A previous blog entry discussed the use of a cast-and-retrieve watertight GPS unit to track streamflow data through a bridge opening, and mentioned a comparison of modeling results to come. The applicable project involved 1D hydraulic modeling of the tidal estuary and bridge using HECRAS. Without going into the limitations of this modeling approach too much (compared to 2D), here is a short summary the effort. The peak velocities for each track were extracted and reduced by 20% to convert from surface data to an approximate depth-averaged velocity. Peak velocity predictions from the applicable tide cycle in the model were extracted for comparison per location along the crossing. In most cases, the GPS recorded up to 2-3 times faster peak velocities than the model predicted. This bust is primarily due to two issues with the 1D approach: the model is not capturing the convergent (2D aspects) flow characteristics through the crossing (significant at this site), and the streamtube approach of the model is smoothing/averaging the data, removing the peaks. A 2D model such as RMA2 would better capture those localized peaks around the substructure, as well as angles of attack. Pulling the tracks into google earth and doing a simple averaging of the velocities between start and finish points confirmed the logging was good and gave velocities much closer to the model predictions. Luckily, model calibtration was accomplished via on-site tide gage data. In short, I look forward to using this field method on a future 2D estuary flow project and comparing ... oranges to oranges... exuce me - had to say it.
