Laboratory Study of the Dynamics of a Pollutant Within a City Block During Urban Flooding

Abstract

During urban flooding, pollutants from point releases (e.g., sewer overflows) transported in street runoff can invade city blocks, exposing residents to contamination risks. The present laboratory study focuses on the intrusion of pollutants from flooded streets into a porous city block under steady flow conditions. The modeled city block is rectangular, with a variable number of openings in its sidewalls. Five pollutant release durations are considered. This study aims at assessing the impact of block porosity and release duration on pollutant dynamics within the block by measuring the spatial distribution of concentration using a colorimetry technique, and assuming a uniform distribution over the depth. For a continuous release of pollutant, the spatially‐averaged concentration reaches an equilibrium. The spatio‐temporal variability in concentration, and the filling timescales, strongly vary with the number of openings. With two openings, pollutant transport is driven by two recirculating mean flow cells and slow turbulent diffusion toward the cell cores, resulting in long filling timescales and a homogeneous equilibrium concentration. With four and six openings, the large‐scale oscillations of the multiple jets speed up the transition toward equilibrium. As multiple jets feature variable inlet concentrations and high unsteadiness levels, the variability in the spatio‐temporal distribution of concentration remains high at equilibrium. For the four short release durations, the subsequent flushing kinetics is independent of the release duration. The concentration follows a first‐order exponential decay. This data set could serve as benchmark data for evaluating the numerical models that solve the two‐dimensional depth‐averaged advection‐diffusion equations.