**5. Conclusions**

In this study, the combined Bragg reflections of oblique water waves by periodic surface-piercing and submerged breakwaters were solved using the eigenfunction matching method (EMM). In the solution procedure, the surface-piercing structures and bottom profiles are sliced into a number of shelves separated by abrupt steps. The solution on each shelf is composed of eigenfunctions with unknown coefficients. Then, a system of linear equations is obtained by applying the conservations of mass and momentum. The present method was validated under three problems of oblique wave scattering by a rectangular surface-piercing structure over a flat bottom, behind a parabolic breakwater, and Bragg reflection by periodic rectangular/triangular surface-piercing structures over a flat bottom. Then, the proposed method was applied to solve the Bragg reflections of oblique water waves by periodic surface-piercing structures over periodic bottoms. For oblique Bragg reflections by periodic surface-piercing structures, numerical results indicated that the case with a larger incidence angle results in a more intensive Bragg reflection and the secondary resonance is stronger than the primary resonance. However, the results are completely opposite for oblique Bragg reflections by periodic submerged breakwaters. In addition, Bragg's law of oblique waves was applied in the numerical experiments. Some theoretical and physical studies on the resonance intensity and incidence angle are required in future investigation.

**Author Contributions:** Conceptualization, I.-F.T. and C.-C.T.; methodology, C.-C.T.; software, C.-C.T.; validation, C.-S.Y. and C.-C.T.; formal analysis, C.-S.Y.; investigation, C.-S.Y.; resources, I.-F.T. and C.-C.T.; data curation, C.-S.Y.; writing—original draft preparation, I.-F.T. and C.-S.Y.; writing—review and editing, I.-F.T. and C.-C.T.; *J. Mar. Sci. Eng.* **2020**, *8*, 522

visualization, C.-S.Y.; supervision, I.-F.T. and C.-C.T.; project administration, I.-F.T. and C.-C.T.; funding acquisition, I.-F.T. and C.-C.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** Ministry of Science and Technology, Taiwan: 107-2221-E-992-045-MY2.

**Acknowledgments:** The Ministry of Science and Technology of Taiwan is gratefully acknowledged for providing financial support to carry out the present work under the Grant No. MOST 107-2221-E-992-045-MY2.

**Conflicts of Interest:** The authors declare no conflicts of interest exist.


**2. Effects of oil spills to the water environment**

Oil and petroleum substances represent the basic raw material for the needs of industry, and their impact on all components of the environment is not negligible [8]. The consequences of environmental pollution with oil substances can appear immediately or after a longer period of time.

An oil spill usually affects the entire affected region, destroys the aquatic environment and life in it, and if it reaches the coast, great damage appears there as well (**Figure 1**).

**Figure 1.** *Deepwater Horizon oil spill on May 12, 2010 [9].*

*Possible Oil Spills Disposal for Environmental Water-Body Protection DOI: http://dx.doi.org/10.5772/intechopen.107106*

**Figure 2.** *Oiled bird (© Guardian Unlimited) and oiled seal (© Tom Loughlin, NOAA) [10].*

Oil spills directly or indirectly kill fish and aquatic organisms, birds, plants, affect the oxygen regime, disrupt the natural cycle of ecosystems, and change the physical and chemical properties of the aquatic environment (**Figure 2**).

### **2.1 Mechanism of oil spill pollution in water bodies**

When oil leaks into an open water body, it forms a thin layer on the water surface. In the case of strong winds and strong currents, the oil layer can quickly cover a relatively large area. For example, 1 ton of oil can cover an area of up to 12 km2 with a 1 mm thick layer [11]. The created layer is affected by various factors such as wind, waves, water currents, UV light, the presence of various types of microorganisms, and the oil weathering process occurs [11–13]. Weathering is a process in which leaked oil is gradually lost from the formatted spill depending on the current environmental conditions. The weathering process can be understood as a combination of various physical and chemical changes such as evaporation, dispersion, emulsification, sedimentation, oil entrainment to the water column, biodegradation [11–13]. The study in the year 1992 in the Exxon Valdez accident area shows that 20% of oil evaporated, 50% biodegraded, 14% was cleaned up, 13% remained in subtidal sediments, 2% remained on shorelines, and less than 1% remained in the water [1, 14].

