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June 1972601 Kalil We,tmirvter Dredging Group nY.
20 Rosmolenweg, Papendrecht, Holland.This Report has been compiled by: Hydronamic N.V.,
Port and Waterway Engineers, 34 Merwestraat, Sliedrecht, (Holland). in close consultation with
Dredging Investigations Ltd.
Port Causeway, Bromborough, Cheshire (England), and other member companies of the
Bos Kalis Westminster Dredging Group NV. Copyrights are reserved by the
S
ummary
.
The Report deals with the building of islands in the North Sea, a proposal motivated by the fact that numerous industrial, traffic and other the environment burdening activities in densely populated coastal areas cause considerable problems. A solution for these problems, mainly of an environment-technical nature, must, in the view of the compilers of this Report, be found mainly in a clear separation of residential and working areas.
Many of these industrial activities are, to a larger or smaller degree, tied to sea transport, and situated along the North Sea coast. The North Sea, being a comparatively shallow sea, is therefore suitable for the building of artificial sea islands which should be able to offer a solution to the many such environmental problems.
The Report, compiled in terms of a very general nature and based on schematised physical and nautical basic design conditions, deals in broad out-line with the civil engineering aspects of a number of types of islands with an area of approximately 50, 300 and 1,000 hectares.
Apart from the design, attention has also been paid to the important aspect of the costs involved.
The object of the Report is to make a positive contribution towards discussions on the possibilities of building islands in the North Sea.
By combining efforts of the authorities and the industries concerned and thereby bringing together available information and experience, an early and from a technical point of view optimal realisation will be possible.
Table of contents.
Summary.
1.
Introduction.
Page 5 9 1.1. The motives for building islands in the North Sea. 9 1.2. Similar land reclamation projects. 10 1.3. Division of the contemplated islands. 102. Basic design requirements for
islands to be built in
the North Sea.
132.1. The location of the islands. 13
2.2. Depth and soil conditions. 14
2.3. Wind and waves. 17
2.4. Water-levels and current velocities. 18
2.5. Nautical design conditions. 22
3. Considerations with regard to the
construction of islands to be
built in
the open sea.
253.1. General. 25
3.2. Design levels of sites and sea-defence walls. 25 3.3. General considerations in regard to some
sea-defence constructions. 26
3.3.1. Sand beaches.
3.3.2. Slope-defence construction. 28
3.3.3. Monolith-construction with the use
of caissons. 29
3.3.4. Other possibilities. 29
3.3.5. Costs incidental to the construction of
sea-defences. 30
4. Considerations ,
concerning the design
and building of various sea islands
and the costs incidental thereto.
324.1. General. 32
4.2. The design of the various islands. 32
4.3. Possible building method. 34
4.4. Construction costs per surface-unit. 40
5. Review.
421.
Introduction.
1.1. The motives for building islands in the North Sea.
In densely populated Western Europe and -especially in the countries grouped around the southern part of the North Sea, the rapid industrial -development accompanying economic growth creates
more and more conflict situations in respect of the use of available space.
Despite continuous efforts by Authorities and ·industries to counter environmental problems and pollution, people living and working in densely populated areas of Western Europe tolerate an environment incompatible with certain industrial and
related activities. This is due to many factors, including:
1. the effect on the environment by chemical, petro-chemical and steel industries in the vicinity of populated areas combined with the problem of the treatment of often dangerous waste matter liberated in production processes;
2. the ever increasing tendency to build larger vessels which require harbours of great depth, there-by frequently necessitating deep excavations in coastal areas with such resulting problems as increasing salination, loss of existing land areas, etc.; 3. the increasing demand for electrical energy and the cooling water problem of the power plants incidental thereto;
4. the rapidly growing air traffic with its attendant noise problems in the vicinity of large airports (e.g. Kennedy International Airport alone, which can be said to be located at a reasonable distance from the City of New York, causes an immense noise nuisance to over half a million people living in the surroundings of the airport);
5. The rapidly increasing build-up of a large variety of waste matter both by industry and by the popula-tion, the disposal of which creates an enormous problem in densely populated industrialised areas. To preserve Western Europe's prosperity and notably that of countries situated along the North Sea, it is of paramount importance to create a favourable industrial
climate. Holland.principally due to its location at the mouth of the Rhine, Europe's most important water-way, will obviously play an important part in future industrial development.
Such situations as mentioned above will, of course, have their effect on an increasing scale on the policies concerning establishment and may lead to the total rejection of certain activities - such as the building or extension of airports, deep water harbours and the setting up of new industries, if no proper guarantees can be given. It is, therefore, of the utmost importance to develop plans, whereby the problems referred to may, to a certain degree, be reduced.
Because many industrial activities are associated with and are users of sea transport, the possibility of the establishment of artificial islands in shallow seas, isolated from the residential centres, can in certain cases be a good solution. In addition it has become very evident during recent years that the exploration of the North Sea, especially for the winning of gas and oil is becoming more and more a reality. On the British side of the Continental Shelf, gas is already being obtained and transported to the mainland via pipe-lines, and on the Norwegian part, gas-winning is due to commence shortly.
Naturally, suitable technical environmental requirements will have to be set up on islands of this nature and there must be no question here of any direct effects on the residential environment. More-over, this solution offers the possibility of ensuring that, in a more·efficient way, the observance of rules relating to the burdening of the environment through the concentration of the industrial and other activities in a relatively small area.
Although the realisation of such artificial islands involves various problems, such as civil engineering; problems related to communication with the mainland for the supply and removal of products, the trans-portation of operating staff; the problem of energy and fresh water supplies and problems of legislation, it is expected that interest in such offshore land reclamation projects will in the very near future be high.
