Select level of
AUP Chapter E36.9 Flood Hazard Risk Assessment Report
assessed risk
Site Address:
The Strand, Auckland
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Prepared by (company name)
Beca Ltd
Application No:
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each cell below
(a) The frequency, duration and scale of the flooding hazard;
State If the site being developed will be impacted by flooding in more frequent events than 1 % AEP
The entire project extent is within the 100yr ARI floodplain associated with two major Overland Flow Path
If assessment is for overland flow, determine trigger event as well as 1% AEP scenario.
(OLFPs) and a flood prone area. The total contributing catchments are approximately 200ha from the
An assessment of the duration of the flooding hazard for the 1 % AEP event should be made supported
west and 72ha from the east. The western OLFP conveys runoff from Auckland Domain and the upper
with a study of the hydrology of the contributing sub catchments* that is appropriate for the scale of the
catchment along Stanley Street and Beach Road. The eastern OFLP runs along The Strand from east to
risk.
west and converges with the western OLFP at the intersection of Beach Road and Ronayne Street.
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Describe extent of flooding on site along with discharge rates, depths and velocities at critical points on the The Strand and many of the surrounding properties were flooded during the January 2023 Auckland
developed site.
Anniversary weekend torrential storm event, which was known to be well above the 100yr ARI storm for
that area.
Auckland Council’s Stanley Catchment Modelling results for the maximum probable development (MPD)
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scenario including climate change (completed by Tonkin and Tayler in 2015) indicate that the existing
stormwater pipe network does not have sufficient capacity to convey the 2yr flow under the backwater
effect and some of the nearby properties are at risk of flooding during the 2 and 5yr ARI storm events.
There have been reported road flooding and manholes popping during the 5 to 10yr ARI storm events in
the past (Stanley FHM Model Build Report, Tonkin and Taylor, 2015), which generally aligns with the
medium
Stanley catchment flood modelling results and indicate that the road is at risk of flooding during the 10yr
ARI storm and possibly smaller storm events due to insufficient capacity of the primary drainage.
Most of the site and some of the surrounding properties are also within the 50yr and 100yr ARI (with 1m
sea level rise) coastal inundation zone.
As per Auckland Council GeoMaps OLFP layer, the estimated 100yr ARI peak flow for the existing
scenario is 28.6 m3/s and 14.3m3/s for the western and eastern OLFPs respectively. However, this does
not consider the flow conveyed by primary drainage and detention effect within the flood prone areas.
There are a few major stormwater lines (including a 2300mm by 2100mm box culvert and a DN2350mm
brick pipe) running under The Strand. For the 100yr ARI (1%AEP) MPD scenario, the modelled flow (0.2
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– 2.3 m3/s) and velocity (0.01 - 0.15 m/s) for the overland flow exceeding the pipe capacity are very low
due to flat grade and detention affect within the widespread flood prone area. The 100yr flood depth
within the road reserve varies between 50mm to 500mm with the average flood depth of 250mm.
(b) the type of activity being undertaken and its vulnerability to flooding events;
Identify the activity or activites incorporated in the proposed development as listed in table E36.4.1.
The proposed road improvement works include a 4m wide shared path by widening the existing footpath,
Described the vulnerability (exposure) of the activity or activities to the flood events determined by the
super-elevation upgrade and pavement reshaping at the St Georges Bay Road intersection, new zebra
investigation into the flooding hazards impacting the site described in E36.9(a). This should include
and signalised pedestrian crossings, and minor changes to the existing stormwater network such as
whether the building footprint , any vehicle parking area and means of egress are within the flooding extent catchpit relocation and additions. The entire work extent is within the 100yr floodplain, except the section
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to the north of Sudbury Terrace.
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As per AUP E36.4.1 (A52) and (A53), renewal, repair and minor infrastructure upgrade, and construction,
operation, maintenance, renewal, and repair of road network activities within legal road in the 1% AEP
coastal inundation area, floodplain and OLFP are Permitted activities.
the
The proposed road improvement works will not alter the road network activities but will provide a safer
passage for pedestrians and cyclists (i.e. separation from vehicles).
