Evaluation of Runoff Detention Function of the Paddy Field Installed with Runoff Control Devices

Abstract

Since 2002, Kamihayashi district in Niigata prefecture has been undertaking flood mitigation measures using paddy fields by installing runoff control devices in drainage pits. The purpose of this study is to quantitatively evaluate the effect of this runoff control device on runoff detention at the paddy field scale. The factors controlling the rate of surface water drainage from a paddy field during ponding season are the width of the drainage outlet (rectangular weir) and the diameter of the drainage hole on the floor of the drainage pit (orifice). On the other hand, during midsummer drainage season, the surface of the paddy field is furrowed for promoting quick drainage. Calculation of runoff rate with and without runoff control devices is conducted using the formulae of rectangular weir and orifice in the case of ponding season, and an orifice formula incorporated in tank model for midsummer drainage season. The result shows that the peak discharge from a paddy field plot decreases by 48% for ponding paddy field and 55% for midsummer drainage paddy field with an observed rainfall event (daily precipitation of 101.8mm), and 73% and 71% respectively with a 50-year return period rainfall.