The Report gives a summary of the results of a preliminary investigation into the viability of the building of islands in the North Sea. The
nature of the investigation is clearly one of orientation and it does not aim at launching new spectacular ideas, but rather to summarise existing information in such a way that it can serve as a contribution towards further discussions regarding the problems.
The character of orientation of this study is basically of a schematic nature. No research with hydraulic models, which will be absolutely necessary in the case of any definite project, have been carried out. For the purpose of the Report a certain area in the Southern part of the North Sea has been contemplated for the building of such islands. Conse-quently, the costs estimate, although in the correct order of magnitude, must be looked at with necessary caution.
1.2. Similar land reclamation projects.
In the North Sea the reclamation of land for industrial purposes has already become a reality, with the construction of the Maasvlakte, the western -most part of the Rotterdam harbour and industrial area. (Fig. 1, page 12). Such plans for land reclamation have already been put forward in Holland including: - the further extension of the Maasvlakte, in a southerly direction within Plan 2000+ by the Municipality of Rotterdam.
- the construction of an airport linked with the Goeree-coast, south-west of Rotterdam, by the Inter-national Port and Harbour Construction Company N.V. - the construction of industrial sites to the South of the jetties of IJmuiden by the Municipality of Amsterdam.
These projects coincide in that they link up with the coast and as such can be considered directly as land reclamation and can make use of the existing infrastructure.
Other countries located around the North Sea also have plans for project such as:
- Maplin Sands near Foulness, to the North-East of London, for a large international airport and a deep sea harbour.
- the so called 'Sea City' on the Haisborough Tail. the Plan Zeestad, (Sea-City), in Belgium. - the plans for an island on Thornton and Akkaert Bank off the Belgian coast, for a terminal for large tankers.
- in Germany the investigation of the possibilities for construction of an artificial harbour for use as a large oil tanker terminal near Heligoland.
10
1.3. Division of the contemplated islands.
Considering the developments referred to in the preceding paragraph it may be assumed that the realisation of islands for industrial and related pur-poses will become a reality. Dependent on their function, such islands can be classified in three groups, i.e.:
a. Small islands with an area of about 50 hectares. Such islands may be used for highly specialised activities, such as centralised waste treatment, central storage or, as the case may be processing of gas and oil extracted in the vicinity of the island, the generation of energy together with a fresh water plant and, probably as a first building phase for larger islands. Another possible use of an island is as a fresh water supply link for Holland. Here drinking water supplied in gigantic plastic containers from other countries, can be pumped by pipeline to the mainland.
b. Islands with an area of about 300 hectares, which could be used for the construction of oil terminals, the establishment of specialised industries and for harbours where emergency repairs to ships and vessels could be undertaken.
c. Very large islands with an area of 1,000 hectares or more, which could be used for extensive deep water bound industries, including power plants
and fresh water plants.
The construction of offshore airfields is also within the bounds of possibility, coupled with appropriate transportation facilities for the carriage of passengers to and from the mainland.
This grouping is in no way complete and it is conceivable that in the case of the construction of a large island by stages, that other groups would be introduced. Furthermore, the site area required for certain industries depends largely on the nature of the industry in question.
For the construction of islands in the North Sea, sand will be the most important building material and it may be assumed that the island will be constructed by the building of a sand body, which has to be protected against the influence of the sea (currents and waves) by surrounding it with a sea defence wall. Consequently the costs will consist of two main sections, namely the costs incidental to the building of this sand body and the costs incidental to the construction of the sea defence wall.
Commencing from a schematised standard form for an island and a sea defence wall partly built up from sand, the quantities of sand to different water depths for a number of ground surfaces and the· lengths of the sea defences have been calculated. This shows, that in the construction of islands of under 250 hectares, the quantity of sand per sq.m., especially in the case of greater water depths, increases rapidly because of the volumes of sand required for the slopes. Consequently the cost per sq.m. also increases. For smaller islands the unfavour-able relation between the site area and the length of the sea defence wall also affects those costs. For islands
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12
of over 1,000 hectares it appears that the quantity of
sand per sq.m., remains almost unaffected by the quantity required for the slopes and the costs of the
sea defences become a relatively less decisive factor.
On the basis of these considerations it was
decided to take islands of 50, 300 and 1,000 hectares
as a starting point for the investigations. For each island a schematic plan with an estimate of the costs
has been made, using design criteria set out in the following sections.
2.
Basic design requirements
for islands to be
".
built in the North Sea.
2.1. The location of the islands.
The choice of the location of the islands will be determined by a number of factors which can be
subdived into two main groups:
1. the proposed use of the island concerned;
2. the civil-engineering requirements relating to the
constructional possibilities of the island concerned.
The consideration of these factors will eventually lead to an optimal location.
Relative to the proposed use of the islands the following factors may have an important influence on the choice of the location:
- The effect on the environment. One of the most important considerations for the building of islands is the creation of sites for industrial and other
activities that disturb the environment. A specified minimum distance from the coast, taking into account the predominant directions of the wind and currents, will therefore be necessary.
- The distance with regard to existing ports and industrial centres on the mainland. An essential factor is the speed of supply and transportation of materials, products and personnel between the island and the mainland and the existing industrial areas. It is obvious that transportation between the island and the population areas in the case of islands being used for the construction of airports is important, as is the noise nuisance of such airports which for this reason alone must be situated at an adequate distance from the mainland.
- The water depth in connection with the supply and removal of products by ship. The larger islands with seaport activities will require deep fairways and
approach channels. A location further from the coast and in the immediate vicinity of shipping routes for
deep-draught vessels will, therefore, be necessary.
- The availability of natural sources of energy in the
form of gas or oil can also be an important factor in the choice of the location.
In respect of the civil-engineering aspects it is
clear that:
- The location must be chosen in such a way that the existing coast will not, or will as little as possible, be influenced by silting up or erosion.
