Akira Mano

Downscaling Global Weather Forecast Outputs Using ANN for Flood Prediction

Downscaling global weather prediction model outputs to individual locations or local scales is a common practice for operational weather forecast in order to correct the model outputs at subgrid scales. This paper presents an empirical-statistical downscaling method for precipitation prediction which uses a feed-forward multilayer perceptron (MLP) neural network. The MLP architecture was optimized by considering physical bases that determine the circulation of atmospheric variables. Downscaled precipitation was then used as inputs to the super tank model (runoff model) for flood prediction. The case study was conducted for the Thu Bon River Basin, located in Central Vietnam. Study results showed that the precipitation predicted by MLP outperformed that directly obtained from model outputs or downscaled using multiple linear regression. Consequently, flood forecast based on the downscaled precipitation was very encouraging. It has demonstrated as a robust technology, simple to implement, reliable, and universal application for flood prediction through the combination of downscaling model and super tank model.

Application of 2D numerical simulation for the analysis of the February 2014 Bolivian Amazonia flood: Application of the new HEC-RAS version 5

Abstract Llanos de Moxos are vast plains in the Bolivian Amazonia that are continually flooded by the Mamoreriver. The flood lasts for several days affecting important cities like Trinidad, drowning people, drowning cattle and swamping arable land. Because of the cloudy skies, remote sensing observations are limited to some areas and few days. Thus, there is huge uncertainty about characteristics of flood events and possible consequences. Two-dimensional (2D) numerical simulation proved to be an important tool for understanding flood events. The HEC-RAS model is one of the most popular hydraulic models. In 2014 a new version of HEC-RAS (HEC-RAS-v5) was released including 2D capabilities. The present study applied the new HEC-RAS-v5 to simulate the February 2014 flood event in the Bolivian Amazonia. The flood simulated shows good performance when compared with satellite image of the flood event. In addition, the simulation provides information like water depth, flow velocity and a temporal variation of the flood. Specific locations where water begins to overflow were identified. Over most of the flooded area the water velocity is lower than 0.25 m s−1. During first ten days of the flood the flood extent increases rapidly. The flood depth allows identifying areas exposed to different hazard levels. The west plain of the Mamore river is the most exposed to the flood; it shows bigger flood extent, longer flood duration and deeper water depth. The flood that threatens the city of Trinidad originates in two locations; one located 32 km at the north and other located 10 km at the south west. The flood from the north gets close to Trinidad twelve days after it begins to overflow, while the flood from the south gets close to Trinidad seven days after it begins to overflow. Although the flood from the north is deeper than the flood from the south, the flood from the south begins flooded before the north. Thus, water borne and vector borne diseases may originate at the south earlier than the north. The city of San Javier gets covered by flood five days after the water begins to overflow. The study shows the applicability and the value of the 2D capabilities of the new HEC-RAS for flood studies.

EROSION CHARACTERISTICS OF SEA CLIFF ON THE FUKUSHIMA COAST

The Fukushima Coast composed of a series of soft rock cliffs had suffered from severe erosion due to two factors: high wave attacks and weak rocks. This study attempts to quantify these factors, respectively, by the onshore component of wave energy flux at the breaking point, F, through the refraction analysis, and by Youngs modulus, E, through the measurement of the elastic wave. The dimensional consideration yields a dimensionless parameter ΠC≡ qEL/F where q is the recession rate, L is the cliff height. On this coast, the value of ΠC=0.082 obtained from the best fit of the present data-set gives the recession characteristic q=0.082F/EL.

A TURBULENT AND SUSPENDED SEDIMENT TRANSPORT MODEL FOR PLUNGING BREAKERS

Extensive studies have suggested a high concentration of suspended sediment near the free surface induced by strong plunging breakers. However, prediction by models have not been successful because of the localized interactions among the plunging jet, turbulent production, and sediment movement. This study proposes a new calculation model for turbulent flow and suspended sediment transport in the surf zone. A numerical model was developed to simulate the flow and sediment motion in connection with plunging breakers in the surf zone. The Reynolds-Averaged Navier-Stokes (RANS) equations in two spatial dimensions were employed to simulate the flow field together with a κ-ε model for the turbulence and the Volume of Fluid (VOF) method for multiple free-surface tracking. An advection-diffusion equation was used for the suspended sediment concentration with a bottom boundary condition following the reference concentration formulation. Performance of the suspended sediment transport model under plunging breaking waves were examined through the comparison with experimental data. Good agreement between the model and experimental data was obtained for the surface elevation and velocity, turbulent kinetic energy, eddy viscosity and suspended sediment concentration. The overturning waves, plunging jet and transport of high concentration of suspended sediment near the free surface are reproduced by the present model with selected fine mesh resolution. The study shows the applicability of the present model in the turbulent and suspended sediment dominated region induced by strong plunging breakers.

Boundary Layer Developed Near Surging Front

ABSTRACTWave velocity profiles near the surging front on a dry bed are investigated both theoretically and experimentally. The Rayleigh problem on flows due to the sudden movement of a wall is extended to include the arbitrary wall movement and the convective term, and then is solved numerically. Measurements are performed by the flow visualization technique using the time lines of hydrogen bubbles and a high speed camera. The theoretical solutions are compared to the actual measurements and found to be in close agreement. The original Rayleigh problem proves to be an accurate model for flow fields in the immediate vicinity of the front. It also proved that the velocity profiles are affected by both the convection and deceleration of the upper flow. Singular bottom shear stress is dominant near the surging front, and this is related to the singularity at the wave front, an obstacle to wave runup analyses.

