H.J. Verhagen
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1
Innovative coastal risk reduction through hybrid design
Combining sand cover and structural defenses
Worldwide, sand dunes and hard coastal structures help to minimize loss of lives and property from storm impact and flooding along or behind coastlines. Both sand dunes and hard coastal structures have their benefits and shortfalls in terms of protective capacity, cost, flexibility, and impact to coastal systems. Combining these two inherently different coastal risk reduction measures into a single hybrid system can preserve some of the individual benefits of each while creating an Engineering-with-Nature™ system that fulfills the requirements of high levels of protection, adaptability to future challenges related to climate change, sustainability, and pleasing natural aesthetics. Although such hybrid systems have the potential to become viable alternatives to conventional coastal risk reduction schemes, there are still many unknowns related to the interaction between the soft and hard structural components and their effectiveness in storm surge mitigation and flood prevention that require targeted research efforts to create acceptable design guidelines. Specifically, the combination of hard and soft alternatives into a single structure has not been studied in detail. Here, hybrid coastal risk reduction systems consisting of traditional hard structures (levees, revetments, and sea walls) covered by sand layers attempting to mimic dunes are investigated. It is emphasized that these sand covers can look like natural dunes, but they cannot evolve like natural dunes in the long term due to spatial constrictions along developed coasts and lack of natural sediment supply in eroding coastal systems. Just like any engineered beach and dune system, these hybrid structures require episodic maintenance nourishment, particularly after storm impact. The present overview covers design advances and issues related to both hard structures and engineered sand dunes for coastal risk reduction and investigates existing hybrid approaches. Future research goals to better understand hybrid coastal risk reduction systems and to create applicable design guidelines are discussed.
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Worldwide, sand dunes and hard coastal structures help to minimize loss of lives and property from storm impact and flooding along or behind coastlines. Both sand dunes and hard coastal structures have their benefits and shortfalls in terms of protective capacity, cost, flexibility, and impact to coastal systems. Combining these two inherently different coastal risk reduction measures into a single hybrid system can preserve some of the individual benefits of each while creating an Engineering-with-Nature™ system that fulfills the requirements of high levels of protection, adaptability to future challenges related to climate change, sustainability, and pleasing natural aesthetics. Although such hybrid systems have the potential to become viable alternatives to conventional coastal risk reduction schemes, there are still many unknowns related to the interaction between the soft and hard structural components and their effectiveness in storm surge mitigation and flood prevention that require targeted research efforts to create acceptable design guidelines. Specifically, the combination of hard and soft alternatives into a single structure has not been studied in detail. Here, hybrid coastal risk reduction systems consisting of traditional hard structures (levees, revetments, and sea walls) covered by sand layers attempting to mimic dunes are investigated. It is emphasized that these sand covers can look like natural dunes, but they cannot evolve like natural dunes in the long term due to spatial constrictions along developed coasts and lack of natural sediment supply in eroding coastal systems. Just like any engineered beach and dune system, these hybrid structures require episodic maintenance nourishment, particularly after storm impact. The present overview covers design advances and issues related to both hard structures and engineered sand dunes for coastal risk reduction and investigates existing hybrid approaches. Future research goals to better understand hybrid coastal risk reduction systems and to create applicable design guidelines are discussed.
In this paper, protection options for a high-value, industrial area along the coast of West Bengal (India) are investigated. The options are designed to protect against cyclone surges with a probability of 1/100 per year. Two alternatives are compared, a classical solution of a dike with a revetment and a solution with a mangrove belt in front of the dike. The results reveal that from a pure infrastructural cash-flow point-of-view, the mangrove solution is at least 25% cheaper than the classical solution with a rock revetment. An important finding is that this conclusion does not need the financial evaluation of the obvious additional ecological advantages that mangroves offer. It is postulated that these results are generally valid for high-value coastal areas under the attack of waves during storm surges.
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In this paper, protection options for a high-value, industrial area along the coast of West Bengal (India) are investigated. The options are designed to protect against cyclone surges with a probability of 1/100 per year. Two alternatives are compared, a classical solution of a dike with a revetment and a solution with a mangrove belt in front of the dike. The results reveal that from a pure infrastructural cash-flow point-of-view, the mangrove solution is at least 25% cheaper than the classical solution with a rock revetment. An important finding is that this conclusion does not need the financial evaluation of the obvious additional ecological advantages that mangroves offer. It is postulated that these results are generally valid for high-value coastal areas under the attack of waves during storm surges.
At many locations, especially in deltaic areas, dikes and other flood defence structures are needed to protect the hinterland against flooding by surges (typhoons, hurricanes) and/or tsunamis. Dikes are quite costly, and mangroves in front of the dike will lower the costs of the dike. Mangroves in general allow dikes to be lower and narrower. In this chapter the hydrodynamic effects of mangroves on dikes are elaborated as well as the financial impact on the costs of dikes. The main advantage of a mangrove forest in front of a dike is that the forest decreases the wave height just in front of the dike, and therefore the freeboard needed to cope with these waves can be considerably less. Because the waves in front of the dike are small due to the mangrove forest, often no costly revetment structure is needed any more. In this chapter computational methods are presented to determine the direct benefit for dike construction.
