Reexamining Runoff Coefficients for Evolving Land Uses

Runoff coefficients are often parameters that engineers take for granted because they are easily looked up in tables in criteria manuals or textbooks. However, many of the tables of runoff coefficients are decades old and do not include emerging land uses and types of ground cover that did not exist at the time the runoff coefficients were originally derived. In addition, redevelopment and current, denser development patterns have tended to shift runoff coefficients for common land uses higher due to a greater percent impervious cover. This paper will describe recent work by the Mile High Flood District, the City of Aurora, and the Town of Castle Rock in Colorado to reexamine runoff coefficients based on current and future projected development. This includes developing runoff coefficients for different types of xeriscaping, native plantings, and artificial turf which are ground covers that are becoming more common in Colorado due to water conservation needs. This research also examined runoff coefficients for solar installations in Colorado’s semi-arid environment and developed recommendations based on modeling solar fields as disconnected impervious area. Finally, this research has led to improvements in design guidance for applying runoff coefficients and typical imperviousness values at both the master planning and site design scales.