ThamesWater Tida l T hames Defence Levels Preliminary Report on River Lee Flows and Levels August 1987 A R CHI VE : PLEA SE D O N OT DES TR O Y ThamesWater Tidal T ha mes Defe nce Levels Preliminary Report on River Lee Flows and Levels August 1987 Sir William Ha !crow & Partners Ltd Institute of Hydrology PREL IMINARY RIVER LEE ANALYSIS CONTENT S Page No GLOSSARY SUMMA RY 1. INTRODUCT ION 1 1.1 Objectives 1 1.2 The Lower River Lee Sys tem , 1 1.3 Da ta Availability 2 2 . DATA COLLECT ION 3 2.1 Tida l Wa ter Level Data 3 2 .2 Flood Da ta for the Lower Lee and Tribu taries 3 2 .3 Sub-catchment Area s 3 2 .4 Urban Area s 4 3 . TR END ANALYSES 5 3 .1 Water Leve l Trend s 5 3 .2 Flood Trend s 5 4 . FREQUENCY ANA LY SES OF T IDAL WATER LEVELS 6 4 .1 Frequen cy of Tida l Wa ter Leve ls 6 4 .2 Adj ustment to Lee Mouth 6 7 . FLOOD FREQUENC IES AT RIV ER GAUGING STAT IONS ON THE LOW ER LEE 7 5 .1 General Approach 7 5.2 Flood Frequencies Based on Data Ana lyses 7 5.3 Flood Frequencies Using FSR P rocedures 8 5.4 Propo sed Flood Frequenc ies for Ga uging Station s 9 6 . FLOOD FREQUENC IES IN LOW ER LEE CHA NNELS 10 6 .1 Flood Freq uencies in Lee Flood Relie f Channe l 10 6 .2 Flood Frequenc ies in Lee Nav iga tion Chan ne l 11 FIGURES 1.1 Diagramma tic Sys tem o f the Lower Lee 3.1 Flood Trends 4 .1 Concurren t Peak Water Leve ls at Brunsw ick Wha rf and Tow er P ier 5 .1 Feildes Weir Con current Flood s , Instan taneous and Mean Da ily 5.2 Flood Frequency Ana ly ses 5.3 G row th Fac to rs fo r Main Lee 5.4 G rowth Fac tors fo r Low er Lee Tributaries 6 .1 Low Hall and Fei ldes We ir Concurrent Floods 6 .2 Corre lation o f Ruralised Spec ific MAF 's with Area TABLE S 2 .1 Ava ilab ility o f Ann ua l Max imum T ida l River Leve ls 2.2 Ann ua l Max imum T ida l Water Leve ls 2 .3 Flood Data Ava ilability on Low er Lee and Tr ibutaries 2.4 Annua l Max imum Instan tane ous Flood Peak Data 2 .5 Catchment Areas and Urban Per cen tages 3.1 Corre lation and Rise in Ann ua l Max im um Leve l 4 .1 Quan tiles for Thames T ida l Stations (m AOD ) 4 .2 Quantiles for Tham es T ida l Stations (m AOD) (U pda ted to 1986 us ing loca l trend ) 5 .1 Flood Frequenc ies on the Lower Lee and Tr ibutar ies , based on Da ta Ana lyse s 5 .2 Flood Frequencies us ing MAF and FSR growth factors only 5 .3 Proposed Flood Frequencies for Gaug ing Station s on Lower Lee and T ributaries 5 .4 Grow th Fac tors for Low er Lee Catchments (Ca lcu lations) 5 .5 Ca tchmen t Parameter s and Urban ised Grow th Factor s (SE and 6/ 7 Reg ions ) 6 .1 Fe ildes Weir and Low Ha ll Observed MAF 's 6 .2 MA F 's us ing FSR Ungauged Catchmen t Proced ure 6 .3 Observed and Com put ed MA F's 6 .4 Proposed Flood Frequencies in the Lee Navigat ion Chan ne l GLO SSARY AM ann ua l max imum flood peak ser ies AREA catchment area (km2) CW I catchme nt wetness index EVI extreme va lue Type I distr ibution FSR Flood Stud ies Report (NERC , 19 75) GEV genera l extrem e va lue distr ib ution instan taneou s flood peak (m 3/s ) MAF mean o f AM instantane ous flood ser ies (m 3/s ) MA FD mean o f annual max imum mean da ily flood series (m3/s ) MAFr MA F for rura l catchmen t (m3/s) MA Fu MAF for ca tchment includ ing urban deve lopment (m 3/s ) MD mean daily flood peak (m3/s) length o f reco rd (years ) POT peaks—over—thresho ld flood peak ser ies PRr percentage runo ff from rura l ca tchmen t PRu percen tage runo ff from catchmen t including urban dev e lopmen t Q50r 50—year flood from rura l ca tchm ent (m3/s ) Q50u 50—year flood from catchmen t inc lud ing urban deve lopmen t (m3/s) SAA R standard (19 16-50 ) annua l averag e rain fall (mm ) SO IL so il index (FSR ) STM FRQ stream