Many neighborhoods and the central business core in the City of Seattle contain combined sewer systems that include runoff from roof downspouts. Paying for roof runoff to be sent to the wastewater treatment plant is extremely costly, and combined sewer overflows to receiving waters are highly polluting events in the natural marine waters of the Seattle waterfront.

As an idea to help reduce the volume and occurrences of combined sewer overflows, the City of Seattle instituted a green roof monitoring program to address the question: “How much can green roofs contribute to reducing stormwater runoff?”

Seattle Public Utilities tasked Cardno TEC to develop monitoring guidelines that would ensure that high-quality, credible data is collected from the green roofs being studied. The “Green Roof Monitoring Guidelines” identified a water balance approach to assessing green roofs. At two sites, Cardno TEC installed complete meteorological monitoring stations to measure rainfall, wind velocity, temperature, insolation, and humidity as the measured hydrologic inputs to rainfall patterns and evapotranspiration. Measuring runoff from the roofs involved installation of an innovative controlled piping system to collect all the flow and to route it to flow meters appropriate for either low or high flows. The operation of these systems is remotely tracked, and data downloaded, via telemetry to the Cardno TEC offices. The resulting data sets have been high-quality and largely complete with few of the data gaps associated with conventionally deployed monitoring equipment.

The next step in evaluation of the roofs’ effectiveness is data analysis and preparation of the data for use in calibrating a continuous hydrologic model to analyze the range of possible benefits from the use of green roofs in the City of Seattle.