Appendix B
Coastal Processes Report
SH20 Manukau Harbour
Crossing
REPORT
S92 Response to ARC
Coastal Processes
Prepared for:
Qantas House
191 Queen Street
Auckland
28 August 2006
PHONE 09 427 3600
FAX 09 427 3601
EMAIL [email protected]
OFFICE 208 Grand Drive, Orewa
WEB www.northerngateway.co.nz
POST PO Box 697, Orewa
Contents
1
Introduction ------------------------------------------------------------------------------------------------- 1-1
2
Physical environment ----------------------------------------------------------------------------------- 2-1
2.1
Setting
2.1.1
2.1.2
2.1.3
2-1
2-1
2-5
2-5
Mangere Inlet
Onehunga Bay
Tararata Creek
2.2
Sediment properties
2-6
2.3
Wind
2-9
2.4
Water levels
2.4.1
Astronomic tides
2.4.2
Extreme water levels
2-10
2-10
2-12
2.5
Wave effects
2-14
2.6
Currents
2-15
2.7
Future climate change
2.7.1
Sea level rise
2.7.2
Climate variations
2-16
2-16
2-16
2.8
Water quality
2-17
2.9
Sedimentation
2-18
2.10 Discussion
3
2-18
Assessment of the proposal -------------------------------------------------------------------------- 3-1
3.1
Duplicate Mangere Bridge
3.1.1
General arrangement
3.1.2
Construction process
3.1.3
Scour
3.1.4
Contraction scour
3.1.5
Local Scour
3-1
3-1
3-2
3-2
3-2
3-6
3.2
Replacement footbridge
3-6
3.3
Orpheus Drive
3-6
3.4
Tararata Creek
3-7
4
References -------------------------------------------------------------------------------------------------- 4-1
5
Drawings ----------------------------------------------------------------------------------------------------- 5-1
i
List of Tables, Figures, Plates & Appendices
Tables
Table 2-1: Surficial sediment properties in the vicinity of Mangere Bridge and Orpheus Drive............... 2-8
Table 2-2: 10 minute and 1 hour duration wind speeds based on AS/NZS1170.2 ................................... 2-9
Table 2-3: Tide levels from the Port of Onehunga (LINZ, 2006) ............................................................. 2-11
Table 2-4: Occurrence of extreme water levels from 1926 to 1973 ........................................................ 2-13
Table 2-5: Details of most significant extreme water levels from 1926 to 1973...................................... 2-13
Table 2-6: Estimation of annual exceedance from records between 1926 and 1973 ............................. 2-14
Table 2-7: Predicted significant wave height, period and wind frequency, typical conditions................. 2-14
Table 2-8: Potential wave heights and periods with extreme winds ....................................................... 2-15
Table 2-9: Tidal gauging data from Mangere Footbridge........................................................................ 2-16
Table 2-10: Summary of receiving water quality at Mangere Bridge (ARC, 2004) ................................. 2-18
Table 3-1: Results of tidal prism analysis.................................................................................................. 3-5
Figures
Figure 1-1: Location plan........................................................................................................................... 1-1
Figure 2-1: Bathymetric information of the general project area (Source: NZ4315) ................................. 2-2
Figure 2-2: Detailed bathymetric information in vicinity of bridges............................................................ 2-2
Figure 2-3: Extract from 1853 bathymetric chart ....................................................................................... 2-3
Figure 2-4: Elevation and plan of existing motorway bridge (M.D. 14608) ............................................... 2-4
Figure 2-5: Orpheus Drive seawall/revetment types ................................................................................. 2-5
Figure 2-6: Sediment sample locations (Source: Ryan, 1989).................................................................. 2-7
Figure 2-7: Wind rose from Auckland Airport hourly wind data from 1996 to 2004. Centre figure
represents occurrence of calm or variable winds. (From Reid et.al.2005) ................. 2-9
Figure 2-8: Components of sea inundation (from Frisby and Goldberg, 1981)....................................... 2-10
Figure 2-9: Probability of exceedance curve for extreme predicted high water levels at Onehunga Wharf
for the period from Jan 2000 to Jan 2100 (Bell, 2000). Levels relative to LINZ datum
(RL) based on 1946 MSL.......................................................................................... 2-12
Figure 2-10: Recorded tide levels at the Port of Onehunga in terms of LINZ datum, June to December
2005 .......................................................................................................................... 2-13
Figure 2-11: Tidal currents off Onehunga Wharf (source: NZ4314, LINZ).............................................. 2-15
Figure 2-12: Summary of the relative magnitudes of long period sea level fluctuations excluding storm
driven fluctuations and global warming effects......................................................... 2-17
Figure 3-1: Representation of a tidal inlet (Source: CEM, 2006) .............................................................. 3-4
ii
SECTION 1
Introduction
1
Introduction
The Manukau Harbour Crossing Project involves the construction of a new Duplicate Bridge over the
Manukau Harbour, the replacement of the Old Mangere Bridge with a new footbridge, a bridge over
Tararata Creek, coastal reclamation and shore protection along Orpheus Drive in Onehunga Bay (see
Figure 1-1) and remediation works along the coastal edge of Waterfront Road Reserve.
Figure 1-1: Location plan
This report provides an additional assessment on coastal processes and the effects of the proposed
development on that area in response to Auckland Regional Council’s (ARC) request for further
information dated 29 June 2006, supplementing the original coastal process report included as Appendix
15 of the May 2006 AEE.
1-1
SECTION 2
Physical environment
2
Physical environment
2.1
Setting
The three key areas affected by the proposed development are;
•
the entrance to Mangere Inlet
•
Onehunga Bay
•
Tararata Creek.
2.1.1
Mangere Inlet
The Mangere Inlet is a rectangular shaped estuary of shallow tidal creeks and large expanses of intertidal
mud-flats which extend 4 km east of the old Mangere Bridge. The surface area of the inlet is around 6.6
km2 which at low tide reduces to a narrow shallow channel, extending some 2 km east of the old
Mangere Bridge.
Mangere Inlet is linked to Onehunga Bay by a narrow channel which extends westward some 2.5 km to
White Bluff as a narrow (approx 250 m wide) channel flanked by intertidal mud-flats.
General bathymetric information in the vicinity of the proposed development is available from
hydrographic chart NZ4315 (refer Figure 2-1). This chart provides seabed levels in terms of Onehunga
Chart Datum, some 2.201 m below LINZ datum. The approach to Onehunga Wharf and Mangere Bridge
is through a natural throat from White Bluff to the west and the existing motorway bridge abutment. The
natural throat at White Bluff is around 900 m wide, with the historic channel width in the vicinity of the
existing bridges being of a similar width. Currently the narrowest part of the throat is at the footbridge,
where there is an opening some 234 m wide at mean sea level and a cross-sectional area (not including
bridge pile obstructions) of around 1,420 m2 below MSL based on the levels provided in NZ 4315. The
width of the channel under the existing motorway bridge is around 302 m (see Figure 2-2) and has a
slightly larger cross sectional area of 1,524 m2.
