2. Mangatawa catchment consents for earthworks, storm water ...

It should be noted that while the TCC District Plan specifies a 2% AEP flood level for Papamoa of
4.5m RL recent work in the Wairakei catchment has focussed also on the 1% AEP events with
maximum flood level of 4.6m RL. Both have been considered in the performance target for this
work.
2.5 Results
The mitigation and engineering measures listed in Section 2.1 and performance results are reported
in this section. The table below summarises the 10%, 2% and 1% AEP storm event results. The
long section taken on the centreline of the highway is included in Appendix D. Illustrated on the
long section are the 1% AEP hydraulic grade line and the stop bank levels at Top spill and Bruce
spill. The results reported in the following two Tables below shows that the spill rate across the
highway at Bruce Road will increase with mitigation but the flood depth will decrease. The reason
for this result is that more water is diverted downstream with the mitigation measures and the spill at
Bruce will occur over a longer section of the highway.
Table 6 – 10% AEP Storm Event
Top Spill
Bruce Road
Existing Future Existing Future
Flood depths over TEM (mm) 280 0 290 0
Spill rate (m 3 /s) 16.68 0 2.86 0
Spill volume (m 3 ) 144,154 0 35,576 0
P1 - Central P2 – East P3 – West
Existing Future Existing Future Existing Future
Flood levels
(mRL)
8.19 8.07 8.23 8.22 4.96 4.91
Flood
residence
time (hours)
12.0 8.5 9.0 8.7 10.4 4.9
The flood residence time is based on time above the following levels. These levels were taken in each catchment as follows: P1: 7.15mRL; P2: 7.5mRL
and P3: 4.6mRL and represent time that ponding occurs out of channel, upstream (south) of the railway
Beca // 17 March 2009 // Page 11
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