Post-storm Profile Research Articles

Bar and berm dynamics during transition from dissipative to reflective beach profile

Bar and berm morphology characterize the seasonal beach evolution, and determine the protection against storm erosion as well as the touristic use of beaches. Thus, they are of particular interest for coastal management and engineering in the nearshore zone. This study used large-scale wave flume experiments to observe the transition from fully dissipative to fully reflective beach profile at a high level of detail. Starting from a post-storm profile generated under energetic waves, a very low energy wave condition caused dissipation of the outer and the inner bar, shoreline recovery, and berm accretion. Measurements revealed feedback between hydrodynamics and beach profile evolution with an onshore shift of the wave breaking location. As a result, the magnitude and cross-shore evolution of wave asymmetry-related bedload net onshore and suspended net offshore transport changed. The relative magnitudes of the two transport components and the way they shifted relative to each other caused the observed beach recovery. Additionally, a link between bar and berm morphology (surf-swash sand exchange) was observed. The shifting breakpoint enabled sustained, wave asymmetry-related onshore transport in the inner surf zone, feeding the berm accretion which occurred through advective swash zone processes including berm overwash.

Effects of storm clustering on beach/dune evolution

Impacts of storm clustering on beach/dune morphodynamics were investigated by applying the state-of-the-art numerical model XBeach to Formby Point (Sefton coast, UK). The adopted storm cluster was established by analysing the observed winter storms from December 2013 to January 2014 using a storm threshold wave height. The first storm that occurred during this period is regarded as exceptionally intense, and the occurrence of such a cluster of events is very unusual. A 1D model was setup for the highly dynamic cross-shore at Formby Point. After initial calibration of the model parameters against available post-storm profile data, the model was used for the simulation of the storm cluster. It was assumed that no beach recovery occurred between adjacent storms due to the very short time intervals between storms. As a result, the final predicted post-storm profile of the previous storm was used as the pre-storm profile of the subsequent storm. The predicted evolution during each storm was influenced by the previous storms in the cluster. Due to the clustering effect, the bed level change is not proportional to the storm power of events within the cluster, as it would be in an individual storm case. Initially, the large storm events interact with the multi-bared foreshore enabling the subsequent weaker storms to influence the upper beach and lower dune system. This results in greater change at the dune toe level also during less severe subsequent storms. It is also shown that the usual water level threshold used to define dune erosion is over predicted by about 1m for extreme storm conditions. The predicted profile evolution provides useful insights into the morphodynamic processes of beach/dune systems during a storm cluster (using Formby Point as an example), which is very useful for quantifying the clustering effects to develop tools for coastal management.

Comparison of storm cluster vs isolated event impacts on beach/dune morphodynamics

Two-dimensional numerical simulations were used to investigate the impacts of storm clustering on the beach/dune evolution of the Sefton coast, Liverpool Bay, UK. A storm cluster consisting of a series of closely spaced seven events was identified using observed wave and surge data during the 2013/2014 winter period. First event in this cluster is regarded as exceptionally intense and the occurrence of seven storms within a very short time period, is unique. The XBeach coastal area model was used to simulate beach change from 1) the storm sequence (Clustered events) and 2) the same storms considering them as isolated events. Offshore metadata was transformed to the nearshore area using the Delft3D and SWAN models. Resulting evolution was first compared with the available post-storm profiles measured at a number of locations along the Sefton coast. Analysis of the Clustered and Isolated simulations showed the effect of clustering on the Sefton beach/dune system when compared to the impact of isolated events occurring on a fully recovered beach system. Morphological change occurred during each storm in the Cluster was influenced by the preceding storm(s), such that the evolution is not proportional to the storm power of the event, as it would be for Isolated events. Both storm cases resulted in heavy erosion at Formby Point (i.e. central of the Sefton coast) and accretion in the north and south. The Cluster prevented system recovery with the area of erosion continually extending south along the coast compared with that in Isolated events. The initial storm within the Cluster caused large bed level changes in the nearshore ridge-runnel system, enabling the subsequent storms to penetrate further south. The local convex geometry of the Sefton coast is found to have more influence on the beach/dune morphodynamics than the clustering effect. This study enhances the understanding beach/dune response to storm clusters, to interpret observed morphological changes and to develop tools for sustainable coastal management particularly in the Sefton coast and generally in similar systems worldwide.