The speed of the weathering process significantly depends on the strength and cohesion of the oil film on the surface. Yan et al. [12] studied the forces affecting the entrainment ability of petroleum substances into the water column. According to them, the difference between the surface tension of water and oil and the viscosity of oil are decisive. Waves on the water surface act on the oil layer, which "break" it. A single strong wave can work, but weaker repeated waves are also effective. A "sandwiching" of water and oil occurs, the oil film becomes thinner until it disintegrates [12, 13].

Oil pollution of seawater also affects the formation and properties of marine snow [15]. After the Deepwater Horizon accident (2010), a high formation of marine snow was observed in the Gulf of Mexico. Passow et al. [15] assume three possible ways of formation of marine snow: increased production of mucous webs by oil-degrading bacteria combined with floating oil particles on the surface; coagulation of oil particles produced by the interaction of oil with sedimenting particles; coagulation of phytoplankton with oil droplets into larger aggregates. They also pointed out that oil significantly affects the sinking and breaking up of marine snow, while environmental conditions also interact. Snow in the natural environment of the Gulf of Mexico, composed mostly of oil particles and low-density mucus, disappeared in about a

month after its appearance, while under laboratory storage conditions in the dark and at about 4°C, it remained on the surface for up to 2 years [15].

### **2.2 Oil substances in the water environment**

The change in physical and chemical properties is significantly influenced by free oil substances that create an oil film on the surface, which reduces the transfer of oxygen to the water. Another process in which oxygen is pumped out of the water is the ongoing microbial oxidation of oil pollution. The result of both processes is a change in the concentration of dissolved oxygen, which causes a change in the course of chemical reactions in the process of photosynthesis, which is very unfavorable for the life of aquatic organisms, algae and other plankton are strongly affected [16].

Chemical composition of oil is very variable, present are straight and branched alkanes, alkenes, aromatic compounds, oxygen, and sulfur compounds and in small amount also nitrogen compounds and some metals. Polycyclic aromatic hydrocarbons have a significant presence, which have toxic, carcinogenic, and mutagenic effects. The easier ones remain in the oil film on the surface or are dispersed in the water column, the more difficult to settle in the sediments, and represent long-term hidden dangers. It is important to know the localization and persistence of these substances in nature [16, 17]. When analyzing the composition, it is possible to focus on some specific substances or to monitor the entire group according to normative methods, for example, PAHs or nonpolar extractable substances (NES) [16].

The acceptable concentration limit of these compounds can be determined using bioassays on appropriately selected sensitive organisms. Ecotoxicological studies have shown the impact of harmful effects of chemical substances on living organisms [18] and are one of the main tools suitable for assessing the effects of specific chemical compounds on environmental components. For acute and chronic toxicity tests, test organisms such as fish, daphnia, rats, birds, and seeds are suitable choices [19]. Due to their low price and good sensitivity (germination), the seeds of suitable plants are important toxicological tests for assessing the effects of toxic substances or organic inhibitors [20]. Tamada [21] monitored biodegradability by respirometric tests and compared different levels of toxicity of lubricating oils using toxicological tests. Tests were performed using earthworms (*Eisenia andrei*), arugula seed (*Eruca sativa*), and lettuce seed (*Lactuca sativa*) in mineral, synthetic, and used lubricating oil for different periods of their biodegradation in soil. Toxicity tests were used to indirectly measure the biodegradation of pollutants. The used lubricating oil proved to be the most toxic. Mineral and synthetic oils were efficiently metabolized in the soil, although they were still toxic after 180 days [21]. In their work, Cecutti and Agius [22] present the results of a study in which they successfully applied test organisms such as algae, pearl oysters, and fish to assess the ecotoxicological properties of various oils, including bio-oils new—before use and used—after 1000 hours of use in the aquatic environment. Bordulina [23] report the results of laboratory experiments, where they studied the effect of different concentrations of oil in the water environment on the algae *Chlorella vulgaris* Beijer and the abalone *Daphnia magna* Straus in model samples. They found that as the concentration of the oil increased, the survival of the test organisms decreased. Martinez [24] found acute toxic damage to daphnia after a 48-hour test. Authors, Bordulina [23] and Hybská et al. [25], confirmed that as the concentration of oil in the water increases, the survival rate of abalone decreases. They found that at a concentration of 0.2 mg of oil/1 l of water, regardless of its

origin (mineral and bio oil—sunflower oil), there was no difference in the number of immobilized individuals [24, 26].