Morphology of small-scale submarine mass movement events across the northwest United Kingdom

<p>Given the potentially devastating consequences of shallow submarine landslides on infrastructure and human lives, it is imperative that we understand potential slope stability issues within marine coastal regions. In Scottish waters, our lack of knowledge regarding the nature of the seabed within the fjords and coastal inlets is concerning given that these sea lochs have similar morphological features and settings to global examples (e.g. Norway) where recent slope failures have had such highly devastating results. Global examples from similar physiographic settings also demonstrate the temporal aspect of these events, highlighting that they are caused by active modern processes and therefore represent contemporary geohazards. In addition, previous studies have highlighted that there tends to be a scale bias towards the mapping and reporting of large-scale events, and there is a requirement for studies that focus on small-scale (≤1 km<sup>3</sup>) mass movements which can still have damaging consequences on seafloor and coastal (both nearshore and onshore) infrastructure.</p><p>In this study, a review of multibeam echo sounder (MBES) survey datasets from five locations around the United Kingdom northwest coast has led to the identification of a total of 14 separate submarine mass movement scars and deposits within the fjords (sea lochs) and coastal inlets of mainland Scotland, and the channels between the islands of the Inner Hebrides. In these areas, Quaternary sediment deposition was dominated by glacial and glaciomarine processes. Analysis of the morphometric parameters of each submarine mass movement has revealed that they fall into four distinct groups of subaqueous landslides; Singular Slumps, Singular Translational, Multiple Single-Type, and Complex (translational & rotational) failures. The Singular Slump Group includes discrete, individual subaqueous slumps that exhibit no evidence of modification through the merging of several scars. The Singular Translational Group comprise a single slide that displays characteristics associated with a single translational (planar) failure with no merging of multiple events. The Multiple Single-Type Group incorporates scars and deposits that displayed morphometric features consistent with the amalgamation of several failure events of the same type (e.g. debris flows or slumps). Finally, the Complex (translational & rotational) Group comprises landslides that exhibited complex styles of failures, including both translational and rotational mechanisms controlling the same slide. The submarine mass movements that comprise this dataset are then discussed in relation to global fjordic and glaciomarine nearshore settings, and slope failure trigger mechanisms associated with these environments are described with tentative links to individual submarine landslides from the database, where appropriate. It is acknowledged that additional MBES data are needed not only to expand this database but also to create a more statistically robust study. However, this initial study provides the basis for a much wider investigation of submarine mass movements and correlations between their morphometric parameters.</p>

Modelling and prediction of water current using artificial neural networks: A case study of the commodore channel

Water current modelling and prediction techniques along coastal inlets have attracted growing concern in recent years. This is largely so because water current component continues to be a major contributor to movement of sediments, tracers and pollutants, and to a whole range of offshore applications in engineering, environmental observations, exploration and oceanography. However, most research works are lacking adequate methods for developing precise prediction models along the commodore channel in Lagos State. This research work presents water current prediction using Artificial Neural Networks (ANNs). The Back Propagation (BP) technique with feed forward architecture and optimized training algorithm known as Levenbergq-Marquardt was used to develop a Neural Network Water Current Prediction model-(NNWLM) in a MATLAB programming environment. It was passed through model sensitivity analysis and afterwards tested with data from the Commodore channel (Lagos Lagoon). The result revealed prediction accuracy ranging from 0.012 to 0.045 in terms of Mean Square Error (MSE) and 0.80 to 0.83 in terms of correlation coefficient (R-value). With this high performance, the Neural network developed in this work can be used as a veritable tool for water current prediction along the Commodore channel and in extension a wide variety of coastal engineering and development, covering sediment management program: dredging, sand bypassing, beach-contingency plans, and protection of beaches vulnerable to storm erosion and monitoring and prediction of long-term water current variations in coastal inlets. Keywords: Artificial Neural Network, Commodore Channel, Coastal Inlet, Water Current, Back Propagation.

