Modeling urban sewers with artificial fractal geometries

Abstract

Sewer models are used to simulate complex urban hydrology. However, the development of empirical models can be difficult given the limited availability of sewer plans and the time required to incorporate the system layout. In contrast, fractal geometries can be used to overcome some of these constraints. In this study, two highly impervious residential urban catchments (54 ha and 24 ha) serviced by a combined sewer in East Boston, Massachusetts are modeled using the Storm Water Management Model (SWMM). Two different modeling techniques are compared. The first is an empirical model using the physical characteristics of the network obtained from municipal sewer maps; the second is an abstract conceptual model incorporating fractal scaling laws often used to describe natural river basins. Both modeling approaches were calibrated with 1 month of empirical 5 min interval sewer flow measurements. The models predicted similar total discharge volumes and peak flows over the course of 10 observed rainfall events (0.5 mm to 12.7 mm). Model resolution was tested by simulating the 54 ha catchment as 1, 10, 24 and 173 subcatchments; accurate simulations could be produced for all of the resolutions.