Cross-shore Sediment Transport Rate Research Articles

Photogrammetric Investigation of Storm-Induced Erosion Process on Sandy Beach Profile in Medium-Scale Flume

In this study, laboratory experiments were conducted to investigate the influence of changes in storm wave height and water level on beach response in a medium-scale wave flume. A schematic storm was simulated (rising, apex, and waning phases). A non-intrusive photogrammetric method was used to collect high-resolution and synchronous data regarding the free surface water elevation and bed level, from which shoreline location, sandbar position, cross-shore sediment transport rates, and nonlinear wave parameters were derived. The cross-shore sediment transport was in agreement with previous laboratory measurements, including the monotonous exchange from foreshore erosion to shoaling zone accretion in most stages of the storm simulation. The surf zone was the main region supplying sediment for beach morphology modification and sandbar generation. The degree of storm erosion was not completely determined by the largest wave height and water level or the cumulative wave power of the apex phase. The largest gradients of the wave parameter sequence change occurred in the rising phase, and this was the main factor generating efficient beachface erosion. It induced an increase in sandbar size, accompanied by the cross-shore motion of maximum velocity amplitude, more violent disturbances of wave nonlinearity, and increased surf zone erosion, with these factors increasing beach instability and leading to more severe storm erosion. The large wave height and water level resulted in shoreline retreat, with a more significant swash zone erosion under a higher runup. The offshore sediment transport turned toward the onshore direction as the original large sandbar deteriorated under the decreasing wave parameter sequence in the waning phase.

Cross-Shore Sediment Transport for Modeling Long-Term Shoreline Evolution

A process-based parametric closure model for estimating cross-shore sediment transport (CST) rate has been developed for simulating long-term shoreline evolution. The formulations of this closure model include calculations of various subaqueous cross-shore transport components induced by waves, currents, and gravity. To better calculate asymmetrical near-bed orbital velocity that is a key to predict the cross-shore transport rate in the nearshore zone, a nonlinear wave-shape model is adopted. By formulating wave nonlinearity to cross-shore transport, this closure model can predict the net on- and offshore transport rates in response to shoreline accretion and erosion driven by waves and currents during low- and high-energetic conditions. This new capability for cross-shore shoreline change has been implemented into a one-line model, primarily driven by longshore sediment transport (LST). Calibration of model parameters and model validation were performed by simulating shoreline changes for a 5 km section of coastline in Duck, North Carolina, over a 14-year period from 2000 to 2013. The simulation results reproduced seasonal shoreline recoveries during low-energetic fair weather and also captured rapid retreats during storms. This shoreline model with inclusion of LST and CST has significantly improved the prediction accuracy of long-term shoreline changes at the study site. Impact analysis of a pier located within the domain reveals that this shoreline model with CST can better simulate sediment bypassing through the structure that depends on multiple local variables such as shoreline positions, updrift longshore transport, waves, and water levels. Therefore, this new shoreline model will facilitate long-term management of sediments and shoreline erosion and impact assessment of coastal structures.

Modeling the hydrodynamics and morphodynamics of sandbar migration events

A number of models are available for science and engineering purposes that numerically simulate nearshore hydrodynamics and the corresponding morphological evolution. However, the models include adjustable coefficients in parameterizations for physical processes that need to be calibrated, and thus there remains room for improvement by including additional physics. One such model is XBeach, which can simulate erosion during storms with proper calibration based on observations. The modeled sediment transport, especially in the cross-shore direction, is sensitive to the adjustable coefficients, with preferred values that are site and event specific. Here, the skill of XBeach is investigated by comparing 1-dimensional (cross-shore) depth-averaged simulations with observations of waves, currents, and sandbar migration across an Atlantic Ocean beach. Calibration of coefficients improved the agreement of the computed results with observed wave heights, offshore-directed mean currents (undertow), the wave-orbital-velocity third moments (skewness and asymmetry), and onshore/offshore sandbar migration although the proposed coefficient values depend on the parameterizations used. For example, including a variable breaking-wave roller energy model resulted in more skillful predictions of undertow than using the default constant coefficient value. Using the calibrated roller coefficients and corresponding undertow, XBeach simulated the observed offshore migration of the sandbar. Onshore transport in XBeach is driven by non-sinusoidal wave-orbital velocities, and proposed values for coefficients depend on the parameterization used to estimate skewness and asymmetry and the associated transport, as well as on incident wave conditions. XBeach calculations of cross-shore sediment transport rates were compared with those estimated by a commonly used sediment transport formula based on laboratory experiments. The inter-comparison suggests that using a wave-induced onshore transport parameter 3 or 4 times larger than the default value may at least in part compensate for the lack of bottom-boundary-layer-streaming-driven-onshore transport in XBeach.

