Modeling Consistency for Small and Large Watershed Studies

When conducting a master drainage plan for an urban area, catchments are varied from small to large in size. Although there are many hydrologic methods developed for storm water predictions, the size of catchment often serves as the basis to select a proper method. As always, the common question is how to quantitatively define the size of small watershed, and how to establish the modeling consistency between the rational method for small catchments and the unit hydrograph method for large watersheds. In this study, the volume-based runoff coefficients used in the rational method are theoretically derived, and then calibrated to achieve the best agreement with the unit hydrograph method. The example of the rational method versus the Colorado urban hydrograph procedure was used to demonstrate how to achieve the modeling consistency for the metro Denver area. The same procedure can be extended into the relationships between the rational and kinematic wave methods or the rational and SCS unit hydrograph methods using the local design rainfall depths and soil loss functions. With the established model consistency, the master drainage plan can be implemented using both the rational and unit hydrograph methods for all sizes of watersheds used for drainage designs.