• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.16 no.4
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• pp.223-237
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• 2011

The intermittent freshwater discharge has an critical influence upon the biophysical environments and the ecosystems of the Yeongsan Estuary where the estuary dam altered the continuous mixing of saltwater and freshwater. Though freshwater discharge is controlled by human, the extreme events are mainly driven by the heavy rainfall in the river basin, and provide various impacts, depending on its magnitude and frequency. This research aims to evaluate the magnitude and frequency of extreme freshwater discharges, and to establish the magnitude-frequency relationships between basin-wide rainfall and freshwater inflow. Daily discharge and daily basin-averaged rainfall from Jan 1, 1997 to Aug 31, 2010 were used to determine the relations between discharge and rainfall. Consecutive daily discharges were grouped into independent events using well-defined event-separation algorithm. Partial duration series were extracted to obtain the proper probability distribution function for extreme discharges and corresponding rainfall events. Extreme discharge events over the threshold 133,656,000 $m^3$ count up to 46 for 13.7y years, following the Weibull distribution with k=1.4. The 3-day accumulated rain-falls which occurred one day before peak discharges (1day-before-3day -sum rainfall), are determined as a control variable for discharge, because their magnitude is best correlated with that of the extreme discharge events. The minimum value of the corresponding 1day-before-3day-sum rainfall, 50.98mm is initially set to a threshold for the selection of discharge-inducing rainfall cases. The number of 1day-before-3day-sum rainfall groups after selection, however, exceeds that of the extreme discharge events. The canonical discriminant analysis indicates that water level over target level (-1.35 m EL.) can be useful to divide the 1day-before-3day-sum rainfall groups into discharge-induced and non-discharge ones. It also shows that the newly-set threshold, 104mm, can just separate these two cases without errors. The magnitude-frequency relationships between rainfall and discharge are established with the newly-selected lday-before-3day-sum rainfalls: $D=1.111{\times}10^8+1.677{\times}10^6{\overline{r_{3day}}$, (${\overline{r_{3day}}{\geqq}104$, $R^2=0.459$), $T_d=1.326T^{0.683}_{r3}$, $T_d=0.117{\exp}[0.0155{\overline{r_{3day}}]$, where D is the quantity of discharge, ${\overline{r_{3day}}$ the 1day-before-3day-sum rainfall, $T_{r3}$ and $T_d$, are respectively return periods of 1day-before-3day-sum rainfall and freshwater discharge. These relations provide the framework to evaluate the effect of freshwater discharge on estuarine flow structure, water quality, responses of ecosystems from the perspective of magnitude and frequency.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.16 no.4
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• pp.246-253
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• 2011

In this study, we investigated the effect of freshwater discharge on benthic community in Nakdong estuaries. The sediment was dominated by sand except few channels where heterogenetic sediment composition was observed. Sediment composition was rarely affected by freshwater discharge during the rainy season; however, organic content in the sediment slightly increased after the rainy season. Macrobenthic species composition differed spatially. For instance, species composition decreased after the rainy season near the barrage. Benthic community analysis revealed three groups, in which the first group was found between barrage and sand bars located at the mouth of estuary, the second group was observed outside the sand bars, and the last group was found in the channel. Opportunistic benthic species indicative of organic pollution, such as Sinocorophium sinensis, Magelona japonica, and Heteromastus filiformis, dominated areas close to the barrage. Organic pollution by freshwater discharge appears to be responsible for the emergence of opportunistic benthic species, and this influenced areas from the mouth of bay to sand bars. Outside the sand bars, freshwater discharge did not seem to have affected species composition.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.19 no.1
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• pp.41-52
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• 2014

The impact of freshwater discharge on plankton was investigated in a semi-closed Cheonsu Bay during the rainy season. Field observations for environmental factors (seawater temperatures, salinities, chlorophyll a concentrations and nutrient concentrations) and zooplankton sampling were carried out from June 27 to September 1, 2012 at about 10 days interval. The relationship between the measured parameters and estimated values of zooplankton abundance were examined. After freshwater discharge, nutrient concentrations increased up to about twice and resultant increase of chlorophyll a followed within about 10 days. Both positive and negative impacts on zooplankton were observed. Positive one was the numerical response of zooplankton on this increased food supply, and negative one was the increased mortality caused by sudden changes in salinity. Maximum mortality of copepods was found to be up to 40%, although fast sinking of carcass made it difficult to estimate reliable mortality caused by salinity shock.

