The monitoring of Wanlixi Barrier Lake has been upgraded again! Unmanned ship sonar reveals underwater blind spots for the first time

The barrier lake of Wanli Creek in Wanrong Township is currently overflowing steadily. The Hualien Branch of the Forestry and Nature Conservation Department cooperated with the Chinese Disaster Prevention Society to use an unmanned ship equipped with sonar sounding equipment for the first time to detect the underwater terrain of the barrier lake and re-examine it. Based on the "water level-water storage volume" relationship, the original estimate of water storage volume was about 5.17 million cubic meters, which was revised to about 4.52 million cubic meters. The Hualien Branch pointed out that this is the first time that an unmanned ship has been used to detect the underwater topography of a barrier lake in Taiwan.

The Hualien Branch of the Forestry and Conservation Department of the Ministry of Agriculture said today that as of 9:50 a.m. on the 8th, the water level of the lake was approximately 1,086.34 meters, and the water storage capacity was approximately 4.5397 million cubic meters. The Hualien Branch analyzed that the reason for the decrease in water storage is the continuous collapse of earth and rocks on the right bank slope into the lake area, coupled with the inflow of soil and sand from the upstream and sedimentation.

Hualien Branch Director Huang Qunze explained that barrier lakes often occur in Taiwan, but large-scale ones like Matai'an Creek and Wanli Creek are rare. Disaster prevention and scientific research are very important after they are formed. Although water level gauges have been deployed in the lake area before, water level gauges alone cannot accurately grasp the continuous volume changes in the mountains. After the barrier lake is formed for a period of time, it is still necessary to measure the underwater topography in order to truly accurately grasp the water body and predict future impacts on the downstream.

Wanlixi Barrier Lake is located in a steep V-shaped canyon deep in the mountains. There are no external roads. In the past, it mainly relied on airborne LiDAR, drone aerial photography, real-time images and water level gauges to understand the appearance of the dam, the undercut of the spillway and the changes in water levels. The underwater terrain has always been a blind spot for monitoring.

In order to overcome the terrain constraints, the Chinese Disaster Prevention Society held discussions with air service and special search instructors for many times. On the 5th, the Air Service Corps of the Ministry of Interior dispatched helicopters to transport a combined manned ship and an unmanned ship equipped with sonar sounding equipment to the lake area using a belly-sling method. Five researchers were deployed, and the special search personnel assisted in safety protection and equipment unloading.

Huang Qunce said that the lake area is surrounded by nearly vertical collapsed exposed cliffs, and the environment is dangerous, so the plan must go through rigorous sand table deductions. The mission was originally scheduled to be carried out in October, but Typhoon Ropit was encountered on the eve of departure. It was postponed to ensure the safety of the crew and ground personnel. It then took about two weeks to re-evaluate and confirm the hanging points, assembly process and launching method. The unmanned ship was fully equipped and set off in the early morning of the 5th. Under the guidance of manned ships and water safety guards, the unmanned ship successfully completed comprehensive surveying and mapping.

The research team took advantage of the stable weather period, conducted intensive measurements, collected water depth and lake bottom topography data, and completed the data calculation in 7 days. In addition to recalculating the "water level-water storage capacity" relationship, they also simultaneously grasped the distribution of mud and sand sedimentation and soil and sand migration trends on the lake bottom, and explored the stability of the natural dam's underwater slope.

The Hualien Branch pointed out that the measured data has been incorporated into the hydrological calculation model, which will help improve the simulation accuracy of overflow peaks, flow rates and possible downstream impact areas under extreme rainfall. It will serve as the basis for subsequent rolling reviews of the warning benchmarks, and can also serve as the basis for scientific research on disaster prevention in Taiwan's alpine barrier lakes.

According to the forecast of the Central Meteorological Administration, the cumulative rainfall in the upstream catchment area in the next 12 and 24 hours is estimated to be 0 mm, which does not meet the rainfall warning standard. Aerial photography this morning showed that the dam body is intact and the lake water is overflowing steadily, but the impact on the downstream is still limited to the river channel. The Hualien Branch reminds that there are still about 4.529 million cubic meters of water in the lake area, and potential risks continue to exist. The red alert in the river has not yet been lifted. People should not enter the warning areas around the Wanli Creek embankment, riverbed and other areas announced by the Hualien County Government.