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At many locations, especially in deltaic areas, dikes and other flood defence structures are needed to protect the hinterland against flooding by surges (typhoons, hurricanes) and/or tsunamis. Dikes are quite costly, and mangroves in front of the dike will lower the costs of the dike. Mangroves in general allow dikes to be lower and narrower. In this chapter the hydrodynamic effects of mangroves on dikes are elaborated as well as the financial impact on the costs of dikes. The main advantage of a mangrove forest in front of a dike is that the forest decreases the wave height just in front of the dike, and therefore the freeboard needed to cope with these waves can be considerably less. Because the waves in front of the dike are small due to the mangrove forest, often no costly revetment structure is needed any more. In this chapter computational methods are presented to determine the direct benefit for dike construction.
Single layer concrete armor systems are being widely used nowadays in the design of rubble mound breakwaters. Recently, a new concrete armor unit has been developed and applied as single layer armor system in the repair works of one damaged breakwater at Al Fujeirah, UAE. It has a symmetrical shape, in contrast to most other units. Modern single layer concrete armor units that exist at this moment have design guidelines in terms of placement, stability and overtopping. However, because of lack of laboratory research and the little experience of using Crablock, no design guidance exists yet for this new single layer block compared to other existing one layer units. Being a new armor unit, the placement was investigated first. Then physical model tests were performed in a wave flume to come up with results on stability and wave overtopping. Furthermore, to determine the interlocking properties of armor units, pull tests were also conducted in this research. The placement tests showed that uniform placement was best achieved with a rectangular grid on a relatively small underlayer of rock. Test results on stability showed that longer waves affected the armor layer a little more, with more rocking and earlier start of damage. Packing density as well as placement pattern showed no influence on wave overtopping. The overtopping tests gave larger overtopping than expected, which might be due to the fairly steep 1:30 foreshore that gave a large ratio of significant wave-height from the time domain and the spectral wave-height.
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Single layer concrete armor systems are being widely used nowadays in the design of rubble mound breakwaters. Recently, a new concrete armor unit has been developed and applied as single layer armor system in the repair works of one damaged breakwater at Al Fujeirah, UAE. It has a symmetrical shape, in contrast to most other units. Modern single layer concrete armor units that exist at this moment have design guidelines in terms of placement, stability and overtopping. However, because of lack of laboratory research and the little experience of using Crablock, no design guidance exists yet for this new single layer block compared to other existing one layer units. Being a new armor unit, the placement was investigated first. Then physical model tests were performed in a wave flume to come up with results on stability and wave overtopping. Furthermore, to determine the interlocking properties of armor units, pull tests were also conducted in this research. The placement tests showed that uniform placement was best achieved with a rectangular grid on a relatively small underlayer of rock. Test results on stability showed that longer waves affected the armor layer a little more, with more rocking and earlier start of damage. Packing density as well as placement pattern showed no influence on wave overtopping. The overtopping tests gave larger overtopping than expected, which might be due to the fairly steep 1:30 foreshore that gave a large ratio of significant wave-height from the time domain and the spectral wave-height.
Conference paper
(2017)
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G. Guillaume Carpentier, P. Piero Silva, W Allsop, D. Mouazé, Henk Jan Verhagen, L. Gautier-Chevreux, M. Bagieu
This paper describes a unique design project devised to teach and apply technical skills in port design, but also in the practical cooperation between students in a realistic design project. The 4-week long project now involves students of up to 19 nationalities from 7-10 universities, and has been based each year on real design projects. Student teams of 8-10 per team have to deliver a realistic design to examiners from both academia and industry. The students select from a number of optional modules. No student can take all of them, so appreciation of the skills available within the team are important.
...
This paper describes a unique design project devised to teach and apply technical skills in port design, but also in the practical cooperation between students in a realistic design project. The 4-week long project now involves students of up to 19 nationalities from 7-10 universities, and has been based each year on real design projects. Student teams of 8-10 per team have to deliver a realistic design to examiners from both academia and industry. The students select from a number of optional modules. No student can take all of them, so appreciation of the skills available within the team are important.
