Coastal Levees Research Articles

The potential of global coastal flood risk reduction using various DRR measures

Abstract. Coastal flood risk is a serious global challenge facing current and future generations. Several disaster risk reduction (DRR) measures have been posited as ways to reduce the deleterious impacts of coastal flooding. On a global scale, however, efforts to model the future effects of DRR measures (beyond structural) are limited. In this paper, we use a global-scale flood risk model to estimate the risk of coastal flooding and to assess and compare the efficacy and economic performance of various DRR measures, namely dykes and coastal levees, dry-proofing of urban assets, zoning restrictions in flood-prone areas, and management of foreshore vegetation. To assess the efficacy of each DRR measure, we determine the extent to which it can limit future flood risk as a percentage of regional GDP to the same proportional value as today (a “relative risk constant” objective). To assess their economic performance, we estimate the economic benefits and costs of implementing each measure. If no DRR measures are implemented to mitigate future coastal flood risk, we estimate expected annual damages to exceed USD 1.3 trillion by 2080, directly affecting an estimated 11.5 million people on an annual basis. Low- and high-end scenarios reveal large ranges of impact uncertainty, especially in lower-income regions. On a global scale, we find the efficacy of dykes and coastal levees in achieving the relative risk constant objective to be 98 %, of dry-proofing to be 49 %, of zoning restrictions to be 11 %, and of foreshore vegetation to be 6 %. In terms of direct costs, the overall figure is largest for dry-proofing (USD 151 billion) and dykes and coastal levees (USD 86 billion), much more than those of zoning restrictions (USD 27 million) and foreshore vegetation (USD 366 million). These two more expensive DRR measures also exhibit the largest potential range of direct costs. While zoning restrictions and foreshore vegetation achieve the highest global benefit–cost ratios (BCRs), they also provide the smallest magnitude of overall benefit. We show that there are large regional patterns in both the efficacy and economic performance of modelled DRR measures that display much potential for flood risk reduction, especially in regions of the world that are projected to experience large amounts of population growth. Over 90 % of sub-national regions in the world can achieve their relative risk constant targets if at least one of the investigated DRR measures is employed. While future research could assess the indirect costs and benefits of these four and other DRR measures, as well as their subsequent hybridization, here we demonstrate to global and regional decision makers the case for investing in DRR now to mitigate future coastal flood risk.

Extraction of Coastal Levees Using U-Net Model with Visible and Topographic Images Observed by High-Resolution Satellite Sensors.

Coastal levees play a role in protecting coastal areas from storm surges and high waves, and they provide important input information for inundation damage simulations. However, coastal levee data with uniformity and sufficient accuracy for inundation simulations are not always well developed. Against this background, this study proposed a method to extract coastal levees by inputting high spatial resolution optical satellite image products (RGB images, digital surface models (DSMs), and slope images that can be generated from DSM images), which have high data availability at the locations and times required for simulation, into a deep learning model. The model is based on U-Net, and post-processing for noise removal was introduced to further improve its accuracy. We also proposed a method to calculate levee height using a local maximum filter by giving DSM values to the extracted levee pixels. The validation was conducted in the coastal area of Ibaraki Prefecture in Japan as a test area. The levee mask images for training were manually created by combining these data with satellite images and Google Street View, because the levee GIS data created by the Ibaraki Prefectural Government were incomplete in some parts. First, the deep learning models were compared and evaluated, and it was shown that U-Net was more accurate than Pix2Pix and BBS-Net in identifying levees. Next, three cases of input images were evaluated: (Case 1) RGB image only, (Case 2) RGB and DSM images, and (Case 3) RGB, DSM, and slope images. Case 3 was found to be the most accurate, with an average Matthews correlation coefficient of 0.674. The effectiveness of noise removal post-processing was also demonstrated. In addition, an example of the calculation of levee heights was presented and evaluated for validity. In conclusion, this method was shown to be effective in extracting coastal levees. The evaluation of generalizability and use in actual inundation simulations are future tasks.

