Uma Bhandaram

In urban settings, stormwater runoff (precipitation flow over streets, parking lots, and roofs) finds its way into waterbodies in two ways: 1) municipal separate sewer systems (MS4s) and 2) combined sewer systems. MS4s collect sewage and stormwater in two separate pipes and only treat sewage before discharging. Combined sewer systems collect and treat both sewage and stormwater into one pipe. Combined sewer overflows (CSOs) occur during periods of heavy rainfall, when runoff exceeds treatment capacity and untreated excess sewage and stormwater are discharged into the nearest receiving waterbody. This untreated stormwater runoff from CSOs and MS4s causes water quality problems. Runoff from impervious surfaces can have a high velocity and entrain pollutants. For example, runoff flowing over roads can pick up oil and grease from cars. Redirecting flow away from sewer and storm drains and treating runoff through green infrastructure (GI) is one way of improving water quality. In compliance with New York State’s requirements to reduce CSOs, the NYC Department of Environmental Protection (NYCDEP) and the NYC Department of Parks and Recreation (NYCDPR) are building GI across NYC. Efficacy of GI performance can be dependent on various factors, including location. This paper demonstrates how to use spatial analytics, specifically Geographic Information Systems (GIS), to identify GI locations in public lands within the Alley Creek watershed and sewershed (Study Area, see Figure 1) in Queens, New York. Of the various types of GI, NYCDPR is most interested, of the various types of GI, in rain gardens. Rain gardens catch and detain runoff and allow for infiltration, evapotranspiration, and filtration. Infiltration is the process by which water seeps into the ground – it slows down runoff velocity, diverts runoff away from the drains, and treats runoff through pollutant removal. Evapotranspiration (ET) is the process by which plant roots uptake water and transpire it through their leaves. ET reduces the runoff flowing into the sewer systems by moving water into the atmosphere. Plant material and soils filter out pollutants in runoff through absorption, microbial degradation, and other processes. Rain gardens are composed of flood-tolerant plants in the center and drought-tolerant plants on the outer edges growing on permeable soils. This ensures infiltration and evapotranspiration in wet and dry seasons. Using plants with a wide range of inundation tolerances also ensures that rain gardens will stay vegetated and functional. Figure 2 shows how stormwater can be diverted and contained in rain gardens. Placing rain gardens in the appropriate locations maximizes these benefits. This paper uses a two-tier method for choosing locations: biophysical and programmatic. Biophysical variables can include surface type, depth to groundwater, and the presence of bedrock. These variables determine whether locations are physically suitable to rain garden placement. Programmatic variables depend on the regional, management, regulatory, and political context. These can range from design objectives to management priorities. These variables were selected based on fieldwork, collaboration with local and regional stakeholders, and input from the Natural Resources Group (NRG) housed within NYCDPR. This paper presents a set of GIS methods for identifying and prioritizing locations for rain garden placement within public lands in the Study Area (Figure 1). A customized GIS model was created to show locations that meet both biophysical and programmatic criteria. Locations that meet biophysical criteria are then ranked by priority depending on how many programmatic criteria were met. This research provides NRG and NYCDPR with a tool that can allow for a systematic and clear way to manage stormwater by using GIS for rain garden site selection.