Impacts of storm chronology on the morphological changes of the Formby beach and dune system, UK

Abstract. Impacts of storm chronology within a storm cluster on beach/dune erosion are investigated by applying the state-of-the-art numerical model XBeach to the Sefton coast, northwest England. Six temporal storm clusters of different storm chronologies were formulated using three storms observed during the 2013/2014 winter. The storm power values of these three events nearly halve from the first to second event and from the second to third event. Cross-shore profile evolution was simulated in response to the tide, surge and wave forcing during these storms. The model was first calibrated against the available post-storm survey profiles. Cumulative impacts of beach/dune erosion during each storm cluster were simulated by using the post-storm profile of an event as the pre-storm profile for each subsequent event. For the largest event the water levels caused noticeable retreat of the dune toe due to the high water elevation. For the other events the greatest evolution occurs over the bar formations (erosion) and within the corresponding troughs (deposition) of the upper-beach profile. The sequence of events impacting the size of this ridge–runnel feature is important as it consequently changes the resilience of the system to the most extreme event that causes dune retreat. The highest erosion during each single storm event was always observed when that storm initialised the storm cluster. The most severe storm always resulted in the most erosion during each cluster, no matter when it occurred within the chronology, although the erosion volume due to this storm was reduced when it was not the primary event. The greatest cumulative cluster erosion occurred with increasing storm severity; however, the variability in cumulative cluster impact over a beach/dune cross section due to storm chronology is minimal. Initial storm impact can act to enhance or reduce the system resilience to subsequent impact, but overall the cumulative impact is controlled by the magnitude and number of the storms. This model application provides inter-survey information about morphological response to repeated storm impact. This will inform local managers of the potential beach response and dune vulnerability to variable storm configurations.

LAND COVER EFFECT ON DUNE EROSION AND OVERWASH

In this study, a numerical morphodynamic model eXtreme Beach behaviour (XBeach) was used to simulate the effects of land cover on the shape of the post storm profile s at two test sites on barrier islands at Outer Banks, North Carolina. The simulations included dune erosion and overwash regimes and were carried out with and without the inclusion of land cover effects. The model results were assessed by comparing post storm profiles extracted from XBeach and Light Detection And Ranging (LiDAR) data. The simulations showed that the morphological response of the profiles was sensitive to land cover and inclusion of these effects improved the model results.

Modelling storm-induced beach/dune evolution: Sefton coast, Liverpool Bay, UK

Storm-induced dune evolution on a sandy coastal system is investigated using a nested modelling approach applied to the Sefton coast, Liverpool Bay, UK. Real-time offshore water levels and waves were used as model boundary forcings. A Delft3D coarse grid setup is used to simulate time and space varying sea surface elevations on which offshore waves are transformed (by applying the SWAN model) to establish the wave boundary for the high resolution morphological model (XBeach). Statistical comparisons between model predicted and measured post-storm profiles at a number of locations along the coast suggest that XBeach successfully captures storm-induced beach change along the Sefton coast. Predicted bed evolution of the beach/dune system shows alternate erosion and sedimentation areas in the nearshore. Strong bed level changes are found at the northern part of the Sefton coast when north-westerly (NW) extreme waves and winds coincide with spring-high tide. Morphological changes in the southern part are significantly lower than that in the north as a result of NW wave dissipation on the shoals located to the north of the Crosby channel, which creates low wave actions in that area. In addition, erosion of the dune foot is observed at some locations along the beach. Temporal simulation of beach/dune evolution as a result of variable forcing conditions during storms provides useful insight into the morphodynamic processes of beach/dune systems during storms (using Sefton as an example), which is very useful for developing coastal management strategies over the existing conceptual tools.

In Situ Erosion Testing and Clay Levee Erodibility

Earthen levee damage from storm surge overtopping in the New Orleans area was observed and documented after Hurricane Katrina. Some levee reaches were undamaged and some were slightly eroded, while other portions were completely breached. The in situ jet index test method was applied along selected post-storm reconstructed levee reaches to assess levee fill erodibility. Jet index testing was also conducted at three undamaged (un-eroded) levee surface locations. Pre-storm levee profiles (elevations) were compared to post-storm modeled stormwater elevations to estimate overtopping depths at the test sites. Also, pre-storm levee profiles were compared to post-storm profiles to estimate erosion depths at the test sites. The jet index test results were integrated with pre-storm soil boring data (i.e., stratigraphy and soil classification), post-storm erosion depths, and overtopping depths to investigate soil surface erodibility relationships. The results showed the validity of using the jet index test method for levee soil erodibility assessments.

