Patterson Britton & Partners Pty Ltd - Hastings Council - NSW ...

Sydney Office 

level 4 

104 Mount Street 

North Sydney 2060 

Newcastle Office 

14 Telford Street 

Newcastle East 2300 

PO Box 515 


Australia 

PO Box 668 

Newcastle 2300 

Australia 

telephone (02) 9957 1619 

facsimile (02) 9957 1291 

email: reception@patbrit.com.au 

A.C.N. 003 220 228 

A.B.N. 89 003 220 228 

telephone (02) 4928 7777 

facsimile (02) 4926 2111 

email: mail@newcastle.patbrit.com.au 

Patterson Britton 

& Partners Pty Ltd 

. 

 

Mr Michael Mowle 

lr6715djt_crt070306-LLG Encroachment Asessment.doc 

Hopkins Consultants 

PO Box 1556 

PORT MACQUARIE NSW 2444 9 th March 2007 

Dear Michael, 

AREA 13: THRUMSTER – LEWIS LAND GROUP HOLDINGS, PORT MACQUARIE 

FLOODPLAIN ENCROACHMENT ASSESSMENT 

I refer to Lewis Land Group’s (LLG) 350 hectare parcel of land which forms part of the Area 13 

Urban Expansion Area at Thrumster near Port Macquarie. I also refer to the ‘Thrumster 

Integrated Water Management Plan – Flood Study’ (in draft, February 2007), which was prepared 

by Maunsell Australia and defines the extent of flood liable land across Area 13 during a range of 

design floods. 

The results presented in the Flood Study indicate that some sections of land where development 

is proposed as part of the Area 13 Urban Expansion are predicted to be inundated during the 

design 100 year recurrence flood. This includes sections of land owned by LLG. 

The potential flood liability of these sections of land does not necessarily preclude them from 

being developed in the future. The NSW Government’s ‘Floodplain Development Manual’ (2005) 

indicates that development in areas of the floodplain delineated as flood fringe …’would not have 

any significant effect on the pattern of flood flows and/or levels.’ Therefore, it is possible that 

development could be undertaken across some flood liable sections of the floodplain without 

adversely impacting on existing flood behaviour. 

Accordingly, Hopkins Consultants, acting on behalf of LLG, engaged Patterson Britton & Partners 

to use the results from the Maunsell Report and supplementary investigations, to determine 

which sections of the LLG land have the potential to be developed or filled in the future. This 

involved using the HEC-RAS model that was developed for the Maunsell Report to undertake a 

floodplain encroachment assessment for those parcels of land that adjoin Partridge and 

Karikaree Creeks. The assessment was targeted toward identifying those sections of the 

floodplain where development should be excluded to allow for the conveyance of flood flows. 

The results of the floodplain encroachment assessment are presented in the following report. 

 

 

Principals 

Senior Associates 

Associates 

The ‘Floodplain Development Manual’ (2005) characterises flood prone land according to 

three hydraulic categories, namely floodway, flood storage and flood fringe. Definitions for 

each hydraulic category, which have been taken directly from the Manual are provided in 

Table 1. 

Greg Britton BE MEngSc FIEAust Andrew Chitty BE MIEAust CPEng Peter Coltman BE MEngSc MIEAust 

Bruce Druery BE Dip Sc(Geol) M AppSc MIEAust Paul Harvey-Walker BE FIEAust David McConnell BSc MIEAust 

Joe Marson BE MEngSc FIEAust Andrew Patterson BE FIEAust Christopher Thomas BE MEngSc MIEAust 

Mark Tooker BSc(Eng) MEngSc FIEAust CPEng Michael Wright BE MEngSc MIEAust 

Steve Barrett Simon Batt BE MIEAust Paul Macinante BE MEnvEngSc MIEAust Ben Patterson BE MIEAust 

Marc Roberts BE Michael Shaw BE MIEAust CPEng 

Stephen Aebi BE MIEAust Neville Boyes OMIEAust Scot Cranfield Cameron Druery BE MIEAust 

Adam Knight BE MIEAust CPEng Cameron Smith BE MEngSc MIEAust CPEng 

Alexandra Stone BE MIEAust Chris Yates BE MIEAust



. 