frequen cy (junctions/km 2 ) S 108 5 10-8 5% stream slope (m/km ) URBAN fraction o f catchment in urban deve lopmen t SUMMARY In this study , the freq uenc ies have been estimated o f Tham es h igh tida l water levels at the Lee /Thames con fluence , and of flood pea ks in the Low er Lee . A summ ary of these results is presen ted in Tab les A and B . These water leve l and flood frequen cies rep resent the prelim ina ry inputs need ed for the Tham es Tida l Mode l. However , at th is stage the water leve l frequencies a re based only on histo ric records , and modifica tion s are be ing carried out to take proper account o f the e ffects of the Thames Barrier on future wa ter leve l frequenc ies . The historic water leve l frequenc ies show n in Tab le A for the Lee /Thames con fluence have been derived from an in terpo la tion be tween the freq uenc ies ind icated by the long -term water level ga uging sta tions at North Woo lwich (5.7 km downstream ), and Tow er Pier (10 .6 km upstream ). A lso show n fo r com pa rison are the wa ter leve l frequenc ies at long-term stations Sou thend and Tilbury , and frequencies at B runsw ick Wha rf , where 30 years o f Thames water leve l records ex ist for a fo rmer recording station 0 .5 km ups tream o f the Lee /Thames con fluence . The effec ts o f the linear upwa rd trend in tida l water levels a t the se station s were computed and compa red in the course of the study , and th is trend has been taken into account in the water leve l frequenc ies shown in Tab le A so tha t they rep resent 1986 cond itions. Flood frequencies have been computed for bo th o E the Lee channe ls wh ich discha rge into the tida l reach near the Lee /Thames con fluence . The easte rn cha nne l , the Flood Relie f Channe l , carries flood runo ff from the grea t bulk o f the 1370km 2 catchment , inc lud ing most o f the rura l ca tchm en t. Its flood freq uenc ies are based on the sho rt reco rd o f floods recorded at the lower end of the Flood Re lie f Channe l at Low Hall , using the long-term rec ord upstream at Fe ildes We ir for verifica tion . The weste rn channe l o f the Lower Lee is the Lee Navigation Channel. Th is channe l carries flood flows from a number o f tributar ies d raining 116 .5 km 2 o f high ly urbanised land in the N and NE reg ions of London . No river gauging sta tion reco rds floods on this channe l , and so flood s have been estimated from an ana ly sis of flood records for a number of Lower Lee trib utaries . Flood freq uenc ies for the se Lower Lee tributaries , which were derived in the course o f the study , are a lso presented . Although the area d rained by Lee Nav iga tion Channe l is re lative ly sma ll , because o f the high ly urbanised nature of its ca tchment and its steepness , its flashy ins tan taneous flood peaks a re quite high in com pa rison w ith the m uch flatte r and the m uch more volum ino us flood pea ks o f the Flood Re lie f Channe l. It sho uld be no ted tha t the presented flood peak s in the two channe ls for any particular return pe riod wo uld no t norma lly be expec ted to co inc ide , though w ith the righ t combina tion o f storm ra infa ll seq uence and movement it is possib le on rare occa sion s . Flood da ta ana lysed in the course o f de riving the se flood frequencies ind ica te gene rally upward trends , re flec ting the inc reasing urbanisation o f the ir ca tchments . Data periods a re too sho rt to take m eaning ful acco unt of the trends in flood freq uencies in this study , but further investiga tion into the historic rates of urba nisa tion with in Lee sub-ca tchm ents may ena b le re finements to be made in this context .