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Flood study

Mona Vale - Bayview Flood Study

The Mona Vale - Bayview Catchment has a total area of approximately 5.2km2 with two major sub-catchments subdividing the catchment into the Mona Vale Drain subcatchment of 1.6km² and the Cahill Creek catchment at Bayview of. 3.6km².
A large proportion of the catchment is urbanised with a mix of residential, commercial, light industrial and a smaller proportion of forested areas. Bayview Golf Course is a major feature of the Cahill Creek floodplain.

Flooding in the Mona Vale - Bayview floodplain has historically resulted in inundation
of and damage to, private and commercial property and has restricted road and property access. The recognition of these flooding issues has prompted Pittwater
Council, through the Pittwater Coastal, Estuary and Floodplain Management Committee to prepare a Flood Risk Management Plan for the Mona Vale - Bayview floodplain. The Mona Vale - Bayview Flood Study is the first part of the Flood Risk Management process, which aims to reduce the impact of flooding and flood liability for flood prone land in the catchment.

A flood study of the Mona Vale - Bayview floodplain was undertaken to determine the
existing flood behaviour of flood prone areas for a range of flood risk levels from the 20% Annual Exceedance Probability (AEP) event through to the Probable Maximum Flood (PMF).

Data has been collated for the Flood Study from a variety of sources. This information includes data describing historical flood behaviour that has occurred on the floodplain and data essential for the development and verification of the hydrologic and hydraulic modelling tools. The advice of the local residents ·of the floodplain was sought via a community newsletter and survey circulated in January 2001 for the purpose of collecting historical flood information and community opinion regarding flooding and drainage on the floodplain.

Design rainfall intensities and temporal patterns for the required range of flood risk events were obtained from 'Australian Rainfall and Runoff', Institution of Engineers Australia, (2001) and 'The Estimation of Probable Maximum Precipitation in Australia: Generalised Short-Duration Method,' Bureau of Meteorology (1994).

Flood behaviour was determined for flood prone areas using mathematical modelling tools developed specifically for the study. Catchment runoff was estimated using the hydrological model, RDII, DHI (2001). Predicted catchment flows from the RDII model along with tide levels for the Pittwater were then used as boundary conditions to the hydraulic model MIKE11, DHI (2001), for the determination of flood levels, flows and velocities in the flood prone areas.

The models were calibrated to the April 1998 flood event, which was considered the most suited event for model calibration because of available rainfall and· historical flood records (Pittwater Council). The models were verified using the October 1987 and January 1989 flood events.

The flood model predictions indicate that in many areas of the floodplain the capacity of existing flow channels, major piped stormwater drainage conduits and road culverts are exceeded resulting in overland flooding for a range of simulated flood events.
In the Cahill Creek catchment, overland flood flows occur in the WaIter Road area at shallow depths for the 20% AEP and larger flood events. Overland flooding may also occur when the Peninsula Gardens Detention Basin capacity is exceeded with flood flows passing through several residential properties in Old Samuel Street, Samuel Street and Parkland Road. If the basin outlet remains unblocked, the basin capacity is predicted to be exceeded for events greater than the 5% AEP flood event. Should the basin outlet become completely blocked, flooding of properties downstream of the basin would occur for all the design events tested. While blockage of the basin outlet by debris has exacerbated flooding in past flood events, there is evidence to suggest that blockage of the outlet has not been an issue for every historical flood. Design flood levels presented for the basin in this report are for the unblocked case.

Further downstream on the Cahill Creek floodplain, the Bayview Golf Course and several properties along its periphery in Kunari Close and Parkland Road are predicted to be inundated. Parkland road and Cabbage Tree Road are predicted to be impassable to traffic in several locations under 1% AEP conditions. Pittwater Road is predicted to be inundated for events greater than 1% AEP on the southern side of the floodgates on Cahill Creek.

In the Mona Vale Drain catchment, property flooding is predicted to occur in Darley St East, Seabeach Avenue, Heath Street and Bassett Street on the eastern side of Barrenjoey Road due to flows exceeding the capacity of the existing stormwater drainage system for events including the 20% AEP event and greater. On the western side of Barrenjoey Road, the light industrial area is likely to be flooded for events including the 20% AEP flood event and greater when the capacity of the open channel behind Polo Avenue is exceeded. Anecdotal evidence suggests that the culverts at the downstream end of the open channel behind Polo Avenue block each time there is a storm event with a combination of rubbish, reeds and sediment. The design flood levels are presented in this study with the culverts on the downstream end of the Polo Avenue open channel blocked to approximately 5% of their total capacity. All other stormwater pipes and culverts are modelled at 100% capacity for the presented design flood levels.

Inundation plans showing the approximate extent of flooding for the 1% AEP, 2% AEP, 5% AEP 20% AEP and the PMF events are included as Appendix D (Volume1). Tabulated model results are also provided for reference in Appendix I (Volume 2). These results quantify the predicted existing flood behaviour in terms of flood level, flood depth, flow distribution and flow velocity.

The developed flood modelling tools and reported flood behaviour for existing catchment conditions can be used as the basis for developing a Flood Risk Management Plan for the floodplain. The hydraulic model developed for the flood study may generally be used to assess the hydraulic impact of any proposed structural flood mitigation works on flood behaviour. However the suitability of the model to assess individual options should be assessed.

Additional Information

Field Value
Title Mona Vale - Bayview Flood Study
Publication Date 1 May 2002
Themes Land and Resource Management
Spatial Extent
Map data © OpenStreetMap contributors
Council/LGA Northern Beaches Council
Author/ Prepared by DHI Water & Environment
Publish date 11 September 2017
Update date 19 September 2017
Place Name Mona Vale-Bayview
Approval State Approved
Submitted for approval 11 September 2017
Submitted by John Silk
Approved 20 September 2017
Approved by dstazic
Data Comment

Report only; no other data. Legacy data; may have been superseded.

Identifier 30724950-3835-40df-b086-2512b74ce1ff