Table 1 

DEFINITIONS FOR HYDRAULIC CATEGORIES 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

" # 

! 

 

Based on the definitions provided in Table 1, it can be concluded that filling in floodway 

areas will typically generate unacceptable impacts on flood behaviour. These impacts 

include significant increases in upstream flood level, increases in peak flow velocity and 

major changes in flood behaviour. Therefore, development in floodway areas should not be 

undertaken. 

Filling in flood storage areas can also have adverse hydraulic impacts on flood behaviour. 

Hence, filling in flood storage areas is typically discouraged. However, some filling can occur 

provided it does not cause a significant increase in peak flood levels. 

Table 1 also indicates that development in flood fringe areas does not typically generate any 

significant impacts on existing flood behaviour. 

Therefore, the minimum extent to which development can encroach into a floodplain can be 

assumed to correspond to the extent of the flood fringe zone. Development can extend 

beyond this zone, but it is necessary to show that the impact of the additional extent of 

development does not cause an unacceptable impact on flood characteristics; that is, peak 

level and peak flow velocity. 

Accordingly, the extent to which urban development can occur across the LLG land at 

Thrumster can be established by defining the corridor of land adjacent to Partridge and 

Karikaree Creeks, which if filled would cause an unacceptable increase in peak flood level. 

This extent of developable floodplain will typically lie somewhere between the extent of the 

flood fringe zone and the upslope boundary of the floodway zone. 

!" # # # # $ % 

As discussed, development can generally be undertaken in flood fringe areas without 

significantly impacting on existing flood behaviour. Development can also be undertaken in 

flood storage areas, provided the impact of that development is not significant or 

compensatory works are undertaken. The Manual defines a significant impact as an 

increase in peak flood level of greater than 0.1 metres (refer Table 1). 

2



Therefore, it is possible to determine the extent of the floodplain that could potentially be 

developed by defining the extent to which encroachment into the floodplain will not result in 

an increase in peak flood level of more than 0.1 metres. 

The HEC-RAS hydraulic models that were developed by Maunsell Australia to define the 

extent of flood liable land across Area 13 were obtained and used to undertake a floodplain 

encroachment assessment. The purpose of the floodplain encroachment analysis is to 

determine the limit of development/filling that will cause a maximum change in water surface 

elevation of no more than 100 mm. This procedure can be undertaken with the HEC-RAS 

software by using an iterative (i.e., trial-and-error) approach. 

The floodplain encroachment was undertaken for the design 100 year recurrence flood for 

the three major creeks draining through the LLG land, namely: 

• Partridge Creek (referred to as Catchment A in the Maunsell Flood Study Report); 

• Unnamed Creek (referred to as Catchment C in the Maunsell Flood Study Report); and, 

• Karikaree Creek (referred to as Catchment D in the Maunsell Flood Study Report). 

Only those HEC-RAS model cross-sections falling within the LLG land were considered as 

part of the floodplain encroachment analysis. 

An example of the output from the encroachment assessment is presented in Plate 1. This 

shows the position of “floodplain encroachment stations” which define the maximum extent of 

filling or development that is possible along Karikaree Creek at HEC-RAS model crosssection 

814. The position of the floodplain encroachment stations was determined by 

simulating trial encroachment extents until the encroachment caused increases in peak flood 

level of less than 100 mm. 

. 

Catch Drevch210207 Plan: Plan 01 3/5/2007 

11 

10 

.06 .05 .06 

Legend 

WS 100 Yr Encroachm 

WS 100 Yr Existing 

Ground 

Levee 

Bank Sta 

Encroachment 

9 

Elevation (m) 

8 

7 

6 

5 

0 100 200 300 400 500 

 

Station (m) 

& ' ( ) ) ) 

 

3



The floodplain encroachment stations that were determined for each model cross-section 

were extracted from the HEC-RAS model results and transferred to a spatial (i.e., GIS) 

framework and used to develop a plan of acceptable encroachment extent. The 

encroachment extent between model cross-sections was determined by interpolating 

between the encroachment extent for successive cross-sections relative to the ALS 

topographic data. Peak flood level estimates at each model cross-section were also 

combined with the ALS data to determine the extent of flood liable land at the peak of the 

design 100 year recurrence flood. 