The maximum channel depth is around 7.6 m CD, although this is within the scour hole evident between
the footbridge and the motorway bridge. The typical channel depth at this location is between 4 to 5m CD
which is deeper than the historic channel depths of around 3 m CD based on surveys of the harbour
carried out in 1853 (see Figure 2-3). Scaling from this chart the original cross-sectional area of the
channel in the vicinity of the tuff ring is around1600 m2. This area was based on the assumption that
mean sea level was at the shoreline due to lack of detailed levels in this area. This is a larger area than
currently observed. This may be due to the lack of detailed information, although it is also possible that
there was a larger surface area and hence tidal prism operating prior to the edge reclamation that
occurred along the northern shores of the inlet. We estimate that as much as 1.2 km2 could have been
reclaimed within the Mangere Inlet.
2-1
SECTION 2
Physical environment
Figure 2-1: Bathymetric information of the general project area (Source: NZ4315)
Figure 2-2: Detailed bathymetric information in vicinity of bridges
2-2
SECTION 2
Physical environment
Figure 2-3: Extract from 1853 bathymetric chart
The general elevation and plan of the existing motorway bridge is indicated on Figure 2-4. This is sheet 4
of the M.D. 14508 approval. The existing bridge has 8 sets of piles within the CMA, 7 of which are
situated within the main channel. These piles occupy an area of around 130 m2. Figure 2-4 shows the
existing bridge was designed taking into account potential enlargements to the port with the provision of a
shipping berth and an additional navigation channel to enable passage to Pykes Point as well as a
significant reclamation that provided additional port land, a motorway extension and a cooling water
outlet. It is likely that these potential plans were the reason the Ports of Auckland Limited (POAL)
ownership of the existing northern abutment was not modified at the time, with POAL retaining land
ownership of this abutment to this day.
Discussion with Geoff Higgens, POAL’s property manager has indicated that there is no immediate plans
for enlarging the existing port area, although they are constrained for space and would like additional land
area. However, they wish to retain the possibility of navigation access to Pykes Point for barges and tugs
at some time in the future.
2-3
SECTION 2
Physical environment
Figure 2-4: Elevation and plan of existing motorway bridge (M.D. 14608)
2-4
SECTION 2
Physical environment
2.1.2
Onehunga Bay
A combination of revetments and seawalls fringes the coastal boundary along Orpheus Drive along the
eastern shores of Onehunga Bay. The revetment comprises a conventional sloping rock rip rap along the
majority of Orpheus Drive, with the slope varying from around 2.5(H):1(V) at the northern end to the
Manukau Cruising Club, with larger diameter rock and steeper slopes from the boat club to the scout hall.
The revetment typically extends some 1 m higher than the adjacent road levels and plants and trees are
established on the seawall crest along the northern end of the wall. An offshore rock breakwater is
present immediately to the south of the boat club that provides a more sheltered environment to the
seawall in this area.
Figure 2-5: Orpheus Drive seawall/revetment types
Between the boat club and scout hall there is also an area where concrete and construction debris have
been used as coastal armour. This material has not proven successful, with evidence of slumping and
ongoing erosion.
South of the scout hall there is a dressed bluestone basalt sloping seawall (approx. 1(H):1(V)) with a 1.2
m high vertical upstand. This wall is in reasonable condition, although the upstand is showing signs of
overturning type failure along some 15 m of wall length.
2.1.3
Tararata Creek
Tararata Creek (see Figure 1.1) discharges into Mangere Inlet some 1 km to the south east of the existing
motorway bridge. Its catchment of some 651 hectares is bounded by Mangere Mountain and the
Manukau Harbour to the north, Favona and Roberson Road to the east, Massey Road to the south and
Kirkbride Road to the west. The terrain of the creek catchment is flat to gently rolling with most slope
gradients of less than 10% and the water course gradients of less than 3% (Riley, 2001).
Little remains of the natural creek, apart from its outlet downstream of Hinua Road, although even this
area is heavily impacted upon by the Walmsley Road and Southwestern Motorway bridges and the
Manhunga Road embankment and culverts. Downstream of Walmsley Road the creek is tidal and
mangroves are abundant, leaving only a small channel clear of obstructions. The riparian margins largely
consist of steep grassy and scrub banks (Riley, 2001).
2-5
SECTION 2
Physical environment
Studies carried out by Manukau City Council (1999) indicate that the creek is in poor condition both in
terms of water quality and also bank erosion, although they identified the channels naturalness was the
watercourses best feature, with a high level of channel naturalness and reasonable standards of riparian
vegetation.
2.2
Sediment properties
Surficial sediment sample information has been obtained from a thesis considering the sedimentological
impact of dredge spoil in the Purakau Channel (Ryan, 1989). Samples were obtained at 20 locations in
the vicinity of the channel, port and on the intertidal flats adjacent to Orpheus Drive (see Figure 2-6).
Table 2-1 provides a summary of the physical properties of the sediment.
The mean grain size is very fine, reflecting the predominantly silty clay composition of sediment in this
area. There are pockets of slightly coarser sediments, typically in the scour hole locations and adjacent
to the port. The latter is likely to be due to the dredging and possible terrigenous sources of larger gravel
sizes. The sediments are all poorly sorted to very poorly sorted indicating a depositional environment.
Previous studies of the environment have indicated reasonably high levels of pollutants within the
sediments of the Manukau Harbour (Ryan, 1989). DDT levels were between 1.2 and 2.3 ppb, with the
upper level reflective of the Mangere Inlet. Lindane also had high concentrations in the Mangere Inlet
with values of between 1.5 and 2 ppb. Heavy metal concentrations have indicated strong to moderate
anthropogenic inputs of lead, zinc and copper in the Mangere Inlet and while copper and chromium
concentrations were of similar orders of magnitude as those found in other New Zealand Harbours lead
concentrations, particularly in the vicinity of Mangere Bridge were considerably higher (98-247 μg/g).
2-6
SECTION 2
Physical environment
Figure 2-6: Sediment sample locations (Source: Ryan, 1989)
2-7
SECTION 2
Physical environment
Table 2-1: Surficial sediment properties in the vicinity of Mangere Bridge and Orpheus Drive
Sample
reference
Graphical Method (Folk & Ward)
Moment method
Textural size class (%)
Mean
(mm)
Median
(mm)
D35
(mm)
D65
(mm)
Mean
(mm)
Sorting
Skewness
Kurtosis
Gravel
Sand
Silt
Clay
MK
36
0.006
0.006
0.012
0.003
0.014
2.09
0.59
2.21
0
11
47.75
41.25
MK
37
1.000
0.005
0.129
0.002
0.058
3.12
1.25
2.41
0
41
12.25
46.75
MK
39
0.007
0.009
0.018
0.003
0.015
2.14
0.9
2.5
0
15
48.39
36.61
MK
40
0.005
0.005
0.011
0.002
0.014
2.37
0.53
2.27
0.17
15.83
38.51
45.49
MK
42
0.091
0.117
0.177
0.027
0.089
3.11
0.38
2.25
4.33
52.67
27.71
15.29
MK
43
0.090
0.131
0.158
0.101
0.096
1.88
1.85
6.01
0
80
12.88
7.12
MK
44
0.007
0.006
0.014
0.003
0.014
2.31
0.68
2.2
0
16
42.79
41.21
MK
45
0.003
0.004
0.008
0.002
0.009
2.13
0.22
3.73
0
5
42.81
52.19
MK
46
0.008
0.006
0.012
0.002
0.022
3.42
-0.45
2.7
10.36
9.64
36.95
43.05
MK
47
0.011
0.009
0.027
0.004
0.025
2.59
0.53
1.87
0
30
35.03
34.97
MK
48
0.038
0.100
0.137
0.021
0.056
2.51
1.13
2.8
0
61
19.52
19.48
MK
49
0.158
0.379
0.607
0.163
0.198
3.17
0.87
2.76
13.8
59.2
16.03
10.97
MK
50
0.013
0.016
0.034
0.007
0.024
2.11
0.94
2.43
0
29
43.68
27.33
MK
51
0.030
0.036
0.183
0.009
0.042
3.05
0.59
1.89
0
48
27.27
24.73
MK
52
0.133
0.156
0.165
0.118
0.110
1.6
3.04
11.97
0
90
4.64
5.36
MK
53
0.016
0.019
0.113
0.006
0.034
2.87
0.74
2.01
0
46
23.47
30.54
MK
54
0.005
0.004
0.009
0.002
0.016
2.94
-0.19
2.58
0
15
36.93
48.07
MK
55
0.116
0.154
0.218
0.104
0.131
3.06
0.67
2.67
5.32
64.68
17.04
12.97
2-8
SECTION 2
Physical environment
2.3
Wind
Wind is important both in terms of locally generated sea conditions as well as for windage effects on
vessels and structures. The prevailing surface wind direction is predominantly from the south-west
(26%), west (10%) and from the north to north-east (24%). Sustained winds over 15 m/s rarely occur
(0.2%) while strong gusts are also rate. The dominant winds speeds occur in the range of 2 to 4 m/s
(42%) wile speeds in excess of 9 m/s occur for 15% of the time (Tonkin & Taylor, 1986).
