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LEP013/16 - Planning Proposal

2A - 8 Hawkesbury Valley Way and 383 - 395 George Street

WINDSOR NSW 2756

FLOOD RISK ASSESSMENT

Flood Risk Assessment 2 – 8 Hawkesbury Valley Way and 393 – 395 George Street

LEP013/16 1 Hawkesbury City Council

EXECUTIVE SUMMARY

Council received a planning proposal from Urban City Consulting Pty Ltd (the applicant) which seeks

to amend the Hawkesbury Local Environmental Plan 2012 (the LEP) to rezone Lots 7-12 DP 1093,

383-395 George Street, Lot A DP 108510, 2A Hawkesbury Valley Way and Lots 11-13 DP 1662, 4-8

Hawkesbury Valley Way, Windsor (the subject site) from R2 Low Density Residential to B1

Neighbourhood Centre under the LEP to allow development of the subject site for

business/commercial purposes.

In December 2016 Council forwarded the planning proposal to the Department of Planning and

Environment (DP&E) for a Gateway determination.

In March 2017 Council received a Gateway determination from the DP&E advising to proceed with the

planning proposal. As per the Gateway determination requirements, Council amended the planning

proposal with the inclusion of a cap limiting the number of residential dwellings (shop-top housing)

permissible with consent on the subject site to a maximum of 10 dwellings.

In accordance with the Gateway determination, Council also prepared a flood risk assessment in

consultation with the NSW Office of Environment & Heritage (OEH) and Roads and Maritime Services

(RMS). Relevant State and Council’s plans/policies, studies and flooding information were taken into

consideration in the preparation of a flood risk assessment for the subject site.

A flood risk assessment undertaken for the subject site against the current flood planning provisions

reveals that the majority of the subject site is subject to Low Hazard flooding and a small area of

approximately 13% of the subject site near the intersection of Hawkesbury Valley Way and George

Street is subject to High Hazard flooding. The assessment also reveals that the subject site

experiences Low Hazard flooding during a 100 year flood event and therefore the risk to life at the

subject site is low during a 1 in flood event.

The assessment reveals that it would be possible to locate any future retail/commercial or mixed use

development within the Low Hazard flooding area. Therefore, any shop top housing subject to an 10

dwellings cap contained in a future mixed use development on the subject will be located on the first

floor or floor above (approximately 21M AHD which is well above 1:500 ARI flood level. Therefore,

future mixed use development comprising any shop top housing on the subject site at first floor level

or above is considered to be a better development option than development of each lot (other than

Lot A DP 108510, 2A Hawkesbury Valley Way) for a single dwelling house at ground level under the

current flood planning provisions in the LEP and Council’s Development of Flood Liable Land Policy.

An assessment undertaken against the PMF flood planning level reveals that the entire site is subject

to High Flood Risk during a PMF flood event, and therefore this Flood Risk Assessment suggests that

development of the subject site for future business/commercial and shop top housing purposes is not

appropriate if Council assess the planning proposal against the PMF flood planning level. However,

given the flood planning level is currently defined as the level of a 1:100 ARI (average recurrent

interval) in Clause 6.3 of the LEP, the current flood planning provisions contained in the LEP will be

taken into consideration when determining the planning proposal.

The subject site has readily available access to the Jim Anderson Bridge evacuation route and any future occupants of the subject site can quickly and easily evacuate with prior warning from the Sydney Bureau of Meteorology prior to inundation of Hawkesbury Valley Way and George Street. However, the NSW State Emergency Service will be consulted on this matter as per the Gateway determination.



Should the proposed amendment to the LEP be made to give effect to the planning proposal, a

detailed assessment can be undertaken in close consultation with relevant government agencies

including the SES, the RMS, NSW Office of Environment & Heritage and Infrastructure NSW -

Hawkesbury-Nepean Valley Flood Management Directorate during the development application

stage.

Background

In December 2016 Council considered a planning proposal from Urban City Consulting Pty Ltd (the

applicant) which seeks to amend the Hawkesbury Local Environmental Plan 2012 (the LEP) to rezone

Lots 7-12 DP 1093, 383-395 George Street, Lot A DP 108510, 2A Hawkesbury Valley Way and Lots

11-13 DP 1662, 4-8 Hawkesbury Valley Way, Windsor from R2 Low Density Residential to B1

Neighbourhood Centre under the LEP to allow development of the subject site for

business/commercial purposes. Council at its Ordinary Meeting of 13 December 2016 resolved to

forward the planning proposal to the Department of Planning and Environment (DP&E) for a Gateway

determination.

In December 2016 Council forwarded the planning proposal to the NSW Department of Planning and Environment (DP&E) seeking a Gateway Determination, and in March 2017 Council received a Gateway determination from the DP&E advising to proceed with the planning proposal subject to a number of conditions, with Condition 1 being:

1. “In relation to S117 Direction 4.3 Flood Prone Land, prior to public exhibition, Council is required to:

a. determine the number of dwelling houses permissible with consent under

current development controls; b. amend the planning proposal to indicate that a cap will be applied limiting the

number of residential dwellings permissible with consent on the site as determined above and specify dwelling yield under that cap;

c. undertake a Flood Risk Assessment, including evacuation plans approved by the NSW State Emergency Service, in consultation with Roads and Maritime Services and the Office of Environment Heritage, that demonstrates consistency with this Direction; and

d. during exhibition provide a copy of the Flood Risk Assessment, including the proposed dwelling cap, to Infrastructure NSW’s Hawkesbury-Nepean Valley Flood Management Directorate (Directorate) for consideration and comment”.

