P r o p o s a l
I n n o va t i ve c o n c e p t f o r a n
o ve r t o p p i n g d i k e
P r o p o s a l
I n n o va t i ve c o n c e p t f o r a n
o ve r t o p p i n g d i k e
file x1498-01-000
registration number WG-SE20050625 version 1
CONTENTS PAGE
1 OVERTOPPING DIKES 3
1.1 Why do we need overtopping dikes? 3
1.2 Our contribution 3
1.3 Conditions 4
1.4 DHV’s expertise 4
2 INNER SLOPE PROTECTION 7
2.1 Introduction 7
2.2 Hydraulic load 7
2.3 Failure mechanisms 7
3 CREST DRAINAGE DIKE 9
3.1 Description and innovation 9
3.2 Technical aspects 11
3.3 Construction phase 12
3.4 Maintenance aspects 13
3.5 Environmental aspects 13
3.6 Cost indication 13
4 APPROACH FURTHER DEVELOPMENT CREST DRAINAGE DIKE 15
4.1 Theoretical Phase 15
4.2 Testing Phase/ further development 17
5 PLANNING AND ESTIMATION 18
5.1 Planning and products theoretical phase 18
5.2 Cost estimation theoretical phase 18
5.3 Project team 19
6 REFERENCES 21
7 COLOPHON 22
Appendix A: Curricula Vitae
1 OVERTOPPING DIKES
1.1 Why do we need overtopping dikes?
Based on several separately initiated studies on possible advantages and opportunities for overtopping dikes, a European project called ComCoast (Combined Functions in the Coastal Zone) was set up in 2004. The ComCoast project is carried out in the framework of the Interreg IIIb- North Sea programme. The objective of the ComCoast project is to study the possibilities of a wider coastal defence zone. Instead of raising and strengthening the dike, the coastal defence zone is widened. The wide coastal defence zone gives opportunities for new spatial developments and different types of functions within the zone.
The ComCoast concept involves measures in seaward and landward direction. One of the options in landward direction is an overtopping dike. In this concept the crest and/or inner slope of the dike is strengthened so that more overtopping of the dike can be allowed. Advantage is that heightening of the dike is not necessary and furthermore this can be combined very well with a wet and brackish zone behind the dike. This zone provides opportunities for nature development, recreation and possible concepts like ‘living in the water’ (houses on poles).
Figure 1: Ellewoutsdijk, ComCoast pilot area
In a State of the Art Study [reference] an inventory was made of all available and at present often applied types of inner slope protections. It occurred that the designs of the inner slope protections are usually based on methods and philosophies developed for the outer slope. Hence optimisation of the design methods and philosophies and a study on innovative types of inner slope protections for overtopping dikes are set as targets in the ComCoast project.
1.2 Our contribution
DHV was asked by CUR to develop and present an innovative concept for the inner slope protection, and to make an offer to further work out these concepts. The innovative concepts
must meet certain demands concerning e.g. costs, social acceptance, nature development and of course overtopping resistance.
The proposed innovative concept must be applicable for two locations along the Dutch coastline: Hondsbossche Zeewering and Westkapelle. These locations have both been marked as ‘weak location’ along the Dutch coast where the dike does not or will not in the near future meet the Dutch requirements.
1.3 Conditions
The minimum conditions, which the concept must meet, are described below:
• The concept must be able to resist an average overtopping of at least 15 l/s/m1 (with a maximum of 2300 l/s/m1). The more overtopping resistance, the better.
• Storm duration taken into account is three hours.
• The concept must be applicable for a inner slope gradient of 1:3 to 1:4.
• Heightening or widening of the dike is not allowed, the concept must fit within the present cross-sectional profile.
• Effects on nature, landscape and cultural values must be minimized.
• Maintenance efforts must be minimized during the first fifty years. Used materials must have a minimum life expectance of fifty years.
• The costs for construction and maintenance must be acceptable compared to the costs for traditional heightening of the dikes.
• The concept must meet the current legal standards, such as the ‘Bouwstoffenbesluit’. 1.4 DHV’s expertise
DHV has extensive experience in projects concerning flood protection in general (see Box: ‘DHV and Flood Protection’) and more specific concerning all phases in the design of dike improvements and the safety assessment of different types of dikes and dike revetments. For example, design and technical specifications were prepared for dike improvement projects such as the Waalbandijk Heumen-Dreumel, the Maasbandijk Afferden-Dreumel, the Maaskaden Limburg, the pitched revetment at Schiermonnikoog and the Oostvaardersdijk Noord. Furthermore, project management and supervision of the realisation of several dike improvement projects was executed by DHV; for example the Waalbandijk Heumen-Dreumel, the Maasbandijk Afferden-Dreumel, the Vossemeerdijk and the Ketelmeerdijk. In several of the dike improvement projects, the development of nature areas on and around the dike was included in the design and realisation.