Figure 2-7: Wind rose from Auckland Airport hourly wind data from 1996 to 2004. Centre figure
represents occurrence of calm or variable winds. (From Reid et.al.2005)
Estimates of the extreme duration limited wind speeds for the study area are presented in Table 2-2. The
estimates are based on the extreme 3 second gusts presented in AS/NZS 1170.2:2002 (Standards
Australia, 2002) at 10 m above the water surface. AS/NZS1170.2:2002 provides 3s gust wind speeds for
8 directions for a range of annual exceedance probabilities adjusted by multipliers for direction and terrain
multipliers. The various wind speeds are then converted to sustained mean winds speeds over 1 hour
and 10 minutes based on the approach set out in Figure II-2-1 of Part II of the USACE Coastal
Engineering Manual (2006).
Table 2-2: 10 minute and 1 hour duration wind speeds based on AS/NZS1170.2
1 hour duration wind speed
wind speed
Direction
(m/s)
N
NE
E
SE
S
SW
W
NW
V5
15.0
16.7
23.7
22.6
15.0
22.6
23.7
16.7
V10
15.9
17.8
25.2
24.0
15.9
24.0
25.2
17.8
V20
17.3
19.3
27.5
26.1
17.3
26.1
27.5
19.3
V50
18.2
20.4
28.9
27.5
18.2
27.5
28.9
20.4
V100
19.2
21.4
30.4
28.9
19.2
28.9
30.4
21.4
V500
21.0
23.5
33.4
31.7
21.0
31.7
33.4
23.5
10 minute duration wind speed
wind speed
Direction
(m/s)
N
NE
E
SE
S
SW
W
NW
V5
22.0
24.6
34.9
33.2
22.0
33.2
34.9
24.6
V10
23.4
26.1
37.1
35.3
23.4
35.3
37.1
26.1
2-9
SECTION 2
Physical environment
V20
25.4
28.4
40.4
38.4
25.4
38.4
40.4
28.4
V50
26.8
30.0
42.6
40.5
26.8
40.5
42.6
30.0
V100
28.2
31.5
44.8
42.5
28.2
42.5
44.8
31.5
V500
31.0
34.6
49.1
46.7
31.0
46.7
49.1
34.6
2.4
Water levels
Water levels within the Manukau Harbour include phenomena that are both deterministic and
stochastic. Astronomical tides that result from the influence of the sun and moon on the earth
are deterministic while the effects of storm surge, climate cycles and nearshore processes are
variable and vary from year to year. For this study the sea level that includes astronomical tide,
barometric setup, wind setup, wave setup and wave runup is termed the sea inundation level.
The components of the sea inundation level are presented in Figure 2.8.
Figure 2-8: Components of sea inundation (from Frisby and Goldberg, 1981)
The following sections present a discussion of each of the sea inundation level components for the study
area.
2.4.1
Astronomic tides
Astronomic tide information is available from the Port of Onehunga. These tide levels are based on full
analysis of the relevant tidal constituents at this location.
2-10
SECTION 2
Physical environment
Table 2-3: Tide levels from the Port of Onehunga (LINZ, 2006)
Event
CD, m
RL (LINZ), m
Highest recorded level (21/06/1947,
31/07/1965)
HRT
4.95
2.75
Highest Astronomic Tide
HAT
4.54
2.34
Engineering High Water
EHW
4.4
2.20
Mean High Water Springs
MHWS
4.17
1.97
Mean High Water Neaps
MHWN
3.32
1.12
Mean Sea Level
MSL
2.42
0.22
LINZ (Auckland 1946) datum
RL
2.201
0.00
Mean Low Water Neaps
MLWN
1.45
-0.75
Mean Low Water Springs
MLWS
0.56
-1.64
Lowest Astronomic Tide
LAT
0.12
-2.08
Chart Datum
CD
0
-2.20
Notes
CD = Chart Datum at Onehunga
LINZ datum = 1946 Mean Sea Level
MHWS = level exceeded by 10 to 12% of high tides
HAT = Highest predicted to occur under average meteorological conditions during the period 1/01/2000 to
31/12/2018
EHW established prior to 1983
2-11
SECTION 2
Physical environment
Figure 2-9: Probability of exceedance curve for extreme predicted high water levels at Onehunga
Wharf for the period from Jan 2000 to Jan 2100 (Bell, 2000). Levels relative to LINZ datum (RL)
based on 1946 MSL.
2.4.2
Extreme water levels
Observed levels
There has been no recent assessment of extreme water levels of the Port of Onehunga’s data set.
However, based on published information, the highest recorded storm-tide to date since measurements
started at the Port of Onehunga was RL 2.75 m, which occurred on the 21 June 1947 and on 31 July
1965. We are aware of more recent significant events, particularly 17 April 1999 and 18 September
2005. The 17 April 1999 storm peak was not recorded at Onehunga due to failure of the tide gauge.
However, significant flooding of Kiwi Esplanade occurred and the event was a combination of a high
spring tide (2.2m RL) with low pressure of around 992.8 mb and 35kn south westerly winds. Based on
data from NIWA’s open coast tide gauge at Anawhata, the estimated storm surge magnitude was
approximately 0.6 m resulting in an estimated storm tide level of RL 2.8 m, or 5.1 m above Chart Datum
(Goring & Bell 1999).
The September 2005 event had a low pressure record of 979.2 mb and the tide level reached RL 2.52 m
(see Figure 2-10), some 0.42 m above the predicted high tide level.
2-12
SECTION 2
Physical environment
Figure 2-10: Recorded tide levels at the Port of Onehunga in terms of LINZ datum, June to
December 2005
Additional information on extreme levels was sourced from previous studies that considered extreme
water levels over the period 1926 to 1973, comprising 35053 records (MoEED, 1980).
