Yoshiaki Kuriyama

A one-dimensional model for undertow and longshore current on a barred beach

A one-dimensional model was developed for undertow and longshore current. The model predicts time- and depth-averaged undertow and longshore current velocities with longshore uniformity in depth, waves and current. It was calibrated with field data obtained over longshore bars at Hazaki Oceanographical Research Station (HORS). Verification with the other field measurements and large-scale experiments showed that the present model predicted well the undertow velocities over the longshore bars, while a previous model, calibrated with small-scale experiment data, underestimated the velocities. The verification also showed that although the cross-shore distributions of the longshore current velocity over the longshore bars predicted with the previous model had peak velocities seaward of the bar crests, the distributions predicted with the present model had peaks shoreward of the crests and fitted those measured in the field.

MODELS OF WAVE HEIGHT AND FRACTION OF BREAKING WAVES ON A BARRED BEACH

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

A ONE-DIMENSIONAL PARAMETRIC MODEL FOR UNDERTOW AND LONGSHORE CURRENT VELOCITIES ON BARRED BEACHES

A one-dimensional parametric model for undertow and longshore current velocities assuming a triangular velocity distribution in a surface roller was developed. This model as well as a parametric model with the assumption of a uniform velocity distribution in a roller was compared with field data obtained on barred beaches at Hasaki in Japan and at Duck in the USA. The comparisons showed that the present model predicted the velocity fields at the two sites reasonably well, and the prediction accuracy of the present model is slightly better than that of the other model. However, the present model underpredicted the undertow velocities on the trough regions, and overestimated the longshore current velocities near the shorelines.

FIELD MEASUREMENTS OF UNDERTOW ON LONGSHORE BARS

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

Numerical investigation of the influence of the enhancement of cyclones on long-term shoreline movement

ABSTRACT Kuriyama, Y. and Banno, M.., 2013. Numerical investigation of the influence of the enhancement of cyclones on long-term shoreline movement The enhancement of tropical and extratropical cyclones caused by global warming is expected to induce morphological changes in the nearshore zone. Hence, the influence of the increase in offshore wave energy flux Ef due to the enhancement of cyclones on long-term shoreline movement on the Hasaki coast of Japan was examined using a numerical model for predicting the shoreline changes caused by cross-shore sediment transport. The shoreline movements during a 100-year period were predicted with 4-month intervals by increasing the means and/or standard deviations of Ef during the 4-month seasons. Under each wave condition, the predictions were implemented 1000 times with Ef estimated using a Monte Carlo method. The prediction results showed that the shoreline retreated 18 m with 20% increases and 33 m with 50% increases in both the means and standard deviations of Ef in the 100–year period. The increases in both the means and standard deviations of Ef during the periods from May to August and from January to April influenced the shoreline retreat most and least, respectively. This was probably due to the nonlinear relationship between the shoreline change rate and Ef.

The effects of offshore wave energy flux and longshore current velocity on medium-term shoreline change at Hasaki, Japan

The relationships between medium-term shoreline fluctuations, offshore wave energy flux and longshore current velocity were investigated on the basis of data sets from 1987 to 2001 (15years). In this analysis, the spectral density of the data was calculated, and the frequency components from the investigated period (15 years) to 1000 days were recomposed and defined as the medium-term fluctuation. A multiple linear regression analysis was performed to evaluate the associations between the medium-term fluctuations of shoreline change rate and that of the major dependent variables, which are the offshore wave energy flux and the longshore current velocity. The results suggested that a medium-term fluctuation of the shoreline change rate at the field was affected by both the medium-term fluctuations of the offshore wave energy flux and the longshore current velocity at approximately the same rate. The fluctuation range of the medium-term shoreline fluctuation was nearly the same range of the seasonal shoreline fluctuation, i.e. approximately 20 m. This indicates that for beach management, not only seasonal shoreline fluctuation but also medium-term shoreline fluctuation is needed to be considered.