The dike safety assessment, as is legally required every five years in the Netherlands, was also executed by DHV for a range of dike sections in areas such as Rivierenland, Vallei en Eem, the Afsluitdijk, Noorderzijlvest, Salland, Zeeland and West-Brabant. Checking of the dike stability and height as well as revetments on the dikes was executed many times. We also have been involved in several projects concerning the development of hydraulic conditions for the dikes in the Netherlands (development of Hydra-modules and Hydrascope- a project about the consequences of changes in the hydraulic conditions).
DHV and Flood Protection
Furthermore, DHV has a broad experience and knowledge of the Dutch coast. The projects along the Dutch coastline include dike improvements and checks, coastal policy development (e.g. integral maintenance plan Zeeland), innovation projects (see paragraph below) and at present for example we are working out the planning phase of two ‘weak locations’ along the coast where the dike does not or will not in the near future meet the Dutch requirements. This concerns the location Delfland and the location Flaauwe Werk.
Besides these national experiences, DHV also has experience with coasts all over Europe through the EUROSION project, for which we produced tens of case studies about eroding coast locations in Europe and an analysis report. In the case studies the causes of erosion and the technical solutions and effectivity of these solutions are described but also the socio-economic values of these locations are studied extensively. In the analysis report, based on the experiences at 60 case study locations, an analysis was set up per water system (North Sea, Atlantic, Baltic, Mediterranean) resulting in policy recommendations for handling coastal erosion.
As stated above, DHV has been involved in several innovation projects along the coast. At present, DHV for example is already involved in the ComCoast project. For WP 1, spatial sensing, we are making an inventory of locations along the Dutch Coast where the ComCoast concept could be applicable. This project gives us a good understanding of the objectives and background of the ComCoast project. DHV was also involved in the INSIDE project (within WINN innovation framework), that stands for Innovations on Stability Improvements enabling Dike Elevations. Three innovative techniques were developed, and DHV was involved with several experts in the development and testing of the technique Expanding Columns.
Furthermore, DHV takes its own initiatives in coastal innovation projects; in a joint venture with contractors in 2004 we executed a Quick Scan of the technical and financial feasibility of the Katwijk Sea Marina. In this project, the combination of the realisation of a sea marina with the creation of new areas for housing and other commercial exploitation and the protection of the already existing landward area was studied. One of the options that was studied by DHV
DHV advises in the complete field of flood protection and -prevention; from institutional and legislative policy formulation at regional, national and international levels including applicable EU Directives and inventories of potential flood-risk areas through to the implementation of large-scale complex flood protection works, barriers, dikes, specially assigned inundation polders and operational and maintenance works. DHV has much experience in both river and coastal flood protection projects and warrants an integral approach in which all possible aspects, issues and disciplines are identified and studied.
DHV’s broad experience ranges from probabilistic / statistical analyses of potential dike failures (geo-technical and hydraulic failure mechanisms), to the calculation of economical and social consequences of inundation on surrounding polders, settlements and industrial areas and innovative propositions and implementations of effective and efficient solutions. The problem analyses are based on varying present and future meteorological and hydrological conditions up and downstream of the potential failure area.
was the realisation of a sand spit in front of the Katwijk Boulevard, the spit provides 1) an increase of safety for the boulevard area, 2) extra space for housing and recreation and 3) a water area in between the spit and the present boulevard where a marina can be situated.
In the table below, an overview of some of the relevant reference projects executed by DHV as described above is shown. More information about these projects can be submitted to the client on request.
Tabel 1: Overview some relevant DHV projects
Project Period Contract
sum
Dike improvement Waalbandijk Afferden-Dreumel 1996-1998 ¼
Dike improvement Maasbandijk: Heumen-Dreumel 1996-2002 ¼
Inventory groundmechanic situation Spaarndammerdijk 1999 ¼
STOWA flood defence research 1999 ¼
Safety assessment dikes Zeeland 1999-2001 ¼
Safety assessment West Brabant en Geertruidenberg 2000 ¼
Design Oostvaardersdijk Noord 2001-2002 ¼
Safety assessment pitched revetment Houtribdijk 2001 ¼
Hydraulic conditions 2001 and development module Hydra-2001 2001-2002 ¼
Geotechnical safety assessment dikes Terneuzen 2001-2003 ¼
Safety assessment dikes Waterschap Vallei en Eem 2001-2003 ¼
Maintenance and policy plan primary flood defences Zeeland 2002-2003 ¼
Eurosion, coastal zone management in Europe 2002-2003 ¼
Quickscan feasibility marina Katwijk 2002-2003 ¼
Dike improvement Maaskaden 2003 ¼
Inventory possibilities in coastal innovation 2004 ¼
Inventory of possible ComCoast locations 2004-2005 ¼
Plan Study ‘weak coastal location’ Delfland 2004-2005 ¼
2 INNER SLOPE PROTECTION
2.1 Introduction
To develop a functional innovative concept it is essential to fully understand the relevant hydraulic loads and the failure mechanisms of the inner slope. Therefore, in this chapter these aspects are described in detail for the inner slope of a dike.