• Korean Journal of Ecology and Environment
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• v.44 no.4
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• pp.337-346
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• 2011

We investigated nutrient loading and trophic states in a coastal estuarine system in the Asan estuary by assessing phytoplankton biomass and using the trophic index (TRIX). The monthly and yearly nutrient loading (TN, TP) from freshwater discharge from the Asan and Sapgyo reservoirs into the estuary were estimated and analyzed with related factors. Monitoring data (physio-chemical and biological variables) collected at five estuary stations were used to assess trophic states. Descriptive statistics of total phytoplankton cells, chl a concentrations and primary productivity were also used to assess seasonal trophic status. N loading from freshwater ranged $1.0{\sim}1.3{\times}10^4$ ton yearly. The yearly P loading ranged between 350 and 400 ton during 2004~2006, increasing to 570 ton in 2007. Regression results suggest that DIN and DSi were correlated with freshwater discharge at the upper region. Based on phytoplankton biomass and total cell abundance, the trophic state of the estuary was found to be eutrophic during spring due to phytoplankton bloom. Primary productivity level was remarkably high, especially in summer coinciding with high nutrient loading. Pheopigments increased during warm seasons, i.e. summer and fall. Trophic index results indicate that the trophic state varied between mesotrophic and eutrophic in the estuary water body, especially in the upper region. The results suggest that phytoplankton production was regulated by nutrient loading from freshwater whereas biomass was affected by other properties than nutrient loading in the Asan Estuary ecosystem.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.17 no.3
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• pp.139-148
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• 2012

The short-term variation of salinity and temperature in a dyked estuarine environment is mainly controlled by the freshwater discharge from the dyke. We examined the distribution of salinity and temperature by the freshwater discharge in the Yeongsan River estuary using the CTD data obtained from 8 stations through three surveys in June (weak discharge) and August (intensive discharge), 2010. During the weak discharge in June, the surface salinity showed 30-32.5 psu and its horizontal gradient was relatively high around Goha-do (0.25~0.32 psu/km). On the other hand, the salinity of the bottom layer was almost constant in the range of 33 psu. Water temperature ranged $19{\sim}21^{\circ}C$ and displayed higher gradient in north-south direction than the gradient of east-west direction. During the intensive freshwater discharge on August 12, the salinity dropped to 9~26 psu. The maximum horizontal gradient of surface salinity reached 3.8 psu/km in the north of Goha-do where the strong salinity front was formed, and the horizontal salinity gradient of bottom layer was 0.28 psu/km. The horizontal gradient of water temperature was $-0.45^{\circ}C/km$ in the surface and $-0.12^{\circ}C/km$ in the bottom with high surface temperature near the dyke and decreasing gradually to the river mouth. After 3 days of the intensive discharge ($3^{rd}$ survey), the surface salinity increased to 22~26 psu. However, there still existed relatively high horizontal gradient around Goha-do. In the mean time, the bottom salinity decreased to 26.5~27.5 psu, but its gradient was not big as much as the surface gradient. According to time series of CTD profile near the dyke, the discharged fresh water jetted down temporarily and then recovered gradually with the recovering speed of 0.4 m/hour for the discharge case of $13{\times}10^6$ ton. Due to the combined effects of freshwater discharge and surface heating during the summer of 2010, the Yeongsan estuary, in general, underwent intensified vertical stratification, which in turn caused the inhibition of vertical mixing, especially inside area of estuary. Based on the spatial distribution of salinity and temperature, the Yeongsan estuary can be divided into three regions: the Goha-do area with strong horizontal gradient of salinity and temperature, inner estuary from Goha-do to the dyke with low salinity, and outer estuary from Goha-do to the coasts with relatively high salinity.

• Journal of Environmental Science International
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• v.9 no.5
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• pp.375-384
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• 2000

It is very important to interprete and simulate the variation of phytoplankton maximum region for the prediction and control of red tide. This study was composed of two parts first the hydrodynamic simulation such as residual current and salinity diffusion and second the ecological simulation such as phytoplankton distribution according to freshwater discharge and pollutant loads. Without the Nakdong river discharge residual current was stagnated in inner side of this estuary and surface distribution of salinity was over 25psu. On the contrary with summer mean discharge freshwater stretched very far outward and some waters flowed into Chinhae Bay through the Kadok channel and low salinity extended over coastal sea and salinity front occurred. From the result of contributed physical process to phytioplankton biomass the accumulation was occurred at the west part of this estuary and the Kadok channel with the Nakdong river discharge. When more increased input discharge the accumulation band was transported to outer side of this estuary. The frequently outbreak of red tide in this area is caused by accumulation of physical processes. The phytoplankton maximum region located inner side of this estuary without the Nakdong river discharge and with mean discharge of winter but it was moved to outer side when mean discharge of the Nakdong river was increased. The variation of input concentration from the land loads was not largely influenced on phytoplankton biomass and location of maximum region. When discharge was increased phytoplankton maximum region was transferred to inner side of the Kadok channel. ON the other hand when discharge was decreased phytoplankton maximum region was transferred to inner side of this estuary and chlorophyll a contents increased to over 20$\mu\textrm{g}$/L Therefore if any other conditions are favorable for growth of phytoplankton. decreas of discharge causes to increase of possibility of red tide outbreak.

