Effect of slit-type barrier on characteristics of water-dominant debris flows: small-scale physical modeling

Slit-type barriers, one of open-type barriers, are widely used as active measures to mitigate potential risk and damage by debris flows, and those are designed and installed to reduce the flow energy by only passing relatively small debris. However, the mechanisms of slit-type barriers in reducing the debris flow velocity and debris volume remain poorly understood because of the lack of well-controlled and reliable physical modeling results. This study explored the influence of various arrangements of slit-type barriers, including P-type barriers in which each rectangular barrier was placed in parallel and V-type barriers where the barriers were placed in a V-shape, on characteristics of water-dominant debris flows via small-scale model experiments. The debris flow events were reproduced against the slit-type barriers, where the velocity reduction and trap ratio were monitored, varying the angle and shape of barrier arrangements. The velocity reduction and trap ratio appeared to increase as the angle of the barrier wall decreased because of the decreased opening ratio. The V-type barriers resulted in higher velocity reduction and trap ratio than the P-type, primarily because of the smaller effective opening ratio and the more backwater effect. In addition, as the debris contained more boulders, the extent of velocity reduction and debris trap became greater in all barrier types. Two types of opening ratios, the projected and effective opening ratios, were correlated to the interactions between debris and walls. The obtained results provide baseline data for the optimum design of slit-type barriers against debris flow and suggest that the slit-type barriers can effectively manage the risk of damage by debris flows.     

Geophysical flows impacting a flexible barrier: effects of solid-fluid interaction

Flexible barriers undergo large deformation to extend the impact duration, and thereby reduce the impact load of geophysical flows. The performance of flexible barriers remains a crucial challenge because there currently lacks a comprehensive criterion for estimating impact load. In this study, a series of centrifuge tests were carried out to investigate different geophysical flow types impacting an instrumented flexible barrier. The geophysical flows modelled include covered in this study include flood, hyperconcentrated flow, debris flow, and dry debris avalanche. Results reveal that the relationship between the Froude number, Fr, and the pressure coefficient α strongly depends on the formation of static deposits called dead zones which induce static loads and whether a run-up or pile-up impact mechanism develops. Test results demonstrate that flexible barriers can attenuate peak impact loads of flood, hyperconcentrated flow, and debris flow by up to 50% compared to rigid barriers. Furthermore, flexible barriers attenuate the impact load of dry debris avalanche by enabling the dry debris to reach an active failure state through large deformation. Examination of the state of static debris deposits behind the barriers indicates that hyperconcentrated and debris flows are strongly influenced by whether excessive pore water pressures regulate the depositional process of particles during the impact process. This results in significant particle rearrangement and similar state of static debris behind rigid barrier and the deformed full-retention flexible barrier, and thus the static loads on both barriers converge.     

Surge impact behavior of granular flows: effects of water content

Understanding the fundamental dynamics of interaction between multi-phase geophysical flows and engineering structures is crucial for mitigating geophysical hazards. Specifically, liquid phase between particles induces matric suction which could play a significant part in regulating flow dynamics and warrants further consideration. In this study, flume model tests were conducted to investigate the effects of water content (0–30%) on the impact behavior of granular flows. The particle image velocimetry technique was adopted to visualize the impact kinematics and the impact force was measured through a model barrier system. Results revealed that, besides geometric effects (kinetic sieving), mechanical effects (shearing and collision) are also vital for the mechanism of reverse segregation. At higher water contents, 20 and 30% in this study, discrete-surge impact, rather than a progressive impact process, was observed. The discrete surges induce impulses on the barrier. The discrete surges result from self-organization of unsaturated granular flows to overcome the enhanced shear strength induced by matric suction. Finally, a dimensionless index, namely the suction number, is used to quantify the effect of suction on the dynamic behavior of granular flows. Even for large-scale geophysical flows, if the content of fine particles is high, effect of suction should not be neglected.     

Evapotranspiration models of different complexity for multiple land cover types

A comparison between half‐hourly and daily measured and computed evapotranspiration (ET) using three models of different complexity, namely, the Priestley–Taylor (P‐T), the reference Penman–Monteith (P‐M) and the Common Land Model (CLM), was conducted using three AmeriFlux sites under different land cover and climate conditions (i.e. arid grassland, temperate forest and subhumid cropland). Using the reference P‐M model with a semiempirical soil moisture function to adjust for water‐limiting conditions yielded ET estimates in reasonable agreement with the observations [root mean square error (RMSE) of 64–87 W m^?2 for half‐hourly and RMSE of 0.5–1.9 mm day^?1 for daily] and similar to the complex Common Land Model (RMSE of 60–94 W m^?2 for half‐hourly and RMSE of 0.4–2.1 mm day^?1 for daily) at the grassland and cropland sites. However, the semiempirical soil moisture function was not applicable particularly for the P‐T model at the forest site, suggesting that adjustments to key model variables may be required when applied to diverse land covers. On the other hand, under certain land cover/environmental conditions, the use of microwave‐derived soil moisture information was found to be a reliable metric of regional moisture conditions to adjust simple ET models for water‐limited cases. Further studies are needed to evaluate the utility of the simplified methods for different landscapes. Copyright © 2011 John Wiley & Sons, Ltd.     

