Eiichi Baba

Effects of Wave Breaking Action on Flows in Tidal-flats

In this paper, we construct a mathematical model for flows above and under the tidal-flat. First we construct a mathematical model for flows above and under the tidal-flat. Then we unify the flow equations defined in different regions into single equation by means of fictitious domain method via singular perturbation. Thanks to the unified equation, numerical simulation becomes much simpler. We obtain several observations from our numerical simulations of wave breaking action on the tidal-flats by use of some extracting techniques. First technique is a time-averaging technique, in which we compute a time-averaged flow field and draw time-averaged streamlines. Time averaged streamlines show how the seawater circulates in the tidal-flat. This phenomenon affects to distribution of silts (small sand particles) in sand, which is important for bacteria to live. Our observation is in good agreement with an experimental observation. Second technique is a streak lines technique, by which we trace the loci of fluid particles. We can see a radial direction flow starting from a source point, which is located in a little bit lower than maximum run-up point. This observation is also in good agreement with the experimental results.

Unified model for wave breaking action and internal flow in a tidal flat

What kind of correlation exists between waves on the beach and tidal flat ecosystems? This chapter clarifies the above problem using mathematical science. Concretely, we simulate phenomena involved in the problem by building a mathematical model.

Theoretical study of oil pollution

We discussed effects of waves on a tidal flat ecosystem in Chap. 3 and understood remarkable works of nature in the neighborhood of the margin. In this chapter, we analyze what effects the intervention of oil gives to the harmonious relation between waves and tidal flat ecosystems.

Oil pollution: human damage on hydraulic regime and benthic communities in tidal flat ecosystems

Waves and tides are two major driving forces to supply seawater into sandy beach and tidal flat sediments. Contrary to the role of waves on seawater infiltration, tides are major force than waves for oil penetration into sediments as shown in Chapter 4. This chapter deals with infiltration of stranded oils into tidal flat sediments and its effects on seawater infiltration.

What is tidal flat

Wave and current energies are important factors in determining the physicochemical and biological conditions in coastal ecosystem. Sandy beach develops in the area of very high physical energies facing open ocean where coarse grained sands make beach and sediments are completely oxidized. On the other hand, tidal flat develops under sluggish currents in river mouth and semi-enclosed coastal area where finer particles are trapped in the flat and sediments show strong reducing conditions sometimes to sediment surface.

Oil pollution: human damages on hydraulic regime in sandy beach ecosystems

Various anthropogenic pollutants ultimately threaten coastal areas. One of the most significant anthropogenic pollution is oil pollution. It is estimated that 1.7- 8.8 X 106 tons of petroleum hydrocarbon are released into marine environment annually (Natural Research Council, 1985). Oil enters marine environment in many ways like natural seeps, refinery emission, ship cleaning operations and accidental spills. In particular, tanker accidents that may result in the release of relatively large amount of oils near sensitive coastal environments are of great concern. For example, the Amoco Cadiz discharged 0.2 X 106 tons of Kuwait crude oil into the waters along the Brittany coast in March 1978; the Exxon Valdez released 0.04 X 106 tons of Alaskan North Slope crude oil into Prince William Sound in March 1989. A part of the spilled oil into sea has drifted to the coastal area. About 30% of the Amoco Cadiz spilled oil contaminated 320 km of coastline. In case of the Exxon Valdez, about 50% of the spilled oil stranded on 2,000 km of shorelines along the Gulf of Alaska (Swannell et al., 1996; Wolfe et al., 1994; Vandermeulen et al., 1979).

Decomposition mechanism of spilled oil by bacteria

The unified model formulations developed in Chapters 3 and 5 make it possible to clarify mechanisms of oil decomposition by adding crucial devices to them; this has not been experimentally clarified yet. Multi-phase flow formulation with decomposition contributes to this study. Main components of decomposed materials by bacteria are water, soluble components, and carbon oxides. We deal with the case in which oil decomposes into water. A characteristic of this case is appearance of a jump boundary condition for normal component of velocity at the decomposition surface between oil and water, which is based on density difference between them.

Methodologies for theoretical studies

In this chapter, we explain the fundamental methodologies for theoretical studies stated in Chaps. 3, 5, 7 and 8, which correspond to facilities utilized in experimental studies in Chaps. 1,2,4 and 6. These methodologies consist of a mathematical part and a numerical one, which have been developed by the authors to promote the studies concerned. These methodologies are combined to obtain fruitful results in the computer which can be observed in the form of the many figures included in this book. They have contributed to deeper understanding of remarkable works of nature at environmental margins.

Effects of waves and tide on tidal flat ecosystems

Coming wave to coastal area has an essential effect on ecosystems. In shallow water region such as sandy beach and tidal flats, wave run-ups due to breaking waves, whose height are around 5 cm to 10 cm, are often observed at the slope. In addition to the tidal motion, these small waves may affect water flow inside the seabed. Internal flow in the seabed has close relation with biological activities in these areas. Seawater is a kind of transport medium for oxygen and nutrients. As stated in Chap. 1, the number of bacteria inside the seabed is known to closely correlate with the silt content because the presence of such small partic1es increases the wet surface area for bacteria habitation. The small particles easily move along with the flow. Therefore, to determine the internal flow characteristics of tidal flat is important for understanding of its ecological role. Studies on seawater transport in the surf zone have been carried out mostly for large waves of several meters: (Riedel 1971; McLachlan 1982). However, studies for small waves of several cm high are scarce, hitherto.

Breaking waves and ecosystem dynamics

The role of waves at margins of a sandy beach or a tidal flat to sea water for determination of species survival in a coastal ecosystem was discussed in Section 2.3.3 from a viewpoint of environmental experiments. This means that different kinds of life appear under different habitat conditions such as with waves and without waves, i.e., the wave at the margin constitutes an important factor for determination of species composing coastal ecosystems.