WD.1368 RevC WCD dam break assessment report with appendices

Discharge (m 3 /s)
Waimea Community Dam: Dam Break Analysis
Waimea Water Ltd
boundary, and resultant spillway outflow routed through the inline structure. The hydrograph for
this PMF inflow is shown in Figure 3-2. The initial reservoir level was set to normal operating level.
The inflow causes the water level in the reservoir to increase up to the PMF flood level at 202.53 m,
where the dam is set to breach. A summary of the scenario water levels, dam breach characteristics
and failure modes are shown in Table 5-2. The downstream boundary condition was set to a normal
depth dependent on the slope of the river. The slope was estimated using LiDAR data and adapted
until the downstream water profile appeared stable. This boundary condition was the same for all
three scenarios.
5.3 Dam breach hydrographs
The breach was simulated using a one-dimensional unsteady flow simulation for each scenario. With
the boundary conditions and breach parameters defined, the dam was set to breach in the discussed
fashion and time. For the sunny-day scenario, the failure mode was set to piping at the plinth base
with the breach forming at the start of the simulation. For the rainy-day cases, one failure was set to
base piping, and one overtopping. The dam was then set to fail as soon as the flood caused the
reservoir level to reach the PMF level.
Sensitivity and stability tests were conducted to investigate the effects of changing time steps and
section spacings. It was determined that a time step of 1 second and a cross-section spacing of 20 m
was sufficient for a stable breach model. Figure 5-2 shows the resultant breach outflow hydrograph
for the sunny-day scenario, and Figure 5-3 the rainy-day scenarios.
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Time
Figure 5-2: Breach outflow hydrograph for sunny day scenario.
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