Table 2-4: Occurrence of extreme water levels from 1926 to 1973
Range (from, to) m CD
Range (from, to) m RL
Occurrence (No)
4.27
4.42
2.07
2.22
646
4.42
4.58
2.22
2.38
237
4.58
4.73
2.38
2.53
50
4.73
4.88
2.53
2.68
9
4.88
5.03
2.68
2.83
3
Table 2-5: Details of most significant extreme water levels from 1926 to 1973
Date
Measured
storm tide
(m above LD461)
Predicted high
tide level
(m above LD-46)
Storm surge
(m)
22 June 1947
2.74
2.0
~0.75
31 August 1965
2.74
2.1
~0.65
6 or 7 September 1948
2.72
2.2 or 2.1
0.5 or 0.6
This information was converted to exceedance data and tabulated below. The results show that levels
greater than RL 2.73 m occur every 25 years based on historic data.
1
Note LD-46 or LINZ MSL Datum-1946 is 2.201m above Chart Datum
2-13
SECTION 2
Physical environment
Table 2-6: Estimation of annual exceedance from records between 1926 and 1973
Level, RL m
Total No. of events over period.
Events/year
2.22
299
6.099
2.38
62
1.265
2.53
12
0.245
2.68
3
0.061
2.73
2
0.041
Based on this data, the recent September 2005 event, with a level of RL 2.52 m, has a return period of
around 3 to 5 years. The April 1999 event, reaching RL 2.8 m exceeded the other known large storm tide
events.
2.5
Wave effects
Wave conditions are fetch limited and dominated by locally generated wind-waves, characterised by short
period, steep waves. The longest fetch length of some 17.5 km extend to the south-west which
corresponds to the prevailing surface wind direction (average 26% of the time from the south-west), and
west (with winds occurring around 10% of the time). However, wave conditions experienced along the
Manukau shoreline are highly dependent on water level, with depth limiting and friction losses occurring
as the waves propagate across or are stymied by the wide inter-tidal flats within the harbour. Nearshore
seabed levels along Orpheus Drive are around 1.8 m above CD or RL - 0.4 m and the majority of the
shoals to the south west have levels reaching MSL.
Wave development is dependent on four factors: wind speed, wind duration, fetch length and water
depth. The large tidal range and extensive area of low gradient inter-tidal flats results in a constantly
changing wetted surface area and water depth over a tidal cycle in the Manukau Harbour. This means
the variables used to predict wave development for a particular wind speed are dynamic. Developing an
accurate nearshore wave climate under these conditions would require numerical modelling (Gorman and
Neilson, 1999).
However, despite the complicated natural processes involved in wave generation, an approximate wave
climate can be developed using simple hindcast (Bretschneider) methods, given awareness of the
assumptions the scenario includes. Typical wind speeds over fetches from the south, west-southwest,
west and northwest (bearings as shown in Table 2.2) were used to estimate the local wind generated
wave climate at the proposed site in Onehunga. Wave heights in Table 2.2 were derived by assuming a
constant water level set at MHWS to maximise the depth of water and area covered by water extensive
inter-tidal flats. The method also assumes constant wind-speed of sufficient duration to generate a fully
developed sea over a fetch with varying depth.
Table 2-7: Predicted significant wave height, period and wind frequency, typical conditions
Wind speed
3m/s
7m/s
10m/s
15m/s
Direction
Hs
(m)
Tp (s)
%
/year
Hs
(m)
Tp (s)
%
/year
Hs
(m)
Tp (s)
%
/year
Hs
(m)
Tp (s)
%
/year
S (180)
0.06
0.9
7
0.15
1.4
2
0.23
1.7
0.75
0.36
2.1
0.25
WSW (260)
0.11
1.3
9
0.29
2
10
0.42
2.4
4
0.64
3.1
3
W (270)
0.08
1.1
4.5
0.2
1.6
5
0.29
2
2.5
0.45
2.4
1
NW (310)
0.06
1
4
0.17
1.5
2
0.25
1.8
1
0.38
2.2
0.25
2-14
SECTION 2
Physical environment
Table 2-8: Potential wave heights and periods with extreme winds
Wind Speed
25m/s
35m/s
40m/s
Hs (m)
Tp (s)
Hs (m)
Tp (s)
Hs (m)
Tp (s)
WSW (260)
0.98
4.8
1.17
5.2
1.25
5.4
S (180)
0.63
2.7
0.89
3.1
1.02
3.3
The results of the hindcasting indicate waves arriving at Onehunga from the southwest will be higher than
waves from other directions, with potential for maximum significant wave heights over 1 m. However,
typical significant wave heights from the southwest are likely to between 0.1 and 0.6 m. Waves of up to 1
m from the south are also possible, however strong southerly winds are less frequent than strong
southwesterly winds.
2.6
Currents
Tidal current information is available from the hydrographic chart at a location off Onehunga Wharf
(Figure 2-11). There is also gauging information at Mangere Bridge carried out for previous studies on
harbour dynamics (MEED, 1980). The LINZ data shows spring tide currents are higher than neap tide
currents, with higher velocities during flood flow conditions. At high water there is a slight return flow out
of the Mangere Inlet and the ebb flows have a lower peak, but a longer duration.
The historic gauging at the footbridge was carried out on 5 July 1978 during neap tide conditions and a
spring tide gauging was carried out on 19 June 1978.
Tidal Streams at Onehunga
Springs
Neaps
1.20
1.00
0.80
Current speed (m/s)
0.60
0.40
0.20
0.00
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
-0.20
-0.40
-0.60
-0.80
-1.00
Time, before (-) and after (+) High Water
Figure 2-11: Tidal currents off Onehunga Wharf (source: NZ4314, LINZ)
2-15
6
SECTION 2
Physical environment
Table 2-9: Tidal gauging data from Mangere Footbridge
Variable
Neap Tide (05/07/1978)
Springs (19/06/1978)
Flood
Flood (m/s)
Ebb
Ebb (m/s)
Max velocity at
point
0.53 m/s
0.66 m/s
N.D.
N.D.
Max velocity.
mean in vertical
0.47 m/s
0.49 m/s
N.D.
0.92
Max velocity.
mean in section
0.35 m/s
0.36 m/s
N.D.
0.68
Discharge (max.
mean in section
536 m3/s
560 m3/s
N.D.
1,160 m3/s
Tidal flushing occurs predominantly through the main channels, with water on either side of the channel
being less well flushed.
Observations from previous hydraulic investigations identified that on a flood tide there is a separation of
flows at White Bluff due to the large shoal present at this location. One stream following the main
channel and a second flow swinging into Onehunga Bay and flows around the bay approaching the wharf
from the north west. During ebb tide conditions flow is confined to the main channel.
2.7
Future climate change
2.7.1
Sea level rise
Sea Level Rise (SLR) estimates has been taken from the latest findings of the Intergovernmental Panel
on Climate Change (IPCC, 2001a). According to the IPCC, the world’s climate has changed over the
past century. Over the last century, global mean surface air temperature has increased by 0.6 ± 0.2 °C
and global sea level has risen between 0.1 m to 0.2 m (IPCC, 2001a).
