César Vidal

The Prestige Oil Spill in Cantabria (Bay of Biscay). Part I: Operational Forecasting System for Quick Response, Risk Assessment, and Protection of Natural Resources

Abstract In this paper, we present the operational forecasting system developed to assist in the response to the 2002 Prestige oil spill in Cantabria. The objective of the system developed was to forecast the wave climate, tidal and wind currents, and oil spill trajectories to provide a technical assessment to decision makers for a response to the oil spill. The two main components of the system were data collection and processing and integration with numerical models for forecasting. The information from overflights received daily became essential in achieving a correct initial position of the oil slicks. Meteorological and oceanographic data were also received daily by means of an emergency protocol established between Puertos del Estado (Spain), the Naval Research Laboratory (USA), and the University of Cantabria (Spain). These data were used to run the trajectory model, the wave propagation model, and the shallow depth-integrated flow model. The information generated by the numerical simulations was presented to the decision makers every day in the form of maps that were easy and quick to interpretation as a tool to help in the response planning. In addition, to develop a defensive or protection strategy for sensitive areas like estuaries and marshes, a hydrodynamic study was carried out by the University of Cantabria in all the estuaries of the region. The result of this study consisted of a boom deployment plan for each.

A morphological model of the beach profile integrating wave and tidal influences

Abstract Meso- and macrotidal beaches constitute a significant proportion of the world’s beaches. However, by far, they have been less studied and understood than tideless beaches. Even though in recent times several local studies have addressed this problem, few attempts to model tidal beaches exist. A morphological model capable of predicting the beach profile behaviour under different wave and tide conditions is proposed. It is based on the concept of the two-section equilibrium beach profile, and has been validated with field and laboratory data. This is achieved by means of two parameters: the modal tidal range and the dimensionless fall velocity. Tide is considered a local variable whose principal effect is the lengthening of the intertidal profile. The greater the tidal range, the wider the intertidal profile, here defined as the surf profile. The dimensionless fall velocity defines the transition from dissipative to reflective situations in beaches of any given tidal range. Wave height is the controlling parameter in seasonal beach changes: as the wave height decreases, the beach profile changes from erosive to accumulative. These morphological changes in the surf and shoaling sections of the profile occur in the opposite direction. Whilst in the surf profile the slope of the upper part of this section becomes steeper and the concavity of whole section increases; in the shoaling profile, the upper part flattens resulting in a less concave section. In this transition, the slope break between surf and shoaling profiles, here defined as the discontinuity point, becomes smoother and difficult to identify. The main morphological parameter of the model, xo, describes the length of the surf profile. This parameter is capable of expressing slightly different variability in micro-, meso- and macrotidal beaches. In microtidal beaches, the length of the surf profile decreases proportionally to the dimensionless fall velocity; in meso- and macrotidal beaches xo decreases from the dissipative to the intermediate state, but increases from intermediate to reflective. This results from the flattening of the lower part of the surf profile in the reflective case, where a small change in tidal range generates a high stretching of the surf profile. This beach morphological model is presented as a framework to understand the first-order behaviour of beaches under the action of waves and tides. This becomes a useful and easy-to-apply tool in coastal management and prediction of equilibrium beach profile under diverse conditions.

Temporal and spatial relationship between sediment grain size and beach profile

Abstract Sediment samples and beach profile evolution data collected along one profile line at “El Puntal” Spit, Santander, Spain, are used to analyze the spatial and temporal structure of the grain size distribution variability and its relationship with the beach profile changes. Standard principal component analysis (PCA) and three-way PCA is applied to determine the temporal and spatial scales of variability of the data. Results indicate that the sediment grain size distribution varies markedly along the beach profile both spatially and temporally. These variations are shown to be strongly related to morphological changes in the beach profile. The spatial eigenvectors determined from the profile data and those from the sediment data exhibit similar patterns with their maxima and minima located at the same position. Since eigenvectors may be regarded as representative of uncorrelated modes of variability it is concluded that the spatial variability of both sediment and profile data are strongly related. In particular, it is shown that the location of the highest variability of grain size corresponds to that of the beach profile. Also, different grain sizes are shown to exhibit a distinct degree of variability which leads to the conclusion that each sediment size responds to the same hydrodynamics differently. The temporal eigenvectors determined from the profile and the sediment data shown a seasonal dependency. However, their maxima and minima are not located at the same position. It is shown that this temporal shift is due to the different response of each sediment size to the hydrodynamics, and in particular, that the recovery of the profile starts with fine material from the bar. It is inferred that models for beach profile evolution which do not take into account the sorting processes involved in the sediment transport cannot be fully succesful. A “master” grain size sample, constructed by adding all the grain samples taken over the profile, is used to further examine the cross-shore redistribution of the sediment. The following working hypothesis is suggested: “For a beach profile within a physiographic unit the master grain size does not depend on time”.

