Open Research Archive EASTBOURNE BOROUGH COUNC IL EASTBOURNE PARE DISTR ICT PLAN - REV IEW OF HYDROLOGY AND HYDR AULICS MAY 1987 Binn ie and Partners in association with the Institu te of Hy dro logy PLAN REVIEW OF HYDROLOGY AND HYDRAULICS CONTENTS CHAPTER 1 TERMS OF REFERENCE 2 SUMMARY AND CONCLUS IONS Sch em e proposed in the EPDP report Con clusion s of rev iew study 3 DEFIN ITION OF THE STUDY AREA AND FLOO D PRO BLEM Th e Park Ca tchm en t Dra inage ou tlets The flood p rob lem R isk of flood ing 4 CATCHME NT INFORMAT ION Lan d use Geo logy / soils Top og raph ic surveys of the Park FSR catchment character istics 5 FLOOD EST IMATES Summ ary Catchment area Percen tage runo ff Allowance for storage effects Ra infa ll on the lak es In flows from the Bourne Stream catchment . • A;L:AkY 198 5 F LV A EVL Ji Av a ilab le water level,data Lang ney Bridge wa ter levels Assessmen t o f reservo ir lag time Estima te of effective ou tlet capac ity OUTLET CAPACITY In troduction W est Langney Sewer hydraulics Crumbles Sewer hydrau lics Improvemen ts to ou tlet capac ity HYDRAULICS Gen eral Pr incip les for normal control of lake levels Op erat ion o f the W illingdon Levels during floods Sh inew ater and West Langney Sectors Broadw ater and Southbourne Sectors S iltat ion and debr is problems CONSTRUCT ION OF THE AMENITY LASES Lak e d imensions and dr ift deposits Method of excavation Stab il ity and protec tion of the lak e marg ins , Seep ag es to an d from the lak es 10 WATER QUALITY AND ENV IRONMENTAL ISSUES Ava ilab le da ta Qua lity o f the stream flows Water qu ality in the tr ial pon ds an d boreh oles Effec ts o f urb an dra inage Lan dscap ing and tree growth Water qu ality requ irem en ts for the Park APPEND iGi : A H ISTOR ICAL REV IEW OF FLOODING Search of ra infall records An teceden t cond ition Flood repor ts 100 YEAR FLOOD CA LCULATIONS Des ign flood based on a 31 hour storm Design flood based on a 53 hou r sto rm REFERENCE S 41 41 41 1 . TERMS OF REFERENCE 41 • 1 .1 The following report con tains an appraisa l of the hydrolog ical assump tions and polic ies which are summar ised in a document 41 entitled 'Eas tbou rne Park District Plan - Policies and Proposals ' da ted • May 198 5 , hereafter referred to as the EPDP report (Refl). 1 .2 The original Terms of Reference as contained in your letter 41 to us dated 22 July 1986 stated tha t :- • "The following sub jec ts are to be checked by the consultan ts to form the bas is of a repor t :- 41 1 . The assumption s used in compiling the hydrolog ical 411 model . 2 . The estima te of the maximum flood storage required . 41 3 . The feasibility of the distribution of the flood • storage areas w ith in the Park in the form of lakes and 41 water meadow s . 4 . The suggested maintained normal water level in the 41 lakes and rivers in the four sec tors of the Park . • 5 . To confirm that all the hydro logical policies • incorpora ted in the Plan are sound and tha t they will together allev iate and prevent further flooding . 41 6 . That the 2m deep lakes can be phys ically constructed 41 at reasonable expense , that they w ill retain their 41 water at all times and will not silt up or the banks w ill not suffer from excessive eros ion . 41 7 . Tha t the lakes will be suitable for the water 41 recreational uses sugg ested . 41 8 . That the mound ing of the excavated material from the lakes and its distribution will have no adverse affect 41 on the lakes and water meadows . • 9 . The suitab ility of the excavated material for the • growing of trees . 41 ID 41 411 ID 10 . Sugg est way s in which the lakes can be c on n e c t e d to 40 the rivers and the method of control , especially those in the Sh inewater Sector . 