E a f e t y a i n s t i n u n a l a t i o n dutcb approacb
Hêndrik J. verhagen a n d W i l l e n r F . V o t k e r *
abstract
The most urbanized part of the Netherlands is situated below sea level and is protected by dikes. By act of parliament allowable inundation frequencies for each part of the coun-try are fixed. In case of two dangers (river run-off and stonn surge) it is a problem to deterrnine design values for dikes. Guidelines are deveLoped to solvê these problens.
Bêcause econonical , social and cultural problens with the dike improvênênt prograrn, design hêights of dikes havê to be determined vêry accurately. To overcome many problêms, it has bêen decided to build a storrn surge barrier of 380 m vridê and a threshold depth of 17 m bê1ort sea fevef in the navigation fairway to the port of Rotterdam.
introduction
The Nêtherlands are nainly Located in a floodplain. Especi-aIly the industrializêd and nost urbanized part of the coun-Èry is located in areas which are lying below high sea water levêI . See figure 1. Thêsê arêas (poldêrs) are protected by dikes and dunes. Dikes wêrê constructed along the sea, along rivers and along estuarj.es. The dikes should have such a quality that inundation of the polders is prevented.
one of thê rnost irnportant paramêters of the quality of a dike is its height. Dikes can never be constructed in such a way that the probability of inundation is zero. However this valuê should have such a lorÍ value that the inhabitants have thê feeling that they can live safely behind the dikes. In this paper the philosophy to deterninate the height of a dike is discussed.
history
desiqn practice for river dikes until l-953
The height of dikes was in history always a problem. usually *coastal engineers of the ninistry of public lrorks
(rijkswaterstaat), hydraulic division
p.o. box 5044r 2600 GA Delft, The Nêtherlands a 9
thê
0
50
100km
-4
t-L 3 f l o o d p l a i n , i n u n d a t e d b y s t o r m s u r g e s -b y absence oÍ dikes
figure 1. Àrea of the Nêtherlands prot.ected by dikes
dikes were designed at a crest Lêvel of 0,5 m above the highest known water 1eve1 , with a surcharge for wave run-up. After a serious flood. much dikes in the coastal- zone $rere
inproved. The height was increased up to a level related to the highest stonn. However, because of botton subsidence the
V p r h r r r o n / \ T O I k e ! .
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crest height of the dikes became also lower in respect to leveL of the sea. Thê river dikes in thê Netherlands werê also designed on a hiqhest known water-Ieve1 . Also spêcial
rrgrêen riversrr were in operation to divert water if the water i-n the river raisêd to a too high 1evel . Because after
l-926 there s/ere no serious probl-ems with river dikes, therê was less political pressure to improve the dikes than it was
in thê coastal area. After the second norld war engineers and mathematicians warned that this approach is not correct, and that an êxtreme value statistic should be applied. The consêquence of this approach lrouLd have considerabl"e finan-cial consequences, and no political decision was made.
the 1953 storm surge disaster
on february Lst, L953 it stonned. The \^rater-l-eve1 raised to a level- of 0.6 m higher than the highest observed. storm surge (of 1-894), with as consequence that t-365 krn2 was inun-dated and L835 peoplê were killed. 47300 houses lrere damag-ed, the totaL damaqe to real estate was 160 rnillion guilders
( i n 1 " 9 5 3 l - U . S . $ w a s a p p r o x . 3 . 6 0 D u t c h g u i l d e r s ) . I n conparison, in 1916 6874 km2 and in j.894 306 ktn2 was inun-dated. Because nost of this area was also below mean sea 1eveI, after passage of the storrn, the polders were still coverêd s/ith water. Repairing l_60 kn of dike took rnore than a year and it cost 380 nillion guilders. The total direct costs of thê disaster $rere 2OOo nillion guilders, which was l-4 I of the gross national product in 1952.
The nain conclusion was: this should never happen again, anc a comnittee of specialists (the delta-comnittee) was instal-led to find a solution
solution of the Delta-cornrnittee
The dêlta-committee concluded in its final report that dikes should be designed on a design stom-surge levet wj.th a given probability of occurrencê. From economic considerati-ons foLlov/ed that storm-surge 1evêIs with a probability of less than 1/10000 a year are the optinun for the densely populated central part of Hol1and. For the other provinces this value is 1/4000 a year.
The L/10000 storrd surge levêl was deternined frorn extrapola-tÍon of all knovrn water-levels at Hook of Holland (see fig. 2), and resulted in a design water-level of 5 m above mean sea level .
This figure has to be corrected for the various locations along the coast.
