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 . The
ava ilable data show tha t the critical storm duration , with a flood
alleviation scheme U 5 outlined in the EPDP report , is at least 31 hou rs and qu ite possib ly 53 hours •r longer . The storage needed to limit the maximum flood level to 2 .0 m OD dur ing the critica l 100 yen:. flood would be abou t :-
11/ 960 10 if the average effective outlet capacity over a tidal cycle were 7m1/ 6 1 and,
(2 ) 1h20 Mi. if th e avarage effective e utlet capacity over a tidal cyle were only .3 ml/s .
2 .5 Runoff to Willihgdon Levels dra P S t O the sea via West Langhey and Crumb les Sewers , The EPDP report e a/euleitiens ove res timate the effec tive outlet cap acIty of the West Lungney Sewer primarily because :-
• (1) the adop ted 1 in 1;7'0 yeae weeer ievels at Fence Bridge , where the sewer discharges into Pevensey Haven , ere too low ; and ,
l ‘ t ; o f inflows to the sewer • frOJI Mouneney Leec].
IT 2 .6 The effec ts of infl3w8 from Mountn ev Level and the likely p higher flood water levels i n Peven sey Haven probably serea to reduce the
average effec tive ou th ", c ap ac i ty of West lan toey Sew n' to beY»e‘ni 1 .0 l'iC ? -05 -14 12 : 7 5 B INN JE e.HHH I Pt t eo . Iff eS e Ye eb y H . 0 4
and 2 .0 e / s during critics] 100 year flood . A pre lim inary appr aieal of the capacity o f Crumb les Sewer revealed that it migh t discharge up to 1 .5m'/s from Will ingdon Levels during floods , Together these two sewers can probably prov ide an average outlet capacity over a tida l cycle of between about 2 .0 and 3 .5 m3 ,/s from the Willingdon Levels during the 1
A n n . . - - .... f l n n A
c . / With the existing o utlet capacity ther e is insufficient storage available in the lakes shown on the Proposals Map of the EPDP report g iven the proposed max imum flood m id normal reten tion levels .
Therefore to des ign a workable s c heme it will be necessary to cons ider :-
tae prov ision of add itional ou tlet capac ity ,
relaxing the design standards and allow ing higher maximum water levels on W illingdon Lovels or reducing the return period of the design flood ;
(3) adopting lower normal reten tion levels in the storage lakes ; and
(4 ) cons truc ting additional storage lakes .
2 .3 Although a comt nIna tion of mcd sures are 9 1) 1) the cons truc t ion of a land dra inag pump ing station and assoc iated main dra ins ; or (2) the provision of a new gravity oetlet to the sea near Langney Poin t (end possib)y via the proposed Crumb les M ar ina ) ; w ou ld appear to be the mos t viab le alterna tives . However , each would need fur ther de ta ile d s t lAdy to establi sh th eir feasib ility . 1987-05-14 12 :25 BINN IE 8 PARTNERS . 0737 72767 P .05 41 2 .9 The hydraulic struc tures required to convey flnwn through the Willingdon Levels will need to be relative ly large to minim ize head 41 losses and maximize the use o f the ava ilable s torage. There would need 41 to be a large channel to convey flood water across the Shinewater a nd • West Langney Sec tors . There would be little benefit in transferr ing flood wa ter from lakes in the Wes t Langney and Shinewater Sectors to the 41 lakes proposed for the 2roadw3ter and Southbourne Sectors in ::EPDP 41 iepwi-t . The wctira ane4 ou 'ver 'e ?ink ing tha me in rh one ls anti the lakes 41 and also interlink ing the differen t lakes will need to be care fully designed so that the lake system opera tes in the most effective manner . 41 S e c t i o n 8 con tains an ind ication of the types o f structures requ ired . 41 The des ign of tllese s truc turee cannot proceed until the o ver's ): 41 parameters defining the des ign of the lake sys tems have been satisfactor ily established . 41 41 2 .10 The lakes in the Shihewate r Sec tor cou ld no t be cons tructed 41 in Isolation to allev iate 'pr imary ' flooding of the surrounding built up 41 aren during majtn- floods . The lakes m ight poss ibly be used to a lleviate any second ary flooding in minor storms wh ich s tems from the existing 41 nigh water leve ls in the receiving channels of the Willingdon Levels . 41 However if the lakes were to be used in th is way , it i t; like ly that wa ter qua lity problems constrain the range of poss ible 41 wou l d rec rea tional uses . 41 5 2 .11 Sill:at:Loc. within th; lah2 cyet n- is nn i. tritely to be 41 significan t except in the immediate vicinity of the points where water -- a . . 41 might require dredg ing vory occasionally . Trash boo:re might also be requ ired around these areas tc con tain debris and poss ibl y o il slicks . ID if an add itiona : ou tlet W .Is provided into the proposed Crumb les Mari na , its effec t on silta tion sod wa ter qua lity in the Ma rina would need to be 41 assessed . How ever with careful design (and reasonable coop era tion 41 between the relevan t author:ties ) we do no t anticipa te that such an 41 ou tfall would cause any Insurmoun table probleme . 41 41 e t ) 41 41 41 41 13 6 ?-U5- 1.4 12 210 b INN I t 6 P IAK INt M5 . O rir 7 e rb? r Adb 0 0 2 .12 The limited load bearing capabili ty of the,g roui;d will. mean 41 that the proposed flood storage lakes will have to be excavated from 41 their rims and haulag e roa ds w ill need to be buil t . In sp ite of these 41 con stra ints , we see no reason why conventional methods cannot be adopted . Although earth mcv ing costa will be above m in inum rates we 41 would not expec t the overa ll excavation cos ts to b e unacceptable . 41 410 2 .13 The ava ilab le data suggest that the lakes w ill retain their water . Should 41 a n y :o c tal discon tinuities be revea led during excavation there is arp le m a ue r ia l available to construc t a clay blanket. However, 41 par ticular care will be needed with the design of the lake edges to 41 prevent :- 41 (1) erosion and s h imping ; and 41 41 (2) the water in the lakes becom ing ilisco loured by clay 41 particles . 41 2 .14 The ava ilab le data suggest that the landfill operations have 41 had iittle adverso effect on the qua Lity of ehe serfac.e and groundwa ter within ilillingdon Lev els . ::•htinued mon itor ing of the lendfill sites is 41 requ ieed , bu t even if traces of leachate ere de t e c t ed , r emed i a l action 41 to protect the propose& lakes wou ld be feasible et reasonaiqe coat. 41 41 2 .15 rho use o f the subso ils tc co nt tr t ISnd S.7ape m o un ds w il 1 no t ha ve a dethiwen tal affect on the water quolity of the przT o s e i lakes 41 prov ided the mound s a re =ni e g watt e : y dra ired . It w ilt lso he poss ible to 41 establish a w ide ran ge o f tree species on th e mou nde . 41 . 1 t, 41 the proposed la:ces hecause of their chellow dep th , long residence times 41 an d n u tr ien t lo a d in gs,. In most years wa ter qua lf.ty in the lakes will 41 deteriorate during the sunmer nonths and it wiii be necessary to in s ta n and rec ircula tion sys tem, Even w ith 41 an app ropriate mech an ica l aeratioa such a sys tem it is poss ible tha t storit runo ff from the urban areas wi l l 41 O n o cca s iO n v. oau u e sudden deter ioration in water qua lity . 41 k a b r . . 1 AL i t • Pit . - I W V s yes mea d 0 75 7 a 7 Pri ES 41 Nor d arm* elmerAA 7 41 D.E. hft 00W14a 41 t s 3 . DEF INIT ION OF THE STUDY AREA AND FLOO D PROBLEM Th e Par k 3 .1 Th e Eastbou rn e Park D istrict Plan bounda ry enc ompas.ses some 4 .9 km2 of largely agricu ltural lan d , mu ch of it acu tely low-lying . The typ ical field height in th e Park area - som e times referr ed to as the W illing don Levels - is abou t 2 mOD . For compar ison , th e Mean High Water Spring tide is about 3 .7 mOD . In this review study , the exten t of the Park has been taken from F igu re 5 of the EPDP repor t . The EPDP report d ivides the Park into four par ts: the Shinewa ter , Broadwater , Sou thbou rne an d West Langney Sectors (See Fig .3 .1a ). Catchm en t 3 .2 We have assessed the topograph ic area o f th e catchmen t dra ining to the Park as 27 .9km2 . 3 .3 Site insp ection of the nor th-west par t of th e catchment fa iled to confirm whether the downland immedia te ly so uth-west of Fo lking ton dra ins to the Cuckmere or to the Park ; th e latter has been assumed . Deta iled con tour information was no t av a ilable for Central Eastbourne and the e ffec tiv e sou thern boundary in f lood conditions is un clear . How ever , it is be lieved that much o f the se afron t area drains direc tly to the sea th rough sands and gravel . Wh ile th is ar ea has been exc luded , we have a llowed for a contribution fro m the neighbouring Bourne S tream ca tchm ent - wh ich is piped into the Pa rk dra inage system via the Horsey Sewer . This p ipe a lso receives some local storm water in times of intense ra infa ll . O ther storm water impor ts (eg , from the Old Town an d Down s ide dis tric ts ) have been ignored . 3 .4 Th e ab ov e find ings are in close ag reemen t w ith the ca tchmen t boun da ries shown in F igure 4 o f the EPDP report and th e areas quoted in Section 2 .12 . of the sam e report . How ever , the va lu e o f 27 .9km 2 is s ign ifican tly less than th e value of 33 .59W W used to calculate flood runoff to W illingdon Levels in the Hydro logy an d Hydrau lics calcu la tions set ou t in Append ix A o f a docum ent w e rece ived from y ou en titled "E as tb ourne Park Lan dscap e S tudies" (Ref .2 ). Dra inage oJtlets ID 3 .5 There are two drainag e outlets from the Park : • 1) West Langney Sew er wh ich is an arterial drain lead ing ID from Langney Br idge on the southeast marg in of the ID Park , to Fen ce Bridge some 4km to the eas t. At Fence ID Bridge the sew er discharges into Pevensey Haven which in turn discharges to the sea at the Pevens ey Bay ID outfalls , which are loca ted app roximate ly 1 .4km 111 downstream of Fence Bridge . 2 ) Crumbles Sewer which is a carr ier of smaller dimensions than West Langney Sewer . Crumb les Sewer leaves the Park at Crumbles Sluice (which is again on the sou th eas t marg in of the Park ) and flow s to Crumbles Pond in Princes Park , and thence to the sea . ID 3 .6 Flows in both the Crumbles and the West Langney Sewers are sub ject to tidelock . A de tailed discussion of the ou tlet capacities of the two sewers is con tained _in Section 7 . The flood prob lem 3 .7 To determ ine the nature of flooding in the Park we rev iewed II the available flood da ta . The review which is detailed in App end ix A revealed that there ar e two types of flood prob lem in the area :- 1) A 'primary ' flood problem which occu rs when high water levels throughout Willingdon Levels directly threaten widesprea d flood ing of property . Such flooding is generally the result of long dura tion s torms (or a sequence of storms ), as exemplified by the events of October 1949 , Nov ember 1960 and January 1986 . 2 ) Local sur face wa ter dra inage problems in the perl;:,ry of the Park wh ich arise from the limited sizes and grad ients of various storm water sewers . These prob lems , wh ich usually resu lt from 'short duration storms of high rainfall inten sity are linked to , and 40 exacerbated by , concurrent h igh wa ter levels in the ar terial dra inage system . • 3 .8 The flood allev iation proposals ou tlined in the EPDP report focus on attempting to prov ide a solution to the 'primary ' flood • prob lem . 40 • Risk of flooding 3 .9 In the section of EPDP report which presen ts the evidence of flood ing it states tha t :— 1 . "A flood up to a level of 2 .0 metres A .O.D. is not an unusua l occurrence on the lev els" ; 2 . "Floods o f up to 2 .3 metres A .O.D . appear to have occurred during the last 25 y ears" ; and , 3 . "Though the Sou thern Water Authority has no records of water en tering living accommoda tion . . . it is ack now ledged that under certain conditions such an event is possible , and properties w ith floor levels as low as 2 .9 metres A .O.D. cou ld be at risk unless steps are taken to prov ide adequate flood storage capacity". 3 .10 Our h istorical review of flooding in the Park (see App end ix A ) con firmed tha t , although low lying house gardens near the margins of Willingdon Lev els have been flooded , to da te there has been no widespread primary flooding of property . Th is is because historically • the available na tural flood plain storage has been sufficient to store any flood runoff to W ill ingdon Levels in exces s of the combined 40 capacities of Crumbles and West Langney Sewers . 