Analysis of Urban Flood Inundation Patterns According to Rainfall Intensity Using a Rainfall Simulator in the Sadang Area of South Korea

An urban flood in the Sadang area located in South Korea was reproduced using a rainfall simulator. The rainfall simulator was developed to be able to demonstrate the rainfall intensity in range of 80–200 mm/h, and the artificial rainfall was created using 42 full cone type nozzles in the urban model. The uniformity coefficient of the rainfall distribution was 89.5%, which indicates the rainfall simulator achieved the high requirements for spatial uniformity. The flood experiments in the 1/200 scale model of the Sadang area were conducted using the rainfall simulator, and the flood patterns were investigated by changing the rainfall intensity. The rainwater mainly accumulated in the lowland of the crossroad where the entrances to the subway station are located. The flow velocity and the inundation depth were sharply increased until the rainfall intensity became 160 mm/h. Furthermore, the unstable human activities based on the moment and the friction instabilities also occurred from 160 mm/h. These results suggest that the study area requires flood damage mitigation facilities considering a rainfall intensity exceeding 160 mm/h

Flow-induced vibration of a radial gate at various opening heights

This study proposes a method for mitigating the flow-induced vibration of a radial sluice gate to ensure operational safety and long-term stability. To this end, a supplementary plate was attached to the lower rear surface of the gate and its effect on vibration reduction is investigated using a fluid–structure interaction (FSI) analysis. Specifically, a three-dimensional finite-element method (FEM) model of the gate partially submerged in water was constructed and then validated by comparing the numerical responses of the gate with experimental results under the conditions of steady-state discharge. The FSI analysis showed that the vertical vibration acceleration of the gate diminished by 70% after the supplementary plate was attached, thereby reducing the swirling strength calculated from the vortex shedding behind the gate. The Strouhal number describing the intensity of the gate vibration in the flowing fluid was also reduced by 57% after the supplementary plate was attached. The results of this study can be used to inform the design and development of radial gates, as well as maintenance plans for their economical operation

Measuring Inundation Depth in a Subway Station Using the Laser Image Analysis Method

Subway station platforms are vulnerable to flood damage. Thus, investigation of inundation properties in subway platforms is required to ensure the safety of citizens against flooding. In this study, the evacuation time and safety were analyzed in a subway station model using inundation depth measurements. The subway station model contained shallow water depth conditions, which did not allow for contact-type measurement devices. Instead, an image analysis procedure using laser images was proposed to measure the inundation depth. The proposed laser image analysis method can recognize a boundary line between the water and air by visualizing the water surface using a laser sheet. The inundation depth measurements using the image analysis method were reasonably accurate, resulting in differences of 2.97–7.67% compared to the results obtained using a digital point gauge. When inflow positions and flowrates of rainwater were changed, the measured results showed that the inundation depth increased in areas in which the rainwater inflow was relatively small or collided when moving in the direction opposite to the rainwater. The calculated evacuation time from the subway station showed that a drainage system is required to decrease the inundation depth in areas of inflowing rainwater collision. Furthermore, the estimated results of evacuation safety showed that safety handles are necessary even in low depth regions to prevent people from falling down due to increased flow velocity, during evacuation

Comparisons of Two Types of Particle Tracking Models Including the Effects of Vertical Velocity Shear

In this study, two types of particle tracking models were presented to investigate the applicability in the two-dimensional solute mixing simulations. The conventional particle tracking model, denoted as PTM, was developed based on Fick’s law, which adopted the dispersion coefficient to calculate the random displacements. The other model is the particle dispersion model (PDM), which computes the shear dispersion process by dividing into two computation procedures as the shear translation and the vertical mixing. The PTM and the PDM included the effects of vertical profiles of velocity in the computation of dispersion coefficients and the shear translation step, respectively. The main difference between the two models is whether the shear dispersion process is reproduced using Fick’s law or the direct computation method. These differences were clearly revealed by comparing with the analytic solution of the advection-dispersion equation. The concentration curve resulting from the PTM shows the Gaussian curves, which were well-fitted with the analytic solution in both initial and Taylor periods. Meanwhile, the PDM presented skewed curves in the initial period and gradually turned to the symmetric shape in the Taylor period. The inherent differences of the two particle tracking models were scrutinized against the two-dimensional tracer test results, which show the non-Fickian mixing properties. The comparisons of concentration–time curves reveal that the PDM reproduced a more accurate shape of the curves than the results by the PTM by demonstrating skewed concentration curves

Experimental Studies on Surface Vortex Mitigation Using the Floating Anti-Vortex Device in Sump Pumps

The maintenance of the performance of sump pumps is important to mitigate flood damage in urban areas and lowlands. However, the air-entraining vortex in the sump leads to undesirable performance degradation. Thus, in this study, the newly designed floating anti-vortex device (F-AVD) was employed in the intake pipe to enhance the efficiency of water intake in the sump by decreasing the surface vortex. The performance of the F-AVD was evaluated from the model experiments, in which the sump model was designed to represent the pump station that operates in Korea. The flow in the sump was measured using the particle image velocimetry (PIV) technique, and the velocity and vorticity distributions were compared both with and without the adoption of the F-AVD. The experimental results indicated that the vortex structures behind the intake pipe were effectively mitigated by installing the F-AVD. The vorticity magnitude behind the intake pipe was reduced in range of 24.8–52.5% after the installation of the F-AVD. However, in the case of a flow rate increase, the efficiency of the F-AVD decreased because of the strong vortex. Thus, an additional anti-vortex device (AVD), which is attached to the backwall or the floor in the sump, is required to prevent the air entrainment in conditions with high flow rates

Experimental Studies on the Formation of Air-core inside the Drop Shaft

In this study, the drainage efficiency of the multi-stage intake structure, which transports flood to the underground storage, was investigated from the laboratory experiments. The multi-stage intake structure was designed based on the tangential intake and the steps on the bed were purposes to decrease the energy of approaching flow. The experimental results show that the maximum water depth was effectively decreased in the entrance of the drop shaft. The measurements results of the air core width in the drop shaft show that the flow was stably drained without the choking. Furthermore, the air core width tends to increase with the Froude number, and these results indicate that the multi-stage intake structure is applicable to convey the approaching flow with relatively high velocity

Two-dimensional Pollutant Transport Simulations in Natural Streams with Horizontal Recirculation Zone

In this study, flow and pollutant transport simulations were conducted in the tidal reach of the Han River where the horizontal recirculation zone induced by the tidal cycle causes the storage effect. The flow analysis results show that the flow direction was repeatedly changed due to the downstream water level and the horizontal recirculation zone was occurred at the point where the flow direction was changed. Thus, the pollutant cloud was advected back and forth by the periodically occurred reversal flow. The storage effects of the horizontal recirculation zone cause that the concentration-time curve shows oscillatory increasing patterns, and the retention time of pollutant cloud was also increased

Two-dimensional Pollutant Transport Simulations in Natural Streams with Horizontal Recirculation Zone

In this study, flow and pollutant transport simulations were conducted in the tidal reach of the Han River where the horizontal recirculation zone induced by the tidal cycle causes the storage effect. The flow analysis results show that the flow direction was repeatedly changed due to the downstream water level and the horizontal recirculation zone was occurred at the point where the flow direction was changed. Thus, the pollutant cloud was advected back and forth by the periodically occurred reversal flow. The storage effects of the horizontal recirculation zone cause that the concentration-time curve shows oscillatory increasing patterns, and the retention time of pollutant cloud was also increased