Wave reflection on natural beaches: an equilibrium beach profile model

Waves are the most energetic phenomena that control beach morphology. The beach profile mostly depends on the way in which the incident wave energy distributes along the profile, dissipation and reflection being the main mechanisms. While the dissipation phenomena have been widely studied, the effect of wave reflection on the beach profile has attracted much less attention and is still poorly known. In order to evaluate its importance, a new equilibrium profile model that includes reflection is proposed. The model is based on a two-section profile scheme, largely corresponding to the surf and shoaling-dominated zones of the beach profile. The obtained formulations are represented by the expression of two terms. One of the terms accounts for the dissipation effect and coincides with the Dean profile. The other term integrates the reflection process. The model and its coefficients have been calibrated using measured profiles along the Spanish coast. The validation shows a significant improvement of the fitting parameters with respect to the most popular equilibrium profiles model. Moreover, additional empirical expressions that relate morphology and hydrodynamic in the equilibrium profile model are also presented in this study as a novel contribution to this topic.

Modelling of wave loads and hydraulic performance of vertical permeable structures

Abstract Using an eigenfunction expansion method, the potential flow around and inside vertical porous breakwaters has been solved for regular waves. The extension to irregular waves is indicated in this paper. Analytical expressions for functional performance variables (reflection, transmission and dissipation) and for stability (horizontal and vertical forces, including the corresponding overturning moments) have been obtained. These expressions have been numerically exploited to demonstrate the capability of the model for design purposes. The parameters required in the porous flow model are expressed in terms of the porous material characteristics and have been investigated experimentally. Some additional experiments using a vertical permeable structure with an impermeable backwall are carried out to validate the model.

Relationship between Beach Morphodynamics and Equilibrium Profiles

In this paper a new beach equilibrium formulation, that treats the shoaling portion of the profile independently from that of the breaking portion, is proposed. The two portions are matched at the breaking point. The formulation considers wave reflection from the beach and the tidal variation of the sea level. Both profiles are represented by a similar expression that depends on two parameters: A and C, which account for wave dissipation and B and D, which account for beach reflection. The parameters are then calibrated using over 50 profiles from 13 beaches along the Spanish Coast.

Armor Damage Analysis Using Neural Networks

This paper compares to methods to estimate the rubble-mound breakwater armor damage evolution in nonstationary wave conditions. The first method is based on the exponential model on individual waves proposed by Medina (1996). The second method is based on Neural Networks pruned using Evolutionary Strategies. Both methods give reasonably good agreement with laboratory observations.

Standing edge waves on a pocket beach

Field measurements of edge waves obtained on a narrow pocket beach are described. The beach (named Usgo) is located on the north coast of Spain immediately to the west of the city of Santander and is exposed to the Atlantic Ocean. The edge wave field on this beach is analyzed utilizing data from a longshore array of current meters located in about 3 m of water depth. Frequency spectra of longshore velocities display several significant peaks, suggesting that the edge wave field on this pocket beach consists of several dominant modes. This is in contrast to observations on open coastal beaches [e.g., Oltman-Shay and Guza, 1987], which show the existence of a broad-banded edge wave field. We isolate four frequency peaks and examine the longshore variation of the motions at these frequencies and find convincing evidence of a nodal structure. Utilizing the measured bathymetry, we compute the cross-shore profiles and theoretical wavelengths of edge waves at the dominant frequencies using a numerical solution of the shallow water equations by Howd et al. [1992]. We find that edge waves at these frequencies are theoretically confined between the headlands since their amplitudes decay to insignificant levels within a cross-shore distance that is less than the offshore extent of the headlands. In addition, assuming perfect sidewall reflections, the theoretical nodal locations closely correspond to those inferred from the measurements. We conclude that discrete standing edge waves resulting from resonances related to the longshore width of the beach are present on Usgo Beach.

