Akihiko Nakayama

Contribution of Trapped Air, Deck Superelevation, and Nearby Structures to Bridge Deck Failure during a Tsunami

Failure of the Utatsu concrete girder highway bridge in Minamisanriku, Miyagi Prefecture during the 2011 Great East Japan Tsunami was puzzling because the bridge decks were not pushed off their piers but rather were flipped off the landward side of the bridge piers after being deeply submerged by the surging tsunami. To determine what caused this to happen, two simulations were conducted. The first was a large-scale Delft shallow-water simulation (beginning with published tsunami source free surface deviation) to determine the behavior of the tsunami (time series of flow depth and speed) at the bridge site. The second was a small-scale two-dimensional (2D) (profile view) software volume-of-fluid (VOF) simulation of flow over the bridge deck, with boundary conditions taken from the Delft model. The VOF model then allowed calculation of lift force, drag force, and overturning moment on the bridge deck. Results show that factors contributing to failure included the presence of a seawall near the bridge, inclination (superelevation) of the deck upward toward the ocean, sediment entrained in the water, and air trapped between girders.

Mechanisms of Damage to Coastal Structures due to the 2011 Great East Japan Tsunami

Abstract Computational fluid mechanics, field measurements, and empirical methods are used to determine the reasons for the failure of the Kamaishi Breakwater, the Utatsu highway bridge, a concrete girder bridge in Iwate Prefecture, and many small seawalls and buildings during the 2011 Tohoku Tsunami. Results show that the Kamaishi Breakwater failed due to the pressure at the caissons heel exceeding the bearing capacity of its rubble mound foundation, along with scour of the unarmored landward side of the rubble mound due to the jet resulting from overtopping of the caisson. The Utatsu highway bridge failed due to trapping of air between the bridges girders, superelevation of the bridge deck toward the ocean, and the presence of a seawall under the bridge. Scour was also liable for the failure of many smaller seawalls and buildings; in some cases this scour was enhanced by rapid-drawdown-induced liquefaction of sandy soils.

Residual effects of treated effluent diversion on a seaweed farm in a tidal strait using a multi-nested high-resolution 3-D circulation-dispersal model

A high-resolution 3-D model was developed to assess the impact of a diversion outfall at the Tarumi Sewage Treatment Plant (TSTP) on an adjacent seaweed farm in Osaka Bay, Japan. The model was extensively validated to ensure a reasonable agreement with in situ observations. The western part of the farm is largely influenced by tidal currents, whereas the eastern area is mainly affected by subtidal residual currents that are primarily due to surface wind stress. The released effluent is transported by counterclockwise residual circulation formed off the TSTP. The model reveals that the diversion adequately suppresses the influence on the farm. While the instantaneous effluent concentration is diminished by about 50%, the effluent accumulated on the farm decreased from 2.83 × 104 m3 to 2.01 × 104 m3 due to the diversion, demonstrating an approximately 28% reduction of the effluent from the TSTP by the diversion outfall.

Development of smoothed particle hydrodynamics method for analysis of high-speed two-phase flows in hydropower spillways

The basic formulation of the smoothed particle hydrodynamics (SPH) has been re-examined for analysis of gas-liquid two-phase flows with large density differences. The improved method has been verified in the calculation of dam-break flow and has been applied to an open-channel flow over steep sloped stepped spillway. In the calculation of the flow over the steps, not only is the trapped air but entrained air bubbles and water droplets are reproduced well. The detailed variation of the time-averaged mean quantities will have to be further examined but overall prediction with relatively small number of particles is done well.

STUDY ON THE BED LOAD TRANSPORT BY STEEP TUNNEL

Sediment bypass system has been proposed as a countermeasure against sedimentation and turbid effluence that occur in reservoirs over long period of operation. In ordcr to provide a guide for reasonable design of such bypass tunnels, the flow with a movable sediment bed in a steep-sloped pipe has been studied both experimentally and numerically. The phenomenon observed in the experiment indicates that the flow is highly unsteady and non-uniform. An analysis method considering both of these aspects has been developed. The unsteadiness is not only local but it induces variations in the overall discharge. The energy loss due to rapidly varying bed shape is significant and a model for it has been formulated. The present analysis method is found to reproduce the main characteristics observed in the model experiments fairly well.