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Mangrove forests reduce wave attack along tropical and sub-tropical coastlines, decreasing the wave loads acting on coastal protection structures. Mangrove belts seaward of embankments can therefore lower their required height and decrease their slope protection thickness. Wave reduction by mangroves depends on tree frontal surface area and stability against storms, but both aspects are often oversimplified or neglected in coastal protection designs. Here we present a framework to evaluate how mangrove belts influence embankment designs, including mangrove growth over time and failure by overturning and trunk breakage. This methodology is applied to Sonneratia apetala mangroves seaward of embankments in Bangladesh, considering forest widths between 10 and 1000 m (cross-shore). For water depths of 5 m, wave reduction by mangrove forests narrower than 1 km mostly affects the slope protection and the bank erodibility, whereas the required embankment height is less influenced by mangroves. Sonneratia apetala trees experience a relative maximum in wave attenuation capacity at 10 years age, due to their large submerged canopy area. Once trees are more than 20 years old, their canopy is emergent, and most wave attenuation is caused by trunk and roots. Canopy emergence exposes mangroves to wind loads, which are much larger than wave loads, and can cause tree failure during cyclones. These results stress the importance of including tree surface area and stability models when predicting coastal protection by mangroves., Hydraulic Structures and Flood Risk

This report investigates how embankments in Bangladesh could be upgraded with no or limited footprint increase. Many Bangladesh embankments need to be upgraded to improve flood protection. Traditionally, embankments are upgraded by adding more soil, heightening the crests while simultaneously widening the embankment maintain slope stability. However, in many cases there is insufficient space for embankment widening. Case studies show that in Bangladesh, private assets often directly border embankments at both the landward and seaward side. Also land is scarce in general and the land directly adjacent to embankments is often in use by the communities. For traditional embankment heightening and widening, these buildings need to be (re)moved. So owners of private assets and land need to be identified and compensated and the land must be acquired leading additional project costs and delays. This problem could be mitigated by using embankment upgrade techniques with a limited footprint increase. This requires knowledge of the available techniques and their suitability in the Bangladesh situation. Therefore, this study aims to provide an inventory of low-footprint embankment up-grade techniques used around the world, their (dis)advantages in terms of land use, risks, costs and O&M aspects, and their suit-ability for the situation in Bangladesh., Hydraulic Structures and Flood Risk

Coastal cities combine intensive socioeconomic activities and investments with high exposure to flood hazards. Developing effective strategies to manage flood risk in coastal cities is often a costly and complicated process. In designing strategies, engineers rely on computationally demanding flood simulation models, but they can only compare a limited number of strategies due to computational constraints. This limits the efficacy of standard flood simulation models in the crucial conceptual phase of flood risk management. This paper presents the Flood Risk Reduction Evaluation and Screening (FLORES) model, which provides useful risk information in this early conceptual phase. FLORES rapidly performs numerous simulations and compares the impact of many storms, strategies, and future scenarios. This article presents FLORES and demonstrates its merits in a case study for Beira, Mozambique. Our results demonstrate that expansion of the drainage capacity and strengthening of its coastal protection in the southwest are crucial components of any effective flood risk management strategy for Beira., Civil Engineering and Geosciences, Hydraulic Structures and Flood Risk, Policy Analysis

Since the catastrophic flooding of New Orleans due to Hurricane Katrina in 2005, the city's hurricane protection system has been improved to provide protection against a hurricane load with a 1/100 per year exceedance frequency. This paper investigates the risk to life in post-Katrina New Orleans. In a flood risk analysis the probabilities and consequences of various flood scenarios have been analyzed for the central area of the city (the metro bowl) to give a preliminary estimate of the risk to life in the post-Katrina situation. A two-dimensional hydrodynamic model has been used to simulate flood characteristics of various breaches. The model for estimation of fatality rates is based on the loss of life data for Hurricane Katrina. Results indicate that – depending on the flood scenario – the estimated loss of life in case of flooding ranges from about 100 to nearly 500, with the highest life loss due to breaching of the river levees leading to large flood depths. The probability and consequence estimates are combined to determine the individual risk and societal risk for New Orleans. When compared to risks of other large-scale engineering systems (e.g., other flood prone areas, dams and the nuclear sector) and acceptable risk criteria found in literature, the risks for the metro bowl are found to be relatively high. Thus, despite major improvements to the flood protection system, the flood risk to life of post-Katrina New Orleans is still expected to be significant. Indicative effects of reduction strategies on the risk level are discussed as a basis for further evaluation and discussion., Hydraulic Engineering, Civil Engineering and Geosciences