Regime shifts in the multi-annual evolution of a sandy beach profile: Regime shifts in the multi-annual evolution of a sandy beach profile

Extreme storms occasionally induce extraordinarily large morphological changes, which may have major impacts on coastal resilience, tourism and the environment. Some of the changes are persistent, and they are defined as regime shifts. During a 28-year period from 1986 to 2014, the foreshore and the inner transition zone of the Hasaki coast of Japan underwent four morphological stages separated by regime shifts in beach profiles. From stage 2 to stages 3 and 4, over a relatively short period between 2006 and 2008, the beach profiles in the foreshore and the inner transition zone drastically changed, mainly because of large offshore waves. From stage 2 to stage 3, the foreshore and the inner transition zone were greatly eroded. Then, from stage 3 to stage 4, although the inner transition zone was further eroded, the foreshore underwent accretion. The severe erosion of the inner transition zone over the 2-year period formed a steep slope, which was inferred to have led to accretion in the foreshore in stage 4. Stage 4 persisted for approximately 7 years, and the beach profile had not returned to a similar morphology to those of stages 1 or 2 by the end of the study period.

Development of Shoreline Change Model Considering Time Heterogeneity and Tidal Impact and Prediction of Future Shoreline Position under Global Climate Change

測し,モデルへ反映させることは難しい.時系列データ において,そのようなモデルに反映できない未知変数は 時間的に異なり,しばしば目的としている観測値のばら つきを増大させる.このような未知変数に起因した観測 値にばらつきを生じさせる影響のことを統計学的にラン ダム効果(Random effects)と呼び,このランダム効果を 考慮した統計モデルを一般化線形混合モデル(GLMM) と呼ぶ(例えば,久保,2012).時系列データにおける 上述のようなランダム効果は「時間的異質性」と呼ぶこ とができ,時間的異質性を有する現象をモデリングする 際には,ランダム効果を考慮しなければモデルパラメー タを正確に推定できないという問題が生じる. そこで本研究では,波浪エネルギーを主な汀線変動の 外力としたモデルを発展させ,潮位による影響と時間的 異質性を考慮した長期的な予測が可能な汀線変動モデル を構築した.具体的には,波浪エネルギーに対する汀線 の応答特性に潮位や未知変数が与える影響を考慮したモ デルであり,統計的手法を応用し,未知変数による影響 をランダム効果とした線形混合モデルである.また構築 されたモデルを用い,地球温暖化に伴う海面上昇や波浪 条件の変化による汀線位置の将来予測を行った. なお,本研究における汀線変動モデルは岸沖方向に一 次元のモデルであり,汀線位置とは現在のH.W.L.の地盤 高の岸沖位置である.つまり,海面上昇によって汀線位 置の基準となる地盤高は変化させないことから,本研究 における汀線位置の定義はいわゆる水際ではなく,汀線 変動は前浜の侵食や堆積の状況を表す指標として取り扱 うことができる(図-1). 潮位による影響及び時間的異質性を考慮した 汀線変動モデルの構築と将来予測 Development of Shoreline Change Model Considering Time Heterogeneity and Tidal Impact and Prediction of Future Shoreline Position under Global Climate Change

CONTINUOUS OBSERVATION OF LONGSHORE CURRENT VELOCITY BY USING A PENDULUM-TYPE FLOAT

茨城県波崎海岸に位置する波崎海洋研究施設において，直径20cmの受感球（フロート）を用いた常時観測可能な沿岸流速計を観測桟橋上に設置した．受感球は直径22mmの円柱鋼材に固定させたうえで，平均海水面下約1.2mの海中に留まらせた．この鋼材の中央付近から桟橋に向けて細いワイヤーを3方向に張り，それぞれにロードセルを接続し，受感球および海中に没している円柱鋼材に加わる力を張力として計測した．この張力の関係を解析することにより沿岸流速を算出した．沿岸流速は観測期間中，高波浪時も含めて欠測することなく計測された．また，ほぼ同地点に設置した電磁流速計の出力値を用いてその精度を検討した結果，砕波帯内での観測にも関わらず全体的に高い精度で評価できることが分かった．