Incorporating a risk assessment procedure into submarine outfall projects and application to Portuguese case studies

Resumo

Coastal waters are an integral part of the natural environment. Careful planning and management is needed to protect and conserve them, and to ensure that the water supply is useful for a variety of uses. The project of submarine outfalls is a complex problem for solving because equal significance should be given to the environment, economy and social aspect of the problem. Moreover, according to the new paradigm of water pollution, water quality is closely connected to aquatic ecological and biological characteristics. This is reflected in the new European Union Water Framework Directive (EU WFD 2000/60), where the ecological health of aquatic ecosystems is described not only in terms of the concentration of specific physico-chemical substances but also by biological indices indicating the status of the aquatic ecosystems. The above means that, when designing a submarine outfall, solutions must be economically acceptable, both for population and stakeholders, and should contribute to the improvement of environmental protection and sustainability. The solutions should also be flexible enough to be constantly upgraded and improved in order to fulfill expected environment protection requirements. Submarine outfall projects generally include specifications pertaining to the conception, design, construction, exploitation, maintenance, and repair of the outfall. Nevertheless, they rarely include a systematic assessment of risks. This signifies that the design methods used are essentially deterministic. A common step of a deterministic or a risk design approach for submarine outfalls, as well as other maritime structure, is the definition of the possible failure modes. The verification of the structure against failure and operational stoppage modes, as well as the estimation of the joint probability of failure of the subset of the structure during each of the project phase should be estimated. The aim of this work is the development of an application of probabilistic and optimization methods in the context of a risk management approach to the project of submarine outfalls concerning outfall exploitation (discharge, dispersion and pollutant transport). The risk assessment method developed aims to specify the probability that the outfall fails or stops operating, stating the possible consequences of such a failure or stoppage to populations and environment. For the verification of failure and operational stoppage modes probabilistic Level III verification methods are proposed. The first step of the study was the development of an engineering procedure, adapted from the Spanish Recommendations for maritime structures, ROM 0.0, for the specifications of requirements and target design levels of submarine outfall projects focusing on their influence on the environment, economy and served populations. The procedure for calculating target design levels determines if a project satisfies the safety, serviceability, and exploitation requirements for the recommended levels of reliability, functionality, and operationality during all of the project phase. The identification of these design levels makes it possible to estimate the useful life of the structure, the maximum admissible joint probability of failure against the principal failure modes, the minimum operationality, the admissible average number of technical breakdowns and the maximum admissible duration of an operational stoppage. The engineering procedure developed for the specification of requirements and target design levels of submarine outfall projects is supported and bound to next step of the study: the development of a risk assessment procedure for operational failure estimation and application to project design alternatives. The procedure aims to verify if the proposed design alternatives for a submarine outfall satisfies the design target levels dependent of the operational intrinsic nature of the structure. The risk assessment procedure relies on three main topics: environmental legislative framework, climate agents on the coastline and effluent fate and distribution. The probability of occurrence of failure in the useful life is calculated by applying Level III Verification Methods (Monte Carlo simulations) using the methodology developed by Solari and Losada (2013).The results help identifying the structure¿s probability of failure or stoppage and the definition of operational target design levels enabling a multi-criteria and an adaptive design of these structures assuring that they will remain operational during their useful life. Moreover, in order to achieve appropriate discharge management, the authorities and the entities that are developing and managing submarine outfall installations should be provided with appropriate tools to improve discharge efficiency and to increase the effectiveness of effluent dilution into the sea. An operational forecast methodology is here proposed for the management of submarine outfalls providing information to deal with the marine environment problems and to satisfy needs for coastal communities. The risk assessment procedure, described above, is adapted from a management perspective to support decision making: short-term forecasts of maritime climate and hydrological conditions along with foreseen effluent characteristics (depending on seasonality and population) are used to predict the plume behaviour near the coastline. The methodology aims at contributing to an adaptive management in the operationality of these structures and, when fully developed, can also be used as a powerful alert and information tool for companies operating the installations and the pertinent environmental authorities. The last step of the methodology is the development of a risk assessment procedure for aquatic systems induced by submarine outfalls. The objective is to incorporate marine biodiversity life cycles in the design and management of submarine outfalls offering an understanding of stressor levels that can cause significant impact on marine communities and a more rigorous basis for these structures projects preserving marine resources and effectively conserve coastal biodiversity. The above is accomplished with the development of an encounter-probabilistic model, based on Reynolds transport theorem, to estimate residence times of marine species in effluent plumes and the respective number of individuals, helping answer the broader question of how marine species are influenced by environmental perturbations. The overall methodology aims to provide the first step of a rational and systematic procedure for automatic and optimal design of submarine outfalls granting a cost optimization of this type of projects, reducing accidents and their environmental dramatic consequences.