Model for designing infiltration basins in tropical and subtropical climates with a focus on unpaved roads

Controlling runoff from impervious surfaces such as roadways is an important part of storm water management in both urban and rural societies. Infiltration basins are one type of structure used to control storm water runoff. Design of these structures often involves using a single design storm and the assumption that the basin is initially dry. However, in tropical and subtropical climates where rainfall occurs frequently, the assumption of a dry basin is invalid. The objective of this study was to develop a new infiltration basin model that includes the effects of closely spaced storms on basin size. The model was used to design an infiltration basin for a section of unpaved road in João Pinheiro, Minas Gerais, Brazil. The performance of the model was evaluated with respect to infiltration rate, storm return period, and basin dimension parameters. Results showed that the infiltration rate was the most important input variable affecting basin size and that small but closely spaced storm events more strongly influenced basin size than isolated large events. Thus, while infiltration basins are often sized to capture small storm events (e.g., 25 mm or 90th percentile) to capture the most pollutants, this model indicates that such a sizing guideline may be inadequate if the basin is not empty. For unpaved roads, it is recommended that basins have an infiltration rate 5-10 mm h^-1 to balance hydrologic performance and size.