After the catastrophic flooding of New Orleans due to hurricane Katrina in the year 2005, the city's hurricane protection system has been improved to provide protection against a hurricane load with a 1/100 per year exceedance frequency. This paper investigates the risk to life in post-Katrina New Orleans. In a risk-based approach the probabilities and consequences of various flood scenarios have been analyzed for the central area of the city (the metro bowl) to give a preliminary estimate of the risk to life in the post-Katrina situation. A two-dimensional hydrodynamic model has been used to simulate flood characteristics of various breaches. The model for estimation of fatality rates is based on the loss of life data for Hurricane Katrina. Results indicate that – depending on the flood scenario – the estimated loss of life in case of flooding ranges from about 100 to nearly 500, with the highest life loss due to breaching of the river levees leading to large flood depths. The probability and consequence estimates are combined to determine the individual risk and societal risk for New Orleans. When compared to risks of other large scale engineering systems (e.g. other flood prone areas, dams and the nuclear sector) and acceptable risk criteria found in literature, the risks for the metro bowl are found to be relatively high. Thus, despite major improvements to the flood protection system, the flood risk of post-Katrina New Orleans is still expected to be significant. Effects of reduction strategies on the risk level are discussed as a basis for further evaluation., Hydraulic Engineering, Civil Engineering and Geosciences

Increases in typhoon intensity and sea level rise could pose significant challenges to coastal defences around Tokyo Bay. In order to analyse the extent of future problems the authors determined the increase storm surge that could result from an increase in typhoon intensity and sea level rise to this area around the turn of the 21st century. Results show how the various settlements around Tokyo Bay are at considerable risk of storm surges and sea level rise in the future. If defences are breached the potential direct economic consequences could be significant, potentially in excess of 100 trillion yen, with the indirect costs likely to be even greater. As a result it is likely that sea defences will have to be strengthened around Tokyo Bay in the future, which could cost in the order of 370bn yen to defend against a 1 in 100 year storm by the year 2100. Alternatively, a storm surge barrier could be built, which would be more expensive (possibly in the range of 700-800bn yen), though it could increase the protection level and would be able to cope with 1 in 200 or 500 year events, amongst other benefits., Hydraulic Engineering, Civil Engineering and Geosciences

The Houston - Galveston area is at significant risk from hurricane induced storm surges. This paper summarizes ongoing studies on flood risk reduction for the region. Firstly, based on a simplified probabilistic hurricane surge model , the return periods of surges within the bay have been estimated. This model framework can be used to assess the effectiveness of several risk reduction interventions. Sketch- and conceptual designs have been made of a storm surge barrier in the Bolivar Roads, that would be part of the Ike Dike coastal protection concept. Such a storm surge barrier would consist of two parts, an environmental section for flow requirements (consisting of caissons with vertical gates) and a navigation section of the barrier (consisting of a barge gate), which would allow unhindered passage of navigation during normal conditions. Future questions and challenges for flood risk reduction in the Bay are identified., Hydraulic Engineering, Civil Engineering and Geosciences

This paper explores an alternative method to determine extreme surge levels at the Dutch Coast. For this exploration, specific focus is on the extreme water level at Hoek van Holland, The Netherlands. The alternative method has been based on a joint probability model of the storm characteristics at the North Sea. The intent of this method is to provide a better physical and statistical insight into the effects of meteorological characteristics on surge levels and surge duration, especially for surges of more extreme storms currently not captured in existing water level measurement records. The meteorological part is an analytical parametrical model based on the Holland model for hurricanes, which results in time- and space-varying wind and pressure fields of North Sea storms. The wind and pressure forcing is then applied in the hydrodynamic model which numerically solves the nonlinear depth-averaged shallow water equations in a one-dimensional domain from the edge of the continental shelf between Scotland and Norway to Hoek van Holland. Validation against wind observations from historical storms at one location in the entire domain shows good results. Results of the calibrated surge level model are reasonable if peak surge levels are considered. The surge duration, however, is underestimated by the model. Next, the model has been applied to define extreme surge levels using Monte Carlo Analysis. Probability density distributions for the storm parameters based on historical data have been used as input. The computed surge level (including tide) with a statistical return period of 10,000 years appears to be close to the value from statistical extrapolation of surge levels. The output also indicates that the average duration of computed surges with a return period of 10,000 year is roughly two hours longer than the storm duration currently adopted., Hydraulic Engineering, Civil Engineering and Geosciences

