Analysis of Wind-Driven and Water-Driven Transportation of Gross Soilds in Urban Watersheds

Abstract

Watershed health is heavily influenced by urbanization and the export of associated pollutants such as nitrogen (N) and phosphorus (P) that leach into stormwater and accelerate the effects of eutrophication. In urban watersheds, gross solids materials have been shown to contribute significant amounts of P to stormwater runoff. Positive correlations have been identified between precipitation patterns and influxes of gross solids to stormwater runoff. However, the role of wind-driven transportation as a contributor of organic material to urban stormwater solids remains poorly understood in comparison to the role of water-driven transportation. Wind-blown leaf litter has the potential to travel beyond the hydrologically connected areas that bound much of the research on stormwater solids. To address this knowledge gap, a detailed analysis was conducted on identifiable tree material from gross solids samples captured at two different rain garden sites - one parking lot site and one residential street site - with low canopy cover in the Minneapolis-Saint Paul metropolitan region in Minnesota, USA, to assess the possible mechanisms behind their deposition into the rain garden sites. It was assumed that material from outside of the watershed was transported by wind. Findings showed that material from outside the hydrologic watershed plays a significant role in the prevalence of gross solids, with an average percentage of 29% of material from outside the watershed and 71% from inside the watershed. Material from outside the watershed represented the majority of identified material from the residential street site. Material from within the watershed appeared to rise and fall along with precipitation patterns while material from outside the watershed appeared to fluctuate opposite them, suggesting that material from outside the watershed is more dominant in dryer conditions. Results of this study suggest that material from outside the watersheds represents a significant portion of leaf litter and other plant materials that constitute the organic fraction of urban stormwater solids.