Assessment of integrated green infrastructure-based stormwater controls in small to large scale developed urban watersheds

Abstract

Green infrastructure (GI) stormwater control approaches and techniques store, infiltrate, evapotranspire, and in some cases reuse stormwater to reduce runoff quantity and to improve overall environmental quality. The literature review indicates substantial benefits provided by GI stormwater controls in small scales including reduced stormwater runoff volumes, enhanced groundwater recharge, reduced pollutant discharges to water bodies, and decreased combined sewer overflow events. The main objective of this dissertation research was to examine the benefits of individual and integrated GI stormwater control practices at small to large scales in urban watersheds. The hypothesis of this research is: "Retrofitting integrated green infrastructure controls in large areas served by separate or combined sewers can result in significant runoff volume reductions." Three case studies which were extensively monitored and evaluated have been selected for this dissertation research; 1) Millburn, NJ with dry wells monitored at a small scale, 2) Kansas City, MO with various GI practices including biofilters, curb extension biofilters, cascade biofilters, porous pavement, rain gardens monitored at small scales (individual GI performance) and monitored at large scales (overall integrated GI performance and their impact on combined sewer overflows (CSOs), and 3) Cincinnati, OH with three study sites including Cincinnati State College, the Cincinnati Zoo, and the Clark Montessori High School, which have several GI stormwater control types with monitoring at large scales. Analyses were conducted at infiltration facilities and at combined and separate sewer flow monitoring locations in the study areas to calculate the benefits of green infrastructure-based stormwater controls. The analyses conducted as part of this dissertation research were aimed at showing that monitoring results for runoff volume reductions from isolated small-scale stormwater controls can be scaled-up for use in typical drainage area benefit predictions, but only if sufficient information is available (such as soil characteristics, land development, actual runoff treated, etc.). The analyses at the small scales at Millburn, NJ and Kansas City, MO, indicated that there were varying levels of infiltration performance in the areas, but most dry wells and biofilters were able to completely drain within a few days. However, several had extended periods of standing water that may have been associated with high water tables, poorly draining soils (or partially clogged soils), or detrimental effects from snowmelt on the clays in the soils. At large scales at the Kansas City and Cincinnati test areas, direct measurements of flows by the in-system flow monitors in the combined or separate sewers on or adjacent to several of the green infrastructure components were used to directly measure whole system performance. The results at the large scales indicated that for most flow monitoring locations, there was a statistically significant difference (p<0.05) between the "after" construction period data and the "before" construction period data, which supports the hypothesis of this dissertation research. The runoff volume reductions for the large-scale studied areas ranged from 20% (for the Clark Montessori High School that has about 25% of its drainage area treated by green infrastructure controls) to about 85% (for Cincinnati State College where most of the area's runoff was treated by the treatment devices). The results showed that the green infrastructure locations and coverage in the watersheds directly affected the runoff reductions in the areas. The watersheds should have most of their flows treated by the green infrastructure stormwater control practices to result in large runoff volume reductions in the watersheds. Some of the flow monitoring results appears to be faulty and since the monitoring period has concluded and the equipment removed, it is not possible to verify the calibrations. Therefore, an important part of this dissertation research was to develop and demonstrate an effective monitoring and evaluation strategy and QA/QC process. This dissertation research also utilized a calibrated version of WinSLAMM for each study area that can be used to determine likely long-term benefits under a large variety of conditions, as well as recommendations for flow monitoring of green infrastructure stormwater controls.