Mangroves have proved to be an asset to coastlines owing to their various advantages in terms of coastal protection and stability. Along tropical coastlines, as mangrove tracts recede and threats of coastal erosion mount, mangrove rehabilitation projects are being seriously contemplated. The design of such projects, however, is in an incipient phase, and several technical and economic challenges are to be faced. The plantation and growth of mangroves requires a protected coastal environment with proper drainage of the soil substratum. Hence, in a sample design undertaken for a mangrove rejuvenation project along the eastern coast of Mumbai (India), various layouts have been studied for a protective coastal structure and drainage system. One such design uses bamboo pile walls in creating shielded compartments (with multiple compartmental layouts) for mangrove growth, along with bamboo drains. The bamboo-based structure was found to be environmentally and economically advantageous over other designs such as sand dykes, which are many times more expensive. Moreover, employing natural material such as bamboo helps the structure integrate with the developing mangrove habitat, allaying concerns about dismantling the structure after mangrove growth. A cost-minimising and eco-friendly bamboo structure such as this therefore promises to pave the way for large ecological projects in the future, spanning over 1000 ha.
...
Mangroves have proved to be an asset to coastlines owing to their various advantages in terms of coastal protection and stability. Along tropical coastlines, as mangrove tracts recede and threats of coastal erosion mount, mangrove rehabilitation projects are being seriously contemplated. The design of such projects, however, is in an incipient phase, and several technical and economic challenges are to be faced. The plantation and growth of mangroves requires a protected coastal environment with proper drainage of the soil substratum. Hence, in a sample design undertaken for a mangrove rejuvenation project along the eastern coast of Mumbai (India), various layouts have been studied for a protective coastal structure and drainage system. One such design uses bamboo pile walls in creating shielded compartments (with multiple compartmental layouts) for mangrove growth, along with bamboo drains. The bamboo-based structure was found to be environmentally and economically advantageous over other designs such as sand dykes, which are many times more expensive. Moreover, employing natural material such as bamboo helps the structure integrate with the developing mangrove habitat, allaying concerns about dismantling the structure after mangrove growth. A cost-minimising and eco-friendly bamboo structure such as this therefore promises to pave the way for large ecological projects in the future, spanning over 1000 ha.
Accidental use of earth bodies as flood defence
The Vlaardingen case study
An overview of the upgrade of the railroad dike in Vlaardingen to a full sea defence, creating in this way a multifunctional flood defence.
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An overview of the upgrade of the railroad dike in Vlaardingen to a full sea defence, creating in this way a multifunctional flood defence.
Grass covers have been applied as an effective measure for protecting river levees and sea dikes. We conducted experiments to show how roots considerably improve the shear strength of soil on dike slopes. Roots of 1-year-old Bermuda and Carpet grass may increase the total shear strength of up to 20 kPa. Exposed to severe overtopping flow, dike slopes may possibly fail in various manners including ‘head-cut’, ‘roll-up’ and ‘collapse’. The ratio between shear strength of the grass cover and its subsoil layer would get a value of two to distinguish the first two manners and would be zero for the last one. To some extent, the findings contribute to the basis for thoughtfully investigating the strength and failure mechanism of grass-covered slopes.
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Grass covers have been applied as an effective measure for protecting river levees and sea dikes. We conducted experiments to show how roots considerably improve the shear strength of soil on dike slopes. Roots of 1-year-old Bermuda and Carpet grass may increase the total shear strength of up to 20 kPa. Exposed to severe overtopping flow, dike slopes may possibly fail in various manners including ‘head-cut’, ‘roll-up’ and ‘collapse’. The ratio between shear strength of the grass cover and its subsoil layer would get a value of two to distinguish the first two manners and would be zero for the last one. To some extent, the findings contribute to the basis for thoughtfully investigating the strength and failure mechanism of grass-covered slopes.
The strength of the grass sod is an important factor for the stability of a dike in the Netherlands during wave overtopping conditions. Many tests have been performed the last few years with the Wave Overtopping Simulator, leading to the Cumulative Overload Method and a critical velocity. This velocity is a strength parameter of grass on a dike under loads induced by overtopping wave volumes. A new method has been developed to determine this critical velocity, by measuring the force while lifting the grass sod perpendicular to the slope out of the sod. This force is rewritten into the critical grass normal stress which is one of the input parameters for determining the critical velocity of a grass sod. When the critical velocity resulting from this method is compared with the determined critical velocities with the Wave Overtopping Simulator, there is good correspondence between the results for the tested locations. Therefore the sod pulling test could provide results that are reliable enough to determine the critical velocity of a dike section.
...
The strength of the grass sod is an important factor for the stability of a dike in the Netherlands during wave overtopping conditions. Many tests have been performed the last few years with the Wave Overtopping Simulator, leading to the Cumulative Overload Method and a critical velocity. This velocity is a strength parameter of grass on a dike under loads induced by overtopping wave volumes. A new method has been developed to determine this critical velocity, by measuring the force while lifting the grass sod perpendicular to the slope out of the sod. This force is rewritten into the critical grass normal stress which is one of the input parameters for determining the critical velocity of a grass sod. When the critical velocity resulting from this method is compared with the determined critical velocities with the Wave Overtopping Simulator, there is good correspondence between the results for the tested locations. Therefore the sod pulling test could provide results that are reliable enough to determine the critical velocity of a dike section.