Analysis of Water Quality Variation by Lowering of Water Level in Gangjeong-Goryong Weirin Nakdong River

낙동강 강정고령보 수위저하 운영에 따른 수질 변동특성 분석

Abstract

The objectives of this study were to construct a three-dimensional water quality model (EFDC) for the river reach between Chilgok Weir and Gangjeong-Goryong Weir (GGW) located in Nakdong River, and evaluate the effect of hydraulic changes, such as water level and flow velocity, on the control of water quality and algae biomass. After calibration, the model accurately simulated the temporal changes of the upper and lower water temperatures that collected every 10 minutes, and appropriately reproduced changes in organic matter, nitrogen, phosphorus, and cyanobacteria. However, the simulated values were overestimated for the diatoms and green algae cell density, possibly due to the uncertainties of the parameters associated with algae metabolism and the lack of zooplankton predation function in the simulations. As a result of scenario simulation of running the water level of GGW from EL. 19.44 m to EL. 14.90 m (4.54 m drop), Chl-a and algae cell density decreased significantly.In particular,the cyanobacteria on the surface layer, which causes algal bloom, declined by 56.1% in the low water level scenario compared to the existing management level. The results of this study are in agreement with the previous studies that maintenance of critical flow velocity is effective for controlling cyanobacteria, and imply that hydraulic control such as decrease of water level and residence time in GGW is an alternative to limit the overgrowth of algae.

본 연구의 목적은 낙동강에 위치한 칠곡보와 강정고령보 구간을 대상으로 3차원 EFDC 모델을 구축하고, 보 구간의 운영 수위 저하 및 유속 증가와 같은 수리학적 특성 변화가 수질과 조류 생체량에 미치는 영향을 평가하는데 있다. 보정결과, EFDC 모델은 10분 단위의 고빈도로 측정된 상층과 하층 수온의 시간적 변화를 적절하게 모의하였고, 유기물, 질소, 인계열 수질항목과 남조류의 시계열 변화를 적절히 재현하였다. 하지만, 규조류와 녹조류 세포 밀도에 대해서는 모의값이 실측값을 과대 산정하였다. 규조류와 녹조류 예측의 오차 요인은 조류의 신진대사와 관련된 매개변수의 불확실성과 동물플랑크톤에 의한 포식기능이 모의에 포함되지 않은 것에 기인한 것으로 유추된다. 강정고령보의 보 운영 수위를 관리수위(EL. 19.44 m)에서 하한수위(EL. 14.9 m)까지 약 4.54 m 낮추어 운영하는 시나리오 모의결과, Chl-a와 조류 세포수 밀도가 급격히 감소했다. 특히, 녹조를 발생시키는 남조류 세포 수는 기존 관리수위에 비해 하한수위 시나리오에서 표층 기준 56.1% 급감하였다. 연구결과는 임계유속 유지가 남조류 제어에 효과적이라는 선행연구들과 일치하며, 강정고령보에서 수위 저하와 체류시간 감소와 같은 수리학적 조절은 조류의 과잉 성장을 제한 할 수 있는 대안임을 시사한다.

Figure 1. Location of study site, and numerical grid system from Chilgok weir to Gangjeong-Goryong weir.

Figure 2. Temporal variations of surface and bottom water temperature along with river discharge from May to November, 2017.

Figure 3. Comparison of observed and simulated water surface elevation.

Figure 4. Comparison of simulated water temperatures with observed high-frequency temperature data at (a) upper layer and (b) lower layer in Gangjeong-Goryong Weir.

Figure 5. Comparison of simulated water temperature and water quality concentration with observed data (ME, K-water).

Figure 6. Comparison of simulated chl-a and cell density of each algae group with observed data (ME, K-water).

Figure 7. Comparison of water temperature and water quality concentration according to operation water level scenario of the weir.

Figure 8. Comparison of chl-a and cell density of each algae group according to operation water level scenario of the weir.

Figure 9. Spatial distribution of cyanobacteria biomass as carbon unit at water surface according to the operation water level scenario of the weir on (a) 2017-06-04 (julian day 155), (b) 2017-08-04 (julian day216), (c) 2017-10-27 (julianday 300).

Figure 10. Temporal variations of cyanobacteria biomass as carbon unit at water surface according to the operation water level scenario of the weir at (a) middle point (A1) and (b) downstream point (A2).

Table 1. Calibrated model parameter values for each algae group

Table 2. Error analysis between simulated results and observed data

Table 3. Basic statistics analysis of simulation results according to operational water level of the weir

Table 3. Continued

Table 4. Normality and Wilcoxon signed-rank test, and median difference of water quality variables

Table 5. Normality and Wilcoxon signed-rank test, and median difference of algae and flow velocity

Table 6. Changes of flow velocity at mid-stream and downstream according to the operation water level scenario