Analysis of annual mean sea levels recorded at the Port of Auckland since 1899 indicates a linear rise of
0.13 ± 0.01 m per century (Bell et al., 2000; Hannah, 2004). There is no similar analysis of the Port of
Onehunga gauge, but short term cyclical variations experienced on the east coast are also observed on
the west coast, suggesting that any rise in water level shown in Auckland should also occur within the
Manukau Harbour.
Future climate change effects include possible accelerated increase in global mean sea level ranging
from 0.09 m to 0.88 m by 2100. Published “most likely” mid-range values are between 0.31 m and 0.49
m (IPCC, 2001a, b & c). The recently published Ministry for the Environment guidance manual on coastal
climate change for local government (MFE, 2004) recommends values for increases in mean sea level for
an additional 0.2 m by 2050 and 0.5 m by 2100.
2.7.2
Climate variations
The mean level of the sea can fluctuate due to the influence of wider climatic variations such as the
annual heating/cooling cycle, the 2–4 year El Niño—Southern Oscillation (ENSO) cycle and the 20–30
year Inter-decadal Pacific Oscillation (IPO) cycle.
The annual warming/cooling cycle accounts for a mean sea-level variation of ±0.03–0.08 m, with the
maximum occurring in late summer or autumn. At ENSO timescales (2–4 years), the mean level of the
sea varies around ±0.12 m, with the higher positive value occurring in intense La Niña episodes. Finally,
the IPO is a longer “El-Niño like” 20 to 30 year climate cycle that acts across the Pacific in the
background, enhancing or diminishing the ENSO effects. The IPO is thought to have switched phase in
1998, which suggests that sea levels may temporarily be elevated by around 0.05 m on average over the
next two decades. Presently, across these climate cycles of at least 6-months period, the actual mean
2-16
SECTION 2
Physical environment
level of the sea can fluctuate by up to ±0.25 m (see Figure 2-12), and should be taken into account to
provide for present climatic variability (Bell, et al. 2000; MFE, 2001; MFE, 2004).
The maximum combination is likely to occur in summer months during La Niña episodes for decades
when the 20 to 30-year Interdecadal Pacific Oscillation (IPO) cycle is in a negative phase. In this respect,
we are now in a negative IPO phase, which appears to have started in, and may last for the next 20 to 30
years. Consequently, this “sea-level fluctuation” factor should be considered when assessing coastal
inundation hazards over the next few decades.
Figure 2-12: Summary of the relative magnitudes of long period sea level fluctuations excluding
storm driven fluctuations and global warming effects
2.8
Water quality
Auckland Regional Council has a baseline water quality monitoring point at Mangere Bridge. This site
has been monitored since 7 October 1987. The following table summarises key data from baseline
surveys carried out from January to December 2003 and the long term range of data from October 1987
to December 2003. The table shows the median levels of key water quality parameters, the ratio of interquartile range (IQR) over the median for 2003 and the maximum and minimum values recorded from
1991 to 2003. The ratio of IQR/M demonstrates the variability in the data, with a large value indicating
greater variability than a small number. This is the latest published summary of saline water quality data
for this location. Many of the water quality results have improved since the Mangere Treatment Plant
upgrade in 2002. Therefore the long term range is not necessarily reflective of the current water quality
situation.
2-17
SECTION 2
Physical environment
Table 2-10: Summary of receiving water quality at Mangere Bridge (ARC, 2004)
Component
Median
Period01/03 to 12/03
IQR/Median (%)
Period01/03 to 12/03
Temperature (°C)
15.9
35.2
10.0 to 26.0
pH
8.0
0.4
7.3 to 8.4
Suspended solids
(mg/L)
26.0
100
10 to 150
Turbidity (NTU)
11.4
112
1 to 95
Secci Depth (m)
0.5
40
0.15 to 1.5 m
Chloride (g/L)
17.2
8
13 to 20.5
Salinity (ppt)
31.2
8
24 to 38
BOD (mg/L)
1.0
0.0
0.4 to 7.2
DO (mg/L)
7.5
17
4 to 11
DO (saturation) %
91.3
12
55 to 145
Ammonia nitrogen
(mg/L)
0.12
98
0.02 to 2.5
Nitrite nitrogen (mg/L)
0.021
83
0.002 to 0.227
Dissolved reactive
phosphorus (mg/L)
0.17
44
0.02 to 0.4
Total phosphorus
(mg/L)
0.23
27
0.16 to 0.58
Chlorophyll (μg/L)
6.6
129
0.6 to 16.8
Presumptive coliforms
(MPN/100 mL)
35
593
4 to 20,000
Faecal coliforms
(MPN/100 mL)
18
185
4 to 20,000
2.9
Long term range
(10/87 to 12/03)
Sedimentation
As described in the section above, the suspended sediment rates are high. In addition there are large
intertidal areas comprise of fine sands and silts, providing a large supply of potential sediment.
Sedimentation has been assessed by various authors in the vicinity of the port, but there is no detailed
assessment of sedimentation rates in the adjacent parts of the harbour. However, it is generally
acknowledged that the intertidal areas have experienced reasonably significant accretion particularly
those areas adjacent to the causeway and bridge abutments. This is particularly evident in areas that
have experienced rapid mangrove growth in recent years.
POAL maintains a berth adjacent to the wharf of 5.5 m below Chart Datum and have an annual
deposition rate of around 10,000 m3/yr, or around 1 m sedimentation depth. In the vicinity of the existing
footbridge there appears to have been some scour of the channel, but recent diver inspection did not note
any significant scour features, indicating the seabed in this area had adjusted to the change in crosssection brought about by the bridge and abutment construction.
2.10
Discussion
The area can be divided into three specific area:
•
Onehunga Bay
2-18
SECTION 2
Physical environment
•
the throat to Mangere Inlet
•
Mangere Inlet/Tararata Creek.
Onehunga Bay is a low energy environment in terms of tidal currents, but is affected by wind and wind
generated waves, with the highest waves occurring during strong south-westerly winds at high tide levels
and affects the more exposed central and southern parts of Orpheus Drive. The inlet is higher energy,
due to the presence of strong tidal flows although there is slightly more sheltering from wave effects.
Both areas have been significantly modified due to a combination of reclamations and edge
developments that have progressively encroached into the Coastal Marine Area.
The outlet of Tararata Creek discharges into Mangere Inlet. This inlet almost fully drains each tide,
providing full flushing of any inflows into the greater area of Manukau Harbour.
Previous studies of storm surge around New Zealand’s coastline have concluded that storm surge
appears to have an upper limit of approximately 1.0 m (Hay, 1991; Heath, 1979; Bell et. al, 2000). Given
the perceived upper limit of storm surge for New Zealand, a standard storm surge of 0.9 m is considered
representative of a return period of 80 to 100 years (MFE, 2004). Based on the work of de Lange (1996),
a storm surge of 0.8m represents a reasonable upper bound for a return period of 50 years.
The recently completed MfE guidelines on climate change suggested extreme water levels representative
of a 1% AEP event be based on a combination of MHWS tide level and a default value of a 0.9 m storm
surge, along with estimates of wave set-up and wave run-up and future climate change allowance. At
Onehunga this would result in a 1%AEP storm tide (exclusive of wave processes and climate change
effects) of RL 2.87 m (RL 1.97 m + 0.9m). Site specific data suggest storm surge levels with a return
period of around 25 years are in the vicinity of RL 2.75 m and one event in the last 80 years reached RL
2.8 m. Therefore, the 2.87 m RL may represent a reasonable assessment of a 1%AEP event without the
requirement to include wave set-up.
