Masters Geelong Stormwater Management Plan

Transcription

1 ` Masters Geelong Job Number: CG111748

2 ABN Oxford Street, Collingwood Victoria 3066 Australia Telephone: Facsimile: International: victoria@cardno.com.au Document Control Version Date Description Prepared Reviewed Principal Approval V1 V2 06 March March 2012 Masters Geelong SMP DB YA / MN Masters Geelong SMP DB MN " All Rights Reserved. Copyright in the whole and every part of this document belongs to and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person without the prior written consent of. i

3 Table of Contents 1 Introduction and Background Proposed Development Stormwater Management Stormwater Quantity Management Internal Drainage Downstream Flooding Stormwater Quality Treatment Catchment Delineation of Stormwater Quality Treatment Rainwater Tanks Stormwater Quality Modelling Bio-retention Basin Alternative Biofiltration System Conclusions and Recommendations... 9 List of Tables Table 1: Catchment Properties... 6 Table 2: Water Quality Objectives - Best Practice Guidelines... 7 Table 3: Bio-retention Basin Properties... 8 List of Figures Figure 1: Subject Site... 1 Figure 2: Site Layout... 2 Figure 3: Existing 1 in 100 year Flood extents... 3 Figure 4: Location of Floodway and Existing Retarding Basin... 5 Figure 5: Catchment Plan... 6 Figure 6: Proposed Development MUSIC Model Layout... 8 ii

4 1 Introduction and Background Cardno were engaged by Hydrox Nominees Pty Ltd in February 2012 to undertake a stormwater management plan for the proposed Masters store at Corio, Geelong. The location of the site is shown in Figure 1 and extends approximately 5.33Ha in area and is bound by the Geelong Ring Road to the north, Bacchus Marsh Road to the west and open space to the south and east. The site to the south, 240 Bacchus Marsh Road is also proposed to be developed shortly. Figure 1: Subject Site 2 Proposed Development The proposed development comprises of a large scale retail hardware store with associated car parking. Additional tenancies will also be accommodated on the site. Access to the site will be via Bacchus Marsh Road towards the southern boundary of the site. The entry way will have culverts installed to allow access over the proposed floodway. All drainage assets internal to the site will be maintained by the body corporate. The proposed development layout for the site is shown in Figure 2. 1

5 Figure 2: Site Layout 3 Stormwater Management 3.1 Stormwater Quantity Management This site, and the adjoining property to the south, is currently subject to inundation from a large upstream catchment. The south western portion of the site currently acts as a broad floodway for flows discharging from the existing 5 x 2.4m x 0.9m culverts under Bacchus Marsh Road. Figure 3 illustrates the extent of flooding for the Masters site and downstream for the existing conditions. 2

6 Figure 3: Existing 1 in 100 year Flood extents BMT WBM 3

7 It is proposed to construct a formalised floodway, parallel to Bacchus Marsh Road to carry the entire 1 in 100 year ARI flow expected at that location. The floodway is proposed to extend south to an existing floodway parallel to Plantation Road before entering an existing retarding basin. BMT WBM have carried out a flood study for the area, analysing existing and proposed conditions. The 1 in 100 year flood depth estimated by BMT WBM within the floodway adjacent to the Masters site ranges from 1.0 to 1.3m Internal Drainage An internal underground drainage network will be provided on site to cater for the 1 in 10 year ARI flow. All flow within the drainage network will be directed to the proposed shallow bio-retention basins, for water quality treatment purposes, located onsite prior to being discharged to the proposed floodway at the west of the site. Flows in excess of the 1 in 10 year ARI up to the 1 in 100 year ARI will be managed as overland flow through the proposed car park and open space. All overland flow will be directed to the proposed floodway. Spillways from the bio-retention basins will be provided to ensure the unimpeded flow of excess stormwater. Detail of the bio-retention basins are provided in Section Downstream Flooding A flood Impact Assessment has been carried out by BMT WBM for the proposed Masters site and 240 Bacchus Marsh Road site to the south. The report analysed the proposed development of the two sites and the formation of a formal flood way constructed parallel to Bacchus Marsh Road. The flood impact was assessed for the downstream flow path to Princess Highway via Kosciusko Avenue. The existing retarding basin, located at the corner of Plantation Road and Hendy Street has been included in the hydraulic model. Figure 4 shows the location of the proposed floodway and the existing retarding basin. 4

8 Figure 4: Location of Floodway and Existing Retarding Basin The BMT WBM report found that the development of the Masters site and 240 Bacchus Marsh Road site will cause an increase to the level of flooding within the vicinity of Kosciusko Avenue by up to 0.07m. In order to reduce the expected level of flooding to predevelopment levels, stormwater detention will need to be provided upstream of Kosciusko Avenue. Detention may be provided on site within the proposed developments, or alternatively, the capacity of the existing retarding basin increased to accommodate the additional stormwater runoff from the two sites. For the purposes of this investigation, Cardno have assumed that the capacity of the existing retarding basin will be increased to accommodate the increased flows and as such, onsite detention is not proposed. Further work will need to be carried out to assess the extent of enlargement required to attenuate the developed flows to existing levels in either the downstream retarding basin or onsite. The City of Greater Geelong should be consulted to ascertain the viability of this option. 3.2 Stormwater Quality Treatment Due to the topography of the site and the proposed layout, three separate bio-retention basins are proposed for stormwater quality treatment, as shown on Figure 2. Cardno undertook a review of the stormwater treatment proposed for the site and evaluated the physical size and treatment effectiveness of the proposed bio-retention basins. Best practice industry standards call for a separation of gross pollutants, treatment of stormwater on site and reduction of outflow pollutants such as Total Nitrogen (TN), Total Phosphorus (TP) and Total Suspended Solids (TSS) to 45%, 45% and 80% respectively. 5