- A location must be aimed at where tidal influences - water level and currents - are not too great and where also the waves are not too high.
- The water depth must not be too great as problems grow with the increasing depths and, naturally, the costs per sq.m. of site area will increase.
- The occurrence of building materials, such as sand
and gravel.
Within the framework of this orientation study of the building of islands in a general sense, it is impossible to make a definite choice of the location. However, in connection with further considerations an area has been designated within which the design conditions have been determined in a schematised
7 I
-form. Fig. 1, (page 12) shows the limitation of this area, motivated as follows:
- The first limitation is obtained by involving only the Dutch part of the Continental shelf, with respect to the legal aspects:
- the suitability from a point of view of water depth, physical conditions and existing shipping routes to the ports along the southern coast of the North Sea, with the location restricted to the area south of 53° 30' North latitude.
- Its acceptability regarding distances for transporta-tion, effects on the coasts, factors concerning the environment and depth of the fairway.
- Its suitability when the island is used on an inter-national basis, especially by England, where a location more in the central area of the North Sea would obviously be more favourable. For this investigation the limitation to the East has, therefore, been fixed at 4° east longitude.
On the basis of these motives an area has ultimately been determined as shown in Figures 1 and 2.
The following paragraphs contain a more detailed explanation of the schematised design conditions for this area. In this respect it should be realised, how-ever, that for a definite choice of the location the conditions referred to should be defined in detail on the basis of data obtained from accurate measuring campaigns.
At the same time it should be mentioned that the foregoing considerations do not exclude any other location, outside this area, which possibly offers even more favourable design conditions.
2.2. Depth and soil conditions.
For the construction of artificial islands in the North Sea considerable quantities of sand, increasing in proportion to the water depth, will be required. In general sand is available for winning in most areas of the North Sea and it is essential for the carrying out of extensive dredging operations to know the water-depth and the quantities of sand available for winning in any given area and also the properties of the sand.
Moreover, it is also of importance to know the structure of the deeper layers in order to be able to make predictions about the settlement of the reclaimed island and the stability of the banks. At the same time it is possible that due to the changed hydraulic condition by the island, deeper layers will be exposed to erosion, therefore the current-resistance of the sea-bottom in the area under review must be known.
14
Data concerning the depth of the North Sea have been taken from nautical charts. They show the mini -mum water-depth in a certain sector in respect of low low water spring (L.L.W.S.), and its importance to ship-ping. Figure 2 shows the most important depth lines of the contemplated area. As is detailed, the average water-depth in the southern part of the
conti-nental shelf varies roughly from 10 to 30 metres. However, sharp variations in the water-depth may occur locally, due to shallows, banks and sand-dunes.
The shallows, such as Dogger Bank, occur more frequently in the part of the North Sea to the north of the contemplated area.
The banks often have an elongated form in the direction of the maximum tidal currents. These banks may have been formed by sand accumulation, as, for instance, the Flemish banks, or alternatively through erosion of older sedimentary deposits, such as in the case of the Brown Ridge.
Sand dunes occur in a large area of the North Sea and may even reach heights of over 5 m. They are asymmetrically shaped with in front of the Dutch coast the steepest slopes pointing mostly in northerly and north-easterly direction. This corresponds with the resulting direction of the current. The presumption is that these sand dunes shift slowly in the direction of their steepest slope. This may be an explanation for sand transportation.
Detailed studies of shallows, banks and sand -dunes are necessary in connection with the sand winning operations, the stability of the island and the current resistance of the bottom.
The structure of the material of the North Sea bottom has already been studied in many previous investigations.
In this connection the studies of Eisma and Jarke are of particular importance. Fig. 3 (page 16) shows a survey made by the latter of the average grain dia-meter of the surface layer of the sea-bottom, with an added indication of the layer thickness.
Oele roughly composed a longitudinal profile, shown in Fig. 4. According to other investigators the thickness of the surface layer appears to vary from 10 m in the South to approximately 5 m in front of Hook of Holland and to only a few metres near Den Helder.
Apart from the layer thickness decreasing, the grain diameter towards the North decreases from 300 to 400µ off the Zeeland coast and to 150 to 200µ near Den Helder.
Investigations carried out by Veenstra, Van Eerde and Houbert have shown that the banks in the
southern bend of the North Sea are covered by layers of young sea-sand with an average grain dia-meter of 200µ to 400µ. Near the Brown Ridge, how-ever, this layer is very thin.
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A B C D E F G 20m BROWN RIDGE _J (/) ~ 30m • ::::: 0 _J LU [lJ 40m LEGEND ACCORDING TO E. OELE YOUNG SEASAND SUBSOIL CONSISTING
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FINE SAND AND CLAY (COURSE A-G, FIG. 3)Fig. 4. A characteristic geological profile in the area under consideration.
In general, the sand dunes appear to consist
exclusively of the material of the surface layers.
Knowledge of the deeper layers of the North Sea
bottom is limited. The general assumption is that al ter-natively layers of sand and clay or sometimes layers
of peat will occur.
Summarising, it can be stated that extensive soundings and soil tests will have to precede any definite choice of location of an artificial island in the North Sea from areas suitable for that purpose.
2.3. Wind and waves.
On board the Dutch lightships, as well as on board many freighters, wind and wave observations have been carried out over an extensive period. The
Board of Roads and Waterways has carried out detailed wave measurements along the Dutch coast, where also the K.N.M.1. (Royal Netherlands Meteorological Institute), has recorded wind data in various locations.
On the basis of an analysis of this information, the design wave heights necessary for the design of sea-defences for the contemplated area have been laid down. Insomuch as both the location of the island in the contemplated area and, consequently, the water-depth have not been defined definitely, the result must be seen as a first, strongly schematised approximation.
