Files\Riverstone-East\Water Cycle Management and Flood Analysis Water Cycle Management Report-Riverstone-East_Rev G.docx 10 May 2016 Water Cycle Management Report. This report undertaken by Mott MacDonald (MM) details the procedures used and results obtained from analyzes undertaken in the development of the water cycle management strategy for the Riverstone East area.
The Site
Topography and Geology
The district is located in the South Creek sub-basin of the Hawkesbury-Nepean River and consists of undulating terrain with elevations ranging from 20-80 m AHD. The main ridge runs in a north-south direction through the center of the district, with the two main natural drainage paths running parallel, First Ponds Creek to the west and the southern tributary of Killarney Chain of Ponds to the east.
Developed Layout – Indicative Layout Plan (ILP)
The design of local and collector roads will be guided by the information in this and other preliminary reports and will be documented in the Indicative Plan Plan. A common objective of the Water Cycle Management Strategy and the Infrastructure and Development Plan is to advance the planning process and provide planners with robust engineering information to enable a considered Indicative Planning Plan.
Rainfall Data .1 Rainfall Records
A preliminary road hierarchy plan provided by ARUP has allowed the development of a flood evacuation plan and improved compliance with the principles of water cycle and flood risk management. Under the road hierarchy plan, Garfield Road East will operate as the main east-west road, with Clarke Road and Tallawong Road identified as future sub-arterial roads for north-south travel.
Additional Information used in the Assessment .1 Drainage Information
Cadastre
Creek Categories
Growth Centres Development Code (October 2006)
State Environmental Planning Policy (Sydney Region Growth Centres) 2006 This legislation provides a set of controls on the planning process and on the development of land within
NSW Floodplain Development Manual (April 2005)
Floodplain Risk Management Guideline: Practical Consideration of Climate Change – Department of Environment and Climate Change (2007)
Stream Classifications for the North West Growth Centre
Australian Rainfall and Runoff – Volume 1 (2001)
NSW Department of Environment and Heritage
Managing Urban Stormwater: Environmental Targets
Under its former name, NSWEPA, DECC published Managing Urban Stormwater: Environmental Targets, outlining recommended environmental targets for stormwater. Suspended solids 85% retention of the annual average load 85% retention of the annual average load Total Phosphorus 65% retention of the annual average load 65% retention of the annual average load Total Nitrogen 45% retention of the annual average load 45% retention of the annual average load Source: Managing Urban Stormwater: Environmental Targets; and Blacktown City Council DCP 2006.
Managing Urban Stormwater: Source Control
Managing Urban Stormwater: Soils and Construction
Blacktown City Council (BCC) Control Documents .1 Blacktown City Council DCP 2006
Blacktown City Council Engineering Guide for Development
Blacktown City Council Developer Handbook for Water Sensitive Urban Design Council’s Developer Handbook for Water Sensitive Urban Design sets out their requirements for the design
Blacktown City Council Growth Centre Precincts DCP 2010
Water Sensitive Urban Design and Flooding – Riverstone and Alex Avenue Precincts, GHD 2008 and Post Exhibition Report 2010
The integrated Riverstone and Riverstone East analysis took into account the Riverstone East development potential and draft basin strategies were developed, but this would be reconsidered in more detail during the preparation of the Riverstone East ILP. Finally, the potential impact of climate change was not originally considered in the 2008 report due to the preliminary nature of the study.
Water Cycle Management – Box Hill/Box Hill Industrial Precinct, JWP 2011 The Box Hill and Box Hill Industrial precincts border Riverstone East to the north east along Windsor road
Water Cycle Management Strategy & Flood Study – Area 20 Precinct, JWP 2010
Design storms were applied to these models to provide estimates of 500-year ARI discharges as well as maximum probable precipitation (PMP) discharge, which are discussed next. Evaluation of these models then allowed the sizing and configuration of proposed detention basins and documentation of their requirements.
XPRAFTS
Parameters
Aerial photographs provided information on current land use for runoff modeling in the existing scenario. GIS information on the extent of the existing Rouse Hill Regional Park and the extent of the proposed adjacent dedicated green space has been provided by the Department of Planning and Environment. In the existing scenario, a Manning's roughness parameter of 0.04 was adopted for the former and 0.025 for the impervious portion component of the entire 'undeveloped' soil.
The existing and proposed models both adopted a default 'b' value of 1.0 and no further calibration was considered necessary based on comparisons with other approved models (see section 5.4).
Existing Scenario
Since both the Riverstone and Alex Avenue Precincts pass through the Riverstone East Precinct, they have been considered part of the assessment. The Alex Avenue district has been assessed as an existing worst case as any development should have no worse or better runoff flow. "Dummy Nodes" were used where two or more existing sub-basins converged, allowing assessment of both inflow and outflow hydrographs.
External catchments, which are channeled through the site, are included as part of the assessment.
Model Calibration
Here, each node is representative of the catchment and is divided into both permeable and impervious values (see Appendix B). "Layer connections" were used as connections between the nodes and were modeled to give the travel time (in minutes) for the peak flow to travel the length of this reach. Existing dams are assumed to be filled so as not to cause streamflow attenuation.