In this study an overview of existing and planned storm surge barriers in the world is provided. A systematic analysis relates functional requirements (e.g. navigation and tidal exchange) to the main barrier characteristics (e.g. gate type, dimensions). Furthermore, as the costs of barriers are an important issue in the planning, available cost information is analyzed and related to main barrier characteristics. An approach to provide a preliminary cost estimate of new barriers is presented. Finally, some critical technical challenges are discussed and related to functional requirements and boundary conditions. Overall, the results of this study can assist in the initial design and planning phase of storm surge barriers for new locations., Hydraulic Engineering, Civil Engineering and Geosciences

Eind oktober 2012 gebeurde wat al jaren was voorspeld: een orkaan treft een groot gebied in het noordoosten van de VS, met uitzonderlijk veel schade en meer dan 130 dodelijke slachtoffers. De vraag is of de Amerikanen meer aandacht gaan krijgen voor preventieve maatregelen., Hydraulic Engineering, Civil Engineering and Geosciences

The objective of this study is to provide suitable candidate solutions for a storm surge barrier in the Bolivar Roads as part of the Ike Dike system and provide recommendations for further research., Hydraulic Engineering, Civil Engineering and Geosciences

In het najaar van 2011 is Thailand zwaar getroffen door overstromingen, met meer dan 800 dodelijke slachtoffers en ongeveer 45 miljard dollar directe schade. Nederlandse expertise werd ingeroepen voor het crisismanagement om de schade te beperken., Hydraulic Engineering, Civil Engineering and Geosciences

Hydraulic Engineering, Civil Engineering and Geosciences

This paper reports a reconnaissance level study of a storm surge barrier in the Mississippi River. Historical hurricanes have shown storm surge of several meters along the Mississippi River levees up to and upstream of New Orleans. Future changes due to sea level rise and subsidence will further increase the risk of flooding due to hurricane storm surge. A surge barrier downstream of New Orleans has been considered as an alternative to levee raise along the Mississippi River. Hydraulic computations show that the build-up of water behind the barrier due to the Mississippi River flow is (much) lower than the hurricane surge protruding up the river in the no-barrier situation. The barrier will probably eliminate the need to upgrade the system upstream of the barrier while providing the same level of hurricane risk reduction. A hybrid barrier (a combination of different gate types) with a primary swing gate for navigation (and flow) and secondary lift gates to accommodate for flow is a technically feasible alternative. The barrier remains open for almost the entire year and would only to be closed during severe tropical events (say once every 2 - 3 years). Several measures are included in the conceptual design to mitigate the navigation impact. The construction costs of the barrier are estimated at $1.6 - 2.6 billion. It is recommended to compare the investment costs of a barrier including adjacent tie-ins to the existing HSDRRS to the costs of upgrading and maintaining the levee system throughout the city of New Orleans., Hydraulic Engineering, Civil Engineering and Geosciences

After the catastrophic flooding of New Orleans due to hurricane Katrina in the year 2005, the city’s hurricane protection system has been improved to provide protection against storms with at least a 100 year return period. The aim of this article is to investigate the risk to life in the post-Katrina situation for the New Orleans metro bowl. In a risk-based approach the probabilities and consequences of various flood scenarios have been analyzed to estimate the risk to life. A two-dimensional hydrodynamic model has been used to simulate flood characteristics. Results indicate that – depending on the flood scenario – the estimated loss of life in case of flooding ranges from about 100 to nearly 500. The highest life loss value is found for breaching of the river levees. Probability and consequence estimates are combined to evaluate the individual risk and societal risk for New Orleans. When compared to risks of other large scale engineering systems (e.g. other flood prone areas, dams and the nuclear sector) and acceptable risk criteria found in literature, the evaluated risk exceeds acceptable risk levels. Thus despite major improvements to the flood protection system, the flood risk of post-Katrina New Orleans is expected to be significant. Effects of risk reduction strategies on the risk level are investigated to assist in providing bases for decision making. Results indicate the necessity of further discussion regarding the management and reduction of the city’s risk to flooding., Hydraulic Engineering, Civil Engineering and Geosciences

