In recent years, sheetflow sand transport regime has attracted the attention of many coastal engineers and scientists as it is predominant in the surf zone. Sheetflow conditions develop when the near bed velocity is large enough to wash out sand ripples and transport sand in a thin layer with high sand concentration along the bed. This sand transport regime involves very large net transport rates and thus results significant changes of the beach topography.

When waves propagate to the nearshore zone, their shapes gradually change primarily owing to the combined effects from wave shoaling, breaking, and nonlinear interactions. As waves enter the shallow water; their shapes evolve from sinusoidal to the pure velocity asymmetric waves with sharp crests separated by broad, flat wave trough in intermediate water depths. As waves continue to shoal and break, they transform through asymmetrical, pitched-forward shapes with steep front faces in the inner surf, to a pure acceleration asymmetric waves (pitched-forward) near the shore. In addition to the change of wave shapes, the interaction of nearshore waves and currents is also an indispensable hydrodynamic element in coastal regions. For example, the offshore-ward near-bottom current, referred to as undertow, develops to compensate the onshore flux caused by waves. This type of waves-currents interaction, however, is quite weak. In contrast, a strong interaction can be observed in the vicinity of river mouth. The existence of different wave shapes and their interactions with different magnitude currents may lead to the different sediment transport behaviors. Many laboratory studies have been conducted in the oscillatory flow tunnels with sinusoidal, pure asymmetric velocity waves and pure asymmetric acceleration waves. However, it is hardly found any experiment conducted with the combined velocity-acceleration asymmetric waves and with strong opposing currents. Thus, new prototype scale laboratory tests (53 tests) using different wave shape conditions with and without the presence of strong opposing currents were performed. These experiments were motivated by the fact that most natural waves in surf zone produce mixed skewed-asymmetric oscillatory flows (Ruessink et al., 2009) and sand transport at the river mouth is influenced by the interaction of nearshore waves and strong river discharge.

Experimental results reveal that in most of the case with fine sand, the "cancelling effect", which balances the on-/off-shore net transport under pure acceleration/velocity asymmetric waves and results a moderate net transport, was developed for combined asymmetric-skewed shaped waves. However, under some certain conditions (T > 5s) with coarse sands, the onshore sediment transport was enhanced by 50% under combined asymmetric-skewed waves. Additionally, the new experimental data under collinear waves and strong currents show that offshore net transport rate increases with decreasing velocity skewness and acceleration skewness.

Image analysis technique was employed to investigate major aspects of sediment transport under asymmetric-skewed waves and currents. Measured maximum erosion depths were found larger for shorter wave periods and for wave profiles with shorter time to maximum velocities. This suggested that faster flow acceleration could produce higher bed shear stress. In addition, it is found that the presence of a strong steady current which results in larger ratio uc/uw also increases the sheetflow layer thickness. It is because the appearance of currents in the opposite direction with waves could enlarge the available time length for flow erodes the sand bed and raises up sand to the maximum possible elevation. Thus, as a consequence it enlarges the sheetflow layer thickness.

A two phase flow model with calibrated turbulence closure terms was employed to get further insight sand transport mechanism. The simulated results agree well with observations. Analysis of forces acting on sand precisely shows that an increase of flow acceleration will increase applied forces on sand particles and hence the sand velocity travelling in the upper sheetflow layer. However, inside the pick-up region, due to high sand concentration, sand motions will be absorbed by the intergranular stress and as a result it increases the bed shear stress. In addition, the mobile bed effect were also confirmed by the two phase flow model and the Nikuradse bed roughness that is often estimated as of the order of the sheetflow layer thickness appears to be corrected.

