Hydrologie study of artificial flooding tactics at Bicchutakamatsu Castle by Hideyoshi based on a flood-inundation simulation

Abstract

In May 1582 (lunar calendar, Tensho 10). the battle called Artificial Flooding Tactics at Bicchutakamatsu Castle by Hideyoshi broke out at Bicchutakamatsu, in Okayama prefecture, Japan. For the battle, Hideyoshi Hashiba (Toyotomi) reportedly built a bank 7 m in height 21 m in bottom width, 10m in top width, and 3 km in length. He dammed the Ashimori River to direct water toward Bicchutakamatsu castle, causing flooding around the castle. Hideyoshi is said to have built the bank in only 12 days. However, previous studies pointed out that it would have been difficult to build such a large bank in such a short time. According to recent studies, the use of a natural levee located west of the castle enabled the completion of the bank within 12 days. On the other hand, no previous report described Artificial Flooding Tactics at Bicchutakamatsu Castle by Hideyoshi from a hydrologie science viewpoint. Therefore, we simulated the flooding event based on a hydrologie model, mainly comprising artificial flooding and an inundation flow simulation. After analyzing the simulated results, we inferred the necessary conditions for the tactics. Around Bicchutakamatsu, a flat plain extends below 10 m a.s.l. with small asperities and with moimtains higher than 300 m a.s.l. circumjacent to the plain. Considering these geographic conditions, we developed a hydrologie model. We used a kinematic wave model to simulate runoff from the mountains and a dynamic wave model for the inundation flow simulation. We created a high-resolution digital elevation model (DEM) using fundamental geospatial data scale level 2,500 spot elevation to represent the small asperities around Bicchutakamatsu. Using this high-resolution DEM, along with public reports of a previous excavation and our own field survey, we recreated the DEM that was presumed to have existed at the time of the tactical event We hypothesized several scenarios with and without the existence of banks, with differing heights of banks, and with the present and historic DEM. Based on these scenarios, we conducted several simulations of the tactical event From the results, we inferred that the bank around Kawazugahana, and inflow from the Ashimori River were necessary for Artificial Flooding Tactics at Bicchutakamatsu Castle by Hideyoshi, in addition to the usage of a natural levee located west of the castle. With this finding and the historical records, we inferred that the height of the bank around Kawazugahana was about 3 m.