Wind force
Various investigations show that gales with a
maximum wind force of 40 m per second and over occur, although their occurence is rare. Fig. 5 (page 18) shows data taken from the K.N.M.I. about the frequency of occurrence of certain wind forces dependent on the duration of the wind (longer than 3, 12, 24 and 36 ho.urs, respectively) off the lightship Goeree. These data apply to all directions of the
wind together.
On the basis of the above mentioned investiga
-tions and this information, the following can be said in its generality:
- gales with wind-velocities of 22 to 24 m/sec with a duration of the wind of approximately 12 hours occur on an average once a year.
- gales with wind-velocities of 30 to 35 m/sec, with a duration of approximately 3 hours only occur once every 1 00 years.
- a maximum value of around 40 m/sec for the same duration of the wind comes within the bound of possibilities.
By far during the greater part of the time the direction of the wind appears to be in the quadrant between 210° and 300° in regard of the northerly direction and at higher wind velocities chiefly between west and north-west.
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Fig. 5. Frequency curve of wind forces from measurements obtained from the Dutch lightship Goeree.
Waves.
Most data about waves have been obtained
through visual observations. For the information
about high waves obtained in this way, the usual assumption is that Hvisual more or less corresponds with H10 p.c. Concerning the relation factor between this H10 p.c. and the significant wave height which is of importance for the project, values are indicated of between 1. and 1.1. For the purpose of the following considerations, the former figure has been taken as a
starting-point.
On the basis of the available information of the lightships Goeree and Texel and taking into account
the greater distance from the coast - greater fetch
for easterly directions - a wave rose has been
composed for the contemplated area. The result
thereof is shown in Fig. 6.
For the purpose of the sea-defence construction the design wave heights for the various directions must be determined in the contemplated area. For the
westerly and northerly directions, considering the
assumed water-depth, the criterion for breaking waves
will have to be taken into consideration for this purpose. By way of approximation it is often assumed that the criterion for breaking waves corresponds with the highest wave, which, in fact, breaks when occuring in a recording lasting 10 minutes. This
wave height almost corresponds with the H1p.c. which
is approximately 1 ½ times the value of the significant wave-height.
The relation between the significant wave-height and the wave period incidental to it is shown in Fig. 7. This information taken from visual observations
confirms the thesis often put forward that the average
18
wave period (Tm) for the part of the North Sea con
-cerned can be considered less than 12 sec.
This, Tm more or less corresponds with 2/3 to 3/4 times the period of H
5_
On the assumption that wind force and fetch are
not restrictive factors, the frequencies of the wave
-heights can be determined for various depths. This has formed the basis for Fig. 8 (page 20), in which data
recorded by a number of observation stations have
at the same time been included. Considering the
purpose of this preliminary investigation a design
-wave height of approximately 10 m has been taken as
the starting-point for the design of the sea-defence
construction along the most heavily attacked side of
the island.
For the construction to be made on the other
sides lower values may be used. For the east side, for instance, approximately 5 m. It is obvious that this latter value is dependent upon the eventual distance between the island and the coast.
2.4. Water-levels and current velocities.
For a long time water-levels along the Dutch coast have been recorded. There is, in fact, a
suffi-cient amount of information available to these
shore-stations to determine the trend of the water-levels and
the frequencies of certain high-tide and low-tide levels. For the contemplated area, situated at a dis-tance of 50 to 100 km from the Dutch coast, these data cannot just simply be used as the tidal amplitude and the water level variation due to wind set up
N
w
s
Fig. 6. Assumed distribution of the significant wave height for all directions in the area under consideration.
considerably differs from the coastal situation.
In the southern basin of the North Sea there is a rotating tidal motion which propagates anti-clockwise around an area set in the middle of the sea off
IJmuiden. In this area the tidal amplitude is small. It
becomes apparent from literature that for the contem-plated area the total tidal amplitude will be roughly 25 per cent lower than the amplitude of the shore-stations situated on the same latitude.
Fig. 7. Relation between significant wave height and
the accessory mean wave period.
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Fig. 8. Assumed frequency curve of wave heights for various water depths.
For the purpose of general information the
exceeding frequency of the tidal amplitude for Hook of
Holland is shown in Fig. 9. The average tidal amplitude
here is 1.66 m. The amplitude at IJmuiden and Den Helder deviates only slightly from this figure.
On the basis of these considerations an average
tidal amplitude of 1.00 to 1.25 m has been assumed for
the time being for the contemplated area.
For the purpose of determining the wind set up in
the southern part of the North Sea various calculation
methods are available. From these, often schematised
methods, it becomes apparent that at wind velocities
of 30 m/sec and over, from a north-westerly direction,
the difference in the set up of the water-level over the
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of 1 m.
On the basis of these considerations in regard
of the tide and the so-called wind effect in the
con-templated area it has been assumed, for the sake of
simplicity, that the extremely high water-levels in this
area will be approximately 1 m less than in front of
Hook of Holland (Fig. 10).
Finally, gust bumps lasting up to a few minutes
will be able to cause an extra rise of the water-level of some tens of centimetres. When determining the height of the site this should be borne in mind.
<( 1.0 0 166 A MEDIUM_Jl.Q_AL AM___ELJIUDL _ _ _ f--0.9 1.50 0.8 1.25 07
06 1.00 REFERENCE: TIDE TABLE FOR THE NETHERLANDS. 1971
99.8 99 95· 75 50 25 5 03
PERCENTAGE OF TIME INDICATED TIDAL AMPLITUDE IS EXCEEDED
Fig. 9. Frequency curve of tidal amplitudes in Hook of Holland.
'
"
Fig. 10. Frequency curve of indicated water levels.
All water-levels have been referred to the mean
sea-level (M.S.L.). On the coast this almost
corres-ponds with N.A.P. (Amsterdam Ordnance Datum).