9 All stormwater runoff from the site, up to and including the 1 in 10 year ARI storm event, will be collected and transferred to one of the three bio-retention basins via an underground pipe network Catchment Delineation of Stormwater Quality Treatment The site has been separated into three catchments, each with their own treatment device. Figure 5 below is the catchment plan for the site. Figure 5: Catchment Plan The catchment areas and the percentage impervious are shown below in Table 1. Table 1: Catchment Properties Catchment Area (Ha) Impervious Area A % B % C % Treated stormwater from Catchment A will be piped through Catchment C to the proposed Floodway but will bypass the proposed Bio-retention Basin C. Stormwater outflow from Bio-retention Basin B and C will discharge directly into the proposed floodway Rainwater Tanks No rainwater tanks have been considered in the stormwater modelling for the Masters site due to the uncertainty of the final arrangement for stormwater reuse. It is, however, encouraged to utilise rainwater tanks to capture the runoff generated from the roofs of the building and use that water for open space irrigation and toilet flushing. The use of rainwater tanks has the potential to reduce stormwater runoff from the site, reduce potable water use within the development and reduce the size of the proposed bio-retention basins. Further 6

10 modelling will be required to assess the potential reduction of the bio-retention basin area with the inclusion of the rainwater tanks Stormwater Quality Modelling In order to assess potential water quality improvements provided by the site s proposed treatment measures, the Model for Urban Stormwater Improvement Conceptualisation (MUSIC) version 5 was been used. MUSIC is the current industry standard for the conceptual design of stormwater filtration systems and other stormwater quality treatment facilities. It is capable of generating flows via a rainfall runoff model and water quality data in addition to being able to assess a variety of stormwater treatment facilities. The parameters adopted for the MUSIC model were provided in the City of Greater Geelong MUSIC Guidelines. The treatment train effectiveness has been assessed using the land uses as source nodes and rainfall data for The Geelong Salines (Moolap) rainfall station (station 87023) (1985) which has an annual rainfall of 513mm. The generation of annual pollutant loads (in kg) and the expected reduction provided by the proposed stormwater treatment measures were derived using model default mean and standard deviations for each land use. The proposed development has been assessed using the procedure set out in the Best Practice Environmental Management Guidelines (CSIRO 1999) as per council requirements with Best Practice Guidelines set out in Table 2. Table 2: Water Quality Objectives - Best Practice Guidelines Pollutant Type Total Suspended Solids Total Phosphorus Total Nitrogen Water Quality Objectives 80% reduction in average annual load of pollutants leaving the developed unmitigated scenario compared to the developed mitigated scenario 45% reduction in average annual load of pollutants leaving the developed unmitigated scenario compared to the developed mitigated scenario 45% reduction in average annual load of pollutants leaving the developed unmitigated scenario compared to the developed mitigated scenario 7

11 The MUSIC model layout for the proposed development is shown in Figure 6. It incorporates three catchments each with an associated bio-retention basin. Figure 6: Proposed Development MUSIC Model Layout Analysis has shown that the proposed stormwater treatment measures provide significant improvements in stormwater quality and exceed the Best Practice Guidelines for water sensitive urban design Bio-retention Basin The MUSIC modelling indicates that the following bio-retention basins are required for the site to meet best practice guidelines. Table 3: Bio-retention Basin Properties Basin Total Surface Area (m 2 ) Filter Area (m 2 ) EDD (m) Filter Media Depth (m) A B C Total Discharge from the bio-retention basins will be directly into the proposed floodway at the west of the site. It is noted that the outlet to the bio-retention basins will be submerged during high flow events within the floodway, reducing the outlet capacity of the basin. As bio-retention basins are primarily designed for regular, low flow, events, the reduced outlet capacity during high flow events is not anticipated to compromise the treatment performance of the bio-retention basins. In the event that the extended detention depth (EDD) within the bio-retention basins is exceeded, the overflow mechanism will be in operation. For Basin B and C, this can be a weir directly flowing into the proposed floodway. For Basin A, a grated pit is proposed at the EDD level to divert excess flow into the underground drainage pipe network. 3.3 Alternative Biofiltration System Alternative proprietary treatment systems exist such as Biofilta Pty Ltd s stormwater system which can reduce the treatment footprint from 290m 2 to a single 50m 2 bio-retention basin with stormwater storage underneath. Detailed analysis of this system is outside the scope of this report, however the preliminary sizing is included for information purposes. 8

12 4 Conclusions and Recommendations Cardno have analysed the proposed bio-retention basins proposed for the Masters site at Bacchus Marsh Road, Corio. Minimum sizes are given for the three basins in order to treat the stormwater runoff from the site to best practice guidelines. Further detail design is required to determine if the three proposed bio-retention basins are required, or if alternative designs are to be integrated to reduce to one treatment facility. A flood assessment carried out by BMT WBM, assuming no onsite detention, indicates an increase of 70mm in the 1 in 100 year flood level. Guidance is required from the City of Greater Geelong as to the viability of upgrading the existing retarding basin downstream to attenuate this and the adjoining site s stormwater runoff. 9