How connected are urban catchments?: Estimating the effective impervious area

In estimating runoff from urban catchments, 3 separate types of surfaces are generally recognised: (i) impervious areas (e.g. roofs and paved areas) which are directly connected to the drainage system (ii) additional impervious areas which are not directly connected, runoff from which flows over pervious surfaces before reaching the drainage system (e.g. a roof that discharges onto a lawn); (iii) Pervious areas consisting of lawns, gardens and parkland. The identification of directly connected impervious areas and indirectly connected impervious areas in urban areas is a key issue that informs any modelling assessment of runoff in urban catchments. Key questions to be resolved in this regard are: - Whether the ratio of directly connected impervious area to total impervious area is broadly constant, irrespective of catchment location and age; or - if it is a factor of the age of a catchment, whether this may be an indicator of the historical versus current drainage practices An analysis of seven gauged urban catchments across Australia has been undertaken to establish if directly connected impervious areas are the same as effective impervious areas or if effective impervious areas reflect runoff responses from connected impervious areas and indirectly connected impervious areas draining onto pervious areas. These analysis comprised a statistical analysis of rainfall and streamflow, and a desktop GIS analysis using mainly aerial photography. The analysis of rainfall and streamflow indicated that across the study catchments, the Effective Impervious Area is approximately 30% - 40% of the urbanised area, and is around 75%-85% of the Directly Connected Impervious Area estimated using a desktop GIS analysis method. It is concluded that the GIS procedure overestimates DCIA and that impervious areas are less connected than assumed to date by hydrologists.