A comprehensive understanding of the complex runoff configuration within a catchment in urban areas is a prerequisite for successfully planning green infrastructure (GI) programs to meet wet-weather control goals. Given the nature of most GI controls, they are typically associated with much smaller catchments than traditional stormwater quantity controls. For example, the catchment associated with a green roof is the roof itself, whereas the catchment associated with an 18-inch storm collector pipe may cover multiple City blocks. Because of this difference in scale, modeling of GI controls requires a different catchment delineation approach than modeling of traditional stormwater quantity controls. Traditional runoff catchment delineation defines one catchment per one or multiple manholes with one total area and one set of runoff parameters. This practice is appropriate when the goal of a study is to obtain the total flow from the lumped catchment, or for evaluating system improvements downstream of the catchment. However, it does not provide enough detail for accurate evaluation of the hydrologic impact of GI solutions identified for placement inside the catchment.
In this paper, the traditional catchment per manhole delineation procedure is improved to increases the accuracy of the hydrologic model when applied to the scale of GI controls. GIS data, combined with field observations, are used to identify and delineate features of the catchment that have fixed runoff characteristics with unique runoff configurations within the catchment. For example, roof areas are delineated separately - house roofs are completely impervious and have constant/high slopes and negligible depression storage. Also, the sheet flow configuration for the roof subarea can be defined with high accuracy. Field observations are then used to determine if runoff from these roofs is routed over pervious lawns, to the impervious streets, or directly connected to a combined sewer system. Similarly, slope parameter and sheet flow width for parking lots, streets, and alleys are easily calculated from the GIS data.
Splitting the catchment into smaller subareas of fixed hydrologic parameters not only improves the model to closely resemble field conditions, it also simplifies and improves the accuracy of planning and evaluating GI programs. As an example, by isolating commercial and industrial buildings as individual runoff catchments, it allows for accurate evaluation of green roofs applied to the low slope roof surfaces of these buildings. Similarly, singling out streets and lawn areas during the delineation process will allow for more accurate evaluation of permeable pavement and porous curb and gutter units, since they intercept runoff from streets and lawns before it arrives at the storm inlets. By investing additional effort in the catchment delineation process, the proposed catchment delineation method reduces the uncertainty in the runoff parameters during the modeling analysis. In doing so, there are many benefits: the accuracy of the hydrologic model to mimic observed data is increased; the calibration process is accelerated and will help ensure that calibrated parameters are kept within their physical limits; the hydrological impact of the GI program on the catchment’s runoff is better understood; and an understanding of the performance of the GI program in meeting regulatory objectives can be evaluated with a higher level of accuracy.