High-order shallow water expansions in free surface flows: Application to steady overflow processes

Free surface flow modeling in the natural environment is frequently conducted within a depth-integrated framework using the non-linear shallow water equations or Saint-Venant equations. However, these equations underestimate the discharge in overflow processes over obstacles, such as during the flooding of coastal levees. Alternative models are constructed by expansions in terms of a shallowness parameter σ, with the 2nd-order results being the so-called Serre-Green-Naghdi theory, widely used in ocean research. Higher-order expansions are required in some instances to obtain more accurate results or to set the validity limit of the leading order term of these perturbation results. However, most of the high-order results available apply to water wave problems over horizontal beds, yet not to the overflow over obstacles of generally uneven bathymetry. High-order shallow water expansions for open channel flows over uneven beds are developed in this work by an iterative procedure to generate the corresponding asymptotic expansions up to the high-order O(σ6), resulting in high-order ODEs when the Bernoulli equation at the free surface streamline is invoked to include gravity effects. These new equations are applied to the overflow process, as occurs during the coastal flooding in a levee-protected area. A perturbation solution to this problem is presented up to O(σ6) for the main overflow variables, which are successfully and systematically compared with a new set of experiments conducted in a large scale obstacle model. The novel theoretical solutions presented are further demonstrated to be better than former theories available in the literature.

Multi-functional urban design approaches to manage floods: examples from Dutch cities

ABSTRACT This practice paper identifies and examines four multi-functional urban design approaches dealing with urban flooding. These combine flood protection measures such as levees and water reservoirs with parking, retail, plazas, and parks. This paper explains the emergence of these multi-purpose solutions to flood management in the Netherlands. It then examines the characteristics of four types, including inland multi-purpose levees, coastal multi-purpose levees, water plazas, and integrated carparks and water reservoirs. It discusses the challenges of these approaches compared to mono-functional flood strategies. Finally, it identifies their potential applicability for urban design activities elsewhere.

Influence of Soil Heterogeneity on the Contact Problems in Geotechnical Engineering

Contact problems are widely encountered in geotechnical engineering, such as the contact between soils and concrete used in earth and rockfill dams, tunnels and coastal levees. Due to the unknown contact region and contact forces, the contact problems have strong boundary nonlinearity. In addition, soils have been recognized as heterogeneous materials in geotechnical engineering. The existence of the soil heterogeneity increases the nonlinearity of the contact problems. Currently, the contact problems are mostly analysed without considering the soil heterogeneity, which may not reflect the contact behavior well. In order to investigate the influence of soil heterogeneity on the contact problems, in this paper, a simple plane-strain contact problem is analysed as an example. In this example, Young’s modulus is taken to be a spatially variable. The local average subdivision (LAS) is used to model the heterogeneity of Young’s modulus. The penalty method is utilised to determine the contact behavior. By the first use of linking the penalty method with the LAS, the proposed approach can be used to analyse the contact problems considering soil heterogeneity. The results show that the influence of soil heterogeneity on the elastic contact problems is significant. The contact forces of the heterogeneous case present apparent variation compared to the results of the homogeneous case. The distribution of the contact force at a specific point is also normal when Young’s modulus is normally distributed, moreover, the coefficient of variation (COV) and the horizontal scale of fluctuation of Young’s modulus affect the extent of variation of the normal contact forces. The standard deviation of the normal contact force increases with the increase of the COV and decreases with the increase of the horizontal scale of fluctuation of Young’s modulus. From the analyses, to better predict the deformation/stress in the contact problems, heterogeneity needs to be considered.

Comparison of Pile-type and Gravity-type Coastal Levees in Terms of Resilience to Tsunami

Comparison of Pile-type and Gravity-type Coastal Levees in Terms of Resilience to Tsunami

Comparison and investigation on tidal levels designed for the coastal embankments in Guangdong-Hong Kong-Macao Greater Bay Area

Tidal levels designed for coastal levee in Pearl River estuary have been provided by Department of water resources of Guangdong Province in 2002 and played important role in ensuring the security of coastal cities in recent decades. However, big changes have taken place in the hydrological regime of the estuary as the sea water level rise and Frequent storm surges in recent years. The proportions of the coastal levee lengths in Guangzhou and Zhuhai city occupied a large part of the portion in the Bay Area, about 30% and 23% respectively, while the shortest in Shenzhen city, accounting for only 5%. The designed tidal levels used for coastal levees in the past were compared with those proposed by PRWRC in the year 2011, showing lower about 0.2∼0.5m totally in the mouths and open waters including Shiziyang of the estuary. It would reach to about 0.6∼0.7m lower in the waters around Nansha district after considering the influence of the typhoon Mangkhut and Hato, while there was little variation in the net river area upstream in any case. The controlled tidal levels designed for coastal levees were analyzed for its applicability and it showed that those used in Guangzhou, Shenzhen and Jiangmen city meet the requirements and were somewhat lower in Dongguan, Zhuhai and Zhongshan city under variation of hydrological regime recently.

