{"help": "https://data.nsw.gov.au/data/api/3/action/help_show?name=package_show", "success": true, "result": {"author": "Northern Beaches Council", "author_email": null, "contact_info": "False", "contact_point": "manly@northernbeaches.nsw.gov.au", "creator_user_id": "8d2ac61a-d63a-4456-a00d-b35fee4c4025", "data_state": "inactive", "id": "82fed73c-bf52-49ac-a0c8-1132994f11e4", "isopen": true, "jurisdiction": "New South Wales Government", "license_id": "cc-by", "license_title": "Creative Commons Attribution", "license_url": "http://www.opendefinition.org/licenses/cc-by", "maintainer": null, "maintainer_email": null, "metadata_created": "2025-12-17T02:16:15.654931", "metadata_modified": "2026-01-13T05:56:33.157545", "name": "fdp-manly-lagoon-flood-study-final-report2013", "notes": "#Conclusions\r\nProvided below is a summary of the key findings of the Flood Study, in particular some of the\r\nimportant considerations for future floodplain risk management in the catchment:\r\n\r\n* The design flood conditions documented in the report typically provide for a small increase in\r\npreviously adopted design flood conditions for Manly Lagoon. The main contributing factor to this\r\nchange is the way the entrance condition has been modelled. In addition to advances in the\r\nsoftware to simulate entrance breakout response, the initial conditions in respect to berm\r\nelevations and initial water levels in the Lagoon have been represented in the model according to\r\ncurrent Council entrance management practices.\r\n* Longer duration events (6-9 hours) typically provide for the worst case flooding conditions in\r\nManly Lagoon. With the Lagoon water body providing flood storage, events of longer duration are\r\nrequired to generate sufficient flood runoff volumes from the catchment to elevate Lagoon water\r\nlevels. In the lower reaches of all the tributary catchments, flood levels are dominated by the\r\nLagoon flooding conditions. The peak flood water level in the Lagoon extends a significant\r\ndistance up the tributary channels. In the upper reaches of the tributary catchments, shorter\r\nduration events of the order of 2-hours provide the critical flood condition in terms of peak flood\r\nwater level.\r\n* The rise in flood water levels can be relatively fast from the catchment\u2019s response to rainfall.\r\nEven for the longer duration events providing for the highest peak flood water levels in the\r\nLagoon, the main period of rise in Lagoon water level can occur over a few hours. The April 1998\r\nflood event (used for model calibration in the current study) is an example of such a response in\r\nthe catchment. Flood levels in the tributary catchments may also rise significantly faster owing to\r\nthe shorter critical durations in these catchments. This potentially rapid inundation has\r\nimplications for flood warning and emergency response, particularly in flood situations where\r\nproperty and access roads may be quickly inundated.\r\n* Catchment derived flooding events represent the dominant flooding mechanism in Manly Lagoon.\r\nWhilst some ocean flooding scenarios will provide for inundation of some foreshore areas, the\r\nextent and severity of flooding is significantly less than the corresponding catchment derived\r\nevent magnitude. The entrance condition has some influence on catchment flood behaviour with\r\nhigher entrance berm levels providing for higher peak flood levels. The existing entrance\r\nmanagement policy provides for manual breakout of the Lagoon entrance at defined trigger levels\r\nin preparation for imminent flooding. Irrespective of the successful implementation of a manual\r\nentrance breakout, significant flood inundation may be expected during major catchment flood\r\nevents.\r\n* There are a number of areas within the Manly Lagoon catchment which represent the most\r\nsignificant flood risk exposure to existing property. The worst affected areas are typically in the\r\nlower parts of the catchment and most severely impacted on by major flooding in Manly Lagoon.