2.2 Hydraulic load
The main hydraulic load at the inner slope is wave overtopping. Wave overflow of the dike crest will never be allowed for the Dutch dikes.
Wave overtopping
The quantity of wave overtopping is dependent on the significant wave height (Hs), the peak wave period (Tp), the crest level of the dike, the geometry of the outer slope of the dike and the revetment on the outer slope. For the determination of the wave overtopping the numerical model ‘PC overslag’ can be used. Wave overtopping can be reduced by heightening the dike, increasing the roughness of the outer slope, applying a berm in the outer slope or decreasing the slope gradient of the outer slope. In the Netherlands normally a maximum average wave overtopping of 0,1 to 1,0 l/s/m1 is allowed, which can be resisted by a normal grass revetment at the inner slope.
2.3 Failure mechanisms
Due to wave overtopping, different types of failure mechanisms can be of importance for the inner slope. First of all, the waves overtopping the dike cause a hydraulic load on the inner slope, which can cause erosion of the top layer. Furthermore the overtopping water will infiltrate into the dike, which can lead to instability of the inner slope in different ways.
Infiltration due to wave overtopping
Due to wave overtopping, infiltration into the dike crest and the inner slope will occur. During
WKH LQILOWUDWLRQ WKH PDVV YROXPH ZLOO LQFUHDVH IURP dry WR wet. As long as the infiltrated water
can flow through the dike, the water stresses is negligible. The quantity of infiltration is dependent on the amount of wave overtopping, the period of overtopping and the permeability of the inner slope layer.
When the infiltration zone is saturated, the water stresses in the dike will increase and the grain stresses and thus strength will decrease. Furthermore, because of the saturation of the soil, the load on the inner slope will increase. The combination of these effects decreases the stability of the inner slope and can cause shearing of the revetment. Instability of the inner slope will first show by deformations of the slope protection, the slope will move in the direction of the dike toe and bulge a bit. In the upper zone of the inner slope, due to the slope movement a crack can occur parallel to the crest. When overtopping waves find their way into this crack, the inner slope will soon become instable and shear.
Micro stability
Due to the high water level during a storm event at the outer side of the dike, the freatic water line in the dike will rise. This can cause water pressure on the inner slope revetment and lift the revetment. Furthermore, because of these water pressures from inside the dike, the revetment could also shear before even being lifted. This is caused by a decrease of grain stresses and thus strength on the interface of dike core and revetment; due to the weight of the revetment the revetment can shear.
The micro stability effects are found mainly at the toe of the inner slope, when lifting or shearing of the revetment occurs here, shearing of the rest of the inner slope due to infiltration will occur easier and faster. This is a combined failure mechanism for the inner slope.
Erosion of the inner slope revetment
Erosion of the inner slope top layer can be caused by water run off due to wave overtopping. The erosion is a dependent on the quantity of wave overtopping (resulting in a certain flow velocity on the inner slope), the revetment strength, the type of material of the revetment (permeability), the slope gradient of the inner slope, the core material of the dike (permeability) and the geometry of the inner slope.
3 CREST DRAINAGE DIKE
3.1 Description and innovation Background of the concept
In the State of the Art study [Haskoning, 2005] all sorts of different types of revetments are described. The hard type of revetments, such as pitched revetments, concrete, gabions and asphalt, on the one hand have a high erosion resistance but on the other hand the costs for these alternatives are relatively high and the effects on landscape and nature can be very negative. In our opinion, instead of increasing the revetment strength of the entire inner slope, as is the case in all the State of the Art alternatives, it is also possible to decrease the overtopping loads at the inner slope without alteration of the cross section of the dike. We see a challenge in trying to find a way to decrease the loads on the inner slope by taking more local measures at the crest of the dike. Because of the smaller physical scale of measures at the crest, this will have much less negative effects on LNC-values and the costs will be lower because at the inner slope the grass revetment can be maintained.
In short, we propose an alternative where the wave-overtopping load on the inner slope is reduced to an acceptable level by measures taken at the crest of the dike.