The horizontal levee: a multi-benefit nature-based treatment system that improves water quality and protects coastal levees from the effects of sea level rise

Municipal wastewater treatment plants in coastal areas are facing numerous challenges, including the need to provide a cost-effective approach for removing nutrients and trace organic contaminants from wastewater, as well as adapting to the effects of climate change. The horizontal levee is a multi-benefit response to these issues that consists of a sloped subsurface treatment wetland built between a coastal levee and tidal marshes. The wetland attenuates storm surges and can provide space for wetland transgression to higher elevations as sea levels rise, while simultaneously removing contaminants from treated wastewater effluent. To assess the ability of the horizontal levee to improve water quality and to identify optimal operating conditions, a 0.7-ha experimental system was studied over a two-year period. The removal of nitrate and trace organic contaminants was particularly sensitive to hydrology; rapid and near complete removal (>97%) of these contaminants was observed in water flowing through the subsurface, whereas surface flows did not exhibit measurable contaminant removal. Removal of F+ coliphage also appeared to be sensitive to hydrology, with up to 99% removal of these indicator viruses in subsurface flow. For phosphate, removal was not as sensitive to hydrology, but significant removal (>83%) was still observed when overland flow was eliminated. Although removal of contaminants did not appear to be sensitive to other design considerations, parameters such as soil texture and planting regimes affected the maximum subsurface flows, which in turn controlled contaminant mass loadings. Rapid subsurface removal of contaminants suggests that water quality benefits of these systems are limited by physical constraints (i.e., the ability of the system to maintain subsurface flow) and not chemical or biological conditions in the subsurface.

Collapse of concrete-covered levee under composite effect of overflow and seepage

During the Great East Japan Earthquake and Tsunami of 2011, enormous coastal levees collapsed as a result of tsunami overflows. Concrete-covered levees, where concrete blocks are used to protect levees made of soil, showed a specific failure mode in which back slope blocks and internal soils were dislodged. Although several hydraulic experiments have been conducted in an attempt to understand the failure mechanism, the complexity of the failure requires that both hydro-dynamical and geotechnical points of view be taken into account for a deeper understanding. This study uses a centrifuge to identify the failure behavior of levees from those points of view. The centrifuge can reproduce the stress, water pressure, and overflow velocity of the prototype-scale ground. Overflowing at sufficient time periods was produced in the centrifuge by generating differences in water height for seaside and landside areas. The test results showed that the levees collapsed due to a combination of the effect of water flow on the back slope and seepage inside the levees. In addition, the effectiveness of several proposed countermeasures for preventing collapse was confirmed.

Experimental Study on Relation between Breach Process and Velocity Distribution on Levee due to Overflow

Ko, D.; Kang, J., and Ryu, Y., 2019. Experimental study on relation between breach process and velocity distribution on levee due to overflow. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 16-20. Coconut Creek (Florida), ISSN 0749-0208.Coastal levee is an defensive structures important in low-lying coastal areas prone to floods, but is vulnerable to overflows when water levels increase extremely. Due to storm surge and high waves, water overflow and wave overtopping occur over coastal levee. Likewise, river levee is to protect lowland areas from floods. However, levee breaches due to aging of levees have caused huge damages on properties and human casualties. To advance as a society, it is essential to pursue meaningful implementations of solutions that prevent the erosion of levees during floods. To find a resilient structure of levee against overflow, this study has tested an environmentally friendly substance for reinforcing levee. With intent to testify the new method, a series of hydraulic experiments were carried out for to verify a flow state and surface velocity distribution according to levee breach progress by building a semi-prototype levee with a bottom width of 5 m, a slope of 1:2, a crown width of 1 m, and a height of 1 m in a 3 m bottom-wide natural channel. By building a measuring system using a number of imaging devices on site, the surface velocity distributions on the earthen levee and the levee reinforced with the new environmentally friendly substance were measured using a LSPIV technique. It is expected to be able to examine the characteristics of flow velocity distribution following breach processes on levees to evaluate the initial points of failure and the weak points during overflow.

Using H, O, Rn isotopes and hydrometric parameters to assess the surface water-groundwater interaction in coastal wetlands associated to the marginal forest of the Río de la Plata

The interaction between groundwater and surface water flows determines the ecohydrological characteristics of coastal environments which are periodically flooded by tidal flows. The Punta Lara Natural Reserve constitutes the southernmost vestige of the marginal forest of the Río de la Plata, which develops on the margins of Las Cañas stream. The aim of this work is to assess the interaction between groundwater and surface water in the reserve. For this purpose, isotopic data and hydrometric parameters for surface water and groundwater were obtained. δ18O, δ2H and 222Rn activity were measured from three sampling campaigns performed in winter of 2017 and 2018 and early summer of 2017. A monitoring network composed of shallow groundwater boreholes and surface water measuring points was designed to record groundwater and tidal levels, together with data of daily rainfall. The results obtained show that during high tides or extraordinary high tides, water spreads through the Las Cañas stream and floods the coastal levee infiltrating in the sandy sediments. Thus, the water of the Río de la Plata constitutes a source of periodic recharge to the groundwater within the levee, being of greater magnitude than the recharge registered during the events of intense rains. Conversely, groundwater discharge processes dominate during low tide. These results may contribute to the understanding of surface water - groundwater interaction processes as conditioning factors of the environmental characteristics.