Taking into account sea level rise, this storm surge level should be increased by 0.2 m for allowance of
2050 estimates, or 0.5 m for 2100 year potential increases.
2-19
SECTION 3
Assessment of the proposal
3
Assessment of the proposal
There are four areas that have a direct influence of the coastal marine area:
•
duplicate Mangere Bridge
•
replacement footbridge
•
Orpheus Drive
•
Tararata Creek bridge.
3.1
Duplicate Mangere Bridge
3.1.1
General arrangement
Conceptual drawings for the duplicate bridge have been developed to show the span arrangement,
section depths, overall height and structural form of the bridge. In addition construction methodology
drawings have been developed to show sequentially the major construction operations involved with
building the bridge. The works are shown on the following drawings included in Section 5.
•
B-098-5001
Plan and Long Section
•
B-098-5002
Cross Section and Pier Details
•
B-098-5003
Construction Methodology – Sheet 1 of 3
•
B-098-5004
Construction Methodology – Sheet 2 of 3
•
B-098-5005
Construction Methodology – Sheet 3 of 3
The conceptual design is generally consistent with the documents already submitted for the resource
consent application, their purpose is to show in greater detail the bridge and how it will be constructed.
However, there no longer being a requirement to reclaim land adjacent to the northern abutment
As shown on drawing 5001 the duplicate bridge runs more or less parallel with the existing bridge and at
the same height above water level. The pier locations, in general, do not match those of the existing
bridge. There are sound technical and economic reasons to justify not doing so. The existing bridge is
in-fact 4 smaller bridges in a line, each separated by an expansion joint. In all there are 5 expansion
joints on the existing bridge, these are located at mid-span of the longer spans and at the abutments.
Locating deck joints at mid-span on the long spans has led to excessive creep deflection of the
cantilevers this has caused noticeable sagging of the bridge near the joints resulting in an uneven ride
and appearance. Expansion joints are also not desirable since they effect ride comfort, are noisy and
require road closures for maintenance. Modern bridge design practice seeks to minimise the number of
expansion joints by using a continuous superstructure with joints only at the beginning and end limits of
the bridge, this is proposed for the duplicate Mangere Bridge.
The alternating long span – short span arrangement of the existing bridge is strongly linked to separating
the existing bridge into its 4 discreet sub-bridges and keeping each sub-bridge stable during its
construction. Since we do not wish to repeat the structural form of the existing bridge the span
arrangement is no longer logical to repeat for the duplicate bridge, nor economical.
The longer spans on the existing bridge make allowance for eastern expansion of the Onehunga Wharf
(Port) and a navigation channel in the deepest part of the Harbour. The 45m wide channels, as
envisaged in the original design, are shown on drawing 5001, even though plans for the inner harbour
development have diminished
The shorter spans of the existing bridge make allowance for Onehunga Harbour Road, Onehunga Branch
Rail line and a future East-West motorway (which is no longer planned).
The pier locations proposed for the duplicate bridge avoid the navigation and berthing channels, as
originally proposed, and still provide for clearance to Onehunga Harbour Road and the Rail line. In doing
3-1
SECTION 3
Assessment of the proposal
so the duplicate bridge remains faithful to the original designers intention but in a way that reflects
modern bridge design practice.
Within the coastal marine area, the top of pile cap level on the existing bridge is at mean sea level (MSL).
It is proposed to do the same with the duplicate bridge. Structural requirements dictate that deeper pile
caps are desirable for the duplicate bridge and so the underside of the pile caps will be deeper into the
water (or mud) than those on the existing. On land the pile caps will be higher than MSL but will always
be buried over their full extent.
3.1.2
Construction process
Bridge construction requires the installation of the piles and pile caps, which requires access to and
disturbance of the seabed, then the superstructure of the bridge is constructed at a higher level above the
CMA.
A range of construction options are possible for the piles and foundations, including the construction of
temporary causeway/bridges or using barges. However, it is likely that the temporary bridge would be the
most cost effective approach and require the shortest construction time.
The proposal is to install low level temporary staging, some 9 m wide extending out from the southern
reserve area adjacent to the existing bridge. The bridge would be founded on driven steel sections by a
pile driver or vibrating hammer situated on the deck of the previously constructed section. By this manner
of construction there is limited disturbance to the seabed, with the only occupation being the footprint of
the steel pile.
The temporary staging extends out over the edge of the main channel and will enable the three southern
most foundations to be constructed. The bridge will then be progressively dismantled and reassembled
on the northern side of the inlet to construct the remainder of the foundations. Using this approach the
deepest part of the channel is never obstructed and the temporary obstruction of the staging does not
have such a significant effect on cross-sectional area. Assuming the pile sections are 300 m wide the
total occupation of the channel is around 48 m2 for the southern extension, including the temporary sheet
piles surrounding the pile caps and around 103 m2 for the northern extension, although at this point, there
will also be two additional pile caps at the southern end, which increases the obstruction to 138 m2.
The pile spacing will be no closer than 7 m centres, possibly increasing to around 10 m, with the piles
driven into the seabed by a hammer or vibrating hammer from the deck of the constructed temporary
staging. The disturbance to the seabed on installation and subsequent removal of the temporary piles is
minimal and we anticipate full restoration of the seabed within 7 days due both to the limited area of
disturbance and the strong tidal flows that are present at this location.
3.1.3
Scour
The key effect of the proposed duplicate bridge will be the modification of tidal currents and the
associated change to the seabed these changes in tidal current will create. There are two key scour
effects, contraction scour and local scour (Melville and Coleman, 2000). Contraction scour is as a result
of the overall reduction in cross-sectional area of the channel, typically by abutment encroachments and
the presence of piles within the water way. Local scour occurs as a result of the physical interaction of
the currents with the structure that can cause local deepening adjacent to the abutments and piles.
The proposed duplicate bridge does not have any additional abutments confining the water way and
therefore the main change to the system is the additional blocking that may occur within the channel due
to the presence of the bridge piers.
3.1.4
Contraction scour
Numerical modelling methods typically simulate constriction scour type effects, with pier effects simulated
by locally applying increased flow drag resistance at the pile location. This approach is because the
typical grid size used for the numerical model is significantly larger than the individual piles requiring the
presence of piers to be simulated by a sub-grid scaling technique (DHI, 2006).
3-2
SECTION 3
Assessment of the proposal
Published contraction scour formula (Melville and Coleman, 2000) for bridge and bridge abutments are
not readily applicable as they are based on relatively uniform river channel sections.
A first order assessment can be made by evaluating the change in velocity and the associated change in
seabed level using empirical methods based on tidal inlets, with the overall reduction in area of the piles
used to reduce the available cross-sectional area. This is a conservative approach as the piles do not
restrict flow over their entire area. However, the results of this first order assessment will establish the
likely order of change and potential effects, with a decision made to carry out the numerical modelling
subject to the severity of the likely effects.
To assess the change in velocities through the inlet we carried out assessments using the methods
presented in EM 1110-2-1100 (Part II), (CERC, 2006) for velocities in tidal inlets using the method of
Keulegan (CEM, 2006).
