Modeling of Stormwater Drainage and Overflow Process Considering the Volume Capacity of Ditches in Urban Area

To predict risk level of urban inundation caused by heavy rainfall, so many numerical models have ever been developed. However, there are few models which considers the effects of ditches on inundation flow, which play a significant role in the process of stormwater drainage and overflow. In this study, assuming ditches and drainage boxes on both sides of all streets, a revised inundation flow model is proposed where the storage effects of the ditches are incorporated into stormwater drainage and overflow process.

The inundation model used here consists of 2D overland inundation model and 1D sewer pipe network model. In our previous model, these two models were connected through stormwater drainage boxes located at the center of each street mesh. In this study, this interaction model is revised. Ditches are assumed to be located on both sides of each streets, and the drainage boxes are located at each vertices of street mesh. The revised stormwater drainage/overflow process proposed here is through the overland surface, ditches, a drainage box, and sewer pipes. This model is applied to the Mikatsuki area, Kochi City, Japan. The hyetograph observed on August 3, 2014, was used for the simulation.

The inundated area of the actual flood event on August 3, 2014, was reproduced well in the simulation cases of the revised model considering the capacity of ditches. From the comparison of the simulation results between two cases considering and not considering the capacity of ditches, significant difference can be found, which concludes that the total volume capacity of ditches is not negligible in urban inundation events.

As the next step, the volume capacity of ditches is incorporated in the volume of stormwaer drainage boxes in our previous model. From the comparison of two models, e.g. the above revised model with the detailed ditch structure and the existing model with the volume of ditches, those two models have no significant difference of the peak water volume though small difference can be found in the increasing/decreasing process.

Although the volume of ditches has significant effects on the inundation flow, it is possible to simplify the stormwater drainage process by incorporating the volume of ditches into that of stormwater drainage boxes in the interaction model between the overland surface and the sewerage system.