Manly Lagoon Flood Study

Conclusions

Provided below is a summary of the key findings of the Flood Study, in particular some of the
important considerations for future floodplain risk management in the catchment:
<ul>
<li>The design flood conditions documented in the report typically provide for a small increase in
previously adopted design flood conditions for Manly Lagoon. The main contributing factor to this
change is the way the entrance condition has been modelled. In addition to advances in the
software to simulate entrance breakout response, the initial conditions in respect to berm
elevations and initial water levels in the Lagoon have been represented in the model according to
current Council entrance management practices.</li>
<li>Longer duration events (6-9 hours) typically provide for the worst case flooding conditions in
Manly Lagoon. With the Lagoon water body providing flood storage, events of longer duration are
required to generate sufficient flood runoff volumes from the catchment to elevate Lagoon water
levels. In the lower reaches of all the tributary catchments, flood levels are dominated by the
Lagoon flooding conditions. The peak flood water level in the Lagoon extends a significant
distance up the tributary channels. In the upper reaches of the tributary catchments, shorter
duration events of the order of 2-hours provide the critical flood condition in terms of peak flood
water level.</li>
<li>The rise in flood water levels can be relatively fast from the catchment’s response to rainfall.
Even for the longer duration events providing for the highest peak flood water levels in the
Lagoon, the main period of rise in Lagoon water level can occur over a few hours. The April 1998
flood event (used for model calibration in the current study) is an example of such a response in
the catchment. Flood levels in the tributary catchments may also rise significantly faster owing to
the shorter critical durations in these catchments. This potentially rapid inundation has
implications for flood warning and emergency response, particularly in flood situations where
property and access roads may be quickly inundated.</li>
<li>Catchment derived flooding events represent the dominant flooding mechanism in Manly Lagoon.
Whilst some ocean flooding scenarios will provide for inundation of some foreshore areas, the
extent and severity of flooding is significantly less than the corresponding catchment derived
event magnitude. The entrance condition has some influence on catchment flood behaviour with
higher entrance berm levels providing for higher peak flood levels. The existing entrance
management policy provides for manual breakout of the Lagoon entrance at defined trigger levels
in preparation for imminent flooding. Irrespective of the successful implementation of a manual
entrance breakout, significant flood inundation may be expected during major catchment flood
events.</li>
<li>There are a number of areas within the Manly Lagoon catchment which represent the most
significant flood risk exposure to existing property. The worst affected areas are typically in the
lower parts of the catchment and most severely impacted on by major flooding in Manly Lagoon.</li>
</ul>

These areas include the foreshore areas of the Lagoon around Riverview Parade. Much of the
lower floodplain area is however occupied by park lands / golf courses such that flood risk
exposure of existing property is limited. Elsewhere, the Warringah Mall and Balgowlah Industrial
Estate are located on the alignments of Brookvale Creek and Burnt Bridge Creek respectively.

When drainage system capacities in these areas are exceeded, there is potential for overland
flow through these areas:
<ul>
<li>Peak design flood water levels are expected to progressively increase as the impacts of climate
change manifest. For the Manly Lagoon catchment, potential sea level rise will provide for a
worsening of existing flood conditions through higher ocean water levels (tide and storm surge),
higher entrance berm and higher initial water levels in the Lagoon. Robust land use planning and
development policies will be required to ensure future flood risks are not unduly exacerbated in
light of predicted flood behaviour under potential climate change scenarios.</li>
<li>Council’s existing entrance management policy is to open the entrance at a defined trigger water
level (currently 1.4m AHD). With potential sea level rise, normal tide levels in the Lagoon will
approach and eventually exceed the current trigger levels. Future openings would need to be at
significantly higher trigger levels to be effective. Low-lying land currently impacted by flooding
may also be subject to regular (or permanent) tidal inundation at some time in the future.</li>
</ul>

Model Development

Computer models are the most accurate, cost-effective and efficient tools to assess a catchment’s
flood behaviour. Traditionally, for the purpose of the Flood Study, a hydrological model and a
hydraulic model are developed.

The hydrological model simulates the catchment rainfall-runoff processes, producing the
stormwater flows which are used in the hydraulic model.

The hydraulic model simulates the flow behaviour of the overland flow paths, creeks and lagoon
producing flood levels, flow discharges and flow velocities.

Information on the topography and characteristics of the catchments and floodplains are built into the
hydraulic model. Recorded historical flood data, including rainfall and flood levels, are used to
simulate and validate (calibrate and verify) the model. The model produces as output, flood levels,
flows (discharges) and flow velocities.

With consideration to the available survey information and local topographical and hydraulic controls,
a linked 1D/2D model was developed extending from the Lagoon entrance in Queenscliff at the
downstream limit, to the head of the catchment. The floodplain area modelled within the 2D domain
comprises a total area of approximately 18km2 which includes the Manly Lagoon catchment in its
entirety.

Model Calibration and Validation

The selection of suitable historical events for calibration and validation of flood models is largely
dependent on the availability of relevant historical flood information. Ideally the calibration and
validation process should cover a range of flood magnitudes to demonstrate the suitability of a model
for the range of design events to be considered.

Review of the available rainfall and water level data for the Manly Lagoon catchment highlighted two
relatively recent flood events with sufficient data to support a calibration process – the April 1998, and
March 2011 event. The April 1998 event resulted in the highest recorded Lagoon water levels since
the installation of the MHL water level gauges.

The models were found to provide a reasonable representation of the observed flood behaviour in the
catchment.

Design Event Modelling and Output

The developed models have been applied to derive design flood conditions within the Manly Lagoon
catchment. Design rainfall depth is based on the generation of intensity-frequency-duration (IFD)
design rainfall curves utilising the procedures outlined in AR&amp;R (2001). A range of storm durations
using standard AR&amp;R (2001) temporal patterns, were modelled in order to identify the critical storm
duration for design event flooding in the catchment.

A suite of design event scenarios was defined that is most suitable for future floodplain management
planning in Manly Lagoon. Consideration was given to flood events driven by both catchment and
ocean processes. The catchment derived events were found to be the critical events in terms of
determining maximum flood levels.

The design events simulated include the PMF event, 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, 10%, 20% and
50% AEP events for catchment derived flooding and the 0.5%, 1%, 2%, 5%. 10% and 20% AEP
events for ocean derived flooding.

The model results for the design events considered have been presented in a detailed flood mapping
series for the catchment (see Appendix A). The flood data presented includes design flood
inundation, peak flood water levels and depths and peak flood velocities.
Provisional flood hazard categorisation in accordance with Figure L2 of the NSW Floodplain
Development Manual (2005) has been mapped in addition to the hydraulic categories (floodway,
flood fringe and flood storage) for flood affected areas.

Sensitivity Testing

A number of sensitivity tests have been undertaken to identify the impacts of the adopted model
conditions on the design flood levels. Sensitivity tests included:
<ul>
<li>The modelled lagoon entrance berm conditions;</li>
<li>The coincident catchment and ocean flooding conditions;</li>
<li>Structure and stormwater pipe blockages; and</li>
<li>Changes in the adopted roughness parameters.</li>
</ul>

Tags: Manly Lagoon, hydraulic model, hydrologic model
Organization: Northern Beaches Council