A copy of the Gateway Determination is contained in Attachment 1 of this Flood Risk Assessment Report. In line with Condition 1(a) of the Gateway Determination, the number of dwellings permissible with consent under current development controls has been determined as 11 (see Attachment 2 of the Flood Risk Assessment Report). In addition, the planning proposal has also been amended with the introduction of a cap limiting the number of dwelling units (shop top housing) permissible with consent on the subject site to 11 as per Condition 1(a) of the Gateway Determination (see Attachment 3 of the Flood Risk Assessment Report). In line with Condition 1(c) of the Gateway determination, this Flood Risk Assessment (FRA) for the subject site has been prepared in consultation with the NSW Roads and Maritime Services (RMS) and the Office of Environment Heritage (OEH) prior to public exhibition of the planning proposal.



Subject Site The subject site consists of 11 properties, and has a total area of approximately 0.53ha as shown in Table 1 below: Table 1: Subject Site Properties and Current Land Uses

Property Description

Street Address

Area

Current Use

Lot 13 DP 1089172 383 George Street 329m2 Open car yard




Lot 9 DP 1093 391 George Street 285m2 Open car yard and car sale office

Lot 8 DP 1093 393 George Street 285m2 Car sales office and car workshop

Lot 7 DP 1093 395 George Street 285m2 Car sales office and car workshop

Lot A DP 108510 2A Hawkesbury Valley Way 277m2 Open car yard

Lot 13 DP 16626 4 Hawkesbury Valley Way 1088m2 Dwelling



Total 5,331m2

The subject site is bounded by Hawkesbury Valley way to the north, George Street to the east and R2 Low Density Residential zoned properties to the south and west, and therefore it can be accessed via both Hawkesbury Valley Way and George Street as shown in Figure 1 below. The subject site has an approximately 75m wide frontage to Hawkesbury Valley Way, and 79m wide frontage to George Street. Approximately 22% of the subject site covering 383-389 George Street and part of 391 George Street at the northeast corner is currently being used as a car sales yard, and there are three single storey dwellings fronting Hawkesbury Valley Way as shown in Table 1 above and Figure 1 below. A car sales office and a car workshop occupy the rest of the subject site covering part of 391 George Street and 393-395 George Street.

Figure 1: Aerial View of the Subject Site



Current Residential Development Potential The subject site is currently zoned R2 Low Density Residential under the LEP. The current minimum lot size for subdivision of the subject site is 450m

2 and a 10m maximum building height provision

applies to the subject site. Dwelling houses are permitted with Council’s consent within the R2 Low Density Residential zone, and all other types of residential development (other than secondary dwellings) are not permitted within this zone. A “Gateway” determination advising to proceed with the planning proposal received from the NSW Department of Planning and Environment requires the determination of the number of dwelling houses that are permissible with consent under current development controls and an amendment to the planning proposal to identify a cap to limit the number of residential dwelling permissible with consent, on the subject site. In line with the “Gateway” determination, a dwelling cap has been determined as a maximum of 10 dwellings on the subject site (refer to Attachment 1 of the Flood Risk Assessment Report). Future Development The planning proposal seeks to rezone the subject from R2 Low Density Residential to B1 Neighbourhood Centre under the LEP to allow future retail/commercial development on the subject site. Given shop top housing is a permitted land use with consent in the proposed B1 zone, the applicant may develop the subject site as a mixed use development incorporating shop top housing above ground floor retail or business premises. Currently the maximum building height provision applying to Business zoned land within the Hawkesbury LGA is 12m, and therefore the planning proposal seeks to amend the Height of Buildings Map of the LEP to specify a 12m maximum building height provision for the subject site to ensure consistency with the Business zoned land within the Hawkesbury LGA. Therefore, any future development of the subject site is subject to a maximum 12m height or generally a four-storey development. However, given the size and location of the subject site, some site constraints such as the 25 - 30 ANEF contour range, flooding and relevant development controls applying to the subject site, any future mixed use development may not be able to yield 10 dwellings on the subject site. Flood Affection All 11 properties shown in Table 1 above that form the subject site are flood prone land as shown in Figure 2 below, and therefore Clause 6.3 Flood planning of the LEP and Council’s Development of Flood Liable Land Policy applies.

