Prefeasiblity – Northern Future Access Road & Flood Management Huntley Golf Course / Residential Development – Considerations of a Flood Management Strategy for Future Access Road to Site 1

29 July 2014

Our Ref: S1021-29B

Contents 1

Introduction .............................................................................................................................................................. 3

2

Available Flooding Information ........................................................................................................................ 3

3

Existing Flood Behaviour..................................................................................................................................... 3

4

Potential Flood Management Measures ........................................................................................................ 4

5

4.1.1

Incorporate a culvert / bridge design into future access road ......................................... 4

4.1.2

Provide Additional Floodplain Storage ...................................................................................... 5

4.1.3

Road & Bridge Design Levels .......................................................................................................... 5

4.1.4

Flood Impacts ........................................................................................................................................ 5

4.1.5

Stay in Place Policy.............................................................................................................................. 5

Conclusion.................................................................................................................................................................. 6

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1 INTRODUCTION The following pre-feasibility report has been prepared for Urbis to address flooding at the future access road to be located at the northeast corner of the proposed Huntley development. This future access road is proposed to connect the residential area, ‘Site 1’, to Cleveland Road as part of the proposed Huntley development (refer Appendix A). Due to the existing flood affectation of the subject site, the future access road will incorporate measures to allow vehicle access to Site 1 during all flood events up to and including the 100-year ARI flood event. This report identifies opportunities and flood management measures that may complement the future access route design and provide an effective flood management strategy for the northern extent of the subject site. Additional opportunities / design measures not identified in this report may also become apparent during subsequent planning / design phases and should be included in any subsequent assessments.

2 AVAILABLE FLOODING INFORMATION The subject site is located in the Mullet and Brooks Creek Catchment. A Floodplain Risk Management Study and Plan of Mullet Creek and Brooks Creek has been developed for Wollongong City Council by Bewsher Consulting (FRMS&P, 2010). A key component of this study was to define existing flood behaviour with the establishment of flood models. Council subsequently identified the need to extend these flood models to include areas adjacent to the foothills of the escarpment and engaged Bewsher Consulting to complete the ‘Mullet Creek, West Dapto Extension of Flood Model (2011)’. The subject site and location of the future access road to Site 1 is included within the extents of the extended flood model (2011). Flooding behaviour in the vicinity of the future access road has been reviewed using flood maps provided in the Mullet Creek, West Dapto Extension of Flood Model (2011). Due to the scale of these flood maps (catchment-wide) at relatively low resolution, it is difficult to accurately define flood behaviour at this location. Nevertheless, these flood maps provide an indicative understanding of the flood behaviour at the site and allow for a general understanding of potential flood management measures that should be considered in the design of the future access road.

3 EXISTING FLOOD BEHAVIOUR Flood extents for both the 100-year ARI and PMF events are observed to inundate a portion of the area proposed for future access road location and surroundings. The approximate 100 year ARI flood extents provided in the ‘Extended Flood Model (2011)’, have been superimposed onto a plan that includes the proposed Site 1 and the future access road (refer Appendix A). Flood contours during the 100-year ARI flood event range from approximately +40m AHD to +41mAHD in the vicinity of the future access road location (refer to the Mullet Creek, West Dapto Extension of Flood Model, 2011). Site specific detailed 2D flood modelling of the site will need to be undertaken to more accurately define these flood levels and extents at the future access road location.

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Results of site specific modelling undertaken by BRS as part of Trunk Drainage Proposal (2014), demonstrate the 100-year ARI flood event to inundate a portion of the site area proposed for future access road. However, it is understood that this additional flood modelling is yet to be approved by Council. Flooding in the vicinity of the access road during the 100 year ARI (and PMF) event, may be due to a combination of flooding from a minor convergent tributary and overland flooding of the main watercourse which traverses the site. Floodwaters during the 100 year ARI and PMF events inundate a portion of Cleveland Road. It appears that some attenuation of 100 year ARI floodwaters occurs immediately upstream of Cleveland Road, before weir flows overtop a substantial portion of Cleveland road adjacent to the bridge. A backwater effect resultant from the under-capacity bridge on Cleveland Road may influence flood levels at the location of the future access road. Site specific detailed 2D flood modelling would be required to confirm this to be the case.

4 POTENTIAL FLOOD MANAGEMENT MEASURES Based on results of existing flood models relevant to the subject site, the future access road will need to incorporate design measures to provide a trafficable access road to Site 1 during a 100-year ARI flood event. Ensuring the future access road is trafficable in the 100 year ARI flood event is consistent with the West Dapto Section 94 Development Contributions Plan (2011), which identifies Cleveland Road (adjoining the future access road) to be trafficable in the 100 year ARI flood event. Consideration of the following design measures will be reviewed during future planning / design phases as part of an overall flood management strategy at the northern extent of the site: -

Incorporate a bridge / culvert structure exceeding 6m diagonal opening into the future access road Design new bridge decks to be elevated above the 100 year ARI level with freeboard Incorporate additional floodplain storage using excavated areas / potential watercourse modifications Incorporate bunding to contain flood extents and flooding impacts Possible road raising and upgrading of the Bridge capacity at Cleveland Road Ensure no adverse flooding impacts off-site. Stay in place policy for flood events exceeding the 100 year ARI flood event

Each of the above-listed measures may influence the design parameters of a future access road to Site 1 and is discussed in further detail under the headings below.