(c) the consequences of a flooding event in relation to the proposed activity and the people likely to be involved in that activity;
Identify the impacts on the proposed activity during a flood event e.g. if the building footprint is fully or
The shared path can be flooded with 50mm to 500mm of water during the 1% AEP storm event. The
partially within the flooded area what level will the flooding reach in respect to the living areas and other
flooding is not expected to cause any damage to the shared path or road infrastructure, but is likely to
components of the dwelling. If egress from the building will be flooded . . to what depth and for what
affect the trafficability of the road and safety of the road users occasionally/for short durations.
period of time. Identify any potential for damage to, or deterioration of, the stuctural and functional integrity According to AUSTROADS and AIDR Australian Disaster Resilience Guideline, the flood hazard is low
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of the building resulting from the intensity and or frequency of flooding.
for most of the project extent where the flood depth is less than 300mm and velocity is less than 0.15
m/s. At a few localised areas where the flood depths reach 500mm, it is still low hazard for pedestrians
but unsafe for small vehicles.
(d) the potential effects on public safety and on other property;
Describe effects on publc safety if the activity will include public use. Identify any potential flooding of
Based on the modelled flood depths and velocities for the 1%AEP MPD scenario, the estimated flood
upstream or downstream properties that may be affected by the proposed activity
hazard is generally safe for people, vehicles, and buildings for most of the project extent and unsafe for
small vehicles for localised areas where flood depth reaches 0.5m.
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The proposed road improvement works will not increase the flood risk to the public safety or other
properties.
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(e) Any exacerbation of an existing flooding hazard risks or creation of a new flooding hazard risk;
Describe results of investigation into any potential effects on other property if the activity results in
Since the existing impervious cover of the road reserve is very high (100% imperviousness for Stanley
diversion of flood flow or overland flow. Identify any new activity that results in an increase to the number
Street) and the proposed shared path expands along the existing footpath, the overall increase in
of people exposed to an existing flood risk.
impervious area is very minor, and will be distributed over a long distance. The major stormwater
pipelines running along Stanley Street and The Strand receive runoff from more than 3 km2. The
increase of 990 m2 is approximately 0.03% of the wider catchment area. In addition, as the site is located
at the downstream end of the Stanley catchment, it is expected that peak runoff from the road reserve
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would drain to the pipe before the upstream peak flow arrives due to the difference in time of
concentration. Therefore, the impact from the proposed road works on the performance of the existing
primary drainage network and flood risk is negligible.
The estimated change in 100yr flood depth is less than 10mm due to the proposed super-elevation
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upgrades and pavement reshaping at the corner of St Georges Bay Road. There will be no exacerbation
of existing flood hazard risks or creation of new flood hazard.
(h) the design and construction of buildings and structures to mitigate the effects of flooding
Decribe how the potential flooding effects identified above, determined by investigation and described in The proposed road works generally follows the existing ground surface and maintains the existing OLFPs
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detail in a flooding report, will be mitigated by the design and materials of the building.
within the road reserve. Where overland flow paths are changed due to the super-elevation upgrades and
pavement reshaping, capacity of the proposed road cross section is checked to ensure there is no
increase in flood risk to buildings. Catchpits relocation and addition are proposed where the existing
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primary drainage network is affected by the road works.
(j)
site layout and management to avoid or mitigate the adverse effects of flooding hazard, including access and exit during a flooding event;
Decribe how the potential flooding effects identified above, including any effects on upstream and
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downstream properties, determined by investigation and described in detail in a flooding report, will be
mitigated by the design form of any structures and site works. Describe measures proposed to provide
safe egress from property
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(l) any measures and/ or plans proposed to mitigate the flooding hazard or the effects of the flooding hazard.
Describe any other measures to mitigate the flooding hazard which can include information about future
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works planned by Auckland Council in the wider catchment that will reduce the flooding risk.
Include any other measures to mitigate effects that are not described above.
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