Best management practices for coastal inlets

Coastal inlets separate individual barrier islands or barrier spits and adjacent headlands (Hayes and Fitzgerald 2013). Inlets modify longshore transport and store sediment in flood and ebb shoals leading to dynamic adjacent shorelines. For example, 80% to 85% of the beach erosion in Florida can be attributed to inlets (Dean 1991). In some cases, structured inlets are designed to trap sand in a preferred location to minimize interference with navigation and facilitate its removal through dredging. Sound coastal engineering practice requires that this sand be placed on adjacent eroding beaches (NRC 1995) to protect coastal resources. This paper provides a brief overview of coastal inlet management and identifies Best Management Practices (BMPs) intended to balance human needs for inlet navigation with the natural systems adjacent to tidal inlets. Today’s conservation measures, which are a result of considerable monitoring, numerical modeling, and other science-based methods, demonstrate that BMPs improve management of sand resources and reduce impacts associated with tidal inlet dredging. For some inlet conditions, BMPs include use of inlet sediment sinks as cost-effective and eco-friendly sand sources for beach nourishment projects located close to the inlet. For optimal coastal inlet management, the ASBPA Science and Technology Committee recommends the following BMPs and conservation measures: • Limit frequency and duration of impacts, • Follow environmental windows, • Implement regional sediment management, • Use beach-compatible sand, • Conduct pre-, during-, and post-dredging monitoring, • Modify dredging equipment/practices, and • Design rechargeable, low-impact inlet borrow sites.

TIME-VARYING WAVE EFFECTS ON FLOWS AND DYNAMICS AT AN UNSTRATIFIED INLET

Surface gravity waves alter discharge and circulation near and within coastal inlets, affecting the exchange and transport of water masses, nutrients, sediments, and pollutants between inland waters and the ocean. Field observations and numerical simulations suggest that, during storms, wave forcing (radiation-stress gradients) owing to wave dissipation across the ebb shoal can enhance fluxes into the inlet (Bertin et al. 2009; Wargula et al. 2014). As a result, water levels may increase inside the bay (Olabarrieta et al. 2011; Dodet et al. 2013), creating an offshore-directed pressure gradient that may balance onshore fluxes during energetic waves, and may enhance offshore fluxes after the waves decrease. Spatial and tidal variability in water depths on the ebb shoal lead to complex wave breaking patterns that drive spatially and tidally asymmetric flows. Here, field observations and numerical simulations are used to evaluate the effects of waves on discharge and circulation, and the relative importance of wave radiation-stress and pressure gradients at an unstratified inlet during and following energetic waves.

MODELLING WAVE INTERFERENCE PATTERNS USING THE SWAN MODEL

This study presents an efficient approach of resolving wave interference patterns in spectral wave models (e.g., SWAN). Such interference patterns, which frequently occur in coastal waters (e.g., near headlands, harbor entrances and coastal inlets), may lead to rapid changes in wave statistics, and thus, can affect wave-driven flow and transport processes. Therefore, prediction of wave conditions for coastal applications should account for these effects. Presently, operational wave models compute the mean wave properties by solving the action balance equation, which describes the transport of wave energy through geographic and spectral space, augmented with source terms to account for non-conservative and nonlinear processes. This model equation, initially intended for deep water conditions, is derived under the assumption that waves propagating at angles are mutually independent so that the wave field changes its mean properties (e.g. wave height) over many wavelengths. However in nearshore areas, the interaction of waves with variable bathymetry and currents can result in interference zones where crossing wave trains are statistically correlated and wave heights change rapidly.

THREE-DIMENSIONAL NUMERICAL MODELING OF SEDIMENT TRANSPORT NEAR COASTAL INLETS

Simulation of the hydro- and morphodynamic processes near coastal inlets is important but challenging, due to multiple interacting forces (waves, wind, tides, river flows, density currents, etc.) on a wide range of spatial and temporal scales. A number of 2-D models have been used in the past, but recently 3-D models have gained more and more attentions because better accuracy and reliability are desired. A 3-D model has been developed in this study to simulate the complex flow, sediment transport and bed change near coastal inlets.

Hydrodynamics and Sediment Transport of Benoa Bay, Semi-Enclosed Bay in Bali, Indonesia

Benoa Bay is a micro tidal semi-enclosed water with a size of 10 km x 15 km of the inner bay with a narrow gap formed by Serangan Island and Benoa Peninsula. Benoa Bay is a strategic area in south of Bali Province linked to the Indian Ocean through a narrow entrance. Recently, a large amount of siltation occurred in this area, which is caused by sedimentation from five rivers discharged into the bay. The sedimentation in the lagoon is of ecological and physical concern due to the mangrove ecosystem disturbing and operation of Benoa port for the necessity of increased dredging. This paper deals with tide-induced flows in a semi-enclosed water body in a micro-tidal environment that is subjected to tide and river discharge conditions. In this study, The Coastal Modeling System (CMS) developed by Coastal Inlets Research Program was used for simulating flow, waves, sediment transport, and morphology changes in coastal areas. This CMS has effectively calibrate the data set supplemented by field observations.