SMEC Northern Beaches Hospital Connectivity and Network Enhancements Stage 2 Environmental Impact Statement: Surface Water Assessment -XO\ 2015 GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | i Table of contents 1. Introduction ................................................................................................................ 10 1.1 Introduction ....................................................................................................... 10 1.2 Background....................................................................................................... 10 1.3 Project location .................................................................................................. 11 1.4 Project Description ............................................................................................. 11 1.5 Secretary’s Environmental Assessment Requirements............................................. 12 1.6 Purpose and scope ............................................................................................ 15 2. Regulatory context ...................................................................................................... 17 2.1 Secretary’s Environmental Assessment Requirements............................................. 17 2.2 Water Management Act 2000............................................................................... 17 2.3 Environmental Planning and Assessment Act 1979 ................................................. 18 2.4 Protection of the Environment Operations Act 1997 ................................................. 18 2.5 Policy and guidelines .......................................................................................... 19 3. Methodology .............................................................................................................. 26 3.1 Data collection and field work............................................................................... 26 3.2 Existing conditions and impact assessment ............................................................ 27 3.3 Surface water management strategy ..................................................................... 29 3.4 Assumptions and limitations................................................................................. 31 4. Existing environment ................................................................................................... 33 4.1 Catchment overview ........................................................................................... 33 4.2 Major waterways ................................................................................................ 33 4.3 Ecology ............................................................................................................ 34 4.4 Geology............................................................................................................ 34 4.5 Stormwater drainage network............................................................................... 35 4.6 Erosion, sediment and stormwater quality .............................................................. 40 4.7 Water quantity and stormwater ............................................................................. 45 4.8 Water sources and supply catchments .................................................................. 46 4.9 Groundwater and surface water interactions ........................................................... 46 5. Proposed drainage description ...................................................................................... 48 5.1 Slot drainage ..................................................................................................... 48 5.2 Cross-drainage upgrades .................................................................................... 49 5.3 Detention storages ............................................................................................. 50 5.4 Other network upgrades ...................................................................................... 51 6. Impact assessment ..................................................................................................... 54 6.1 Overview of changes to catchment hydrology ......................................................... 54 6.2 Erosion, sediment and water quality impacts .......................................................... 55 6.3 Water quantity and stormwater impacts ................................................................. 63 6.4 Cumulative impacts on water sources ................................................................... 65 ii | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 6.5 Interaction with the Stage 1 Project and the Northern Beaches Hospital ..................... 69 6.6 Climate change.................................................................................................. 69 7. Surface water management strategy .............................................................................. 71 7.1 Measures to manage erosion, sediment and water quality impacts ............................ 71 7.2 Measures to mitigate water quantity and stormwater impacts .................................... 74 7.3 Cumulative impact management........................................................................... 75 8. References ................................................................................................................ 77 Table index Table 1: SEARs related to the Stage 2 Project Surface Water Assessment ................................... 13 Table 2 WMA 2000 principles relating to the Stage 2 Project ...................................................... 18 Table 3: Stormwater pollution reduction targets as set in the Northern Beaches Stormwater Management Plan (1999) .................................................................................... 20 Table 4: Water quality objectives (Engineers Australia, 1999) ..................................................... 22 Table 5 River flow objectives (Engineers Australia, 1999)........................................................... 23 Table 6: Interpreted geological conditions (eastbound slot location) ............................................ 35 Table 7: Existing stormwater quality treatment devices .............................................................. 37 Table 8: Removal rates for Trash Racks SWS0018 and SWS0041 .............................................. 38 Table 9: Results of 2004 water quality sampling ....................................................................... 45 Table 10: Proposed culvert upgrades ...................................................................................... 50 Table 11 Proposed below-ground detention storages as part of the Stage 2 Project ....................... 51 Table 12 Summary of changes to existing drainage lines under the proposed Stage 2 Project ......... 52 Table 13: Summary of catchment areas under existing and proposed conditions ........................... 55 Table 14 MUSIC model results under existing and proposed conditions ....................................... 60 Table 15: Potential impacts of changes to groundwater seepage flows, dry periods........................ 65 Table 16: Music Modelling Average Flow Outputs .................................................................... 68 Table 17: CSIRO indicative change in rainfall one-day totals (CSIRO, 2007) ................................. 69 Table 18: Fraction impervious assumptions by land use zone ..................................................... 81 Table 19: Rainwater tank properties........................................................................................ 82 Table 20: DRAINS model overland flow paths .......................................................................... 83 Table 21: Antecedent Rainfall Depths ..................................................................................... 84 GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | iii Figure index Figure 1: Stage 1 and Stage 2 Network Enhancement Location .................................................. 16 Figure 2: Study process overview ........................................................................................... 26 Figure 3 Assessment of potential erosion hazard ...................................................................... 28 Figure 4: Sediment trap/trash rack downstream of Currie Road (looking upstream) ........................ 39 Figure 5: Sediment trap/trash rack downstream of Currie Road (looking downstream) .................... 39 Figure 6: Sediment trap/trash rack at Rabbett Reserve (looking downstream) ............................... 39 Figure 7: Sediment trap/trash rack at Rabbett Reserve (looking upstream) ................................... 39 Figure 8: Bioretention system (western)................................................................................... 40 Figure 9: Bioretention system (eastern) ................................................................................... 40 Figure 10: Waterway upstream of Currie Road in Catchment 4 ................................................... 41 Figure 11: Undermining of maintenance access track at the sediment pond and trash rack downstream of Currie Drive in Catchment 4 ..........................................................