The encroachment (i.e., development/filling) extent for Catchment A is shown in Figure 1 as 

the light blue shaded area. The encroachment extents for Catchments C and D are 

presented in Figure 2. The red areas provided in Figures 1 and 2 show the extent of the 

floodplain that needs to be retained for the conveyance of flood flows. It is likely that a large 

proportion of the red areas would be considered to fall within floodway zones. Development 

or filling of these areas is likely to generate unacceptable increases in peak flood levels, and 

therefore, should be avoided. 

In general, Figures 1 and 2 show that the majority of those areas where urban development 

is proposed are located outside of the red areas. Notwithstanding, there are some places 

along Partridge Creek, the unnamed creek and the northern arm of Karikaree Creek, where 

development is proposed to extend into the red areas. Consideration should be given to 

excluding development from these areas or providing suitable measures to mitigate any 

adverse flood impacts should development of these areas proceed in the future . 

 

Investigations undertaken for this report have determined the maximum potential extent of 

filling that could be undertaken across flood liable sections of the LLG site at Area 13 without 

adversely impacting on existing flood behaviour. The maximum extent of filling or 

encroachment along the primary tributaries of Catchment A (i.e., Partridge Creek) is 

represented by the light blue areas shown in Figure 1. The maximum extent of 

encroachment along Karikaree Creek is represented by the light blue areas shown in 

Figure 2. 

Figures 1 and 2 show the flood liable sections of the LLG land that could potentially be filled 

without significantly impacting on existing flood behaviour. They also show those areas of 

the floodplain where development should be excluded to allow for the conveyance of flood 

flows. These areas are shown in red on each of the figures. 

It should be recognised that the encroachment extents presented in Figures 1 and 2 should 

be used as a guide to the maximum potential extent of development that could be 

undertaken. Furthermore, the boundaries of the red coloured areas on both figures should 

be “smoothed” to more realistically represent “floodway” zones. 

It should also be recognised that the encroachment assessment documented in this report 

has been completed using a broad-scale, one-dimensional, steady state hydraulic model. It 

is considered that more detailed two-dimensional hydrodynamic modelling will be required to 

assess individual development proposals during the subsequent Development Application 

phase of the project. 

. 

4



. 

* 

Port Macquarie - Hastings Council (April 2006), ‘Area 13 Investigations Area: Structure 

Plan – Urban Design Guidelines’; prepared by Deicke Richards. 

Port Macquarie - Hastings Council (6 th October 2006), ‘Area 13: Thrumster’; 

http://www.hastings.nsw.gov.au/www/html/2393-area-13-thrumster.asp, accessed 11 th 

October, 2006. 

Port Macquarie - Hastings Council (August 2006), ‘Hastings River Flood Study’; prepared 

by Patterson Britton & Partners. 

Port Macquarie - Hastings Council (February 2007), ‘Thrumster Integrated Water 

Management Plan – Flood Study’; prepared by Maunsell Australia. 

New South Wales Government (2005), ‘Floodplain Development Manual: the 

management of flood liable land’; ISBN 0 7347 5476 0. 

--------------------------------- 

We trust that the above report addresses your needs. Please feel free to contact me should you 

require any further information. 