Crest drainage dike
This concept consists of a concrete construction in the crest of the dike (in the form of a wide U-profile), see Figure 2 for a conceptual sketch. Most of the overtopping water will be caught in this construction and will be discharged through drains either to the inner side of the dike or if necessary also (partly) to the outer side of the dike. For the ComCoast concept it should be studied further in a following phase first of all how much salt water input is desired and can be stored at the landward side of the dike, and second if the option of also discharging water towards the seaward side of the dike could be preferable in extreme situations. If it is, the seaward discharge should be adjustable, for example by installing a valve.
Figure 2: Conceptual sketch Crest Drainage Dike (not scaled!)
Concrete U-profile (prefab) Secondary use as promenade
CORE
DRAIN
CLAY CLAY
Seaside
Concrete U-profile (prefab) Secondary use as promenade
CORE
DRAIN
CLAY CLAY
Because the crest construction will catch of a significant part of the overtopping water, this water will not run off the inner slope, resulting in a reduction of the loads on the inner slope to a normal level. In extreme situations, the capacity of the drains might not be sufficient, causing more water to run off the inner slope. However, also in this situation the crest construction will cause a load reduction for the inner slope. The overtopping waves will not pass the crest in a sheet flow (as is the case for smooth dike crests) but will loose its energy in the crest construction and finally flow over the edge towards the inner slope if the construction’s capacity is exceeded. The water that eventually reaches the inner slope will have less energy because of the crest construction, even if the occurring wave overtopping exceeds the construction’s capacity.
The discharge takes place under natural drop through for instance a simple synthetic PP pipe (Poly Propyleen). The pipe will be buried superficially to take it out of sight. The concrete U-profile’s bottom should have a small gradient towards the discharge pipes, which are located at the inner side of the crest. The concrete construction can function as a promenade for walking and cycling with a nature area on one side and the sea on the other side.
A variation to be worked out in a later stage is to replace the concrete U-profile with for instance two lighter and easier to manufacture L-profiles on a body of gravel, if necessary partly penetrated with an open structure of colloidal concrete or bitumen in order to resist the loads by the water mass on the crest. This alternative is shown in Figure 3.
Figure 3: Conceptual sketch variation Crest Drainage Dike (not scaled!)
2 Concrete L-profile (prefab) on gravel bed Secondary use as promenade
CORE
DRAIN
CLAY CLAY
Seaside
2 Concrete L-profile (prefab) on gravel bed Secondary use as promenade
CORE
DRAIN
CLAY CLAY
Seaside
A variation that was considered for the discharge method was to apply concrete channels instead of synthetic pipes to discharge the water from the crest construction. However it is expected that the costs for this option are much higher and the effects on nature, landscape and cultural values are expected to be higher. Furthermore when wave overtopping exceeds the design value, overflowing of the channel might cause local scour of the grass slope next to it.
3.2 Technical aspects Inner slope
Since the occurring wave overtopping is caught before it reaches the inner slope, the design of inner slope is according to the proven methods and design philosophy (1-10 l/s/m1 for a grass revetment). The failure mechanisms are described in Chapter 2, these do not change when the crest drain concept is applied, as long as clay is applied under and around the new structures (see also construction aspects in this paragraph).
Overtopping capacity
The allowed quantity of wave overtopping depends on the capacity of the discharge from the crest construction. A simple calculation is made with the Darcy-Weisbach formula (see below). Because of the relative long length of the pipe, the entrance and exit losses are neglected in this first estimation. g u g u D L dH 2 2 2 2 + =
λ
and: 2 )) 7 , 3 /(log( 25 , 0 k D =λ
with:dH= natural drop pipe line [m] L= length of pipe [m]
D= diameter of pipe [m]
u= flow velocity in the pipe [m/s] g= gravitational acceleration [m2/s]
For a pipe diameter of 200 mm, D/k= 25.000 (very smooth PP pipe), a natural drop dH of 13 m (Hondsbossche Zeewering) and a approximate pipe length of 50 m, the resulting flow velocity is 9,18 m/s which leads to a capacity under natural drop of 0,26 m3/s (Q = u.A).
For a pipe diameter of 250 mm, D/k= 25.000 (very smooth PP pipe), a natural drop dH of 13 m (Hondsbossche Zeewering) and a approximate pipe length of 50 m, the resulting flow velocity is 9,18 m/s which leads to a capacity under natural drop of 0,45 m3/s (Q = u.A).
For a pipe diameter of 0,30 m, D/k= 25.000 (very smooth PP pipe), a natural drop dH of 13 m (Hondsbossche Zeewering) and a approximate pipe length of 50 m, the resulting flow velocity is 9,18 m/s which leads to a capacity under natural drop of 0,70 m3/s (Q = u.A).
