Harshinie Karunarathna

An experimental study of rip channel flow

A laboratory study of the flow over a bar with a single rip channel has been performed. First, the well-known pattern of a bar circulation cell with a strong offshore-directed current out through the rip channel and a weaker onshore-directed return flow over the bar is documented. Then measurements of the three-dimensional structure of the flow in the area where the rip channel, the bar and the trough meet and well inside the rip channel are presented. These measurements reveal that 3D effects play an important role, and that a depth-integrated viewpoint may not always be sufficient for predicting the flow in the near bed region. Particle-tracking experiments illustrate the near bed flow pattern over the entire area. These demonstrate how the overall trajectory pattern changes as a function of the distance of wave breaking from the bar crest: For some conditions, the rip current is fed from the trough and for other conditions it is fed directly from the bar. Both the 3D measurements and the trajectory tests show the existence of a weaker onshore-directed near-bed drift in the area where the rip current ceases. Finally, in a series of sensitivity tests, measurements of the rip current intensity for different wave climate and water level conditions reveal a strong correlation between the rip current intensity and the wave height (both normalized).

A Boolean Approach to Prediction of Long-Term Evolution of Estuary Morphology

Abstract A systems method based on a Boolean approach is used to predict medium- to long-term behaviour of estuary morphology. Boolean networks are formed for various types of generic estuaries by taking into account the feedback involved between various estuary elements and external forcing which drive morphological evolution. A logical framework is then developed, and the Boolean matrix is derived. Various different potential morphological evolutionary pathways are then selected from the Boolean matrix, analysed, and discussed. The Boolean method provides a complementary approach for qualitative modelling of complex estuary systems. The method was applied to the Ribble Estuary in northwest England, and the predictions from the Boolean approach were in very good qualitative agreement with the observed morphological evolution of the Ribble Estuary during the last 150 years.

Annual down-glacier drainage of lakes and water-filled crevasses at Helheim Glacier, southeast Greenland

Supraglacial lake drainage events are common on the Greenland Ice Sheet. Observations on the west coast typically show an up-glacier progression of drainage as the annual melt extent spreads inland. We use a suite of remote sensing and modelling techniques in order to study a series of lakes and water-filled crevasses within 20km of the terminus of Helheim Glacier, south east Greenland. Automatic classification of surface water areas shows a down-glacier progression of drainage, which occurs in the majority of years between 2007 and 2014. We demonstrate that a linear elastic fracture mechanics model can reliably predict the drainage of the uppermost supraglacial lake in the system, but cannot explain the pattern of filling and draining observed in areas of surface water downstream. We propose that the water levels in crevasses downstream of the supraglacial lake can be explained by a transient high-pressure wave passing through the subglacial system following the lake drainage. We support this hypothesis with analysis of the subglacial hydrological conditions, which can explain both the position and interannual variation in filling order of these crevasses. Similar behaviour has been observed in association with jokulhaups, surging glaciers, and Antarctic subglacial lakes, but has not previously been observed on major outlets of the Greenland Ice Sheet. Our results suggest that the behaviour of near-terminus surface water may differ considerably from that of inland supraglacial lakes, with the potential for basal water pressures to influence the presence of surface water in crevasses close to the terminus of tidewater glaciers.

Analysis of Climate Change Effects on Seawall Reliability

Crown heights of seawalls should be designed to suppress overtopping discharge to a permissible level. The permissible level is determined from viewpoints of the structure types of coastal seawalls and hinterland use. It is usually difficult to design the crown heights of seawalls, especially in the present time where climate change due to global warming is expected. This study analyzes climate change effects such as sea level rise (SLR) and increase of waves and surges on the failure probability of seawalls under various conditions of crown height, toe depth and slope by using a Level III reliability analysis. It was found that the difference of SLR trends (fast, medium or low) has less impact on overtopping rates than the differences in wave height change for a seawall at a target location.

Modelling the Effects of Marine Energy Extraction on Non-Cohesive Sediment Transport and Morphological Change in the Pentland Firth and Orkney Waters

This paper considers the process of modelling sediment transport and morphological change in the Pentland Firth and Orkney Waters using coastal area models. This region is atypical of regions commonly modelled using such techniques: it is high energy with limited and highly variable regions of mobile sediment. This causes challenges with regards both model capability and availability of data. Computational time restrictions for fully coupled modelling solutions should also be recognised which limits practical duration of simulation. Impacts to modelled bed level change over test periods are noted for both wave and tidal energy extraction scenarios. In both cases the magnitude of difference is equal to the magnitude of the change itself, however, lack of data and poor validation of the morphological modelling for the wave energy test case means confidence is low in these results. Based on the difficulties faced in conducting these modelling exercises and the high cost of additional data collection, it is recommended that regulators take a pragmatic approach when requiring such modelling for environmental impact assessments at some locations where changes to morphodynamics are unlikely to be critical to key receptors. Other approaches such as conceptual modelling or consideration of bed shear stresses could be undertaken rather than fully coupled morphodynamic modelling.

Improving decadal coastal geomorphic predictions: an overview of the iCOASST project

Coastal areas are already at high risk from a range of geohazards. The cumulative effect of human intervention on soft coastlines has frequently left them far from equilibrium under today’s conditions, especially in densely populated areas. Future changes in marine forcing due to climate change reinforce the need to understand and predict processes of change in shoreline position and configuration at management (decadal) scales. The UK-based iCOASST project is developing new and improved methods to predict decadal geomorphic evolution, linked to coastal erosion and flood risk management. This is based on a framework that links several components to develop a system-level understanding of this change. The framework includes: (1) new methods for system-level analysis and mapping of coast, estuary and inner shelf landform behaviour; (2) well validated ‘bottom-up’ hydrodynamic and sediment transport shelf models which can be applied at shelf scales to investigate inner shelf-coastal interactions; and (3) model compositions formed of existing or new ‘reduced complexity models’ of selected coastal landforms and processes that are suitable for multiple decadal length simulations. This will ultimately allow multiple simulations of coastal evolution which can explore uncertainties in future decadal-scale coastal response, including the effects of climate change and management choices. This paper outlines the current state of progress in the iCOASST Project.