3 .11 It shou ld be noted , however , that no even t approach ing the severity of a 100 year flood has occu rred in recent decades during wh ich time much of the catchment urbanisation has occurred . Each new • • ID development adds to the r isf of prim ary the volume of flood runoff to W ill ingdon Levels and in some cases also reducing the ava ilable natural flood plain storage . It is qu ite conceivab le that should a long duration 100 year flood occur , w ith the ex isting level of catchment urban isatiOn and current ou tlet arrangements, then widespread primary flooding would result. To confirm ID this statement it would be necessary to :— (1) Carry out a deta iled topographic survey of the whole of • Willingdon Levels and its margins to the standard of the recent 1 :500 scale survey of the Shinewa ter Sector . This • wou ld allow the natura l flood plain storage below selected • levels to be comp uted . 411 (2 ) Adopt the best estimate of the effect ive outlet cap acity • from Willingdon Levels (see Section 7). • (3 ) Compute the 100 year flood runoff for a range of differen t storm durations and to undertake trial routings to define 4111 the critical storm duration (see Section 5 .10) • (4 ) Route the 100 year flood hydrograph based on the cr itical storm duration through the Willingdon Levels to determine • the max imum flood leve l . ID 1111 ID 40 1110 40 • • • • • • • • S. ore • ,.. - %. ty . ,,4 -./ . . . . % .0. . C % S. A a iat osater . S. . . . • -/ I 7 S. t r e e / n u* B Southbourne . • • • 1/4 C West Lengney 1 x B i % % . D Shinewater % A % I • % % , o N I % e Cr um b les $ on g % % / 1 0 1 kr11 • \ .. Y 1:50 .000 e • • 1 \ 1 • \ • • F ig .3.1a •Ea s tbo urne Park c a tc h me nt - Sec to rs d e fine d in E PDP re po rt • • • • • 46, • 4;,4 • ‘ 1/4.41/4 • • et • • 1. 3/4 tr i m , star G ona ve SW•cs • i C IV I S O IPS See r • i t ‘ ‘ . 0‘ ‘ " • cro a k. Pen • . , 1 0 1 km • 1:50,000 • • F ig .1.1b Eas tbour ne P ar k c atch me n t - ar terial d r a inage s ys te m 4 . CATCHMENT INFORMA TIM Land use 4 .1 The Park itself is a low- ly ing we tland , partly used for summ er grazing , with some s lightly higher parts g iven over to allotments and other uses which can tolerate occasional flood ing . The catchment drain ing to the Park is h igh ly urbanised and includes Willingdon and Hampden Park , most of West Langney , much of Polegate and some northern parts of Cen tral Eastbourne (See Fig .4.1). Urb anisation of the ca tchment has progressed unceasingly over the last 40 years . Fur ther • deve lopment is foreseen both in the fringes of the Park and beyond (eg . Polegate ). The rura l par t of the catchm ent is largely in agricultural use and includes down land and a small amount of woodland. A rela tively recen t development fea ture of the Park is the large landfill site off Lottbridge Drove . Geology/so ils 4 .2 The under lying geology of the catchment can be summ aris ed in three parts (see F ig .4.2): ( ) permeable area s of the Chalk escarpment of the South Downs and the outcrop of the Lower Greensand (which occurs a t the foo t of the scarp slope ), (ii) impermeab le areas of Gault Clay (outcropping between the Chalk and Lower Greensand ) and Weald C lay (in the north-eas t of the catchment ); and , low- lying areas of river and mar ine alluvium . The latter areas are overlain by largely clayey soils wh ich are seasonally waterlogg ed. 411 4 .3 The urban parts of the catchment are situated primar ily on • the Gault Clay and Weald Clay but s ub E im a t l a l areas between Will ingdon and Eastbourne overlay the Chalk , and parts o f Polegate are on the Lower .Green san d . Dev elopment of the low- lying alluvial areas has occurred in 0 0 sevt:r;::: ti;v :! ceps , most :K, • an d 1...'ct t 40 around Hampden Park . Hy drogeo log ical mapp ing of the reg ion ind icates tha t the groundwater catch men t is broad ly in accord w ith the topog raph ic boun dary . • Topog raphic surv eys of the Par k • 4 .4 A recen t h igh-quality 1 :500 top ograph ic survey o f the Sh inewater Sector was ava ilab le to the study . From th is , 24 fields were 40 iden tified hav ing land at or below 2 .4 mOD . Reference was made to a • to tal of 466 spot heigh ts an d area/leve l and vo lume/ lev el tab les thereby 40 constructed for the Sh inew a ter Sec tor : 40 4 .5 O lder 1 :2500 p lan s were prov ided fo r the Sh inew ater , Broadw ater and Sou thbourn e Sectors bu t no t the W est Langney Sector (wh ich is poss ibly the most low-lying of the four ). Comparison o f the • 1 :2500 and 1 :500 p lan s sugg ests that the former overestimate field levels in the Sh inewater Sec tor by about 0 .25 metres . Th is sign ifican t 40 discrepancy cas ts doubt on the validity of fie ld levels sh own on the 1 :2500 p lans elsewhere in th e Park and demands fur ther investigation . • FSR catchmen t characteristic s 40 4 .6 The s tandard FSR ca tcl:Hm t charac teristics such as av erage annual rain fall , ma instream leng th and slope are w ell de fined on the stan dard maps and require no spec ific commen t . • • • C Z :3 & ono • C ross • Fo lkinolon • • • • • • ••••••• •• [77 77:CC-I Urba n • 0 1 km • t e0.000 • F i g .4. 4 Lo c a t i o n o f u r b a n a aa a a • • • P r . • We ald Clay r ms . Gault C lay Cha Low er G reanmand Alluvium ON" 0 1 km 11:50.000 Ca t c hment g e o l o g y • • 5 . F I X ) I; Summ ary 411 41 5 .1 To check the validity of the 100 year flood calculations carr ied out for the EPDP report we have made an estiam te of the critical • 100 year flood runoff to Willingdon Levels using the Flood Studies • Report (FSR ) rainfall/runoff method , as up dated by Flood Studies • Supp lementary Repo rt No 16 (See Refs 3 and 4 ). The differences in des ign storm construction recommended in F lood Stud ies Supp lementary .41 Report No 5 (Ref 5 ) FSSR 5 for heavily urban ised catchments have not • been applied . Th is is because the Park catchm ent is sensitive to long 41 duration storms for wh ich the assump tion of a relative ly 'peaky ' summer pro file would be inappropriate. 41 • 5 .2 The critica l storm duration , wh i c h is dependent both on the 41 ava ilab le flood storag e and the effective outlet capacity, is uncertain but probably lies in the range between 31 and 53 hours . The calculation • of 100 yea r flood hydrographs based on storm durat ions of 31 and 53 41 hours are shown in Appendix B . • 5.3 The 100 year flood hydrograp h based on a 31 hour storm is 41 shown in Figure 5 .1 . It is of a longer durat ion and has a significantly • larger volume than the event adopted for use in the EPDP report . There 41 are several reasons for this : some methodolog ical and some interpretat ive . The follow ing paragraphs con sider in detail the more 41 important aspects of the flood estimation . • 41 Catchment area 41 5 .4 As ou tlined in paragraphs 3 .2 and 3 .3 we consider that an • area o f 27 .9km2 dra ins directly to the two outlets from Willingdon 41 Leve ls (i .e Langney Sewer at Langney Bridge and Crumbles Sewer at Crumb les Slu ice ). A small add itional area dra ins to the levels via the 41 Bourne Stream and Horsey Sewer (see Fig 4 of the EPDP report) . In the • hydro logy compu ta s for the EPDP report the catchment area dra ining • to W illingdon levels is taken to be 33 .59km2 . • • • • runufi 41 5 .5 It is often the case that the assessment of percen tage 40 runo ff to be expec ted from given soils and land use is the most crucial 41 asp ect o f flood estimation . Par ticular care has therefore been taken in assess ing the soils and underlying geo logy of the Park catchment and their interrelationship with urban isation . 40 41 5 .6 The basic FSR map of W inter Rainfall Acceptance Potential (WRAP ) b roadly .distinguishes the three parts o f the catchment referred 41 to in Section 4 .2 and ass igns them to WRAP classes 1 (Chalk etc ,), 3 41 (Alluv ium ) and 4 (Weald Clay ) respectively . However , it is known from 41 the analysis of runoff data from catchments located on the Wea ld Clay that the response is more characteristic of a WRAP class 5 so il. 41 Moreover it is known that the Gau lt Clay is , if anything , more 41 impermeab le than the Weald Clay and therefore also warrants assignment to WRAP class 5 . 41 5 .7 The most contentious aspect concerns the ability of the 41 Alluvial Clays of the Park to accept winter rainfall . There are few 41 catchments consisting of such young soils on which runoff response has been assessed sc ientifically . Moreover where such exp erimen ts have been 41 carried out - for examp le at Newborough Fen in the North Level Internal 41 Dra inage Board - it is obv ious that the ability of the soils to absorb 41 w inter ra infal l is only ma intained by a rigorous management system incorpo rating deep drains and pumping stations . This is man ifestly not 41 the case in the Park area , where large par ts are sea sonally waterlogged 41 and local ponding occurs in winter . It is therefore, concluded that this 41 area should also be interpreted as WRAP class 5 . 41 Allowance for storage effects 41 41 5 .8 The hydro logy studies carried out for the EPDP report assume tha t the Willingdon Levels catchment is sensit ive to the 100 year flood 41 derived from a 9 hour des ign storm . Our stud ies of the available data 41 reveal tha l : 7; mnjo r events , because of the a ttenua tion and delay on 41 runoff caused by the storage of water of the flood plain , the Park • • • ID 411 sens itive to relatIvc i:. long d l ins storms (0 : ID sequ ences o f s torms). 5 .9 In the FSR procedure for an unreservoired catchment , the design storr duration (D) is calculated from : 11 D = (1 + SAAR/1000 ) Tp (1) • where SAAR is average annual rainfall (mm ) and Tp is a characteristic response time (hr ) of the catchment to heavy rainfall . Because the Park 40 catchment is reasonably compac t (in s tream structure ) and heavily urban ised the characteristic response time is only about 5 hou rs . • Applica tion o f Equation 1 y ields a design storm duration of 9 hours . 5 .10 For a reservoired catchm ent it is necessary to calculate the des ign sto rm duration by reference to a charac teris tic response time • wh ich includes the delay imposed by the storag e effect. Fo llowing the ICE Gu ide to Floods and Reservoir Safety (Ref 6), the design storm 111 duration is ca lcu lated from : D = (1 + SAAR /1000 ) (Tp + RLAG ) (2) 411 where RLAG denotes reservoir lag , the time delay (hr) between the peak in flow to , and outflow from , the reservo ir . • 5 .11 For an impound ing res ervoir , RLAG is generally estima ted 'from the storag e and discharge characteristics of the reservo ir . For 411 the W illingdon Lev els catchment bo th the size of the storage , wh ich is prov ided by the extensive flood plain area upstream of Langney Bridge , ID and the discharge capacities of the two outlets are uncertain (see 40 Sect ion 7). Thus it has been necessary to est imate RLAG (and hence an approp riate design storm durat ion from Equation 2) by other means . 