Effect of Tidal Currents at Amphidromes on the Characteristics of N-Wave Type Tsunami

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

A Study on Regional Sea Level Variation Along the Indian Coast

Mean sea level variation is a global phenomenon with spatial variations in trends on regional levels. Among the key findings of IPCC, global mean sea level has increased and it will continue to increase in the coming century. The amount of sea level rise or fall and its effects at any given location is highly unpredictable. Increase in intense spells of precipitation, uneven spread of rainfall in space and time, damage due to storm surges and coastal flooding are increasing in frequency with increase in global warming trends and the changing sea level. Increasing trend in mean sea level may cause inundation of low lying areas especially affecting small islands. In case of storm surges the scenario may be even worse. Bay of Bengal being an active breeding area of tropical storms, it might become devastating to cope up with the situation if the increasing trend in sea level continues. Mangroves, the natural barrier of coastal flooding and erosion, will be at high risk of submergence due to sea level rise. There is a need to understand the statistics of sea level changes and adapt to the situation in all possible ways. This study focuses on changes in the mean sea level on a regional scale. The available tide gauge data (along the coasts of North Indian Ocean (NIO)) and satellite altimetry data are analysed to understand the changing trend of the sea level in the NIO. Though most of the tide gauges show a positive trend there exists a few which also shows a negative trend in the sea level. Also there lies a gap in the global estimates of sea level variation and the local average. For instance, based on the available long tide gauge data, it is found that the sea level at few stationsshowed a rise of nearly 1 to 8 mm/year whereas few stations experienced decrease in sea level by as much as 3 mm/year. It is therefore important to understand the increasing/decreasing trend of various tide gauge records individually before considering them in regional sea level variation studies. In this study only the tide gauges with longer periods of data with minimum gaps in data set are considered for analysis. Satellite altimetry data available since 1993 is used to investigate the changes in the sea level at locations where tide gauge data are not available

Wave climate projections along the Indian coast

Future changes in wave climate will influence the marine ecosystem, coastal erosion, design of coastal defences, operation of near‐ and off‐shore structures, and coastal zone management policies and may further add to the potential vulnerabilities of coastal regions to projected sea level rise. Many studies have reported changes in the global wave characteristics under climate change scenarios, but it is important to project future changes in local/regional wave climate for smooth implementation of policies and preventing severe coastal erosion and flooding. In this study the regional wave climate along the Indian coast for two time slices, 2011–2040 and 2041–2070, is reported using an ensemble of near‐surface winds generated by four different CMIP5 general circulation models (GCMs), under RCP4.5 scenario. Comparison of the wave climate for the two time slices shows an increase in wave heights and periods along much of the Indian coast, with the maximum wave heights increasing by more than 30% in some locations. An important finding is that at most locations along the east coast, wave periods are expected to increase by almost 20%, whereas along the west coast an increase of around 10% is expected. This will alter the distribution of wave energy at the shoreline through changes in wave refraction and diffraction, with potential implications for the performance and design of coastal structures and swash‐aligned beaches. Furthermore, the computations show material changes in the directional distribution of waves. This is particularly important in determining the longshore transport of sediments and can lead to realignment of drift‐aligned beaches, manifesting itself as erosion and/or siltation problems. This study is a preliminary contribution towards regional climate projections for the Indian Ocean region which are needed to plan and mitigate the impacts of future climate change

Numerical Investigation of Breaking Focused Waves and Forces on Coastal Deck Structure with Girders

In the present study, breaking focused wave groups were simulated using open-source Computational Fluid Dynamics model REEF3D in order to investigate the breaking wave impact on scaled (1:10) two-dimensional coastal deck structure with girder. The effect of environmental parameters, such as bottom slope and wave steepness on the breaking and geometric properties of high-crested spilling breakers, was investigated. The effect of the wave breaking location on the impact forces acting on the deck structure located at different airgap positions was studied for three wave impact scenarios: (i) when the wave breaking starts, (ii) when a slightly overturning crest is formed, and (iii) when the wave breaks and a fully overturning crest is formed just before hitting the preceding trough. The peak horizontal impact force was found to be higher when the wave breaks ahead of the structure and the overturning wave crest hits the deck positioned above the still water level. Additionally, the peak vertical impact force attains the peak when the deck is placed at the still water level for different stages of breaking. The peak horizontal impact force shows a parabolic trend, whereas the peak vertical impact forces show a decreasing linear trend with an increase in airgap. Finally, force coefficients are derived for calculating the peak impact force on deck with girders subjected to high-crested spilling breakers

Numerical Investigation of Breaking Focused Waves and Forces on Coastal Deck Structure with Girders

In the present study, breaking focused wave groups were simulated using open-source Computational Fluid Dynamics model REEF3D in order to investigate the breaking wave impact on scaled (1:10) two-dimensional coastal deck structure with girder. The effect of environmental parameters, such as bottom slope and wave steepness on the breaking and geometric properties of high-crested spilling breakers, was investigated. The effect of the wave breaking location on the impact forces acting on the deck structure located at different airgap positions was studied for three wave impact scenarios: (i) when the wave breaking starts, (ii) when a slightly overturning crest is formed, and (iii) when the wave breaks and a fully overturning crest is formed just before hitting the preceding trough. The peak horizontal impact force was found to be higher when the wave breaks ahead of the structure and the overturning wave crest hits the deck positioned above the still water level. Additionally, the peak vertical impact force attains the peak when the deck is placed at the still water level for different stages of breaking. The peak horizontal impact force shows a parabolic trend, whereas the peak vertical impact forces show a decreasing linear trend with an increase in airgap. Finally, force coefficients are derived for calculating the peak impact force on deck with girders subjected to high-crested spilling breakers

