Asunción Baquerizo

An analytical method to evaluate the efficiency of porous screens as wave dampers

Abstract The linear theory for water waves impinging obliquely on dissipative multilayered media is used to evaluate the reflection and transmission coefficients. The case of periodic medium consisting of alternating layers of upright porous walls and water of equal or different thicknesses is examined. Wave propagation in these media exhibits Bragg reflection. Using a plane wave assumption, which involves neglecting the evanescent eigenmodes that exist near the structure boundaries (to satisfy matching conditions), the problem can be reduced to a matrix equation which relates the complex amplitudes of the incident and reflected plane waves to the complex amplitude of the transmitted wave. The problem of wave propagation is analysed for an infinite channel, and for a wave flume in which a paddle generates the waves and a backwall limits waves propagation. The variation of the magnitude of the reflection coefficient, | R |, with k 1 Λ is discussed, where k 1 is the wavenumber in the water and Λ is the width of a unit cell consisting of two layers, one porous and another water. Increasing the porous layer width or decreasing the wave period broadens the range of k 1 Λ values for which resonance occurs. Furthermore, with increase in the angle of wave incidence, θ, the value of | R | decreases and the dependence of | R | on k 1 Λ is damped; for large angle of incidence the reflection is almost constant and negligible. On increasing the number of absorber units, an overall decrease in the reflection coefficient is achieved, but an increase in the number of oscillations between resonant peaks and in the peak amplitude also occurs.

On the development of large-scale cuspate features on a semi-reflective beach: Carchuna beach, Southern Spain

Abstract Carchuna beach, located on the southeast coast of Spain, has a series of natural permanent horns of time-dependent horizontal dimensions, which do not appear to be periodically spaced, and do not propagate alongshore. Until at present, it has been assumed that the permanency of these forms is related to the existence of bed rocks; nevertheless, analysis of the configuration of the depth contours and the bed sediment composition does not justify this hypothesis. This paper explores the formation of cuspate features by the effects of the wind wave dynamics and their development by nearshore circulation and infragravity waves, including edge waves. Three levels of wave energy flux conditions are considered. In all likelihood, wave refraction of severe storms on submerged fluvial valleys is the main cause for longshore variation of breaking wave height, which seems to be one of the initial conditions necessary for the cuspate to initiate. Moderate storms enhance the deepening of the embayments, whereas mild sea states are able to maintain the features by self-organization. The possibility of the occurrence of edge waves due to the non-linear interaction between the components of a gravity wave spectrum approaching the shore under moderate storm conditions is explored. Preliminary analysis of video images taken by an Argus station seems to confirm this hypothesis.

Coastal zone management with stochastic multi-criteria analysis

The methodology for coastal management proposed in this study takes into account the physical processes of the coastal system and the stochastic nature of forcing agents. Simulation techniques are used to assess the uncertainty in the performance of a set of predefined management strategies based on different criteria representing the main concerns of interest groups. This statistical information as well as the distribution function that characterizes the uncertainty regarding the preferences of the decision makers is fed into a stochastic multi-criteria acceptability analysis that provides the probability of alternatives obtaining certain ranks and also calculates the preferences of a typical decision maker who supports an alternative. This methodology was applied as a management solution for Playa Granada in the Guadalfeo River Delta (Granada, Spain), where the construction of a dam in the river basin is causing severe erosion. The analysis of shoreline evolution took into account the coupled action of atmosphere, ocean, and land agents and their intrinsic stochastic character. This study considered five different management strategies. The criteria selected for the analysis were the economic benefits for three interest groups: (i) indirect beneficiaries of tourist activities; (ii) beach homeowners; and (iii) the administration. The strategies were ranked according to their effectiveness, and the relative importance given to each criterion was obtained.

Coastal Evolution, Sea Level, and Assessment of Intrinsic Uncertainty

Abstract The climate variability, its implication in coastal processses and the uncertainty that it therefore introduces in the morphological evolution of the coast are addressed. Historical evidences in the Iberian Peninsula allow relating the occurrence of significant variations in sea level position during the Holocene and its effect on the morphology, to natural climate changes. It is still unknown the way that long term climate variability will affect sea level position and the severity of other meteorological agents, which is a source of uncertainty that adds to the stochastic nature of coastal long term proccesses. In a decadal scale, under the assumption that sea level and other parameters that describe the climatic forcing remain stationary, the methodology by Baquerizo and Losada (2008) is used to predict the impact of the construction of a reservoir in the delta of the river Guadalfeo (Spain) and to illustrate how to deal with the uncertainties of the prediction for management purposes. Among other results, it is found that the probability that the shore retreats more than 120 m at any location is about 0.95, which allow to conclude that the construction of the dam will have a severe impact.