411 These are discussed in Section 6 .6 . • Rainfall on the lakes 5 .12 When considering rese rvo irs w ith a large surface area , it is 40 appropriate to assume 100% runoff from rain falling directly on the water surface . Because the flood plain is currently only an informal flocd , Lorage arca - with an extensive lak e area only in major floods - ra infall on the lake surfaces has been ignored for simplicity . How ever 411 • led de s ign stage , ra ll on thc :.e ctrvoir su rface w ill need to be taken into accoun t as 100 ha of water surface will increase • the 100 year flood runoff volume by at least 40 to 50 r41. Inflows from the Bourne Stream catchment • 5 .13 The Bourne S tream catchment dra ins to W illingdon Levels via 40 Horsey Sewer . From the dimens ions of the limiting section , we estimate that the carrying capacity of the pipe linking the two areas is about 0 .75 m 3 /s . We hav e allowed for the con tribution of the Bou rne S tream • catchment by adding 0 .75 m 3 /s to the ca lculated baseflow for the 27 .9 • km:direct ca tchment to Willingdon Levels . • • • 40 40 40 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 30 This analysis- see APPIPlOis S - - EP14 report - for ca rp er iSa l 1 1 1 1 1 1 F i g i l Es t i ma t e s, o f 1 0 0 - y o o r f l o o d h y d r o g r a p h 1 2 0 1 1 1 • 1 • 1 LOW 1 t rilobit e 1 1 4 Excess Volume above 3 cumi n 1 is 1220 M I 1 10 1 4 2 3 0 36 0 6 12 18 24 Tim e • • 6 ANALYS IS OF THE JANUARY 1986 FLOOD EVEN T 411 Available water level data • 6 .1 It was thought at the start of the study that the water 10 level data availab le for the Park area were lim ited to the da ily records 411 for Lottbridge Pum p ing Station plus spo t wa ter level rea dings at a 4111 number of the major sluices within the Levels . However , we were informed by Sou thern Water that continuous water level readings are ava ilable for Langn ey Sew er at Langney Bridge for the per iod January 411 1980 to date . No mention is made of these da ta in the hydro logy or hydraulic studies for the EPDP report. • Langney Bridge water levels 411 • 6 .2 Figur e 6 .1 shows Langney Bridge water levels an d ca tchment rainfalls for the period 1-6 January 1986 . Catchm ent rainfall was 411 assessed by reference to four daily raingauges (each loc ated w ithin 7km 410 of the catchment cen troid ) and a recording ra ingauge at W ish Valley , Eastbourne . 6 .3 Two features are of particular note in F igure 6 .1 :- 0 • (1) the tidal influence is marked throughou t the period of flood runo ff ; and , • (2 ) the effec tive response time of the catchment and • flood plain storage to the heavy rainfall, far exceeds the duration of a sing le h igh tide . Therefore the.degree of synchronization of the runoff peak with • the tidal cycle wou ld se em to be relatively 40 un important . 40 Assessment of reservo ir lag time • 6 .4 Based on a hydrau lic analysis using the availab le data it 4111 was not possible to define prec isely the per formance of W est Langney Sewer during the floo d of 2 to 6 January 1986 (See Section 7 ). Both the • • mE,gn ituce and tim ing of ou tf lows from W illingdon Leve ls c ;) v ZA : assessed within fairly wide limits . However , s imply by smoothing over 40 the tidal fluctua tion in the Langney Bridge water lev els , as shown by • the broken line in Figure 6 .1, it is possib le to get a reasonable 4111 estimate of the tim e of the effective peak disch arge from the Park . ID 6 .5 Compar ison of the peak discharge time w ith the centro id of the corresponding period of flood-producing rainfall (see Fig .6) y ields a tota l characteristic response time of about 17 hou rs , of wh ich 5 hours can be attributed to the catchment response and l hours to the delay 40 imposed by the flood plain storage . • 6 .6 The calcu lation of the critical design flood hydrograph is strong ly influenced by the assump tion made abou t RLAG . It is possible ID that the 'observed ' lag time of 12, hours is typ ica l on ly of modest flood events and that during an extreme event the delay effec t wou ld be rather greater . In this respect , the historical ev idence - tha t the Park • catchment is generally sensitive to sequences of heavy sto rms over a 40 number of days - g ives cause for concern that for a 100 yea r even t the ID critical design storm duration may be longer than 31 hours . To guard • agains t the poss ib ility tha t 31 hours is too short a s torm duration for the critical design flood we have calculated also the 100 year flood • runoff to Willingdon Levels from a 53 hour storm (see Append ix B ). Such 4111 an even t would be critical if the lag due to flood p lain storage were 24 hours . • Estima te of effectiv e outle t capacity ID 6 .7 A simp le water balance approach can be used to obtain a first estimate of the average effective discharge capacity of the two 10 outlets from W ill ingdon Lev els dur ing the flood of 2 to 6 January 1986 . • It can be seen from Figure 6 .1 that most of the flood runoff generated by rainfall period s A and B was discharged w ith in 5 days . As these • storm s were preceded by heavy rainfall at the end of December , a 411 relative ly high percentage runoff would be expected for the even t. ID Tak ing a con serva tively high value of 60% leads to an estimate ("..: of runoff in 5 day s or 9mm /day . For a 27 .9km 2 catchmen t this • co rresponds to a mean discharge of 2 .9 m3fs . ID • • • • • • • • • • • • • • • • • • • • • • • - • • • • • • • • • • • • • 0 1 RAINFALL Aprils 2 PERIOD 'A' 3 4 1•5 • 1•1 ••• % I see % I F • . 0 SemothoW W a a m m a le N M cycle I 0 • 3 VIRYCR LEVEL Of A00 0 25 0 2/1/00 3/1/40 4/1/1113 5/1/28 6/1/04 F ig . 6 1 C a tc h m e n t r a in f a l l a n d L a n g n e y B r id g e w a te r le ve l d a t a fo r J a n u a r y 19 8 6 e ve n t 41 41 • OUTLET CAPAC iTY 41 Intro duction • • 7 .1 Outf low from W illingdon Levels is v ia Crumbles and/or West • Langney Sewers . The work ca rr ied out for the E PDP rep ort su gg es ts that during a 100 year floo d :- • • (1) the average ou tflow via West Langney Sewer at low tide • w ill be 7 .75 m3 /s ; and , )• (2 ) there w il l be no ou tflow via Crumb le s Sewer . 41 In the follow ing parag raphs we rev iew the calculat ions and comm en t on 41 the assumptions that they are based upon . 41 • West Langney Sew er hydraulic s 41 7 .2 In the s tudy (Ref 2 ) carried ou t for th e EPDP report the 41 capac ity of Wes t Langney Sew er between Langn ey Bridge and Fence Bridg e 41 (ie across Moun tney Level ) was estimated using Man n ings equation . In 41 th is equation :- (1) the channel param eters were b ased on i'he average of 411 n ine survey ed cross sections ; • (2 ) a fric tion factor (n) o f 0 .035 was adopted ; and , 41 41 (3 ) the wa ter surface slope was derived on the basis of a level o f 1 .9 m OD at Langney Bridge an d an everag e low 41 water level of 1 .4 m OD at Fen ce Br idge . 41 • 7 .3 For the most p ar t , the bas ic approach adopted is reasonab le ev en though it inco rpora tes sev eral simplify ing assump tions . Th e choice 41 of fr ic tion fac tor is rea li!“-ic and , a lthough we have not been ab le to 41 ob tain a draw ing show ing o f the survey ed cross sections , we have 41 no reason to doub t the calcu lation of the chann el prop erties . On e small • • ID modification tha t is requ ired is to allow for the loss o f kinetic energy 4111 as the flow påsses into Pevensey Haven via the constric ted ch anne l section at Fence Bridge S lu ice . Th is wou ld reduce th e sewer disch arge , wh en the water level is at l . 4 mOD , from 7 .75 to 7 .3 re /s . ID ID 7 .4 Two other facto rs hav e a much greater bear ing on the effective capacity of West Langney Sewer as a carrier of ou tflows from W illing don Levels :- • (1) the ac tual w ater surface slope betw een Lan gn ey Bridge 4111 and Fence Br idge ; and , (2 ) the effects of runoff from Moun tney Level , wh ich was • not men tioned in the EPDP studies . The effect of • inflows from th is 7 .2 kin2 catch m en t is importan t because th e runoff from Moun tney Lev el will en ter West 411 Langney Sewer in preference to the outflow Willingdon • Levels . • 7 .5 In a tide-locked reach , the average discharge over a tidal ID cy cle is very sensitive to variations in the wa ter level a t the down stream end o f the reach . To improve the estimate of th e effective 1111 d ischarg e capacity of Wes t Langn ey Sewer at Langney Br idge an d to assess its sensit ivity to the water surface slope we hav e made s imp lified ID calcu la tions to allow for the temporal var iations in water leve ls at e ither end o f the reach and for inflows from Mountney Leve l . In these calcula tions we have assumed :- 4 P (1) tha t the chan nel parameters are as calcula ted for the • EPDP an d rem a in con stan t over the tid al cyc le ; and 411 (2) all the in flow from Moun tney Level enters the Sewer ID halfway be tween Langney Bridge and Fence Bridge . ID 4111 -7 .6 The rc,s-i.L o f our calcula tions are summarised on F igure 7 .1 . The figure shows how sensitive the discharge is both to the level d ifference between Langney Br idg e and Fence Bridge and to the inflow • • • ID from Moun tney Level . For example , if the level difference increases from 0 .3 to 0 .7m , the discharge wou ld rise from 5 .7 to 8 .6 m /s in the • 3 absence of any inflows from Mountney Leve l . Alternatively , w ith a head difference o f 0 .5m, the presence of a 5 m3 /s inflow from Mountney Leve l wou ld reduce the discharge capacity at Langney Bridge from /7.3 to 4 .2 m3 s. 7 .7 During a 100 year flood based on a 31 hou r storm we estimate that the average discharge from Mountney Level would be 6 .25 m3 /s dur ing 411 the peak 6 hour period and 4 .15 m3 / 8 over the peak 24 hour period . Assuming tha t the water level difference was 0 .5 m , all through the low ID tide period , the average low tide outflow at Langney Bridge at the height of the design flood would be about 3 .2 m3 /s as compared with the 411 value of 7 .75 m3 /s assumed in the EPDP report . 7 .8 Th e effect of the water level var iations at Fence Bridge • throughout the design event are difficult to take account of as there is no information ava ilable to us on 1 in 100 year flood conditions . We have attemp ted to gain some idea of poten tial conditions , including the ID per iod of tid elock , from an analys is of the flood of January 1986 . 411 7 .9 Th e water levels recorded at Langney Br idge and Pev ensey Depo t on 3 January 1986 are shown on Figure 7 .2. Th is figure also shows 110 the level at Fence Bridge at 9am as reco rded by the sluice keeper . The 4111 observed valu e was 0 .4m higher than the contemporary level recorded at Pevensey Depot . A possible tide curve for Fence Br idge based on this observed level has been ske tched on F igure 7 .2. The level records ID indicate tha t levels at Pevensey Depot were higher than those at Langney Bridge for a total of 8 hours on 3 January 1986 . 7 .10 We hav e assessed the discharge at Langney Bridge for 3 111- January 1986 using Figure 7 .1 on the bas is of the observed or inferred water leve ls and assuming that direct drainage from the catchment 411 downstream of Langney Bridge was half the runoff anticipated in the 100 year desigr T 7nrm . Th e results shown on Tab le 7 .1 suggest an average 111 discharg e at Langney Bridge of between 2 .7 and 3 .9 m3 /s depending on ID which water level curve is assumed to app ly at Fence Bridge . This 0 0 estima te allows for the storage of flows draining directly to the Sewer during the h igh tide period bu t makes no allowance for any reversa l of flow that may have occurred at Fence Bridge . The estimated average daily discharge at Langney Bridge has to be compared with the • independent estimate of 2 .9 m3 /s for the com bined capacities of West Langney and Crumbles Sewers based on flood volume and the time taken to evacuate the flood (See paragraph 6 .8). The results suggest that the low 40 water level a t Fence Bridge was probably significantly higher than that recorded at Pevensey Depot. This difference would ar ise from headlosses in Pevensey Haven downstream of Fence Br idge whilst the outfall to the sea at Pevensey was discharg ing freely at low tide . II II 7 .11 Since the levels at Fence Br idge are not known during a 100 II year design flood we have assumed that the lev els at both Fence Bridge and Langney Bridge on 3 January °1986 would be raised 0.25m to g ive a peak level o f 2 .0 m OD at Langney Bridge . Using these levels , and the 100 year inflows from Moun tney Lev el sugg ests that the average flow at • Langney Bridge dur ing the peak 24 hours wou ld be between 1 .1 and 2 .2 e / s depend ing on which of the two tide curves assumed for Fence Bridge • is the more rea listic . Th is analysis is ou tlined on Table 7 .2. By contrast in the unlikely event that the levels at Fence Bridge are the sam e as on 3 January 1986 , the average daily discharge wou ld rise to between 4 .7 and 5 .5 re /s . Tab le 7 .2 indicates the number of hou rs of tidelock for each case and the max imum water level difference at low tide . 