Figure 2: Flood Affection



Given the height of the subject site proposed to be rezoned varies from approximately 18.5m AHD to the southwest of the subject site to approximately 16m AHD to the northeast, part of the subject site is above the flood planning level which is defined as the level of a 1:100 ARI (average recurrent interval) in Clause 6.3 of the LEP and equivalent to 17.3m AHD at the subject site as shown in Figure 2 above. Development of Flood Liable Land Policy The Hawkesbury City Council Development of Flood Liable Land Policy (the Policy) was adopted on 31 July 2012 and makes provisions for the assessment of development on flood prone land within the Hawkesbury LGA that are affected by the 1% Annual Exceedance Probability (AEP) flood (1 in 100 year flood event). Under the Policy, new buildings can be constructed on the subject site with a ground level above 14.5m AHD, but any habitable floors of any development on the subject site are to be located above the 1 in 100 year ARI event which is defined as the flood planning level in the LEP. Given the applicant seeks to develop the subject site as a mixed use development with shop top housing, habitable floors associated with the shop top housing can easily be located above flood planning level. Given Hawkesbury Development Control Plan 2002 currently makes no provisions for mixed use development, future development of the subject site for mixed use development purposes will be the subject of a merit assessment. Draft Hawkesbury-Nepean Valley Regional Flood Study The Study uses best practice and the latest techniques in flood estimation to define flood behaviour in the Hawkesbury-Nepean Valley and provides an update to the publicly available flood information for the Hawkesbury- Nepean Valley which was previously undertaken in 1996. It provides contemporary information on flood risk important for increasing community awareness of their flood risk and building resilience and detailed analysis of flood behaviour to assist flood managers and Council in management and development of flood prone land.

The Study adopts the following revised hazard classifications identified in ‘Managing the floodplain: a guide to best practice in flood risk management in Australia’ (AIDR 2017):

• H1 - No constraints

• H2 - Unsafe for small vehicles

• H3 - Unsafe for all vehicles, children and the elderly

• H4 - Unsafe for all people and all vehicles

• H5 - Unsafe for all people and all vehicles. Buildings require special engineering design and construction.

• H6 - Unsafe for people or vehicles. All buildings types considered vulnerable to failure. The study provides spatial maps of flood levels as a result of main river flooding. The Study states that flood levels for the 1 in 100 AEP have changed a minor amount due to the change in methodology and increased knowledge of flood behaviour since the 1990s. However, the 1 in 100 AEP level at Windsor has not changed. Section 117 Direction 4.3 Flood Prone Land Given the subject site is identified as being flood prone, Section 117 Direction 4.3 Flood Prone Land applies to the subject site. The objectives of this Direction are to:

(a) ensure that development of flood prone land is consistent with the NSW Government’s Flood Prone Land Policy and the principles of the Floodplain Development Manual 2005, and



(b) Ensure that the provisions of an LEP on flood prone land are commensurate with flood hazard and includes consideration of the potential flood impacts both on and off the subject land.

Given the subject site is flood prone land, Clause 6.3 Flood planning of the LEP and Council’s Development of Flood Liable Land Policy will apply to future development of the subject site. The Hawkesbury Floodplain Risk Management Study and Plan (HFRMSP) divides flood prone land into five flood risk categories ie. Extreme, High, Medium, Low and Very Low and suggests that commercial and industrial development is suitable in all but the Extreme Flood Risk area. The Extreme Flood Risk area is defined as all land below the 1 in 20 year flood event level, which for the subject site is 13.7m AHD. The height of the subject site proposed to be rezoned varies from approximately 18.5m AHD to the southwest of the subject site to approximately 16m AHD to the northeast, and therefore rezoning of the subject site to B1 Neighbourhood Centre zone is considered consistent with this Direction. If the proposed LEP amendment to give effect to this planning proposal is made, a detailed flood impact assessment could be undertaken if required when assessing a future development application for the proposed development on the subject site. Table 1 below demonstrates how the planning proposal is consistent with Section 117 Direction 4.3 Flood Prone Land.

Clause Justification

A planning proposal must include provisions that give effect to and are consistent with the NSW Flood Prone Land Policy and the principles of the Floodplain Development Manual 2005 (including the Guideline on Development Controls on Low Flood Risk Areas).

The subject site is flood prone land, and therefore Clause 6.3 Flood planning of the LEP applies to the proposed development on the subject site. Given Clause 6.3 is consistent with the NSW Flood Prone Land Policy and the principles of the Floodplain Development Manual 2005 (including the Guideline on Development Controls on Low Flood Risk Areas), the planning proposal does not propose to include provisions that give effect to and are consistent with the NSW Flood Prone Land Policy and the principles of the Floodplain Development Manual 2005.

A planning proposal must not rezone land within the flood planning areas from Special Use, Special Purpose, Recreation, Rural or Environmental Protection Zones to a Residential, Business, Industrial, Special Use or Special Purpose Zone.

The subject site is currently zoned R2 Low Density Residential under the LEP and the planning proposal seeks to rezone the subject site to B1 Neighbourhood Centre under the LEP. As such, the planning proposal is not inconsistent with this clause.

