Coupling Monitoring and Mathematical Modelling of Beaches to Analyse a Problem of Harbour Sedimentation: Case Study (original) (raw)
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Darwin Harbour, Northern Territory, Australia, is a semi diurnal macro-tidal embayment that is situated in a cyclone prone area. The tidal variations range up to 8 m with a mean tidal range of 3.7 m. The coastal area consists of mangrove fringes, sandy beaches, tidal flats, rocky shore platforms and coastal cliffs. The main morphological changes are movement of the sandbars and erosion of beaches and coastal cliffs. Sea level rise due to climate change and more intense cyclones and storm surges may exacerbate these processes with detrimental impacts on the coast and the adjacent city, particularly when occurring at high tide. To assist with coastal erosion management, a greater understanding of morphological changes is required. A two-dimensional depth averaged finite-element hydrodynamic model (RMA-2), coupled with a sediment transport model (RMA-11) from Resource Modelling Associates, have been used to deduce the sources and spatial patterns of sand erosion and deposition in the harbour. Geochemical analysis is also used to characterize the sand source(s). This paper presents hydrodynamic simulations focusing on culturally and recreationally significant beaches in Fannie Bay. Simulations indicate that the Cullen Bay sandbar is an indirect sand source replenishing Fannie Bay beaches. Respective geochemical results also show similar Rare Earth Element contents of the sand in the area. Considering the fast pace of development in and around Darwin Harbour, this study is essential in providing a fundamental understanding of coastal processes and to assist coastal and shoreline management in a tropical estuary.
Modelling of Bypass of Sediment at Harbours
2008
The conditions for bypass of sediment and the sedimentation is studied by composite modelling where physical and numerical modelling have been used to support and complement each other. A harbour placed on an exposed straight coastline with a slope of 1:25 is considered. The mathematical modelling is made by the area model system MIKE 21. The physical modelling is made in a 25 m by 35 m wave basin with directional wave generators along one side of the basin. The sediment transport is illustrated by fine sand acting as tracer over a hard concrete bed; this illustrates the bypass of sediment and the intrusion into the harbour. Further, experiments are made with a moveable bed over part of the model domain. The physical model provides details on the flow and the wave field, on the areas of sedimentation and the mechanisms for bypass of sediment. The numerical models run in parallel to the physical modelling simulating the detailed flow and wave patterns and are used to provide the basi...
Water and sediment movements in Harbours
Abstract Water movements in five harbour-models were assessed using (i) a Sontek-ADV to measure the velocity distributions and (ii) fluorometers to determine the local solute-concentrations under both tidal and steady flows. The width of the harbour entrance and the water-depth's amplitude influence significantly the flushing process for the narrow-entrance harbours. To achieve a high rate of flushing, as required in harbours in order to reduce the pollution concentration therein and the need for dredging, it is recommended that as wide as is feasible entry into the harbour is used: if a separate entry and exit are to be introduced, they should be well separated. Less expenditures of commercially-purchased energy will then be required for achieving these aims.
Short-term observation of beach dynamics using cross-shore profiles and foreshore sediment
Ocean & Coastal Management, 2012
Cross-shore beach profiles and textural characteristics of foreshore sediment were analyzed for understanding an annual cycle of intertidal beach dynamics at Devbag, an Island sheltered estuarine coast. This study is carried out as response to the shoreline management project proposed by Ministry of Earth Science, India. Cross-shore transects were monitored in a monthly interval during February 2008 to February 2010 along with collection of beach surface sediment samples. Change in beach volume over an annual cycle showed a slow rate of sediment accretion at the Devbag beach where the maximum accretion was found at dissipative zone. During the year 2008e09, the volume of sediment accretion was 24.23 m 3 /m with an average beach width and slope of 91 m and 3.3 respectively. During 2009e2010, the accreted beach volume was 33.54 m 3 /m with an average beach width and slope of 132 m and 2.6 respectively. The beach along the coast was varying from extremely gentle to a slightly steeper slope which showed dissipative to reflective nature. The Beach was dominated by both fine and medium sands. More quantity of unimodal and bimodal sediment was found with moderately well sorted and moderately sorted nature. No significant correlation in beach morpho-sedimentary characteristics was found due to the non-uniform trend in beach morphodynamics and textural characteristics. Present study emphasizes the need to observe the beach changes over inter-annual seasons before planning coastal protection measures. The results of the study will be helpful for formulating an effective coastal zone management plan at Devbag, Kali estuary, west coast of India.