\r\n\r\nThese areas include the foreshore areas of the Lagoon around Riverview Parade. Much of the\r\nlower floodplain area is however occupied by park lands / golf courses such that flood risk\r\nexposure of existing property is limited. Elsewhere, the Warringah Mall and Balgowlah Industrial\r\nEstate are located on the alignments of Brookvale Creek and Burnt Bridge Creek respectively.\r\n\r\nWhen drainage system capacities in these areas are exceeded, there is potential for overland\r\nflow through these areas:\r\n\r\n* Peak design flood water levels are expected to progressively increase as the impacts of climate\r\nchange manifest. For the Manly Lagoon catchment, potential sea level rise will provide for a\r\nworsening of existing flood conditions through higher ocean water levels (tide and storm surge),\r\nhigher entrance berm and higher initial water levels in the Lagoon. Robust land use planning and\r\ndevelopment policies will be required to ensure future flood risks are not unduly exacerbated in\r\nlight of predicted flood behaviour under potential climate change scenarios.\r\n* Council\u2019s existing entrance management policy is to open the entrance at a defined trigger water\r\nlevel (currently 1.4m AHD). With potential sea level rise, normal tide levels in the Lagoon will\r\napproach and eventually exceed the current trigger levels. Future openings would need to be at\r\nsignificantly higher trigger levels to be effective. Low-lying land currently impacted by flooding\r\nmay also be subject to regular (or permanent) tidal inundation at some time in the future.\r\n\r\n#Model Development\r\nComputer models are the most accurate, cost-effective and efficient tools to assess a catchment\u2019s\r\nflood behaviour. Traditionally, for the purpose of the Flood Study, a hydrological model and a\r\nhydraulic model are developed.\r\n\r\nThe hydrological model simulates the catchment rainfall-runoff processes, producing the\r\nstormwater flows which are used in the hydraulic model.\r\n\r\nThe hydraulic model simulates the flow behaviour of the overland flow paths, creeks and lagoon\r\nproducing flood levels, flow discharges and flow velocities.\r\n\r\nInformation on the topography and characteristics of the catchments and floodplains are built into the\r\nhydraulic model. Recorded historical flood data, including rainfall and flood levels, are used to\r\nsimulate and validate (calibrate and verify) the model. The model produces as output, flood levels,\r\nflows (discharges) and flow velocities.\r\n\r\nWith consideration to the available survey information and local topographical and hydraulic controls,\r\na linked 1D/2D model was developed extending from the Lagoon entrance in Queenscliff at the\r\ndownstream limit, to the head of the catchment. The floodplain area modelled within the 2D domain\r\ncomprises a total area of approximately 18km2 which includes the Manly Lagoon catchment in its\r\nentirety.\r\n#Model Calibration and Validation\r\nThe selection of suitable historical events for calibration and validation of flood models is largely\r\ndependent on the availability of relevant historical flood information. Ideally the calibration and\r\nvalidation process should cover a range of flood magnitudes to demonstrate the suitability of a model\r\nfor the range of design events to be considered.\r\n\r\nReview of the available rainfall and water level data for the Manly Lagoon catchment highlighted two\r\nrelatively recent flood events with sufficient data to support a calibration process \u2013 the April 1998, and\r\nMarch 2011 event. The April 1998 event resulted in the highest recorded Lagoon water levels since\r\nthe installation of the MHL water level gauges.\r\n\r\nThe models were found to provide a reasonable representation of the observed flood behaviour in the\r\ncatchment.\r\n\r\n#Design Event Modelling and Output\r\nThe developed models have been applied to derive design flood conditions within the Manly Lagoon\r\ncatchment. Design rainfall depth is based on the generation of intensity-frequency-duration (IFD)\r\ndesign rainfall curves utilising the procedures outlined in AR&R (2001). A range of storm durations\r\nusing standard AR&R (2001) temporal patterns, were modelled in order to identify the critical storm\r\nduration for design event flooding in the catchment.