A planning proposal must not contain provisions that apply to the flood planning areas which:

(a) permit development in floodway areas, (b) permit development that will result in

significant flood impacts to other properties,

(c) permit a significant increase in the development of that land,

(d) are likely to result in a substantially increased requirement for government spending on flood mitigation measures, infrastructure or services, or

(e) permit development to be carried out without development consent except for

The planning proposal is only a map amendment to rezone the subject site and does not contain any provisions that apply to the flood planning area. The subject site is not located within a floodway, and the majority of the subject site is subject to Low Hazard flooding based on the current flood planning level. The planning proposal does not seek to amend the Land Use Table of the LEP to permit development without development consent or insert new provisions to permit development that will result in significant flood impacts to other



the purposes of agriculture (not including dams, drainage canals, levees, buildings or structures in floodways or high hazard areas), roads or exempt development.

properties or increased development on the subject site.

A planning proposal must not impose flood related development controls above the residential flood planning level for residential development on land, unless a relevant planning authority provides adequate justification for those controls to the satisfaction of the Director-General (or an officer of the Department nominated by the Director-General).

As previously mentioned, the planning proposal does not contain any flood related development controls.

Clause Justification

For the purposes of a planning proposal, a relevant planning authority must not determine a flood planning level that is inconsistent with the Floodplain Development Manual 2005 (including the Guideline on Development Controls on Low Flood Risk Areas) unless a relevant planning authority provides adequate justification for the proposed departure from that Manual to the satisfaction of the Director-General (or an officer of the Department nominated by the Director-General).

Clause 6.3 of the LEP applying to the subject site defines the flood planning level as the level of a 1:100 ARI which is taken into consideration when determining the planning proposal. There will be no change to flood planning level as a consequence of this planning proposal.

Having considered the above, the planning proposal seeking rezoning of the subject site to B1 Neighbourhood Centre zone is considered to be consistent with this Direction. Flooding in Windsor Area The probability of various key floods occurring in Windsor, and to what extent the subject site would be affected is shown in Table 2 below: Table 2: Key Floods Occurring in Windsor

Probability Windsor Flood Level (at Windsor Bridge) m AHD

Subject Site’s Flood Affection

AEP ARI

20% 1 in 5 11.1 Not inundated




1% 1 in 100 17.3 Partially inundated

0.50% 1 in 200 18.9 Totally inundated



0.001 1 in 100,000 26.4*

Totally inundated

Source: Hawkesbury City – Local Flood Plan (December 2010) * Probable Maximum Flood (PMF) Level



Probable Maximum Flood The Probable Maximum Flood (PMF) is an extremely rare flood event having a probability of around one chance in 100,000 per year and this is the upper limit of flood planning. According to the SES, the river level in Windsor that could be reached at Windsor in a PMF is 26.4m AHD. The main impact at this flood level is likely to be 100% of Windsor being inundated. However, this is considered to be the worst flood that could conceivably occur. The PMF defines the extent of the floodplain and the area that requires planning for possible evacuation and therefore all floods up to the PMF would need to be taken into consideration, even though a PMF flood event is extremely rare. However it is noted that the flood planning level adopted for the recently released Stage 1 of the Vineyard Precinct in the North West Growth Centre within the Hawkesbury Local Government Area is the post-development 1:100 year flood event plus 0.5m freeboard. To avoid risks to people and properties, any future residential development within the Vineyard Precinct will not be permissible below the 1 in 100 flood planning level. Currently, Clause 6.3 Flood Planning of the LEP also defines the flood planning level (FPL) as the level of a 1:100 ARI flood event. This flood risk assessment has undertaken assessments of flood risks from floods up to PMF flood events. This flood risk assessment is described below. Flood Risk Assessment As previously mentioned, the subject site is identified as being flood prone. Therefore, a flood risk assessment for the subject site has been undertaken for the subject site to determine how the subject site would be impacted by a 100 flood event and an extremely rear flood event like a PMF flood event. The flood risk assessment and assessment findings are outlined below. Flood Risk from floods up to the 100 Flood Event In order to assess the flood risk to the subject site, relevant parameters such as average ground level, flood depth and velocity (100 Year) were determined for 11 properties that form the subject site using Council’s WaterRide information. The peak flood levels, depths and velocities of these 11 properties during a 1 in 100 flood event and average ground levels of the properties are shown in Table 3 below. Table 3: Average Ground Level, Flood Depth and Flood Velocity and 100 Year Flood Level

Lot Description 100 Yr Flood Level

(m AHD)

Average Ground Level

(m AHD)

100 Yr Average Flood

Depth (m)

100 Yr Average Flood Velocity

(m/s)

Lot 13 DP 1089172 17.3 16.0 1.3 0.07

Lot 12 DP 1093 17.3 16.1 1.2 0.06

Lot 11 DP 1093 17.3 16.4 0.9 0.06

Lot 10 DP 1093 17.3 16.6 0.7 0.06

Lot 9 DP 1093 17.3 16.8 0.5 0.06

Lot 8 DP 1093 17.3 17.1 0.2 0.03

Lot 7 DP 1093 17.3 17.3 0.0 0.02

Lot A DP 108510 17.3 17.0 0.3 0.04

Lot 13 DP 16626 17.3 17.2 0.1 0.04

Lot 12 DP 16626 17.3 17.5 -0.2 0.04

Lot 11 DP 16626 17.3 17.7 -0.4 0.04

The peak flood velocities, depths, levels, and hazards of these 11 properties during a 1 in 100 flood event are also shown in Figures 3, 4, 5 and 6 respectively. As shown in Figure 3 below and Table 3 above, flood velocity across the site is generally low ranging from 0.02m/s to 0.07m/s, and the average velocity across the subject site at the 100 flood event is 0.05m/s. The flood velocity is comparatively higher near the intersection of Hawkesbury Valley Way and George Street.



