Application Continuity Equation to Analyze the Hydraulic Performance of Nature Water Retention Measures

Flood is one of the most frequent and costly natural disasters worldwide. In many cases, implementing flood protection measures is limited due to land or resource availability. Natural Water Retention Measures (NWRMs), a more cost-effective approach, have recently drawn the attention of many researchers. Instead of infrastructure construction, NWRMs aim to reduce the risk of flooding and economic loss by land use and water management practices without many construction applications. Much previous literature qualitatively investigates the mechanisms of NWRMs, however, only a few focus on the hydraulic characteristic and the effectiveness of flood reduction of NWRMs. To improve the understanding of NWRMs, this study clarifies and analyzes the hydraulic performance of NWRMs. We consider the triangular inflow hydrograph based on the continuity equation with the Muskingum-Cunge method to derive the outflow of the channel, as well as the weir equation to the outflow of the retention area. Following, the continuity equations are formulated as a first-order ordinary differential equation in dimensionless form. The conceptual model built from the equations could denote the primary hydraulic mechanism in the original channel and the additional retention area. Two important parameters include the ratio of peak maximum outflow and peak inflow, and the ratio of maximum storage and total flood volume can be obtained by solving the equations. The results show that the relationship between two dimensionless parameters are nonlinear. Also, in the channel, the relationship is sensitive to the shape factor in the Muskingum-Cunge method, especially in a lower ratio of maximum outflow and peak inflow. With this model, the study following examined the different proportional of flood volume flowing in retention areas and calculate the downstream outflow. The result shows the effectiveness of flood reduction and the proportional of flood volume in retention areas are nonlinear relationships. Briefly, there is an optimal operation of NWRMs by balancing the flood volume in the river and retention could induce the minimum outflow. The findings in this study represent the hydraulic performance of NWRMs. The results can also improve the design and operation of NWRMs appropriately.