본 연구에서는 섬진강의 염분 침투 특성을 분석하기 위하여 섬진강 및 광양만에 환경변화와 기초자료를 활용하여 EFDC 모델을 구축하였다. 모델 보ㆍ검증을 실시하여 모델의 신뢰성을 확보한 후 송정유량 및 다압취수량에 대하여 28개 시나리오에 대해 수치모의 실험을 수행하였다. 주요 4개 지점(두곡, 신비, 목도, 화목)에 대하여 염분농도를 전체기간, 대조기, 소조기로 나누어 분석하였고 염분의 공간적 평면 및 성층 분포를 분석하고 염분과 유량 모델을 구축하였다. 또한, 하구로부터 거리에 따른 염분침투 종단분포를 분석하고 염분 최대 침투거리 분석 및 염분과 침투거리 산정 모델을 구축하였다. 마지막으로 염분저감 시나리오를 구성하고 시나리오별 염분저감 효율을 정량적으로 산정하여 제시하였다.
염분농도 변화는 송정유량 5~40 cms에서 두곡 4.7 psu, 신비 16.0 psu, 목도 19.2 psu, 화목 28.2 psu로 나타났으며, 대조기는 염분 변화 폭은 증가, 평균 농도는 감소, 소조기는 염분 변화 폭은 감소, 평균 농도는 증가하였다. 신비, 목도, 화목은 소조기에 비해 염분농도가 크게 증가하여 염분 성층화 현상을 보여주었다.
다압취수량 2.523~4.630 cms에서 두곡이 다압취수량에 따라 염분차이가 가장 크게 나타났고, 하류로 갈수록 염분차이가 가장 적게 나타났다. 다압취수량에 따라 염분차이는 1 psu 미만으로 나타나 다압취수량이 염분농도에 미치는 영향은 작은 것으로 판단하였다. 대조기에 다압취수장이 미치는 영향은 상대적으로 다소 크고, 소조기에는 염분 성층화 현상이 발생하고 있다.
염분의 공간적 평면 분포에서 염분 최대농도는 송정유량이 증가할수록 감소하였다. 대조기보다 소조기에는 감소폭이 작아 송정유량 영향이 감소하였고, 대조기 및 소조기 염분차이가 커져 염분 성층화 현상이 뚜렷이 나타나는 것으로 판단하였다. 대조기 보다는 소조기에 고농도 염분이 상류쪽으로 이동하였다.
염분의 공간적 성층 분포에서 대조기에는 송정유량이 증가함에 따라 성층현상이 발생하였고, 소조기에는 강한 성층현상이 발생하고 있으며, 송정유량이 증가할수록 성층현상 정도가 커지며, 하구로부터 멀어질수록 성층현상이 강하게 나타났다.
염분 침투 농도는 하구기점과 15~20 km 지점의 염분 차이는 송정유량이 증가할 수록 크게 나타났으며, 송정유량이 25 cms이상에서는 염분차는 거의 변화가 없는 것으로 분석하였다. 다압취수량에 따라서는 최대 염분차는 약 1 psu로 나타나 변화가 매우 작은 것으로 분석하였다.
두곡, 신비, 목도, 화목지점에 대해 정량적 예측 모델을 구축하였으며, 전체기간 지수 값이 상류 두곡지점에서 하류 화목지점까지 갈수록 –1.114에서 –0.123으로 절대값이 감소하여 하류로 갈수록 송정유량의 영향이 감소하며 전체기간보다 대조기 값이 더 크고 소조기 값은 더 작아 송정유량은 소조기보다 대조기에 더 영향을 많이 주고 있음을 알 수 있었다.
목표 염분농도 달성을 위한 하천유지유량을 산정한 결과, 전체기간 두곡은 7 cms, 신비 12 cms, 목도 21 cms, 화목 92 cms로 나타났다. 대조기에는 두곡지점은 전체기간과 동일하였으나, 신비, 목도, 화목은 감소하였다. 소조기에는 신비, 목도, 화목지점이 모두 크게 증가하였으며, 특히 화목지점을 20 psu 달성하기 위해서는 송정유량이 100 cms이상 필요한 것으로 산정하였다.
송정유량(염분 1 psu 기준) 5 cms에서 24.1 km, 40 cms에서 18.2 km로 침투 거리 변화는 5.9 km로 분석하였고, 다압취수량 증가에 따라 염분침투 거리 영향은 작은 것으로 분석하였다.
