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Layer: Lower_Namoi_Valley_FMP_Floodway_Network (ID: 33)

Parent Layer: Lower Namoi Valley FMP 2020

Name: Lower_Namoi_Valley_FMP_Floodway_Network

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Description: Used as part of the Healthy Floodplain Lower NamoiValleyFloodplain Management Plan.The Lower NamoiValleyFMP floodplain boundary, which covers 570,700 hectares, was defined to capture the floodplain areas inundated during flooding of the major rivers running between Narrabri and Walgett and to include any works that may affect floodingThe floodway network map represents those areas on the floodplain that have adequate hydraulic capacity and continuity to effectively convey flood waters. It is comprised of:a coordinated and integrated network of floodways where a significant discharge of floodwater occurs during floodsthe inundation extent of small and large design floodsareas that preserve floodplain connectivityareas that provide sufficient pondage for floodwater.The Lower Namoi floodway network is comprised of three hydraulic categories:defined floodways (133,600 ha or 24 % of the floodplain), which are areas where a significant discharge of floodwater occurs during design floods and are important for the continuity of flood flows over the floodplain ill-defined floodways (21,400 ha or four % of the floodplain), which are areas where a significant discharge of floodwater occurs during design floods. They are overland flow paths with no defined channels or riverbanks that are important for the continuity of flood flows over the floodplaininundation extent (221,700 hectares or 39 % of the floodplain), which includes areas of the floodplain that are important for the temporary storage of floodwaters during the passage of a flood and for secondary flood discharge.The remaining 189,700 hectares or about 33 % of the floodplain was outside of the inundation extent of the large design flood and was therefore not included as part of the floodway network. About half of this area is not flooded because it is protected by flood works. Design floods were selected for the Burren, Merah, Molle, Reach 2 and Narrabri model reaches of the Lower Namoi Valley FMP:· large design flood – January/February 1971 (4% AEP @ Namoi River at Mollee gauging station GS 419039)· small design flood – December 2004 (13% AEP @ Namoi River at Mollee gauging station GS 419039).A flood frequency analysis was undertaken to understand the relative size of the historic floods Hydraulic model outputs used include a depth-velocity product map from the large design flood and inundation extents of the small and large design floods.Hydraulic criteria for mapping the floodway network are summarised in the table below.Hydraulic categoryCriteriaMajor Discharge Areas, Defined FloodwaysMajor discharge areas that have a depth-velocity product of ≥ 0.2 m2/s for the large design flood (1971 – 4% AEP).Floodplain connectivity was provided for by incorporating:parts of the small design flood extent (2004 – 13% AEP)for and/orfloodplain areas that have a depth-velocity product of ≥ 0.05 m2/s for the large design flood (1971 – 4% AEP).To ensure that conditions on the ground are adequately represented, the above criteria were checked against the following additional data:flood aerial photography and satellite imageryspatial watercourse layers, topographical mapping, ADS DEM and LiDARprevious floodplain management plans and development guidelineslocal knowledge obtained from floodplain communities and floodplain/environmental managers.Major Discharge Areas, Ill Defined FloodwaysMajor discharge areas that have a depth-velocity product of <0.05 m2/s for the large design flood (1971 – 4% AEP).The location of ill-defined floodways is guided by:areas with a depth-velocity product of <0.05 m2/s for the large design flood (1971 – 4% AEP) and/orflood imagery and/orlocal knowledge and/orparts of the small design flood extent (2004 – 13% AEP).The width of ill-defined floodways is determined by the width of the upstream defined floodway and a maximum 500 metre buffer either side of this floodway (minimum width is one kilometre).To ensure that conditions on the ground are adequately represented, the above criteria were checked against the following additional data:flood aerial photography and satellite imageryspatial watercourse layers, topographical mapping, ADS DEM and LiDARprevious floodplain management plans and development guidelineslocal knowledge obtained from floodplain communities and floodplain/environmental managers.Flood Storage and Secondary Flood Discharge AreasFlood storage and secondary flood discharge areas of the floodplain:are areas not already identified as an ill-defined or defined floodway and areincluded within the extent of the large design flood (1971 – 4% AEP) or areenclosed by existing Part 8 approved flood works that are overtopped during moderate to large floods.To ensure that conditions on the ground are adequately represented, the above criteria were checked against the following additional data:flood aerial photography and satellite imagery from design floodsspatial watercourse layers, topographical mapping, ADS DEM and LiDARprevious floodplain management plans and development guidelineslocal knowledge obtained from floodplain communities and floodplain/environmental managers.Areas outside floodway network (known as Flood Fringe and Flood Protected Areas)Areas outside of the floodway network include the flood fringe areas of the floodplain which have not been mapped as floodways or flood storage and secondary flood discharge areas. These areas of the floodplain are:outside the extent of the large design flood (1971 –4% AEP) and/or are enclosed by existing Part 8 approved flood works that are not designed to be overtopped during flooding To ensure MZ C represents on-ground conditions, the above criteria were road tested against the following additional data: flood aerial photography and satellite imagery from design floods spatial watercourse layers, topographical mapping, ADS DEM and LiDAR previous floodplain management plans and development guidelines local knowledge obtained from floodplain communities and floodplain/environmental managers.

Copyright Text: Office of Environment and Heritage: Healthy Floodplains Unit

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