Coastal morphology changes are commonly experienced phenomena. While several coastal countermeasures are applied along the coasts to control the morphological changes, the interference of these human activities upon the coasts might make it hard to achieve its perceived goals. In order to prepare an overall and sustainable strategy for the coastal management, reliable long-term shoreline change data and forecasting of the future changes are required. Investigations of historical coastal changes are essential to understand the physical mechanisms of the changes and to predict the future shoreline evolutions. It is thus crucial and worthwhile to quantitatively estimate the areas of erosion and accretion, sediment sources, sediment transport rates, and sediment characteristics along the coast.

The general aim of this research is to study the long-term shoreline changes through the analysis of shoreline locations, investigations of sand materials, and application of numerical models. As a case study site, this research focuses on the Nile Delta, where is suffering large scale and long term coastal erosion. The study site has 250 km stretch of the coast line from Iduku lake in the west end to Port-Said in the east and faces to the Egyptian Mediterranean Sea. To fully capture the physical characteristics of the coastal dynamics, this study applied multiple different methodologies: analysis of the land-sat images, investigations of sampled sand grains around the swash zone, and the numerical analysis based on the shoreline model.

First, the past coastal line changes were quantitatively monitored around the study site. The coastal lines were automatically extracted based on the local XY-coordinates from Land-sat satellite images over 37 years from 1973 to 2010 with unequal time intervals. The extracted coastal lines were quantitatively investigated based on the linear regression technique as well as the empirical orthogonal function analysis.

Second, a field survey was conducted in March 2011. Sand samples were collected at more than 60 various locations along the shoreline. Samples were collected from the layer at 5-10 cm under the ground surface in the inter-tidal zone. These samples were used for the thermo-luminescence (TL) test. This study measured TL intensity of the feldspar extracted from the sand samples. Consistency between the results of this method with the interpretations based on the analysis of land-sat images was achieved. Using the same samples, sand characteristics were further investigated. The grain size distribution; the median sand size, D50 was measured using the laser diffraction particle size distribution measuring apparatus. The color of sand grains was also identified by naked eyes and the mineral composition based on color was interpreted using the automated analysis of images captured by a digital microscope.

Last, shoreline change model was applied to reasonably explain physical procedures of the observed shoreline changes, accounting for sediment size distributions. The calibrated model is then used for predictions of the future shoreline changes accounting for different climate scenarios. The present model accounted for the quantitative characteristics of coastal structures and river mouth. The improved numerical model simulates the wave transformation, regional sediment transport, sediment size, and shoreline changes. The model composed of two parts, Energy Balance Equation model and One-line model. Due to the high curvature of the shoreline along the study site, the One-line model was formulated in terms of local coordinates normal to and tangential to the actual shoreline. Based on Kumada et al. (2002), alongshore distributions of the sediment grain sizes were also computed and compared with the observed results.

Approximately 46% of the Nile Delta Coast (NDC) experienced erosion while 52% undergo some accretion. The general features along the NDC are the erosion at the three headlands of Rosetta, Burullus and Damietta, and the accretion for the embayment, near the structures, and the sand spit (Eastward Damietta).

As a result of the TL test, it was found that the sand grains near the Rosetta and Damietta branches present a higher TL signal and TL intensity gradually decreases with increasing distance from the two river mouths, which indicates sediment alongshore transport features. The small difference of TL intensities between the sand grains near the river mouth and the ones on the natural coast indicates that the sediment supply from the river is limited. The estimated sediment transport directions based on the TL analysis are consistent with the interpretations based on the analysis of land-sat images around Rosetta promontory while these two estimations were contrary to each other on the west side of Damietta promontory. Observed inconsistency may be due to fluctuating errors of TL measurements

Through the analysis of sand characteristics, clear difference was noticed by the naked eyes in the sand color, and size between different areas. Around Rosetta promontory the sand color is light and average size is 0.28 mm, while in the middle part of the Delta the color is lighter and grain sizes are coarser with average size of 0.6mm, and from Damietta to Port Said the color is darker and average grain size is 0.23mm. The sediment comes from the river contains blackish sand with small size and other colors with bigger size. The dark sand is dominant in the sand dunes. The sand samples near the large traps i.e., Idku jetty, El Burullus jetty, and Damietta breakwater are characterized by dark color. This illuminates that the dark grains is smaller than other grains which moves easily and accumulated near the coastal structure.