The Delta-corunittee advised that, a dike should be desiqned
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in suctr a way that every cross-section can withstand this waÈer-Ievel, with the accompanying wave-run up in such a way that no serj-ous datÍrage to tbe dike will occur' Thê nunber of overtopping waves strould be lêss than 2 g'
J'-
\ry-l
I -:---< 0 Êaffi'ï t,ro.ooo
E l 1 : 4 O O O E l 1 : 1 2 5 O Hook ol 100km ) I i i Í ') I ) Ilemgrli.l sh!icas tót6)l
l1
(.-.,i
* dunasr protactlng poldc?s
l
- , - ]
figure 2. dike-circles and inundation frequêncy
Àround 1975 it was realized that aLso river dikes had to be irnproved, in order to guarantee an identical safety to thê people living behind river dikes. Thê 'tBêcht-committêêrr was
forned to detennine the height of river dikes. This
tee concluded that inundation by rivers is less serious than inundation by sêa water. This is caused by the fact that:
* inundation with frêsh htater causes l-ess prob]ems
* the warning t irne for high-water run-off is longer than for a storn surge from the seai
* the polders along the rivers are mainly above the nor-ma1 nonnal water level, and conseguently the water flows out of the polder after passing of thê high-water in the river
* there is no tidal fl-oer through the gap in thê dike, and repair is therefore more easy.
Based upon thesê considerations the Bêcht conmittee decj.ded that river dikes should be designed on a run-off with a probability of occurrence of L/ f25O per year. The rrBecht-conmitteerr was influenced by the fact that the public dit not like the r,rorks for raising dike-levels
probabilistic consideratioDE
À s d i s c u s s e d a b o v e , t h e d ê s i g n o f a s e a - d e f e n s e w a s i n a l l cases based upon a representative load (r^rater-leve] and s/avês ) ltith a defined probability of occurrence. The fact t h a t t h e r e i s a l s o a v a r j , a t i o n i n t h i s l o a d , a s s / e 1 l a s a variation in the strength of the sea-defense was neglected. Studj.es Itêre perforned in order to find a probabilistic method in which also these variations could be takên into account. Iíith a probabilistic approach it is possible to a c h i e v e t h i s . l v r o u w e n v e ] d e r , L 9 8 7 1
the dlike-circle
U s i n g t h e p r o b a b i l i s t i c c o n s i d e r a t i o n s , r n e n t i o n e d a b o v e a l l o w e d t o q i v e a p r o b a b i l i t y o f f a i l u r ê o f d i k e s e c t i o n . ï n fact a dike has also a given length. The inhabitant of a polder arêa it is not interested what is the probability of
failure of a give dike section, but lrhat is the probability of getting rdet feet or worse. The low lying part of the N e t h e r l a n d s i s d e v i d e d i n s o - c a l l e d d i k e - c i r c l e s . A d i k ê -c i r -c l e i s a 1 o w - I y i n g a r e a , w h i c h i s s u r r o u n d e d b y d i k e s , dunes and/or high grounds. Failure of one section of the sea-defense usually rêsults in the inundation of a Í,./hole d i k e - c i r c l e . E a c h d i . k e c i r c l , e h a s a g i v e n a l 1 o w a b 1 e p r o b a b i -I i t y o f i n u n d a t i o n . I n f i g u r e 2 s o n ê o f È h e 4 0 d i k e - c i r c t e s of the Netherl,ands are presênted with the a1Iowab1e inunda-t i o n f r e q u e n c i e s .
The allowable inundation frequencies are laid down in the Larr/ on Sêa Defênse. The choice of an allowable inundation frequency is fundanentally a political decision.
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More recênt studies are performed to find the optimal aLlov/-able inundation frequency, based upon the econornic value of real estate and infrastructurê in the dike circle' However ' the values of human life, natural environrnent, historical and cultural values' etc. made it inpossible to definê the o p t i m a l v a L u e i n a n o b j e c t i v e w a y '
I t i s i n t e r e s t i n g t o n e n t i o n t h a t t h e r e s u l t s o f t h ê s e s t u d i e s i n d i c a t e t h a t t h e e c o n o m i - c a I I y o p t i n a l v a l u e s a r e i n t h e o r d e r o f t 0 - 4 t o l - o - 5 p e r y e a r . T h e f r e q u e n c i e s p r o p o s e d in the nêtt Lal,t on sea Defênse are a factor 10 higher'
I n t h i s L a w i t i s a l s o s t a t e d t h a t t h e b o u n d a r y v a ] . u ê s ( S u c h
as water-leve]s) have to be recalculated every five year' and that dike nanaqing authorities have to certify every f i v e y e a r s t h a t t h e i r d i k e s t i l l f u t f i l l s t h e r e q u i r e n e n t s .