According to Figures 4 and 5 below, approximately 28% of the subject site area is above the 1 in 100 flood planning level. During a 1 in 100 year flood event, the depth of inundation across the subject site would range from approximately 0.1m and 1.3m and average inundation depth across the subject site is 0.42m. According Table 3 above and Figure 4 below, 1.3m maximum inundation depth occurs near the intersection of Hawkesbury Valley Way and George Street. Generally, flood hazard is used to assess the potential risk to personal safety and physical damage to properties. The extent of a flood hazard is generally based on the combination of depths and velocities. As previously mentioned the subject site experiences shallow inundation depths and low velocities. Figure 6 below presents the 1% AEP flood hazard at the subject site which indicates that the majority of the site is subject to Low Hazard flooding and a small area of approximately 13% of the subject site near the intersection of Hawkesbury Valley Way and George Street is subject to High Hazard flooding. The low flood velocities and shallow flood depths associated with Low Hazard flooding as shown in Figures 3 and 4 and Table 3 above indicates that the risk to life at the subject site is low during a 1 in 100 chance of flood event. Therefore, this assessment reveals that the subject site experiences Low Hazard flooding during the 100 year flood event.

Figure 3: Flood Velocity



Figure 4: Flood Depth

Figure 5: Flood Levels



Figure 6: Flood Hazard Flood Risk from floods up to the PMF Flood Event Hawkesbury-Nepean Valley Regional Flood Study states that estimating Probable Maximum Flood (PMF) levels is a complex and uncertain task as there is limited physical evidence or data on such extreme floods. As previously mentioned, the river level in Windsor that could be reached at Windsor in a PMF is 26.4m AHD. The main impact at this flood level is likely to be 100% of Windsor being inundated. However, this is considered to be the worst flood that could conceivably occur. The PMF defines the extent of the floodplain and the area that requires planning for possible evacuation and therefore all floods up to the PMF would need to be taken into consideration, even though a PMF flood event is extremely rare. In order to assess the flood risk to the subject site from a PMF flood event, relevant parameters such as average ground level, flood depth and velocity (at PMF level) were determined for 11 properties that form the subject site using Council’s WaterRide information. The peak flood levels, depths and velocities of these 11 properties during a PMF flood event and average ground levels of the properties are shown in Table 4 below. As shown in Table 4 and Figure 7 below, flood velocity across the site is high and ranging from 0.27m/s to 0.31. The average velocity across the subject site is 0.29m/s which is approximately six times faster than the average velocity across the subject site during a 100 flood event. Figure 8 shows that the subject site is completely inundated with water during a PMF flood event, and the average depth of inundation across the subject site is 9.47m which is approximately 22 times the average inundation depth at the subject site during a 100 flood event.



According Table 4 below and Figure 8 below, 10.4m maximum inundation depth occurs near the intersection of Hawkesbury Valley Way and George Street, and Figure 10 shows that the entire site is subject to High Flood Risk during a PMF flood event. Table 4: Average Ground Level, Flood Depth and Flood Velocity and PMF Flood Level

Lot Description PMF Flood Level

(m AHD)

Average Ground Level

(m AHD)

PMF Average Flood

Depth (m)

PMF Average Flood Velocity

(m/s)

Lot 13 DP 1089172 26.4 16.0 10.4 0.28

Lot 12 DP 1093 26.4 16.1 10.3 0.28

Lot 11 DP 1093 26.4 16.4 10.0 0.28

Lot 10 DP 1093 26.4 16.6 9.8 0.28

Lot 9 DP 1093 26.4 16.8 9.5 0.27

Lot 8 DP 1093 26.4 17.1 9.3 0.28

Lot 7 DP 1093 26.4 17.3 9.2 0.28

Lot A DP 108510 26.4 17.0 9.3 0.28

Lot 13 DP 16626 26.4 17.2 9.1 0.30

Lot 12 DP 16626 26.4 17.5 8.8 0.31

Lot 11 DP 16626 26.4 17.7 8.5 0.31

Figure 7: Flood Velocity (at PMF Level)



Figure 8: Flood Depth (at PMF Level)

Figure 9: Flood Level (at PMF Level)



Figure 10: Flood Hazard (at PMF Level)