Geomorphology, 2002
Redesign of Port Gisborne for the 21st century has encompassed a broad interdisciplinary approach. This procedure has taken into account the operational requirements of the port, effects of dredging and construction upon the benthic fauna, and wave activity within the port confines after the proposed development. Added amenity value of the development to the local community is an important ancillary redesign consideration. Initially, a major research project into the environmental impacts of the developments has been undertaken.The project, which commenced in 1996 and is still continuing, involves an iterative approach integrating the initial design and development options with the operational feasibility, construction constraints, environmental constraints, social acceptability, and economic practicality of the port; all of these require in-depth assessment to obtain the necessary planning and development approvals. This requires close liaison between the professional environmental research scientists, port management, port operation staff (pilots), construction engineers, planners, and the community interest groups.Numerical modelling of the hydrodynamics of Poverty Bay, simulating waves and current effects on the various initial designs options, and calibrated against data from a substantial field program, has been a fundamental tool. It was applied experimentally to determine the best option for the port layout, as well as to assess sedimentation impacts. Modelling results indicated a significant increase in maintenance dredging expected as a result of deepening the navigation approach channel. Because this may have an impact on the nearby sandy beach by inducing erosion, the best option for disposal of the sandy dredged material was determined to be disposal in the surf zone for subtidal beach profile renourishment. Textural analysis of the sediments trapped in the navigation channel demonstrated that they were suitable for this purpose. D . Lagrangian modelling techniques simulating wave and sediment dynamics determining sand-body equilibria. In: Harff,
Morphological modeling of a nourished bayside beach with a low tide terrace
Coastal Engineering, 2013
We present a simple one-dimensional process-based morphological model for predicting medium-long term beach profile evolution of a bayside beach with a low tide terrace. Bayside beaches are characterized by low wave energy. In regions with a large tidal range, a beach face is slowly eroded due to erosional waves inside the surf zone which shifts along the beach profile over a wide range during a tidal cycle. The model is developed using an existing model framework proposed by Mariotti and Fagherazzi , who formulated a model for sediment suspension and transport due to waves and tidal currents in a tidal flat regime. The new developments in this study include an implementation of the bedload sediment transport and modification of the suspended load transport for noncohesive particles. The model parameters were calibrated using beach profile data collected over a one-year span at Xiamen Beach. Numerical experiments indicate that waves suspend sediment in the narrow surf zone, inducing pulse-like high sediment concentration in shallow water. The suspended sediment is advected by the tidal current and wave-induced undertow, causing erosion of the steep beach face at the upper portion and deposition of the flat beach face at the lower portion of a beach profile. The erosion on the upper beach face was not predicted by a bedload-only transport model, suggesting that the sediment suspension associated wave breaking may be the major mechanism for the erosion. Tide-only induced sediment transport is insignificant, but tides play a role in distributing the wave-induced erosion over a wide region on a beach profile.
Modelling of Marine Structures' Contribution in Coastal Sediment Transport – Case of Ouranopoli Bay
2006
The construction of marine structures in coastal environments introduces a variety of changes in the morphology of the sea bottom and the coast itself. Alteration of budget by the interposition of structures in the surf zone produces plethora of potential problems that require proper investigation during the designing phase. The evolution of the morphology of seabed and coastline can be predicted by the application of appropriate mathematical models simulating the combined wave-current activity, its affect on the bottom shear stress conditions and the associated mobilization of sediments in the swash zone. The simulation regards determination of the existing sediment budget conditions in the annual cycle compared to the predicted state after construction.
Journal of Coastal Research, 2020
The main objective of this study is to investigate the effect of sediment flushing on the morphology of artificially embayed beaches using field data and numerical modelling. The sandy stretch of Cova-Gala in the West Coast of Portugal, characterized by a high-energy wave climate, is critical regarding erosion-flooding risks. The site was monitored from August 2018 to February 2019, including the wave climate, the sea level and the beach topo-bathymetric elevation via sonar-single beam, GNSS-RTK and drone surveys. During this period several erosive events caused the retreat and lowering of the upper beach profile. The XBeach model was applied to simulate the morphological evolution during the monitored period using synoptic wave and sea level data as the hydrodynamic forcing. The results of the data-model analysis reveal that i) the alongshore extension of the sedimentological cells and the cross-shore structures length have a relevant influence on the lee-side erosion patterns, which affect the overall surrounding morphology, ii) the model overestimates the scouring of the seawall toe, particularly with the 1D mode; and iii) the beach backshore typology, dune and seawall, has a great influence on beach dynamic processes.
Beach Morphodynamics and Profile Sequence for a Headland Bay Coast
2001
This paper presents a sequence of beach profile for a headlands and bay coast. Shape analyses of the embayed beach, identification of the predominant wave direction, beach and nearshore profiles, sedimentology characteristics, hydrodynamic conditions at the beaches, morphodynamics and morphometric data for 17 beaches on central-north coast of Santa Catarina, Brazil, were obtained. Beaches are classified in three main groups: (1) exposed; (2) semi-exposed; and (3) sheltered. The exposed beaches had an indentation ratio smaller than 0.39 and the dominant south waves are approximately parallel to the coast (angle smaller than 40°). The beaches can be divided into three mainly groups. (a) Reflective beaches have coarse sand (0.59mm-0.94mm) and steeper nearshore slope (1:40) associated with a very narrow coastal plain <1Km). (b) Intermediate beaches with one nearshore bar have medium sand (0.30mm-0.45mm) and gentle nearshore slope (1:100-1:200) and a developed coastal plain-island bar...