Maria Graça Neves

Designing a Preliminary Multifunctional Artificial Reef to Protect the Portuguese Coast

Abstract Portugal is one of many countries in the world that suffers from coastal erosion. Conventional ways of protecting a coastline appear to entail some disadvantages. An innovative and interesting way of protecting a local coastal zone by means of multifunctional artificial reefs avoids some of them. A multifunctional artificial reef is a submerged breakwater which, besides helping to protect the local coastline, can have other purposes; in particular it may enhance the surfing possibilities and the environmental value of the local area. The structure has several positive side effects: first, it provides an unimpaired visual amenity; second, it offers tourist and economic benefits by improving the surfing conditions. A preliminary design, achieved step-by-step, is proposed for a multifunctional artificial reef making use of the theory and state of the art multifunctional artificial reef design, a preliminary design, achieved step-by-step, a priliminary design is proposed. The proposed reef geometry, together with numerical and physical tests, allows the analysis of a multifunctional reef breakwater designed to protect a stretch of the northwestern coast of Portugal. Taking into account the condition that the proposed geometry will only function properly on a sea slope bottom of less than 1 : 50, the main choices are as follows: the upper part of the structure is delta shaped with an angle of 66° and a side slope of 1 : 10, and the lower part consists of a platform whose slopes are as steep as possible. The position of the reef should be such that the distance from the apex of the structure to the undisturbed shoreline is greater than 1.5 times the natural surf zone width.

Comments on “Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SW Spain” by E. Poizot et al. (2013) Geo-Marine Letters 33(4)

This Discussion provides comments on the application of grain-size trend analysis to Camposoto beach (SW Spain) reported by Poizot et al. (2013) in Geo-Marine Letters 33(4):263–272. Some of their results are updated or complemented by existing data from other studies carried out on Camposoto and other nearby beaches. For example, a detailed breakdown of beach nourishment volumes and costs is presented, and the influence of a horizontal reef flat on the tilting of the beach profile around the mean or the low water level is highlighted. Moreover, data from the displacement of dyed samples are used to evaluate the relationship between sediment transport speed (va) and current speed (V), the corresponding ratio being consistent with the range of values reported by several other authors. Finally, some minor, although still significant, differences are detected in some granulometric parameters as well as in the profile shape. Determining the reason for these discrepancies could enhance our current knowledge about the factors controlling short-term beach profile responses.

Risk assessment in submarine outfall projects: The case of Portugal

Submarine outfalls need to be evaluated as part of an integrated environmental protection system for coastal areas. Although outfalls are tight with the diversity of economic activities along a densely populated coastline being effluent treatment and effluent reuse a sign of economic prosperity, precautions must be taken in the construction of these structures. They must be designed so as to have the least possible impact on the environment and at the same time be economically viable. This paper outlines the initial phases of a risk assessment procedure for submarine outfall projects. This approach includes a cost-benefit analysis in which risks are systematically minimized or eliminated. The methods used in this study also allow for randomness and uncertainty. The input for the analysis is a wide range of information and data concerning the failure probability of outfalls and the consequences of an operational stoppage or failure. As part of this risk assessment, target design levels of reliability, functionality, and operationality were defined for the outfalls. These levels were based on an inventory of risks associated with such construction projects, and thus afforded the possibility of identifying possible failure modes. This assessment procedure was then applied to four case studies in Portugal. The results obtained were the values concerning the useful life of the outfalls at the four sites and their joint probability of failure against the principal failure modes assigned to ultimate and serviceability limit states. Also defined were the minimum operationality of these outfalls, the average number of admissible technical breakdowns, and the maximum allowed duration of a stoppage mode. It was found that these values were in consonance with the nature of the effluent (tourist-related, industrial, or mixed) as well as its importance for the local economy. Even more important, this risk assessment procedure was able to measure the impact of the outfalls on human health and the environment.

INVESTIGATING THE LENGTHS OF SCALE MODEL TESTS TO DETERMINE MEAN WAVE OVERTOPPING DISCHARGES

This paper analyses the influence on the measured mean overtopping discharge of the duration of physical model tests of wave overtopping, bearing in mind the practical purpose of the studies, and the required accuracy of the measurements. The case study of the South Breakwater of Póvoa de Varzim Harbor, Portugal, is used to investigate this subject. During the two-dimensional physical model tests, three main target test conditions were used. For each one, different wave trains were utilized, all conforming to the same target JONSWAP spectrum, and three different test durations were employed. The number of random waves ranged from about 300 to 2400. Then, one of the three main test conditions was again used, but for twelve different test durations. In this case, the number of waves ranged from about 150 to 1900. The results suggest that the convergence of the mean overtopping discharge to a constant value with increasing test duration is not obvious. Regardless of the test duration, the information obtained with a single test gives limited information about the expected mean discharge, as the mean overtopping discharge varies even for the same wave and structure characteristics. Consequently, more information is obtained on the mean discharge if several tests of the same short duration (but with different time series) are undertaken rather than if one test of long duration is carried out.