Therefore, this Flood Risk Assessment suggests that development of the subject site for future business/commercial and shop top housing purposes is not appropriate if Council takes into consideration the PMF flood planning level in the determination of the planning proposal. However, given the flood planning level is currently defined as the level of a 1:100 ARI (average recurrent interval) in Clause 6.3 of the LEP, the current flood planning provisions contained in the LEP will be taken into consideration when determining the planning proposal. If the proposed amendment to the LEP is made to give effect to the planning proposal, a detailed assessment can be undertaken in close consultation with relevant government agencies including the SES, the RMS, NSW Office of Environment & Heritage and Infrastructure NSW - Hawkesbury-Nepean Valley Flood Management Directorate during the development application stage. Development of the subject site Under the current flood development provisions in the LEP and the Council’s Development of Flood Liable Land Policy, the subject site containing 11 lots zoned R2 Low Density Residential can be developed for 10 dwelling houses and one secondary dwelling with Council consent. The planning proposal is subject to a cap of 11 dwellings, and therefore it does not increase the dwelling yield on the subject site. Hence, it does not contribute to regional evacuation traffic or impact the NSW State Emergency Service (SES)’s evacuation capacity for Windsor. The subject site varies from approximately 18.5m AHD to the southwest of the subject site to approximately 16m AHD to the northeast. Given the existing ground elevations and the size of the subject site, any shop top housing on the first floor level of a future mixed use development with a finished ground floor level +0.3m on the subject site would be above the 1: 200 ARI flood level (18.9m AHD). Therefore, future business/commercial development containing shop top housing above 1:200 ARI on the subject site is considered an improved land use outcome compared with development of the subject site for 11 dwellings and one secondary dwelling at or near ground level just above 1 in 100 ARI flood planning level.



The OEH also acknowledged in its response to this Flood Risk Assessment, the planning proposal seeking rezoning of the subject site to allow development of the subject site for a mixed use development containing shop top housing preferably located at or above the 1887 highest flood of record (observed 19.7m AHD and adjusted 19.3m AHD) in line with the Hawkesbury Flood Risk Management Study and Plan 2012 is an improvement to any single storey residential development at or near ground level currently permissible with Council consent. Shop Top Housing The planning proposal seeks to rezone the subject from R2 Low Density Residential to B1 Neighbourhood Centre under the LEP to allow future retail/commercial development on the subject site. Given shop top housing is a permitted land use with consent in the proposed B1 zone, the applicant may develop the subject site as a mixed use development incorporating shop top housing above ground floor retail or business premises. As shown in Figure 6 above, it would be possible to locate any future retail/commercial or mixed use development within the Low Hazard flooding area with appropriate setbacks to both Hawkesbury Valley Way and George Street. Climate Change Climate change could affect flood behavior in the Hawkesbury Local Government Area through:

(i) increased sea levels; and/or (ii) increased severity of flood producing storms or other weather systems.

However, the impact of climate change on rainfall is a topic of greater uncertainty. An assessment of the potential impacts of climate change on flood behavior in the Hawkesbury-Nepean behavior has been conducted by WMSwater (2009) following the protocol described in DECC’s 2007 Guideline. Sea level rises of 0.18m, 0.55m and 0.91m, increases in peak rainfall and storm volume of 5%, 10% and 20%, and a combined high level ocean rise and high level rainfall increases have been assessed for the modelled 20, 100 and 200 year ARI events. Sea level risers produced no significant increases in peak flood levels for the majority of the floodplain. Small risers have been recorded downstream of Lower Portland (0.19m at Wisemans Ferry for the 20 year ARI high sea level rise scenario. However, peak flood levels are highly sensitive to increases in rainfall. As shown in Table x below, The 100 year ARI event, peak flood levels at Windsor Bridge would rise by 0.47m (for a 5% rainfall increase), 0.9m (for a 10% rainfall increase) and 1.71m (for a 20% rainfall increase) at Windsor Bridge. Table x: Effect of Increased Rainfall Intensities on Peak Flood Levels at Windsor Bridge

In line with the above Table x, to maintain a 100 year ARI protection, the current minimum floor level of 17.3m AHD (without freeboard) would need to increase in the future to: 17.7m AHD with a 5% rainfall increase 18.2m AHD with a 10% rainfall increase 19.0m AHD with a 20% rainfall increase In line with the recommendation in Volume 2 of the Hawkesbury Flood Risk Management Plan and Study December 2012, it is considered appropriate for such circumstances outlined in Table x above to provide a second storey to a new residential developments that have their lower storey at the existing 100 year ARI flood level, provides a good buffer against possible climate change risks.

Rainfall Increase (%)

+5%

Rainfall

+10%

Rainfall

+20%

Rainfall

+5%

Rainfall

+10%

Rainfall

+20%

Rainfall

+5%

Rainfall

+10%

Rainfall

+20%

Rainfall

Peak Flood Level Rise (m) 0.50m 0.98m 1.90m 0.47m 0.9m 1.71m 0.46m 0.91m 1.73m

Item20Y ARI 100Y ARI 200Y ARI

ARI Event



The Hawkesbury-Nepean Valley Regional Flood Study states that:

“A 1 in 100 AEP event at Windsor under existing climate becomes a 1 in 80 AEP event with a 4.9 per cent rainfall increase and a 1 in 65 AEP event with a 9.1 per cent rainfall increase”.