Key parameters used in this model are:
Ab
= surface area of the bay
T
= tidal period
Ao
= ocean tide amplitude
K
= entrance and exit energy loss coefficients (Ken and Kex)
L
= inlet length
R
= inlet hydraulic radius
The conceptual representation of an inlet is shown in Figure 3-1 which is a reasonable replication of the
situation at this location, with a large water body connected to the sea via a narrower access channel. In
this instance the bay area is shallower than the access channel and therefore the tidal range is limited by
the seabed level.
It is noted that this method is a simple analytical technique to determine average and maximum velocities
in a channel cross section due to the ocean tide and tidal elevation changes in the bay. The results are
expected to be conservative but provide the correct order of magnitude results. Several assumptions are
required as set out below, with our evaluation of their applicability for this location in brackets:
•
the walls of the bay are vertical (typically applicable as cliff or hard edge fringe the Mangere Inlet)
•
there is no significant inflow from rivers or streams (generally applicable)
•
no density currents are present (applicable)
•
tidal fluctuations are sinusoidal (reasonable approximation, although in this instance limited by the
shallow bay area)
•
water level rises uniformly (some friction effects likely which will affect speed of propagation)
•
inlet channel flow area is constant (reasonably applicable)
•
inertia of the mass of the water in the channel is negligible (reasonable)
3-3
SECTION 3
Assessment of the proposal
Actual situation in Mangere Inlet
Figure 3-1: Representation of a tidal inlet (Source: CEM, 2006)
The area of Mangere Inlet was taken to be 6.6 km2 and the entrance channel dimensions were taken at
both the footbridge and the existing motorway bridge. The inlet length was taken to be the distance
between the two bridges.
Historic flow gauging provided us with additional information to assist in the determination of the
appropriate tidal range due to the shallow bay area, particularly the mean velocity and the tidal flows
(refer Table 2-9). The initial 1978 gauges were used to calibrate the tidal inlet model for the situation of
no motorway bridge. The resulting tidal range was then used to assess local velocity changes at the
existing motorway bridge and the proposed bridge duplication.
The area blocked for each bridge was measured from the design drawings, taking the pier area between
Mean Sea Level and the seabed within the channel defined below MSL. For the existing bridge there are
5 piers with a cross-sectional area of 130 m2 some 8.5% of the over all area. There are four groups of
piles within the main tidal channel supporting the duplicate bridge, characterised as that portion of the
harbour below Mean Sea Level. Based on a conservative assessment of cross-sectional area, the
proposed works also encroach some 130 m2 into the channel section area. However, as can be seen on
Drawing B-098-5001, the proposed piles are generally situated in close proximity to the existing bridge
3-4
SECTION 3
Assessment of the proposal
piles, reducing their potential effect. The actual encroachment area, taking into account the existing
bridge piles, is expected to be less than 60 m2.
A total of 9 cases were analysed and the boundary conditions and results summarised in Table 3-1.
Case 1 and 2 comprised the situation that would have existed in 1978 prior to the completion of the
current motorway bridge for both Springs and Neap tidal cycles. Case 1 was used to establish the spring
tide range that produced the peak flow rate measured in 1978. Case 2 was for the neap condition of the
same situation and the inlet area had to be reduced to achieve similar peak flows. As there is 0.85 m
difference between high water springs and neaps this appeared a reasonable assumption and the
resulting velocities and flows compared well with the measured result. Case 3 was for the 1859 situation
where there was no reclamation or bridges. This indicates that spring tidal velocities were only slightly
less (0.06 m/s) than the existing situation as a result of the reduced tidal prism.
Table 3-1: Results of tidal prism analysis
Case
Description
Tide
Channel
area (m2)
Piers
(m2)
Inlet
area
(km2)
Max.
velocity
(m/s)
Peak
flow
(m3/s)
1
Mangere footbridge
Springs
1420
158
6.6
0.92
1,160
2
Mangere footbridge
Neaps
1420
158
5.2
0.48
603
3
Original situation
Springs
1600
0
7.8
0.86
1,370
4
Current motorway bridge
Springs
1524
130
6.6
0.83
1,160
5
Proposed bridge
encroachment
Springs
1524
190
6.6
0.87
1,160
6
Proposed bridge
Springs
1594
130
6.6
0.79
1,160
7
Proposed bridge
(conservative approach)
Springs
1524
260
6.6
0.92
1,160
8
Proposed bridge,
southern temporary works
Springs
1524
130+48
6.6
0.86
1,160
9
Proposed bridge, northern
temporary works
Springs
1524
130+138
6.6
0.92
1,160
Case 4 represents the current motorway bridge and shows the tidal velocities at this location are around
0.1 m/s less than at the footbridge. This is due to the wider channel area present at this location.
The effects of the proposed duplicate bridge is assessed in the next 5 cases. Case 5 assumes that the
duplicate bridge acts to further reduce the available cross-section at the current bridge site by 60 m2,
which results in an 0.04 m/s increase in peak velocity. Case 6 represents the cross sectional channel
area in the vicinity of the proposed bridge, which provides a slightly larger cross-section, due to the
greater width of channel. The velocity of 0.79 m/s represents the lower bound of expected peak velocity
at this location, assuming no interference or blockage is felt by the combined bridges. Case 7 is the most
conservative approach, reducing the cross-sectional area at the current bridge by both its pile area and
the pile area of the proposed bridge. This assumes full interference, with no benefit of sheltering or
alignment of the piles. In this situation the velocity matches the velocity under the footbridge.
Construction effects are considered in Case 8 and 9, with the worst case of the northern temporary bridge
providing a similar effect to the conservative assessment of case 7 and of the velocities at the existing
footbridge.
The result of this assessment indicates that the proposed duplicate motorway bridge will not increase
velocities more than that currently experienced at the existing footbridge. There will be a slight increase
in local velocities, in the order of 0.1 m/s (peak), with between 60 and 130 m2 of encroachment of the
channel.
3-5
SECTION 3
Assessment of the proposal
The likely effect of this slight contraction will be a lowering of the seabed in the vicinity of the bridge, with
the channel deepening to increase cross-sectional area. Equilibrium conditions will occur when the
cross-section deepens sufficiently to reduce velocities to existing levels. This will require between 60 m2
to 130 m2 of profile adjustment, which equates to between 0.2 m to 0.5 m average depth change based
on 60 m2 to 130 m2 additional contraction. In reality this would manifest itself with a greater change in the
centre of the channel and smaller changes along the sides of the channel.
The total volume of material potentially able to be scoured has been determined assuming the bed level
will reduce uniformly between the pile caps that are 10 m wide, and that there will be no change in level to
the west of the bridge, as the channel in this area tends to be deeper, but lowering of the seabed could
extend up to 30 m from the edge of the piles. A greater influence is unlikely as the inlet becomes
significantly wider east of this point which would have the effect of further reducing the tidal velocity.
Based on the area of potential modification, there is likely to be some 2,000 m3 of erosion from this area,
with the sediment redistributed both within the inlet and placed within the port area, as this is an area of
reduced velocity. In the most extreme situation of full blocking and no beneficial sheltering taken into
account, the localised erosion would be up to 4,000 m3. This adjustment in sea bed level is likely to occur
reasonably quickly (in the order of weeks to months) and then no further significant adjustments would
occur.
