Taisuke Ishigaki

Assessment of Safety on Evacuating Route During Underground Flooding

Heavy rainfalls over 50 mm/hr have been observed frequently in the last decade in Japan. If a sewage system in urban area could not drain such rains, flood water would be flooding and flowing into underground spaces such as subway or basement of building. As the area of underground space is limited, flood water fills up the area in a short time. It is very dangerous situation and people should evacuate from there as soon as possible in order to save their life. To draw up an evacuation plan, it is important to assess the safety of evacuating route. In this paper, the method of safety assessment on the evacuating route is discussed on the basis of experimental results and flood simulation data in an underground space. In order to assess the safety on an evacuating route, the value of the specific force per unit width, M=0.125m2, is used in this study, of which value has been obtained in the previous work. Using the relation between M values and evacuating speed, the assessment of safety on evacuating routes during underground flooding has been carried out well and the data for preparing an evacuation plan have been shown. The data are very useful for the people to evacuate from when and through which route.

PIV and LDA Measurements of Secondary Flow in a Meandering Channel for Overbank Flow

Secondary flow in a compound meandering channel with straight floodplain banks for overbank was investigated by a visualization method and velocity measurement using three-component laser Doppler anemometor (LDA). The secondary flow in a cross section was visualized by the neutral buoyant tracer method with a submergible video camera. Secondary flow vectors in a cross section were obtained by using PIV software with captured frames from video source through PC and also by LDA measurements. From the comparison of the PIV and LDA results, it is found that PIV data show good agreement in quality with LDA measurements when the secondary flow is strong and stable as shown in this paper.

Evaluation for emergency escape during stair climbing in a simulated flood evacuation

The objective of this study was to measure muscle activity during stair-climbing in order to estimate the water depth at which people can evacuate safely from urban underground. A life-size model of a staircase with a water pump that generated water flow from the top of stairs was constructed. In the study, electromyogram data were measured for six muscles on the right lower extremity, namely, the rectus femoris, vastus lateralis, long head of the biceps femoris, medial gastrocnemius, tibialis anterior, and gluteus maximus. From the results, the GA exhibited the highest muscle activity in the latter part of the stance phase, suggesting that the subjects tended to avoid stumbling against the stream of the water during ankle planter flexion. The results obtained from this study would be used for the construction of a guideline for evacuation from urban underground in flood disaster.

NUMERICAL STUDY OF STORM SURGES IN THE SETO INLAND SEA BY MULTI PHYSICS MODEL

This study proposes a multi physics model which consists of atmosphere, tide, surge and wave physics. It was applied to storm surge prediction in the Seto Inland Sea with complex topography and large tidal fluctuation. The storm surges generated by Typhoon Chaba and Songda were hind casted well by the numerical model of SuWAT using the mesoscale weather model output under fluctuating tide condition since the pressure and wind field is well estimated by the weather model. The hind casted water level variation by an empirical typhoon model under high water sea level is underestimated in comparison with the measurements.

Secondary flow in compound sinuous/meandering channels

Secondary flow and its related problems in compound sinuous/meandering channels are discussed. The structure of secondary flow in sinuous/meandering channels during floods isillustrated by detailed velocity measurements using a fibre optic laser Doppler anemometer (FLDA) and an advanced flow visualisation technique using a small submergible video camera. Some data analyses were carried out in order to estimate energy expenditure by secondary flow and its effect on the macro flow structure. The results clearly shows relatively large effect of secondary flow on estimating conveyance of the channel system.

Flow visualization in a transverse cross section of an open-channel flow

Abstract Secondary flow is of prime importance in investigations of the three-dimensional structure of turbulent flow in an open channel. Since Nikuradse showed the influence of secondary flow on the velocity distribution in a cross section and Prandtl discovered that there are two kinds of secondary flows, many researchers have investigated these flows experimentally or theoretically. Because mean values were obtained by velocity measurements, secondary flow was elusive because of its intermittent production and instability. Flow visualization is an effective technique for observing the three-dimensional motion of flow. In this paper, a direct way of visualizing the instantaneous motion of secondary flow in an open channel is shown and the characteristics of the flows are investigated.

STUDY ON VULNERABILITY OF MEGA-UNDERGROUND SPACE TO EXTREME FLOOD

Japanese cities have many underground spaces such as subway and shopping malls. These cities locate in lowland of coastal zone near river mouths and are prone to extreme floods. The vulnerability of underground spaces to extreme floods are treated in this paper by using the results obtained by numerical method in densely urbanized area, Osaka, Japan. Pluvial, fluvial, storm-surge and tsunami flood cases were calculated. It is found that up to 60% of flooded water intrude into underground spaces if no counter measures were taken. Inundation depth and area in the underground shopping mall are different in the four types of extreme floods, and the flooded water enters into connected subway stations. This means that administrators of underground space have to make different counter measures for each type of extreme floods.

1-D and 2-D Simulation of the Flow Around Discontinuous Dyke

There are many kinds of traditional flood management in Japan. Some traditional flood managements still work as a disaster reduction system. A discontinuous dyke discussed here is one of them. The discontinuous dyke has some functions by which the river water inundated through it to the surrounding flood plains during floods. However, its hydraulic function has not yet fully clarified. In this paper, the flow around discontinuous dyke was calculated by one-dimensional model and two-dimensional model. In one-dimensional simulation, it is necessary to determine an overflow discharge equation with experiment data. From the result of previous work, the overflow discharge equation has been investigated experimentally. A good agreement between the calculated and measured data can be seen. Then, the flood flow in Kameoka Basin, Kyoto, Japan is calculated by two-dimensional model without any experimental equations of overflow discharge, and the inundation process of this area is simulated. Inundation process in this area can be simulated.

Influence of Channel Width on the Flood Flows in a Stream Valley

Investigative reports of flood disaster in a stream valley state that the magnitude of flood damages depends on the stream configuration. A meandering stream runs in mountainous basins through narrow valley. Flood damages were serious around the narrow apex of a bend. On the other hands, the damages were little serious around the wide apex. From this point of view, the stream width at the apex of a bend was focused on. The experiments with a doubly meandering compound channel were conducted to investigate the influence of channel width on the flood flows. Water surface flows were visualized, and water level was measured. The channel width influences on the magnitude of water surface velocity and divergence, implying that the scale of bank erosion can be difference. The magnitude is large in the case of narrow apex. On the other hand, the magnitude is small in the case of wide apex. This means that the flood damages can be explained by experimental results. As a result, it is supposed that appropriate river width must be taken into account to reduce the flood damages in a stream valley.

THE EFFECT OF CONFIGURATION ON FLOOD FLOW IN STREAM VALLEY

The flood disaster in a stream valley is discussed with experimental results by water surface flow visualization and water level measurement. From a field survey, it is found that the magnitude of damage is dependent on valley width, which implies the change of flood flow depth on the alluvial plain, and river width at the bend apex. As experimental results, the magnitudes of water surface velocity and divergence around the apex are large for deep condition on the flood plain and narrow channel width at the bend apex, implying that significant erosion can occur around a bend. Thus it is important to understand the effect of configuration on flood flows and appropriate river width at the bend apex to reduce flood damages in a stream valley.