EIS 1384
Dora Creek floodplain management plan for Lake Macquarie
City Council. sw DEPT PRIMAP INDUSTRiES I ABOI 8200 I L/\ND 0 WATER I CONSERVATION I I I I
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LAKE MACQUAE CITY COUNCIL
I June 1998 I I
I 0 LPAThF)T OF
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I DORA CREEK FLOODPLAIN MANAGEMENT PLAN
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I LAKE MACQUARIE CITY COUNCIL I June 1998 I I I I I I I I NSW Department of Land and Water Conservation i ISBN 0 7313 0373 3 I FOREWORD
I The State Government's Flood Policy aims to reduce the impacts of flooding and flood I liability on individual owners and occupiers, and to reduce private and public losses resulting from flooding. The Policy encourages the development of solutions to existing flood problems in developed areas, and strategies for ensuring that new development is compatible I with the flood hazard and does not create additional flooding problems in other areas.
Under the Policy, local government has responsibility for managing flood liable land. Lake I Macquarie City Council, with the assistance of its Floodplain Management Committee, is responsible for this task at Dora Creek. The Committee membership comprises a wide range I of technical and community based skills. The Policy provides for technical and financial support by the State Government for a I number of activities that include: 1. Preparation of a Flood Study to determine the nature and extent of the flood problem. The l Dora Creek Flood Study was prepared by the Public Works Department in 1986.
2. Preparation of a Floodplain Management Study to evaluate management options for the I floodplain in respect of both existing and future development. Assessments of subdivision options and flood mitigation options for Dora Creek were undertaken by NSW Public Works in 1991 and 1992. I 3. Preparation of a Floodplain Management Plan that involves formal adoption by Council. I The plan may include provision for structural and non-structural options. 4. Implementation of the Plan.
I The Dora Creek Floodplain Management Plan constitutes the third stage of the floodplain management process for Dora Creek. This plan has been prepared by the Department of Land and Water Conservation in conjunction with Lake Macquarie City Council. It provides I the basis for the future management of flood liable lands and the management of development within the Dora Creek floodplain.
I I I I 1 I SUMMARY I
The main objective of the NSW Government's Flood Policy is to reduce the impact of I flooding and flood liability on individual owners and occupiers and to reduce private and public losses resulting from flooding. I Lake Macquarie City Council sought to examine a range of floodplain management options ' for Dora Creek. These options were intended firstly, to protect the existing development as far as possible, and secondly, to ensure that any new development would be reasonably protected and would not have major adverse impacts on existing development. Council's design standard for the Dora Creek area is the 1% AEP (annual exceedance probability) I flood.
About 180 houses and 11 commercial properties in Dora Creek would be inundated above I their floor level by floodwaters in the 1% AEP event. Up to 120 of the inundated houses ' would be in an area of high hazard. The estimated potential direct damage in the 1% AEP flood is $1.3 million, with average annual damage of $113,000 (1992 estimates). The flood behaviour characteristics of the 1% AEP event, including flood hazard, are shown on Sheets 1 to 5 which accompany this report.
in accordance with the State Government's Floodplain Development Manual, Council's Floodplain Management Committee assisted in the development of the Floodplain I Management Plan for Dora Creek, following examination of floodplain management options. Structural options considered by the Committee included dredging of Dora Creek, a diversion channel to Lake Macquarie, and a diversion levee upstream of Kalang Road. I Structural options would generally have low benefit-cost ratios. However the diversion levee would have considerable merit because it would reduce flood velocities and hence the hazard I to existing development in the Kalang Road area. Non-structural options included flood warning and evacuation planning, an alternative road access for evacuation of the Baker Street area, voluntary purchase of properties in Kalang I Road, voluntary house raising, and planning and development controls. A voluntary purchase scheme for Kalang Road would have prohibitive cost and would be unlikely to achieve a high level of acceptance by residents of the area. The other non-structural I measures would have high merit in terms of reducing potential flood damages, as well as the hazard in developed areas of Dora Creek. I With regard to future development, options for subdivision of floodplain areas adjacent to Dora Creek were considered by the Committee. Options which would generally involve I intensive residential development along both sides of the creek would lead to significant flood level increases which are likely to be unacceptable to the community.
I I ii I Subdivision of residential areas east of the Main Northern Railway to a minimum lot size of I 700m2, as allowed under Council's current policy, would be likely to cause only small increases in flood level provided that there are appropriate controls to limit the additional obstructions to overland flow paths in major floods. Any proposals to change the use of non- I urban land on the south side of Dora Creek east of the railway would need to be assessed by specific studies.
I Council's policy which prohibits further subdivision of residential areas west of the railway line appears to be appropriate because these areas contain important floodways andlor experience high hazard conditions during major floods. The large non-urban areas south of I Baker Street and west of Kalang Road currently function as flood storage areas. Any proposals to change the use of these areas would need to be assessed by specific studies.
The Dora Creek Floodplain Management Plan contains the following key measures, with estimated implementation costs as indicated: I ' Flood forecasting system - feasibility study $ 20,000
Flood awareness and public education program $ 10,000 1 Flood access roadway from Baker Street $ 200,000 Diversion levee upstream of Kalang Road $ 700,000
I Voluntary house raising $1,080,000
Planning and development controls (including - I Development Control Plan for flooding) I TOTAL $2,010,000
The plan will provide benefits in terms of reduced flood damages and reduced hazard to existing residential areas, as well as an acceptable level of protection for new development. Thepotential adverse effects of future development on existing residential areas will also be controlled. I I I I I I
I TABLE OF CONTENTS I I FOREWORD SUMMARY
1 1. INTRODUCTION 1 I 2. SUMMARY OF PREVIOUS STUDIES 2 2.1 GENERAL 2 2.2 FLOOD LEVELS, HYDRAULIC CATEGORIES AND FLOOD HAZARD 2 I 2.2.1 General 2 2.2.2 Flood Levels 2 2.2.3 Hydraulic Categories 3 I 2.2.4 Flood Hazard 4 2.3 FLOOD DAMAGES 4 2.4 ASSESSMENT OF SUBDIVISION OPTIONS 5 I 2.5 ASSESSMENT OF FLOOD MITIGATION OPTIONS 7 2.6 COMMUNITY CONSULTATION 7 2.7 IMPLEMENTATION OF FLOOD WARNING ALARM SYSTEM 8 2.8 LAKE MACQUARIE LOCAL FLOOD PLAN 8 I 2.9 SOUNDING SURVEYS OF DORA CREEK 8 I 3. EXISTING PLANNING AND DEVELOPMENT CONTROLS 9 3.1 LAKE MACQUARIE LOCAL ENVIRONMENTAL PLAN 1984 9 3.2 DESIGNATION OF FLOOD LIABLE LAND 9 I 3.3 INTERIM DORA CREEK FLOODPLAIN MANAGEMENT PLAN 10 3.4 DEVELOPMENT CONTROL PLAN NO.9- MEDIUM DENSITY 10 RESIDENTIAL DEVELOPMENT I 3.4.1 Dual Occupancy Development at Dora Creek-East of the Railway Line 10 3.4.2 Integrated Housing Development 11 3.5 FLOOR HEIGHT CONTROLS 11 I 3.6 SUBDIVISION CONTROLS 12 3.7 DEVELOPMENT ON LAND ABOVE THE 1% AEP FLOOD LEVEL 12 3.8 PREFERRED BUILDING MATERIALS 12 I 3.9 FILLING 13 3.10 MINE SUBSIDENCE PROVISIONS 13 3.11 HERITAGE 13 I 3.12 CONDITIONS OF CONSENT FOR DEVELOPMENT ON FLOOD LIABLE 13 LAND 3.13 ADVICE PROVIDED IN SECTION 149 CERTIFICATE 14 I 3.14 SEPP 14 WETLANDS 14 3.15 CONCLUSION 14
I 4. FLOODPLAIN MANAGEMENT PLAN 16 I 4.1 GENERAL 16 I iv I I
4.2 FLOOD WARN[NG SYSTEM 17 I 4.2.1 Description 17 4.2.2 Benefits 19 4.2.3 Costing 19 1 4.2.4 Implementation 19 4.3 FLOOD ACCESS ROADWAY FROM BAKER STREET 19 4.3.1 Description 19 I 4.3.2 Benefits 20 4.3.3 Costing 20 4.3.4 Implementation 20 I 4.4 DIVERSION LEVEE UPSTREAM OF KALANG ROAD 20 4.4.1 Description 20 4.4.2 Benefits 21 I 4.4.3 Costing 21 4.4.4 Implementation 21 4.5 VOLUNTARY HOUSE RAISING SCHEME 21 I 4.5.1 Description 21 4.5.2 Benefits 22 4.5.3 Costing 22 I 4.5.4 Implementation 22 4.6 PLANNING AND DEVELOPMENT CONTROLS 22 4.6.1 Description 23 4.6.2 Benefits 23 I 4.6.3 Costing 24 4.6.4 Implementation 24 I 4.7 CHANNEL IMPROVEMENTS TO DORA CREEK 24 5. IMPLEMENTATION OF PLAN 26
I 6. ACKNOWLEDGMENTS 27
I APPENDIX A - CATCHMENT DESCRIPTION 28
LIST OF FIGURES
1 Locality Plan Flood Profiles Peak Flooding Conditions for 1% Probability Flood I 1% Probability Flood - Floodway and Flood Storage Areas for Existing Conditions
LIST OF TABLES
Floor Height Controls Applying to New Development 1 Floor Height Controls Applying to Existing Development Program to Implement the Dora Creek Floodplain Management Plan I I 1. INTRODUCTION I Lake Macquarie City Council, through its Floodplain Management Committee, is developing a Floodplain Management Plan for Dora Creek in accordance with the New South Wales I Government's Floodplain Development Manual (1986).