Current velocities.
Fig. 11 shows the current representations for four
points in the vicinity of the area under consideration.
This shows that in the contemplated area vel o-cities of 0, 75 to 1,00 m/sec at spring-tide and
0,50 m/sec at neap-tide should be taken into account,
both for the flood-current and the ebb-current. The
northbound current is, in general, a little stronger
N
+
;r
+6,
,
~-
:
J I :I
I/f
/,
//
270° // '-..J-4 !,,! I 900 +21.:
:
I
1/
\'
At
/:
(!1
/-
2 0 -1 180°I
POINT AI
LEGEND NI
POINT BI
LOCATION POINTS OF MEASUREMENTS IN FIG.2 MEAN SPRING TIDE MEAN NEAP TIDE N N / / / / 90°
-
s/1 ;/
I /,
/j
/
_,
,>
"
-2:
180° 180°I
POINT CI
I
POINT DI
0 0.5 1.0 111/sec-
I
+
HOURS BEFORE AND AFTER HIGH WATER AT DOVERFig. 11. Measurements of current velocity in the locations A, B, C and D.
2.5. Nautical design conditions.
As mentioned in the foregoing paragraphs a dis-tinction will be made between three types of islands with site areas of 50, 300 and at least 1.000 hectares, respectively.
In view of the specific use to be made of each of these types of island different nautical design condi-tions will apply for the project.
- for islands with a site area of approximately
50 hectares:
Islands of this type may be used for the pro-cessing of waste matter mentioned in the other volume of this publication.
Large quantities of waste matter will be transported in various forms by small freighters and perhaps by sea-going push-barge units specially designed for the purpose.
22
It will be possible also to employ the same vessels for the carriage of products from the islands, such as scrap metal.
The length of these vessels or push-barge units will be approximately 100 metres at the most, their width and draught being approximately 15 and 6 metres, respectively.
If, for instance, these islands are used exclusively as working islands for building into larger islands, large quantities of building material and equipment will have to brought in. The types of vessels to be used for that purpose will be the same as those men-tioned above.
Furthermore, the harbour will have to serve as a shelter during periods of bad weather.
water is a pre-requisite, so that it will be necessary to
build and fit up a sheltered harbour. The harbour and
especially the approach to the harbour must more
-over be aligned in such a manner that a safe entering
manoeuvre will be possible and that sufficient shelter
is ensured. In addition to that, there must be sufficient
stopping length available for the vessels. There must
also be sufficient space for the vessels to swing.
These latter factors determine the minimum length and
width of the harbour.
If activities are to be carried out on the islands
involving the handling of large vessels (e.g. tankers);
these will have to be handled with the aid of
single-buoy mooring systems. This implies, of course, that
in the immediate vicinity of the island the water must
be sufficiently deep.
- for islands with a siie area of approximately 300
hectares.
Islands of these dimensions will be able to serve
for special industrial activities, such as the storage
and transhipment of oil and as damage repair harbours.
Apart from the nautical provisions for the small island mentioned above facilities for larger vessels will be required. For islands of this small size the constru
c-tion of a fully sheltered harbour for large vessels will be, relatively, a costly matter.
In this case the construction of jetties and/or
single-buoy moorings on the lee-side of the islands
must be contemplated.
The use of these facilities will be subject to
restrictions due to weather conditions, especially
because tug-assistance will be required for the
mooring-manoeuvre and these tugs must operate in
the open sea.
Also in this case the island must be built near
sufficiently deep water.
- for large industrial islands of at least 1,000 hectares.
Inasmuch as in the case of this type of islands large
-scale industrial activities tied to deep water are
envisaged, full harbour facilities will have to be
real-ised for the largest vessels that can sail the North
Sea.
Such islands therefore, will have to be built at
locations most favourable in relation to shipping routes for deep-draught vessels. The harbour-entrance must then be aligned preferably in the direction of the
greatest current component, to minimize the influence
of the transverse current components and,
conse-quently, the manoeuvring speed.
With a view to the manoeuvrability of the
vessels this speed should, in fact, be at least
approximately 4 to 5 times the transverse component
of the current in front of the harbour-entrance.
At the same time the stopping length of such
large vessels should be taken into account. In the
way of general information Fig. 12 shows some
Furthermore, turning circles will have to be provided for the swing-manoeuvre of the tankers near
the berths (quays or jetties).
Naturally, it will also be necessary to have
accom-modation available for tugs, pilot-boats, etc.
(f) 10
l-o
z ~z
0 UJ UJ ()._ (/) (/) ()._ I (/) 8 6 4 2 0 0 WITH TUG ASSISTANCE 2 3The shape and lay-out of the harbour will
finally depend on the total quantity of products to be
brought in and to be transported out and on the
dimen-sions of the vessels involved.
REFER·ENCE:NETHERLANDS SHIP MODEL BASIN
WATER DEPTH 1.2 x DRAUGHT
2 r.p.m.
',,
-...____ ...,
-~-\ \ \ 4 \'t, \,i
· \0 \ \ 5 6 7 8 STOPPING DISTANCE IN kmFig. 12. Stopping distances of tankers in relatively shallow water.
24
l
I I I I I--3. Considerations with regard to the
c
onstruction of islands to be built
in the open
s
ea
.
3.1. General.
For the building of islands in the open sea sand will be used in large-scale quantities. In fact, this
material is available in sufficiently large quantities in
the immediate vicinity. By nature, sand is a material
with a low resistance to the eroding action of current
and waves. A reclaimed sand-plateau will, therefore,
deform and flatten under the influence of currents and
waves and as a result show a tendency to shift, which
phenomenon also occurs, for instance, in the case
of the Frisian Islands.
