smiths lake planning study volume 1: text - Great Lakes Council

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FLOODING AND DRAINAGE 28 Based on Australian Rainfall and Runoff (IEAust, 1987), the design rainfall intensities for the SLA and SLVA are listed in Table 3-1 below. Average Table 3-1 Design Rainfall Intensities (mm/h) Duration Recurrence Interval 1 hour 12 hour 72 hour 2 years 40.0 8.0 2.5 50 years 76.0 13.6 4.6 Regional skew is listed as 0.01 and F2 and F50 values are listed as 4.32 and 16.15 respectively. Willing and Partners (1996) states that DLWC and NSW Department of Works and Services (through Manly Hydraulics Laboratory, MHL) do not operate any gauging stations or have any data on discharge of flood levels relating to the Smiths Lake catchment. 3.1.3 Groundwater Characteristics Groundwater within the study area occurs in aquifer systems within either fractured rock or unconsolidated sediments. The Department of Land and Water Conservation has indicated that there are no registered groundwater sources or bores within the study area. The Carboniferous Age rocks have been subject to considerable folding and faulting and this has resulted in a network of joints and fractures that can store and transmit water. Groundwater generally infiltrates fractures near the surface in topographically high areas and drains through the fractures and joints until it discharges into the surface drainage system. Groundwater quality within fractured rock aquifers is likely to be variable with higher salinity values in areas where the sedimentary rocks have a marine origin such as the Yagon Siltstone. Areas of unconsolidated sediment within the study area comprise: • alluvial sediments (Unit A) along drainage paths. These areas are characterised by clayey sediments and as such groundwater availability is likely to be very low. • Estuarine (Unit E) sandy and muddy sediments in low lying coastal backdune areas. Groundwater availability and quality in these areas can vary considerably due to the complex depositional history and the interaction between marine and fluviatile environments. In general, groundwater occurs as lenses of fresh water overlying denser brackish or saline waters with recharge from rainfall. Groundwater availability in these areas is likely to be restricted to low yield non potable (drinking) supplies. • Aeolian (wind blown) coastal sand beds and dunes (Unit S). These sediments are usually characterised by low salinity, high yield and potable groundwater reserves with recharge from rainfall. The extent of clean sand beds within the study area is restricted to sandhills at Sandbar. A potential constraint on groundwater usage in the Sandbar area would be the potential for salt water intrusion with over pumping. An assessment of groundwater quality is presented in Section 2.3.7. SMITHS_LAKE_PLANNING_STUDY.DOC O C E A N I C S A U S T R A L I A
FLOODING AND DRAINAGE 29 3.2 Flooding and Drainage Issues Flooding and drainage issues in the SLA and SLVA is dominated by three types of flooding resulting in inundation: • elevated ocean levels; • creek flooding; • combination of both of these. These issues are discussed further below. 3.2.1 Elevated Ocean Levels Elevated ocean levels have the potential to result on rises in Smiths Lake water levels due to sea water flowing from the sea to Smiths Lake via the entrance (either open or closed). These elevated ocean levels can be the result of any one or a combination of the following natural causes, briefly described below: • Wind Set-up: When strong wind blows over an open body of water (eg. during a cyclone as wind blows from the east over the ocean), drag forces result in a transportation of water towards the coast causing a rise in water level. • Inverse Barometric Effect: Cyclones and other low pressure systems are accompanied by a drop in atmospheric pressure resulting in a flow of water from high pressure areas to the areas experiencing low pressure. When combined with wind se-up, this effect is called Storm Surge; • Wave Set-up: Wave set-up occurs shoreward of the breaker zone resulting in a concave upward shape of the water surface. At the shore this causes a rise in water level. • High Tides: Normal tidal variations result in two high tides per day. When these coincide with the other phenomena described here, then the peak water levels can occur; • Eustatic Changes (ie. The Greenhouse Effect): Increasing concentrations within the earth’s atmosphere of various gases, largely derived from the burning of fossil fuels, are trapping solar radiation. The resulting global warming (enhanced Greenhouse effect) has the potential to change weather systems, rainfall patterns, wind velocities and, significantly, cause mean sea level to rise. CSIRO (1998) states that sea level rise by 2050 is expected to be between 10cm and 40cm, with a ‘best guess’ of 20cm. It is a large and complex study to consider the effects of all of these factors to result in a peak ocean elevated level for planning purposes. A study of this size is beyond the scope of this study. However, estimates can be drawn from nearby studies of a similar nature. In 1989 AWACS assessed the elevated ocean levels at the two Manning River entrances at Harrington and Old Bar (Farquhar Inlet). Given that the latter entrance is similar in nature to the Smiths Lake entrance (ie. shallow and sometimes closed) as well as only 50km to the north of the study area, it is reasonable to assume that the elevated ocean levels will be similar. SMITHS_LAKE_PLANNING_STUDY.DOC O C E A N I C S A U S T R A L I A
Page 1 and 2: SMITHS LAKE PLANNING STUDY VOLUME 1
Page 3 and 4: CONTENTS I CONTENTS 1 INTRODUCTION
Page 5 and 6: CONTENTS III 11.1 Sewer 128 11.2 Ot
Page 7 and 8: LIST OF FIGURES V Figure 8-1 Visual
Page 9 and 10: INTRODUCTION 1 1 INTRODUCTION 1.1 G
Page 11 and 12: INTRODUCTION 3 • visual quality;
Page 13 and 14: SOILS AND GEOTECHNICAL 5 2.2.2 Fiel
Page 15 and 16: SOILS AND GEOTECHNICAL 7 2.3.3 Geol
Page 17 and 18: SOILS AND GEOTECHNICAL 9 Table 2-3
Page 19 and 20: SOILS AND GEOTECHNICAL 11 rather th
Page 21 and 22: SOILS AND GEOTECHNICAL 13 soils wit
Page 23 and 24: SOILS AND GEOTECHNICAL 15 The resul
Page 25 and 26: SOILS AND GEOTECHNICAL 17 • RCA11
Page 27 and 28: SOILS AND GEOTECHNICAL 19 to steepe
Page 29 and 30: SOILS AND GEOTECHNICAL 21 Based on
Page 31 and 32: SOILS AND GEOTECHNICAL 23 • Mediu
Page 33 and 34: SOILS AND GEOTECHNICAL 25 There are
Page 35: FLOODING AND DRAINAGE 27 3 FLOODING
Page 39 and 40: FLOODING AND DRAINAGE 31 3.2.3 Floo
Page 41 and 42: FLOODING AND DRAINAGE 33 the perime
Page 43 and 44: STORMWATER MANAGEMENT 35 This study
Page 45 and 46: STORMWATER MANAGEMENT 37 Figure 4-4
Page 47 and 48: STORMWATER MANAGEMENT 39 4.4.7 Sedi
Page 49 and 50: STORMWATER MANAGEMENT 41 4.5.4 Swal
Page 51 and 52: STORMWATER MANAGEMENT 43 Table 4-1
Page 53 and 54: FLORA 45 characteristics. Some vege
Page 55 and 56: FLORA 47 mahogany (Eucalyptus robus
Page 57 and 58: FLORA 49 Distribution: This forest
Page 59 and 60: FLORA 51 Distribution: This forest
Page 61 and 62: FLORA 53 5.2.23 Scrub (224) Structu
Page 63 and 64: FLORA 55 Species Significance & Dis
Page 65 and 66: FLORA 57 Species Significance & Dis
Page 67 and 68: FLORA 59 5.4.7 Swamp Oak (32) Distr
Page 69 and 70: FLORA 61 5.4.17 Spotted Gum (70) Di
Page 71 and 72: FLORA 63 Forest Type Description Co
Page 73 and 74: FAUNA 65 6.1.4 Swamp Oak (32) Habit
Page 75 and 76: FAUNA 67 heathland shrub layer woul
Page 77 and 78: FAUNA 69 of the tree adjacent to th
Page 79 and 80: FAUNA 71 Common Name Scientific Nam
Page 81 and 82: FAUNA 73 Common Name Scientific Nam
Page 83 and 84: FAUNA 75 6.4.1.1 Community Habitat
Page 85 and 86: FAUNA 77 6.4.2 Problems with the Me
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FAUNA 79 6.6.3 Development of Mediu
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WATER QUALITY SENSITIVITY MAPPING 8
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VISUAL AND SCENIC QUALITY 87 Rural
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VISUAL AND SCENIC QUALITY 89 • mi
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VISUAL AND SCENIC QUALITY 91 8.3.1
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VISUAL AND SCENIC QUALITY 93 Sensit
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LAND CAPABILITY ASSESSMENT 95 9 LAN
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LAND CAPABILITY ASSESSMENT 97 Time
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LAND CAPABILITY ASSESSMENT 99 Envir
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LAND CAPABILITY ASSESSMENT 101 Envi
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ABORIGINAL AND EUROPEAN HERITAGE 10
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INFRASTRUCTURE 129 11.2 Other Servi
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BUSHFIRE MANAGEMENT INVESTIGATION 1
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LAND SUITABILITY ASSESSMENT 139 13
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LAND SUITABILITY ASSESSMENT 141 lin
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LAND SUITABILITY ASSESSMENT 143 13.
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LAND SUITABILITY ASSESSMENT 145 The
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LAND SUITABILITY ASSESSMENT 147 13.
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LAND SUITABILITY ASSESSMENT 149 The
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REFERENCES 151 Floyd, A.G. (1990)
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