Dora Creek is located about 30km south west of Newcastle. The creek rises in the Sugarloaf I Ranges, some 460m above sea level, and flows in an easterly direction to its mouth at Lake Macquarie. The township of Dora Creek, which is located about 3km from the mouth, has been developing at a rapid rate in recent years. Significant flooding of Dora Creek has been I experienced on several occasions, including floods in 1927, 1949, 1953, 1962, 1974, 1977, 1978, 1981, 1989 and 1990. i The area covered by this Plan is the floodplain of Dora Creek from the junction of Stockton Creek to the mouth. Figure 1 shows the location of the Dora Creek catchment and the study I area. Flooding of the area results from heavy rainfall over the catchment of Dora Creek and its I tributaries. Inundation of overbank areas occurs when the canying capacity of the Dora Creek channel is exceeded. Overbank flows where floodwaters do not return to the main creek channel occur at several locations in the vicinity of Dora Creek township. Upstream of the I Main Northern Railway, there are overflows to the north to Muddy Lake. Downstream of the railway, overflows occur to Muddy Lake and Lake Eraring in the north and to Bonnells Bay in the south.
Flooding in the lower reaches of Dora Creek may also be influenced by high water levels in 1 Lake Macquarie. Council has adopted the 1% AEP (annual exceedance probability) flood as the flood standard (or designated flood) for the study area. This determines the area of land that should be subject I to flood related development and building controls. The study area is covered by Council's flood maps (1:2000 Urban Cadastral Series - Dora Creek 43, 44, 53, 54,72 and 81). ,E I I I L I I 1 2. SUMMARY OF PREVIOUS STUDIES I 2.1 GENERAL
1 Several investigations have been carried out in relation to the flooding problems of Dora Creek. These investigations have included studies to determine flood levels, hazard and damages, and I floodplain management studies which assessed the impacts of development options and flood mitigation options. In addition, the State Emergency Service (SES) Local Controller has developed a local flood plan for flooding within the Lake Macquarie City Council area. I The most recent studies include the following:
I "Dora Creek Flood Study", Public Works Department NSW, Report No. PWD 85019, May 1986
I "Dora Creek Floodplain Management Study - Hydraulic Analysis of Subdivision Options", NSW Public Works for Lake Macquarie City Council, Report No. 90015, February 1991
I "Dora Creek Floodplain Management Study - Hydraulic Analysis of Flood Mitigation Options", NSW Public Works for Lake Macquarie City Council, Report No. 91016, I February 1992 . "Dora Creek, Kalang Road: Two Dimensional Study and Preliminary Design of a Flood I Deflector Levee", Patterson Britton & Partners Pty Ltd for Lake Macquarie Council and NSW Public Works, February 1994 I . "Lake Macquarie Local Flood Plan - A Sub-Plan of the Lake Macquarie Local Disaster Plan (DISPLAN)", State Emergency Service, April 1996.
2.2 FLOOD LEVELS, HYDRAULIC CATEGORIES AND FLOOD HAZARD
I 2.2.1 General
Flooding of Dora Creek results predominantly from heavy rainfall over the upstream I catchment. Information on the characteristics of the catchment is provided in Appendix A.
The major impacts of flooding are related to the depth and velocity of floodwaters. The floodwaters may also carry substantial sediment and debris loads from the various land uses in the catchment, including debris from timbered areas, agricultural areas and urban areas.
2.2.2 Flood Levels
I Design flood levels were determined for various events. The flood profiles for the 1% AEP and 5% AEP events are shown in Figure 2. The peak flood levels and creek velocities for the 1% I AEP flood are shown in Figure 3. 1 2 I
The estimated peak flood levels for an extreme flood event are up to 3m higher than the levels I for a 1% AEP event. I Design flood levels near the outlet of Dora Creek to Lake Macquarie are determined by backwater flooding from the lake, which includes the effect of rainfall-runoff events in the catchment. The flood levels near the creek outlet shown in Figures 2 and 3 were established in I the Lake Macquarie Flood Study undertaken recently by Council. I 2.2.3 Hydraulic Categories The hydraulic categories of flood liable land are:
I Floodway - areas where a significant volume of water flows during floods Flood storage - areas that are important for the temporary storage of floodwaters during the passage of a flood I Flood fringe - the remaining area of land affected by flooding. I The hydraulic categories of the Dora Creek floodplain for the 1% AEP event were assessed based on the results of hydraulic modelling and are shown in Figure 4. Three floodway areas I were identified, in addition to the Dora Creek channel: The Kalang Road area The front portions of the properties in Baker Street which are adjacent to Dora Creek I The floodway through the low level areas of Watt Street, Minnie Street and Newport Road which allows floodwaters to flow into Muddy Lake. Part of this area has been designated I for floodway purposes by Council. Council has also designated a number of areas downstream of the railway for floodway purposes, in recognition of the importance of maintaining dedicated overflow paths from Dora I Creek to Lake Eraring and Bonnells Bay. The Council designated floodways which should be retained for this purpose are shown in Figure 4.
I The remaining floodplain area downstream of the railway is categorised as flood storage. This is because extensive filling of the area would lead to significant increases in upstream flood 1 levels. It should be recognised that this area also carries overflows from Dora Creek to Muddy Lake, Lake Eraring and Bonnells Bay. Retention of this function will be important in consideration of development proposals in the area.
Upstream of the railway, the large flood liable area south of Baker Street and the area adjacent to the Kalang Road floodway are categorised as flood storage because complete filling of these areas would be likely to result in increased flood levels.
The broadly defined flood storage areas upstream of the railway will contain some areas which U can be considered as flood fringe. These are the areas where filling could be permitted without adverse effects on flood behaviour. The precise extent of flood fringe areas would need to be I defined in conjunction with any development proposals for the areas. U I 3 I
1 2.2.4 Flood Hazard Hazard categories were assigned to the Dora Creek floodplain under 1% AEP conditions in I accordance with the Floodplain Development Manual. The preliminary determination of hazard category is based on hydraulic conditions, particularly the depth and velocity of floodwaters. Where hydraulic considerations are not obviously dominant, the preliminary I categorisation may be varied according to other factors such as effective evacuation time and evacuation difficulties.
The above floodways would generally be in high hazard areas, based on the high depth and velocity of floodwaters, the limited available evacuation time, and likely difficulties experienced during evacuation.
Parts of the flood storage area adjacent to Baker Street are also considered to be high hazard areas under existing conditions. These are the areas which would be required to be evacuated I via Baker Street where floodwater depths may be great, particularly under the railway. The hazard could be reduced by construction of an alternative access roadway for evacuation during I floods, in conjunction with effective flood warning and evacuation procedures. The flood storage area adjacent to Kalang Road would also be considered high hazard if any I development in that area was required to be evacuated via Kalang Road. The hazard could be reduced if the proposed development provided access from the area to high ground to the west.