Het stormseizoen is weer begonnen in Nederland. De duinen en dijken langs de Nederlandse kust kunnen in deze periode zwaar belast worden door een combinatie van hoge stormvloed en zware golven. Op dit moment worden deze waterkeringen getoetst en ontworpen met waterstandsgegevens die vooral gebaseerd zijn op statistische extrapolatie van langjarige tijdreeksen. Een parametrisch model, ontwikkeld bij de Technische Universiteit Delft samen met Royal HaskoningDHV, geeft meer inzicht in de achterliggende stormkarakteristieken die horen bij deze maatgevende condities. Ook biedt deze methode perspectief om op termijn de extreme combinaties van waterstand en golven beter te voorspellen., Hydraulic Engineering, Civil Engineering and Geosciences

Recent research done the IPCC (2007) working groups and other organizations has sparked global concern over the possible impacts of climate change and corresponding sea level rise (SLR) upon coastal communities. In reaction studies were done (for example by Nicholls et al., 2008) to assess the vulnerability of coastal regions and get an indication of the magnitude of the potential global impacts. However, most of these studies did not address the development of climate change adaptation designs to protect the coastline. In this paper it is demonstrated how a localised coastal vulnerability assessment could guide the development of conceptual coastal protection designs in an African context. The overall aim of this paper is the appraisal of climate adaptation measures and coastal management strategies for Durban, South Africa. This is illustrated using a case study, for a coastal section along Durban´s central beaches. The case study is an example of how the vulnerability to coastal hazards could be assessed, for different SLR scenarios and should provide guidance for developing conceptual coastal protection designs. A recent extreme storm event indicated that significant damage can be sustained from coastal hazards in Durban under the current conditions. A 1-in-100 year storm is shown to already affect the operations of critical infrastructure under current conditions at the case study site. The projected vulnerability increases significantly for future SLR scenarios., Hydraulic Engineering, Civil Engineering and Geosciences

Tidal basins, as found in the Dutch Wadden Sea, are characterized by strong spatial variations in bathymetry and sediment distribution. In this contribution, the aim is at simulating the spatial sand-mud distribution of a tidal basin. Predicting this spatial distribution is however complicated, due to the non-linear interactions between tides, waves, sediment transport, morphology and biology. To reduce complexity, while increasing physical understanding, an idealized schematization of the Amelander inlet system is considered. Delft3D is applied with a recently developed bed module, containing various sediment layers, combined with formulations for both cohesive and non-cohesive sediment mixtures. Starting with uniform mud content in the spatial domain, the development of the sediment distribution is simulated. Realistic sand-mud patterns are found, with accumulation of mud on the tidal flats. The schematization is further used to determine the sensitivity of the sand-mud patterns to changes in tide, while assessing the influence of tidal dominance on the large-scale sand-mud patterns. The patterns are enhanced/diminished under the influence of higher/lower tides., Hydraulic Engineering, Civil Engineering and Geosciences

The Lagos coast has been suffering high rates of erosion since the construction of three harbour moles, i.e. the West Mole, East Mole and the Training Mole, at the tidal inlet connecting the Lagos Lagoon to the South Atlantic Ocean. To provide for a permanent erosion mitigation measure and to create residential and commercial area for circa 400,000 people, the Eko Atlantic City project has been initiated in 2008. In front of the eroded coast, approximately 9 km² of land will be reclaimed and protected by a revetment. In this study the long-term and large-scale morphological behavior of the Lagos coast is investigated and subsequently the long-term morphological impact of the project is assessed. First, a conceptual model is created, in which the historical development of the coast is discussed. The long-term morphological behavior of the coast downstream of the inlet is determined by two main factors: sediment accumulation at the West Mole and sediment import into the tidal inlet and the lagoon, induced by disturbance of the morphological equilibrium by sea level rise and dredging activities. Using the numerical simulation model Unibest, the long-term impact of Eko Atlantic City is assessed. It is concluded that the construction of Eko Atlantic City will not change the total erosion volumes downstream of the inlet. However, as the revetment of the project retains the coast, the erosion will be shifted towards downstream. Downdrift of the project, the erosion rates are locally relatively high. The shape of the sea defence has been designed to minimize the local erosion effect. A monitoring and mitigation strategy has been recommended to monitor this effect and instruct coastal protection management actions to be implemented if required., Water Management, Civil Engineering and Geosciences