7 .12 The results shown on Table 7 .2 ind icate the importance of water level differences between Fence Bridg e and Langney Bridge . Increasing the dif ference at low tide will inc rease the discharge . At 40 high tide , the most important factor is the duration of any tide locked periods when discharge at Fence Bridge is not possible . In summary these results suggest that, if during the critical 100 year flood water leve ls at Fen ce Bridge are app rox imately 0 .25 m above those inferred for the 3 January 1986 from the sluice gaug e rea ding , the discharge past Lane:LJ% ?r idge will be severe ly res tricted to a 24 hour averag e o f around 1 .1 m3/s . This value is less than one third the amount (3 .9 m3 /s) implied in the EPDP report . 7 .13 We have not made any allowances for the transient effects wh ich arise because water levels at both ends of the Sewer are continually chang ing in response to the level fluctuations in Pevensey Haven nor have we made any allowanc e for flooding of land downstream of Langney Bridge . These aspects cou ld be analysed using a trans ient backw ater model bu t we do not think the refinemment is warranted at this stage . 7 .14 The height of the sea tides at Peven sey seem likely to have only a sma ll effect on the high wa ter level in Pevensey Haven as the durat ion of the period when the tida l doo rs are tide locked will probab ly be fairly similar whatever the tide range . This aspect could be considered further by exam ining the tide curves and the level of the tidal doo rs at Pevensey Crumbles Sewer hydraulics 7 .15 We hav e examined the availab le da ta for the Crumbles Sewer to see if it would be able to increase sign ificantly the outflow from the Willingdon Levels during floods . No outflow was assumed in the EPDP report . 7 .16 The sewer discharges to the sea through a 243 m squa re box culver t wh ich is protected by a tidal flap . The culvert invert is set at — 0 .03m OD , which will allow free discharg e from the Crumbles Sewer whenever sea level is below about + 0 .5 m OD , perhaps 7 hours each tide . 7 .17 The water level in the sewer channel upstream of the tidal doors during low tide periods will be de term ined by the head requ ired to pass the discharge through the tide flap s . The flow through the ou tfall sluice at Princes Park boating lake will also be affected by this level . Overa ll we estimate that flows of up to about 2 m3 /s can be passed throug h the boa ting lake and out to sea at low tide without over topp ing either the inlet or the outle t weirs at the boa ting lake . At flows al,ove ab ou t 2 .4 Tn3 /s , the level in the boa ting lake will exceed the inlet and ou tle t weir level of 0 .86 m OD . 7 .18 If discharges through the boating lake are higher , the lake • level will rise to allow sufficient flow 'over the weir. The 11-12m leng th of this we ir limits the rise , so that at a flow of 4 1/13/s the • lake leve l is likely to be 1 .1 m OD , just over 0 .2 m above the weir leve l. At higher flows , the level in the lake will be contr011ed by the • backwa ter from the tidal flap , so that at a flow of 8 F113/s , the lake 0 leve l will rise to about 1 .6 m OD , about 0 .75 m above the weir level . • 7.19 The flow entering the Pr inces Park boating lake is con trolled by the Crumbles Sluice on the W illingdon Levels and by the overa ll capacity of the Crumbles Sewer channe l, includ ing the effects of culve rts and other structures . From a prelim inary assessmen t of the chann el properties we consider the low water discharge capacity of the • sew er may be about 3 m 3 /s with a level of around + 1.9 m OD in the • W ill ingdon Levels and a level of around 1 .1m OD in Princes Park lake . Since discharge can only take place for just over half the tidal period , the sewer may be able to discharge an average of around 1 .5 m 3/s with flo od levels of 4 1 .9m OD on the W illingdon Levels . In this assessment we have not made any allowance for runoff which drains direc tly into the sewer downstream of Willingdon Levels. 7 .20 One advantage of discharging throu gh the Crumbles Sew er is tha t the ta ilwater level outside the tide flaps is d irectly related to sea level, which is fairly predictable , and not dependent on flood run off from another catchm ent . • 7 .21 The backing up of wa ter in the sewer at high tide wou ld necess itate flood banks in this sewer tha t were high enough to con tain • the peak levels throughou t its length . These flood banks may already • ex ist . Improvemen ts to ou tlet capacity • 7 .22 The forego ing ana lysis of the West Langney and Crumbles Sewers suggests tha t during a 100 year design flood , the average ou tflows wh en the water level on Willingdon Levels is 2 .0 m OD w ill be be tween 1 .0 and 2 .0 ril'/s through the West Langney Sewer and up to 1 .5 0 • • 411 4111 m3 /6 through the Crumbles Sewer . The comb ined average outlet capacity ID of these two sewers will certain ly be less than the average of 3 .9 m3/s assum ed for West Langney Sew er in the EPDP report. ID 7 .23 We have attempted to estimate the interrelationsh ips between ID storage capacity , outlet capacity , flood inflows and peak water levels 111 by rou ting the design flood through storages of var ious sizes with a range of outlet capacities . The in itial level of the storage was set at 1 .36 m OD , the average level of all lakes proposed in the EPDP report. • 7 .24 The analyses in itially used a 100 year flood based on a 31 ID hour storm , but a check was made using a 100 year flood base d on a 53 411 hour storm as this was found to g ive higher water levels for outlet • capacities of less than 7 e f s at a level of 2 .0m OD. • 7 .25 The outlet ratings were based on a pipe runn ing full with a crown at + 1.0m OD which was able to discharge freely . The rating was • • A similar rating could be de fined for a free surface discharge through tidal doors . The precise rating for an actual structure wou ld depend on 411 its size , its design and its level . 0 • 7 .26 The e ffec ts o f rou -,ing the upd:ttel.: 100 y ear des ign flooi through the 112 ha lake are a proposed in the EPDP report is illustrated • in Figure 7.3 . This demonstrates that the ex isting 24 hour average • comb ined outlet capacity of around 3.0 m3/s through West Langney and Crumbles Sewers at 2 .0 m OD would cause peak levels in the Willingdon Leve ls lake system of about 2 .6 m OD . In practice som ewhat lower levels would be expected because o f the flooding of low grou nd around the lake • system . This figure also suggests that the ex isting outlet capacity wou ld need to be trebled to prevent water rising above 2 .0 m OD in Willingdon levels in the design storm . • 7 .27 The extra outle t capacity requ ired in this situation could not eas ily be prov ided by modifications to either the West Langney or Crumbles Sewer channels . An alternative would be larg e tidal doors or a pump ing station discharg ing into the proposed Crumbles Marina . • A 411 pump ing station wou ld need to be capable of discharg ing upto 8 m3/s dur ing flood per iods . Tidal doors wou ld need to be capable of discharg ing up to 16 m3 /s if they were tide locked for hal f the time . • The size of the doors would depend critically on their des ign , • par ticular ly the period o f tidelock , but we anticipate that an effective area of at least 5m2 below + 1 .0 m OD would be requ ired . 410 • 7 .28 The provision of additional storage or the relaxa tion of the • des ign top water level in W illingdon Levels wou ld perm it a reduction in the capac ity of the outlet structures. The effect is illustrated on Figure 7 .4 . For example the provision of an additional 230 Ml of 41 storage below 2 .0m OD , would reduce the required nom inal average outlet 4111 capacity from 11 to 7 rf13/s . Allowing for the existing ave rage outlet capacity in the two sewers , this would halve the amoun t of add itional capacity needed from 8 to 4 m3/s . • 40 7 .29 The adoption o f lower normal retention levels wou ld also ID • increase the availab le floo d storage and so reduce the requ ired outlet capacity . In addition a lower normal retention lev el wou ld increase 4111 local gradien ts in the storm water sewers around the marg ins of the W illingdon Levels and so ten d to improve surface water drainage from the ID urban area s (See Section 3 .7). ID 41 411 41 • • • 7 .30 Irrespec tive of whether the ou tlet capacity of tne Willingdon Levels is improved by the prov ision of a new tidal outlet , or by a lan d dra inage pump ing station , appropr iately sized main channels • will need to be constructed to link Willingdon Levels to the sea . The proposed Crumbles Marina cou ld be utilized for the seaward .portion of the chan nel . Since a pumping station cou ld discharge all through the tide cycle , its requ ired cap acity would be abou t half that necessary • with grav ity flow tidal doors . The importance of min imizing head losses ID in a grav ity system wou ld necessitate connecting channels that were three or four times the size of those required for a pumping station . 411 These advantages of a pumping station wou ld be offset to some extent by the running costs of the station and the costs nec essary to ensure ID reliable opera tion of a station that would only be required on rare occasions . 411 • • • • • 111 ID ID ID ID ID ID 411 ID • 41 • • • 41 41 • Notes 41 (1 ) In Case A the Fence Bridge lev els are assum ed to b e the same as the levels a t Pevensey Depot . • • (2 ) In Case B the Fence Bridg e levels h ave b een in ferred from 41 Peven sey Depot levels taking into accoun t an ob served spo t level at Fence Br idge (see Figure 7 .2 ). 41 41 41 41 Alternative es timates of average discharge cap acity of Wes t Langney • Sewer at Langney Bridge on 3 January 1986 . • 41 41 41 41 TABLE 7 .1 41 41 41 • CASE • C D * E 41 HWI at Lan gney Bridge (mOD ) 2 .00 2 .00 2 .00 2 .00 • HWI at Fence Br idg e (mOD ) 2 .12 2 .12 1 .87 1 .87 • Per iod of T idelock (hrs ) 8 .00 8 .00 0 .00 0 .00 LWI at Langney Bridge (mOD ) 1 .70 1 .70 1 .80 1 .80 41 LWI, a t Fence Bridg e (mOD ) 0 .90 1 .30 0 .65 1 .05 • Max imum lev el d ifference (m ) 0 .86 0 .50 1 .20 0 .83 • (Langney Bridg e - Fence Bridge ) 41 • Average inflow from Mountney Level (m 3 /s ) 4 .1 4 .1 4 .1 4 .1 • Pe ak inflow from Moun tney Level (m 3 /s ) 6 .5 6 .5 6 .5 6 .5 41 • Av erage discharge at Langney Bridge • ov er 24 hr at heigh t of flood ( 0 / s ) 2 .2 1 .1 5 .5 4 .7 41 • NOTES 41 Case C : Fence Br idge levels : 0 .25 m ab ove levels recorded at 41 Pevensey Dep o t on 3 Jan 1986 41 Lang ney Br idge levels : 0 .25 m abov e leve ls recorded on 3 Jan 41 1986 Case D : Fen ce Br idg e lev els : 0 .25 m ab ove lev els inferred for 41 Fence Bridge on 3 Jan 1986 41 Langney Br idge levels : 0 .25 m above leve ls recorded on 3 Jan • 1986 Cas e E : Fence Bridg e levels : As recorded at P ev ensey Depot on 3 41 Jan 1986 41 Langney Br idge levels : As in EPDP repo rt 41 Case F : Fence Br idge lev els : As in feriec for Fen ce Bridge on 3 Jan 1986 41 Langney Bridge levels : As in EPDP report 41 • Estimates of West Langney Sewer discharge at Langney Bridge during a 100 year flood . 41 41 TABLE 7 .2 1.5 Inflow from Mountney Leve l (m3/s) 1 5 3 1 0 • • • • 1.0 • • • • 0.5 • 0 • Ls • 0 • 0 2 4 6 8 10 12 • F low in Langney Sewer at Langney Bridge (m3/s) • • • • Gra h showin calcu lat ion of flow • in Lan ne Sewer at Lan ne Brid e • Figure 1.1 • Pevensey Depot (recorded) Langney Bridge (recorded ) Fence Bridge \( inferred) ti 1 ti 1 ti FenceBridge (observed) ti 0.5 21 00 03 06 09 12 15 18 21 00 03 Time (hrs) GMT • Recorded Leve l s on 3 Janua 1986 • Figure 1. 