Seasonal to Decadal Variability of Shoreline Position on a Multiple Sandbar Beach

ABSTRACT Umeda, S.; Yuhi, M., and Karunarathana, H., 2018. Seasonal to Decadal Variability of Shoreline Position on a Multiple Sandbar Beach. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 261–265. Coconut Creek (Florida), ISSN 0749-0208. Seasonal to decadal variability of shoreline position at a long multi-barred beach with gentle slopes was investigated using a dataset of biannual shoreline surveys of 23 years. An empirical orthogonal function (EOF) and spectral analysis on 15.2 km long stretch were conducted to describe characteristic patterns of shoreline variation at multiple scales in time and space. A relatively alongshore uniform shoreline migration toward offshore in summer and onshore in winter was dominant in the first EOF mode which secondarily exhibited a long-term trend of shoreline recession. Mid-term shoreline variations related to shoreline sand wave and longshore sandbars were represented by the combinations of the second to sixth EOF modes filtered through two bands of dominant frequency. The shoreline pattern reconstructed on a longer time scale represented a large scale shoreline undulation whose scale and migration speed were comparable to shoreline sand waves observed in various coasts. On the otherhand, the shoreline pattern reconstructed on a shorter time scale represented alongshore uniform shoreline migration toward onshore and offshore in about five-year cycles, which corresponded to the average cycle of net offshore sandbar migration observed at the site. The periodic shoreline recession-advance related to sandbar migration was estimated to be comparable to seasonal scale shoreline variation. A correlation analysis between seasonal scale shoreline variation and wave statistics exhibited that shoreline advance in summer increased with low wave heights. It was suggested that a moderate waves were responsible for seasonal scale shoreline advance in this coast.

Investigation of wind and tidal forcing on stratified flows in Greenland fjords with TELEMAC-3D

Abstract Many researchers have analysed the effect of wind on fjord dynamics by using two-dimensional numerical models. This paper investigates the wind and tidal forcing effects on the strong stratification and circulation by application of the three-dimensional model TELEMAC-3D. The capability of the model to reproduce the physical processes has been investigated by applying it to Sermilik Fjord, Greenland. This study shows that the major changes in the fjord dynamics are wind induced. Any changes in the wind speed have an instant impact on the water surface velocities. Also, the diffusively driven circulation produced by the horizontal pressure gradients at coast plays an important role in fjord dynamics. After calibration, the tidal model produced matching results with the measured wave height. The tidal analysis shows that the strength and direction of the tidal currents are sensitive to any changes in the width and shape of the domain. The effect of the tidal propagation on the salinity profile has been shown to play an important role, with salinity picks being 5–6 h behind the tidal flood and ebb picks, along the wind forcing, on the fjord circulation. TELEMAC-3D is a suitable model to simulate accurately such complicated dynamics as in presented case study.

Analysis of multi-scale morphodynamic behavior of a high energy beach facing the Sea of Japan

Monthly cross shore beach profiles measured at the Ogata Wave Observation pier located in Joetsu-Ogata Coast, Niigata Prefecture, Japan, was analysed to investigate multi-scale morphodynamic beach behaviour. The Ogata beach, facing the Sea of Japan, is subjected to high energy wave conditions with that has a strong winter/summer seasonal signature. The measured beach profiles at the beach show very significant variability where cross-shore movement of shoreline position and lowering of the beach at the location of measurements exceed 20 m and 4 m respectively. The shoreline position seems to follow the seasonal variability of incident wave climate where a correlation coefficient of 0.77 was found between monthly averaged incident significant wave height and the measured monthly shoreline position. During the summer months, the beach variability mostly concentrated to in the sub-tidal part of the profile, while a significant amount of upper beach change was observed during the winter months. The beach profile shape was found to rotate between three different beach states in time; (i) concave reflective profile; (ii) profile with sub-tidal berm; and (iii) gentle, dissipative profile. Empirical Orthogonal Function (EOF) analysis of the profiles show that the variability of beach profile shape is dominated by (a) upper shoreface steepening; (b) sub tidal berm development and dissipation; and (c) variability of the overall profile slope, which have some longer than seasonal cyclic signatures. Comparison of temporal EOFs with climate indices such as Southern Oscillation Index and Pacific Decadal Oscillation index shows notable some correlations between profile change and climatic variability in the region. The analysis also shows that the morphological variability of Joetsu-Ogata Coast has similarities and some distinct spatial and temporal differences to beaches of similar kind found elsewhere.

Sediment dynamics of Negombo Lagoon Ooutlet, Sri Lanka.

This paper presents a 2DH modelling approach used to investigate sediment dynamics and potential morphological changes of the Negombo lagoon outlet, Sri Lanka. Schematised wave, wind and river-inflow parameters were imposed representing the present day forcing scenario. The model was initially calibrated against the measured water level and velocity data, and then implemented to observe sediment dynamics and morphodynamics. Northward alongshore transport in this area changes to the west-east orientation at the outlet and is interrupted by the lagoon sediment export. Channels at the lagoon end have different transport capacities. Residual transport showed a sediment exporting system in the lagoon. One-year period of morphological evolution suggested that the narrow connecting channel between the lagoon and the sea experiences erosion and sedimentation in front of the lagoon entrance. Also, erosion at the apex of the western headland provides sediment to south and north therein.