Taking into account the effects of mobile bed and the flow acceleration, empirical formulas have been proposed to estimate bed shear stress, the maximum erosion depth and the sheetflow layer thickness. Sand transport mechanism was investigated by comparing the bed shear stress and the phase lag parameter for each half cycle. The "phase lag parameter" was modeled as the ratio between the sheetflow layer thickness and the settling distance. By analyzing the temporal brightness distribution at different elevations which corresponds to the distribution of suspended sand concentration, it is precisely found that phase lag is considered to be significant once it value exceeds 0.9. In such circumstances, the so-called "cancelling effect", will occur. In contrast, in cases phase lag is small; the bed shear stress plays a more fundamental role as it causes an onshore enhancement for mixed shaped waves.

The new net transport rate measurements were compared with several net transport rate models (i.e, Watanabe and Sato, 2004; Silva et al., 2006; Van der A et al., 2010) and found that those approaches fails to deliver an accurate prediction. The reason is pointed out due to the inappropriate estimates of the representative suspension height in their models. Thus the new estimation for sheetflow layer thickness was incorporated in a new net transport rate model, based on Watanabe and Sato's concept. The new model has been examined with comprehensive sheetflow experimental data and prediction skill over a wide range of hydraulics and sediment conditions shows that the new model fulfills for practical purposes and can be integrated into numerical morphodynamic models.

海浜変形を予測するうえで重要となるシートフロー状の漂砂現象においては,砂の移動量が格段に大きくなる砕波帯において移動量と移動方向を評価することが重要である.砕波帯においては,波の非線形性と砕波による乱れの発達により,底面付近の流速変動は,流速も加速度も非対称な波形となる.さらに底面付近には戻り流れが発達するため,底質の移動は,非対称な波動流速と沖向きの定常流れが重なった複雑な条件で生じることになる.このような複雑な底質移動現象に対して,縮尺効果の少ない振動流装置における実験が進められ,流速が非対称な条件と加速度が非対称な条件のそれぞれに対して,実験データが蓄積されてきた.本研究では,流速と加速度の両者が非対称な条件に実験条件を拡張し,さらに沖向き流れが重なった現実的な条件のもとでのシートフロー漂砂に関する実験を実施した.これにより,従来のモデルを修正するとともにその適用範囲を飛躍的に拡大することに成功した.また,砂粒子と流体をそれぞれの相互作用を考慮したうえで,質量と運動量の保存則を個別に扱う二層流モデルにより,シートフロー漂砂の移動機構を検討し,実験データの解釈とモデルに用いるパラメタを物理的に解釈した.従来の研究では不明確であった流速の非対称度と加速度の非対称度の役割を実証的に明らかにし,数値モデルも併用することにより移動機構の物理に基づいたモデルを提案している点が本研究の独創的な点である.

砂移動量の計測実験では,加速度非対称性は砂を岸向きに輸送するのに対し,流速の非対称性は条件によっては,沖向きの砂移動に貢献し,両者の非対称性が混在する実際の条件では,これらのバランスにより砂移動量が決定されていることがわかった.漂砂の移動方向の判定には,波の周期と底質の沈降速度によって評価できる非定常性指標である位相遅れ指標を用いることで統一的な解釈が可能であることが確かめられた.底質の濃度計測では,高速ビデオカメラを活用した画像解析手法を開発し,砂粒子からの反射光強度に基づく濃度推定に加えて,シートフロー層の厚さの時間変化や侵食深さの時間変化など,漂砂のモデル化において本質的となるパラメタの実証的な評価が可能となった.

以上,要するに,本研究により,従来,水槽実験や現地調査のみでは十分な精度で議論することが不可能であった砕波帯のシートフロー漂砂現象に対して,流速波形を制御した緻密な実験と二層流数値モデルを併用したモデル化により,既存のモデルに比較して格段に精度が高く適用範囲の広い漂砂量算定手法を提案することができた.また,本研究で開発された二層流概念に基づく数値モデルは,シートフロー現象における本質的な移動機構に立脚しているうえ,広い範囲の実験データと比較して再現精度が高いことが確かめられたため,これをベースにして漂砂量の直接的な算定手法にまで高めることも期待でき,発展性・実用性が高い.

よって本論文は博士(工学)の学位請求論文として合格と認められる.