Air–sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats

Measurements of the air–sea fluxes of N 2 and O 2 were made in winds of 15–57 m s − 1 beneath Hurricane Frances using two types of air-deployed neutrally buoyant and profiling underwater floats. Two “Lagrangian floats” measured O 2 and total gas tension (GT) in pre-storm and post-storm profiles and in the actively turbulent mixed layer during the storm. A single “EM-APEX float” profiled continuously from 30 to 200 m before, during and after the storm. All floats measured temperature and salinity. N 2 concentrations were computed from GT and O 2 after correcting for instrumental effects. Gas fluxes were computed by three methods. First, a one-dimensional mixed layer budget diagnosed the changes in mixed layer concentrations given the pre-storm profile and a time varying mixed layer depth. This model was calibrated using temperature and salinity data. The difference between the predicted mixed layer concentrations of O 2 and N 2 and those measured was attributed to air–sea gas fluxes F BO and F BN. Second, the covariance flux F CO( z) = 〈 wO 2′〉( z) was computed, where w is the vertical motion of the water-following Lagrangian floats, O 2′ is a high-pass filtered O 2 concentration and 〈〉( z) is an average over covariance pairs as a function of depth. The profile F CO( z) was extrapolated to the surface to yield the surface O 2 flux F CO(0). Third, a deficit of O 2 was found in the upper few meters of the ocean at the height of the storm. A flux F SO, moving O 2 out of the ocean, was calculated by dividing this deficit by the residence time of the water in this layer, inferred from the Lagrangian floats. The three methods gave generally consistent results. At the highest winds, gas transfer is dominated by bubbles created by surface wave breaking, injected into the ocean by large-scale turbulent eddies and dissolving near 10-m depth. This conclusion is supported by observations of fluxes into the ocean despite its supersaturation; by the molar flux ratio F BO/ F BN, which is closer to that of air rather than that appropriate for Schmidt number scaling; by O 2 increases at about 10-m depth along the water trajectories accompanied by a reduction in void fraction as measured by conductivity; and from the profile of F CO( z), which peaks near 10 m instead of at the surface. At the highest winds O 2 and N 2 are injected into the ocean by bubbles dissolving at depth. This, plus entrainment of gas-rich water from below, supersaturates the mixed layer causing gas to flux out of the near-surface ocean. A net influx of gas results from the balance of these two competing processes. At lower speeds, the total gas fluxes, F BO, F BN and F CO(0), are out of the ocean and downgradient.

Vulnerability Indicators for Coastal Dunes

This paper describes the development of a new parameter to characterize dune vulnerability to storm-induced erosion. Existing indicators of dune erosion vulnerability are examined, including expected cross-sectional erosion values calculated using storm characteristics. We extract a series of 110 pre- and poststorm profiles at cross-shore transects, spaced at approximately 300 m alongshore on digital terrain models of a North Carolina barrier island. Dune failure and survival are designated based on a percentage of the cross-sectional area eroded—50% or greater erosion indicates failure, less than 50% erosion, survival. Crest height does not prove to be an effective predictor of dune vulnerability. Existing cross-sectional area based parameters show some success in predicting erosion vulnerability. We improve the dune failure and survival prediction success rate using a new parameter, a surrogate moment of inertia, the erosion resistance.

UNDISTORTED FROUDE MODEL FOR SURF ZONE SEDIMENT TRANSPORT

Small scale movable bed wave tank experiments were carried out according to undistorted Froude model laws with the sediment fall time, H/wT, as the governing parameter for scaling the model sediment. Four questions addressed in this study included: (a) the ability to reproduce larger scale model results for both erosional and accretive conditions, (b) the effects of more realistic concave upward initial beach profiles instead of the more usual planar initial slopes, (c) the criterion for onshore-offshore sediment transport, and (d) the capability of the model to simulate post-storm recovery. Based on a comparison with large scale results of Saville (1957), it was found that the model provided good agreement for erosive conditions. For accretive conditions, the results were less conclusive although the general patterns of profile change were similar. The final beach profiles resulting from concave upward initial profiles were found to be substantially different from those for an initially planar profile. It appears that the initially planar profile unrealistically affects the breaker type and results in a more pronounced longshore bar and offshore slopes that are steeper than found in nature. Tests conducted to evaluate the criterion separating onshore-offshore transport suggested a higher value of the fall time parameter, H/wT, than was originally proposed by Dean (1973); this is interpreted to be due to scale effects in most of the model data used in the original development. Tests to simulate post-storm recovery were affected by the presence of "reflection bars" associated with a partial standing wave system. The reflection bars appear to strongly affect the sediment transport limiting the post-storm profile recovery. The most effective recovery was induced by continually changing wave conditions to maintain the wave breakpoint slightly landward of the bar crest.

Coastline Sedimentation in Tectonically Active Geosynclinal Basin, Glacial Outwash-Plain Shoreline of Northeastern Gulf of Alaska: ABSTRACT

The outwash-plain shoreline of southeastern Alaska consists primarily of 2 types of coastal morphologic areas: (1) places where outwash streams border the shore, and (2) beach-ridge plains. The outwash streams provide an abundant supply of sediment to the longshore drift system. Beach-ridge plains develop as a series of prograding spits, most of which indicate sediment transport from east to west. The spits trail away from the streams that originate at the termini of large piedmont glaciers, such as the Bering and the Malaspina. Once a stream channel is abandoned, or a new outlet is found, the beach-ridge plain is eroded back at the rate of several feet per year. The beach processes are dominated by southeasterly storms which generate exceptionally strong longshore drift from east to west. The cycle of erosion-deposition on the beaches is similar to that of the New England coast; that is, the post-storm profile is flat to slightly concave upward, the beach recovers by the landward migration of a series of ridge-and-runnel systems, and the maximum constructional phase is a broad depositional berm. The cycle is shorter on the Alaskan coast, presumably because of greater storm frequency. The End_Page 624------------------------------ large diurnal inequality of the tides has a striking effect on beach morphology, because wave energy is concentrated at 4 different levels during a 24-hour period. End_of_Article - Last_Page 625------------