Effects of substrate depth and precipitation characteristics on stormwater retention by two green roofs in Portland OR

Abstract

This study took place in Portland Oregon, a city of over 600,000 residents located in the Willamette Valley in the state of Oregon in the Pacific Northwest region of the United States. Portland experiences a temperate climate with Mediterranean features. Runoff patterns from two extensive green roofs with substrate depths of 75 and 125 mm, situated on a 5000 square meter retail store, were compared over a one year period. Precipitation, irrigation, and storm water discharge were continuously monitored and the performance of the green roofs for storm water control was investigated in detail. Over the study period, the 125 mm and 75 mm green roofs retained 32.9% and 23.2% of all precipitation by volume, respectively. The hydrologic response of the green roofs during individual storm events was found to depend strongly on the total depth of the storm event as well as the length of the antecedent dry weather period. Differences in performance between the two substrate depths were most pronounced for small storms with long antecedent dry weather periods. Both green roofs showed strong seasonal dependence in storm water retention, with higher percent retention in the relatively dry summer months compared to lower retention in the wetter winter months. These findings have important implications for the effective installation of green roofs for stormwater management in our region. Because of the increased frequency of storm events during the Pacific Northwest winters, it is imperative that efforts to increase storage capacity through increased substrate depth be paired with efforts to ensure rapid removal. If deeper substrates are to be utilized effectively; more research is needed to identify ways to increase evapotranspiration, for example via more informed plant selection, during wet winter months.