I The flood storage area downstream of the railway is considered to be low hazard because the depth and velocity of floodwaters would generally be low enough to allow evacuation of people and their possessions safely. However there may be localised areas where floodwater depths I exceed 1 metre or flow velocities are high. Therefore it is important that effective flood warning and evacuation procedures be implemented and maintained to enable evacuation to I take place as early as possible during a flood event. Tt is also important to recognise that floods greater than the 1% AEP event may occur, with I increased hazards throughout the Dora Creek floodplain.
2.3 FLOOD DAMAGES
About 180 houses and 11 commercial properties would be inundated above their floor level by floodwaters in the 1% AEP event. Up to 120 of the inundated houses would be in an area of I high hazard. Houses of light framed construction could suffer damage due to the velocity of floodwaters, as well as damage due to the depth of inundation.
I The estimated potential direct damage due to inundation in the 1% AEP event is $1.3 million. The estimated average annual damage is $1 13,000 (1992 estimates). These estimates do not include potential direct damage due to the velocity of floodwaters (eg impact damage from I floating debris, distortion of the building frame, movement of the building off its foundations), indirect damage (eg cleanup costs), and intangible damages (eg adverse social and I psychological effects of flooding). I 1 4 I
2.4 ASSESSMENT OF SUBDIVISION OPTIONS
An investigation was undertaken to assess the hydraulic impacts of broad scale development options on flood behaviour. The options were intended generally to represent proposals for I intensification of development in residential areas of Dora Creek township. Two levels of final development were considered: I Complete infilling, which would result in 100% blockage of the floodplain. . Half infilling, which would result in substantial blockage but which would still permit some I overbank flows to occur. 1 A summary of the options and their hydraulic impacts is as follows: Option 1 involved fully infilling the floodplains at Kalang Road, Minnie Street (excluding the existing floodway), Baker Street, Dora Street and Stingaree Point Drive. The maximum effect I of this development would be to raise flood levels above the design flood levels by 40 to ' 120mm.
Option 2 involved infilling only half of the floodplain areas defined in Option 1. The maximum effect was to raise levels above the design flood levels by 20 to 60mm.
I Option 3 involved full infilling as for Option 1, but without infilling of the Kalang Road area. The maximum effect of this option would be the same as for Option 1. I Option 4 involved infilling half the floodplain areas defined in Option 3. The maximum effect of this option would be the same as for Option 2.
The results indicate that complete blockage of the floodplain would lead to flood level increases of up to 120mm, which would be unacceptable. Fifty per cent blockage would lead to flood I level increases of up to 60mm, which would also be likely to be unacceptable. Any subdivision on the Dora Creek floodplain has the potential to lead to flood level increases. I Council's subdivision policy should be compatible with the need to minimise flood level increases and any adverse impacts to existing residents.
I Council resolved on 14 May 1990 to allow subdivision with a minimum lot size of 700m2 east of the railway. For most of the area north of Dora Creek, the resulting development pattern would be a form of dual occupancy involving two detached houses on a subdivided lot. The I additional house and associated filling would be located largely in a line with the existing house on the parent block. This form of development would create some additional obstruction to overflow paths from Dora Creek towards Muddy Lake and Lake Eraring. The effective I increase in obstruction and the resulting increase in flood levels could be limited by means of ' appropriate site controls; for example, the new house to be located in the "shadow" of the existing house, and controls on filling, fences and other structures on the lot.
There are some lots north of Dora Creek which are greater than about 20m wide where I subdivision into two adjoining lots could occur. Development of these lots would result in a significant local increase in the obstruction to overflows. Since the number of such potential
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subdivisions appears to be low, the overall effect on flood levels should be limited. It will be 1 appropriate to apply site controls to retain the greatest practicable width for unimpeded I overland flow paths. The form of development with most impact on flood behaviour would involve areas of continuous filling and/or building across overland flow paths. This would be likely to lead to I significant redistribution of flows and increase in flood levels.
For the area south of Dora Creek, there is unlikely to be subdivision in the existing residential area for about 400m east of the railway, because of the smaller lot size. In the residential area near the eastern end of Stingaree Point Drive, the form of subdivision would likely be similar to that on the north side of Dora Creek.
There are approximately ten (10) lots extending over a length of about 1700m on the south side of Dora Creek east of the railway line which are not currently in urban use. This is in addition I to the two lots designated by Council as floodways and to be retained for that purpose. All lots would function as overland flow paths in major flood events. This important function would I need to be considered in any future use of the area. It is likely that intensive residential subdivision or other use involving extensive filling and/or building would cause flood level increases in existing developed areas. Any proposals to change the land use of the area would 1 need to be assessed through detailed site specific studies.
Council's current policy prohibits subdivision of flood liable areas west of the railway. This I proposal is appropriate in the Watt Street/Minnie Street/Newport Road area which is a high hazard floodway. Further development in that area is likely to be at high risk, as well as lead to flood level increases. The policy is also appropriate for the existing developed area around I Baker Street, which is a high hazard area.
In regard to the large undeveloped area south of Baker Street, complete filling for residential or similar development would be likely to result in flood level increases. The type and extent of development appropriate to the area would need to be assessed through detailed study, taking into account the flood hazard. A similar requirement would apply for the undeveloped area west of the Kalang Road floodway.
In summary, Council's current subdivision policy is likely to lead to small increases in flood levels as a result of further development east of the railway on the north side of Dora Creek. Withappropriate site controls, the maximum increases should be less than the increases for the "half infilling" option outlined above; ie less than 60mm increase in the 1% AEP flood.
It should be recognised that there will be an increase in flood risk and potential flood damages associated with further development in the floodplain. There will be a small increase in damages incurred by existing residents. In addition, damages may be incurred by new residents; for example, to low lying property in the 1% AEP event. I I 1 6 I
2.5 ASSESSMENT OF FLOOD MITIGATION OPTIONS
The Floodplain Management Committee identified the following structural and non-structural flood mitigation options for the lower Dora Creek floodplain area:
Lowering of creek bank at "Miracle Haven" Dredging of Dora Creek C.Diversion channel to the Lake Council controls on development at Kalang Road Voluntary purchase of properties in Kalang Road F.Flood access roadway from Baker Street Diversion levee upstream of Kalang Road Flood warning system 1. Voluntary house raising
Options A, B, C and G are structural options which are designed to reduce the impact of flooding by modifying flood behaviour. All the structural options would have low benefit-cost ratios. Hydraulic analysis indicated that a diversion levee upstream of Kalang Road (Option G) is the only option which would generate a significant reduction of the flood hazard conditions in the Kalang Road area.
Options D, E, F, H and I are non-structural options intended to reduce the impact of flooding without modifying flood behaviour. All five options are positive management measures in I terms of their ability to reduce damage potential and risk to life and limb in hazardous areas. However voluntary purchase of properties in Kalang Road (Option E) would have prohibitive I cost and would be unlikely to be well accepted by the local community. Voluntary house raising (Option I) would be the most effective measure in reducing the number of dwellings with floor levels below the 1% AEP flood level.
2.6 COMMUNITY CONSULTATION
I A copy of the report on hydraulic analysis of subdivision options was placed on display at Council's Library at Morisset in April and May 1991. Copies of the report on hydraulic analysis of flood mitigation options were placed on display at the libraries at Morisset and I Speers Point in October and November 1991.
A number of submissions were received by Council following display of the reports. Most submissions were from residents of the Kalang Road area expressing concern about the provisional categorisation of the area as a high hazard floodway, suggesting structural options I for mitigation of flooding, and seeking approval for construction of dwellings on vacant land or, alternatively, voluntary purchase of the property.
The proposed diversion levee upstream of Kalang Road was one of the measures favoured by the residents, and a preliminary design for the levee was initiated by Council. However the levee option was opposed by at least two of the three landowners on whose land the works, if I approved, would be constructed. I
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2.7 IMPLEMENTATION OF FLOOD WARNING ALARM SYSTEM
A flood warning alarm system for the Kalang Road area was installed by the Department of Public Works and Services in 1993. The system includes a telemetered water level recorder in I Dora Creek near the north end of Kalang Road. Water level recorders were previously established at upstream locations by the Department. Access to water level records from these I stations is available by means of- * Direct telephone contact, receiving a synthesised voice water level from the data logger. Alarm warnings issued at pre-determined water levels, transmitted to ten programmed telephone numbers including the home numbers of four members of the Dora Creek Progress Association. I PC access to the Manly Hydraulics Laboratory VAX computer and a specifically designed Council menu, including neighbouring water level and rainfall stations.