2 • • • • • • • • 2.6 • • • 2.5 a-• • _c —a, • 2.4 L • _C o • 0 - E 2.3 • 0 L 4- 1 >- • -0 0 0 2.2 E • _ _C X C•1 • 2.1 • • 2.0 • 0 2 4 6 8 10 12 Effective out let capac ity (m3/s ) • • • Note: • In above graph effect ive outlet capac ity is the average outf low a tida l cyc le w ith a water leve l of 2.0m 0.0 on W illingdon Leve ls • • Relationshi between effective outlet • ca acit and maximum 100 r flood hei ht on • Willindon Levels • Figure 1.3 • ID 100 150 200 250 • Lake surface area (ha ) • • • • • Relationsh i s between effect ive outlet • ca acit lake surface area and maximum • 100 r flood level • F igure 1.4 • • • 8 . LAKE SYSTEM H':LLAULICS • • General • 8 .1 The relat ionship between the storage requ iremen ts And the 411 outlet capacity for the Willingdon Levels developmen t propOsed in the EPDP report has been considered in paragraphs 7 .21 to 7 .28 . This show ed • that the proposed storag e capacity w ill no t be adequate unless the ID existing outle t capacity is increased or the perm itted maximum water level in the lake system is raised . In this section we consider the internal arrang ements needed to transfer water to the differen t water storage areas in the W illingdon Levels and the arrangemen ts necessary for contro l of sediment and floating trash in the Levels . For this • ana lysis we have used the lake system dev ised for the EPDP report • w ithou t taking accoun t o f the changes tha t wou ld resu lt if the volume of storag e is increased or maximum lake levels are raised . , II • Principles for normal con trol of lake levels 8 .2 In Summer the water level in the Willingdon Leve ls is maintained at + 1.4m OD . This level cou ld be maintained in lak es in the 411 West Langney and Sh inewater Sec tors by maintain ing hy draulic con tinuity with the main streams conveying water across the leve ls . Th is wou ld of necess ity entail a small level difference between the lakes at the ups tream end of the Shinewater Sec tor and the downstream end of the West ID Langney Sector . A major problem will be ensuring an adequate flow of • water through the lakes in summer to maintain their quality . Th is might entail special summer sluices in the main carrier channels . 40 Interconnect ing culverts would be required be tween all the lakes in the ID Shinewater and West Langney Sec tors to ensure each had a supp ly of fresh ID water . • 8 .3 In win ter the wa ter levels in the Will ingdon Levels are allowed • to fal l to aid the discharge of floodw aters . Th is fall in genera l 411 levels will affec t the proposed lakes and wou ld requ ire sluice structures to ensure water levels cou ld not f:.11 r.elow the minimum ID retention level . The flood storage potential of the lakes would be • • • • • • • enhanced if the : en tiont i level U e w in ter mcr It h s 41 when prolonged large volume floods arc mo s t likely . 41 • Operation of the W illingdon Leve ls during floods 41 8 .4 At times of flood the con straints on the operation of the lakes • and chan nels in the Willingdon Lev els will be very different from those • that dominate norm al operation . During floods a major priority is to direct as much water as possible through the main channels rather than 41 through the lakes . • • 8 .5 The majority of in flow to the Willingdon Levels enters the Sh inewater Sec tor from Wea ldon District and then flows on into the Wes t 41 Langney Sector . A sma ller prop ortion enters the Broadwater Sector 41 directly and passes through the Sou thbourne Sector en route to the Wes t • Langney Sector . In our analysis of the behaviour of the Willingdon Levels lake system during floods we have cons idered the Shinewater and 41 West Lang ley Sectors separa tely from the Broadwater and Sou thbourne 41 Sectors . 41 Sh inew ater an d West Langn ey Sectors ID 41 8 .6 In our flood rou ting calculations , we hav e assumed tha t the • levels in all the lakes of the West Langney and Shinewater Sectors , and also in the flooded area upstream , will be the same . In practice a 41 water surface slope will be requ ired to allow water to flow under 41 grav ity through the sy stem . The difference in water level across the • Willingdon Lev els will depend partly on the way the water is transferred across the Lev els but ma inly on the size of the channels , culverts and 41 weirs that link the pr oposed lake system . 41 • 8 .7 With in the W illingdon Levels it will be important to allow as much wa ter as possible to pass straigh t through the Levels in the main 41 chann els , to ensure that the maximum possible volume is discharged early • in the flood w ithout filling the available storage . When the storages 41 need to b e utilized it wou ld be benefic i .2se as far as is pra ctical the storages in the West Langney Sector ahead o f those in the Sh inewater 41 • 0 ID Sector . Th is will requ ire chazinels tha t can pass floodwater around the 411 Shinewater lakes , and allow water levels in the West Langney lakes to rise rela tively early in the flood . This will provide a large water ID sur face to enable full use to be made of the ava ilable low tide ou tflow ID cap acity at Langney Bridge with a m inimum of drawdown . • 8 .8 Other ben efits of large bypass channe ls wou ld be : 4111 • (1) a reduction in the size of culverts and weirs link ing the • lakes together ; and , ID (2 ) preventing 'first flush ' pollutants and bed load sed iments ID from entering the lake sys tem . 8 .9 In our assessments of the app roximate sizes of the dra inage ID struc tures requ ired we hav e assum ed tha t the main channel across the • W illingdon Leve ls from Shinewater Bridge to Langney Bridge w ill require 411 a cro ss section of 20s? dur ing flood conditions . Assuming the leng th of this channel to be about 3500m , it wou ld be able to discharge about 5 ID mz /s w ith a head difference o f 0 .2m from end to end . ID • 8 .10 The weirs allowing flood over flow from this channel into the West Langney lakes might be set at a cill level of + 1.55m OD w ith a 411 leng th of 50 to 70m . The weirs con trolling overflow from the main ID channe l into the Shinewater Lakes wou ld need to be set higher at say + 411 1 .85m OD and be between 90 and 120m long to delay inflow into this sector . The weirs controlling inflows to both the West Langney and ID Sh inewa ter lakes would need to be able to discharge around 10m3 /s . ID • 8 .11 Th e levels and leng ths of the weirs will need to be carefully designed to ensure that the lak es fill in the most effective manner . If 411 the weirs connecting the chann els to the lakes are grassed , care w ill ID have to be taken to ensure that the water velocities over the we irs and • floodways w ill be low enough to avoid damage to the grass . The length of the weirs con trolling overfl:, ID also depend on the distance between the channel and the lak .. i t supp lies , to ensu re tha t the ID headlosses in the .floodway between the channel and the lake are ID • • • accep tab le . We have losses in these floodways will have to be less than 0 .05m . 8 .12 W ith in each sec tor , weirs w ill need to be pro vided to allow the easy transfer of water from one lake to the next . These weirs wou ld probably be set about 0 .15m above the summer reten tion level of the lakes and wou ld be as long as possible to minim ize head losses as water passed from one lake to the next . These weirs would be in add ition to the culverts required to circulate water through the lakes during low flow periods . 8 .13 We estimate that the culvert requ ired to transfer water from the Sh inewate r lakes to the West Langney lakes would need to have a capacity of about 5m2 /s w ith a headloss of not more than 0 .1m . This culvert m igh t require a cross sectional area of around 6 to 7m2 and would be add itional to the main 20 m2 channel under Willingdon Drove . 8 .14 The calculations of the size of the drainage structures is dependent on the sch eme that is finally chosen . The sizes given above are indicative only and must not be used for design . A more detailed study of rou ting and outflow arrangements w ill be required to ensure that s tructures are of an appropriate size to allow the lake sys tem to opera te in the most suitable manner . One major problem will be that the small headlosses which must of necessity occur as flood wa ters move through the lake system will preven t the whole of the storage area being utilized to the perm itted top water level. This will require the provision of extra storage volume or the local relaxa tion of constra ints on top water level . A major objective of the design of the cross dra inage structures w ill be the ach ievement of an econom ically and enV ironmen tally accep table balance between the size and cost of the structures and the volume and cos t of the storage prov ided. Broadwater and Sou thbourne Sectors 8 .15 The lak es in the Broadwater and Sou thbourne Sectors are fed directly by the stream s in to th is par t of the catchment . We have assum ed 25% o f the flood inflow s wou ld en ter the se sec tors • • 41 d irec tly . :looci through the lakes in these sec:prf. ind ica tes tha t the s torage ava ilab le above 1 .1m OD is su ffic ien t to 41 store the who le in flow to these sec tors w ithout exceed ing 2 .0m OD • prov id ing the baseflow can be pass ed into the West Langney S ec tor . The 41 presen t storage ava ilab le in the Broadwater and Sou thbourne Sec tors is insu ff icien t to provide relief for the storages in th e Sh inewa ter and • West Langn ey Sec tors . • 41 S iltation and debris problems • 8 .16 The transfer of as much water as possible through the main 41 channels crossing the Willingdon Levels is necessary for the mos t 41 effective operation of the flood storage . This mode of opera tion will encourage sed iment and floating deb ris that is washed downstream in the 41 ear ly stages of a flood to byp ass the main lake areas . Th is should • significantly reduce the amount of sed iment and debris that would 41 otherw ise collect in the lak es . 41 8 .17 The prov ision of overflow weirs as the main method of water 41 transfer into the lake sys tem dur ing floods will encourage most of the 41 debris travelling downstream at the peak of the flood to enter the lake system . We recomm end tha t appropriate trash screens or booms are placed 41 around the overflow weirs to con tain the debris with in a small section 41 o f the lake from where it can be removed after the flood'has subsided . • Ideally these booms should be placed in 2m deep water and have a length of 50 to 100m . The low averag e velocities passing the screen would 41 ensure minimal head losses even if the screens became partially blocked . 41 The des ign of the screens or booms will depend on whether there is a 41 need to contain sur face oil slicks . 41 8 .18 The use of overflow weirs will ensure tha t the large r sediments 41 wh ich norma lly travel close to the stream bed and may be moved dur ing a 41 flood w ill rema in in the main stream . Most of the finer sed iments flushed through in the early stages of a flood before the leve ls rise 41 enough to ov ertop the we irs w ill also pass down the main chan nels . • • • • • • • 0 8 . IY ::': sed iments suspenci,j .cnv s. : Li c.7. of 41 th e ma in channels w ill be d iverted into the lakes ov er the w eirs . Any • fine sand s in the su spend ed sed iments will settle qu ick ly in the • qu iescen t cond it ions in the lake , mo stly within abou t 100m o f the inlet weir . If the accumula tion o f sand sized material caused a prob lem it 41 could be period ica lly removed from the area ad jacent to the in let w eirs . 41 The deb ris sc reens shou ld b e po sitioned around the area wh ere d redging 41 m igh t be required . The si lts and clay s that enter the lakes during flood s w ill settle in a very thin layer ove r a large po rtion o f the lake 41 bed . 41 • 8 .20 Du ring low flow pe riod s the water circula ting throug h the cu lverts link ing th e lakes wou ld contain some silt and c lay sized 41 sed iments . Mo st o f these sediments wou ld also settle in the lake system • a nd would be add ed to those which settle du ring floods . 41 8 .21 The averag e rate o f accumu lation o f silt s and clay sized 41 sed im ents in the lake sy stem is likely to be very slow . Fo r example if • a ll the runo ff from the catchm ent passed through th e lake system and • conta ined 100 mg/ 1 o f silt and clay , the average ra te of s ilta tion in the lake system would be a round lmm/y r. In p ractice , since both 41 a ssum p tion s are conserva tive the average rate of accumu la tion w ill be 41 much lower , though loca lly co u ld reach 1 or 2 mm/yea r . The accumu lation • o f sed imen t on the lake bed over a 50 yea r period wou ld be unlikely to a ffec t the p ropo sed uses o f th e lake . 41 41 41 41 41 41 • 41 41 41 • • ICTION OF THE AMEN ITY LAKES 41 • Lake dimensions and drift deposits 41 9 .1 In the EPDP report is is env isaged that the proposed amenity 41 lakes will comprise excavations approx imately 3m deep and normally • filled w ith wa ter to a dep th of 2m , so tha t the water surface lies about • 1m below origina l ground level . 