Estimation of paddy field dam effect on flood mitigation focusing on Suse region of Hyogo, Japan

Recently, Hyogo Prefecture in Japan has expanded its paddy field dam activity within the prefecture using wood weir plates. A wood weir plate was placed at the drainage outlet of a paddy field to limit drainage from the field, thus enabling the paddy fields to store rainwater to a certain extent. As the effect on flood mitigation was not appropriately estimated, we carried out a field experiment using a wood weir plate to estimate the weir coefficient, which determines the drainage discharge from the paddy according to the water depth. Then, using the estimated weir coefficient, we developed a 3,000 m2 paddy field dam model. Thereafter, we applied the model to the Suse region (approximately 33 ha) considering the 2009 and 2011 rainfall events. It was observed that the paddy field dam has a relatively large effect on reducing the peak discharge (approximately 25.2% for 2009 and 48.5% for 2011) within the proximal downstream areas of the paddy dam site. However, the simulation showed that when the rainfall is extreme, as was the case in 2009, the paddy dam activity may cause erosion of the paddy levee due to the increase in the paddy water depth.