PROTOTYPE ANALYSIS OF STABILITY OF RUBBLE MOUND PROTECTIONS FOR SUBMARINE OUTFALLS

This paper presents the construction, surveying and results of a prototype experiment on stability of rubble mound protection for submarine outfalls. Taking advantage of the Santander outfall construction, financial support from the European Community was obtained to carry out the experiment that consisted of covering the outer layer of the outfall’s protection with two extra layers of rubble with a stone weight smaller than that of the project. Three stone weights were tested in three different stretches of the outfall. The experiment lasted two years, during which waves and stone movements were surveyed. Damage and wave data were analyzed and the damage parameter is presented here as a function of the mobility parameter. To take into account the random characteristics of waves in the sea states, a Montecarlo simulation is used here to calculate the mobility parameters corresponding to all the waves of all the sea states that reached the experimental sections. The use of the average of the 50 biggest mobility parameters that reached the test sections at the survey date is proposed here to represent damage results against the mobility parameter. Using this mobility parameter, prototype results compare well with laboratory experiments carried out with regular waves. INTRODUCTION Submarine outfalls for wastewater are economical alternatives in coastal areas to wastewater treatment plants. In coastal areas of high wave energy like the northwest of Spain and in those sites were the bottom characteristics do not allow the burial of the pipe, the outfalls are protected by several layers of rubble. 1 Associate Professor, University of Cantabria, Avda. de los Castros, s/n 39005 Santander, Spain. vidalc@unican.es 2 Assoc. Researcher, University of Cantabria, Avda. de los Castros, s/n 39005 Santander, Spain. lomonacop@unican.es 3 Associate Professor, University of Cantabria, Avda. de los Castros, s/n 39005 Santander, Spain. martinfl@unican.es Vidal, Lomonaco, and Martin 1 The existing methods for the assessment of the armor layer stability against wave action are based on extrapolations of formulas developed for submerged breakwaters or for the determination of sediment transport on horizontal bottoms. The existing uncertainty about their applicability to the deep outfall protections may result in conservative, oversized designs. During the 1990’s, the application in Spain of the European Union regulations for wastewater emissions generated a high number of outfall projects for coastal cities in the North Atlantic waters of Spain. Most of these outfalls were designed with rubble protections. The application of the existing formulas showed great differences among the prescribed weights of the stones required in those sections of the outfall where the water was deep enough to prevent the rubble from the direct action of the breaking waves. To cover this lack of stability formulations, the Ocean & Coastal Research Group (hereafter GIOC) of the University of Cantabria started in 1997 several R&D Projects founded by the Spanish Commission for Science and Technology (CICYT) and by the European Founds for Regional Development (FEDER). After extensive experimental work in wave tank, a methodology for the assessment of the stability of near bed rubble protections, based on the mobility parameter, was presented in Vidal et al. (1999). In 1999, the GIOC obtained FEDER founding to carry out the construction and survey of several prototype test sections over the outfall of Santander that was being built at that time. The outfall’s building company, Dragados y Construcciones, S.A, collaborated in the Project, building these test sections after the main protection was completed. The prototype experiments started in September 1999 and ended in August 2001. The objective of the test was to measure the stone movement due to wave action. This paper presents the prototype experiment and their results. SITE DESCRIPTION AND WORK PLAN The prototype experiment was designed to take place over the outfall of Santander that extends 2 Km into the Cantabrian sea from the cliffs of La Virgen del Mar (6 Km to the northwest o the city) up to 40 m water depth, were the diffusion section is located, see figure 1. The pipe is buried in the rocky bottom at a 15-m water depth. Deeper than this, the pipe lays over the bottom, protected by a core of 5 cm gravel, a filter of two layers of 200-300 Kg of quarry stones, and an armor of two layers of 3,000 Kg quarry stones, see figure 2. Vidal, Lomonaco, and Martin 2 ATLANTIC OCEAN PENAS EMOD Diffusion section REMRO Test section Santander Figure 1: Location of test section and wave buoys. The prototype experiment was planned to start just after the protection of the pipe was completed. Test sections were built using the same equipment and type of rock used by the building company.

STABILITY OF NEAR-BED RUBBLE-MOUND STRUCTURES

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.