4.1.1 INCORPORATE A CULVERT / BRIDGE DESIGN INTO FUTURE ACCESS ROAD Due to an existing minor tributary at the location of the future access road to Site 1, a culvert / bridge design will be required. Hydrologic / hydraulic modelling will be required to determine the capacity required to convey flows & subsequently provide a trafficable route during the 100 year ARI flood event. It is noted in accordance with Council’s DCP 2009 ChE14, a bridge / culvert span exceeding 6m diagonal opening would be required for the future access road to prevent a 100% blockage factor being applied (with a 25% blockage factor applied for diagonal openings exceeding 6m). GRINSELL & JOHNS

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The impact of the proposed culvert / bridge on existing flood levels will need to be considered in determining design levels for the access road & Cleveland Road / bridge. The FRMS&P states, ‘all new bridge decks will be located above the 1% AEP flood level’, with an additional freeboard likely to be required. Furthermore, design levels of the access road will need to account for the potential road raising of Cleveland road which is highlighted as a 100 year ARI flood access route in Council’s West Dapto Section 94 Development Contributions Plan.

4.1.2 PROVIDE ADDITIONAL FLOODPLAIN STORAGE The FRMS&P identifies potential excavation areas immediately upstream and downstream of Cleveland Road bridge (refer Figure 7.1 in FRMS&P, 2010). Providing additional floodplain storage upstream and downstream of the Cleveland Road Bridge (and the future access road) may reduce existing 100 year ARI flood levels which currently inundate Cleveland Road. The extent to which existing flood levels are reduced (if at all) from excavation / watercourse modifications, will require detailed 2D modelling. Establishing resultant flood levels from earthwork modifications may influence design levels for the future access Road and Cleveland Road / Bridge. Bunding may be investigated as an alternative to elevating road levels along Cleveland Road (or as an interim solution until Cleveland Road / Bridge is upgraded). This alternative may also provide some additional floodplain storage while containing flood extents within the proposed Huntley development site. Using bunding as an alternative to road raising or as an interim measure should be discussed with Council to determine its feasibility.

4.1.3 ROAD & BRIDGE DESIGN LEVELS Wollongong Council’s West Dapto Release Area Section 94 Contributions Plan (2011) identifies Cleveland Road as a flood access route that will be designed to be trafficable in a 100 year ARI flood event (refer to ‘Flood Access Routes’ map, pg 75). This may require upgrading the existing Cleveland Road bridge to increase flood conveyance capacity in a 100 year ARI flood event. The FRMS&P states, ‘all new bridge decks will be located above the 1% AEP flood level’, with an additional freeboard likely to be required. As previously discussed, design levels of Cleveland Road (& bridge) should be determined in conjunction with other potential flood management measures to ensure cumulative impacts to flood levels are established.

4.1.4 FLOOD IMPACTS In accordance with Council’s DCP ChE13, post-development flood levels for neighbouring properties are required to be maintained the same as or lower than existing flood levels. Some local increases in flood levels may be permitted within the extent of riparian corridor providing Council consider these increases acceptable. Determining a feasible access road design would use detailed flood modelling to ensure no adverse impacts to adjacent properties.

4.1.5 STAY IN PLACE POLICY Where flood free access to Site 1 is restricted during flood events exceeding the 100 year ARI flood event, it may be considered appropriate that the occupants are provided the safe alternative of staying in place (Site 1is elevated above the PMF). It is noted that because flood durations in the GRINSELL & JOHNS

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Illawarra are short, people could ‘stay in place’ during a large flood event. The Floodplain Development Manual (2005) advises that complete isolation may be acceptable during a flood, particularly where floods typically recede within hours.

5 CONCLUSION A flood access route to site 1 would be developed in conjunction with a flood management strategy for the northern extent of the proposed Huntley development. This flood management strategy would take into account a range of measures to ensure the future access road to Site 1 remains trafficable in a 100-year ARI flood event. Opportunities and flood management measures that may complement/ influence the future access road design and provide an effective flood management strategy for the subject site, include: -

Incorporate a bridge / culvert structure exceeding 6m diagonal opening into the future access road Design new bridge decks to be elevated above the 100 year ARI level with freeboard Incorporate additional floodplain storage using excavated areas / potential watercourse modifications Incorporate bunding to contain flood extents and flooding impacts Possible road raising and upgrading of the Bridge capacity at Cleveland Road Ensure no adverse flooding impacts off-site. Stay in place policy for flood events exceeding the 100 year ARI flood event

Each of the above-listed measures may influence the design parameters of a future access road to Site 1 and will be investigated in further detail during future planning / design phases. Where necessary, proposed flood management measures will be assessed using site specific detailed 2D flood modelling to quantify design parameters and flooding impacts. A cumulative impacts assessment is recommended to ensure no adverse flooding impacts resultant from the proposed development areas. Any proposed works within the watercourse will require consent from the NSW Office of Water and should be discussed at an early stage during the planning process.

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Consulting Surveyors

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LOCAL GOVERNMENT AREA :WOLLONGONG

APPROXIMATE 100 YEAR ARI FLOOD EXTENTS BASED ON MULLET CREEK, WEST DAPTO EXTENSION OF FLOOD MODEL (2011) AVONDALE ROAD, HUNTLEY

PLAN No.

S1021/29 AMEND: A

DRAWN: JP

CHECKED: AB

CAD.REF: S1021-29(B)

DATUM: AHD

SCALE:

APPROVED: RG

SURVEY DATE: 8 OCT 2013

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