For PP pipes larger than 250 mm, costs can increase significantly. Optimization of the pipe diameter in combination with the amount of discharge pipes to be applied should take place in a following phase. In Table 1 some possible combinations of pipe diameter and pipe frequency are shown including the resulting capacity to catch of wave overtopping at the crest per meter dike length. In the cost indication (see Paragraph 3.6), the 250 mm discharge pipe every 30 meters is taken into account (resulting in a 15,0 l/s/m1 catch-off capacity, interpolated).
Table 1: Overview possible combinations of pipe diameter and pipe frequency and capacities
Pipe diameter Pipe capacity Pipe frequency Catch-off capacity crest
200 mm 0,26 m3/s = 260 l/s Every 100 m 2,6 l/s/m1 200 mm 0,26 m3/s = 260 l/s Every 50 m 5,2 l/s/m1 200 mm 0,26 m3/s = 260 l/s Every 25 m 10,4 l/s/m1 250 mm 0,45 m3/s = 450 l/s Every 100 m 4,5 l/s/m1 250 mm 0,45 m3/s = 450 l/s Every 50 m 9,0 l/s/m1 250 mm 0,45 m3/s = 450 l/s Every 25 m 18,0 l/s/m1 300 mm 0,70 m3/s = 700 l/s Every 100 m 7,0 l/s/m1 300 mm 0,70 m3/s = 700 l/s Every 50 m 14,0 l/s/m1 300 mm 0,70 m3/s = 700 l/s Every 25 m 28,0 l/s/m1
This concept requires further study into the physical aspects of overtopping at the crest. Can it be assumed that al overtopping water is caught in the 3 to 4 m wide crest as long as the pipe capacity is sufficient or should a certain amount of wave overtopping on the inner slope still be taken into account? And if so, what amount? Maybe the design of the concrete construction should be optimized to catch of as much overtopping water as possible? Questions like these should be studied further in a following phase.
Construction aspects
The concrete crest construction must be packed in clay to prevent instability of the dike, this could also be necessary for the discharge pipes (depending on the stability of the dike, stability computations should be made to determine this).
Critical constructional aspects of this concept are the connection points:
• between the concrete construction and the synthetic pipes, beware of leakage at this point.
• joints between the concrete elements, a flexible chain of elements should be constructed so that small ground settlements can be handled without damage to the construction, also beware of leakage at these joints.
The construction aspects of these connections should be worked out in more detail in a following phase.
The durability of the concrete U-profile or the L-profile is not considered to be critical for the working life of this construction, concrete in the Netherlands is usually prepared for a working life of around 80 years, or more if wanted. For the concrete it should be kept in mind that special demands should be set because of the salty environment and water in the construction. The discharge pipes will be made of durable synthetic material.
3.3 Construction phase
The concrete construction can be made of prefab elements in a U-form or two L-profiles. The topsoil at the crest is excavated to the required depth, if necessary the clay layer is refilled and on top of that the concrete elements are placed. For the discharge pipes, a trench is dug in which the synthetic pipes can be placed. If necessary, a clay layer is placed around the pipes.
During construction, special attention should also be given to the connection points between the concrete elements and between the concrete and the discharge pipes (as mentioned at the
construction aspects before) and it is recommended to at least place an extra clay core around these connection points.
3.4 Maintenance aspects
Cleaning and repairing damage
The maintenance activities for this concept consist on the one hand of keeping the concrete construction and the pipes clean to prevent obstruction of the flow. Especially at the point where the water flows from the concrete construction into the pipes, special attention is needed to prevent blockage (a grid and/or a sink hole could be applied for this purpose). The concrete construction and the connection point should be cleaned at least before every storm season and should be monitored during storm season. For cleaning of the “trench” a cleaning vehicle such as normally used for roads is an option, in combination with a manual check and if needed cleaning of the connection points between the concrete construction and the PP pipes.
Besides cleaning, possible damage to the concrete construction should be monitored and repaired if necessary. Small damage can be repaired, if the damage to an element is of a larger scale the element can also be replaced entirely.
Accessibility and vandalism
For easy cleaning, monitoring and repairing damage accessibility to the construction is of the highest importance. The accessibility is well preserved in this concept, the concrete U-profile functions as a low-lying walking and cycling path in the dike crest. The construction can easily be monitored visually and measures can be taken if necessary.
