The Straddle Stagger method (or progressive average lag method) uses empirical representations of translation and attenuation processes to route water through a reach. Specifically, inflow to the reach is lagged in time and then averaged over a specified duration to produce the final outflow.
Required Parameters
The parameters that are required to utilize this method within HEC-HMS are the initial condition, lag [minutes], and duration [minutes]. Two options for specifying the initial condition are included: outflow equals inflow and specified discharge [ft3/sec or m3/sec]. The first option assumes that the initial outflow is the same as the initial inflow to the reach from the upstream elements which is equivalent to the assumption of a steady-state initial condition. The second option is most appropriate when there is observed streamflow data at the end of the reach. The lag parameter specifies the travel time through the reach; inflow to the reach is delayed in time by an amount equal to the specified lag. The duration parameter specifies the amount of spreading in a flood peak as it travels through the reach. The delayed inflows are averaged over this specified time duration. The duration parameter loses physical meaning when it is greater than twice the lag time. These parameters are most often estimated using observed data and refined through calibration.