MODELING OF BERM FORMATION AND EROSION AT THE SOUTHERN COAST OF THE CASPIAN SEA

Cross-shore beach profile data from field measurements performed at six locations on the southern coast of the Caspian Sea are used to investigate bathymetry change due to various wave conditions. Beach profile measurements are analyzed and subsequently compared with the results of a berm formation and erosion model. The model comprises distinct empirical sediment transport equations for predicting the cross-shore sediment transport rate under various wave conditions. To yield a berm formation and erosion model, empirical cross-shore sediment transport equations are combined with the mass conservation equation. Simulations results obtained from the model compared well with the measurements, proving the capability of the model in simulating berm formation and erosion evolution.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/FTgAr73h5rA

SIMULATION OF LONG-TERM SHORELINE CHANGE DRIVEN BY LONGSHORE AND CROSS-SHORE SEDIMENT TRANSPORT

Driven by wave and current, sediment transport alongshore and cross-shore induces shoreline changes in coasts. Estimated by breaking wave energy flux, longshore sediment transport in littoral zone has been studied for decades. Cross-shore sediment transport can be significant in a gentle-slope beach and a barred coast due to bar migration. Short-term beach profile evolution (typically for a few days or weeks) has been successfully simulated by reconstructing nonlinear wave shape in nearshore zone (e.g. Hsu et al 2006, Fernandez-Mora et al. 2015). However, it is still lack of knowledge on the relationship between cross-shore sediment transport and long-term shoreline evolution. Based on the methodology of beach profile evolution modeling, a semi-empirical closure model is developed for estimating phase-average net cross-shore sediment transport rate induced by waves, currents, and gravity. This model has been implemented into GenCade, the USACE shoreline evolution model.

LONG-TERM COASTAL EVOLUTION MODELLING OF LONGSHORE BARS

An extended version of a numerical model introduced by Larson et al. (2013) to simulate long-term cross shore material exchange for the subaqueous portion of the profile has been developed. Efforts have focused on improving the model to better account for beach systems consisting of two bars (inner and outer bar), as well as simulating the feeder response over time of nearshore dredged material bars, intended to function as beach nourishment. The theory for the evolution of a single-bar to a two-bar system was modeled, considering an inner and an outer bar, where the outer bar is of primary interest with the purpose of predicting the behavior of placed dredged material. The cross-shore sediment transport rate is based on the evolution equation for the bar system response to the hydrodynamic forcing by reference to its equilibrium condition, where the change in the bar volume is based on a set of wave criteria, describing the onset of a new breaking zone when the outer bar forms. Empirical formulas are employed for the bar equilibrium volume and for coefficients determining the bar response rate. In this study, a description of the extended model and the results from the model component validation at two different sites in USA (Duck, North Carolina, and Cocoa Beach, Florida) are presented. Duck measurements have detected that some bars form in the nearshore and move all the way offshore (eventually deflating by non-breaking waves). At the same time, it was equally observed that a lot of inner bars formed in shallow water do not move offshore but remain as inner bars all the time. According to this, the developed model considers that the inner bar will not become the outer bar, but material previously dedicated to the inner bar will be available for the outer bar. Overall, the present study demonstrates the potential for using rather simple models, based on the definition of an equilibrium state that is compared to the current state and the magnitude of offshore wave forcing to drive the changes in the profile. The methodology employed here allowed to quantitatively reproduce the main trends in the subaqueous beach profile response in a long-term perspective as a function of the bar volumes disequilibrium, the magnitude of the incident wave height and the dimensionless fall velocity to move the sand with a time varying forcing.

EFFECT OF NET CROSS-SHORE SAND TRANSPORT ON BEACH PROFILES

The concept of an equilibrium beach profile is convenient and widely adopted for long-term simulations of shoreline changes and beach profile evolutions. The equilibrium profile is regarded as a profile with net cross-shore sediment transport rate qn = 0. Kobayashi et al. (2008) proposed formulas for offshore suspended sediment transport rate qs and onshore bedload transport rate qb and derived the equilibrium profile equation of Dean (1991) for the case of qn = (qs+ qb) = 0. Their model is extended to equilibrium profiles with net offshore (qn0) sediment transport rates. The equilibrium profile with qn=0 is modified by periodic beach nourishment (qn0). The extended model predicts the degree and location of profile modifications as a function of qn and it can be used to improve beach nourishment design and devise barrier beach overwash countermeasures.