Potential impact of climate change at five Japanese beaches

ABSTRACT Yoshida, J., Udo, K., Takeda, Y., and Mano, A., 2013. Potential impact of climate change at five Japanese beaches Coastal erosion caused by sea level rise is a serious problem over the world. Future sea level rise will almost certainly accelerate through the 21st century. Although its extent remains uncertain, some predictions expect the increase to be between up to 30 and 180 cm by 2100 (Nicholls and Cazenave, 2010). Several studies show that sea level rise is responsible for long-term beach erosion. Moreover, change of wave height and non-climate-related processes such as ground subsidence amplify coastal vulnerability associated with climate change. It is a pressing issue to predict shoreline change considering the fact that 10% of the global population lives in the coastal regions within 10 m elevation. This study estimates the potential impact of climate change in terms of the effects of: sea level rise; wave height variation; and land subsidence by comparing the past coastline evolutions at five beaches in Japan. The past long-term shoreline change was found to be related to coastal protection measures according to Japans national policy (Coast Act). Notable erosion was resulted from the construction of coastal facilities that blocked coastal sand drift and the decrease of sediment supply from rivers caused by dam building from 1950 to 1990. Since 1990, shoreline has relatively unchanged because some measures, such as beach nourishment, groins and detached breakwaters, have worked to conserve sand beaches. The estimation results showed that shoreline would retreat over 15 m due to sea level rise and up to 5 m due to wave height variation at the five beaches by 2100. Projection of ground subsidence is uncertain because its rate varies depending on natural forces and human activities. This study suggests that sea level rise would have much greater impact on beach erosion compared with wave height variation and ground subsidence.

Breaking process and mechanism of coastal levees on Sendai Bay Coast hit by the 2011 mega tsunami

ABSTRACT Mano, A., IIda, T., Udo, K., and Tanaka. H., 2013. Breaking process and mechanism of coastal levees on Sendai Bay Coast hit by 2011 mega tsunami The 2011 mega tsunami induced by the East Japan Earthquake, Mw 9.0 hit the Sendai Bay Coast, overtopped the coastal levees and intruded deeply onto the Sendai and Ishinomaki plains by flushing people, houses, trees, etc. The levees with 6 to 7 m height had been constructed along 65 km-long sandy beach to protect the hinterland from the storm surges and wind waves. Eighty percent of the levees were broken by the tsunami in various degrees ranging from scatter of the surface blocks to the complete disappearance. The government decided to rebuild the levees with the target of the local major hazards as the storm surges but in a durable way even for the tsunami overtopping. Based on this background, this study aims to find the breaking process and mechanism of the coastal levees especially for the destructive cases. We conducted field survey, and collected aerial photos, tsunami records and videos, blue prints of the levees, etc. We integrated information of the tsunami records and videos, and matched the spatial information of the various data by GIS. These analyses enabled well understanding on the breaking process and mechanisms composed from two steps: (1) The impact of the leading bore with the maximum amplitude of 6 to 7m broke weaker parts like structural joints, parapets, landward soil slopes. (2) Return flow concentration to the broken parts in the first step or to the channels and swamps expanded erosion to make tsunami channels and bays and to lead levee breach.

Serious Erosion of the Southern Sendai Coast Due to the 2011 Tohoku Earthquake Tsunami and Its Recovery Process

We investigate morphology change of the southern Sendai Coast due to the 2011 Tsunami by analyzing topography and aerial images before and after the tsunami. The results show the characteristics such as erosion in the longshore direction behind seawalls, landward sediment transport during tsunami runup, seaward sediment transport from shore during backwash especially through crevasses of the seawalls, and coastal stabilization by coastal structures such as seawalls, breakwaters and headlands. At the seriously eroded Yamamoto Coast, more than half of the total amount of eroded shore sand above sea level was estimated to be transported seaward due to backwash. After 1 year from the tsunami, the eroded coasts were recovered to form pocket beaches. After another year, the coastal morphology had not changed apparently but seawalls started to be reconstructed. At present, after 3 years from the tsunami, the seawalls with a height of 7.2 m have been reconstructed along the coast. With the reconstruction, the foundation ground of the seawalls has been recovered, but the eroded beaches still remain disappeared. The coast act in Japan was established in 1956 to protect the coast from disasters, and amended in 1999 to also preserve both the coastal environment and its utilization. From the perspective of long-term coastal management, it is strongly required to consider the vision of the future coast.

MORPHOLOGICAL CHANGES ALONG THE ISHINOMAKI COAST INDUCED BY THE 2011 GREAT EAST JAPAN TSUNAMI AND THE RELATIONSHIP WITH COASTAL STRUCTURES

The 2011 Great East Japan Earthquake and Tsunami caused severe damage along the affected coastal area. This study analyzed seabed subsidence and morphological changes along the Ishinomaki Coast based on bathymetry measured shortly before and after the 2011 event. In general, seabed subsidence ranged from 0.32 m to 0.91 m. In addition, massive erosion on the seabed due to the 2011 tsunami occurred locally, in front of the Naruse River mouth and at the west end of the Ishinomaki Coast. Moreover, the depth of closure generated by the tsunami in these areas was higher than in others. The sandspit in front of the river mouth was severely eroded and the riprap protection at the west end was completely destroyed. Thus, there were no buffers in these areas during the tsunami. Therefore, tsunami-induced bed stresses were higher and may have occurred in deeper areas. Other areas, which were protected by relatively unharmed concrete structures, did not suffer from severe erosions. This suggests that the coastal protection structures protected both the land and the seaward side during the tsunami.

A DIMENSIONLESS PARAMETER DESCRIBING SEA CLIFF EROSION

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.