So, they have to check the hêight of the dike, the quality o f t h e s l o p e p r o t e c t i o n , e t c . I n t h i s w a y i t i t s t r i e d t o prevent that the effect of clirnatic changes causes sur-p r i s e s , a n d d i k e s h a v e t o b ê a d a p t e d t o t h e n e w s i t u a t i o n s r e g u l a r l y . T h i s i s t h e r n a i n r e a s o n t h a t d e s i g n $ / a t e r - l e v ê l s ' e t c . , a r e n o t g i v e n i n r e g u J ' a l i o n , b u t o n l y t h e i r p r o b a b i l i -t i e s .
thê problên of tidal rivels
Along tidal rivers there is a conplicatj'ng factor' High water 1eve1s can be caused by hiqh river run-off, but also by storn surges entering frorn the sea. The problem of calcu-I a t i n g t h e w a t e r l e v e L s i s d i s c u s s e d b y V a n d e r M a d e []'9691' The step from a design waterlevel to an inundation frequency of a dike-circle is recently worked out for the guidelines o n d e d e s i g n o f r i v e r d i k e s [ T À ! Í , 1 9 8 9 t T A ! { ' ] ' 9 9 0 1 ' I n
figure 3 and 4 the local water-1evel is presented for t$/o stations at different points along the sane dike circl'e' For t h e l o c a t i o n s o f t h e s t a t i o n s J a a r s v e l d ( J ) a n d S l i e d r e c h t ( s ) s e e f i g u r e 2 .
The l^tater-level is a function of the water-level- at sêa ( H o o k o f H o l t a n d ) a n d t h e r u n - o f f ( R h i n e d i s c h a r g e a t L o b i t h ) . B o t h t h ê h t a t e r - I e v e l a t H o o k o f H o l l a n d a n d t h e river discharge at Lobith have there own probabiJ'ity of occurrence. These variables are not ful1y independent ' For e x a r n p l e a t s t a t i o n J a a r s v e l d t h e h e i g h t o f t h e d i k e i s 6 ' 0 meters above mean sea leveL. This vtater-Ieve1 can be achiev-ed by a storm surgê of 5.5 m above nean sea leve1 ' and a r u n - o f f o f E o O O * 3 7 = o r b y a s e a w a t e r l e v e l i f 3 ' 3 Í r ( o n c ê i n t e n y e a r s s t o r m ) a n d a r u n - o f f o f 1 5 o o o m "/s'
Using both the distribution of extreme storm surges as weII as extrerne river discharges, the exceedance-frequency for
Verhagen/Volker
all stations can be calculated. Exarnples are presented in fig 3b and fig. 4b. Fron these diagralBs follows that in Jaarsveld a water-levêL of 6.0 m is exceeded with a proba-bility of L/ ooo. In sliedrecht 3.75 rn is exceeded with the same frequêncy. , E ! .eao 6009 80s0 rs0g6 t2gsg d i s c h a r g e o f f h e R h i n e a t L o b i t h Í m l l s l e x c e e d a n c e - f r e q u e n r y Í e v e n i s / y e a r l to g l 0
figure 3. Waterlevêls at Jaarsvêld
Verhagen/VoIker
JooísYeld
If for aL1 stations aLong the dike these diagrams are con-structed, for each dike section the water-level vtith a pro-bability of occurrence of l/4ooo pêr year can bê deternined. Unfortunately this does not mean that if the dikes are con-structed in such a way, the probability of inundati-on of the dike-circle is also I/4OOO per year.
] zaae .sag d l s c h e r g e 6900 8000 tggag | 2B€€ o f t h e R h i n e à t L o b i i h I m r , / s l l1€09 r6a9s r 8sg3 3 ' ) .l 3.zs z a e x c e e d ê n c e - f r e q u e n c y l e v e n t s , / y e a r l l o g 1 0
figure 4. waterlevels at Slj.edrêcht
Sliedrecht
when a dike-circLe has only two section, both constructed in such a sray that they have each a probability of failure of I/4OOO per year, and thêy are attacked by fully uncorrelated dangers, thê probability of inundation of the dike-circle is 2 t.imes 7/4OOO per year.
In real j,ty a dike-circle has rnany sections, and they are attacked by partly correlated dangêrs. Mathernatical tech-niques are available to calculate also in those cases the probability of inundation. This is in the order of three tines the probability of failure of a single dike section.
lequired acculacyi politics, ecology ard social problêms In the NetherLands stonn surges with a ten tirTres Less fre-quency of occurrence are approx. three quarter of a mêter hi.gher. This irnplicates that if a dike is made approx. haLf a meter higher, the safety increases approx. with a factor
5. (Wave run-up may change these figures). The cost of
naking a dike a littl,e bit higher are generally not very hi.qh, if the dike is situated in agricultural areas. Àlso a few extra feet increase in height doês not introduce ecolo-gical and social problerns as a ru1e.
