Green Infrastructure Performance Monitoring

Denver Green Infrastructure has partnered with Denver’s Department of Public Health and Environment (DDPHE) and the Mile High Flood District to conduct performance monitoring across its many projects. The objectives of the monitoring program are to confirm facilities are functioning as intended, experiment to improve future design, and use performance data to scale the benefits of green infrastructure up across the city.

The green infrastructure facilities are monitored closely for their stormwater volume and water quality impacts. There are three primary monitoring strategies: 

• Water quality sampling
• Passive hydrologic monitoring
• Synthetics runoffs

Water Quality Sampling

The purpose of water quality sampling is to quantify how much stormwater pollution each green infrastructure facility is removing. Water quality monitoring involves the installation of at least two automated stormwater samplers. Samplers are configured to collect storm runoff through time as it enters and leaves the facility. The integrated, composite water samples approximate an event mean concentration that characterizes the entire storm, but only requires less laboratory analysis. 

Often, the water quality samples are paired with measurements of flow into and out of the facility. Flow measurements enable estimation of water quality mass loads into and out of the facility, as well as mass removal by the facility. If it is not practical to measure flow into the facility explicitly, it can be estimated with rainfall data and assumptions about the runoff from the facility’s tributary area. If flow measurements are not taken, concentrations can still be compared. 

Passive, Hydrologic Monitoring

Passive hydrologic monitoring includes measuring water levels or stormwater flow in a green infrastructure facility during naturally occurring storm events. Hydrologic measurements are typically taken with a pressure transducer that records water depth at one location every 15 minutes. When the pressure transducer is placed near a flow control structure, the water depth measurements can be used to calculate flow rates. In other cases, water level measures are used directly to determine how long a facility takes to drain. 

Many ultra-urban facilities are constructed with outflow control structures where pressure transducers can be inserted at any point after construction, without modification to measure flow through the structure’s orifice and over the structure’s weir. 

Synthetic Runoff Testing (SRTs)

Synthetic runoff tests involve filling a facility with a known quantity of potable water from a water truck or fire hydrant to mimics a natural rain and runoff event. The facility is instrumented so that surface infiltration and outflow can be measured, allowing for calculation of volumetric runoff reduction, peak flow reduction, infiltration rates, and seepage rates. Synthetic runoff tests are especially valuable because they are performed on-demand, so there is no sitting around waiting for rain to come. Also, they are highly controlled so there is a high degree of confidence in the data collected. Synthetic runoff tests are used during targeted experiments and are repeated at a given site annually to monitoring performance changes through time. 

Special Projects

In addition to these three routine strategies, the Division of Green infrastructure also executes monitoring projects to investigate special topics. One topic of interest is to test new growing media blends in bioretention facilities for improved water quality treatment, water retention, and vegetation health. Additionally, DGI will look at different strategies for successful plant establishment and success under lower irrigation regimes in bioretention facilities. 

Monitoring Locations