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Abstract: A 2DH numerical, model which is capable of computing nearshore circulation and morphodynamics, including dune erosion, breaching and overwash, is used to simulate overwash caused by Hurricane Ivan (2004) on a barrier island. The model is forced using parametric wave and surge time series based on field data and large-scale numerical model results. The model predicted beach face and dune erosion reasonably well as well as the development of washover fans. Furthermore, the model demonstrated considerable quantitative skill (upwards of 66% of variance explained, maximum bias −0.21m) in hindcasting the post-storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied. The prediction skill ranged between 0.66 and 0.77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied. The sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay. The model sensitivity to the sheet flow sediment transport limiter, which served as a proxy for unknown factors controlling the resistance to erosion, was significantly greater than the sensitivity to the hydraulic forcing parameters. If no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response. [Copyright &y& Elsevier]

The relationship between alongshore rip channel migration and sediment transport is investigated using time-averaged video images to identify the positions of rip channels at three sites along southern Monterey Bay, California, over 1–3year periods. Daily migration rates are calculated for each site. The basic Coastal Engineering Research Center (CERC) formula for bulk alongshore sediment transport is used to estimate alongshore transport rates at each site, based on hourly input of directional wave spectra measured at an offshore buoy and shoaled and refracted to the nearshore. The wave transformation model is validated by comparison with measured nearshore directional wave spectra in 13m water depth. An expanded CERC formulation that accounts for the additional forcing of alongshore wave height gradients is also tested but rejected. Correlation coefficients between daily rates of calculated alongshore transport and rip channel migration are low: 0.01, 0.44, and 0.38 at the Sand City, Stilwell, and Marina sites, respectively. The problem of digitization noise, associated with video-based rip channel identification, is discussed and several filtering methods are used to increase the signal to noise ratio and obtain improved correlations. Results indicate that higher frequency migration events (on time scales shorter than 8days) will likely be obscured below the digitization “noise floor”, but that longer period oscillations (such as tidal and seasonal cycles) show up clearly in both rip migration and sediment transport datasets. Daily rates are cumulatively summed to generate time series of representative overall rip migration distance and net alongshore transport, and the cumulative summing procedure is shown to be similar to applying a low-pass filter. Mean migration is demonstrated to lag behind net transport in the summertime, suggesting that a minimum level of alongshore forcing is required to generate detectable rip migration. Rip channel migration and net alongshore transport have correlation coefficients that range from 0.76 to 0.94 at the three sites. [Copyright &y& Elsevier]

Abstract: Record flooding on the Santa Clara River of California (USA) during January 2005 injected ∼5millionm3 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 170m of local shoreline (mean high water (MHW)) progradation was observed as a result of the flood, followed by 3years of rapid local shoreline recession. During this post-flood stage, linear regression-determined shoreline change rates are up to −45ma−1 on the subaerial beach (MHW) and −114ma−1 on the submarine delta (6m isobath). Starting approximately 1km downdrift of the river mouth, shoreline progradation persisted throughout the 3-year post-flood monitoring period, with rates up to +19ma−1. Post-flood bathymetric surveys show nearshore (0 to 12m depth) erosion on the delta exceeding 400m3/ma−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 −8m, and accretion in downdrift areas was greatest above –2m. 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. [Copyright &y& Elsevier]

Abstract: Sediment transport and the potential for erosion or deposition have been investigated on the Palos Verdes (PV) and San Pedro shelves in southern California to help assess the fate of an effluent-affected deposit contaminated with DDT and PCBs. Bottom boundary layer measurements at two 60-m sites in spring 2004 were used to set model parameters and evaluate a one-dimensional (vertical) model of local, steady-state resuspension, and suspended-sediment transport. The model demonstrated skill (Brier scores up to 0.75) reproducing the magnitudes of bottom shear stress, current speeds, and suspended-sediment concentrations measured during an April transport event, but the model tended to underpredict observed rotation in the bottom-boundary layer, possibly because the model did not account for the effects of temperature–salinity stratification. The model was run with wave input estimated from a nearby buoy and current input from four to six years of measurements at thirteen sites on the 35- and 65-m isobaths on the PV and San Pedro shelves. Sediment characteristics and erodibility were based on gentle wet-sieve analysis and erosion-chamber measurements. Modeled flow and sediment transport were mostly alongshelf toward the northwest on the PV shelf with a significant offshore component. The 95th percentile of bottom shear stresses ranged from 0.09 to 0.16Pa at the 65-m sites, and the lowest values were in the middle of the PV shelf, near the Whites Point sewage outfalls where the effluent-affected layer is thickest. Long-term mean transport rates varied from 0.9 to 4.8metrictonsm−1 yr−1 along the 65-m isobaths on the PV shelf, and were much higher at the 35-m sites. Gradients in modeled alongshore transport rates suggest that, in the absence of a supply of sediment from the outfalls or PV coast, erosion at rates of ∼0.2mmyr−1 might occur in the region southeast of the outfalls. These rates are small compared to some estimates of background natural sedimentation rates (∼5mmyr−1), but do not preclude higher localized rates near abrupt transitions in sediment characteristics. However, low particle settling velocities and strong currents result in transport length-scales that are long relative to the narrow width of the PV shelf, which combined with the significant offshore component in transport, means that transport of resuspended sediment towards deep water is as likely as transport along the axis of the effluent-affected deposit. [Copyright &y& Elsevier]