Due to complexities of the hydrodynamic model provided (rain on grid plus various fixed inflow locations) and the basin having a large surface area relative to storage volume, an alternative approach to modelling the basin in Breacher with inflows was required. Basin outflow hydrographs and peak elevations were extracted from the hydrodynamic model for each of the events described above.
The basin was then simulated in Breacher as an SDF failure model for all events (even the FFS events) with fixed water levels matching the peak elevations extracted from the hydrodynamic model. If the outflow hydrographs were included in Breacher as inflows, this would cause a “double counting” of basin attenuation effects.
A comprehensive simulation was ran for all events in Breacher to estimate initial parameter ranges (example for SDF shown in Figure 31).
The parameter ranges identified in the comprehensive simulation are summarised in Table 3. Decision making was made around failure mode, with overtopping of the spillway found to be critical for most of the events, and piping of the embankment found to be critical for the PMF.
The simulated PMF runs were imported into Breacher-Post to identify the critical piping elevation, the resulting plot is shown in Figure 32 and binned elevation plot from Breacher-Post in Figure 33.
The breach time and bottom width relative to peak flow is shown in Figure 34.
An example of the combined (breach + release) flow hydrograph, for the PMF, is shown in Figure 35.