fn build-up areas however, this is not true. The dikes cross towns like Rotterdan and Dordrecht. E.g. in Dordrecht the naj.n shopping cenÈer is established in historicaL houses, built on the dike. These buildings date fron the 18th centu-ry and have.a hÍgh historical value. This irnplj.es that the conseqÍuences of L0 cn extra height are considerable. Such a difference costs sonetines 10 rni.Ilion UsS nore for a dike stretch of less than one kilometer.
thê storE surge barriet
Because of the problens of constructing dikes in the tidal_ area, it has been decided by parliarnent in narch 1989 to construct a stonn surge barrier in the entrance channel to Rotterdam. This will be a barrier with a free width of 360 n
(without any piers in the channel) and a threshold depth of 17 n belo!, nean sea level and no li:nitations in height for shippj,ng traffic, in order not to disturb traffic to Rottêr-dam. (Thê harbor-basins for deep-drafting ships, rnore than 17 n, are situated sea$rard of the barrier) This barrier wil1 be closed only during stonn surgês occurring once in approx. 1 0 y e a r s .
Upstream of this barrier dikes in the tidaL area will only be attacked by river run-off and not any more by storn surges. The conseguence is that dikes in the tidaL area do not need to be raised. fn build-up arêas this rnakes it rnore
Verhagen/Vo lker
-easy to find suitable solutions. Financially the construc-tion of a storrn surge barrier i-s rnore or less equal as raising the dikes. The construction of the barrier will cost a lot of money in the next fêr" years (?50 lTtillion US $), but irnproving all the dikes in the tidal area will cost nore tirne (until 2O2O). Especially the long tines required for raising the dikes, and the social and ecological problens of these !íorks are politically unacceptable.
practical guidlelines
In spring 1989 "Guidelines for the design of river dikes, part 2, tidal riversrr has been published. In these guidelin-es an operational ruethod is presented for the dike rnanaging authorities to design dikes. This nethod consists of two parts. In the first part an approximation is given for the height, using a table of water-leveL/wind cornbinations for each place around the dike-circle. In the second step all thê dikês of a dike-circle are enterêd in a computêr pro-gram. This program cal-culates r.tith the abovê nentioned neth-od the probability of inundation of the area. By trial and error the crest levels of the dikes rnay then be found in such a way that the probability of inundation is correct and the total rrcoststi (noney + other problems) are ninimal .
In this version t'inundation'r is defined as a situation in which at aÈ least one locati'on around the dike-ci-rcle the vrater-Ievel and wave run-up exceed the crest height of the dike. So in fact onl.y faj.lure by overtopping is taken into account, Fai.lure by other causes (for exanple by instability of the subsoiJ.) is also taken into consideration. The design should be in such a way that the probabitity of occurring of such a failure is extremely srnall. For thê tirne being it j's estimated that probability of failure is for 60 t deternined by overflow and overtopping (which is conpletely included in the probabilistic approach), and for 40 3 by all' the other mechanisrÍts of f ailure.
It is the intention that in the next version of the guide-lines also the other failure rnechanisro are included in the calculation, so that reatly the probability of inundation is calcuLated.
Àfter running the progran the authorities can see if the systern fulfi11s thê legal requirenents. ff not, somenhêre a section of the dike has to be increased in height. The pro-grarn also indicates which sections contribute to the insecu-rity. Thesê sections should be considered for improvernent ' Àgain by trial and error the nost effective inprovement can be found.
In the computer progran the effect of a storn surge barrier is fully included.
re fêrêDcês
Remark: ltlany references can be given to reports of the Dutch governnent on this subject. Àlthough thêse reports are not confidential , they arê quiÈê difficult to obtain abroad, and they are all in Dutch. In the folJ.owing list only accessible
references in English are given. Interested readers nay
contact the authors of this paper for background information from the Dutch reports.
ItÍade, J.Í{. van deri Design levels in the transition zone betweên the tidal reach and the river regine reach, Àssocia-t i o n I n t e r n a t i o n a L e d ' H y d r o l o g i . e S c i e n t i f i q u e , B u c a r e s t c o n f e r e n c e , 1 9 6 9 , p p 2 5 7 - 2 7 2
Technical Àdvisory comrnj.ttee on waterdefenses [TAW] t cuidê-lj-nes on the design of river di.kes, part 1, non-tidal ri-versi to be published by CUR/Gouda in 1989 (translation of the Dutch guidelines of 1984)
Technical Advisory conmittee Iines on the design of river be published by CuR/couda in g u i d e l i n e s o f 1 9 8 9 ) .
on Waterdêfensês ITAW] t Guide-d i k e s , p a r t 2 , t i Guide-d a l r i v e r s i t o 1990 (translation of the Dutch
Vrour.rênve1der, A.c.l{.M, t Probabilistic Design of Flood de-fences, CuR/couda, 1987
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