WD.1368 RevC WCD dam break assessment report with appendices

Waimea Community Dam: Dam Break Analysis
Waimea Water Ltd
6.7 Extent of flooding on Waimea Estuary
The model boundary was set off the mouth of the Waimea River so that tidal effects could be
considered in the modelling. The modelled flow depths at the boundary are a good match to the tide
levels at the main flow paths, indicating that water ‘exits’ the model in balance. Flows that exit the
model still in reality need to exit the estuary itself though through one of the channels. The increase
in flow depths in these channels is likely to be small, but for completeness the possible influence
area has been hatched on the flood maps. The hatch boundary is set to the LINZ estuary coastline to
indicate that the expected impact is contained to the estuary.
6.8 Arrival Times
Model arrival times are an important result from the analysis as they are vital for emergency
planning. Arrival time information gives an insight to the required warning and evacuation time in a
dam break emergency. An understanding for the arrival times can be gained from the hydrographs
shown in Figures 6-6 to 6-8. A summary of the arrival time model data is shown in Table 6-5. Times
are rounded down to the nearest 5 minutes. Rainy-day scenario times represent the time for the
dam breach flood wave to arrive, not the initial flood flow wave.
Table 6-5: Arrival times for breach flood wave at downstream locations.
River
Chainage
(m)
Location
Sunny-Day Scenario
(Piping Failure)
Time for
first flood
wave to
arrive
Time for
peak water
depth to
occur
Rainy-Day Scenario
(Overtopping Failure)
Time for
first flood
wave to
arrive
Time for
peak
water
depth to
occur
0 Dam Site T=0 min - T=0 min
2910 Lucy Creek
Confluence
+15 min +30 min + 10 min +25 min
8220 Fairdale +30 min +40 min +20 min +35 min
12720 Wairoa River
Confluence
16470 SH6 Bridge
Brightwater
20330 Wai-iti River
confluence
24220 SH60 Coastal
Highway
bridge
+45 min +60 min +35 min +55 min
+60 min +85 min +45 min +70 min
+75 min +120 min +65 min +90 min
+100 min +150 min +80 min +105 min
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