Existing Model Verification .1 Previous Studies
Modeled flow rates were also compared to calculated flow rates determined using the rational method to confirm that they were within the expected range. The rational method is the most widely used empirical technique used for calculating design flow rates within Australia (as recommended in AR&R87). The rational method calculates the peak flow rate corresponding to the specific time of concentration for the catchment.
The above shows that the currents run across the three studies; along with the Rational Method control, vary across the site, but generally align to the primary outlet with a difference of about 5%.
Developed Scenario
Proposed Development Assumptions
For example, the following detention basins from the GHD study have been included in the MM model: F16 (Online detention basin); F25; F28 (Online Detention Pool); F32; and F34. Since Alex Avenue is separated from the Riverstone and Riverstone East Precincts by Schofields Road, it has an independent impact on First Ponds Creek. The graph below shows the hydrograph of the MM-prepared Alex Avenue Precinct under existing conditions, against that of the GHD model under developed conditions.
To accurately account for the Alex Avenue Precinct under the developed scenario, flow hydrographs were exported from the GHD model (node F10a) for all periods and entered into the Mott MacDonald model by replacing the 'existing scenario' nodes (SRBR & SRCU).
100 year, 90 minute storm - Hydrograph Comparison
Management Strategies .1 Major/Minor System
- Detention Basins
- Design Discharges
- Comparison of Existing and Post-Developed Flows
- Probable Maximum Flood
- Climate Change Assessment
Two basins online of First Ponds Creek, previously proposed as part of the Riverstone Precinct study, have been maintained and renovated making a total of seven basins. Since this approval, more detailed analysis of the basins has been undertaken as part of this Riverstone East study. A summary of the proposed detention storage locations for the Riverstone East area is shown in the table below.
However, for the purposes of this report, a sensitivity analysis has been conducted using a 20% increase in the rainfall intensity of the highest 100-year ARI storm event.
Introduction
Existing and Proposed Models .1 TUFLOW Software Package
- Flood Events
- Hydrologic Data
- Digital Terrain Model .1 Survey data
- Boundary Conditions .1 Precinct Catchments
- Hydraulic structures (1D ESTRY component)
- Flood Management Strategies
These hydrographs have been applied to specific locations in the model, as discussed in Section 6.2.5. In the developed scenario, tributary streams entering First Ponds Creek from the Riverstone district are fed into the model with attenuated hydrographs. Losses due to evaporation and infiltration to the soil have been applied in the hydrological model for all catchment areas of the district.
Stage versus stock ratios for these facilities have been replicated in the TUFLOW model as discussed in Section 6.2.4.3.
Results
- Flood maps for design events
- Existing and developed scenario comparison
Detention basins are designed through the hydrological analysis of the developed scenario to dampen developed scenario flows back to existing for events ranging from the 2 - 100 year ARI. The performance of these detention basins can be observed in the attenuated flow rates discussed in the results section below and can be observed on the flood maps for ARI events from 2 to 100 years. The proposed 1st order creek alignments and the effective placement of Basin 1 have improved flood levels across the site, although the site is still not flood-free, there is no worsening for storms through the 100-year event.
As a result of the detention basins, there is generally no worsening flooding for Windsor Rd divers and no greater impact on the regional park.
Channel Stabilization Assessment
The shear stresses have been grouped into the following categories, where a shear stress is recorded in one of these categories, further assessment may be required to confirm that the soil type at that location has a higher shear stress than those recorded. If the recorded shear stress is less than the critical shear stress for the soil type in question, no erosion will occur. If the recorded shear stress is greater than the critical shear stress of the soil type in question, erosion protection may need to be provided at the detailed design stage.
It should be noted that vegetated layers have different shear stress values than those specified below.
Climate Change
Flood Evacuation Strategy
It is recommended that people not attempt to cross the floodwaters in their vehicles and should return to higher ground and follow the procedures of the regional flood evacuation plan. Although clear evacuation strategies have been defined for the district, significant parts of the district remain unaffected by flooding during a PMF event. Evacuation alert when an SES Operations Controller's intent is to alert the community to the need to prepare for a potential evacuation.
A guide to the content of evacuation warnings and order messages is provided in the Blacktown City Local Flood Plan.
Comparison of Modelled Results
The stormwater management systems for the area must meet the requirements of the Growth Center Commission and Blacktown City Council's Development Control Plan. The Council's policy requires improved water quality of the stormwater flow from the developed site before it is discharged into the local stormwater drainage system.
MUSIC Methodology
Model Parameters .1 Rainfall Data
- Catchment Analysis
- Adopted Land Uses
- Pollutant Generation
- Treatment Train
It should be noted that through consultation with BCC and relevant authorities, it has been decided to analyze only four (4) categories in the MUSIC model. Therefore, urban residential areas with medium and high density should be excluded from the study area. The necessary water quality treatment equipment is expected to be incorporated into the on-site development to meet the council's target rates.
In developing the MUSIC model for the proposed scenario, it is our understanding that a rainwater (reuse) tank should be included for each individual lot within the area.
Results
The results at the overall receiving node are evaluated against the BCC pollutant removal targets and show that the proposed treatment train is efficient in achieving the target removal rates, which are shown below.