Dose Estimation in Recycling of Decontamination Soil Resulting From The Fukushima NPS Accident For Road Embankments.

Since the Fukushima nuclear power station accident, large quantities of radiocesium-contaminated soil generated from decontamination activities have been stored in Fukushima prefecture. To complete the final disposal of decontamination soil, reducing the disposal volume through recycling can prove effective. The Ministry of the Environment of Japan has developed a policy of handling low-activity decontamination soil as recycled materials under the management of public authority. The recycling is limited to civil engineering structures in public projects, such as road embankments and coastal levees. However, there has been no practical review or safety assessment of decontamination soil recycling. In this study, to contribute to guideline development for decontamination soil recycling by the Ministry of the Environment, dose estimation was considered as a way of ensuring that the use of recycled decontamination soil for road embankments was safe. First, based on Japanese construction standards, additional doses to workers and the public in construction and service (e.g., use of a road embankment) scenarios were evaluated. From the result, the radioactive cesium concentration level of recycled materials that would result in all additional doses meeting the radiation criterion of 1 mSv y was derived to be 6,000 Bq kg. Then, construction conditions were reviewed to reduce additional doses to the public in a service scenario. To confine doses to the public to below 10 μSv y based on the derived radioactivity level, an additional layer of soil slope protection of 40 cm or more was needed. Finally, additional doses expected in a disaster scenario were confirmed to be below 1 mSv y based on the derived radioactivity level, an additional layer of soil slope protection of 40 cm or more was needed. Finally, additional doses expected in a disaster scenario were confirmed to be below 1 mSv y based on the derived radioactivity level.

Coastal vulnerability analysis during tsunami-induced levee overflow and breaching by a high-resolution flood model

The coastal levee is a crucial defense element in low-lying coastal areas, but is vulnerable to overflow when water levels increase extremely. This study focuses on the failure of the concrete-armored levees caused by local scouring at the toe of their landward slope that was widely observed in the 2011 Tohoku tsunami in Japan. Using a high-resolution flood model with a shock-capturing property, we explore the actual breach processes and their impacts on the protected areas in one of the most devastated districts. The model is based on nonlinear shallow water equations and successfully reproduces detailed flow structures of the tsunami overflow at 2m resolution. However, because we neglect the non-hydrostatic pressure over steep levee slopes, the modeled overflow discharge is slightly underestimated. During simulation, supercritical flow developed on the land side of levee sections with slowly rising tailwater water level due to hinterland topographic features, despite the large overflow discharge. Topographic data analysis revealed large-scale scour holes or trenches at the locations of the supercritical flow development, suggesting that levee vulnerability can be identified by the local flow parameters based on the high-resolution model. The levee breach was modeled as a sudden removal of the corresponding levee section, and the consequences were investigated. The levee breach changed the spatial balance of the flood intensity, but did not significantly worsen the damage in this case, because the narrow low-lying areas were fully inundated even without breach.

Quantitative analysis of dynamic inconsistencies in infrastructure planning: an example of coastal levee improvement

Cost–benefit analysis is considered as an effective means for the government to avoid failures in public projects. However, once cost–benefit analysis becomes mandatory and residents expect a public project to be established based upon it, there is a potential for a dynamic inconsistency problem to arise, where dynamic inconsistency is defined as a difference in the optimal policy before and after a certain time. Taking as an example the coastal levee improvement policy in the city of Rikuzentakata in Japan, the present study clarifies the mechanism behind the dynamic inconsistency problem that is attributable to mandatory cost–benefit analysis and also discusses quantitatively the influence of the dynamic inconsistency problem on social welfare. In addition, through examining the quantitative result, we indicate that, in the projects where the improvement cost increases gradually with the scale, the inefficiency of the dynamic inconsistency problem is incurred on a larger scale.