Laboratory Study on Cross Shore Sediment Budget over Submerged Breakwaters

The objective of this study is to understand the characteristics of the cross-shore sediment transport over the submerged breakwater through quantitative observations of wave breaking, undertow, and the characteristics of near bottom velocity profiles such as skewness, asymmetry around the permeable or impermeable submerged breakwaters. A series of laboratory experiments were conducted in a 2D wave flume with uniformly sloping 1/20 movable bed of uniform sand grains (D50 =0.15 mm). Either permeable or impermeable breakwaters was installed and four regular incident wave conditions were selected so that generated waves cover both daily or stormy wave conditions. Boundary detection technique was used to obtain the temporal and spatial variation of bed profile and then to calculate cross shore sediment transport rate, CTR, from the images captured by multiple high speed and normal video cameras. Tracer particles were used in the experiment and PIV was applied to capture the velocity field around the submerged breakwater. The experimental results indicate that the impermeable breakwater is generally more effective than the permeable breakwater in terms of the reduction of the seaward CTR over the submerged breakwater but the amount of CTR was very sensitive to the breaking point especially in the case of impermeable breakwater. Increase in acceleration skewness caused low CTR in impermeable breakwater case even with a relatively high wave action on the lee of the breakwater.

Erosion and heavy mineral depletion of a placer mining beach along the south-west coast of India: Part I— Nearshore sediment transport regime

The Chavara coast of southwest India is well known for its rich beach placer deposits which are being commercially exploited by the industries. Replenishment of these resources, which consist of heavy minerals of varying densities, by the hydrodynamic processes is essential for maintaining the stability of the coast as well as sustenance of mining. Rich concentrations of heavy minerals were reported consistently in the beach sediments of this coast in the past, but a systematic reduction in the concentration of the heavies has been reported during the past one-and-a-half decades. This paper, the first in a series of three, emanates from a programme of study launched to understand the mechanisms that manifest the reported changes in the morphology and mineralogy along this coast. In this study the longshore and cross-shore sediment transport rates along this coast have been estimated adopting numerical model studies. The validated LITDRIFT and LITPROF modules of the LITPACK modelling system have been used for computing the longshore and cross-shore sediment fluxes in the surf zone and innershelf region. The net annual longshore sediment transport is northerly in the surf zone where as it is southerly in the innershelf. Detailed analysis of the computed results shows domination of onshore transport over offshore transport. The beach volume change estimated from the measured beach profile on the other hand shows a reduction in the annual replenishment. The domination of the onshore flux as seen in the computations is actually not reflected in the field observations, and this can be attributed to the influence of excessive sand mining by the industries.

An investigation on the formation of submerged bar under surges in sandy coastal region

Cross-shore sediment transport rate exposed to waves is very important for coastal morphology, the design of marine structures such as seawalls, jetties, breakwaters etc, and the prevention of coastal erosion and accretion due to onoff shore sediment transportation. In the present study, the experiments on cross-shore sediment transport are carried out in a laboratory wave channel with initial beach slopes of 1/8, 1/10 and 1/15. By using the regular waves with different deep-water wave steepnesses generated by a pedal-type wave generator, the geometrical characteristics of beach profiles under storm conditions and the parameters affecting on-off shore sediment transport are investigated for the beach materials with medium diameters of d50=0.25, 0.32, 0.45, 0.62 and 0.80 mm. The offshore bar geometric characteristics are the horizontal distances from the shoreline to the bar beginning (Xb), crest (Xt), and ending (Xs) points, the depth from the bar crest to the still water level (ht), and the bar volume (Vbar). The experimental results have indicated that when the deep-water steepness (H0/L0) increased, the net movement to seaside increased. With the increasing wave steepness, the bars moved to widen herewith the vertical distances from still water level to the bar beginning (Xb), crest (Xt) and ending (Xs) points and the horizontal distances from the coast line to the bar beginning, crest and ending points increased. It was also shown from experimental results that the horizontal distances from the bar beginning and ending points to the coast line increased with the decrease of the beach slope. The experimental results obtained from this study are compared with previous experimental work and found to be of the same magnitude as the experimental measurements and followed the expected basic trend.

バームの形成侵食に着目した短期海浜地形変化モデルの高精度化

A short-term swash zone beach profile change model proposed by Suzuki and Kuriyama (2009) focusing on berm formation and erosion was improved using the distribution curves of cross-shore sediment transport rates for berm formation and erosion determined by the slope of the curves at the inflection points. The improved model was applied to the calculation of three months beach profile change, which results were compared with field data and the computational results of Suzuki and Kuriyama (2009). The present model was found to well predict not only the shoreline change but also the beach profile change, including the berm formation and erosion, compared to the model by Suzuki and Kuriyama (2009).