• Korean Journal of Ecology and Environment
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• v.42 no.4
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• pp.432-440
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• 2009

During the period of past fifteen years (1992~2006), variations of chlorophyll a in relation with water quality in freshwater reservoirs were investigated. This study compared total nitrogen (TN), total phosphorus (TP), chlorophyll a, Secchi depth (SD) and total suspended solids (TSS) between terrestrial freshwater reservoir and coastal freshwater reservoir systems based on their location. Regression analyses (linear and non-linear regressions) were applied for all study sites to examine relationship and interaction of these factors in the freshwater systems from in-land to coasts. The results demonstrated that chlorophyll a was significantly correlated to total phosphorus ($R^2=0.94$, P<0.0001) and was remarkably related to TSS increase ($R^2=0.63$, P<0.0001) in the selected reservoirs. The TN : TP ratio in the reservoir systems was higher than Redfield ratio (16 : 1) indicating that the reservoirs are potentially experiencing P limitation. Water quality of coastal freshwater reservoir system was more significantly decreased than the reservoirs located in in-land during the past fifteen years. The strict management of nutrient discharge into freshwater systems should implemented in the coastal reservoirs since the freshwater is introduced into coastal estuarine systems.

• Journal of the Korean Society of Marine Environment & Safety
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• v.29 no.6
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• pp.576-586
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• 2023

In this study, a numerical particle tracking experiment was conducted to assess the distribution characteristics of freshwater exclusion resulting from the discharge of Gahwa Stream into Sacheon, Jinju, and Gangjin bays, located downstream of the Namgang Dam. The number of particles discharged into Noryang Channel, Daebang Channel, and Changseon Strait was compared by releasing 1000 particles through Gahwa Stream under three discharge conditions: no discharge, discharge during rainfall, and discharge during flood. Evidently, the percentage of particles in the Noryang Channel increased, whereas that in the Daebang Channel decreased as the discharge from the Gahwa Stream increased. Approximately 95% of the material located downstream of the Gahwa Stream generally escaped through the Daebang Channel. However, as the discharge from the Namgang Dam increased due to rainfall, the percentage of particles in the Noryang Channel increased, reaching 45.5% during floods.

• Journal of Environmental Science International
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• v.22 no.8
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• pp.937-944
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• 2013

Ozone concentrations in water and air, and resulting disinfective properties, were measured following generation by either an ozone generator or a low-temperature dielectric barrier discharge plasma generator. In freshwater, ozone concentrations of 0.81 and 0.48 mg/L $O_3$ were observed after the ozone and plasma generators had been operated for five minutes, respectively. Higher levels of dissolved $O_3$ were attained more easily with the ozone generator. In seawater, both systems were capable of creating concentrations greater than 3.00 mg/L $O_3$ after 5minutes of operation. Higher ozone levels were attained more easily in seawater than in freshwater. Rates of bacterial sterilization in seawater after three minutes were 96% and 88%, using the plasma and ozone generators, respectively. In freshwater, higher concentrations of ozone were released into the atmosphere by the ozone generator than by the plasma generator. In creating equivalent levels of dissolved ozone in freshwater, the plasma generator released 4.5 times more ozone into the atmosphere than did the ozone generator. This shows that ozone generators are more effective than plasma generators for creating ozonated water. For the same concentration of dissolved ozone in seawater, more ozone was released into the atmosphere using the ozone generator than using the plasma generator. Therefore, with regard to air pollution, plasma generators seem to be less expensive than ozone generators.

• Magazine of the Korean Society of Agricultural Engineers
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• v.44 no.6
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• pp.124-136
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• 2002

A freshwater lake water quality management system(FLAQUM) was developed to help regional manager for the water quality of a rural basin. The integrated user interface system FLAQUM written in Visual Basic, includes three subsystems such as a database management system, basin pollutant loads simulation model using SWMM model and freshwater lake water quality simulation model using WASP5 model. Pollutant load simulation model was applied to simulate the discharge and pollutant loading from the watershed, and freshwater lake water quality model was applied to analyze the changes in water quality with respect to watershed pollutant loads, and this model could be used in planning to control watershed pollutant source for water quality management. Database management system was constructed fur all input and output data processing, and it can be used to analyze statistical characteristics using constructed data. Results are displayed both graph and text for convenience of user. The results of FLAQUM application to Boryeong freshwater lake showed that the lake was in eutrophic condition. The major contribution of pollution comes from tributary No.1 and No.4, which have a large number of livestock farms. Therefore, water quality management must be focused on appropriate management of the livestock farming in the two breanchs.