Effect of long-term wave climate variability on longshore sediment transport along regional coastlines

Long-term changes in wave climate have potential impacts on the evolution of regional coastlines. This study investigates the impact of variable wave climate on the temporal dynamics of longshore sediment transport (LST), which plays a major role in defining the overall coastal geomorphology of regional coastlines. The central west coast of India is considered as the study region. ERA-Interim wave hindcast dataset over the period of 1979–2015 is used to derive the contemporary wave climate in this region. The annual average significant wave height, period, and direction are computed and used to estimate LST of the study region. This region experiences oblique waves from the W-SW direction with an annual average significant wave height and wave period of 1.32 m and 8.10 s, respectively, that induces a gross northerly transport of approximately 450,000 m3/year. It is found that the total LST is driven by swell waves and wind waves and shows a decreasing trend of about 5% over the analysis period. The decay in LST is found to be linked with decreases in wave activity in this region. The swell wave induced sediment transport is an order of magnitude higher than the wind wave induced LST. It is observed that the swell generation in the lower latitudes has decreased, resulting in reduced swell wave induced LST in the study area. Both swell and wind wave induced LST show seasonal variation. Finally, a link is established between the seasonal variation of swell induced LST and the cyclogenesis periods. In addition, the wind wave induced LST is observed to have a direct link with the latitudinal position of the inter-tropical convergence zone (ITCZ)

Evaluation of CMIP5 and CORDEX derived wave climate in Indian Ocean

The effects of climate change are currently a widely investigated issue. However, little attention has been paid to the effects of climate change on regional wave climate. With the growing need towards developing future projections of waves to assess climate-driven impacts on coastal processes, it is required to evaluate the performance of General Circulation Models (GCMs) and Regional Climate Models (RCMs) in simulating wave climate independent of their ability to simulate other standard variables. Near-surface winds, derived from GCMs participating in the Coupled Model Intercomparison Project (CMIP5) and RCMs from Coordinated Regional Climate Downscaling Experiment (CORDEX), are used to force a spectral wave model to simulate hindcast waves over Indian Ocean (IO) region. The GCM and RCM forced climatological wave characteristics (significant wave height, mean wave period, and mean wave direction) are compared with reanalysis data derived from ERA-Interim for performance evaluation. RCMs work at fine resolution and are assumed to simulate regional climate better than GCMs. However, we identified that there is no added value in simulating wave climate using RCMs. We also identified that there is no improvement in wave simulation upon choosing a fine resolution GCM (~ 1.4°) over a coarse GCM (~ 2.5°). Seasonality in wave characteristics is an important aspect in the IO. The skill of climate models in capturing seasonality was also evaluated. It is observed that ensemble GCM forced wave simulations capture seasonality better than other models. Finally, it is recommended to use ensemble GCM wind forcing for better wave simulation in the IO region

Future wave-climate driven longshore sediment transport along the Indian coast

Longshore sediment transport is an important nearshore process that governs coastal erosion/accretion and in turn defines the orientation of coastlines. In this study, we assess the changes in longshore transport rates along the Indian coast due to the potential changes in wave parameters under the RCP4.5 climate scenario. The projected wave climate for two time slices, ‘near-term/present’ (2011–2040) and ‘mid-term/future’ (2041–2070) were used to investigate changes in the corresponding sediment transport rates. An empirical model accounting for major wave parameters, longshore current, resulting sediment transport and shoreline evolution was used. It was found that most of the Indian coast exhibited the same drift direction in both time slices, although changes in transport magnitude were present. To give a broad-brush characterisation of the coastline, the shoreline elements were classified as erosive, accretive or stable based on the comparative longshore transport rates of neighbouring elements. Similar characterisations, carried out for both time slices, showed that about 35% of the total coastline would remain unaffected due to the changing wave climate in the future (i.e. there is little to no change); about 20% is expected to ‘worsen’ (i.e. expected to undergo higher magnitudes of erosion wrt present rate) and 45% to ‘improve’ (i.e. expected to accrete/reach stability). It was also observed that the net annual transport rates pertaining to the future period are not expected to change significantly with respect to the current scenario. This indicates that the change in longshore transport rates arising from future changes in wave climate as represented by the RCP4.5 climate change scenario will have a broadly neutral effect

Effect of Girder Spacing and Depth on the Solitary Wave Impact on Coastal Bridge Deck for Different Airgaps

Coastal bridge damage has become a severe issue of concern in the recent past with the destruction of a considerable number of bridges under the impact of waves during tsunami and storm surges. These events have become more frequent, with waves reaching the bridge deck and causing upliftment and destruction. Past studies have demonstrated the establishment of various theoretical equations which works well for the submerged deck and regular wave types but show much scatter and uncertainty in case of a deck that is above still water level (SWL). The present study aims to generate a solitary wave to represent an extreme wave condition like a tsunami in the numerical wave tank modeled using the open source computational fluid dynamics (CFD) model REEF3D and to study the vertical impact force on the coastal bridge deck. A parametric study is carried out for increasing wave heights, girders spacing and depth for varying airgaps to analyze the effect of these parameters on the peak vertical impact force. It is observed that increasing the girder spacing and girder depth is effective in reducing the peak vertical impact force for the cases considered