Afgelopen voorjaar was sprake van een bijzonder hoogwater op de Mississippi. De rivier voerde op haar hoogtepunt in totaal meer dan 65.000 kubieke meter per seconde af naar de Golf van Mexico, ongeveer vier keer zoveel als de 1/1.250 jaar ontwerpafvoer van de Rijn in Nederland, en overtrof op sommige plaatsen het historische hoogwater van 1927. Bovenstrooms in de Mississippi vonden op diverse plaatsen overstromingen plaats. Dijken werden opgeblazen om het water weg te leiden om overstromingen in steden te voorkomen. Benedenstrooms werd onder andere de Morganza Spillway, een grote overlaat die sinds 1973 niet meer gebruikt was, opengezet om New Orleans te beschermen tegen (nieuwe) overstromingen. De totale schade liep in de honderden miljoenen dollars. Dit artikel geeft een overzicht van dit unieke hoogwater en een doorkijk naar mogelijke maatregelen om het hoogwaterrisico te reduceren., Hydraulic Engineering, Civil Engineering and Geosciences

Hydraulic Engineering, Civil Engineering and Geosciences

This paper discusses the application of a simple algorithm for the buffering of fines in a sandy seabed. A second layer is introduced in which fines may be stored during calm weather and from which fines may be resuspended during storms. The algorithm is applied first in a one-dimensional vertical (1DV) point model at a location in the North Sea, Noordwijk 10, 10 km offshore. It is able to reproduce the observed temporal variability of suspended particulate matter satisfactorily. Apart from the second layer, also the applied first order erosion rate is an important element of the algorithm. This allows for an equilibrium sediment mass per unit area for any combination of bed shear stress climate and sediment supply. The classical Partheniades-Krone formulation with zeroth order erosion (i.e. an erosion rate that is independent from the sediment mass per unit area) does not have such equilibrium. As a next step, the algorithm is incorporated into a 3D model for suspended particulate matter (SPM) transport in the Dutch coastal zone. It is demonstrated that the model is able to reproduce the observed spatial and temporal variability reasonably well. An essential feature of the 3D mud model is that it is sufficiently fast to compute equilibrium bed composition. This implies that the results are completely independent from the applied (uniform) initial conditions. Finally, the mud model is applied to assess the impact of a large-scale release of fines in the Dutch coastal zone. The computed impact turns out to be very sensitive to the assumed buffer capacity of the seabed. However, information on transient system response (such as the dissipation of a sediment pulse in the system) from which the buffer capacity may be estimated is most often lacking. For the time being, estimates on the residence time of fines in the seabed and its mixing depth are derived from the literature. Additional field and laboratory test on the exchange mechanisms of fines between the water colu

Volgens de Wet op de Waterkering moet de minister van Verkeer en Waterstaat om de vijf jaar de Hydraulische Randvoorwaarden voor de primaire waterkeringen vaststellen. Voor de IJssel- en Vechtdelta gaat het daarbij om de maatgevende waterstanden waarop de primaire waterkering getoetst moeten worden. Hiervoor moet als onderdeel van een probabilistische benadering een groot aantal berekeningen uitgevoerd worden met het stromingsmodel WAQUA voor verschillende combinaties van afvoer op de IJssel en de Vecht, windsnelheden, windrichtingen, meerpeilen op het IJsselmeer en het al dan niet functioneren van de balgstuw bij Ramspol. In dit project worden (concept) toetspeilen voor de Hydraulische Randvoorwaarden 2006 voor de IJssel- en Vechtdelta bepaald. Grofweg komt deze bepaling neer op drie stappen: • berekeningen met WAQUA om de maximale waterstand te bepalen in het gebied afhankelijk van de volgende parameters: afvoeren, windsnelheden, windrichtingen, meerpeilen en het al dan niet gesloten zijn van Ramspol; • probabilistische berekeningen met HYDRA-VIJ om de toetsrandvoorwaarden af te leiden bij een bepaald veiligheidsniveau. Samengevat zijn de volgende activiteiten aan de orde geweest in dit rapport: • uitvoeren van de berekeningen; • uitvoeren van de controles en aanpassing van de controles; • aanpassing van de modelschematisatie. Ten aanzien van fase 2 van het project WAQUA productiesommen en berekenen van concept-toetspeilen voor de IJssel en Vecht delta kunnen de volgende conclusies getrokken worden: • de 4.500 uit te voeren berekeningen zijn binnen de planning uitgevoerd en goedgekeurd; • de vooraf gestelde controle criteria zijn waar nodig aangepast en uitgebreid maar zijn vrijwel even streng gebleven als de originele criteria; • de afgekeurde berekeningen die “hydraulisch juist” zijn, zijn handmatig goedgekeurd; • op een drietal punten is de modelschematisatie en de barriersturing verbeterd. Uit deze fase komen de volgende aanbevelingen naar voren: • De Ramspolk, HR2006