Figure 11 shows the change in flood level from the existing 1 n 100 AEP flood level for the various climate change scenarios for Windsor

Figure 11: Climate change 1 in 100 AEP at Windsor Source: Draft Hawkesbury-Nepean Valley Regional Flood Study

The Hawkesbury-Nepean Valley Regional Flood Study also states that a 0.9m sea level rise increases the 1 in 100 AEP flood level at Windsor by 0.01m. Flood Resistant Materials and Construction Flooding is one of the natural hazards which can cause significant financial, environmental and social costs. A major flood event can place persons at risk, destroy buildings and cars and pollute the natural environment. Therefore, in order to minimise any property damages and risks to personal safety, appropriate flood mitigation or management measures including flood compatible building materials/components up to the flood planning level to stand the forces of floodwater, debris and buoyancy up to the 100-year ARI flood at the subject site. All future structures/buildings on the subject site should be constructed of flood compatible materials and construction methods in accordance with the Reducing Vulnerability of Buildings to Flood Damage guidelines (2006) prepared by the Hawkesbury-Nepean Floodplain Management Steering Committee. According to the guidelines, materials that are not adversely affected by water (is dimensionally stable and does not deteriorate or lose structural integrity when flooded) and do not absorb water readily − are considered appropriate for use in new construction on the subject land. In addition, a Site Flood Response Plan (SFRP) incorporating procedures and protocols for the evacuation of the occupants on the subject site in a major flood event would need to be implemented.



Road Access As previously discussed, the access to the subject site is from both Hawkesbury Valley Way and George Street. Given Hawkesbury Valley Way forms the northern boundary of the subject site has an average elevation of 16.4m AHD and George Street forms the eastern boundary has an average elevation of 16.2m AHD, the access to the subject site from both Hawkesbury Valley Way and George Street is flood free up to and including the 50 year flood. However, during a 1 in 100 chance of flood event, part of Hawkesbury Valley Way and part of George Street forming northern and eastern boundaries respectively will be under water and this situation will deny access from the subject site to both roads. Therefore, flood evacuation is considered to be a major determining factor in the determination of any future development application for development of the subject site for a mixed use development containing shop top housing, and the subsequent section outlines this issue. Flooding in Windsor The draft Hawkesbury-Nepean Valley Regional Flood Study states that:

“The floodplain at Windsor is the most severely affected by flooding on the Hawksbury-Nepean River. In a 1 in 10 AEP event, flood levels at Windsor Bridge are 11.93m AHD. By the 1 in 10 AEP event, a number of properties are isolated on low flood islands after access roads are cut.

In the 1 in 100 AEP event, the flood level at Windsor Bridge is 17.3m AHD and the flood extent increases substantially from the 1 in 10 AEP event. In this event, Pitt Town is completely isolated as a flood island. The suburb of McGraths Hill is completely submerged in a 1 in 100 AEP event, and while some areas of Windsor and South Windsor are above the 1 in 100 AEP extent, they are isolated as a flood island. Windsor Road is inundated as far as Vineyard Railway Station (seven kilometres from Windsor Bridge). Macquarie Street is overtopped near Windsor Railway Station and again at the low point near Bligh Park”.

In the PMP flood event, flood levels reach 26.6m AHD at Windsor Bridge. There are no high flood islands in the Windsor floodplain in the PMP flood event. In general, on the Windsor floodplain, the depths in the 1 in 100 AEP event exceed two metres.

Flood Evacuation Evacuation planning needs to include all people living in the floodplain. Evacuations will normally take place when there is a risk to public safety. Circumstances may include:

• Evacuation of people when their homes or businesses are likely to flood.

• Evacuation of people who are unsuited to living in isolated circumstances, due to flood water closing access.

• Evacuation of people where essential energy and utility services have failed or where buildings have been made uninhabitable.

The Sydney Bureau of Meteorology currently provides a flood forecasting and warning system for the Hawkesbury Region. This information is used by the NSW State Emergency Service (SES). The Hawkesbury Local Government Area is in the SES Sydney Western Region and for emergency management purposes is part of the Sydney West Emergency Management District. When flooding is likely to cut evacuation routes or inundate property, the SES may issue a prior Evacuation Warning to prepare for possible future evacuation. The SES advises that the most effective means of evacuation is via road using private cars and buses, but the other means of evacuation such as rail, helicopters and buses may also be used as backups. For the purpose of evacuation planning, the SES classifies a flood in the Hawkesbury-Nepean Valley as either a Level 1 flood or a Level 2 flood according to the degree of severity.



