WD.1368 RevC WCD dam break assessment report with appendices

Waimea Community Dam: Dam Break Analysis
Waimea Water Ltd
Another key network found to be affected by the breach flood is roading in the Brightwater
township. In all breach scenarios the flood will overtop the banks surrounding the SH6 bridge at
Brightwater causing the highway and surrounding residential streets to become impassable. In the
sunny day assessment SH6 was found to flood roughly 70 minutes post breach. The flood depths
were shown to reach up to 1.5 m high on the SH6 and subside after approximately 60-80 minutes. In
the rainy-day case the SH6 road was found to flood in PMF conditions before the breach. At this
stage depths of up to 1.5 m were observed, this increasing up to 2.8 m when the breach flood wave
hits. Due to the extended time period of the precipitation flood events, the length of inundation
time lasted until the end of the simulation. This indicated that information on infiltration and
drainage would be required to properly estimate this residence time. None of the cases showed the
river flow overtopping the SH6 bridge.
The second critical roading asset encountered by the flood wave is the SH60 bridge towards the river
mouth. In the sunny-day scenario the breach flood wave was seen to not overtop the surrounding
stop banks or inundate the coastal highway. For the flood failure scenarios, the flood was observed
to begin inundating the road as the breach flood wave arrived. Flood depths were found to reach up
to 1.5 m along the coastal highway in this simulation. This will result in a loss of access between
Richmond and all townships North-of the Waimea River. In all scenarios the SH60 bridge was not
overtopping, with flood flows primarily around the abutments and floodplains.
6.9.2 Water and Wastewater
The Waimea borefield and Waimea Water Treatment Plant are within the inundation zone at the
northern end of Lower Queen Street.
The Nelson-Tasman Regional Sewerage Treatment Plant on Bell Island is on the edge of the
inundation zone. Access to the plant will be limited and it is possible that the rainy day event at high
tide may result in some flooding of the plant or oxidation ponds.
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