I 2.8 LAKE MACQUARIE LOCAL FLOOD PLAN
The Local Flood Plan is issued under the authority of the State Emergency and Rescue I Management Act 1989 and the State Emergency Service Act 1989. The plan covers preparedness measures, the conduct of response operations and the co-ordination of immediate recovery measures for flooding within the Council area, including Dora Creek. The plan covers I operations for all levels of flooding and caters for both State Emergency Service (SES) control of operations and, where appropriate, the handover of control of part or all of operations from I the SES Local Controller to the Local Emergency Operations Controller. The Local Flood Plan contains information on the water level gauges in the Dora Creek I catchment monitored by the SES, the preparation and issue of flood warnings and evacuation warnings, and evacuation procedures.
I 2.9 SOUNDING SURVEYS OF DORA CREEK
In 1997, Council carried out a sounding survey of Dora Creek. Cross-sections were taken at I fifteen (15) locations, and the results were compared with previous soundings taken at the same cross-section locations since 1989. The comparison indicated an apparent shallowing of the I creek bed in recent years, particularly in the lower reaches. A reduction in the creek cross-sectional area due to sediment build-up has the potential to reduce the creek's carrying capacity and lead to increased flood levels. It will be important to I continue periodic sounding surveys of Dora Creek in order to assess the long term trend of sediment movement. I I I 1 8 I I 3. EXISTING PLANNING AND DEVELOPMENT CONTROLS I
Council administers a range of planning and development controls relating to flood liable land. I These are discussed below. I 3.1 LAKE MACQUARIE LOCAL ENVIRONMENTAL PLAN 1984 There are presently no flood-specific controls in Lake Macquarie Local Environmental Plan I 1984 that regulate development anywhere within the local government area. Clause 22A of this LEP enables the preparation of a floodplain management plan. The LEP is a I legal instrument. It is legally binding on Council and developers. Clause 22A(3) of the LEP requires that the format and procedures for preparing a floodplain I management plan follow those relating to the preparation of a Development Control Plan under the Environmental Planning and Assessment Regulation, 1980.
I A Development Control Plan (DCP) usually takes the form of a written statement which may be supported by maps, diagrams and other illustrative materials. A DCP must describe the land to which it applies, and must identify any LEP applying to that land. A DCP must be publicly I exhibited prior to adoption by Council.
While the Floodplain Development Manual shall be the guiding document in preparation of the I floodplain management plan, other matters which the plan may consider include: I • establishing development requirements to deal with the likelihood of flooding or tidal inundation; . specifying relevant matters for consideration in determining development applications; I specifying conditions which should generally be applied to a consent granted under the plan in particular circumstances; and I establishing procedures for consultation with and co-ordination with any public authority. 3.2 DESIGNATION OF FLOOD LIABLE LAND
I The extent of the 1% AEP flood is marked on Council's flood maps (1:2000 Urban Cadastral Series). During the early 1980's, Council and the then Public Works Department designated various floodways at Dora Creek. The locations of these floodways are shown on Council's I flood maps. Of these floodways, the following should be retained for planning purposes:
1. The floodways on the north side of Dora Creek through the Minnie Street area and adjacent I to the Main Northern Railway embankment to Muddy Lake 2. The floodway on the north side of Dora Creek across Dora Street to Lake Eraring, I approximately 1.1 kin east of the railway 3. The floodway on the south side of Dora Creek across Stingaree Point Road to Bonnells Bay, I approximately 1.0km east of the railway 1 9 I
4. The floodway on the south side of Dora Creek across Stingaree Point Road to Bonnells Bay, approximately 1.6km east of the railway
I It is not considered essential that the other areas currently identified as floodways on Council's maps be retained exclusively for that purpose. However the flood liable nature of the land and the flood hazard need to be taken into account in any proposal to change the use of these areas.
The high hazard area adjacent to Kalang Road should also be adopted and maintained as a floodway until such time as a diversion levee to reduce flow velocities in this area is I implemented.
Council is also concerned to prohibit residential development in floodways. Lake Macquarie I LEP would need to be amended to give effect to any such prohibition. I 3.3 INTERIM DORA CREEK FLOODPLAIN MANAGEMENT PLAN Council adopted an Interim Floodplain Management Plan for Dora Creek in December, 1994. I This Plan includes provisions for development controls, evacuation planning, river level monitoring systems, house raising, the proposed diversion levee at Kalang Road, and the I proposed access road from Baker Street. While some of the elements of the Interim Plan will be superseded by the final Plan, the former will form a useful base for consolidating planning, development control, flood warning and I evacuation measures for Dora Creek.
3.4 DEVELOPMENT CONTROL PLAN NO.9- MEDIUM DENSITY RESIDENTIAL DEVELOPMENT
Development Control Plan No. 9 was adopted on 28 September, 1992 and applies to medium I density development across the Lake Macquarie local government area. This DCP contains specific dual occupancy provisions for flood affected land in Dora Creek east of the railway. I 3.4.1 Dual Occupancy Development at Dora Creek - East of the Railway Line
I (1L1ement Objective To allow for the controlled flow of water through the development so that the existing floodplain characteristics can be maintained and unacceptable increases in flood levels and 1 associated damage do not occur.
(2) Performance Criteria The development should be designed to facilitate the free flow of floodwater through the site by considering the following factors:
a) the layout and orientation of buildings; b)the size of buildings; the setback of buildings from the side boundaries; the location and layout of landscaped areas, barbeque areas, clothes drying areas etc.; and the type and location of fencing.
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(3) Deemed-to-Comply Criteria
The maximum site coverage including all out buildings, garages, lawn lockers, raised I swimming pools, filled areas etc. is not to exceed 0.35:1.
Any area filled above Council's required fill level shall be included in the site coverage I calculations. (c) Buildings or any area of the site raised above the required filling level shall observe a I 2m side boundary setback. (d) No solid internal fencing shall be permitted.
NB. The above element of control only applies to dual occupancy developments. I Developments involving more than two dwellings will not be permitted. These controls would not necessarily apply to other categories of residential development in the I Dora Creek floodplain. Dual occupancy development is not intended in flood affected areas west of the railway line. The ability to enforce this restriction may be difficult, given that dual occupancy development I is permissible under Lake Macquarie LEP 1984. LEP 1984 would need to be amended to give effect to prohibition of dual occupancy in flood liable areas to the west of the railway line. I 3.4.2 Integrated Housing Development I DCP 9 allows for subdivision of land for integrated housing having minimum lot sizes of 232m2. DCP 9 does not appear to have prohibited this level of housing density in the Dora Creek floodplain, but clearly it would be desirable to do so. In fact, Council's Interim Plan I seeks to prohibit multiple dwellings and dual occupancies in high hazard or floodway areas.
3.5 FLOOR HEIGHT CONTROLS
Council has from time to time resolved to adopt floor height controls for existing and new residential, industrial and commercial development. Areas subject to floor height control are marked on maps held by Council.
Table 1 Floor Height Controls Applying to New Development I LAND USE HABITABLE ROOM NON HABITABLE ROOM Residential/Commercial Finished floor level to be at Finished floor level to be at I least 500mm above 1% or above 5% flood level flood level Industrial nla Finished floor level to be at I or above 1% flood level fl I Ll
I Council advises applicants to keep electrical fittings above the 1% flood level for safety reasons.