This eroding action is caused under water
mainly by the phenomena to be distinguished as
follows:
- influence of waves in the surf-zone.
influence of tidal currents and waves in the zone
between the original bottom and the surf-zone. Inasmuch as shifting is unacceptable and erosion
is only admissible to a certain extent, an efficient
sea-defence wall will have to be built around the artificial island.
In addition to measures which must be taken to
prevent the above mentioned eroding influences, it is
also necessary to take measures to prevent flooding of the island in the case of storm-surges.
The construction must be capable of resisting
extreme sea conditions and must, consequently, be
raised to a sufficiently high level. Compared with other continental seas the wave-conditions in the North
Sea are rather unfavourable. For this reason the defen-ces will have to be of a rather heavy construction.
Apart from the design conditions mentioned above various other factors will influence the decision regarding the choice of the type of sea-defence and
the general design of the island. The most important
of these are:
the shape of the island.
- the necessity of providing port facilities on the
island.
- the construction-method to be followed.
- possibilities for future extensions.
Eventually the protection of a reclaimed island
will most probably consist of a combination of various
types of sea-defence walls.
In par. 3 of this chapter a description is given
of a number of possibilities of sea-defences. Inasmuch
as no laboratory studies necessary for constructions
of this nature have been carried out, the design must be seen as a first approximation. The main object was,
on the one hand, the mutual comparison and, on the
other hand, the indication of the costs. Although this
is a somewhat rough estimate it may, at this stage, be
considered as sufficiently accurate for the purpose. The starting-point is an elliptically shaped island
with its longitudinal axis parallel to the direction of the
main current, in connection with the dominating influence of the current on the unprotected sea-bot
-tom and the possibly unprotected bot-tom-part of the sea-defence.
Another starting-point is the construction of sites above storm-surge level. In this respect the criteria have been followed which have been applied
in recent years for the construction of harbour and
industrial sites.
The possibility of making polders with high
sea-defences - such as the IJsselmeer polders - has not been contemplated in this study.
3.2. Design levels of sites and sea-defence walls. Insomuch as capital-intensive industries will be erected on the island, the risk of flooding must be reduced to a minimum.
In Holland, the Delta Commission has laid down
as 'basic level' for the design of dams, a level whose chance for overflowing is once every 10,000 years.
Although in the case of an island considerations as
those used in the study of the Delta Commission do
not necessarily apply, the same criterion has been
assumed for the time being. The overflow frequency of high water-levels shown in Fig. 10 (page 21) implies,
therefore, for the island a design water-level of
approximately M.S.L. +4,00 m for the contemplate::] area. In this connection the effect of gust bumps has not been taken into account. For that reason and inasmuch as the costs of some extra fill are compara-tively small, the level of the sites has been projected for the time being at 1 m above this design-level. For the design level of the sites, therefore, M.S.L.
+
5,00 m has been chosen.For the determination of the height of a
sea-defence not only the type of sea-defence is of impor-tance but also the design water-level, the design wave height and the quantity of water by the overtopping
of the waves to be accepted.
The water-level is determined by tidal influences and wind effects. Due to the shape of the North Sea
basin winds from northerly directions will cause a rise
of the water-level (blowing up) and winds from southerly directions a lowering of the water-level.
Due to these factors the toe depth and the crest
height of a defence-construction to be built around the
island will have variable values along the circum-ference.
For the various constructions - to be considered
in detail in the next paragraph - the design levels can
be determined as follows:
- for a sand dam (beach-solution) a crest-height of
3 to 4 m above the design water-level is considered
sufficient, inasmuch as the waves have practically lost all their energy when reaching the crest of the
sand dam.
- for a slope-defence construction consisting of, for instance, concrete blocks, some overtopping of the
waves under extreme conditions will be admissible.
From the theories developed for this purpose it becomes apparent that a minimum height above the design water-level of approximately 0.75 H8 will be necessary to curtail excess overtopping. At a design
wave height of approximately 8 to 10 m the crest height will then be M.S.L.
+
10 m to+
12 m.- for a sea-defence consisting o.f a monolith-construc-tion, (concrete caissons), the crest of the defence,
asuming the same criterion of overtopping, must be higher still. At such a vertically shaped defence standing waves must, in fact, be reckened with.
- for a smooth slope the wave run-up and, conse-quently, the crest-height can be determined with the
aid of corresponding formulae.
Harbour dams, on the contrary, may have a lower height, as much more overtopping can be admitted.
26
3.3. General considerations in regard to some
sea-defence constructions.
This paragraph contains a description of a num-ber of different types of sea-defence constructions,
i.e.:
- sand beaches, both unprotected and partly pro-tected by means of groynes;
- a slope-defence construction built, among other things, of gravel, quarry stone and concrete blocks, (so-called block-slopes);
- a monolith-construction built with the aid of caissons.
Finally, a rough guide is given of the costs
involved in the various constructions, both as regards
the construction and the maintenance.
3.3.1. Sand beaches.
As has already been mentioned, sand is available in sufficient quantities in the contemplated
area of the North Sea. The average grain diameter varies from approximately 200µ to 400p ..
The slope of a sea-defence built exclusively of sand, will have to approximate the equilibrium profile - which is dependent upon the grain diameter of the sand and the combination of current and wave attack.
Various beach-profiles of the Dutch coast have been investigated. Up to a depth of approximately
N.A.P. (Amsterdam Ordnance Datum) - 15 m the
average slope for sand larger than 200µ appears to be in the order of 1 to 100.
Data concerning slopes of American beaches
show that for this type of sand slopes of
approxima-tely 1 to 30 have been measured. In the absence of information about the correct values, it has been
necessary to make an assumption in respect of the slope. The starting-point has been the profile shown in Fig. 13 with an average slope of the beach of 1 to 50.