송정유량과 염분침투 거리 모델을 이용하여 분석한 결과 대조기에는 송정유량의 영향이 큰 것으로 분석하였으며, 소조기에는 송정유량의 영향이 상대적으로 적으며, 이는 염분 성층화 현상에 의한 것으로 판단하였다. 다압취수량과 염분침투 거리의 모델 결과 다압취수량의 영향은 매우 작은 것으로 판단하였다.
염분저감 효과를 4가지 시나리오에 대해 분석한 결과 하상복원(RA-1~3)에 대한 저감 효과는 전체기간에 두곡 4.3~6.3 psu, 신비 5.6~9.4 psu, 목도 4.9~8.1 psu, 화목 1.0~2.0 psu로 분석되어 저감 효과가 크게 나타났다. 또한, 대조기보다는 고염분이 지속되는 소조기에 더 많은 저감 효과가 나타났다.
염분침투 거리는 저감 효과가 0.7~2.6 km로 나타났으며, 하상복원에 따른 염분침투 거리 계산식을 산정하여 향후 염분침투 거리를 산정할 수 있을 것으로 판단하였다.
수중보(RB-1~3), 수제(RC-1~3), 플러싱(RD-1~3)의 저감효과는 1 psu 이하로 나타나 저감 효과가 매우 적은 것으로 분석되었으며, 다만 플러싱은 고농도 염분이 지속되는 소조기에 염분 저감 효과가 있을 것으로 판단하였다.
염분침투 거리는 저감 효과는 3가지 시나리오 모두 0.0 km로 나타나, 침투거리 저감 효과를 기대할 수 없을 것으로 판단하였다.

연구결과를 종합하여 섬진강 염분 침투 특성을 분석한 결과 염분농도는 소조기가 지배하며, 염분침투거리는 대조기가 지배함을 알 수 있었고, 송정유량은 대조기에 지배력이 강하고 소조기는 염분 성층화 현상으로 지배력이 약하였다.
유량과 염분, 유량과 염분 침투거리 모델을 구축하고 목표염분 달성을 위한 하천유지 유량을 산정할 수 있었다.
염분저감 시나리오 중 하상복원 효과가 가장 좋으나, 인위적인 저감 방안은 다른 분야에 영향을 줄 수 있고, 비용적ㆍ현실적인 측면에서 어려움이 있다. 따라서, 섬진강 하상고가 자연적으로 상승하고 있으므로, 자연적인 복원이 필요하다고 판단하였다.

In this study, in order to analyze the characteristics of saltwater intrusion in the Seomjin River, an EFDC model was constructed for the Seomjin River and Gwangyang Bay using environmental changes and basic data. After confirming the reliability of the model by calibration and verification of the model, numerical experiments were conducted with 28 scenarios for the Songjeong flow rate and Daap intake rate.
The salinity concentration was analyzed by dividing into the whole period(1 month), the spring period, and the neap period for the four main points(Dugog, Sinbi, Mogdo, and Hwamog). The spatial plane and stratified distribution of salinity were analyzed and a salinity and flux model was constructed.
The longitudinal distribution of saltwater intrusion according to the distance from the estuary was analyzed, and the maximum saltwater intrusion distance was analyzed and a salinity and saltwater intrusion distance model was constructed.
Finally, salinity reduction scenarios were constructed, and the salinity reduction efficiency for each scenario was quantitatively calculated and presented.
The changes in salinity concentration were 4.7 psu for Dugog, 16.0 psu for Sinbi, 19.2 psu for Mogdo, and 28.2 psu for Hwamog at 5~40 cms of flow rate in Songjeong. In the spring period, the range of change in salinity increased, but the average concentration decreased, and in neap period, the range of change in salinity decreased, but the average concentration increased. Sinbi, Mogdo, and Hwamog showed a salinity stratification phenomenon as the salt concentration increased significantly compared to the neap period.
Between 2.523 and 4.630 cms of Daap intake rate, Dugog showed the largest difference in salinity according to the Daap intake rate, and the smallest difference in salinity was found in the downstream. The difference in salinity was less than 1 psu according to the Daap intake rate, so the effect of the Daap intake rate on the salt concentration was judged to be small. In the spring period, the effect of the Daap intake rate is relatively large, and the salt stratification phenomenon occurred in the neap period.