Yours faithfully 

PATTERSON BRITTON 

Chris Thomas 

Principal 

5

Sydney Office 

level 4 

104 Mount Street 


Newcastle Office 

14 Telford Street 

Newcastle East 2300 

PO Box 515 


Australia 

PO Box 668 

Newcastle 2300 

Australia 

telephone (02) 9957 1619 

facsimile (02) 9957 1291 

email: reception@patbrit.com.au 

A.C.N. 003 220 228 

A.B.N. 89 003 220 228 

telephone (02) 4928 7777 

facsimile (02) 4926 2111 

email: mail@newcastle.patbrit.com.au 



. 

consulting engineers 

Mr Michael Mowle 

lt6715djt_crt070413-Additional Hydraulic Investigations.doc 

Hopkins Consultants 

PO Box 1556 

PORT MACQUARIE NSW 2444 30 th April 2007 

Dear Michael, 

AREA 13: THRUMSTER – LEWIS LAND GROUP HOLDINGS, PORT MACQUARIE 

OUTCOMES OF ADDITIONAL FLOOD INVESTIGATIONS 

I refer to our letter dated 9 th March 2007, which summarised the outcomes of a floodplain 

encroachment assessment that we completed for Lewis Land Group’s (LLG) land holdings at 

Thrumster near Port Macquarie. The purpose of the floodplain encroachment analysis was to 

determine those sections of the floodplains that could potentially be filled for future urban uses 

without adversely impacting on existing flood behaviour. It is understood that the results from this 

analysis are being used to assist in establishing suitable zoning boundaries for the Area 13 

Expansion Precinct. 

Since the submission of our letter, we understand that additional work has been completed to 

refine the zoning boundaries that are being developed for LLG’s land. As a result of this 

additional work, we understand that you require further investigations to be undertaken to 

determine: 

• the potential to undertake further encroachment along the northern arm of Karikaree Creek 

(i.e., Catchment D) and the unnamed creek (i.e., Catchment C); and, 

• the potential to “pipe” minor tributary inflows to Partridge Creek. 

We also understand that you would like us to assess the potential impact that the adopted 

tailwater levels in the HEC-RAS hydraulic model of Partridge Creek may have on peak design 

flood levels across LLG’s land. Specifically, this is to involve an assessment of the potential to 

adopt a lower tailwater level for the design of stormwater structures, sewerage treatment facilities 

etc. 

Accordingly, we have considered the additional issues that you have raised and are pleased to 

provide the outcomes of our investigations in the following report. 

1. IMPACT OF ADOPTED TAILWATER ELEVATION ON PEAK FLOOD LEVELS ALONG PARTRIDGE 

CREEK 

Existing design flood behaviour along Partridge Creek has been defined based on the results 

of flood modelling that has been completed for Port Macquarie – Hastings Council by 

Maunsell Australia Pty Ltd. The flood model of Partridge Creek was developed using the 

steady state, one-dimensional HEC-RAS software. 

Principals 

Senior Associates 

Associates 

Greg Britton BE MEngSc FIEAust Andrew Chitty BE MIEAust CPEng Peter Coltman BE MEngSc MIEAust 

Bruce Druery BE Dip Sc(Geol) M AppSc MIEAust Paul Harvey-Walker BE FIEAust David McConnell BSc MIEAust 

Joe Marson BE MEngSc FIEAust Andrew Patterson BE FIEAust Christopher Thomas BE MEngSc MIEAust 

Mark Tooker BSc(Eng) MEngSc FIEAust CPEng Michael Wright BE MEngSc MIEAust 

Steve Barrett Simon Batt BE MIEAust Paul Macinante BE MEnvEngSc MIEAust Ben Patterson BE MIEAust 

Marc Roberts BE Michael Shaw BE MIEAust CPEng 

Stephen Aebi BE MIEAust Neville Boyes OMIEAust Scot Cranfield Cameron Druery BE MIEAust 

Adam Knight BE MIEAust CPEng Cameron Smith BE MEngSc MIEAust CPEng 

Alexandra Stone BE MIEAust Chris Yates BE MIEAust



In order to define flood behaviour along Partridge Creek, information on design inflows from 

the Partridge Creek catchment is required. The design inflows were estimated using results 

generated by a RAFTS hydrologic computer model of the Partridge Creek catchment. 

In addition to catchment inflows, a water level must be specified at the downstream end of 

the HEC-RAS model in order for a “backwater” profile to be calculated. 