The concrete construction and buried pipes are not vulnerable for vandalism. 3.5 Environmental aspects
By applying concrete prefab elements for the crest construction, any inconvenience during the construction of this concept is minimized. The concrete construction does not allow any nature development at the crest. However, the effects on LNC-values are limited to the crest because at the inner slope a grass revetment can be maintained which provides good opportunities for nature development. Only locally during the digging of trenches and burial of the pipes, the present nature and landscape will be disturbed at the inner slope.
For the aspect of landscaping, the concrete construction could be used as walking and cycling path and be fit in the current landscape. Since the construction is constructed in the crest, the general view of the landscape does not change.
3.6 Cost indication Construction costs
In Table 2 a rough estimate is given of the construction costs of the drainage crest concept with a U-profile. Two L-profiles could be cheaper (less concrete and easier to place). The costs are given per meter dike length, this results in ¼SHUPHWHUGLNHOHQJWK7KLVLVDURXJK
mm placed every 30 meters (resulting in a 15,0 l/s/m1 catch-off capacity of the crest construction, interpolated from Table 1).
Table 2: Rough estimate construction costs drainage dike
Costs / m1
Ground works/ excavation ¼
Bedding ¼ Concrete U-profiles ¼ Discharge pipes ¼ Completion ¼ Subtotal 1 ¼ To be detailed +15% ¼ Subtotal 2 ¼
Extra contractor’s costs +20% ¼
Total ¼
Maintenance costs
For the maintenance of the concrete construction, a cleaning vehicle can be used like normally used on roads. This should be done a few times a year (at least once before storm season!). Furthermore the connections between the concrete construction and the discharge pipes must be checked and cleaned manually with a certain regularity. The visual inspection should include a check if the flow is not blocked anywhere and a check of the condition of the concrete construction. Maintenance costs are very low, a very rough estimate of the maintenance costs is about ¼SHUPGLNHOHQJWKSHU\HDU
4 APPROACH FURTHER DEVELOPMENT CREST DRAINAGE DIKE
4.1 Theoretical Phase
The theoretical phase consists of different activities:
1. Further study and detailing several aspects of the Crest Drainage Dike; 2. Preliminary Design Crest Drainage Dike;
3. Description of required licenses and permits;
4. Participation meeting in preparation of Testing Phase. These activities are described in more detail below.
1. Further study and details design aspects of the Crest Drainage Dike
In the theoretical phase, the aspects as described in Chapter 3 are reviewed and studied in more detail. This results in a Technical Report for the Crest Drainage Dike. An overview and description of the aspects to be studied and described in the theoretical phase is given below. It is stressed that all these aspects are studied qualitatively, based on the available standards and research.
• Wave overtopping
As was mentioned before, in the theoretical phase the allowed quantity of wave overtopping should be studied further for this concept. Special attention should be paid to the quantity of wave overtopping that is caught in the concrete crest construction, and the way this construction can be optimized to catch of as much overtopping water as possible. In this context, a study is necessary on the required width of the crest in order to catch a significant part of the overtopping water.
• The construction’s impact on dike stability
Geotechnical analysis of the dike’s stability with the presence of the concrete crest construction and the discharge pipes. From this it should follow how much fill of the present clay layers is necessary to maintain stability of the dike.
• Structural aspects
Calculations of the required dimensions for the concrete elements with simplified design rules, and further calculations of the required diameter and distance in between the discharge pipes. Furthermore, because of their critical role in this concept, the design of the joints between the concrete elements and of the connection between the concrete construction and the discharge pipes will be worked out in more detail. Leakage and flexibility are the main attention points at these connection points.
Furthermore, more research into the applied materials (concrete and discharge pipes) will be undertaken in the theoretical phase, the durability in salty conditions and low maintenance effort of the applied materials is a very important factor in the final choice of materials. The exact type of concrete and synthetic pipe will be determined and described.
• Construction phase
Further study and description of the constructional phase for the Crest Drainage Dike. Special attention must be paid to the construction of the connection points between the concrete elements and between the concrete construction and the discharge pipes.
• LNC-values
A global assessment method should be set up for the assessment of all the concepts that will be worked out in the theoretical phase. The assessment criteria and weighing factors as set up in the State of the Art study could form the basis of this global method. The applied global assessment method to be used in the theoretical phase will be determined in consultation with the client.
• Maintenance aspects
A description of maintenance activities to undertake for the Crest Drainage Dike will be made, including the required regularity of the different activities and the risks if the required maintenance activities are not executed.
• Cost estimation
Based on the resulting Preliminary Design, see activity 2, a more accurate cost estimation will be made in the theoretical phase for both locations (Hondsbossche Zeewering and Westkapelle). In this cost estimation, an indication will be included of risks and possible spreading in the expected costs per 100 m dike length.
