flood plain is not delineated to expand southerly, but is modeled to terminate based on the assumed gradient of the topography towards the channel. Such complications in accommodating an expanding flood plain when using a one-dimensional model are obviously avoided by using a two-dimensional approach.
The two-dimensional diffusion hydrodynamic model is now applied to the hypothetical dam-break problem using the grid discretization shown in figure 10. The same inflow hydrograph used in K-634 model is also used for the diffusion hydrodynamic model. Again, the Manning's roughness coefficient at 0.04 was used. The resulting flood plain is shown in figure 12 for the 1/4 square-mile grid model.
The two approaches are comparable except at cross-sections shown as A-A and B-B in figure 8. Cross-section A-A corresponds to the predicted breakout of flows away from the Owens River channel with flows traveling southerly towards the City of Bishop. As discussed previously, the K-634 predicted flood depth corresponds to a flow depth of 6 feet (above natural ground) which is actually unconfined by the channel. The natural topography will not support such a flood depth and, consequently, there should be southerly breakout flows such as predicted by the two-dimensional model. With such breakout flows included, it is reasonable that the two-dimensional model would predict a lower flow depth at cross-section A-A.
At cross-section B-B, the K-634 model predicts a flood depth of approximately 2 feet less than the two-dimensional model. However at this location, the K-634 modeling results are based on cross-sections which traverse a 90-degree bend. In this case K-634 model will over-estimate the true channel storage, resulting in an underestimation of flow-depths.