For a beach around an artificial island the
assump-tion must not be an equilibrium between sand accretion and sand decrease but a decrease must be taken into
account. However, by including a sand buffer in the beach profile and by means of a periodical supply of
sand a technically justified sea-defence can be realised.
For the purpose of determining the extra quantity of sand which must periodically be supplied for a certain length of sea-defence a study was made of the move-ment of sand for a beach around an artificial island. A brief summary of the result of this study is given below.
As has already been mentioned sand transporta-tion takes place due to the following phenomenae:
---=--DESIGN WATER LEVEL
ADDITIONAL SAND -BUFFER
TO PREVENT SANO EROSION
BY CURRENTS ANO WAVES
SAND TRANSPORTATION DUE TO
--:t
l
~
CURRENTS AND WAv_~._,,S'---+---l- --=~
z 0 ;:: <l'. f-a: 0 O,_ CJ) z <l'. a: f-0 z <l'. CJ) _J <l'. :::, z z <l'.
Fig. 13. An unprotected beach profile and the estimated distribution of the littoral drift.
1. erosion in the surf-zone, (waves).
2. erosion outside the surf-zone, (tidal currents and waves).
Erosion in the surf-zone.
In this zone the effect of the tidal current can be
left out of consideration. A current develops due to the breaking waves, the so-called littoral current. The
quantity of sand transported is supposed to be in pro-portion to the component of the loss of energy of the
waves aligned along the coast.
The annual sand transportation along the beaches
in both directions has been calculated for the various
beach sections along a rectangular schematised island
with the aid of a transportation formula developed for this purpose, whilst taking into consideration the
chance of occurrence of certain waves.
In this case of an unlimited length of beach the transportation due to the littoral current on the west-side and in both directions will be in the order of 1 OG cubic metres per year. For the east-side the
figures are considerably lower, probably in the order
of 0.1.106 cubic metres in a northerly direction and
0.25 to 0.5.106 cubic metres in the southerly direction.
Regarding the north-side and the south-side of the Island the eastbound littoral sand transportation due to
the wind blowing predominantly from a westerly
direction is of importance. The transportation quantity in this direction is in the order of 0.75 to 1.106 cubic
metres per year, whilst the westbound transportation
appears to be only 0.1 to 0.5.106 cubic metres per year.
Erosion outside the surf-zone.
In contrast to the preceding case the tidal current is, in fact, of influence here. The transportation is now
caused by the combination of current and waves.
- the tidal current. The flow pattern will be deflected
by the island, causing an increased current velocity along the island. For a schematised island these
current velocities, taking into account the flat
sub-merged slopes, have been determined. In these con-siderations influences of water-depth, variations in tidal amplitudes and tidal motion have been taken
into account.
~ DESIGN WATER LEVEL E 0 N I M.S.L.= m. - = - -H.W.S.
Fig. 14. A beach profile, partly protected by means of groynes.
- on determining the wave characteristics the effect of refraction by the slope of the sand body has also been
taken into account.
The current and wave data have been investigated
for various cases, i.e.: 3 islands, 3 depths, 2
beach-slopes, 2 grain diameters and for both longitudinal
sides of the island. For all these cases the sand trans
-portation along the beach-slope outside the surf-zone
has been calculated with the help of recently
developed formulae for bed-load and suspended load.
Conclusions:
Fig. 13 (page 27) shows a total picture of the
sand transportation along the beach-slope. At the
same time a study was made of the minimum distance
necessary for the build-up of the total sand transpor-tation. For that purpose a comparison was made, among other things, of the increasing sand
transporta-tion in currents downstream of a breakwater.
Calculations for various cases have finally shown
that the annual sand losses per running metre will be
in the order of 103 cubic metres for a beach built on
DOUBLE DESIGN WATER LEVEL
the west-side of an island less than half of this
figure for the east-side.
So as to compensate this loss a sand buffer can be added as shown in Fig. 13.
The loss on the north-side and the south-side will
be caused chiefly by littoral transport and will be approximately half the loss on the west-side of the island.
The sand losses can be limited by constructing groynes, for which the use of rubble mound groynes or rows of piles could be contemplated. Thereby the
littoral transport will to a large extent be averted.
Fig. 14 shows the principle of a beach with groynes.
3.3.2. Slope-defence construction.
In the case of a slope-defence consisting of
dumped materials, the outer layer may consist of heavy concrete blocks. An example of a construction of that
1
_T
_
_
_
~~
.
~
.
==::-=
_
__
MSL:0m _ _ _ _ _ _ _ _ _ ~i
I . QUARRY STONE 1-61.I
LAYERS OFGRAVEL AND QUARRY STONE< 11
Fig. 15. Sea defence wall composed of gravel, quarry stone and concrete blocks.
28
1
I
11
type, designed for the most heavily attacked side of the island, is shown in Fig. 15.
The weight of the concrete blocks has been determinGd with the aid of the Hudson formula. A
slope of approximately 1 : 1.5 proves to be the most
economic slope for this type of construction.
Calcula-tions have shown that in the case of a design wave height of 8 to 10 metres, the weight of the blocks,
dependent upon the specific gravity of the concrete
and the damage to be accepted, must be at least
50 tons.
Proceeding from other design conditions applying
to these constructions, the toe of the block defence
must in this case be about M.S.L. - 10 metres. The toe-construction, as also the layer immediately
beneath the concrete blocks, consists of quarry stone
of 1 to 6 tons each.
A filter construction, consisting of several layers
of gravel and quarry stone less than 1 ton, is to be
built between the sand body and the 1-6 ton quarry
stone.
For the composition of the dams required for the
purpose of confining the sand body, finer quarry stone
and gravel or slag-material, or possibly sand asphalt
can be used.
As already mentioned in the preceding paragraph,
a crest height of approximately M.S.L.