I Table 2 Floor Height Controls Applying to Existing Development
LAND USE HABITABLE ROOM NON HABITABLE I ROOM Residential/Commercial Alterations/additions are as Ensure that new structures for new dwellings, although do not adversely affect I extension of habitable existing flow of floodwaters floors below mm. heights will be considered on merit. I Industrial nla Alterations/additions are as for new buildings, although I where a minor addition is unable to meet Council's requirements, the case will I be considered on merit
I These provisions are consistent with, and expand those contained in Council's Interim Floodplain Management Plan. They should be included in a Development Control Plan for 1 flood liable land. 3.6 SUBDIVISION CONTROLS
I Council resolved on 14 May, 1990, that further subdivision of flood-prone areas west of the Fi railway line be prohibited. Council also resolved to restrict residential lot sizes to 700m2 east of the railway line. Each additional lot created attracts a levy of $1,000 (indexed to the Consumer Price Index from I 1990) to assist in funding flood mitigation works in Dora Creek. I 3.7 DEVELOPMENT ON LAND ABOVE THE 1% AEP FLOOD LEVEL Council has a policy for development on land adjacent to land affected by the 1% AEP flood, adopted by resolution on 13 January, 1993. Development on such land may still be subject to Li the floor height requirements for habitable floors even though the relevant lots may not be subject to flooding from the 1% AEP flood. This requirement will cease to exist for land I higher than 500mm above the 1% AEP flood as calculated for the relevant lot. 3.8 PREFERRED BUILDING MATERIALS
1 Council applies the Draft Flood Proofing Code contained in Appendix F of the Floodplain Development Manual as a guide to construction methods and materials to be used in Dora I Creek. I I 12
1-7
3.9 FILLING
Design filling heights in Dora Creek were determined in the late 1940's and many lots have I been filled to Council's requirements since that time. However, there is no resolution endorsing its application. The required filling heights were designed to allow building on low, swampy land and were determined to facilitate the draining of land, not to bring lots above I flood levels. Filling heights are identified on maps held by Council.
Council believes that as much of the land has been filled to its requirements, further filling is I undesirable as this will exacerbate flooding on other land. Filling to above design flood levels is not a desirable option for reducing flood damages in most areas of Dora Creek. This I provision should be included in a DCP for flood liable land. 1 3.10 MINE SUBSIDENCE PROVISIONS Dora Creek township is in a mine subsidence area. Construction in the area should have regard to the advice of the Mine Subsidence Board. Council has held discussions with mining companies regarding the potential for mine subsidence to render areas such as Dora Creek more flood liable. This question is currently being considered by the mining companies.
I Council makes no special provision for mine subsidence in relation to proposed development. ' However Council policy of requiring 5 00mm freeboard for setting floor levels of new residential and commercial development makes some allowance for site variables such as locally elevated floodwaters and land subsidence. 1 3.11 HERITAGE The City of Lake Macquarie Heritage Study identified a number of heritage items in Dora I Creek that are in the floodplain. These are as follows:
. The road and rail bridge across Dora Creek I House at 16 Dora Street . Holmes Store at 3 Watt Street
I Council has advised that the road and rail bridge, and house at 16 Dora Street will be identified as heritage items in a draft heritage Local Environmental Plan, presumably one that will amend Lake Macquarie LEP 1984. The latter includes development controls pertaining to the I maintenance of heritage items. Development proposals affecting heritage items identified under LEP 1984 require Council consent.
There is limited evidence of Aboriginal heritage in Dora Creek. I 3.12 CONDITIONS OF CONSENT FOR DEVELOPMENT ON FLOOD LIABLE LAND
I On receipt of a development or building proposal on flood liable land, Council issues a "Flood Prone Land Advisory Letter" and information on "Precautions Against Risk from Flooding".
1 13 I I The former document sets out details of flood affectation of the land and the applicant's requirements in addressing the flood related aspects of the proposal. I The latter prescribes minimum floor levels for habitable and non-habitable rooms for various categories of land use. It identifies land that has been notified under the Unhealthy Building Land Act, 1990, for which a certificate of compliance from the Environmental Protection I Authority is required in association with any development or building proposal.
Council's resolution of 8 October, 1990 requires all applications for new buildings or major I additions in Baker Street to include a detailed report, from an appropriate Consultant Engineer, demonstrating that the building or structure can withstand the force of flowing flood waters, I including debris and buoyancy forces. Council's resolution of 9 May, 1994 requires all applications for new buildings or major additions in Kalang Road to include a detailed report, from an appropriate Consultant Engineer, I demonstrating that the building or structure can withstand the force of flowing flood waters, I including debris and buoyancy forces. ' 3.13 ADVICE PROVIDED IN SECTION 149 CERTIFICATE
A prospective property buyer may obtain a Section 149 Certificate under the Environmental Planning and Assessment Act. This Certificate provides details of any statutes, plans and I policies that may affect the subject land. Council provides the following advice in relation to flooding in general: "Council has by resolution adopted a policy to restrict the development of the land by reason of I the likelihood of flooding or tidal inundation."
Further information on development restrictions on flood liable land is available for a fee from I Council, upon application for a Development Restrictions Certificate-Flooding/Tidal Inundation.
I Information on the S149 Certificate should refer to the Dora Creek Floodplain Management I Plan, once adopted. 3.14 SEPP 14 WETLANDS I Some areas subject to SEPP 14 controls intersect marginally with land designated by Council as being affected by the 1% AEP flood. Certain consent (of Council) and concurrence (of the I Minister for Urban Affairs and Planning) provisions will apply to development on such land. 3.15 CONCLUSION
I While Council currently has in place several planning and development controls relating to flooding in Dora Creek, these are contained in a number of documents. From an administrative I viewpoint, this is cumbersome both for Council and the applicant. I 1 14 Due to the number of policies and practices dealing with flood liable land, there appears to be an overlap between some of these documents.
The analysis above has also highlighted policy issues affecting planning and development of flood liable land that need to be addressed in this Plan.
The key issues for the floodplain management plan are to consolidate relevant planning and development controls and to resolve policy matters affecting flood liable land. I I I I I
[1 I I I I I I I I 15 I I 1 4. FLOODPLAIN MANAGEMENT PLAN
4.1 GENERAL
The purpose of the Floodplain Management Plan for Dora Creek is to ensure that:
. The use of flood liable land is planned and managed in a manner compatible with the I assessed frequency and severity of flooding. . Flood liable lands are managed having regard to social, economic and ecological costs and 1 benefits, to individuals as well as the community.
. Floodplain management matters are dealt with having regard to community safety, health I and welfare requirements. I • Information on the nature of possible future flooding is available to the public. . All reasonable measures are taken to alleviate the hazard and damage potential resulting I from development on floodplains. . There is no significant growth in hazard and damage potential resulting from new I development on floodplains. . Appropriate and effective flood warning systems exist, and emergency services are available I for future flooding. Structural measures to modify the physical characteristics of flooding would not generally have I significant merit for the Dora Creek floodplain. The results of previous studies have shown that they would have high costs, would confer limited economic and social benefits, and would not I necessarily receive the full support of the community. The construction of a diversion levee upstream of Kalang Road would have merit because it would reduce flood velocities, and hence the hazard, in the Kalang Road area. Therefore the I diversion levee has been included in the current plan, subject to resolution of the concerns held by the owners of the land on which the required works would be constructed.
I The Dora Creek Floodplain Management Plan contains a range of measures which are intended to reduce the impact of flooding on existing development and to reduce the susceptibility of I new development to damage and disruption from floods. The preferred floodplain management measures for the study area are as follows:
I . Flood warning and evacuation planning . Flood access roadway from Baker Street . Diversion levee upstream of Kalang Road I Voluntary house raising scheme Planning and development controls
1 16 I
Channel improvement to Dora Creek is included as an option to be considered if long term I sounding surveys indicate that siltation has substantially reduced the creek channel capacity.
I 4.2 FLOOD WARNING SYSTEM
4.2.1 Description I Flood forecasting, flood warning, evacuation planning and raising awareness will modify community response to enable people to cope with flooding more effectively. The State I Emergency Service (SES) is responsible for the co-ordination of real time responses to flooding and for coordinating the evacuation and welfare of affected communities. The Lake Macquarie Local Flood Plan provides details of the flood warning system for Dora Creek township, the I issue of evacuation warnings, and evacuation procedures. I a) Flood Forecasting System Flood forecasting involves obtaining rainfall and water level data for the catchment to enable I the size of a flood to be predicted rapidly. The prediction of flood levels well in advance of their arrival allows emergency services personnel to assess the severity of the predicted flood, and to respond effectively. It will also allow flood warnings to be issued to advise the I community what level of flooding is predicted, how they might be affected, and what actions to take to minimise the impacts.
