How do I modify the standard curve number for impervious land in an urban area?
Several factors, such as the percentage of impervious area and the means of conveying runoff from impervious areas to the drainage system, should be considered in computing the curve number for urban areas. For example, do the impervious areas connect directly to the drainage system, or do they outlet onto lawns or other pervious areas where infiltration can occur? An impervious area is considered connected if runoff from it flows directly into the drainage system. It is also considered connected if runoff from it occurs as concentrated shallow flow that runs over a pervious area and then into the drainage system. Urban curve numbers presented in standard tables were developed for typical land use relationships based on specific assumed percentages of impervious area. These curve number values were developed on the assumptions that pervious urban areas are equivalent to pasture in good hydrologic condition and impervious areas have a curve number of 98 and are directly connected to the
