Modeling of Streamflow in an Underdrain System of Vegetated Dry Swales

Vegetated dry swales were designed to promote infiltration, retain storm water, and reduce the surface runoff peak flow. As an important part of swales, the underdrain system, filled with gravels, was usually equipped with a perforated pipe to drain water from upper layers during rainfall events. In most existing models (e.g., SWMM) that were widely used to simulate the hydrological process for vegetated dry swales, outflow from the underdrain system was usually simplified as orifice flow related to water head, and the hydraulic process in the gravel layer and in the perforated pipe on the bottom of the swales could not be described. A hydraulic model of vegetated dry swales was developed in this study by combining an overland flow model, an infiltration model, and an underdrain streamflow model. The flow in the underdrain system was divided into two sections, i.e., the flow through the gravel channel and the flow through the perforated pipe. The former was simulated through a permeability model for porous media, and the latter was described by applying an equation. For the development of the function, data were collected for the calibration of parameters in field tests in Shenzhen, southern China, and water infiltration rate from the gravel channel to the perforated pipe was assumed to be proportional to the water head in the gravel channel and the orifice area on the perforated pipe. Combining an overland flow model, an infiltration model, and an underdrain streamflow model, the vegetated dry swale model was performed to analyze surface runoff retention and underdrain streamflow performance in a dry swale with a perforated pipe after calibration and validation with measured data. The study demonstrated that the proposed model could accurately describe the hydraulic process in the underdrain system of a vegetated dry swale and advance better understandings of dry swale hydraulic performance.