Comparing Instruments for Measuring Runoff from Experimental Ecoroof Platforms: A Case Study on Test Plots at Portland State University

Stormwater management is a primary ecological benefit ecoroofs provide to ecosystems. Quantification of runoff from large scale ecoroofs is difficult to replicate, so researchers often utilize smaller experimental ecoroof platforms. This approach is becoming increasingly common, so it is useful to compare and contrast approaches for runoff measurement at the platform scale. This paper uses the four 17.86 m2 ecoroof platforms located on the Science Research and Teaching Center (SRTC) at Portland State University (PSU) in Portland, OR as a case study. A unique condition of these platforms is that they are installed at grade on the roof with no elevation. The expected runoff flow rate range from the SRTC ecoroof platforms was estimated using long-term hydrologic data and site geometry and was determined as 0.001 L/s to 0.170 L/s. Majority of the expected flow rates fall between 1 mL/s and 5 mL/s, so instrumentation that can capture these flow flows is of priority. Assuming that 0% retention is a possibility in the wet season, the runoff instrumentation should ideally be designed to capture flow rates as high as 170 mL/s. The alternatives explored were a tipping bucket rain gage, an impeller flowmeter, and an H-flume. Instruments were compared based on their ability to capture expected flow, installation requirements, and cost. This work suggests that the most effective solution for low-flow measurement for this site is a custom tipping bucket designed for the expected flow rates because all other solutions do not capture the full expected flow range. As an alternative, a combination of devices, such as a tipping bucket linked to a flume, can be considered to capture both peak flow and low flows. Future efforts should focus on adaptable yet effective instrumentation that can accommodate various platform installations