Travel Time Shift

Travel time shift is include in this section because it is presented as an alternative to Storage, Muskingum-Cunge, Convex or Kinematic routing methods. Travel time shift is definitely not in the hydrologic or hydraulic routing category.

Travel time shift simply uses Manning’s equation to compute the travel time from the upper to lower end of a reach. It then shifts the peak of the hydrograph the computed time. The similarity of this method to the kinematic routing method should be recognized. As we said above, the kinematic routing method in theory does not appreciably change the shape the hydrograph or its peak flow rate.

Given that description, it is essentially travel time shift. Travel time shift as implemented by StormShed3G™ has a slight variation. Since the hydrographs are stored in a specified time interval (the default is 10 minutes), a travel time of less than the interval would require a shift to an odd interval. In order to do this correctly, it would mean interpolating the values. Unfortunately interpolating values would compromise the peak over time as one never interpolates up, just down.

To avoid such a situation, StormShed3G™ accumulated the travel time until it exceeds the hydrograph time step. At that point the program shifts the hydrograph the time step duration and carries over the remainder to be accumulated in future reaches. This alleviates the necessity of interpolating peaks between time steps and does not compromise the peak.

Actually, compromising the peak would probably not be a bad thing since the physical reality of hydrograph routing is more accurately modeled by the Muskingum-Cunge type methods, but in this case, compromising the peaks also affects the hydrograph volume, which is unrealistic.