In assessing peak flood levels for Area 13, Maunsell has adopted a tailwater level at the 

downstream boundary of Partridge Creek that corresponds to the peak 100 year recurrence 

flood level for the Hastings River. Maunsell has assumed that this peak 100 year recurrence 

Hastings River flood level occurs in conjunction with a peak 100 year recurrence flood along 

Partridge Creek. That is, Maunsell has assumed that peak 100 year recurrence design flood 

levels in the Hastings River and along Partridge Creek occur simultaneously. 

However, the occurrence of a 100 year recurrence Hastings River flood occurring 

concurrently with the peak of the 100 year recurrence Partridge Creek is likely to have a 

recurrence interval of well in excess of 100 years. This is because flooding along the 

Hastings River generally occurs as a result of extended periods of rainfall (i.e., 72 hours), 

whereas flooding along Partridge Creek occurs as a result of comparatively short duration 

rainfall bursts (i.e., between 1.5 and 9 hours). 

Therefore, it is less likely that both rainfall mechanisms will occur over the Hastings and 

Partridge Creek catchments simultaneously. As a result, the approach adopted by Maunsell 

is considered to be very conservative. 

As part of the ‘Hastings River Flood Study’ (2006), it was determined that a 100 year 

recurrence Hastings River flood occurring in conjunction with a peak 20 year recurrence 

ocean tailwater level, provided a conservative and reliable estimate of design flood behaviour 

along the lower reaches of the Hastings River. Accordingly, it is considered that a similar 

approach may be suitable to define design flood characteristics for the Partridge Creek 

catchment. 

In order to determine the sensitivity of the results generated by the HEC-RAS model to 

variations in the adopted tailwater elevation, the 100 year recurrence Partridge Creek flood 

was re-simulated with a 20 year recurrence Hastings River tailwater elevation. Results 

documented in the Hastings River Flood Study indicate that the 20 year recurrence Hastings 

River flood level along the lower reaches of Partridge Creek is 2.6 mAHD. This is about 

800 mm lower than the 100 year recurrence tailwater level of 3.4 mAHD adopted by 

Maunsell. 

The results of the revised design flood simulations are summarised in Table 1 for selected 

HEC-RAS model cross-sections along Partridge Creek. The location of the model crosssection 

are shown in Figure 1. 

The results listed in Table 1 show that peak 100 year recurrence flood levels along the lower 

reaches of Partridge Creek are sensitive to changes in the adopted tailwater level (refer 

cross-sections 2008 to 3315). Across the LLG land, peak 100 year recurrence flood levels 

are predicted to reduce by up to 300 mm (refer cross-section 2198.4). However, the 

decreases in peak 100 year flood levels across the majority of the LLG land are generally 

less than 200 mm. 

. 

2



. 

Table 1 

IMPACT OF ADOPTED TAILWATER ELEVATIONS ON PEAK 100 YEAR 

RECURRENCE FLOOD LEVELS ALONG PARTRIDGE CREEK 

HEC-RAS MODEL 

CROSS-SECTION 

(refer Figure 1) 

100 Year Tailwater 

(mAHD) 

PEAK DESIGN FLOOD LEVELS 

20 Year Tailwater 

(mAHD) 

Difference 

(metres) 

2008 3.54 3.17 -0.37 

2198.4 3.60 3.31 -0.29 

2293.6 3.65 3.42 -0.23 

2388.8 3.72 3.55 -0.17 

2484 3.82 3.7 -0.12 

2571.5 3.86 3.76 -0.1 

2659 3.89 3.8 -0.09 

2669 3.89 3.8 -0.09 

3012 4.07 4.01 -0.06 

3305 4.36 4.34 -0.02 

3315 4.34 4.31 -0.03 

3628 5.21 5.21 0 

3784 5.61 5.61 0 

3867 5.92 5.92 0 

3950 7.01 7.01 0 

4033 8.31 8.31 0 

4121.6 9.26 9.26 0 

4298.8 10.98 10.98 0 

Reductions in peak flood levels of this magnitude are predicted to reduce the lateral extent of 

inundation along the lower reaches of Partridge Creek by up to 4 metres. Peak flood levels 

and, therefore, the lateral extent of inundation along the steeper upper reaches of Partridge 

Creek are not predicted to change. 