41 9 .2 Although the thickness of the d ifferen t drift deposits • var ies across the sites of the proposed lakes the typ ical sequence is :- (1) topso il ; 41 (2 ) a 1m thickness of firm to soft , brown to grey silty • clay ; 41 (3) a lm thickness of pea t; and (4 ) severa l metres of soft silty clay . 41 41 Thus the par t of the excavation normally above water level will cons ist • of firm clay whereas the submerged sides and floor the lake will be in the soft clay . The peat will typically outcrop in the vinicity of the 41 sho reline . 41 41 Method of excavation 41 9 .3 . The soft clay floor of the excavations for the lakes will l e have insufficient bearing capacity to support the mov ement of excavating 41 or hau lage plan t. Excavations will therefore need to be carried out from the rim o f the excavation using dragline or back hoe excava tors 41 standing on the stronger sur face clay and topsoile and loading into 41 rubber tyred dump trucks . Even so it will prob ably be necessary to use 41 moveab le mats to prov ide a stance for the machine s while they are travelling or wo rking . 41 • 9 .4 Haulage of the excavated mater ial to the landfill sites or • for the landscaped mounds mentioned in the EPDP report w ill requ ire the con struction of a system of temporary roads because the 41 ground surface is generally too weak to sup port h eavy wheel loads 41 • • • • 1E.s:ve rutt. ng . Su :h ro :is wou ld :.yp ically co:.:tH...s : o f ID between 0 .5 and 1 .0m of firm fill laid over a layer o f woven geo textile • to resist punching . 411 9 .5 It will be sen sib le to leave the existing grass and .topsoil in place when con structing such roads so that advantage is taken of the firmer surface crust they prov ide . • Stability and protection of the lake marg ins 40 • 9 .6 Along the rim of the excava tion the presence of the soft • clay beneath the peat and sur face layer of clay may be expected to give rise to slipp ing and slump ing of the face . This has been demonstrated in the trial ponds excavated as part of the site investigation . 411 Considerable care will be needed in the positioning of the excavators 411 and the permanent edge of the excavation will need to be battered back or stepped to an average slope no steeper than , say , 1 in 4 if serious ID slumping is to be avo ided . ID • 9 .7 The EPDP report env isages that there will be landscape mounds at som e po ints along the lake margins . Considerations of stability of the excavated marg in will dictate that such mounds should I • be of limited he ight (say 3m max imum ) and set well back from the 4111 sho reline . 411 9 .8 Where the peat layers outcrop in the lak e shore , close to 411 normal top water level, they w ill erode more qu ickly under the action of wav es than the overlying clay . The resulting overhang s will be unstable and will collapse when the clay layer cracks . Th is process w ill tend to ID lead to a soft clay sho reline with pea t redeposited on the lake bed below the lowest level disturbed by wave action . Other factors which • will come into play include the growth of reeds and shoreline vegetation , which will reduce wave action and act to stabilise th is soft margin . Erosion of the clay margin of the lakes by wave action w ill tend 411 to colour the water and the fine nature of the particles in suspension 411 .,112 cause the colour to persist even when wind ac tion is no longer presen t. • • • • Measures which ca:, Ut: cons iders:: for lim iting the effects of wave action along the lake shoreline inc lude :- (1 ) placing gravel an d pebb les to form artificial beaching ; (2 ) the use of timber revetments ; (3) the encouragem ent of reed grow th in appropria te locations ; and , (4) the use of geotex tile and concrete slab pro tec tion sys tems . O f the above , beaching is the most natural and immediate ly effective measure . It is also likely to prove the most cost effective g iven tha t su itable materials should be available within an econom ic hau l d istance . Seepages to and from the lakes 9 .10 On the evidence of the ava ilable borehole information there is little doub t concerning the ability of the lake to hold water since they are underlain by a con tinuous layer of clay . Should any loca l discon tinu ities be revealed during excavation there is amp le ma terial ava ilab le to construct a clay blanket . 9 .11 Any inflows to or outflows from the lakes through the pea t layer are likely to be small because of the relatively low permeab ility and shallow hydrau lic gradients . Should it be necessary to exclude any undes ireab le seepages , there is amp le clay available to blanket the peat ou tcrop . Alternatively a cutoff cou ld be con structed by excava ting a trench through the pea t , some d istance back from the shoreline and refilling the trench with clay . ID 41 • 10 . WATER QUALITY 41 Available data 41 10 .1 Southern Water have taken water samples at several po ints on 40 the main stream s of the catchment at approximately monthly intervals over a 10 year period . These samples were analysed only for san itary 41 param eters to monitor the affec t of discharges from Polegate sewag e 41 treatmen t works (STW ). Since the work s was take out of serv ice in July 41 1986 , mon itor ing has been reduced . 41 10 .2 Because Polegate STW no longer discharges to the catchm ent • the above data provide only a genera l gu ide to likely future conditions 41 in Willingdon levels. We have therefore restricted our investiga tion to the information for 1985 and 1986 for the sampling points on :- 411 41 (1) Will ingdon Upper Sew er immed iately upstream of Will ingdon 41 Drove , and 41 (2 ) West Langney Sewer at Fence Bridge . 41 • These data are 'summar ised in Table 10 .1. • 10 .3 Sou thern Water have also tak en a few spot samples at various • points around the Lottbridge Drove landfill site which have been 41 analysed for heavy metals . 41 10 .4 Additional data are availab le from six trial ponds and from 41 three shallow boreholes , in the Park area . These data were collec ted as • part of a pollu tion mon itoring study carried out by the Coun tryside Research Unit of Brighton Poly techn ic (Ref . 7). The data from the trial 41 ponds are summarised in Table 10 .2 . • • Quality of streamflows 41 10 .5 The samples taken from the W illingdon Upp er Sewer show the • water to be slightly alkaline , moderately hard to hard (137 to 270 mg/1 41 41 41 41 • • • total hardness as CaCO3 ) and with total alkalinity ranging from 75 to 202 mg/1 . The m ineral and saline constit-Jents of the water are modera te , the chloride rang ing from 40 to 93 mg /l. 10 .6 The median concen tra tions of ammoniacal nitrogen and n itrite n itrogen in the Willingdon Sew er are 0 .47 mg/1 and 0 .10 mg/1 40 respectively . These concentra tions would normally be taken as ev idence 40 for gross con tamination of a surface wate r drainage by the effluen t and • sugg est relatively low diluting flows from the upp er catchment . The organ ic content is substan tia l at times , indicating the cur ren t effect o f urban dra inage or perhaps resuspens ion of material dep osited from Polega te STW . 10 .7 The concentrations of total ox idised nitrog en in Will ingdon Upper Sewer show considerable var iation (from 0 .5 to 11.3 mg/1 ) although the median of 4 .6 mg/1 is not unusual. The concentra tions a t the Fence • Bridge site are reduced by half over the Willingdon Sewer concentrations , almost certainly indicating plan t uptake of the n itrogen . • • 10 .8 The con cen trations of orthophosphate , the other major plant 40 nu trien t, are high for a surface water and almost certainly stem from sewag e discharges . This median concen tration falls from 2 .8 mg /1 at W illingdon Drove to 1 .00 mg /1 at Fence Bridge aga in sugg esting the • up take of phosphorus by algae or macrophy tes. • 10 .9 The pH varia tions at the two sites provide s further evidence • for algal or macrophyte grow th in the sys tem . Also the values for total oxidised n itrogen show tha t n itrogen is at times comp letely removed from solution . This ind icates tha t n itrogen is the limiting nutrien t on algal or macrophyte grow th in this eco sys tem , a somewhat unusual occurren ce in freshwa ter systems where orthoph osphate is commonly the limitation . 10 .10 Figure 10 .1 shows the recorded nitrogen and phosphorus concentrations since January 1985 . The effect of removing the sewage effluen t discharge from the W illingdon drainage is c learly shown . It must be expected that these nutrients will, howeve r , con tinue to be leached from the surround ing dra inag-i fo:- a year or two before failing to more stable and lower con cen tra tions . • Water quality in the trial p its and boreholes • 10 .11 The study by the Countryside Research Unit (Ref . 7 ) was • concerned with detecting any deleter ious effect of the landfill sites on • water quality . The study used phenols and bacteria as indicators of contam inat ion of th'e ground water by industrial trade wastes from the land fill sites . • 10 .12 The choice of pheno l is unu sua l and we are not aware of any other pub lished literature where pheno l has been used as an ind icator of pollution by landfill lechates . However , there were only three 40 occurrences of low leve ls of phenol, traces being fou nd in two of the 40 ponds and one borehole . These could have resulted from a w ide variety of sou rces and we do not fee l they are a cause for concern . We would 40 exp ect any ground water con tam ination , even at low con centra tions , to be presen t at a permanent backgroun d level rather than as pulses of h igher concen tration . 10 .13 The bac teriolog ical data for the surface waters in the trial ponds show that med ian con cen trations of total coliforms flu ctuate from • 500 to 1700 bacteria per 100 ml and peak at 20 ,000 per 100 ml , with E .coli giving occasional positives . These concen tra tions fall well w ithin the expected range for surface waters in an actively cattle g razed dra inage area . There is no evidence tha t these numb ers reflect o ther than normal anima l faeca l con tamination . Salmon ella was found to be abs ent for all tests . • 10 .14 In add ition to the study by the Countrys ide Res earch Unit , Southern Wa ter have analysed spot samples taken from around the • Lottbridge Drove landfill site for heavy metals . Th ese analyses have n ot yielded any values above the expected backgroun d levels for the area . • • • • • • 'Ac do ! : D t cons idtr ph enci s and bacteria arc par L;cu h iriy sensitive ind icators of pollution and would hav e preferred tb have seen ID more tests using indicators such as ammonia and heavy metals (e .g. ID copper , chrome , lead and zinc ). How ever , from the limited data available there is no evidence to sugges t con tamination of the .surface 41 or groundwater of the Park as the result of leachate from the landfill • site . 1110 Effects of urban dra inage ID 10 .16 Approx imately 40% of the catchMen t drain ing to Willingdon • Levels is already urbanised and the lim ited urban developmen ts de tailed in the EPDP report are unlike ly to cause a fur ther sign ificant de terioration in the quality of the catchment runoff . However it should • be recognised that the runoff from high ly urban ised catchments often con tains significant loads of particulate organics , some heavy metals 111 and slicks con taining oil and tar . There are no data for these param eters for Willingdon Levels and it is difficult to predict the • likely loadings because runoff quality is highly site specific . Studies 411 in North America and Australia have shown tha t stormwater runoff from residentia l areas can produce loads equ ivalent to the load from a sewage ID effluent in terms of nitrogen , phosphorus and suspended mater ial . Much of the suspended material will reach the water courses when mob ilised in the first flush following a period of dry weather , typically as the result of a summer thunderstorm . ID ID Water qua lity requirements for the park 10 .17 The development of the Park is primarily for flood storage , 4111 bu t nevertheless it w ill not be seen as a success if water quality in 411 the p lanned lakes and water features is not attractive . Tab le 10 .3 ID lists the water bodies that have been proposed , together w ith their planned recrea tional use , and "key" features . These fea tures are pr imarily the phy sical requiremen ts or objectives necessary to ach ieve 411 suitable water qua lity for the intended uses . 