Spatial and temporal variations in nutrient and chlorophyll-a concentrations in the northern East China Sea surrounding Cheju Island

Nutrients, chlorophyll-a (Chl-a), and environmental conditions were extensively investigated in the northern East China Sea (ECS) near Cheju Island during five research cruises from 2003 to 2007. In the eastern part of the study area, surface waters were characterized only by the Tsushima Current Water (TCW) during all five cruises. However, the western surface waters changed with season and were characterized by the Yellow Sea Cold Water (YSCW) in spring, the Changjiang Diluted Water (CDW) in summer, and the Yellow Sea Mixed Water (YSMW) in autumn. In spring and autumn, relatively high concentrations of nitrate and phosphate were observed in the surface waters in the western part of the study area, where vertical mixing brought large supplies of nutrients from deep waters. Changes in wind direction occasionally varied the inflow of the Changjiang plume in summer, clearly causing the annual variation in surface nitrate and phosphate concentrations in summer. In summer, the surface distribution of nitrate and phosphate did not coincide with that of silicate in the study area, which probably resulted from the significant drop in the Si:N ratio in the Changjiang plume since construction of the Three Gorges Dam (TGD). Despite large temporal and spatial variations in surface Chl-a concentrations, depth-integrated Chl-a concentrations exhibited little variation temporally and spatially. In the study area, surface Chl-a concentration did not well reflect the standing stocks of phytoplankton. The vertical distribution of Chl-a showed large temporal and spatial variations, and the main factor controlling the vertical distribution of Chl-a in summer was the availability of nitrate. The thermohaline front may play an important role for accumulation of phytoplankton biomass in spring and autumn.     

Evaluation of hydrologic data obtained from a local groundwater monitoring network in a metropolitan city,Korea

In the late 1980s, dramatic increases in water use caused over‐exploitation of groundwater resources and deterioration of water quality in Seoul metropolitan city. To monitor changes in quantity of groundwater resources and their quality, the metropolitan government established a local groundwater monitoring network in 1997 consisting of 119 monitoring wells. Groundwater resources in the urban area were affected by various human activities, including underground construction such as subways, pumping for public or private water use, leaky sewer systems and pavements. The variation patterns of the groundwater levels were mainly classified into four types, reflecting natural recharge due to rainfall events during the wet season, artificial recharge from leaky sewer or water supply systems, and heavy groundwater pumping for drainage or flood control purposes at underground construction sites. Significantly decreasing trends of groundwater levels in the suburbs of Seoul indicate groundwater use for various agricultural activities. Subway construction lowered the water level by an average of 25 m. Electrical conductivity values showed a wide range, from 100 to 1800 µS/cm (mean 470 µS/cm). Groundwater temperature generally showed a stable pattern, except for some sensitive increases at relatively shallow monitoring wells. Detailed analysis of the monitored groundwater data would provide some helpful implications for optimal and efficient management of groundwater resources in this metropolitan city. Copyright © 2005 John Wiley & Sons, Ltd.     

Polybrominated diphenyl ethers (PBDEs) in marine sediments from industrialized bays of Korea

Concentrations of polybrominated diphenyl ethers (PBDEs) were determined in coastal sediments from heavily industrialized areas and major harbors in Korea. SummationPBDE(20) concentrations in sediments ranged from 2.03 to 2253ng/g dry weight. PBDE concentrations were higher at estuarine and inner bay locations close to industrial complexes and large harbors, indicating that the PBDE contamination in the sediments is due to local discharges from industrial complexes. Deca-BDE was the predominant congener and the concentrations were comparable to or higher than those reported from other countries. Non-parametric multidimensional scaling (MDS) ordination showed that deca-BDE technical mixture is the main source of PBDE contamination in Korean coastal waters, with minor contamination by octa-BDE product. This result is consistent with the consumption pattern of brominated flame-retardants (BFRs) in Korea. Significant correlations existed among BDEs 28, 47, 99, 100, 153, and 154; however, BDEs 183 and 209 showed little correlation with less highly brominated congeners.     

Report on the Annual Gametogenesis and Tissue Biochemical Composition in the Gray mussel,Crenomytilus grayanus (Dunker 1853) in the Subtidal Rocky Bottom on the East Coast of Korea

Ocean Science Journal - The Gray mussel Crenomytilus grayanus occurs in high density in the shallow rocky subtidal zone on the east coast of Korea, where the mussel dominates the shallow subtidal...     

Spring Distribution of Ciliate Plankton in the Southeastern Yellow Sea in 2019

Ocean Science Journal - As planktonic ciliates have been recognized as important players in marine microbial food webs, relevant studies have been conducted in the western coastal waters of the...     

Marine-Derived Fungi in Korea

Ocean Science Journal - Studies on marine fungi (termed as ‘marine-derived fungi’ in this paper) have been significantly increasing worldwide because of the critical role displayed by...