Tugun Bypass Environmental Impact Statement Technical Paper Number 7 Flooding and Hydrological Assessment Tugun Bypass Alliance Offices Client: Queensland Department of Main Roads Brisbane Prepared By: WBM Oceanics Australia Denver Karratha Melbourne Prepared For: Parsons Brinckerhoff Morwell (formerly PPK Environment & Infrastructure) Newcastle Sydney Vancouver July, 2003 OCEANICS AUSTRALIA November 2004 2134070A-PR049Car- R.B13010.001.05 Tugun Bypass Environmental Impact Statement Technical Paper Number 7 Flooding and Hydrological Assessment Contents Page Number Glossary I 1. Introduction 1-1 1.1Summary of the Technical Paper 1-1 1.2Reporting of Study Findings in the EIS 1-2 2. Purpose and Approach 2-1 2.1General 2-1 2.2Design Criteria and Required Flood Immunity 2-1 3. Existing Flooding Conditions 3-1 3.1Local/Minor Catchments 3-1 3.1.1 Chainage 00 to 2,300 3-1 3.1.2 Chainage 2,800 3-1 3.1.3 Chainage 3,200 3-7 3.1.4 Chainage 3,900 3-7 3.1.5 Chainage 4,900 3-7 3.1.6 Chainage 5,450 to 6,000 3-7 3.2Tweed River Floodplain 3-9 3.2.1 Previous Studies by WBM on Tweed River 3-9 3.2.2 Tweed River Flood Model 3-9 3.2.3 Tweed River Flooding Characteristics 3-9 3.2.4 Peak Flood Levels 3-13 3.2.5 Influence of Elevated Ocean Levels 3-13 3.2.6 Peak Flood Levels for Rare Flood Events 3-15 3.2.7 Summary of Frequency of Inundation of Study Area 3-16 3.2.8 Inundation Times 3-16 3.3Coolangatta Creek 3-17 4. Potential Impacts of the Proposal 4-1 4.1Impacts on Local / Minor Catchments 4-1 4.1.1 Chainage 00 to 2,300 4-1 4.1.2 Chainage 2,800 4-1 4.1.3 Chainage 3,200 4-3 4.1.4 Chainage 3,900 4-3 4.1.5 Chainage 4,900 4-3 4.1.6 Chainage 5,450 to 6,000 4-5 4.2Impacts on Tweed River Floodplain 4-5 4.2.1 Impacts on Peak Flood Levels 4-5 4.2.2 Impacts of Inundation of Houses / Property 4-6 4.2.3 Impacts on Times of Inundation 4-10 4.2.4 Impacts to Rates of Flood Rise and Recession 4-10 4.3Impacts on Coolangatta Creek Flooding 4-11 5. Cumulative Impacts 5-1 5.1Development in the Immediate Area 5-1 5.2Other Developments on Tweed River Floodplain 5-1 6. Mitigation Measures 6-1 References Queensland Department of Main Roads i Tugun Bypass Environmental Impact Statement Technical Paper Number 7 Flooding and Hydrological Assessment Contents (continued) Page Number List of Tables Table 3.1: Peak Flows for Minor Catchments 3-1 Table 3.2: Anticipated Future Sea Level Rise (m), Relative to 1990 3-15 Table 3.3: Existing Times of Inundation for Tweed River Flooding 3-17 Table 4.1: Peak Flows and Structures for Minor Catchments 4-1 Table 4.2: Loss of Floodplain Storage (Tweed River) 4-6 Table 6.1: Summary of Waterway Structures and Drain Re-Routing 6-1 List of Figures Figure 3.1: Hydraulic Concept Plan (Chainage 0 – 2,000) 3-2 Figure 3.2: Hydraulic Concept Plan (Chainage 2,000 – 4,000) 3-3 Figure 3.3: Hydraulic Concept Plan (Chainage 4,000 – 6,000) 3-4 Figure 3.4: Hydraulic Concept Plan (Chainage 6,000 – 7,700) 3-5 Figure 3.5: Existing 100 Year ARI Flood Levels - Pacific Beach Catchment (Chainage 2,800) 3-6 Figure 3.6: Existing 100 Year ARI Flood Levels - Catchments at Chainages 3,200, 3,900 and 4,900 3-8 Figure 3.7: Tweed River 1D Model Verification for March 1974 Flood Event 3-10 Figure 3.8: Tweed River 1D Model Verification for March 1978 Flood Event 3-11 Figure 3.9: Tweed River 1D Model Verification for April 1989 Flood Event 3-12 Figure 3.10: Existing Peak Flood Levels - Tweed River 3-14 Figure 3.11: Inundation Times for Tweed River Flooding 3-17 Figure 4.1: Impacts to 100 Year ARI Flood: Pacific Beach Catchment (Chainage 2,800) 4-2 Figure 4.2: Impacts to 100 Year ARI Flood: Chainages 3,200, 3,900 and 4,900 4-4 Figure 4.3: Impacts on Tweed River Flooding – 100 Year ARI Flood 4-7 Figure 4.4: Impacts on Tweed River Flooding – 20 Year ARI Flood 4-8 Figure 4.5: Impacts on Tweed River Flooding – 5 Year ARI Flood 4-9 Figure 4.6: Impacts to Time of Inundation 4-10 Figure 4.7: Impacts to Rate of Rise and Recession 4-11 Figure 5.1: Cumulative Impacts on Tweed River Flooding with Road, Rail and Runway – 100 Year ARI Flood 5-2 List of Appendices Appendix A Letter from Tweed Shire Council ii Tugun Bypass Environmental Impact Statement Technical Paper Number 7 Flooding and Hydrological Assessment Glossary Term Meaning Afflux The difference in water levels across a waterway structure (e.g. bridge or culvert). Attenuation Reduction of flood peak due to storage effects of the floodplain. Australian Height Datum (AHD) A common national plane of level corresponding