Satellite imagery reveals that North Korea and China have reopened the Supung Dam’s floodgates on the Yalu River, releasing water once more. This action appears to be a preventive measure to control the reservoir’s water level, as increased flow from the river’s upper reaches threatens to exceed the Supung Reservoir’s maximum capacity.
I analyzed flood damage in the Yalu River basin, focusing on the Supung Dam and Sinuiju area, using Synthetic Aperture Radar (SAR) images from the European Space Agency’s Sentinel-1 satellite. These radar sensors can observe Earth in all weather conditions, offering a distinct advantage over optical satellite images that are affected by darkness, clouds, or time of day. By comparing SAR images before and after heavy rains, I examined the flood situation using publicly available data. This analysis showcases the benefits, limitations, and unique characteristics of SAR imagery in assessing environmental changes and disaster impacts.
After heavy rains on July 28 and 29, satellite images captured several instances of the 420-meter-long floodgates being fully opened for about a week until Aug. 4. Due to cloudy and poor weather conditions, the situation at the Supung Dam was not observable until Aug. 7, when satellite imagery indicated that the water release had halted. On Aug. 15, the dam was found to have partially released water by opening some of its floodgates.
The partial release of water from the Supung Reservoir likely aims to control its level as the Yalu River swells with increased flow from upstream North Korean provinces. In adjacent Liaoning province, flooding has submerged a small dam, creating a 6-kilometer diversion channel. This waterway formed when Supung Reservoir overflow followed a low-lying valley, bypassing the Supung Dam and rejoining the Yalu River downstream. The rapid flow through this new channel may have damaged or destroyed Chinese villages and facilities previously located in the valley. Due to the flooding, the small Chinese dam is currently not visible on satellite imagery.
I examined the situation around the Supung Dam before and after the heavy rains using SAR imagery. From the radar satellite images, I can confirm that the width of the Yalu River has increased as the water level has risen. The SAR images also show that part of the tail-shaped land on the riverbank has been submerged where the water flowing down along Gugok Stream from the interior of North Pyongan province meets the Yalu River.
The above SAR images show the Yalu River basin around Sinuiju before and after heavy rains. Radar satellite images are shown in black and white, with areas that reflect less to the satellite sensor, such as the Yalu River water or Sinuiju airport, appearing in black. On the island of Wihwa, which was split into several parts by the heavy rains, the muddy streams created by the rapid currents are clearly visible as thin black lines. I have indicated the place where Kim Jong Un visited by train on July 28.
SAR images are produced by satellites firing radar waves at the ground and recording the strength of the electromagnetic beams reflected from the surface and detected by the satellite’s sensors. The data can be downloaded free of charge from the European Space Agency’s (ESA) web browser (Copernicus Browser) after registration. The imaging cycle is relatively long at 12-day intervals. SAR satellite images show surface features in black and white and have the advantage that they can be acquired regardless of weather conditions, even at night when it’s cloudy or overcast. Optical satellite images, which are widely used, can represent natural colours as we see them with our eyes, but are dependent on sunlight and affected by weather conditions. SAR data represent the rough and smooth states of the surface, so some expert knowledge is required to understand the satellite images. Rough surfaces appear bright due to high reflectivity, while smooth surfaces such as lake water appear dark due to low reflectivity. As shown in the image above, the Supung Reservoir and Yalu River water appear dark due to low surface reflectivity.
The history of SAR satellites dates back to the 1970s, making it as long as that of optical satellites. SAR data has been less utilized because it requires specialized techniques to understand and process surface conditions and has some challenging aspects. As a result, SAR data has been used in a more limited way, often as an adjunct to optical imagery rather than independently. Analysts who interpret and analyze satellite imagery typically avoid SAR images because of the limited information they produce and their inconveniences. However, the images are undoubtedly an important asset of Earth observation satellites.