+
10 m to+
12 m odd in height is considered necessary to keep the overtopping of the waves within reasonable bounds. At that height the blocks are supported by aconcrete crown-piece.
3.3.3. Monolith-construction with the use of caissons.
For the protection of a reclaimed island it is
also possible to use a continuous row of concrete
caissons. These caissons can be constructed in a
special dock on the mainland and be sunk on the
edge of the island onto a previously built threshold
construction.
Fig. 16 shows a principle cross-section of a
con-DESIGN WArER LE=VEL
L
_ _ _
_
~_ _ _
_
_ _
M = • o ~-L.w.s. -~
-E 0
N
QUARRY STONE 1-6 I QUARRY STONE !;:'. 1 t
Fig. 16. Sea defence wall with caissons.
struction of that type, whereby the foundation of the
caissons has been placed on the original sea-bottom.
It would also be possible to place the caissons on a
bottom-elevation previously built of sand and
sea-gravel.
On the front side an efficient protection must be
provided to prevent erosion of the toe-construction.
In the case of such a vertical fence the development
of standing waves must, in fact, be taken into account. This implies, on the other hand, that immediately in front of the caissons the toe-construction will be
attacked violently, while on the other hand at the nodal
point of the wave - situated at a distance of a quarter
of the wave-length in front of the caissons strong
orbital currents will have to be taken into account.
For that purpose heavy quarry stone dumping
at the toe and up to some distance in front
of the caissons will be necessary.
For the calculation of the stability of such
constructions dynamic phenomena due to heavy wave
blows will have to be taken into consideration.
Another disadvantage of this slightly flexible
defence construction is that in the case of heavy gales
large quantities of spray and water swept across by
the wind must be reckoned with. This disadvantage
may be offset to some extent by the fitting of so-called
'wave-walls' on each caisson.
3.3.4. Other possibilities.
In addition to the afore-mentioned slope-defence
construction with concrete blocks, other materials may
be used, e.g. asphalt.
A further possibility is, for instance, the making
o1 beachGs with sea-grnvel.
So called 'tombolos' may also be used in addition
to sand beaches. This implies the construction of
breakwaters in deep water at intervening spaces, with
sand beaches between thGm. Consideration could also
be given to the construction of continuous breakwaters
in deep water parallel to the future shore line, with
sand beaches behind them.
_j - 10 (/) ~ 0 f--(_') -15 z ;:: ~ _J w a: E z -20 I f--c.. w UNPROTECTED 0 BEACH a: w -25 f--~ 3: ·30 COSTS PER m' OF SEA DEFENCE WALL
Fig. 17. Comparison of construction costs of some types of sea-defence walls at varying depths.
Although within the framework of this investiga-tion these possibilities have not been considered, it is obvious that they will have to be investigated in the case of further studies.
3.3.5. Costs incidental to the construction of sea-defences.
For a comparison of the sea-defence construc-tions described in the preceding paragraphs a brief explanation is given below of the result of comparative cost estimates for the types of construction described. The starting-point has been the most unfavourable design conditions in respect of the water-level and waves (west-side of the island) for water depths of between M.S.L. -10 and -30 metres.
The costs estimates are the building costs per running metre for:
a. an unprotected artificial sand beach. b. a sand beach with groynes.
c. a slope-defence construction (concrete blocks). d. a monolith-construction (concrete caissons).
Fig. 17 shows graphically the results of these estimates of costs for the various water depths. It become apparent that the beach-solution is by far the most economic one. With increasing depths the costs increase almost proportionally.
Also the sand slope protected by means of
30
groynes appears to be a more economic solution than the other constructions. At lesser water depths the costs of a slope and caisson defence appear to be almost equal.
At greater depths, however, the caisson-solution becomes relatively very costly.
It is obvious that, when considering the results of this comparison of costs, the maintenance costs should also be taken into consideration. If the building costs plus the capitalised maintenance costs are mini-mal, one can, in fact, speak of the most economic construction.
The maintenance costs in the case of a beach-solution will consist of costs for a periodical extra sand supply. In the case of a beach with groynes they will be lower. To be added to these are the costs of the maintenance of the groynes, but they do not offset the reduced maintenance costs of the beach.
Also for the slope-defence construction and the caisson-solution an estimate has been made of the annual maintenance costs.
Fig. 18 shows the total building costs and the capitalised maintenance costs for the various possibilities. The sand beach with groynes proves to be the most economic solution. However, the differ-ence with the slope-defence construction,
(concrete blocks), is minimal.
The above comparison of costs only applies in the case of the straight parts of the alignment of the sea-defence. On bends, which notably in the case of smaller islands form relatively an important part of the total circumference, the quantity of material and, consequently, the cost price will increase considerably, especially in the case of the beach-solution. Moreover,
_j -10 cr.i ::;;; LL 0 (') z I- -15 < _j w cc E z -20 I I-Q_ w 0 cc w I- -25 < 3::: -30
COSTS PER m' OF SEA DEFENCE WALL
Fig. 18. Comparison of construction costs plus capitalised maintenance of some types of sea defence walls
at varying water depths.
extra sand losses must be reckoned with in these places.
On the basis of these considerations and in
con-nection with the greater uncertainty as regards the
assumptions for, and the inaccuracies of the calcula -tions in respect of the beach-solution, the slope
-defence construction, in particular for small islands,
is for the time being regarded as the most attractive
solution.
It is obvious that other arguments, notably, for
instance, the possibilities of future extensions along certain parts of the shore, speak in favour of the
beach-solution.
These considerations with regard to the costs
apply to the most heavily attacked sides of the island. The costs per running metre of the sea-defence for the east-side will be less, specifically in cases where the defence wall is not far from the mainland. Rough cost estimates have shown, however, also in this particular
case, that a wholly defended slope construction is the most attractive solution.