Numerical Investigation on the Influence of Tide on Cyclic Seaward Bar Migration

The influences of tide and water level rise on the cyclic medium-term seaward bar migration were examined using a one-dimensional numerical model of beach profile change, in which the cross-shore sediment transport rate was assumed to consist of four contributions due to sediment suspension and undertow, velocity skewness, velocity atiltness and beach slope. The model was calibrated with a year-long dataset of beach profiles obtained at the Hasaki coast of Japan in 1989, when the duration of medium-term seaward bar migration was approximately 1 year. The comparison between bar crest positions predicted with and without the tide at Hasaki during a two-year period including the calibration period of 1989 and the following year shows that the bar migrated further seaward without the tide than with the tide. The comparison between bar crest positions during the two-year period predicted with the tide and the water levels 0.5 m higher indicates that the bar crest positions were located further seaward with the higher water levels than with the tide in 1989. The difference between cyclic medium-term seaward bar migrations with and without the tide, and that with the tide and the higher water levels, are attributed to wave breaking on and seaward of the bar crests.

An Experimental Study of the Cross-Shore Evolution of Artificial Submerged Sand Bars

Artificial submerged sand bars have been used as an alternative soft engineering structure for shore protection. To successfully implement the sand bar in an economically beneficial manner, more knowledge is required concerning the evolution of a sand bar under different conditions. A series of experiments is presented to quantify the sediment transport that is induced by artificial submerged sand bars in a wave-driven beach environment. Artificial movable sand bars of various initial geometries were tested on various fixed, inclined bottom slopes by using different incoming regular wave conditions. A new, beneficial parameter, the cumulative transport rate, is defined by integrating the time-dependent cross-shore sediment transport rate from an initial deposition to a quasi-equilibrium state of a sand bar migration. From many tests and analyses, it has been found that the cumulative transport rate of the sand bar is highly dependent on the local Shields number being related to the bed-load transport. Additionally, the Shields-dependent relation is compared to previous field sand bar evolution projects to determine if the sand bar actively migrates onshore or remains stable. An optimal initial bar geometry is suggested to pursue an efficient onshore sediment transport. The effects of the bottom slope on the cumulative transport rate are also discussed.

Dramatic beach and nearshore morphological changes due to extreme flooding at a wave-dominated river mouth

Record flooding on the Santa Clara River of California (USA) during January 2005 injected ∼ 5 million m 3 of littoral-grade sediment into the Santa Barbara Littoral Cell, approximately an order of magnitude more than both the average annual river loads and the average annual alongshore littoral transport in this portion of the cell. This event appears to be the largest sediment transport event on record for a Southern California river. Over 170 m of local shoreline (mean high water (MHW)) progradation was observed as a result of the flood, followed by 3 years of rapid local shoreline recession. During this post-flood stage, linear regression-determined shoreline change rates are up to −45 m a − 1 on the subaerial beach (MHW) and − 114 m a − 1 on the submarine delta (6 m isobath). Starting approximately 1 km downdrift of the river mouth, shoreline progradation persisted throughout the 3-year post-flood monitoring period, with rates up to + 19 m a − 1 . Post-flood bathymetric surveys show nearshore (0 to 12 m depth) erosion on the delta exceeding 400 m 3/m a − 1 , more than an order of magnitude higher than mean seasonal cross-shore sediment transport rates in the region. Changes were not constant with depth, however; sediment accumulation and subsequent erosion on the delta were greatest at − 5 to − 8 m, and accretion in downdrift areas was greatest above –2 m. Thus, this research shows that the topographic bulge (or “wave”) of sediment exhibited both advective and diffusive changes with time, although there were significant variations in the rates of change with depth. The advection and diffusion of the shoreline position was adequately reproduced with a simple “one line” model, although these modeling techniques miss the important cross-shore variations observed in this area. This study illustrates the importance of understanding low-frequency, high volume coastal discharge events for understanding short- and long-term sediment supply, littoral transport, and beach and nearshore evolution in coastal systems adjacent to river mouths.

Prediction of Beach Profile Change behind Submerged Breakwater on the Niigata-West Coast

The numerical model for predicting the beach profile change behind a submerged breakwater on the Niigata-West coast was developed. The model estimates the beach profile change on the basis of the cross-shore gradient of cross-shore sediment transport rate consisting of suspended load due to wave breaking and bed loads due to wave nonlinearity and beach slope. The model was calibrated with the beach profile change on the Niigata-West coast during a one-year period from July 2001 to July 2002. Numerical simulations using the model shows that the increase in the crown height of the submerged breakwater is effective in reducing the foreshore erosion and in making the sediments transported seaward from the eroded foreshore remain in the inner surf zone.