11 . Con firm tha t the lakes in the Shinewater sector can be con structed in isolation to provide flood storage for • the Nor th Langney Area" . • • 1 .3 W ith our letter dated 11 August 1986 we submitted prop osals for meeting the Terms of Reference listed above . Our proposal was accepted by the council in ear ly October with the proviso that we were to carry out the hydro logical aspects of the study in collaboration with • the Institute of Hydro logy and wou ld produce an Agreed joint rep ort on • the hydrology of W illingdon Lev els . 1 .4 We had discussions with Dr . Reed of the Institu te of • Hydrology and con firmed in principle that we would work together to provide an agreed jo int view of the • s oun dn e s s of the hydro log ical assumptions and concepts under lying in the Eas tbourne Park D istrict 40 Plan . How ever the Institute of Hydrology felt th at to reach agreement • on the practica lity of the proposed flood alleviation sch eme it was necessary not jus t to check the previous hydrologica l calcu lation s but to carry out a more rigorous hydrological appraisal. • 1 .5 In your letter dated 10 October 1986 you agreed to the • app roach to the combined study by the Institute of Hydrology and ourselves which we se t out in our letter to you dated 9 October 198 6 . 41 1987-05-14• 12 :23 P INNIE a PARTNERS . 0737 7276? P .02 • FOR : Dr O. REI D ita r sr a re s ig n Yell o s-0 4Y 40 RE E ASTSO44ANE PARK REV'S" s r oOY ID SUMM ARY AND CONCLUSIONS Scheme propoaed in the FPDP report • 2 .1 Th e Eas tbourne Park D istrict P lan (FPDP ) proposes - th e developmen t of the marshland G o r e of the town to prov ide a new area o f recrea tional open space . The dev elopm en t o f the m ershian d core , wh ich 411 is known as W illing:ion Leve ls , is to be ach iev ed by stor ing that portion or the catchment flood ru no ff wh ich is in excess o f the outlet capac ity 10 o f the system in a ser ies of amen ity IRkes . 4I 2 .2 Thc FFDP sets the f lced standar d tha t the iraxir u v. wa ter 40 love) in Park area m u s t n o t exceed 2 .0 r OD in a 100 yea r flood . In the 411 2PP? report , wh leh pro yides de tails of the p roposed deve l men t lt is cons idered tha t- th is s ratidord can be ach ieved b y th e pro v is ion o f 72C M I 411 o f oterage . Th io f in d ing s t e m s from assump tionsi a lco la tions tha t :- ID 11) the crit ical 100 year flood rune ff resu lts from a s to rm o f 9 10 hon r5 duration ; and , ID (,)) the critical Ir?fl: y e a r flood 'runeff ro illingdon Levels ID ID or y Io n o n e • (het '.y c i r . ;:u c i6ch arg e at a r a t e of 7 .7 :' 3 / F, h o u r l o w tide 411 1 1' per iod . Thorp would be no discharge (-b r in g th e rema inder of ID the tide C y c le . 411 4111 Conclusions of review study ID 2 .3 in princ iple , th e concep t of re& lc:ng; maximum flood leve ln ID on W illingdon Levels by rc oyiding add itional storage and/or ou tlet capacity is sound . However the v iability of a sobers such n5 the one proposed the EFDP report is critically dep en dent on the ID Interrela tionships between flo-)d runoff , ou tlet capac ity And riaxiirom storaga requiremen t during the design even t . As the effeetiyo ou tle t ID capacity d e C r e a N e s , .b h e design eturm durat Iot . th e flood runoff volum e ard the m a x im w n storage requ irement increase . Th e h y dr o lo g y and 0 ) 1987-05-14 12:24 B INNIE & PARTNERS . 0737 7276? P.ue hydraulie caleula tions (see hef. 2 ), which under lie the development scheme set out in the EBDP report, underest imate the critical 100 year design storm dora cion and assoc iated flood runoff vo lume . The net resu lt le that the max imum storage requirement to maintain maximum flood levels below 2 .0 m OD has also been significantly underestimated . 2 .4 The critica l storm duration and hence the shape of the critica l 100 year flood hydrograph for the proposed flood alleviation scheme is dependen t on the characteristics of the W illingdon Leve ls catchment, the net out:e t capacity and the flood storage vo lume .