3.1.5
Local Scour
There are well documented empirical relationships for assessing bridge scour (Melville and Coleman,
2000), which can also be used for estimating local scour due to tidal flows. However, it is noted that
”typically less scour develops because some of the material eroded during one tide phase is deposited in
the scour hole during the next phase”. The theoretical maximum local scour depth at piers in tidal
environments can be approximated by 1.4 times the width of the pile, which is conservatively based on
the width of the pile cap (Melville and Coleman, 2000). In this instance, this would suggest maximum
local scour of more than 14 m. At this location this would appear a very unlikely situation and does not
match the observed situation. Recent diver inspection of the existing motorway bridge piles identified no
significant local scour holes around the piles. It is likely that this is due to the finer material being scoured
either leaving an armoured layer or exposing the more competent and erosion resistant rock shelf or
cohesive bed. A prudent allowance of and additional 100 m3 of potential localised erosion should be
taken into account to allow for local scour effects.
3.2
Replacement footbridge
The consent applications relating to the removal of the Old Mangere Bridge and construction of a new
pedestrian / cyclebridge and fender structure have been withdrawn from the current schedule of consents
sought. This is to enable further time to investigate options for the replacement bridge and to consult with
key stakeholders and the community. Replacement of this bridge remains within in the overall project
scope, and new consent applications will be lodged in due course.
Transit has recently committed funding for some immediate repair works on the bridge, thereby enabling
time for consultation and consideration of options.
3.3
Orpheus Drive
Orpheus Drive works have been scaled back considerably from the design as lodged, with a maximum
encroachment into Onehunga Bay of 5,500 m2 compared to the previously stated 1.6 hectares (refer
Drawing P-181-5004 included in Section 5). There is also no temporary reclamation as it was considered
undesirable to have temporary reclamation area due to the disturbance required to create and remove
these structures.
This revised option provides the opportunity of a 5 m wide walkway/cycleway extending from Onehunga
Harbour Road to the Manukau Cruising Club and the relocated Sea Scout Hall with provision for 14 car
parks adjacent to the hall.
3-6
SECTION 3
Assessment of the proposal
The reclaimed area is situated between the existing power pylon to the east of the Manukau Cruising
Club and Gloucester Park interchange affecting some 380 m of existing shoreline. The existing boat
ramps and facilities of the Manukau Cruising Club will not be affected. This approach reduces potential
issues that may have arisen with a larger reclamation, in terms of effects on tidal flows through the gap
between the boat club and offshore breakwater as the proposed works do not extend beyond the
rockwork around the existing pylon to the east of the boat club. The proposed reclamation is considered
the minimum size of area required to address the required roading/traffic needs as well as to provide an
improved pedestrian/cycle access way.
In considering the preferred coastal edge treatment along this area consideration was given to the
existing shoreline that comprises a steeply sloping dressed basalt wall or a more gently sloping rock
armour revetment as well as softer options such as beach nourishment and vegetation controls.
However, beach nourishment was not considered suitable at this location due to the general nature of the
surrounding environment and vegetation controls were also unlikely to be sufficient to protect an
unprotected reclamation edge.
Ultimately a sloping rock armour revetment was chosen, similar to the remaining revetment situated to the
north west of Manukau Cruising Club, although a basalt or concrete upstand wall would be provided
along the landward side of the revetment to reduce wave overtopping effects.
The sloping rock revetment will have improved wave energy absorption capabilities compared to
sreeply sloping rock wall and will provide a more visually consistent shore edge as it will match
revetment to the north-west. There is the opportunity of establishing planting along the crown of
revetment (Flaxes/Pohutukawa) similar to the planting present along the north-eastern end of
revetment.
the
the
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The reclamation is likely to require the following steps:
•
Install floating silt curtain around perimeter of the reclamation area.
•
End tip a run of pit bund around the perimeter of the reclamation area wide enough for a truck to
reverse along (minimum crest width of 3.5 m). At this stage it is assumed that the bund would be
constructed over the shallow depth of silt overlying the rock shelf as this would limit disturbance.
However, if required the silts will be progressively removed by long reach hydraulic excavator and
the excavated silts would be removed to an approved landfill or reworked for returning into the
reclamation area.
•
Progressively shape and armour the seaward slope of the run-of-pit bund. A geotextile filter fabric
underlying a layer of quarried filter rock and rock armour will be placed on the prepared slope of the
run-of pit bund to provide erosion protection of the reclaimed area. This is likely to require importing
of new quarried rock, although portions of existing rock may be stockpiled for reuse. All construction
debris will be placed within the reclamation area and organics and unsuitables removed from site to
an approved disposal area.
•
Dewater and infill the reclamation area with engineered fill. Dewatering will be carried out to drain
the reclaimed area. If required the remaining silts will be removed and then the area infilled with
engineered fill that may included suitable materials excavated as part of this project, imported
quarried material and stabilised fill.
3.4
Tararata Creek
The extent of the permanent works required over the Tararata Creek for this project, are shown on the
following drawings included in Section 5:•
B-091-5001
Longsection)
Tararata Creek Bridge and Walmsley Road Off ramp Bridge (Plan and
•
B-091-5002
Tararata Creek Bridge and Walmsley Road Off ramp Bridge (Typical Sections)
3-7
SECTION 3
Assessment of the proposal
To construct new widening on the existing Tararata Creek Motorway Bridge and to construct the new
Walmsley Road Off ramp Bridge access to the Coastal Marine Area will be needed. It is envisaged that
the temporary works required for this access will comprise a temporary bridge (temporary staging)
constructed between the two bridges. The staging will be built with an end over end construction method
without machinery access in the Coastal Marine Area. In this manner the temporary works will be
undertaken in a very similar manner to that described earlier for construction of the duplicate motorway
bridge across the Manukau Harbour. The temporary staging is envisaged to cross the entire creek,
some 70m in length.
The new bridge works will occupy the following plan areas within the Coastal Marine Area of Tararata
Creek.
•
Tararata Creek Bridge Widening – 1.2 square metres (based on 4 x 610mm diameter piles)
•
Walmsley Road Off ramp Bridge – 2.3 square metres (based on 2 x 1200mm diameter piles)
The temporary works will occupy approximately 1.9 square metres (based on 20 x 310mm square piles)
Permanent and temporary piles will be located outside of the low flow channel. As a result no scour or
realignment of the channel is expected. Piles will be located within the vegetated margins of the creek.
Damage to mangrove roots will be limited to the area of the piles, both temporary and permanent. The
mat of mangrove roots at creek bed level is expected to remain as an intact mat and this will prevent any
significant scour around the piles, as evidenced by similar bridges in similar environments.
The disturbance to the creek bed on installation and subsequent removal of the temporary piles is
minimal and we anticipate restoration of the seabed within 7 days due to a combination of physical
restoration works (manual tidy-up) and normal tidal flushing action.
3-8
SECTION 4
References
4
References
References
MCC, 1991. Tararata Creek Assessment. Manukau City Council
Ministry of Energy Electricity Division, 1980. Auckland Combined Cycle Power Station Investigation.
Hydrolodgy of the Waitemata and Manukau Harbours. Report of the Hydrology Working Group,
September 1980.
Opus, 2006. Coastal Process Assessment. Unpublished report for Transit NZ Ltd by Opus International
Consultants, May 2006.
Riley, 1991. Tararata Creek Catchments: Comprehensive Catchment Discharge Consent Application.
Unpublished report prepared for Manukau City Council, March 2001.
4-1