\r\n\r\nA suite of design event scenarios was defined that is most suitable for future floodplain management\r\nplanning in Manly Lagoon. Consideration was given to flood events driven by both catchment and\r\nocean processes. The catchment derived events were found to be the critical events in terms of\r\ndetermining maximum flood levels.\r\n\r\nThe design events simulated include the PMF event, 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, 10%, 20% and\r\n50% AEP events for catchment derived flooding and the 0.5%, 1%, 2%, 5%. 10% and 20% AEP\r\nevents for ocean derived flooding.\r\n\r\nThe model results for the design events considered have been presented in a detailed flood mapping\r\nseries for the catchment (see Appendix A). The flood data presented includes design flood\r\ninundation, peak flood water levels and depths and peak flood velocities.\r\nProvisional flood hazard categorisation in accordance with Figure L2 of the NSW Floodplain\r\nDevelopment Manual (2005) has been mapped in addition to the hydraulic categories (floodway,\r\nflood fringe and flood storage) for flood affected areas.\r\n#Sensitivity Testing\r\nA number of sensitivity tests have been undertaken to identify the impacts of the adopted model\r\nconditions on the design flood levels. Sensitivity tests included:\r\n\r\n* The modelled lagoon entrance berm conditions;\r\n* The coincident catchment and ocean flooding conditions;\r\n* Structure and stormwater pipe blockages; and\r\n* Changes in the adopted roughness parameters.", "num_resources": 1, "num_tags": 3, "organization": {"id": "bc8b9274-7fc7-4576-8211-9fd11d7f42e5", "name": "northern-beaches-council", "title": "Northern Beaches Council", "type": "organization", "description": "The Northern Beaches Council is a recent LGA on Sydney's northern beaches formed from the merger of Manly, Pittwater and Warringah councils. \r\n\r\nhttp://www.northernbeaches.nsw.gov.au/\r\n\r\nFlood Studies, Plans and Maps available:\r\nhttp://www.pittwater.nsw.gov.au/property/natural_hazards/flooding/where_does_it_flood/flood_studies_and_plans\r\n\r\nCareel Creek\r\nAvalon to Palm Beach Flood Risk Management Study and Plan - current project.\r\nCareel Creek Catchment Flood Study - July 2013\r\nCareel Creek Floodplain Risk Management Plan November 2002 \r\nCareel Creek Floodplain Management Study Sept 2000 \r\nCareel Creek Drainage Catchment Flood Study June 1999 - superseded\r\n\r\nGreat Mackerel Beach\r\nGreat Mackerel Beach Floodplain Risk Management Study and Plan - November 2010\r\nGreat Mackerel Beach Flood Study September 2005\r\n\r\nMona Vale/Bayview\r\nMcCarrs Creek, Mona Vale and Bayview Flood Study - current project\r\nMona Vale - Bayview Flood Study May 2002\r\n\r\nNarrabeen Lagoon\r\nNarrabeen Lagoon Flood Study - September 2013\r\nNarrabeen Lagoon Floodplain Risk Management Plan Vol 1 Pittwater December 2002\r\nNarrabeen Lagoon Floodplain Management Study 1992\r\n\r\nNareen Creek (subcatchment of Narrabeen Lagoon)\r\nNareen Creek Flood Study September 2005 - superseded\r\n\r\nWarriewood Valley (subcatchment of Narrabeen Lagoon)\r\nWarriewood Valley Flood Study April 2005 - superseded\r\nWarriewood Valley Flood Study Addendum 1 July 2005 - superseded\r\nWarriewood Valley Urban Land Release Water Management  Specification - February 2001\r\n\r\nNewport\r\nNewport Beach Floodplain Risk Management Study & Plan February 2004\r\nNewport Beach Flood Study March 2002\r\n\r\nOther Areas\r\nPittwater Overland Flow Mapping and Flood Study - October 2013\r\nMcCarrs Creek, Mona Vale and Bayview Flood Study - current project\r\nAvalon to Palm Beach Flood Risk Management Study and Plan - current project\r\n", "image_url": "2021-03-23-012619.270871northern-beaches.png", "created": "2021-02-17T08:04:20.227446", "is_organization": true, "approval_status": "approved", "state": "active"}, "owner_org": "bc8b9274-7fc7-4576-8211-9fd11d7f42e5", "private": false, "spatial": "{\r\n \"type\": \"Polygon\",\r\n \"coordinates\": [\r\n  [\r\n   [\r\n    151.28953513805754,\r\n    -33.7836973589487\r\n   ],\r\n   [\r\n    151.291327283252,\r\n    -33.78655081348802\r\n   ],\r\n   [\r\n    151.28893947286997,\r\n    -33.786923180201896\r\n   ],\r\n   [\r\n    151.2864006048767,\r\n    -33.78928005448397\r\n  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