Short-term Beach Profile Change Model Focusing on Berm Formation and Erosion

A short-term beach profile change model focusing on berm formation and erosion was proposed using a 2.5-year data set of beach profiles and offshore waves. The investigation area was set from the maximum wave run-up position to the shoreline position at the mean tide level. The cross-shore sediment transport rates for berm formation and erosion were modeled with the offshore wave energy flux and the berm height, respectively. The model was applied for the calculation of three months beach profile change, and the results revealed that the model well predicts the beach profile change including the berm formation and erosion in a qualitative sense.

Inverse Estimation of Sand Transport Rates on Nourished Delaware Beaches

The Delaware Atlantic coast of 40 km length is suffering from beach erosion. The State of Delaware placed approximately 1,100,0001 m3 sand on its four beaches in 1998. Beach profiles were surveyed along fixed 65 cross-shore lines almost semiannually for 9–11 times until 2005. The measured profiles are analyzed to obtain the shoreline displacement and the area changes in the landward and shoreward zones between the two successive profile surveys. The relations among these profile change parameters indicate that a standard one-line model may not be applicable to these nourished beaches because of the dominant summer and winter profile changes. An inverse method based on a two-line model is developed to estimate the cross-shore sediment transport rate and the gradient of the longshore sediment transport rate. The estimated rates indicate the importance of both cross-shore and longshore sediment transport to predict the evolution of these nourished beaches with large variabilities in time and space.

Cross-shore sediment transport and morphological response on a macrotidal beach with intertidal bar morphology, Truc Vert, France

A 3-week field investigation was conducted on a macrotidal beach with intertidal bar morphology with the aim of quantifying cross-shore sediment transport rates and beach response. Cross-shore transport rates were estimated in four different ways: (1) net transport from bed-level changes; (2) measured suspended sediment fluxes; (3) sediment fluxes associated with migrating wave ripples; and (4) total load transport predicted using the Bailard [Bailard, J.A. 1981. An energetics total load sediment transport model for a plane sloping beach. Journal of Geophysical Research, 86, 10938–10954.] model. Excellent qualitative agreement was found between the different measures under all recorded wave conditions, with onshore (offshore) sediment transport and beach accretion (erosion) observed under low-to- moderate (high) wave conditions. The agreement is due to the strong correspondence between the direction of mean cross-shore flows across the intertidal bar surface and that of the sediment transport. Significant onshore mean flows ( u = 0.1–0.2 m s − 1 ) were persistently observed under low wave conditions, and these were strongest in shallow water depths ( h < 1 m). These flows are attributed to the (weak) three-dimensional water circulation in the surf zone associated with the intertidal bar-rip morphology and caused onshore bar migration. Under high wave conditions, on the other hand, strong mean cross-shore flows ( u = 0.2–0.4 m s − 1 ), due to the bed return flow, were consistently directed offshore and resulted in bar destruction. Under low wave conditions, vertically-integrated suspended fluxes, ripple transport rates and predicted total transport fluxes were remarkably similar, but when the fluxes are integrated over a complete tidal cycle, they are considerably smaller than that derived from bed-level changes based on low tide surveys.

Numerical Model for Medium-term Bar Behavior

A one-dimensional numerical simulation model for bar behaviors in medium-term periods was developed. The model estimated cross-shore sediment transport rates with consideration of undertow, beach slope and velocity skewness and atiltness. Beach profile changes every 2 hours for a 2-year period from 1989 to 1990 on the Hasaki coast were predicted with the model. The comparison between the bar movements predicted and measured showed that the duration time of the predicted bar movements was about 1 year, and almost agreed with the measured one, while the predicted bars were more developed than those measured.

Simple Modeling of Cross-shore Sediment Transport Rate for Berm Formation and Erosion

The spatial distributions of the cross-shore sediment transport rate for berm formation and erosion were examined using a 2.5-year data set of beach profiles. The investigation area was set from the maximum wave run-up position to the shoreline position at the mean tide level. The cross-shore sediment transport rates for berm formation and erosion were modeled with the offshore wave energy flux and the berm height, respectively. Moreover, the model was applied for the calculation of a one-month beach profile change. The results revealed that the model well predicts the beach profile change of the berm formation and erosion in a qualitative sense.