0 !0.1 There are no legal requ iremen ts or s tandards for amen ity an:: 41 recreational lakes in the UK , but we have se t ou t in Tab le 10 .4 the 41 criter ia and ac tual s tandards (the latter as concentrations ) wh ich m igh t 41 well be adop ted by a manag em en t team as ob jectives for the three majo r 41 uses of the water for wh ich water quality has importance . These criter ia ar e taken from the EEC D irectives wh ich now apply by law to 41 certain natura l water bod ies . 41 41 10 .19 There is a fou rth use of the water not described in Tab le 10 .4 . The appearance of the water w ill be o f g reat con cern to the 41 pub lic in several of the water bodies . In Sou thb ou rn e Lake and W inkney 41 Lake paren ts an d ch ildren will be major users and the aesthetic quality 41 o f the env ironmen t w ill be importan t for the venu e 's success . In these lak es , in par ticu lar , good clarity will be importan t , as well as the 41 ab sen ce of floating debris . 41 41 10 .20 Th e EPDP report env isages that the lakes : 41 (1) w ill hav e a con stan t dep th of 2 m at norm a l top water level ; 41 and 41 (2 ) w ill be offline in the sense tha t , excep t during times of 41 flood , th e major part of the flow acro ss W illingdon Leve ls 41 w ill be via the water courses . * 41 Runoff from the catchmen t w ill obv iously b e requ ired to fill 41 the lakes in it ially , to make good evapora tion losses an d to ensu re tha t 41 the lak e con ten ts are renew ed . We estimate that th e long term averag e 41 runoff to the Park is between 0 .3 and 0 .4 m 3 /s w ith perhaps 80% of the av erag e annual runoff occu rring du ring the win ter mon ths . Even if all 41 summ er flow from the ca tchmen t upstream of the Sh inewater sec tor were 41 rou ted thr ough the lakes , the ir conten ts would no t b e rep laced in an 41 av erag e summ er . 41 10 .21 Ba sed on ev idence o f the 198 5 and 19 86 data , the water 41 quality in the dra inage chann els enter ing the Park area fa lls w ithin 41 Class 2 o f the Na tional Water Council class ification , although 41 41 41 41 de t,:rioratIL: towaris Class 3 It sn,y,i1c : u 41 tha t the ex ist ing quality (see Tab le 10 .1) exceeds the gu ideline 41 concen tra tion s for coarse fish for ammon ia an d phosphorus . Sou thern • Water consider tha t the occasional deteriora tion in wa ter quality w ithin W illingdon Levels was due to storm water overflows from Polegate STW . 6 This source of pollution has now been remove d and a trend to lower 41 concen tra tions in nitrogen and phosphorus has been observed . There 41 should also be a gradual improvement in the BOD , ammonia and n itrite levela in the runoff from the catchment upstream at the Sh inewa ter 41 sec tor . Nevertheless, the risk that flood runoff from the urban areas , 41 probably as the result of a summ er thundersto rm , will cause a sudden 41 de teriora tion of water quality must be recogn ised . 41 10 .22 The variations in nitrogen and pho sphorus concentrations in 41 the present drainage system provide evidence of algal and/or macrophyte • growth . In the open waters of the proposed amenity lakes , w ith their very long re ten tion times , the poten tial for growth will be much grea ter 41 and particu larly so if measures are taken to ensure the clarity of the 41 waters (see paragraphs 9 .8 and 9 .9). In spite of a general improvemen t • which may occur in the ambient concen tra tions of nitrogen and phosp horus in runo ff from the catchmen t it likely tha t :- 411 41 (I) on occasions dur ing summer algae will flourish in the 41 surface waters ; and 41 (2 ) macrophy tes will rap idly colon ise the whole lake area where 41 light reaches the bottom sed iments . 10 .23 To reduce the amount of maintenance required we recommend 41 that cons ideration is given to :- ID • (1) deepening parts of the proposed lak es ; and 41 (2 ) where possible ensuring a constant throughput of wa ter . 41 • If the middle of Broadwater , Shinewater and West Langney lakes were deepened to 3 to 4 metres this wou ld not only help to lim it 41 the grc ,..- o f ic and macrophy t.::‘ a iso attrac tive hab itat for fish , allow ing them to avo id the hot upper layers in summer and the attentions of d iving b irds . The average dep th of the lakes need no t be altered greatly from 2 metres , as part of the shoreline can be made gradually shelv ing to prov ide natural marshy banksides . Th is would be particu lar ly suitab le , say , on the western shores of Sh inewater Lake and its wildlife reserve island . and on the western shore of West Langney Lake . 10 .24 In spite of the above , with the long res idence times in the lakes , wa ter quality can be expected to decline markedly dur ing mos t summers un less rem edial action is taken . A study will need to be carried ou t to establish the most effective method of maintaining appropr iate water quality standards in ' the lakes . However we expect that it will prove necessary to install some form of mechanical aeration and rec irculation system . It may also '6e necessary touse algicides . Landscap ing and tree growth 10 .25 We do not envisage tha t the earth moving operations to form mounds or emb ankments will result in any longterm wa ter quality problems . Our experience in the construction o f comparable development at Strathc lyde Park , showed that utilisation of the subsoils caused no water quality prob lems , provided they were adequately drained . How ever the establishm ent of a plan t cover on the new ly form ed embankments was slow unless top soiling was undertaken . 10 .26 The use of pre-seeding fertilisers and sub sequen t fertiliser dressings could prov ide undesirable enrichment of the lakes in Eastbourne Par k . The p lant cover should be allowed to estab lish itself on the newly formed land , for at least a year , before the lakes are stocked with fish . 10.27 Ev idence from the Lottbridge Drove exp erimental planting site (Ref . 8 ) suggests that it w ill be possible to establish a wide range of tre e species with in the Park without undue difficulty . da ta in an MSc thesn by A J M orey (Ref. 9 ). No excessive chlor ide or sulphate concentrations were - found in groundwater samples from the area (with the exception of a s ingle sample with su)phate in excess of 1000 ppm ), nor were pH va lues other than around neutrality recorded . 10 .29 It seems unlikely therefore that any water qua lity problem is likely to arise in the Park from use of the indigenous subsoils in the landscaping . Som e waterlogging of some mounds may occur due to the m ixed nature of the sub soils with pockets of peaty deposits in the clay s ilts , but we believe that local remedial drainage using s tandard land drainage techniques will be able to cop e with such difficulties . It w ill probably be useful to employ some of the pioneer species such as a lder , Alnus sp ., and the wet ground tolerant species of w illow , Sa lix spp . as initial plantings to help dry out areas where taller cover trees ar e wan ted in the long term des ign . The initial plant ings can be thinned out when the permanent trees are well established . • • 0 • Wi l U ng dan Upper Seve r FMK. Br i gge T05 2570 0 358 0 T9 64900 04 74 0 • • De t e ria1nand Mean Medi an Mi ni mum Ibu l arim Mean Med i an Min i mum Maxi mum 0 Te mper at ur e 6C 11 . 75 12 . 5 4 . 0 21 . 0 11 . 7 12 .0 6 20 0 pH Va l ue Un : t 7 . 55 7 . 5 7 . 05 8 . 0 1 7 . 6 7 . 5 7 . 09 9 . 05 • Di s s ol v ed Oxygen • Sat u r at i on 1 65 68 40 78 69 70 56 62 • Al k al i n i t y pH 4 . 5 ss CaCO, ma l l 161 180 75 20 2 17 7 18 5 136 208 0 To t a l Har dness as Ca00, mg/ 1 228 28E 137 27 1 230 22 5 12 7 306 • • Ch/ o r i da Me l 61 63. 5 40 9 3 7e 76 45 102 • Ammon i s cal Ni t r ogen ma / I 0 . 82 0 . 47 0 . 06 2 . 50 0 . 26 0 . 12 0 . 02 1 . 2 0 Ni t r i t e Ni t r og en 0 . 19 0 . 10 0 . 0 1 0 . 69 0 . 06 0 . 06 0 . 0 1 0 . 19 • NO • To t a l Ox i di sed Ni t r oge n mg ' l 4 . 09 4 . 6 L70 . 5 11 . 30 2. 0 1 . 4 L70 . 5 5 . 30 0 Or t h oph os pha t e Phos pho r us n o 2 . 50 2. 80 0 . 31 5 . 71 1 . 62 1 . 00 0 . 22 5 . 10 • • Bi oc hemi c al Oxy gen P. Deman d 5 Day s Ne l 3 . 44 2 . 65 0 . 90 13 . 0 2.4e 2 . 15 1 . 0 5 . 5 • Notes • LT • Les s th an 0 • Based on Southern Water da ta for period July 1985 to November 1986 40 • 40 41 41 ID 4111 41 Summ ary of water quality data in • W illingdon and West Langney sewers • • • • • • • • • • • • • • • • • • • • • • • • • • • • • .• • • • • POLLOT IOW MONITOR ING DATA : tASTDOUNKR PANX 1904 Tr ial Ponds (February to July 1904 ) 1 2 3 4 5 Ned1an Range Med ian Range Med len Range Med ian Range Med ian Range Med ian Range Aerob ic bac teria C. In'm1-1 ) 1.3 0 .3-18 2 .0 0 .9-9 .8 1.4 0 .5-11 .0 4 .8 0 .2-6 .6 1.0 0 .2-4 .8 1.0 0 .2-2 .9 Anaerob ic bacteria -1 m l 475 20-1080 495 126-1660 330 30-1013 133 40-280000 140 20-7200 147 67 -2700 Yeasts and mou lds -1 m l 26 6-970 19 10-6 3 14 10-24 9 10-25 18 10-38 22 10-43 po llform bacter ia m 1-1 10 10-38 40 10-43 17 10-200 10 10-16 13 10-700 10 10-250 -1 (- .) R .coll al I- - ) (- •) • (- .) • (- 0 • f- 0 Salmone lla (- - ) 9 (- -) (- - ) C. - 1 C. - ) (- -) 7 .7-6 .3 pH Va lue 7 .8 7 .5-8 .7 7 .8 7.2-0 .3 8 .1 7 .5-8 .0 8 .1 7 .2-8 .3 8 .1 7 .4-8 .3 8 .0 ) . 0 - 5 . 9 5 day DOD mg/ 1 5.6 4 .2-10 .8 3 .3 2 .5-5 .0 2 .7 2 .2-5 .9 3.6 2 .4-14 .7 1.6 1.0-2 .5 4 .5 Pheno ls me l 0 .5 0 .5- 5 .0 0 .5 0 .5- 5 .0 0 .5 0 .5-1.0 0 .5 0 .5- 5 .0 0 .5 0 .5- 5 .0 0 .5 0 .5-1 .0 • lass than (limit of detection ) - • Absent • • Present ke Features or OV ectives Island reserve , WQ for fish Hard shorel ine , shallow , good clarity , WQ for bath ing Island for camp ing (? ), WQ for bathing and fish Natura l shoreline s , WQ not critica l but to fish ery quality Natural sho relines , WQ no t crit ical but to fish ery quality Natura l shoreline s , WQ not critical but to fishery qu ality Island for camp ing , landing facilit ies on one shore , west shore natura l shoreline , WQ for bathing WQ not critical but to fishery quality WQ not critical bu t to fishery quality Natural veg etation , WQ not critical but to f ishery quality Natural shoreline in parts , high clarity , WQ for bath ing Natura l vegetation , WQ not critical Na tura l vegetation Natural appearance , WC? no t critical Some natura l shores , WQ for bath ing Navigable, WQ not critical bu t to fishery qua lity TABLE 10 .3 - - • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • (11 WATER QUALITY CR ITER IA 31fITABLE VCR AM ITY LAKES Concentrations in mR/1 excep t where ind icated as Annua l means fo otnote. 1 Non e of these se ts of criter ia end standout s app ly by law to mmen ity waters (see text ) 2 EEC Standout for wa ter supporting fr ee:heater fish 1114 3 %IC Standards for ba thing water • CRC S tandout s for trace subs tances for protection of freshwater fish end other fr eshwater life . Values shown are for alkalinity 200-2 50 mg/ 1 5 Total asta l va lue (lharticulete • dissolved co mpo nontm ) 8 Concentration is • 05 pe rcent ile 7 Un it is NPR/100 a l 8 unit is IIPM/1 litre 9 Un it is NPR /10 litres N Mandatory standard • O w OU1deline sta igarg OT greater than (6 7) reece l streptococci ' 100.0 (6 9 ) linterov iruses ' 0.11 V OLt aJ r16 !j J ames J addn uopa n w hi u ! s l anal l ua y l nN Tota l ox id ised n itrogen (mgil) a co 8 ND 0 "0 r \ oO 1 n L - T - - o.-. M D I n - 1 CD O — (0 (D 0 D X a - • o aCD 0 Orthophosphate phosphorous (mg/I) 41 • APPE ND IX A HISTOR ICAL REV IEW OF FLOOD ING 411 • Rainfall s ear ch • A .1 A t an early stage of the study , a search o f da ily rainfall 40 record s was made fo r possible h istorica l flood-produc ing ev en ts on the Park ca tchmen t . In this respec t , the long and g en erally con tinuous • record for Eastbourne W ilming ton Sq . was exc ep tionally h elpfu l . A search fo r large 1-day an d 2-day ra infalls in the period 1888-1984 provided a preliminary list of events for wh ich rainfall depths over a range o f durations were exam ined further . Sub sequen tly , tw o even ts in • 1985/1986 w ere added . The ou tcome of the ra infall search w as the list of no tab le ra infall even ts presen ted here as Table A .1 The list is '110 though t to b e rea sonab ly comprehensive bu t is b iase d to include som e • recent ev en ts (for wh ich addition al da ta ar e availab le) that wou ld 411 o therw ise n ot have qualified as no tab le . 