Furthermore, in the present design it is assumed that the concrete construction is made out of prefab U-profiles because of the possibilities for phased construction and maintenance aspects. In the theoretical phase, the cost difference between the prefab elements (U-profiles and L-profiles!) and locally poured concrete will be calculated in more detail. If the prefab elements turn out to be much more expensive than locally poured concrete, a reconsideration of the choice between the two options should be made.
2. Preliminary Design Crest Drainage Dike
Based on the results of further study of different aspects, as described above, a Preliminary Design will be made of the Crest Drainage Dike for two cross sections: the Hondsbossche Zeewering and Westkapelle. The preliminary design will be based on
3. Description of required licenses and permits
Based on the preliminary design, a legal expert will make an overview of all important licenses and permits that must be taken into account for construction of the Crest Drainage Dike. In the description the required time and activities to obtain such licenses will be taken into account also.
4. Participation meeting in preparation of Testing Phase
DHV’s teamleader for this project will participate in this meeting. If requested by the client, another expert can be added to this meeting for additional costs.
4.2 Testing Phase/ further development Crest Drainage Dike
To get a clear insight into the effectivity of the Crest Drainage Dike in reducing the load on the inner slope, testing of this concept in a flume is crucial. The Crest Drainage Dike is constructed in the flume. A set with different types of waves (with different exceedance frequencies) is imposed on the Crest Drainage Dike. The wave overtopping at the inner slope and the discharge through the pipes are measured during the tests. This gives more insight into the relative amount of wave overtopping that is caught by the crest construction in different conditions.
To get more insight in the energy dissipation by the crest construction, some tests could be done in which the drainage pipes are closed off. In this way, the construction only functions as a breaker of the overtopping water while it is not being discharged, therefore it will eventually all reach the inner slope. As a reference for these tests, the same set of waves should also be imposed on a dike set up without the crest construction (smooth crest). A comparison of the amount of erosion at the inner slope between the two situations will give more insight in the occurring energy dissipation and thus load reduction because of the crest construction.
The width and form of the crest construction could also be varied in the tests, to get more insight into the variation of effectivity of the Crest Drainage Dike if these parameters are changed.
Acceptance of overtopping dike in general
To get the concept of an overtopping dike in general accepted by the different stakeholders at a certain location, more insight should be given into the total amount of wave overtopping towards the landward area during a certain event and the frequency of occurrence of this event. The combination of the overtopping amount per event and the frequency of occurrence will give an idea of the amount of overtopping in a planning period of for example 5 to 10 years. This insight is needed when talking about the development of the landward area.
First of all, to maintain safety, it should be possible to at least store the maximum amount of overtopping water during the extreme design conditions in the landward area. This demand gives criteria for the required size of the landward area. Furthermore, if in the ComCoast concept a combination with nature development in the landward area is foreseen, the amount of wave overtopping and overtopping frequency in the planning period will be of great influence on the possibilities to develop certain types of nature in the area.
With theoretical formulas, calculations can be made of the amount of wave overtopping for a certain exceedance frequency for the studied location/ cross profile. This is executed for lots of different exceedance frequencies, an integration of all these calculation results over a certain period gives the total amount of wave overtopping for this period.
The results of the theoretical formulas can be checked by executing some tests in a flume, in which the total overtopping for a certain combination of waves with different exceedance frequencies can be measured for the same cross profile. The results can be compared with the results of the theoretical formulas.
5 PLANNING AND ESTIMATION
5.1 Planning and products theoretical phase
The products to be delivered per activity are described below:
1. Further study and detailing several aspects of the Crest Drainage Dike; Technical report Crest Drainage Dike
2. Preliminary Design Crest Drainage Dike;
Preliminary design for two given cross sections 3. Description of required licenses and permits;
Memo licenses and permits Crest Drainage Dike 4. Participation meeting in preparation of Testing Phase.
Presence and input at the meeting
All above described products will be delivered in English. The report will be written in a clear but concise manner. Assuming we can start the theoretical phase the first week of June in 2005, the delivery of all products for this phase is set at 31st of July 2005. This is also shown in the planning in Figure 4, which shows the activities and deliverables during the project period.
Figure 4: Planning and products theoretical phase Crest Drainage Dike
If the start of the project is delayed because of a delay in the contract award (planned for May 31st 2005), the entire planning will shift backwards equal to the delay period.