I There are several pluviometer and water level stations in the Dora Creek catchment which are potential sources of data for the purpose of flood forecasting. The pluviometer stations are at Dora Creek, Martinsville, Mandalong and Fassifern. The water level stations are on Dora I Creek at Dora Creek township (at Kalang Road) and at Cooranbong Weir, and on Stockton Creek at Morisset. There would be merit in installation of a telemetered water level station on I ligadee Creek in order to provide monitoring of all upstream tributaries. At present, there is no formal methodology for prediction of flood levels at Dora Creek township using upstream rainfall and water level data. A capability for flood forecasting would I be beneficial in terms of providing the maximum available warning time and of estimating the 1 likely maximum flood height at Dora Creek. Because of the small catchment area and the rapid response to rainfall, Dora Creek catchment is considered to be a "flash flood" catchment. The NSW Flood Warning Consultative I Committee's (FWCC) report of 1992 on the Sydney-Newcastle-WollongOng Flash Flood Warning System identified Dora Creek as a high flash flood risk.
I Real time computer modelling of flood behaviour would only be beneficial if the flood warning system was able to predict future rainfall. A radar system is proposed in the FWCC report, which would allow such predictions to be made. However a radar system may not be available I for some considerable time.
Nevertheless, pre-determined relationships between catchment rainfall and flood levels at Dora 1 Creek could provide useful tools to SES and Council personnel for the prediction of flood I 1 17 I
I behaviour during actual events. The hydrologic and hydraulic models used in previous studies would be appropriate for derivation of the relevant relationships.
1 While there may be some scope for improved flood forecasting, it should be recognised that the small catchment size will be a limiting factor with regard to the available time for and accuracy of flood level predictions at Dora Creek. Hence flood warning and evacuation procedures will I be key factors in minimising damages and the risk to life and limb during flood events. I Flood Warning and Evacuation Planning There will be little warning time available for Dora Creek, and the overall effectiveness of I flood warning measures will depend on a number of important factors such as: I the warning time available for people to initiate evacuation procedures; . the level of public awareness so that people will accept and act on evacuation advice; and
I the availability of a flood free refuge that people can go to with their possessions.
The current warning system for Dora Creek is based on monitoring of water level gauges by the SES, with automatic activation of the telephone numbers of SES personnel and members of the Dora Creek Progress Association at pre-determined gauge heights. When the telephones are activated, the SES will monitor, collect and disseminate flood information. The Dora Creek Progress Association members will warn other members of the Dora Creek community.
The Lake Macquarie Local Flood Plan contains a guide to the content of evacuation warning messages as well as the system for disseminating evacuation warnings. Details of evacuation proceduresare also included in the plan. Information sessions should be held to ensure that the evacuation warnings and procedures are properly understood by the residents.
Flood Awareness and Public Education Program
Raising the flood awareness of floodplain users has been shown to reduce future damages and risk to life. It is proposed that Lake Macquarie City Council implement a flood awareness and public education program in conjunction with advice from the Local Emergency Management Committee. Under the program, regular flood awareness campaigns should be conducted, I including measures such as: . Installation of permanent marks showing the highest observed levels in historical floods, based on Council's records. I Issue of flooding information with rate notices emphasising the level of risk that exists.
. Presentations by SES officers to community groups, including photographs of past flood I events, with recognition that larger floods can occur. Flood awareness campaigns should educate the floodplain users on issues such as: I 1 18 I
. Storage of important documents, memorabilia and treasured items as high as practicable or I at least above the 1% AEP flood level 1 . Procedures for lifting and rapid evacuation of possessions Understanding of the warning system and its limitations
4.2.2 Benefits
I The main benefit of such a comprehensive flood warning system for Dora Creek will be a reduction in flood damages and a reduced risk to life and limb during flood events. I 4.2.3 Costing
The estimated cost of a feasibility study involving computer modelling to derive relationships I between catchment rainfall and downstream flood levels, as an aid to flood forecasting, is $20,000. The major equipment required for a simple flood forecasting system is already in I place and operational. The equipment is operated and maintained by the Department of Land and Water Conservation.
The estimated cost of undertaking a flood awareness and public education program is $10,000, with on-going costs of $1,000 per annum.
1 4.2.4 Implementation
All aspects of a flood warning system can be implemented at an early stage, subject to the availability of funding.
4.3 FLOOD ACCESS ROADWAY FROM BAKER STREET
4.3.1 Description i At present, the only land evacuation route from the Baker Street area is Baker Street which runs underneath the Main Northern Railway towards Stingaree Point Drive. The low point in the I evacuation route along Baker Street is about 0.7m AHD under the railway bridge. This point is inundated during the early stages of a flood, and the depth of inundation would reach about 1.4m during a 5% AEP flood, and about 1.9m during a 1% AEP flood. This location presents I an extremely high hazard condition for evacuation of people and their possessions during floods.
I The evacuation conditions would be improved if an access road was provided along the western side of the railway line, thereby eliminating the need to travel underneath the railway bridge during floods. The flood access road would extend south from Baker Street for about 800m to I Moira Park Road. The road would be flood free in the 1% AEP event at its southern end.
The road could be constructed adjacent to the railway embankment on land owned by the State Rail Authority (SRA), subject to approval by the SRA. Because of space constraints, there might also be a need to extend on to adjoining private property, in which case land acquisition
1 19 I
I would be required. Detailed survey and investigation would be required to establish the best road alignment.
I The access road would not be part of the public road system but would be maintained in condition suitable for use during flood events.
1 4.3.2 Benefits
The primary benefit of the access road would be the increase in available evacuation time I during the early stages of a major flood event. This increased evacuation time will facilitate removal of possessions and reduce the risk to life and limb during evacuation.
I It should be noted that Baker Street and the northern end of the proposed access road would still be subject to high hazard conditions during the later stages of a major flood event. I Therefore flood warning is a key requirement to ensure that the limited available evacuation time under low hazard conditions is utilised effectively.
I 4.3.3 Costing
The estimated cost of construction of an access road with gravel surface is $200,000. This estimate includes survey, investigation and design, as well as a small allowance for land acquisition.
I 4.3.4 Implementation
Council should commence discussions with the SRA regarding the possible use of railway land I for the access road. Survey and investigation should also be initiated to determine the total land requirements for the road and the need, if any, for use of private property. The road could be I constructed after completion of negotiations for land acquisition. 4.4 DIVERSION LEVEE UPSTREAM OF KALANG ROAD
' 4.4.1 Description
The Kalang Road area is subject to overbank flooding from Dora Creek. There are 42 properties along Kalang Road, with houses constructed on most properties. Many of these houses would be inundated above floor level in the 1% AEP event, and it is estimated that I average velocities of up to 1.2 m/s would occur between houses. Thus, under existing conditions, the Kalang Road area is a high hazard floodway where the residents will experience I considerable risk to life and limb during floods. A diversion levee constructed upstream of Kalang Road would modify the flood flow behaviour such that the average velocities between houses would be reduced to about 0.2 m/s. I The Kalang Road area would effectively become a flood storage area with reduced hazard.
The levee would be constructed as an earth embankment with grass cover, and reno mattress protection in high velocity areas. The levee would be about 300 metres long, with a 3 metre I 1 20 I
wide crest and side slopes no steeper than 1 in 4 (vertical to horizontal) to facilitate bank I maintenance. The proposed crest level is 3.2m AHD, which is the 1% AEP flood level.
A levee at this location should not have adverse visual impacts. A drainage system would be constructed through the levee, to allow the area behind the levee to drain.
1 4.4.2 Benefits
The main benefit of the diversion levee will be the removal of floodway conditions through existing development on Kalang Road. The flood hazard in the area will be reduced. The levee, together with an effective flood warning system, will greatly reduce the risk to residents in evacuation to higher ground during major floods. With the lower flood velocities, the risk of I structural damage to lightly framed houses will also be reduced.
The levee will not reduce flood depths along Kalang Road. Thus, for a 1% AEP flood event, I the maximum flood depth will remain at approximately 1.5 to 2m. There will be increases in I flood level upstream of the levee, estimated to be in the range of 0.05 to 0.1m. 4.4.3 Costing
I The estimated construction cost of the diversion levee is $600,000, including 15% for survey, investigation and design and 20% for contingencies.