40 A .2 Th e rainfa ll da ta from Tab le A .1 is represented in Fig . A .1 to h ighlight the rela tive severity o f the ev ents over pa rticular du rations • (from 1 to 16 day s ). Comparison o f the depth- duration data for ID particular even ts with 5-year and 100-year de sign v alues prov ides an indica tion of the relative sev erity of particu lar ev en ts'. As a firs t 41 examp le , it is in teresting to no te that the stil l-rememb ered severe • flood o f O ctober/November 1960 (EV ENT M ) had ra infall depth s wh ich were excep tional on ly for durations o f 4 days or grea ter . M oreover , a t 411 long er du rations the even t was still mu ch smaller than those o f October • 19 39 (EVE NT G ) and October 1949 (EV ENT J ). 41 An teceden t condition • A .3 F lood runo ff from heavy ra infall events is affected by the 40 anteceden t catchmen t wetness an d th is is conven ien tly indexed by 411 an teceden t rainfa ll and time of y ea r . (See sec ond an d third co lumn s o f Tab le A .1 ) • • • • !-lood repo rts A .4 Various sou rces of informa tion were used to glean informa tior. abou t the nature of flooding in the park area and its surrounds. These sources included :- • (i)the EPDP report and a compan ion document entitled 'Eastbourne Park Landscape Studies '; 411 (2) a Section 24 Survey produced by Southern Water; (3) correspondence files relating to specific incidents brough t to the attention of Sou the rn Wa ter ; and, 411 • (4) photographs supplied by Eastbourn e Bor ough Council and a local residen t. ID 411 We also under took an independen t search of local newspaper records , guided by the results of the sea rch of the rainfall records . A .5 Fur ther details of some of the events listed in Table A .1 are 40 given below . Attention has been b iased towards the larger and more ID recent events . • 4/15 OCTOBER 1939 (EVENT G ) 111 • No information was found relating to flood ing in this event . The anteceden t condition was dry and it is possible that the resultant flood 411 was insufficient to warrant mention in wartime newspap ers . (From Fig A .1 • it is seen that the long-duration rainfal l dep ths wer e extremely high ). 411 15/26 OCTOBER 1949 (EVENT J ) 411 411 Newspaper accoun ts cen tred on flooding in Cen tral Eastbourne , 411 attribu ting th is to torren tial ra in coinciding with the high tide such 411 that surface sew ers cou ld not cope . A separate reference to "seriou9 flooding at Hampden Park , where the marshes around the Hydney Estate ar e 411 still deep ly flooded right out to Stone Cross" is perhaps the most 411 • • • • • • te lling of all the h istorical information gathe red . (liainfz, l dep t ir this event provide the historical maxima for all dura tions above 2 days - See Fig A .1). 111 19 OCTOBER / 4 NOVEMBER 1960 (EVENT M ) • 40 Newspaper accoun ts of 2 November 1960 refer to : "heading for the wettest ever year " , "stretches of road under water at Wannock", and "Monday evening 's rain was too much for the dyke and streams entering marsh land between Polegate and Hampden Park , the wa ter overflow ing to form large 'lakes ', the one in the p icture isolating a py lon" . The issue of 5 • November 1960 has a photograph show ing flooding nearly to doorstep level in Hampden Avenue on Thursday afternoon . This would seem to confirm the longevity of innundation of the Park area in this event (assuming that 40 the street flooding was indicative of drowned surface water outfalls). • An East Sussex River Board report on the Novemb er 1960 flood indicates 411 that it as "no t particular ly severe esp ecially in the upper reaches , worse cond itions having been experienced in the past few years . In the low land howev er , flood leve ls were h igher as the levels in the Crumbles • 'and W illingdon Upp er Sewers and Langney Haven are affe cted by cond itions 40 at the Pevensey Bay outfalls as the two systems are interconnected . (Elsewhere a water level of 1 .93m is quo ted for Crumbles Slu ice). There 411 is no doub t tha t works carried out in this area during the last few 40 years had a bene fic ial effect in lowering levels" . This quote raises the point that the existing drainage system has evolved over many years and po ints to the sign ificance of upgrades to the Pevensey Bay tidal ou tfalls undertaken in the mid/late 1950 's. The EPD P repor t speaks of this event as being "the last time when conditions arose which gave rise 411 to concern .., but the flood did not materialise " . 4111 24 NOVEMBER / 9 DECEMBER 1960 (EVENT M ) • • Th is event followed soon after Event M and led to reports in the 7 December 1960 issue : "water swirls off Downs" , "wettes t ever year " , "the wa ter was com ing o ff the hills in streams", "worst rainstorm and gale for many years ; 12 hours of non-stop wind and torren tial rain" , I 411 ca ll-ou ts to cellars in Hampden Park area", "drainage ditch at Manor Close , Willingdon overflow ed" and , accompany ing a p icture of flooded • • • 11 E rdens , -the flooding a t Lo-Ac :' V:1:in2don" . ThcL - : \ ID tha t, in som e circumstances , mod era te to rapid response flows fror the • chalk area s may be significant and that a flood bu ild-up in the Park area may be accompanied by gales (wh ich presumably might exacerbate flooding through wave set-up ). 1111 • 9119 JUNE 1971 (EVENT 0 ) 40 Th is event led to a newpaper report (23 June 1971) "wa ter , water , everywhere" bu t little detail other than a well over flowed and flooded • land at Wilm ington , in an adjacent catchment. A Sussex River Authority report indicated that tho se flooding problem s tha t did arise were • con fined to gardens and could be attributed to local features of the dra inage system . However , the report acknow ledged that "follow ing the 410 extens ive developmen t of Eastbourne on the boundaries of the Willingdon development of Eas tbourne on the boundar ies of the Willingdon Marshes • ... the rate of run off w ill inev itably be increased in future years" - 4111 in response to complaints from local residen ts about the "general • tipp ing on the Marsh and filling in of the flood plains". This quote rem inds tha t the exacerbation of flood frequency by developmen t has a long h istory in the Park . Well leve l data for Folkington (in the north-west corner of the catchment) show ed a remarkable rise of 5 metres • during the period of this event. While this demonstrates the capacity of the cha lk to absorb heavy rainfall , the rate at wh ich the well level subsequently fell indicates that the spring flow response on the scarp slope can be relatively rap id. 13/22 NOVEMBER 1974 (EVENT P ) 411 The newspaper of 23 Novemb er 1974 reported a b ig flood after a week of 111 heavy rain and hundreds of houses and gardens inches deep in water . (The reference to inundation o f hundreds of hou ses would seem to be an overstatemen t ; presumably some house flooding occurred due to local surface sewer problems ). The Hamp den Park area appeared to be particu lar ly affected . 2f1 NOVEVEEL / 1 DKCEMB:j J9 :,- 0-.VE11- S ) Newspap er reports were direc ted more at flo oding i t; the Cuckmere ca tchmen t to the north-east . 6/8 JULY 1980 (EVENT T ) The n ew sp aper o f 12 Ju ly 1980 re fers to flood ing of gardens and some roads an d basements , the latter be ing a ttribu ted to ra infall in excess of th e local surface water dra inag e sy stem . (Re ference to F ig A .1 indica tes that the short-duration rain fa ll dep ths in this even t were indeed excep tionally h igh . G iven tha t the an teceden t period was wet , the fa c t that the even t did not lead to w idesp read flooding wou ld seem to con firm the general sensitivity of the Park ca tchmen t to long-du ra tion even ts ). os ? 1984 Accord ing to Southern Water files , a dra inage problem arose at a fa ctory in B irch Road (off Lo ttbr idg e Dro ve). Th is appears to h ave b een a design m iskake - the su rface wate r discharg ing 0 .5m below the summ er retention level in the arterial drainage system . A rep ly from Eastbou rn e B .C . ind icated that the prob lem would be improved by the balan cing reservoirs foreseen in the EPDP which would "enable the water , to be kep t a t a lower level" . 16 DECEMB ER 1984 The ra infa ll an aly sis did no t iden tify this as a notable even t . It wou ld appear from Sou thern Water files that flo oding o f gardens occurred in Sandp iper Walk (West Lan gney ), a ttribu table to back ing up of water in the sur face water sewer "due to th e heigh t o f water in the dykes" (ie the West Langney and W illing don Sewers ). Th is type o f secondary flooding seem s to be thematic o f dra inag e prob lems on the periphery o f the Park wh ich ar ise b ecause o f the very lim ited hydraulic g radien t availab le . 23 DECEMB ER 198 5/7 JANUARY 1986 (EVE NT W ) Deta ils o f the even t are con ta ined in Sec tion 6 . •• N OT A BL E R A IN F A L L E V E NT S - E AS T BOUR N E (18 8 8 -19 8 6 ) R a inf a ll (m m ) i n s ta t e d d u r a t io n (da y s ) E v e n t A n te c ' D a te k e y c o n d it ' 1 2 3 4 6 8 10 12 16 A 2 5 /2 8 O c t 0 9 5 0 66 8 2 8 4 v e t 7 /12 N o v 1 1 36 5 2 73 8 2 1 15 d r y 29 S e p 1 1 O c t 12 3 6 6 9 8 1 D v d r y 9 / 10 J u l 2 3 4 3 5 1 2 4 /2 5 J u l 3 2 4 2 8 1 8 4 8 /15 No v 3 4 4 5 5 4 56 5 7 57 10 7 d ry 4 / 15 O c t 3 9 5 3 58 7 6 10 9 13 5 14 8 16 7 18 0 v e t 16 A u g 4 6 6 5 15 /2 6 O c t 4 9 4 8 7 1 10 3 114 136 17 3 18 4 2 0 3 d r y 18 /2 1 O c t 5 5 3 7 6 9 9 3 10 3 d r y 2 :Ju l 5 7 6 2 d r y 8 /1 1 A u g 6 0 4 9 8 0 10 0 11 1 19 0 c t 14 N o v 6 0 - 2 5 50 7 0 8 5 1 12 126 139 15 1 19 5 m v ve t 2 4 No v /9 D e c 6 0 3 4 4 5 5 4 5 7 6 1 7 3 90 108 12 3 d ry 13 / 15 M a r 6 4 4 9 70 S I O d r y 9 / 19 J u n 7 1 4 0 57 5 7 6 1 10 6 1 15 155 16 3 ve t 13 /2 2 N o v 7 4 3 5 4 5 5 2 5 5 7 1 10 8 12 1 v e t 4 / 1 1 No v 7 6 2 0 3 5 4 5 6 3 8 3 9 0 • 2 8 No v 1 1 De c 7 6 3 7 6 6 8 0 9 0 v e t 6 / 8 J u l 8 0 7 7 8 4 8 9 d r y 1 1/ 14 Au g 8 0 4 6 4 7 6 0 6 5 2 1 /26 No v 8 2 2 3 3 4 49 5 2 69 v d r y 2 3 /2 8 Ma r 8 4 2 3 37 4 0 48 6 5 23 De c 17 J a n 8 6 2 7 4 4 4 9 6 6 77 9 7 97 10 0 13 5 X v e t 17 /26 No v 8 6 2 3 4 1 6 3 7 2 8 1 8 9 93 N B -U nd e r l i ne d va lue s d e no t e t h e h is to r ic a l m a x imu m fo r t he s ta te d d u r a t io n . Ta b l e A. 1 . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 1 110 0 100 • • •• 1 L t 14 * T 0 h " a A- it 1 .1? • li I X 4 0 0- p I r 7 0 • y S i u s h . • t • s it a Iv / I 40 0 v I S •• Sa y TS B I k V Ity v I s n ot 10110n 10 1 2 3 • • 01/110 / 0 •1 ( 011 VS ) — 1. De p t h -d u r a t io n p lo t o f no t a b le ra infa lls at E as t bo u rne (W i lm in g t o n S q u a r e ) in pe r io d 18 88 - 19 86 0 • • 1 . Eastbourne Borough Council 198 5 'Eastbourne Park D istric t Plan 41 -Polic ies and Proposa ls ' • (Abb reviated version of the • Written Sta temen t). 41 2 . Eastbourne Borough Council un dated 'Eas tbourne Park Landscap e • Studies Appendix A • Calcu lation of flood storage requ iremen ts ' 41 • 3 . Natural Env ironment 1975 'The F lood S tudies Report ' Research Coun c il London • 4 . Institute o f Hydrology 1985 'The FSR ra in fall-runoff model parameter estima tion equation upda ted ' Walling ford • 40. 5 . Institute of Hydrology 1979 'Design floo d estimation in ca tchments subjec t to urban isation ' Walling ford • 6 . Th e Institu tion of Civil 1978 'Floods and reservoir sa fety : • Eng ineers An Eng ineer ing Gu ide ' London • 7 . Countrys ide Research 198 5 'A report on pollu tion un it , Br igh ton Polytechn ic mon itoring in the area of the • Lo ttb ridge Landfill , 41, Eastbourn e Park ' • 8 . Coun tryside Research 1985 'Lottb ridge Drove Landfill , Un it , Brigh ton Polytechnic Eastbourne , Tree Plan ting • Tr ials ' • 9 . A J Morey 198 5 'Eng ineering evalua tion o f a 111 recen t infilled estuary , Eastbourn e , Eas t Sussex ', M .S .thes is University of • London •