Numerical modelling of sand beach evolution around coastal structures

A systematic study of waves, nearshore currents and sediment transport related to the coastal evolution of sand beach under the action of wave, has been carried out. On the basis of the re sults, a numerical model of sand beach evolution around coastal structures has been established. To pre dict the bed topography, the sediment transport rates of bedload and suspended load under the action of wave and nearshore current as well as the effects of wave on the sediment are considered. The sand beach evolutions caused by a breakwater and a settled ship near the shore are simulated.

Investigation and Discussion on the Beach Morphodynamic Response Under Storm Events Based on A Three-Dimensional Numerical Model

A well-established 3D phase-averaged beach morphodynamic model was applied to investigate the morphodynamics of a typical artificial beach,and a series of discussions were made on the surfzone hydro-sedimentological processes under calm and storm events.Model results revealed that the nearshore wave-induced current presents a significant 3D structure under stormy waves,where the undertow and longshore currents exist simultaneously,forming a spirallike circulation system in the surfzone.Continuous longshore sediment transport would shorten the sediment supply in the cross-shore direction,subsequently suppress the formation of sandbars,showing that a typical recovery profile under calm waves does not necessarily develop,but with a competing process of onshore drift,undertow and longshore currents.Sediment transport rate during storms reaches several hundreds of times as those under calm waves,and two storm events contribute approximately 60%to the beach erosion.Sediment transport pattern under calm waves is mainly bed load,but as the fine sands underneath begin to expose,the contribution of suspended load becomes significant.

Spatiotemporal Evolution of the Dongting Lake Beach in Recent 90 Years

Using historical topographic maps and aerospace remote sensing data since the 1930s,this study investigates the spatial and temporal evolution of Dongting Lake beach.The evolution characteristics of the beaches in different regions and the related formation mechanism were also analyzed.The results show that Dongting Lake beach expanded from 1622.17 km^(2)in 1938 to 1962.28 km^(2)in 2018.With the addition of 980.96 km^(2)of reclaimed high bay beach,the beach area increased by 1321.07km^(2).However,the change process fluctuated somewhat rather than continuously increased.Substantial expansion of the beach area occurred during 1938-1948 and 1958-1998,while slow contraction of the beach area occurred during 1948-1958 and 1998-2018.Dongting Lake beach was dominated by terrigenous debris,the sedimentary types included lacustrine deposits,river alluvial deposits,floodplain and main channel deposits,and river-lake interaction deposits.The rapid expansion occurred in the estuary delta of the east branch of the Ouchi River,which advanced 38.55 km from the estuary toward the lake over the past 90 years.The causes of the changes in the beach included beach reclamation,sediment changes,and lake sand mining.Seventy embankments(covering 2057.77 km^(2))have been enclosed in the Dongting Lake area since 1930s,of which the high bay beach covered an area of 980.96km^(2).The amount of sediment deposited in Dongting Lake has reached 230857×10^(4) m^(3) since 1950s,which is equivalent to an average deposition height of 0.85 m on the lake's bottom.The mining of lake sand caused the beach to shrink,and the proportion of the beach area decreased from 77.18%in 1998 to 72.60%in 2018.The results of this study provide objective data for protecting the lakeshore's ecosystem and biodiversity and supporting the ecological restoration and environmental protection of the Yangtze River Basin.

Predicting beach profile evolution with group method data handling-type neural networks on beaches with seawalls

A major goal of coastal engineering is to develop models for the reliable prediction of short-and longterm near shore evolution.The most successful coastal models are numerical models,which allow flexibility in the choice of initial and boundary conditions.In the present study,evolutionary algorithms(EAs)are employed for multi-objective Pareto optimum design of group method data handling(GMDH)-type neural networks that have been used for bed evolution modeling in the surf zone for reflective beaches,based on the irregular wave experiments performed at the Hydraulic Laboratory of Imperial College(London,UK).The input parameters used for such modeling are significant wave height,wave period,wave action duration,reflection coefficient,distance from shoreline and sand size.In this way,EAs with an encoding scheme are presented for evolutionary design of the generalized GMDH-type neural networks,in which the connectivity configurations in such networks are not limited to adjacent layers.Also,multi-objective EAs with a diversity preserving mechanism are used for Pareto optimization of such GMDH-type neural networks.The most important objectives of GMDH-type neural networks that are considered in this study are training error(TE),prediction error(PE),and number of neurons(N).Different pairs of these objective functions are selected for two-objective optimization processes.Therefore,optimal Pareto fronts of such models are obtained in each case,which exhibit the trade-offs between the corresponding pair of the objectives and,thus,provide different non-dominated optimal choices of GMDH-type neural network model for beach profile evolution.The results showed that the present model has been successfully used to optimally prediction of beach profile evolution on beaches with seawalls.

Effects of offshore artificial islands on beach stability of sandy shores:case study of Hongtang Bay,Hainan Province

Artificial island-type reclamation often exerts certain impacts on near-shore sandy shoreline resources and coastal ecological landscapes.The relationship between artificial islands and offshore beach evolution has attracted considerable attention in coastal protection and engineering construction.In this study,we consider Hongtang Bay in Hainan Province,China,as the research object.We adopted the Gao-Collins model to investigate the substrate transport trend in this sea area based on the analysis of the measured hydrologic and sediment data.The shore section from Nanshanjiao to Hongtangling(Taling),including the flat and straight shore sections,is dominated by the lateral transport trend of the vertical shore.The near-shore water has a strong lateral sediment transport capacity,while the outer deep-water area exhibits a sediment transport trend consistent with the tidal current movement.Using multi-year topographic data,the shoreline and seabed alterations in Hongtang Bay were analyzed,and the LITLINE beach evolution model was adopted to simulate the effects of three artificial island layouts with different island filling areas,offshore distances,and plan forms on the near-shore shoreline deformation.The results obtained indicate that the artificial island arrangement with a large offshore distance and a small area has relatively substantial advantages,such as minimizing the adverse effects of artificial island implementation on the near-shore beach.

长江荆江河段航道水深条件与河流演变的关系

Given the importance of waterway depths in river development,the effects of the evolution of bars and troughs on waterway expansion play an important role in river management and water depth conservation.This study aims to expand the waterway dimensions of the Jingjiang Reach of the Yangtze River.To achieve this objective,determining the relationship between river evolution processes and the potential for waterway depth improvement and navigation hindrances is vital.Therefore,the sedimentation,hydrological,and terrain data of the Jingjiang Reach from 1955 to 2020 are analysed to elucidate the above-mentioned relationship.Since the commissioning of the Three Gorges Dam,the scouring of the low-flow channel has accounted for 90%-95%of all scouring in the Jingjiang Reach.Furthermore,the central bars and beaches have shrunk by 9.4%and 24.9%,respectively,and 18.3%overall.Considering the bed scouring and waterway regulation projects in the Jingjiang Reach,we investigated the continuity of a 4.5 m×200 m×1050 m(depth×width×bend radius)waterway along the Jingjiang Reach,and find that navigation-hindering channels account for over 5.3%of the waterway length.Furthermore,part of the Jingjiang Reach is an important nature reserve and shelters numerous water-related facilities,which inhibits the implementation of waterway deepening projects.The findings of this study demonstrate that numerous challenges are associated with increasing the waterway depths of the Jingjiang Reach.