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

Table 7 – 2% AEP Storm Event
Top Spill
Bruce Road
Existing Future Existing Future
Flood depths over TEM (mm) 330 70 290 110
Spill rate (m 3 /s) 37.7 8.13 6.06 13.06
Spill volume (m 3 ) 469,026 44,823 94,460 144,826
Flood levels
(mRL)
Flood
residence
time (hours)
P1 - Central P2 – East P3 – West
Existing Future Existing Future Existing Future
8.80 8.70 8.86 8.87 5.61 5.60
17.5 11.4 15.2 13.1 14.1 8.0
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
Table 8 – 1% AEP Storm Event
Top Spill
Bruce Road
Existing Future Existing Future
Flood depths (mm) 360 140 290 120
Spill rate (m 3 /s) 47.25 17.13 9.17 15.29
Spill volume (m 3 ) 638,424 133,103 133,509 204,236
Flood levels
(mRL)
Flood
residence
time (hours)
P1 - Central P2 – East P3 – West
Existing Future Existing Future Existing Future
9.11 9.0 9.15 9.06 5.80 5.79
19.15 12.6 17.94 14.5 15.37 8.9
A property located in the lowest part of catchment P1 has a building floor level at 9.65m RL, and
thus will not be affected. There are no known buildings located in the lower-lying part of catchment
P2 and P3.
In order to report duration of flooding in each catchment, levels were selected close to the top of the
existing drains. This is to report when water is outside the farm drain and thus flooding occurs.
This level was also selected based on available data from the model where a full reporting period
(start and end time) was available.
The spill rate at Bruce Road will increase with the mitigation measures for the future option. This
increase will be accommodated with the provision of additional storage capacity within the Maranui
Swale and providing two additional pipe culverts that will offset the effects and thus convey the
Beca // 17 March 2009 // Page 12
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