Cause analysis of dam break in Zhumadian reservoir

We can analyze the causes of the extraordinary flood in southern Henan in early August of 1975 from the following four aspects:

As far as general circulation is concerned.

1In the first ten days of August, 975, the westerly circulation situation in East Asia was relatively stable, and the equatorial convergence zone advanced northward, which was very active. In particular, Typhoon No.7503 went deep into the inland, moving slowly, even with a slight increase in intensity, and formed a strong water vapor channel in the eastern periphery of the typhoon, which kept the southern part of Henan Province in a continuous strong upward movement, abundant water vapor supply and constantly reconstructed unstable gas layer, creating good conditions for the emergence of continuous heavy precipitation.

Typhoon No.7503 landed in Fujian Province on August 4th, passed through Gannan and Hubei, turned to the northeast at 02: 00 on the 6th, turned to the north on the 6th 14, and moved slowly. It reached Tongbai Mountain at 20: 00 on the 6th, arrived near Biyang County in Henan Province at 08: 00 on the 7th, then turned to the southwest for the second time, and disappeared in the south of Daba Mountain on the 8th 14. Typhoon depression stayed in southern Henan for more than 20 hours. It is extremely rare for a typhoon to go deep inland and last for such a long time.

Weather scale system configuration

The weather scale system configuration has created extremely favorable conditions for continuous heavy rain. The rainstorm system consists of three heavy precipitation processes. The first time was caused by the intersection of the moist southerly airflow on the east side of Typhoon 7503 on the south bank of the Yangtze River and the weak cold air in Central China. The second time was the direct influence of the typhoon. The rainstorm area was located in the northeast of the typhoon, and the typhoon circulation rose the fastest. At 08: 00 on the 7th, the typhoon center reached the northernmost latitude. At this time, the typhoon moved slowly and almost stopped. However, the tropical vortex in the east of Taiwan Province Province moved to the northwest and approached Typhoon No.7503, which obviously strengthened the easterly wind in the rainstorm area of Henan Province, and the convergence and rising speed of water vapor reached the maximum, resulting in the strongest third precipitation process. It is precisely because of the configuration of various weather scale systems during various rainstorms that the necessary conditions for the occurrence of heavy rainstorms (that is, continuous strong upward movement, continuous water vapor supply and continuous potential instability establishment) are close to the maximum. The continuous rainstorm was thus produced.

Good mesoscale circulation conditions

Good mesoscale circulation conditions make its rainstorm intensity rare. The intensity of such heavy rain is mainly caused by strong convective activities. In this rainstorm, convection activities are frequent and intense, especially thunderstorm activities. Thunderstorms occurred one after another in the rainstorm area, causing heavy precipitation. When the thunderstorm disappears, the precipitation is over. In the rainstorm area, a large amount of latent heat of condensation is released, which heats up the gas layer and strengthens the upward movement, thus strengthening the rainstorm. In every rainstorm, the strong convective activity is related to the convergence zone of low air scale airflow (mesoscale gas vortex).

zone

Terrain played an important role in strengthening this rainstorm. The rainstorm occurred in a series of hills from the south of Wuyang County at the eastern end of Funiu Mountain Range to Tongbai Mountain Range, forming an arc-shaped terrain, which has a strong forced convergence of the airflow from the wide easterly wind around the typhoon to the easterly wind around the northeast. To sum up, there are three aspects of terrain influence: the first is the trigger effect caused by large terrain uplift and the quasi-stable convergence area caused by small and medium terrain; Secondly, the arc-shaped terrain opening to the east produces many mesoscale vortices, thunderstorms and shear lines on the left side of the low-altitude easterly jet. Thirdly, rain clouds often move along the river valley, which makes the rainfall in the areas where rain clouds are prevalent particularly heavy, thus causing the rainstorm center to occur on the crosswind slope.

Because of the heavy rain

1975, China was in the late Cultural Revolution. In that special era, everything seemed to be in chaos. After the accident, according to the investigation team who went to investigate the situation, Typhoon No.3 at that time disappeared from the radar of the Central Meteorological Observatory, and the local meteorological bureau should have taken over the monitoring, but the Henan Meteorological Bureau at that time did not turn on the radar for various reasons. Although Nanyang Meteorological Bureau monitored the trend of typhoon, the information could not be released in time because there was no transmission equipment.

On the surface, it seems that only the unfavorable response of the meteorological department led to the final dam break. But is this really the case? Is there no other reason?

According to Chen Xing, a former Henan water conservancy expert, the highest flood level recorded at that time was117.94m, but the dam height was only116.3m.. When the dam breaks, the flood level not only exceeds the crest 1.6 meters, but also exceeds the wave wall by 3 inches. So the water overflowed the dam crest. The composition of this earth dam, its downstream slope is sandy soil, which can not stand the erosion of water, so the sand core is quickly washed away after the dam overflows. Formed a dam break. The dam collapse of Shimantan Reservoir in Banqiao Reservoir is due to the low design flood standard.