In the spatial plane distribution of salinity, the maximum concentration of salinity decreased as the Songjeong flow rate increased. It was judged that the influence of the Songjeong flow rate was reduced in the neap period compared to the spring period, and the salinity stratification phenomenon appeared clearly. The high-concentration salt moved upstream in the neap period than the spring period.
In the spatial stratified distribution of salinity, stratification occurred as the Songjeong flow rate in the spring period increased, and strong stratification occurred in the neap period. The degree of stratification increased as the Songjeong flow rate increased, and the stratification became stronger as the distance from the estuary increased.
As for the saltwater intrusion concentration, the salinity difference between the estuary and the 15-20 km point increased as the Songjeong flow rate increased, and it was analyzed that the salinity difference was almost unchanged when the Songjeong flow rate was 25 cms or more. The maximum salinity difference was about 1 psu according to the Daap intake rate, and the change was analyzed to be very small.
A quantitative prediction model was built for Dugog, Sinbi, Mogdo, and Hwamog points, and the absolute value of the index value for the entire period decreased from -1.114 to -0.123 from the upstream Dugog point to the downstream Hwamog point, and the influence of the Songjeong flow rate decreased in the downstream. It was found that the control period value was larger than that of the entire period, and the neap period value was smaller, indicating that the Songjeong flow rate had a greater effect on the spring period than the neap period.
As a result of calculating the instream flow to achieve the target salinity concentration, the entire period was 7 cms for Dugog, 12 cms for Sinbi, 21 cms for Mogdo, and 92 cms for Hwamog. The instream flow decreased in Sinbi, Mogdo, and Hwamog in the spring period. Sinbi, Mogdo, and Hwamog points all greatly increased during the neap period. In particular, in order to achieve 20 psu in Hwamog, it was calculated that a Songjeong flow rate of 100 cms or more was required.
Songjeong flow rate (1 psu standard) was analyzed as 24.1 km at 5 cms and 18.2 km at 40 cms, and the change in saltwater intrusion distance was 5.9 km. It was analyzed that the effect of the saltwater intrusion distance was small according to the increase in the Daap intake rate.
As a result of analysis using the Songjeong flow rate and saltwater intrusion distance model, it was analyzed that the Songjeong flow rate was significant in the spring period. In the neap period, the influence of the Songjeong flow rate was relatively small, which was determined to be due to the salinity stratification phenomenon. As a result of the model of the Daap intake rate and the saltwater intrusion distance, the effect of the Daap intake rate was judged to be very small.
The salinity reduction effect was analyzed for 4 scenarios. The reduction effect on the river bed restoration (RA-1~3) was analyzed as 4.3~6.3 psu for Dugog, 5.6~9.4 psu for Sinbi, 4.9~8.1 psu for Mogdo, and 1.0~2.0 psu for Hwamog over the entire period. In addition, more reduction effects were shown in the neap period with high salinity than in the spring period. The reduction effect of the saltwater intrusion distance was found to be 0.7~2.6 km. It was determined that the saltwater intrusion distance could be calculated by calculating the saltwater intrusion distance calculation formula according to river bed restoration.
The reduction effect of submerged weirs (RB-1~3), groyne (RC-1~3), and flushing (RD-1~3) was less than 1 psu, and it was analyzed that the reduction effect was very small. However, it was judged that flushing would have an effect of reducing salinity in the neap period.
The effect of reducing the saltwater intrusion distance was 0.0 km in all three scenarios, so it was judged that the effect of reducing the saltwater intrusion distance could not be expected.
As a result of analyzing the saltwater intrusion characteristics of the Seomjin River, the salt concentration was dominated by the neap period and the saltwater intrusion distance was dominated by the spring period.
Songjeong flow rate had strong dominance in the spring period and weak dominance due to salinity stratification in the neap period.
A model of flow rate and salinity, flow rate and saltwater intrusion distance was established, and the instream flow to achieve the target salinity could be calculated.
Among the salinity reduction scenarios, the riverbed restoration effect is the best, but the artificial reduction method can affect other fields and has difficulties in terms of cost and reality.
Therefore, since the height of the Seomjin River bed is rising naturally, it was judged that natural restoration is necessary.