Overview
Weir gate operations on regulated rivers alter hydraulic residence time, water depth, and nutrient dynamics — all of which influence water quality and the occurrence of harmful algal blooms (HABs). This study applied a three-dimensional hydrodynamic and water quality model to evaluate the effects of weir gate control at selected locations in the Nakdong, Geum, and Yeongsan Rivers in South Korea.
Model Setup
The model simulated annual and summer conditions under open- and closed-gate scenarios at upstream and downstream weir sites, including Sejong and Baekje Weirs on the Geum River and Seungchon and Juksan Weirs on the Yeongsan River. Chlorophyll-a concentrations and HAB occurrences were tracked at each location.
Key Findings
For the Geum and Yeongsan Rivers, opening the gates decreased annual and summer chlorophyll-a and HABs at upstream locations but increased summer concentrations downstream. The reduced hydraulic residence time upstream left less time for nutrient consumption, resulting in higher dissolved inorganic phosphorus that fueled algal growth downstream. In the Nakdong River, however, both annual and summer chlorophyll-a and HABs increased at all locations under the open scenario — likely driven by increased light availability from reduced water depths. The contrasting responses underscore that gate control outcomes depend strongly on site-specific physical, chemical, and biological conditions.