Structure of the turbidity field in the Guadalquivir estuary: Analysis of observations and a box model approach

A study is presented on the transport of suspended particulate matter (SPM) in the Guadalquivir estuary during low river flow conditions. Observations show that tidally induced SPM exceeds that associated with catchment-derived inputs. The main mechanisms that contribute to longitudinal transport are identified and quantified by analyzing the tidally averaged and depth-integrated SPM flux decomposition over time and space. The net transport is generally directed upstream, although differences in the direction between spring and neap tides are identified. The transport is largely controlled by the mean advection, the tidal pumping associated with the covariance between SPM concentration and current, and the tidal Stokes transport. The convergence of the transport associated to these mechanisms alone explains the presence of primary and secondary estuarine turbidity maxima. The tidal reflection at the upstream dam appears to play a significant role in their generation, as evidenced by the convergence zones of the M4 and M2 induced tidal pumping transports. The spatial structure of the transport motivates the development of a box model that describes the concentration of SPM and its exchange between different stretches along the estuary at subtidal time scales. The model is fed by the net SPM transport obtained from observations. Analysis of the morphodynamical state of the estuary using the box model indicates that erosion is dominant in the stretches close to the estuary mouth and that this sediment is transported upstream and deposited in the middle part of the estuary. This process is more influential during spring tides than during neap tides.

Edge wave scattering by a coastal structure

The propagation of an incident edge wave on a straight beach with a coastal structure perpendicular to the coastline is analyzed. A simple idealized beach profile with a steep foreshore, a plane slope and a horizontal shelf on an infinitely long straight shoreline is considered. The edge wave is reflected and transmitted at the structure and, by assuming that its width is much smaller than the alongshore wave length, the solution to the problem is obtained by a mode matching method including the head loss at the structure. The existence of the steep foreshore and the shelf modifies the dispersion equation of the trapped modes obtained by Eckart (Wave Report 100, Ref. 51-12, Scripps Institute of Oceanography, University of California, La Jolla, 1951, 99pp). The reflection coefficient, defined by the ratio of the incident and reflected edge wave amplitudes, depends on the friction coefficient at the coastal structure.

Uncertainty assessment: Application to the shoreline

It is impossible to know beforehand the planforms of a stretch of beach without being first aware of the maritime climate affecting it. This article describes a procedure for objectively calculating the uncertainty associated with the prediction of the evolution of a stretch of beach in terms of probability. On the basis of oceanographic data records as well as empirical orthogonal functions (EOF), we propose a procedure for the simulation of possible sequences of storm events. Such sequences were then entered as input for a morphodynamic model with a view to the subsequent generation of possible planforms. EOF methodology was then used to estimate the probability of each of the planforms thus generated. The case study presented here is that of the evolution of an initially straight sand beach where a rectangular tapered fill had been constructed. The beach is located upshore of a groin perpendicular to the coastline, and had blocked all longshore sediment transport. For this analysis we used a one-line model with time-dependent boundary conditions and a non-homogeneous diffusion coefficient.

A global model of a tidal jet including the effects of friction and bottom slope

This paper presents a global model describing the jet plane flow due to a tidal or river discharge. This model characterizes the geometry and the velocity field of the ebb-tidal jet in the whole domain including the effects of friction and bottom slope. The model does not have adjusting coefficients since they are obtained during the integration process. It can be concluded that higher values of the friction coefficient increase the jet width, whereas higher bottom slope reduces the jet width due to the incoming complementary water. Moreover, for small slopes, the area affected by the flow varies, essentially, with the friction coefficient, whereas for higher sloping bottoms this area can be assumed constant.

Frontshear and backshear instabilities of the mean longshore current

An analytical model based on Bowen and Holman [1989] is used to prove the existence of instabilities due to the presence of a second extremum of the background vorticity at the front side of the longshore current. The growth rate of the so-called frontshear waves depends primarily upon the frontshear but also upon the backshear and the maximum and the width of the current. Depending on the values of these parameters, either the frontshear or the backshear instabilities may dominate. Both types of waves have a cross-shore extension of the order of the width of the current, but the frontshear modes are localized closer to the coast than are the backshear modes. Moreover, under certain conditions both unstable waves have similar growth rates with close wave numbers and angular frequencies, leading to the possibility of having modulated shear waves in the alongshore direction. Numerical analysis performed on realistic current profiles confirm the behavior anticipated by the analytical model. The theory has been applied to a current profile fitted to data measured during the 1980 Nearshore Sediment Transport Studies experiment at Leadbetter Beach that has an extremum of background vorticity at the front side of the current. In this case and in agreement with field observations, the model predicts instability, whereas the theory based only on backshear instability failed to do so.

Edge Wave Scattering by Coastal Structures on Arbitrary Bathymetry

Abstract The formation of rhythmic patterns at the Field Research Facility (U.S. Army Corps of Engineers, Duck, North Carolina) under extreme wave climatic conditions is analyzed in terms of the partial standing oscillations formed by the superposition of the incident edge wave and the wave reflected by the structure. The reflection coefficient at the pier is evaluated with an existing model used for the propagation of an edge wave through a permeable structure perpendicular to the coastline, modified here to deal with an arbitrary beach profile. The mass transport patterns under the partial standing edge waves are found to resemble the beach cusps and the crescentic bars observed on the shore, north of the pier, after a series of storm events.