Yasunori Muto

Estimation of Offshore Wind Resources in Coastal Waters off Shirahama Using ENVISAT ASAR Images

Offshore wind resource maps for the coastal waters off Shirahama, Japan were made based on 104 images of the Advanced Synthetic Aperture Radar (ASAR) onboard the ENVISAT satellite. Wind speed fields were derived from the SAR images with the geophysical model function CMOD5.N. Mean wind speed and energy density were estimated using the Weibull distribution function. These accuracies were examined in comparison with in situ measurements from the Shirahama offshore platform and the Southwest Wakayama buoy (SW-buoy). Firstly, it was found that the SAR-derived 10 m-height wind speed had a bias of 0.52 m/s and a RMSE of 2.33 m/s at Shirahama. Secondly, it was found that the mean wind speeds estimated from SAR images and the Weibull distribution function were overestimated at both sites. The ratio between SAR-derived and in situ measured mean wind speeds at Shirahama is 1.07, and this value was used for a long-term bias correction in the SAR-derived wind speed. Finally, mean wind speed and wind energy density maps at 80 m height were made based on the corrected SAR-derived 10 m-height wind speeds and the ratio U80/U10 calculated from the mesoscale meteorological model WRF.

A probabilistic bridge safety evaluation against floods.

To further capture the influences of uncertain factors on river bridge safety evaluation, a probabilistic approach is adopted. Because this is a systematic and nonlinear problem, MPP-based reliability analyses are not suitable. A sampling approach such as a Monte Carlo simulation (MCS) or importance sampling is often adopted. To enhance the efficiency of the sampling approach, this study utilizes Bayesian least squares support vector machines to construct a response surface followed by an MCS, providing a more precise safety index. Although there are several factors impacting the flood-resistant reliability of a bridge, previous experiences and studies show that the reliability of the bridge itself plays a key role. Thus, the goal of this study is to analyze the system reliability of a selected bridge that includes five limit states. The random variables considered here include the water surface elevation, water velocity, local scour depth, soil property and wind load. Because the first three variables are deeply affected by river hydraulics, a probabilistic HEC-RAS-based simulation is performed to capture the uncertainties in those random variables. The accuracy and variation of our solutions are confirmed by a direct MCS to ensure the applicability of the proposed approach. The results of a numerical example indicate that the proposed approach can efficiently provide an accurate bridge safety evaluation and maintain satisfactory variation.

A METHOD FOR ESTABLISHING STAGE-DISCHARGE CURVE BY USING RAINFALL, WATER LEVEL DATA AND RUNOFF MODEL

A method for establishing stage-discharge curve (H-Q curve) using a runoff model to the rainfall observed in a basin and the water level observed in a river channel was discussed. A quadratic function that represented the relation between water level and discharge was built into the runoff model. When the observed water level hydrographs at some floods were reproduced, the H-Q curve was obtained. The method was applied to six water level stations in Shikoku which observed runoff quantity, and the established curve was compared with the H-Q curve based on the runoff observation. Two major findings were obtained: (1) a satisfied H-Q curve in which the error was 5-17% compared with measurement H-Q curve could be set up by using the data of one main flood event when there was a rain-gauge stations for each 30km 2 in the basin or more; (2) a useful H-Q curve was still obtained by using a lot of flood events when the rain-gauge stations were sparse and were unevenly distributed.

AN EXPERIMENTAL STUDY ON EFFECTS OF GROYNE SIZE ON BED EVOLUTION IN A CURVED OPEN CHANNEL

Following the previous report by the authors (Muto et al., 2005), new experiments were carried out in order to study the effects of design of groynes on bed evolution in a curved open channel. How scouring and depositions on the inner sand bar are produced by the groynes, which are known as wando and tamari, were of the main interest.A sand bar which is usually formed along the inner bank is divided into two parts when the groynes are installed. The downstream part of the bar tends to be higher than the case without groynes, since the bed material left from the eroded region is captured by the groynes. The size of scouring area is decided mainly by the height of the groynes among the tested design parameters. By changing the height according to the basic geomorphology of the sand bar, a wider range of the size of scouring hole is produced, which will provide a better environmental background for aquatic natural lives.