I It is expected that the preferred levee alignment will require acquisition of land on three (3) private properties. The total land area to be acquired may approach 2ha, at an estimated cost of I $100,000. 4.4.4 Implementation
I Community consultation should be undertaken at an early stage to address any social and environmental issues and to ensure that the levee is acceptable to local residents. Negotiations I should then be commenced with the relevant landowners regarding acquisition of the land on which the levee will be constructed. I Following completion of the works, residents of Kalang Road should be made aware at regular intervals of the on-going risk of inundation to considerable depths.
I 4.5 VOLUNTARY HOUSE RAISING SCHEME 1 4.5.1 Description House raising involves elevating existing residences such that habitable floor levels conform with Council's flood standard. There are about 90 houses in the lower Dora Creek floodplain I with floor levels below the 1% AEP flood level and which may be suitable for raising. These houses are generally of timber frame construction with fibro-cement or timber cladding, and are I located east of the railway line. I 1 21 I
On 26 July, 1990, Council adopted a resolution to provide assistance for house raising in flood liable areas to a maximum of $8,000, to be indexed to the CPI from 1991 onwards. Some property owners in Dora Creek have sought this assistance from Council. Future assistance I under this scheme will be subject to the availability of funding. 1 4.5.2 Benefits There will be substantial social and economic benefits of voluntary house raising accruing from I the reduced incidence of over-floor flooding. 4.5.3 Costing I The estimated cost of raising a fibro-cement or weatherboard house at Dora Creek is $12,000. The estimated total cost of raising all suitable houses in the study area is $1,080,000.
It may be appropriate for the Council and State Government to offer a subsidy of up to $10,000 towards the cost incurred by the owners in raising the floor level to not lower than the level adopted for new buildings. Such subsidy payments by Council to homeowners will be subject to formal acceptance by the homeowner of Council conditions set out in the Floodplain I Management Plan. 4.5.4 Implementation I To develop a voluntary house raising scheme, Council should:
. Prepare an inventory of houses with floor levels below the 1% AEP flood level and which I would be of suitable construction to allow floor raising.
Establish specific rules and conditions for applicants to qualify for financial assistance to raise their houses. I The conditions should include prevention of the use of the underfloor area, except for parking, in order to limit potential flood damages.
I There could be some objections to the house raising option by residents who feel that it would change the character of their houses or reduce the visual amenity of the area. It will be I important to exercise some flexibility with regard to these matters. 4.6 PLANNING AND DEVELOPMENT CONTROLS I It is recommended that Council: I • Consolidate various provisions relating to plaing and control of development control on flood liable land (prepare a Development Control Plan for Flood Liable land in Dora Creek); . Formally adopt floodways; I Ensure that planning and development control provisions are consistent; and Amend LEP 1984 to give effect to objectives for flood liable land. I 1 22 P1
4.6.1 Description
There are presently a number of provisions relating to the planning and control of development on flood liable land. These are contained in various documents, held by different sections of Council. These provisions could be consolidated into a DCP for flood liable land at Dora Creek. The existing Interim Floodplain Management Plan for Dora Creek would form a useful I basis for preparing the DCP.
Floodways to be adopted for planning purposes include the following: I 1. The floodways on the north side of Dora Creek through the Minnie Street area and adjacent to the Main Northern Railway embankment to Muddy Lake I The floodway on the north side of Dora Creek across Dora Street to Lake Eraring, approximately 1.1 km east of the railway I The floodway on the south side of Dora Creek across Stingaree Point Road to Bonnells Bay, approximately 1.0km east of the railway 4. The floodway on the south side of Dora Creek across Stingaree Point Road to Bonnells Bay, 1 approximately 1.6km east of the railway
The high hazard area adjacent to Kalang Road should also be adopted and maintained as a floodway until the proposed diversion levee to reduce flow velocities in this area is implemented.
I Appropriate controls should be applied to all approved residential development in flood affected areas, in a similar manner to the controls currently applying to dual occupancy I development. Council's LEP 1984 should be reviewed and amended as necessary for consistency with I Council policy relating to flood liable land; in particular, prohibiting dual occupancy and multiple dwelling development in high hazard areas or floodways, and prohibiting further subdivision of flood prone land west of the railway line. Council should maintain its existing I subdivision policy in developed areas to ensure that sufficient clear space is retained between buildings for the conveyance of overbank flows during flood events.
I On adoption of this Plan, Council's Section 149 Certificate format will be updated to refer to the Plan.
I 4.6.2 Benefits
The benefits of consolidating various documents relating to flood liable land are twofold. I Firstly, there will be a reduced burden to Council in administering provisions relating to flooding; and, secondly, it will be considerably easier for the public to understand the range of planning and development controls, if these are consolidated into a single document; ie a DCP. I This DCP could be amended to include provisions relating to other areas in Lake Macquarie, as I similar information is developed through subsequent floodplain management plans. The benefits of updating floodway provisions will be to provide greater certainty about I constraints applying to certain land. 1 23 I
I The benefits of applying appropriate controls to all residential development in the floodplain will be to minimise the increase in potential flood damage and risk to both existing and new I residents. Amending LEP 1984 will give statutory force to Council policy on subdivision and preferred land usage in flood affected areas. In addition, removing inconsistencies between planning and r- development control provisions will enable Council to implement floodplain management policy which would reduce the risk of legal challenge.
I Property owners will be made aware of the latest information relating to development of I floodprone land in Dora Creek. 4.6.3 Costing I There would be no cost involved in implementing recommended planning and development control measures.
4.6.4 Implementation
The recommended planning and development control measures can be implemented by F Council. In the case of proposed amendments to LEP 1984, final approval is subject to the Minister for Urban Affairs and Planning.
LI Community acceptance of this Floodplain Management Plan will be an important prerequisite to preparation of a Development Control Plan. In particular, the community should recognise that continued development on the Dora Creek floodplain will increase the overall level of I flood risk. Existing residents will experience increased risk associated with flood level increases (albeit small increases) due to further development. New residents will experience L~ risk including potential damage to low lying property and risk associated with evacuation during major floods. The risk associated with floods greater than the design standard, ie greater I than the 1% AEP flood, should also be recognised. 4.7 CHANNEL IMPROVEMENTS TO DORA CREEK
I Long term siltation of the Dora Creek channel has the potential to reduce the carrying capacity of the creek and lead to increased flood levels. Sounding surveys of the creek should be continued in order to assess the medium to long term trend of sediment movement. If the I surveys indicate that siltation is progressing to the extent that significant flood level increases may occur compared with existing conditions, a detailed study of sediment transport processes I and remedial options will be appropriate. One option to be considered in such a study is channel improvements to Dora Creek. This option would involve removal of accumulated sediment in order to increase the waterway area. I The dredged area would be subject to infilling over time by material transported from the upper reaches of the creek. Hence the dredging would probably have to be repeated from time to time I if the benefits were to be maintained in the long term. I 1 24 I Dredging may have a very high cost in relation to the benefits obtained. For example, dredging of the lower Dora Creek area was assessed in the i 992 hydraulic analysis of flood mitigation options. Dredging of 190,000 m3 of material at an estimated cost of $2.9 million would reduce I 1% AEP flood levels by up to 0.12 metres between Kalang Road and the railway line, and by up to 0.20 metres downstream of the railway line. This benefit would be lost progressively I over about 20 years as infilling of the dredged area occurred. No statement of the extent, timing and cost of dredging of Dora Creek has been included in this Floodplain Management Plan. Decisions on the need for and scope of dredging should be I based on information obtained from future sounding surveys of the creek together with a detailed sediment transport study.
I I I I P1 I IF I I I I I I 1 25 I I 5. IMPLEMENTATION OF PLAN I The Dora Creek Floodplain Management Plan should be implemented as shown in Table 3 I Table 3 I Program to Implement the Dora Creek Floodplain Management Plan MILESTONE COST I YEAR I YEAR I YEAR I YEAR YEAR I 1 2 3 4 5 Flood Forecasting System - Feasibility study 20,000
I Flood Awareness and Public Education Program 10,000 _...... Annual costs will be about $1,000
I Flood access roadway from Baker Street 200,000
Diversion levee upstream of Kalang Road 700,000 1 Annual maintenance costs will be about $2,000 Voluntary house raising 1,080,000
I Consolidate planning & development control provisions - 1 Amend floodway designations on map Amend S149 Planning Certificates
I Amend Local Environmental Plan - I Prepare Development Control Plan for Flooding - Total Capital Costs 2,010,000 Total Annual Costs 3,000 I Estimated Annual Expenditure ($,000) 220 1 570 1 360 1 110 1 110 Note: For the purpose of estimation of aimual expenditure, it is assumed that voluntary house I raising will take place over a period of 10 years.