Therefore, the adoption of a 20 year recurrence tailwater level is predicted to have only 

relatively minor impacts on peak flood levels and the extent of inundation along that section 

of Partridge Creek draining through LLG land. 

Nevertheless, we consider that the adoption of a 20 year recurrence Hastings River tailwater 

level provides a suitable basis for the design of services and infrastructure within the Area 13 

precinct. However, we would advocate a more conservative approach when establishing 

minimum floor level requirements for buildings. 

Accordingly, we suggest that the 100 year recurrence Hastings River tailwater level should 

be adopted when establishing minimum floor levels for future residential and commercial 

buildings across Area 13. A 20 year recurrence tailwater level could safely be adopted for 

the design of stormwater structures, sewerage treatment facilities etc. 

3



. 

2. POTENTIAL TO “PIPE” TRIBUTARY INFLOWS TO PARTRIDGE CREEK 

As shown in Figure 1, a major tributary of Partridge Creek extends in a northerly direction 

from the Oxley Highway through a section of the Area 13 expansion precinct that is 

designated for potential future urban development. Therefore, additional investigations were 

completed to assess the potential to “pipe” flows (i.e., carry surface runoff along a subsurface 

pipe system) along this tributary in order to maximise the potential developable 

“footprint”. 

Available survey information for the tributary indicates that the creek channel is poorly 

defined. As shown in Plate 1, the tributary comprises a relatively flat depression that is 

approximately 40 metres in width. Plate 1 also shows that the tributary comprises negligible 

riparian vegetation. 

PLATE 1 

VIEW LOOKING NORTH FROM THE OXLEY HIGHWAY SHOWING THE LIMITED EXTENT OF 

RIPARAIN VEGETATION ALONG THE SOUTH-EASTERN TRIBUTARY OF PARTRIDGE CREEK 

Flooding along this tributary was previously considered as part of the floodplain 

encroachment assessment that was previously completed for Partridge Creek (refer 

Figure 1). The results of the assessment indicated that a 10 to 14 metre wide corridor would 

need to be maintained along the tributary to convey flood flows without significantly 

impacting on existing flood behaviour. That is, fill could potentially be placed to within 5 to 

7 metres of the centreline of the tributary. 

Nevertheless, there is potential to further decrease the required creek corridor width by 

conveying a proportion of the total design flow via a sub-surface pipe network. In this way, a 

reduced proportion of the design flow could be conveyed overland, thereby allowing a 

greater degree of encroachment/filling. 

4



It is considered that runoff from the catchment would be managed in accordance with the 

principles of the major / minor concept outlined in ‘Australian Rainfall & Runoff – A Guide to 

Flood Estimation’ (1987). The minor system would comprise the gutter and pipe network 

which will be designed to carry runoff generated during the smaller more frequent storms; 

e.g., up to and including the 10 or 20 year recurrence event. The major system would 

comprise both formal and informal drainage pathways (e.g., grass swales) that convey the 

residual of the runoff to Partridge Creek (i.e., flows in excess of the capacity of the pipe 

system up to and including the 100 year recurrence event). 

It is considered appropriate to provide a formalised overland flowpath for the major system in 

close proximity to the alignment of the existing tributary as there will be a natural tendency 

for surface runoff to be carried to this location. Therefore, even if a piped system was 

implemented, it would still be necessary to provide a “development free” corridor in close 

proximity to the existing tributary in order to safely convey runoff in excess of the capacity of 

the pipe system. 

Notwithstanding, it is considered that the overland flowpath could be engineered to provide 

increased conveyance capacity, thereby increasing the potential to fill within close proximity 

of the creek. 

