New Methods for Quantifying Effective Impervious Area in Urban Watersheds.

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John Gulliver, Ali Ebrahimian, Bruce Wilson

Friday 3 july 2015

13:45 - 14:00h at Asia (level 0)

Themes: (T) Water resources and hydro informatics (WRHI), (ST) Catchment hydrology

Parallel session: 16C. Engineering - Industrial

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The design of stormwater management practices is often performed using the impervious area in a watershed. Recent studies have shown that a better parameter for these designs is the effective impervious area (EIA), or the portion of total impervious area that is hydraulically connected to the storm sewer system. EIA is the most important parameter in determining urban runoff. Knowledge of EIA is therefore critical in rainfall-runoff modeling. The incorrect use of TIA instead of EIA in urban hydrologic modeling leads to an overestimation of runoff volumes and rates. The method of rainfall-runoff data analysis (Boyd et al., 1993) is known among the other EIA estimation methods, because it is based on observed data. Our investigations on the issues of this method and improvements that we made to address those issues, led to development of an improved method called “Successive Weighted Least Square” (S-WLS) method. While this method provides a better understanding of the runoff generation mechanisms in urban watersheds, it also reduces the uncertainty of EIA estimates and produces more accurate results that can be used to verify other EIA estimation techniques. The S-WLS method was applied to 20 urban watersheds mostly in the Twin Cities metro area of Minnesota, USA. In order to have a better comparison with the Boyd, et al. (1993) rainfall-runoff data analysis method, the S-WLS method was also applied to many of the same watersheds. We also used the results of this method and developed a new method called “GIS-CN” for EIA estimation in un-gauged watersheds. GIS-CN method is based on evaluating curve number (CN) at the basin scale. In order to check the integrity of the GIS-CN method, we used several qualified rainfall and runoff data sets from urban watersheds with different sizes and hydrologic conditions throughout the USA. The results are used to evaluate the potential and the limitations of the GIS-CN method. The GIS-CN method is particularly attractive for practitioners involved in computing and modeling runoff from urban watersheds and design of stormwater control measures.