I Financial assistance to fund the above program can be sought from the State and Commonwealth Governments, except for planning and development control measures which I are strictly within the jurisdiction of Council. I I 26 I I
6. ACKNOWLEDGMENTS I
This study was jointly funded on a 2:1 shared arrangement between the New South Wales I Government and Lake Macquarie City Council.
This study has been prepared by the Sustainable Land and Coastal Management Division of the I Department of Land and Water Conservation in association with Lake Macquarie City Council and Council's Floodplain Management Committee.
I I I I I I I I I I I I I
1 27 I I APPENDIX A - CATCHMENT DESCRIPTION I Dora Creek is located on the western side of Lake Macquarie, approximately 120 km north of Sydney and 30 km south-west of Newcastle. It has a catchment area of 196 km2, and is the I largest tributary of Lake Macquarie which has a total catchment of 690 km2. The location of the Dora Creek catchment is shown in Figure 1.
The Dora Creek catchment is fan shaped and has three main streams. Dora Creek extends from the western extremity of the catchment, and is joined by Jigadee Creek and then Stockton I Creek, before draining to Lake Macquarie. The Sugarloaf Range forms the western boundary of the catchment, reaching a maximum I height of about 460 metres above sea level, with slopes of up to 40% occurring in some parts of the upper catchment. However, the predominant topography is gently undulating terrain, generally under 50 metres in height. The surface rocks and soils are composed of sandstone, I shales and conglomerates.
Watercourses above the tidal limit are meandering and small. Below the tidal limit the creeks 1 become broader, with low lying land adjacent to the creek banks. Around the township of Dora Creek the alluvial flats vary from lake level to about 2 metres above lake level, and the creek I has formed a digitate delta into Lake Macquarie. ' The catchment area is mainly timbered, with some areas of rural and urban development, particularly in the east.
Parts of the township of Dora Creek are flood liable, and the severity of flooding depends I largely on the intensity and duration of rainfall over the upstream catchment. A range of storm durations was tested for the 1% AEP event in the Dora Creek catchment, using rainfall data obtained from Australian Rainfall and Runoff (The Institution of Engineers, Australia, 1987).
The Dora Creek catchment has been identified as a high flash flood risk catchment with a critical storm duration of 12 hours (NSW Flood Warning Consultative Committee "Sydney- 1 Newcastle-Wollongong Flash Flood Warning System", May 1992). However flooding may also result from shorter duration storms, particularly if heavy rainfall is concentrated in the I lower part of the catchment. I I I I 1 28 I
I FIGURES I I I I I I I I I I I
I I I I H — — — — — — — — — — — — — — — — — — — —
NEW SOUTH WALES
LAKE MACQUARIE CATCHMENT
91
Toronto Belmont DORA CREEK North
SUB CATCHMENT SI SI cJ)LI 2 Lal 1• ------Martins Sc I ' STUDY cl— ( / AREA ,
Don
Avondste
or
Mandalong / // I —) '9 I. -S im
Wyeo
0 1 2 3 4 5 6 7 8 9 10 LOCALITY PLAN SCALE IN KILOMETRES Figure 1 ------
DORA CREEK FLOOD PROFILES
Figure 2
------
. FlOOd Storage FLOODWAY 1 FLOO(PWAV Council designated floodway to be retained
NOTES DORA These areas are oniy indicative and are based on ground levels estimated from orthophototsaps.
CREEK Delineation of potential flood fringe areas is sabiect to detailed study.
ERA RING
CRFFK.. ...
FLOODWAY 4
FLOODWAY 3 BONNELLS BAY
MORISSET
06 1.0km
SCALE
1% PROBABILITY FLOOD
\ FLOODWAY AND FLOOD STORAGE AREAS FOR EXISTING CONDITIONS
Figure 4 DORA CREEK FLOODPLAIN MANAGEMENT PLAN
Key Plan
Sheets 1 to 5 4
MUDDY
MEN
SHEET 5 BONNELLS mom
B A Y
0 0.2 0.4 0.6 0.8 1km I I I I SCALE
DORA CREEK FLOODPLAIN MANAGEMENT PLAN Key Plan
N I LEGEND: IccI 1% AEP Flood Contour (m AHD) I —'43--. // Floodway (High Hazard) /W/ I Flood Storage (High Hazard) NOTES: Hydraulic and hazard categories are for 1% AEP flood I under existing conditions. Limit of inundation for 1% AEP flood is estimated from I orthophoto mapping and is approximate. I 0 100 200 300 400 500metres SCALE I P'714jbe -W 1 I I I I I I I I I I I I 1 N
0 100 200 300 400 500mtres I I SCALE
7j;- j
NOTES: LEGEND: Cur (m AHD)
::; N eed Flood Storage (High Hazard) orthophoto mapping and is approximate F DORA CREEK FLOODPLAINL\\ MANAGEIV1ENT N PLAN I l Sheet2
- iJL. dk \, \ N SHORT STREET \ ——_—/
/L : L A K E TJIL iIL // / WETLAND/ -- //// / L I / ---/ ---- - —- 4tj / hft0T / / / JIL -\ LEGEND: 1% AEP Flood Contour(m AHD) p,p"*"q 0""' Floodway (High Hazard) Flood Storage (High Hazard) Flood Storage (Low Hazard) LA_4 WETL SEPP 14 Wetlands (approximate) -- / / F - - - - 0 100 200 300 400metres
SCALE I
'N'OTES LIL 881 1. Hydraulic and hazard categories are for 1% AEP flood - under existing conditions. - 2. Low hazard category for flood storage area east of railway is conditional on maintenance of an effective flood -- -. -- - warning and evacuation system. High hazard conditions -Alb- -- may occur in localised areas due to the depth and - : velocity of floodwaters. 3. Limit of inundation for 1% AEP flood is estimated from - 1L orthophoto mapping and is approximate.
IL
I .T :
I STREE I I * /k I 1) 4WET II \ — W14N- DORA CREEK FLOODF AN MANAGEMENT PLAN 1/ /4NI.-_.._i_.. n8 -.\\ 7 SfO8 - 1/ Sheet 3 I
I j L A K / N A I I , ERARING
40
I I / STREET I I / I / I CREE,(
l I zdL --'- - -
I I I / -- -- --- --- -- AL / I / /
ALI I - No. - I AL I Al' 886L '- T 0 100 200 300 400metres AL I SCALE
WELAD - B 0 N N E L L S - ------. ----\ LEGEND: - / --- --- - -No-- --\ I 1% AEP Flood Contour (m AHD)
: Floodway (High Hazard)
1 CO ~IL : :: ::: -.--= EE I Tjj] SEPP 14 Wetlands (approximate) 800NAL NOTES: B A Y I Hydraulic and hazard categories are for 1% AEP flood under existing conditions. RO.4b Low hazard category for flood storage area east of railway is conditional on maintenance of an effective flood warning and excavation system. High hazard conditions may occur in localised areas due to the depth and velocity of floodwaters. DORA CREEK FLOODPLAIN IV1AN GEIVIENT PLAN I Limit of inundation for 1% AEP flood is estimated from orthophoto mapping and is approximate. eet 4 I I I I I I I I I / I
I / M09 DOM I :\ \f I 0 100 200 300 400 500metres SCALE
I LEGEND: DORA 1% AEP Flood Contour (m AHD) Floodway (High Hazard) I 1NGAE - / L Flood Storage (Low Hazard) I SEPP 14 Wetlands (approximate) I Hydraulic and hazard categories are for 1% AEP flood under existing conditions Low hazard categori for flood storage area is conditional on maintenance of an effective flood warning and excavation system. High hazard conditions may I occur in localised areas due to the depth and velocity of floodwaters. 1% AEP flood level of 1.38m AHD near Dora Creek outlet is due to backwater flooding from Lake Macquarie. Higher levels may occur due to wave action. I BONNELLS I Goat Island I