A Level 1 flood is defined as one in which the water level of the Hawkesbury- Nepean River is not expected to exceed 15.0m on the Windsor gauge. For such a flood, the operation is within the scope of normal arrangements detailed in the respective SES Region and Local Flood Plans and the respective District and Local disaster plans (DISPLAN’s). According to the SES, for floods that are predicted to peak no higher than around 15.0m at Windsor (ie, a Level 1 flood) there will be no need to completely evacuate the major population centres in the valley. A Level 2 flood is defined as one in which the water level of the Hawkesbury-Nepean River is expected to exceed 15.0m on the Windsor gauge. For this purpose, additional arrangements are contained in the Hawkesbury Nepean Flood Emergency Sub Plan. The SES suggests that the operations dealing with the flood affected people, are essentially dependent on the level of flooding at Windsor being likely to exceed a level of between 14m to 16m at the Windsor Bridge gauge. It also advises that during Level 2 flood operations a significant percentage of the population (up to 25 percent) may have to be evacuated even for floods less than 15m AHD. If flooding is expected to exceed 17.3m AHD, the entire population will need to be evacuated to assigned evacuation centres. On leaving the Sector these persons will need to be moved under traffic management arrangements (detailed in the Hawkesbury Nepean Flood Emergency Sub Plan) to the evacuation centres. During Level 2 flood operations the road evacuation routes for Richmond, Windsor, Bligh Park, Windsor Downs, McGrath's Hill and Pitt Town Sectors will be managed under arrangements detailed in the Hawkesbury-Nepean Flood Emergency Sub Plan. The SES also can use aircrafts for flood evacuation purposes and the SES designated helicopter landing position for the Windsor Sector is located within McQuade Park on the St Matthews Church side (22m to 24m AHD) terrain level. The SES has advised that they will conduct their operations to ensure that evacuations commence in sufficient time to allow all flood affected communities to leave before egress routes become cut by floodwaters. Flood Evacuation in Windsor Area As part of the Windsor Road Upgrade program, the NSW Roads and Maritime Services (RMS ) has built a 2.6km, two-lane road between Macquarie Street at Windsor, and Groves Avenue/Windsor Road at Mulgrave, with a high-level bridge across South Creek. This is now known as the Hawkesbury Valley Way and Jim Anderson Bridge. The HFRMSP acknowledges that evacuation from Windsor has been greatly improved by the construction of the Jim Anderson Bridge (South Creek Crossing) in 2007. Currently, the Jim Anderson Bridge provides only a single outbound lane for evacuation of traffic. However, according to the HFRMSP, evacuation capacity within the Windsor Section can be significantly improved by reconfiguring the Bridge to allow two outbound lanes for evacuation which was the original design intent of the Bridge. The RMS has confirmed that the Bridge was designed to accommodate two lanes outbound and one emergency lane in bound during evacuation events in case of emergency. The HFRMSP states that for future development in Windsor, assuming dual outbound lanes on the Jim Anderson Bridge could be utilised during flood evacuations subject to the approval of the SES, then the evacuation risk could be immediately lowered from Class D ‘Intolerable/unacceptable’ to Class C ‘Serious’, and once community flood refuges were provided, the evacuation risk could be further lowered to Class B ‘Moderate’.



In Windsor, there is a reasonably sized area above 25.0m AHD near the historic site of St Matthew’s Anglican Church, and at one spot the ground level is higher than the PMF Level of Windsor (26.4m AHD). This would be beneficial and practical to encourage community refuges with floor space above the PMF on one or more of these islands. The SES advises to commence evacuation early enough to ensure sufficient time is allowed for evacuation, and in accordance with a timeline analysis, an evacuation of Windsor must commence at least 15 hours prior to floodwaters cutting access to the Jim Anderson Bridge. For flood evacuation planning operation purposes, the Windsor area has been divided into the following four subsectors:

• Windsor North

• Windsor Central

• Windsor

• South Windsor The subject site is located within the Windsor subsector which becomes a flood island at 17.3m AHD when the Jim Anderson Bridge over South Creek is inundated. However, given the subject site has a frontage to Hawkesbury Valley Way which leads to the Jim Anderson Bridge evacuation route, future occupants of the subject site can quickly and easily be evacuated prior to inundation of both Hawkesbury Valley Way and George Street. According to the SES, a small number of evacuations are required during the relatively frequent floods of Minor and Moderate classifications within the Windsor subsector. The levels of flooding referred to Minor and Moderate classifications are 5.8m AHD and 12.2m AHD at Windsor Bridge respectively. Windsor has a significant range of facilities including a hospital, supermarkets and hotels. Whilst this area can be isolated during a flood, and some of these facilities may be shutdown, a significant number of support facilities will likely still be available during many floods. Conclusion The majority of the subject site is subject to Low Hazard flooding and it would be possible to locate any future retail/commercial or mixed use development within the Low Hazard flooding area. Any shop top housing subject to a 10 dwellings cap contained in a future mixed use development on the subject site can easily be located well above 1 in 100 ARI flood planning level and even above 1 in 500 ARI. Therefore, future development of the subject for a mixed use development incorporating shop top housing is considered to be better than development of each lot (other than Lot A DP 108510, 2A Hawkesbury Valley Way) for a single dwelling just above 1 in 100 flood planning level. Should the proposed amendment to the LEP be made, a detailed flood risk assessment can be undertaken at the development application stage. The subject site has readily available access to the Jim Anderson Bridge evacuation route and any future occupants of the subject site can quickly and easily evacuate with prior warning from the Sydney Bureau of Meteorology prior to inundation of Hawkesbury Valley Way and George Street. However, the NSW State Emergency Service’s advice on this matter will be sought as per the Gateway. ATTACHMENTS AT – 1 Gateway Determination AT – 2 Dwelling Cap AT – 3 Revised Planning Proposal

flood risk assessment - hawkesbury city council · the flood risk assessment report). in line with...

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