In order to provide an indication of the potential increased encroachment that could be 

achieved if a proportion of the runoff is conveyed via a pipe system, a revised encroachment 

analysis was completed. The revised encroachment assessment was completed by 

assuming 50% of the design 100 year recurrence flow was conveyed via a sub-surface pipe 

system. The residual flow was assumed to be conveyed along the existing tributary. 

The results of the revised encroachment analysis are superimposed on Figure 2. This figure 

shows that the required corridor width would reduce from between 10 and 14 metres to 

between 6 and 11 metres. 

Accordingly, although the provision of a piped system will reduce the required floodway/flood 

storage corridor width, it will not remove the need for the corridor completely. Therefore, 

even if a piped drainage system was implemented as part of any future development of the 

area, a corridor with a typical width of about 10 metres will still be required to convey flows 

during large storms. 

3. ENCROACHMENT ASSESSMENT FOR KARIKAREE CREEK AND UNNAMED CREEK 

As discussed, an encroachment analysis was completed for the three catchments draining 

through LLG’s land. The purpose of the encroachment analysis was to determine the extent 

of the floodplain areas that could potentially be filled to accommodate future urban 

development. This information, in conjunction with a range of other information (e.g., riparian 

corridor boundaries) has been used to refine the zoning boundaries that have been 

developed for LLG’s land holdings in the Area 13 Expansion Precinct. 

The revised zoning boundaries that have been developed for urban sections of LLG’s land 

are generally located outside of the “floodway” and “flood storage” areas that were defined 

as part of the encroachment analysis. This indicates that future urban development in these 

areas will not result in unacceptable impacts on existing flood behaviour; i.e., increases in 

peak flood level of more than 100 mm. 

However, one of the proposed urban areas along the northern floodplain of Karikaree Creek 

is predicted to locally extend into an area of the floodplain that was previously delineated as 

a potential floodway/flood storage area. 

. 

5



In addition, some urban areas as well as the proposed realignment of the Oxley Highway are 

predicted to extend into an area of the floodplain of the Unnamed Creek (i.e., Catchment C) 

that was previously delineated as a floodway/flood storage area 

Accordingly, additional investigations were completed to assess whether additional 

encroachment across these areas may generate unacceptable impacts on existing flood 

behaviour. 

The assessment was completed by incorporating the additional encroachment along the 

northern floodplain of Karikaree Creek and along the Unnamed Creek, into the existing 

HEC-RAS model. The 100 year recurrence flood was then re-simulated and peak flood 

levels determined from the revised simulation were compared against peak 100 year 

recurrence flood levels for existing conditions. 

The results of the revised encroachment simulations show that the additional encroachment 

would generate localised increases in peak 100 year recurrence flood levels. However, the 

maximum increase in peak 100 year recurrence flood level along Karikaree Creek is 

predicted to be less than 100 mm. In addition, the additional encroachment is not predicted 

to increase peak flood levels along the Unnamed Creek by more than 100 mm. 

The results of the revised encroachment assessment are presented in Figure 2. The results 

of the original encroachment assessment have been superimposed for comparison. 

Overall, it is considered that the additional encroachment along both Karikaree Creek and 

the Unnamed Creek, can be accommodated without unacceptably impacting on existing 

flood behaviour along each watercourse. 

4. REFERENCES 

• Port Macquarie - Hastings Council (August 2006), ‘Hastings River Flood Study’; prepared 

by Patterson Britton & Partners. 

• Port Macquarie - Hastings Council (February 2007), ‘Thrumster Integrated Water 

Management Plan – Flood Study’; prepared by Maunsell Australia. 

• New South Wales Government (2005), ‘Floodplain Development Manual: the 

management of flood liable land’; ISBN 0 7347 5476 0. 

--------------------------------- 

We trust that the above report addresses your needs. Please feel free to contact me should you 

require any further information. 

. 

Yours faithfully 

PATTERSON BRITTON 

Chris Thomas 

Principal 

6

Patterson Britton & Partners Pty Ltd - Hastings Council - NSW ...

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