Analysis of the mechanism of seabed liquefaction induced by waves and related seabed protection

As one of the most serious offshore hazards, wave-induced seabed liquefaction can trigger massive landslides on the ocean floor and pose a great threat to submarine structures (e.g., coastal levees, oil platforms, drilling platforms and seabed pipelines). In view of the complexity and practicability of the problem, this study systematically analyzes the mechanism, factors and remedial measures of liquefaction. Compared with seismic liquefaction, waved-induced liquefaction varies in many respects, such as the load pattern, loading position, drainage condition and characteristics of pore water pressure, resulting in different mechanisms of seabed liquefaction under the action of waves. Both wave characteristics and soil characteristics, including the wave period, water depth, wave height, degree of saturation, seabed thickness, permeability and stress history, affect the degree of seabed liquefaction. Moreover, to ensure the sustainable development of the ocean, a series of remedial measures against liquefaction, including evaluation of the liquefaction potential, management of disaster prevention, ocean monitoring and forecasting, are proposed.

海岸堤防背後の落堀をモデルにした津波減勢施設とその効果

A facility for dissipating tsunami flood energy that modeled a pool dug by a tsunami flood has developed and evaluated its effect. Reducing tsunami flood energy is one of the important problem for a future huge tsunami counter- measure. The formation of pools at the toe of back slope of the coastal levees caused breaking a lot of coastal levees at the 2011 Great East Japan Earthquake tsunami. However, it is pointed out that the pools also played a roll of reducing flood disaster in backyard of coastal zone. The tsunami flood energy dissipator is settled in the foot of coastal levee. Two types of the tsunami flood energy dissipator that change its slope in the coast side were examined by the hydraulic model experiment. We evaluate the effect of reduction of tsunami flood velocity in three tsunami wave height. Results of the hydraulic experiment indicate that constructing the energy dissipator that has mild slope in the coast side can decrease almost 30% of the tsunami flood velocity.

THE NUMERICAL ANALYSIS OF WAVE OVERTOPPING AND WATER OVERFLOW OVER THE COASTAL LEVEE IN THE STORM CONDITION

Storm surges and high waves occur at the same time. Due to high tide and waves, water overflow and wave overtopping occur over the coastal levee. Numerical simulation based on the two-dimensional Navier-Stokes equations by using the VOF method, CADMAS-SURF, is applied. The overflow and overtopping calculated by numerical simulation is analyzed, the hydraulic characteristics of flow rate over the coastal levee are clarified.

System Analysis of Construction Quality Control Measures of Coastal Levee

The phenomenon of the actual situation leave the design index is very common during the civil project construction especially in the coastal levee construction. As the way to reduce the probability of diseases occurrence, from the aspect of quality accident analysis, construction quality monitoring moisture, quality inspection and accident early warning system, the paper studied the levee construction quality control monitoring system deeply and advanced some useful conclusion.

Breach process simulation of coastal levees broken by the 2011 Tsunami

ABSTRACT Iida, T., Kure, S., Udo, K., Mano, A., Tanaka, H., 2014. Breach process simulation of coastal levees broken by 2011 tsunami. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 302–307, ISSN 0749-0208. On March 11, 2011, a tsunami hit the Sendai Bay Coast and broke 80% of the coastal levees. The areas around the breached levees were scoured deeply, causing serious adverse effects on post-disaster reconstruction efforts. This motivated us to analyze the mechanism of the local scour in the areas around breached levees. We collected and investigated various data such as aerial photographs and tsunami videos. As the results of the investigation, we found the breach points progressing in two steps. Firstly, the brittle structure was destroyed by the first leading wave. Secondly, the concentration of the return flow eroded and breached levees with the broken points from the first step....

Seed bank composition in a secondary forest in the Lower Delta of the Paraná River (Argentina)

The native forests once occupying the coastal levees of the Lower Delta islands of the Parana River (in Argentina) have been replaced by commercial forests. Many of those forests have been abandoned, resulting in secondary forest formation that is subject to numerous invasive exotic species. A priori observations suggest that successional trends do not lead to recovery of the original forest. The aim of the present study was to analyze the role of invasive exotic species in these environments and the likelihood of recovery of the original forest cover. Therefore, we examined the composition of the seed bank and of the standing vegetation, as well as the population structure of tree species. Secondary forests are described as being in an intermediate successional stage, with few exotic species (mainly trees) but with a high abundance of standing vegetation and seed banks. These exotic species will likely continue to successfully predominate in different forest strata over the next stages. Because of the low density of native tree species, it is difficult to predict their future persistence. In conclusion, ecological restoration strategies will be needed in order to increase native tree species richness and biodiversity of the forests in the Lower Delta of the Parana River.