5.2 Cost estimation theoretical phase
Per activity, the required man-days and costs are shown in the cost estimation in Table 3. This results in an offer for the execution of the theoretical phase for the Crest Drainage Dike for the amount of ¼ H[FOXGLQJ%7: HTXDOWR¼LQFOXGLQJ%7:
22 23 24 25 26 27 28 29 30
Contract award/ start of the project 1. Further study aspects Crest Drainage Dike
Technical report Crest drainage dike 2. Preliminary Design Crest Drainage Dike
Preliminary design for two given cross sections 3. Description of required licenses and permits
Memo licenses and permits Crest Drainage Dike
4. Participation meeting preparation Testing Phase to be planned in June 2005
Client’s comment period on concept products
Delivery moment concept product Delivery moment definitive product
Table 3: Cost estimation theoretical phase Crest Drainage Dike
Activity: Man-days Costs
1. Further study and detailing several aspects of the Crest Drainage Dike 20 ¼15.000
2. Preliminary Design Crest Drainage Dike 4 ¼
3. Description of required licenses and permits 2,5 ¼
4. Participation meeting in preparation of Testing Phase 1 ¼
Total 27,5 ¼
The cost estimation is based on overall input by the team leader for 20% of time, senior experts for 20% of time, junior and medior specialists for 55 % of time and project manager for 5 % of time. Rates are based on price level 2005. Per activity this ratio might be a bit different. If additional activities are requested, the daily rates as shown in Table 4 are applied for the calculation of additional costs.
Table 4: Daily rates for additional activities
Function class Daily rate
Team leader/ senior expert ¼
Medior specialist ¼
Junior specialist ¼
Project Manager ¼
This proposal is valid until the 31st of July 2005. The ‘RVOI 2001’ (Regeling van de Verhouding tussen Opdrachtgever en adviserend Ingenieursbureau) applies to this project. The RVOI is available for inspection at the DHV office and can be submitted to the client if requested.
5.3 Project team
The team leader for this project will be Henk van Hemert, he has an extensive experience in all phases and technical aspects of dike improvements. For the hydraulic aspects, Vincent Hombergen is proposed as a senior specialist and for the structural aspects of the concrete construction, Eric Brasser is proposed as a senior specialist. For the overall project management and quality control of this project we propose Tjibbe van Ellen, a project manager with a wide experience in dike safety assessment and dike improvements. A short description of the knowledge fields of the key personnel is given below, the full Curricula Vitae are given in Appendix A. The key personnel will be supported by junior and medior specialists in hydraulics, geotechnics, structures and legislation (licenses and permits).
Henk van Hemert
Mr. van Hemert has over ten years of experience in the area of geotechnical and hydraulic engineering. He started as a geotechnical specialist in all types of projects and broadened his horizon in hydraulic engineering and coastal zone management the last few years. Besides practical work in the field of dike improvement, he has been involved in several research programs concerning this knowledge area. Furthermore, appreciation for his work also shows in several positions he has had in technical committees and brainstorming groups in this area. He has developed a significant competence for working with and supporting of staff in order to achieve project related goals.
Vincent Hombergen
Vincent Hombergen has over 12 years experience in civil and maritime engineering. His particularly expertise is in relation to dikes and dams, river revetments, coastal engineering, flood protection works and risk analysis. He has worked for private and public clients. His expertise extends over the project phases: feasibility, design, contract documents, contracting and contract management. Overseas experience: Europe, Africa, and Central America. Typical projects in which he was/is heavily involved: dike reinforcement projects, flood risk analysis projects, high speed rail link (tunnel engineering / project management) and environmental impact assessment studies.
Eric Brasser
Mr. Brasser is an experienced senior specialist in the area of infrastructure in relation to water (ports, waterways, embankments etc.). He has worked on projects concerning many different types of maritime structures such as quays, locks, bridges, aqueducts etc. In these projects he has gained a lot of knowledge on different types concrete constructions and different types of materials. He has gained experience in every project phase: from the initiation, feasibility, design, technical specification, tendering to project management and supervision during the construction phase. He has handled projects in which not only the technical and financial aspects are of importance but also other aspects such as landscaping, architecture and special use of materials and energy.
Tjibbe van Ellen
Tjibbe van Ellen is a project director in the unit Ports, Waterways and Coastal development. He has about 20 years of experience in hydraulic engineering, with a special focus in the areas of flood defence and depot building. He has great technical knowledge of these areas, and knows how to combine this with all other processes and aspects that can be of importance in large and complex projects.
6 REFERENCES
Haskoning, 2005 ComCoast WP3 State-of-the-Art inventory, CUR RWS, Report 9P8624.A0, March 2005
7 COLOPHON
CUR/Innovative concepts for an overtopping dike WG-SE20050625
Client : CUR
Project : Innovative concept for an overtopping dike
File : x1498-01-000
Length of report : 22 pages
Author : O.E. Nieuwenhuis
Contributions : V. Hombergen
Project Manager : V. Hombergen Project Director : T. Louters
Date : 22 April 2005