The improved model was calibrated and validated against the data from Land-Sat images along the NDC in Egypt. The model successfully simulates the general features along the NDC. The model exhibits its ability in the prediction of the shoreline around Rosetta promontory before and after placing the countermeasure structures. The correlation between the numerical model simulation and the Land-Sat image data was more than 90%. The model was used to check the importance of placing the current coastal structures around Rosetta mouth by simulating the shoreline changes with and without structures. Severe erosion was estimated in the case if there are no structures around the Rosetta mouth. By comparing the grain size distribution around Rosetta mouth in 1988 with the size in 2012, it is noted that the grain size become coarser due to the erosion. The model succeeded to qualitatively simulate such change of the grain size around the Rosetta mouth.

海岸侵食問題は、局所的な対策では恒久的な解決に至らないため、長期的かつ広域的な漂砂移動特性の把握が必要不可欠となる。そのためには、モニタリングが重要となるが、従来の深浅測量のような直接的な測量や現地観測には費用や時間などの制約もある。

本研究では、ナイル川河口デルタを対象地点として、衛星画像や海岸を構成する砂粒子の観察・分析、さらにこれらの観察・分析結果の数値予測解析を通じて、ナイル川デルタにおける漂砂特性の解明を試みた。対象領域はアスワンハイダムの建設に伴う慢性的で深刻な海岸侵食問題に直面しており、また、延長250kmの広域デルタを分析するため、本研究で構築する一連の分析手法の適用性を検証するのにも適していると言える。

まず、1973年以降2010年までの期間を対象に様々な衛星画像を収集し、汀線位置を自動的に抽出して平面直角座標上で比較した。さらに抽出した汀線位置に基づき、経験的固有関数法などを用いた汀線変化のトレンドを客観的に抽出した。その結果、長期的には沿岸方向への土砂移動のモードが卓越的であることが確認され、特に現在の固定された河口部において顕著な侵食傾向が見られるのに対し、突堤や護岸、離岸堤等の海岸防護構造物周辺部などでは、汀線後退量が減少傾向であることなどが分かった。

またナイル川デルタにおいて現地観測を実施し、海岸や河口部における60地点で表層砂のサンプリングを実施した。抽出したサンプルに対し熱ルミネッセンスを計測した。熱ルミネッセンスは河口部から沿岸方向に徐々に減少する傾向が見られ、衛星画像に基づく汀線位置変化の分析結果と符合する結果が得られた。しかしながら、河口部と沿岸部における熱ルミネッセンスの違いは、先行研究による他の地点での熱ルミネッセンスの分析結果と比べる小さく、河口からの土砂供給量が限定的であることも示唆された。さらに、抽出したサンプルの粒度や個々の砂粒子の色の地点による違いを分析した。その結果、河口部からの供給土砂は褐色で粒径の小さい粒子が卓越的であるのに対し、沿岸部の侵食が進行している地点におけるサンプル砂は明るく粒径の大きい粒子が卓越的であることが分かった。さらに河口から離れた地点であっても沿岸部の堆砂促進構造物がある地点の砂粒子は、河口部の土砂と同様の傾向を示しており、粒径の小さい河口からの土砂は優先的に運ばれ、構造物背後に堆積する傾向があることが分かった。

最後に、粒径や比重の違いによる粒子別移動量を考慮することが可能な汀線変化モデルを構築し、ナイル川デルタに適用した。その結果、河口からの粒径の小さな土砂が沿岸部を優先的に輸送され、構造物背後で堆積する過程や、侵食が進行する沿岸部では、粒径の小さい砂がすぐに輸送されてしまう傾向など、サンプル砂の分析結果と符合する再現計算結果が得られた。

以上の研究成果のうち、特に現地サンプルの収集から分析、結果の考察に関わる部分は本研究の根幹をなす部分であり、粒径分布や汀線変化の分析に加えて、熱ルミネッセンスや鉱物組成の違い(粒子色)、数値モデルによる補完的考察などを行った点については、本論文におけるアーメドモスタファ氏による研究の独自性・優位性が大いに認められる。またこれらの研究成果は、3つの概要査読付き国際会議での発表およびその論文集での公開という形でも表れている。以上より、本論文は博士(工学)の学位請求論文として合格と認められる。