31 January 2019
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Wolverton Street Culverts - Design Flow Assessment
Background
A Wolverton Culverts Hydraulic Peer Review meeting was held at Auckland Transport offices on
Wednesday 23rd January 2019. The current design methodology for the two Wolverton Street Culvert
Upgrade Projects was discussed at this meeting, with particular emphasis on the peer review findings
undertaken by GHD. These findings are outlined in the January 2019 GHD report titled “Wolverton
Culverts 1 and 2 - Peer Review of Design Flows”.
At this meeting it was confirmed that the current design flows are based on the Auckland Council Unitary
Plan zonings which retain the existing Maungakiekie and Titirangi Golf Courses land uses. Given the
critical nature of both Wolverton Street culvert sites it was agreed that the possibility of these large
greenfield areas being converted into residential housing should also be considered as part of the design
flow assessment. This memo updates the culvert design flows taking into account this possibility.
Design Flow Assessment
Design flows at both Wolverton Culvert sites have previously been calculated by Auckland Council (refer
to memo dated 23rd November 2018) using the following three methods:
1. TP108 hand calculations incorporating Unitary Plan land zoning and Climate Change rainfall
2. Rapid Flood Hazard Mapping (RFHM) model incorporating a 70% uniform imperviousness and
peak TP108 rainfall (adjusted for Climate Change)
3. Detailed catchment framework model (ICM) incorporating Unitary Plan land zoning, 24 hour
TP108 rainfall profile (adjusted for Climate Change), 10 percentile MHWS tidal level plus
1.0 metre sea level rise and upstream constraining culverts removed.
In addition to this work, TP108 hand calculations have also been calculated by GHD for the Unitary Plan
land zoning plus the residential infil ing of the Maungakiekie and Titirangi Golf Courses. For these
calculations the following assumptions were made:
• The entire 41.69 hectare site of the Maungakiekie Golf Course site is converted to housing with
an assumed average 70% impervious surface coverage
• 80% of the 61.29 hectare site of the Titirangi Golf Course site is converted to housing with an
assumed average 70% impervious surface coverage (the remaining 20% of the site is currently
covered in native bush which is unlikely to be developed).
12505938///Wolverton Street Culverts Design Flow
Assessment
The results of these various calculation methods are summarised in Table 1 and 2 respectively for
Wolverton Culvert Sites No. 1 and No. 2.
Table 1: Design Peak Flows – Wolverton Culvert Site No. 1
Peak Design Flow (m3/s)
ARI Storm
TP108 Calculations
Detailed
Event
RFHM Model
Framework
Supplied to
Model
AT (23/11/18)
Unitary Plan
Unitary Plan
+ GC Dev
Catchment
Imperviousness
61%
66%
70%
61%
-
10 Year + CC
54
56
-
57
57
50 Year + CC
77
79
-
79
79
100 Year + CC
86
88
87
89
89
Table 2: Design Peak Flows – Wolverton Culvert Site No. 2
Peak Design Flow (m3/s)
ARI Storm
TP108 Calculations
Detailed
Event
RFHM Model
Framework
Supplied to
Model
AT (23/11/18)
Unitary Plan
Unitary Plan
+ GC Dev
Catchment
Imperviousness
57%
63%
70%
57%
-
10 Year + CC
48
50
-
44
48
50 Year + CC
70
72
-
61
68
100 Year + CC
78
80
75
68
75
As discussed at the 23rd January 2019 meeting, the critical design storm is the 2500 year ARI storm
event, with the 10 to 100 year ARI flows primarily being used to al ow extrapolation of these higher flows.
The design flows supplied to AT are stil considered acceptable for culvert design for Auckland Council’s
flood protection Level of Service for upstream property floor levels. However, a sensitivity check should
be undertaken to assess the impact of possible future residential infil development within the
Maungakiekie and Titirangi Golf Courses.
Time constraints prevent rerunning the detailed framework model for this alternative development
scenario. Instead, the impact of infil development within the golf course sites can roughly be assessed
by increasing the supplied design flows by the same factor (2 m3/s) as derived in the TP108 hand
calculations. Based on this, the recommended peak flows to be applied for a sensitivity check of
possible golf course infil development are summarised in Table 3 for the Wolverton Culvert Sites No. 1
and No. 2.
Table 3: Recommended Peak Flows to be Adopted for a Sensitivity Check
ARI Storm
Sensitivity Check Peak Flow (m3/s)
Event
Wolverton Culvert Site No. 1
Wolverton Culvert Site No. 2
10 Year + CC
59
50
50 Year + CC
81
70
100 Year + CC
91
77
12505938///Wolverton Street Culverts Design Flow Assessment
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Regards
Technical Lead - Hydraulic Modelling
12505938///Wolverton Street Culverts Design Flow Assessment
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