Wave hydrodynamics around a multi-functional artificial reef at Leirosa

This paper describes an application of the Boussinesq-type COULWAVE model to study the wave hydrodynamics in the vicinity of a multi-functional artificial reef (MFAR). This reef is under investigation and consists of a supplementary protection solution for the Leirosa sand dune system located at South of Figueira da Foz, on the Portuguese West coast. Such installation near the coastline is expected to contribute to enhance the surfing conditions in the area, protect the sand dune system in the surroundings of Leirosa beach, and increase its environmental value. Numerical calculations with the COULWAVE model were performed for four test cases, considering two reef geometries (differing in the reef angle) and two incident wave conditions (storm condition and a common wave condition). Comparisons between the results obtained, in terms of wave heights and breaking line positions allow us to assess the influence of the reef on the hydrodynamics near the beach and around the reef. Moreover, the reef performance was analysed in terms of surfability and coastal protection. The surfability parameters (breaker height, Iribarren number and peel angle) were calculated for each test case using the numerical wave heights, wave directions and wave breaking positions. Comparisons of parameters allow characterizing the most appropriate configuration of the reef to improve the surfing conditions in the study area. A methodology based on numerical free surface elevations and horizontal velocity components was developed to calculate wave directions, since this is not a direct output of the COULWAVE model. Concerning coastal protection, analyses of the mean currents around the reef were used together with observations of the velocity cells near the shoreline as an indication of the sediment transport.

NUMERICAL MODELLING OF BREAKWATER OVERTOPPING USING A NLSW EQUATION MODEL WITH A POROUS LAYER

This paper illustrates the application of a non-linear shallow water numerical model, AMAZON, to study the mean wave overtopping discharge at a breakwater protecting the Portuguese harbour of Povoa de Varzim. The results are compared with two-dimensional physical model data collected at the National Civil Engineering Laboratory, Portugal. The implications are considered of using two different governing equations for flow within the porous layer of the breakwater: firstly, the Darcy equation for stationary laminar flow; secondly, the Forchheimer equation for stationary turbulent flow. The results suggest that the type of flow is of secondary importance in determining the mean overtopping discharge. The parameter which most affects the results is the specified maximum velocity of the flow during the exchange of water between the surface and porous layers. Choosing suitable values of the porous flow parameters leads to a good agreement between the AMAZON results and the data.

Hydrodynamics around an Artificial Surfing Reef at Leirosa, Portugal

As a new alternative countermeasure to protect the coastal zone and to increase the surfing possibilities in the Leirosa area of Portugal, multifunctional artificial reefs were investigated numerically in this paper. The primary surfing parameters used in the design (i.e., breaker type, peel angle, wave height at breaking, and currents around the artificial reef) were analyzed. The reef functionality was also analyzed for coastal protection. Two reef geometries with different reef angles of 45 and 66° were tested, considering two design wave conditions (storm and common) and two tide levels (medium and low). Simulations show that both reef geometries are adequate for surfing, although the reef angle of 66° is more suitable for standard surfers, and the 45° angle is more adequate for advanced/professional surfers. A morphodynamic study should be carried out to analyze the efficiency of the artificial surf reef for coastal protection. DOI: 10.1061/ (ASCE)WW.1943-5460.0000128.

Rubble-Mound Breakwater Inspection in Portugal

In Portugal rubble-mound breakwaters are the most common harbor protection structure. Since 1985, the Portuguese Civil Engineering Laboratory is in charge of the periodic visual inspection of most of these structures. This paper describes the procedures and the results of this work as well as the methodology that is being developed to combine the information from those inspections with that from the surveys of both the above water and submerged parts of the breakwaters in order to get better diagnoses for this kind of structures.

Experimental study of multi-functional artificial reef parameters

Portugal is one of many countries in the world to suffer from coastal erosion. Conventional ways of protecting a coastline appear to entail some disadvantages. An innovative and interesting way of protecting a local coastal zone by means of multi-functional artificial reefs avoids some of them. A multi-functional artificial reef (MFAR) is a submerged breakwater which protects the local coastline and may also enhance the surfing possibilities and the environmental value of the local area. The structure has several positive sideeffects: first, it provides an unimpaired visual amenity; second, it offers tourist and economic benefits by improving the surfing. A 2D physical study is under way at LNEC to investigate the relations between the breaker type and the submergence and the length of a MFAR. Two different geometries were tested for different incident wave characteristics and different water depths. This paper presents the main characteristics of the physical experiments in progress in a wave flume to analyze the influence of the submergence and the length of a MFAR on the wave breaking type. The main preliminary conclusions are that the length of the reef doesn’t have much influence on the breaker type and that the shorter the slope of the reef the more the wave breaks towards the crest.

A 3D Numerical Wave Tank for Coastal Engineering Studies

This paper presents the validation of active and passive, made by a dissipation beach, numerical absorbing methods implemented in RANS-VOF FLUENT® code for modelling long time series of wave propagation interacting with coastal structures. Verification of both numerical techniques was performed in 2D – wave flume, and 3D – wave tank, this one using a multiple active absorption wave makers. The active absorption wave maker allows maintaining the incident wave generation and the mean water level along the time. Good results were obtained for 2D and 3D applications for active absorption wave maker at the generation boundary and both numerical beach and active absorption at the end of the flume/tank.