approximately to mean sea level. This report generally refers to levels relative to AHD. Average Recurrence Interval (ARI) A means of defining the probability / size of a flood event by ascribing the average period (usually in years) between exceedances of a given flood magnitude. Barometric Setup The increase in mean sea level caused by a drop in barometric pressure. Bathymetry The measurement of depths of water, also information derived from such measurements. Catchment The area draining to a site. It always relates to a particular location and may include the catchments of tributary streams as well as the main stream. Discharge The rate of flow of water measured in terms of volume over time. It is to be distinguished from the speed or velocity of flow which is a measure of how fast the water is moving rather than how much is moving. Flood Relatively high stream flow which overtops the natural or artificial banks in any part of a stream or river. Floodplain The portion of a river valley, adjacent to the river channel, which is covered with water when the river overflows during floods. Greenhouse Effect A term used to describe the likely global warming predicted to accompany the increasing levels of carbon dioxide and other “greenhouse” gases in the atmosphere. Peak discharge The maximum discharge occurring during a flood event. Run-off The amount of rainfall which actually ends up as streamflow, also known as rainfall excess. Storm Surge The increase in coastal water level caused by the effects of storms. Storm surge consists of two components: the increase in water level caused by the reduction in barometric pressure (barometric setup) and the increase in water level caused by the action of wind blowing over the sea surface (wind setup). Time of Inundation Period of time a given area is under water due to flooding. Wave Set-up The increase in water level within the surf zone above mean still water level caused by the breaking action of waves. Queensland Department of Main Roads I Tugun Bypass Environmental Impact Statement Technical Paper Number 7 Flooding and Hydrological Assessment 1. Introduction 1.1 Summary of the Technical Paper This technical paper examines the potential flooding and drainage issues associated with the proposed Tugun transport corridor. The proposed transport corridor crosses Coolangatta Creek and a number of local/minor catchments. Existing flooding and drainage characteristics are provided for each of these. In addition, the proposal is located within the floodplain of the Tweed River. Flooding characteristics of this system are identified including peak flood levels, the influence of elevated ocean levels, peak flood levels for rare flood events, frequency of inundation of the study area and inundation times. Flood impacts on Coolangatta Creek, the local/minor catchments crossed by the proposed route and the Tweed River floodplain have been considered on an individual basis. Impacts were assessed on construction of the transport corridor with immunity from inundation during a 1 in 100 year ARI (Average Recurrence Interval) flood event. The proposal would have a negligible impact on the nature and behaviour of flooding in Coolangatta Creek as there would be little, if any, change to the conveyance characteristics of the overflow channel from Coolangatta Creek to the Cobaki Broadwater. This channel would pass over the proposed road route and be re-instated as an open channel. The channel would be sized to ensure that it has the same flood capacity as in its present state. The primary impact of the proposal on the Tweed River floodplain is the loss of floodplain storage resulting from the filling required for the road embankment. This loss of storage has the potential to result in increased flood levels. The loss of floodplain storage was quantified using the 1D flood model of the entire Tweed River floodplain. The results of the this modelling showed that filling for the proposal would result in increases in flood level of less than 3 mm for the 100, 20 and 5 year ARI flood events. The proposal would have a negligible impact on the time that flood prone land is inundated and the rates of flood rise and recession during Tweed River flood events. This is due largely to the loss of floodplain storage being only a very small proportion of the total floodplain storage on the Tweed River floodplain and to the slow rising behaviour of Tweed River flood events at the downstream end of the floodplain.