DELMARV Again: Hydrology of Flatland Watersheds

Hydrologic modeling of watersheds in flat regions can be difficult and results can be highly variable. The two parameters used in development of a unit hydrograph are the peak rate factor (or peaking coefficient) and the watershed response time. The SCS standard dimensionless unit hydrograph uses a peak rate factor (PRF) of 484. Twenty-five years ago, SCS developed the Delmarva unit hydrograph, with a PRF of 284, for use in the Delmarva Peninsula. A method for comparing the Snyder Peaking Coefficient (CP) to the SCS PRF is presented in this paper and indicates that a CP of 0.4 equates approximately to the PRF of 284. As important and difficult to estimate for flatland areas as the PRF is the watershed response time, represented by either the time to peak (TP) or lag time (TL). There have been several studies in addition to the SCS Delmarva study related to the hydrology of flatland watersheds that evaluate the PRF as well as the time to peak. This paper includes a review of previous studies of flat, coastal plain regions, including the Delmarva Peninsula and New Jersey. The PRF and time to peak values for watersheds evaluated in these studies are presented. Equations for computing time to peak from these studies are also presented, including a new equation using data from several studies. Results of the application of these methods to subject watersheds are compared. Historical rainfall/runoff events are presented and used to develop calibrated Snyder Unit Hydrograph parameters for watersheds in coastal New Jersey and the Delmarva Peninsula. 15 pp. 17 references.