Peter Berry & Associates Pty. Ltd.
Civil Engineers
Stormwater Management Strategy & Services Report
Ash Road West – Leopold
Prepared for Paisley Manor Pty Ltd
February 6, 2012
TABLE OF CONTENTS
1.0INTRODUCTION
2.0THE SITE
3.0THE PROPOSED DEVELOPMENT
4.0STORMWATER STRATEGY
5.0BASIN VOLUMES
5.1Northern Catchment
5.2Southern Catchment
6.0WATER QUALITY TREATMENT MEASURES
7.0SERVICES AVAILABILITY AND DEVELOPMENT AFFECT
7.1Drainage & Sewer
7.2Electricity
7.3Water Supply
7.4Road Infrastructure
8.0INFRASTRUCTURE CONSTRAINTS, TRIGGERS & COSTS
8.1Basin Construction
8.1.1Northern Basin
8.1.2Southern Basin
8.2Extraordinary Road Costs
8.2.1Northern Open space
8.2.2Northern Basin
8.2.3Southern Basin/Open space/Landscaping Buffer
9.0CONCLUSION
APPENDICES
1Indicative Plan of Subdivision – St Quentin Consulting
2Pre-Development Catchment Plan – Incorporating SMEC
Urban details
3Post Development Stormwater Management Plans
STORMWATER MANAGEMENT STRATEGY
ASH ROAD WEST – LEOPOLD
1.0 Introduction
This strategy has been prepared having regard to the earlier work undertaken by SMEC Urban in relation to the Horizon@Leopold development (May 2009) and the plan prepared by St Quentin Consulting; Appendix 1, on behalf of a group of Landholders.
This strategy is a preliminary expression of intentand understanding that further DPO conditions will apply and are receiving early consideration.
2.0 The Site -
The Ash Road West proposal fronts Ash Road in the east, abuts the current southern boundary of the developed part of Leopold in the north, abuts the Horizon@Leopold Estate in the west and extends south a distance of approximately 650 metres.
The north east corner of the site is very flat with gradual grades to the south and west.
Apart from two minor catchments which fall towards Ash Road, the majority of the site falls to the west and south west as shown on a pre-development catchment plan; Appendix 2*.
This plan has details of a more simplistic catchment superimposed over it, taken from the earlier referred to SMEC Urban report.
This has been done, as the catchment information from that earlier report has been accepted by the drainage authority, the City of Greater Geelong and has been the basis for drainage outflows from this developable area.
* Contours have been digitized from a Barwon Water’s PROFIS plan and superimposed over those of the Horizon@Leopold development to the west.
3.0 The Proposed Development -
The area is approximately 26 hectares and has the ability to generate approximately 300 lots. This is a culmination of the residential expansion east towards Ash Road initiated by the Horizon@Leopold development.
There will be a mix of sizes from 450 square metres shown in pink on Appendix 1, to higher density lots in the range of 300m2 to 400 m2.
These latter sized lots are located around the two Public Open Space areas which has been place in consultation with Council and in concert with other areas set aside within the adjoining development.
The proposed road layout provides good permeability within the development and coincides with the major through road from the Horizon@Leopold to connect with Ash Road.
4.0 Stormwater Strategy -
Apart from an area of approximately 0.5ha in the very south east corner, the whole parcel can be drained back to the outlets provided in the west by the Horizon @Leopold development.
Appendix 2 indicates the contour information and Appendix 3, the post development Stormwater Management Plan, sets out the piped and overflow drainage intentions.
It is proposed that two retarding/bioretention basins be established as shown and these have been sized having regard to the drainage authority and Water Sensitive Urban Design (WSUD) guidelines and the footprints shown have been computed.
The intention it to direct the 1 in 100 year stormwater runoff by a combination of pipes and overland drainage paths to these basins, provide the required volume within them to retard outflows , treat the water to achieve best practice environmental performance objectives before discharging them in both the 10 year and 100 year quantities previously calculated.
Where road grades do not permit or where areas that are being commanded away from their natural catchment, it will be necessary to cater for stormwater within pipes; the transference of Ash Road Stormwater west along Walkers Road to the north catchment basin is a prime example.
The runoff from the developed south east corner seemingly cannot be commanded back to the southern basin, however the requirements to treat this stormwater component will be allowed for within the basin.
The northern area falling towards Ash Road can be commanded west along Walkers Road while the majority of the southern 1.86 hectares can similarly be drained back to the west.
In arriving at the basin outlet allowance for the southern catchment,an inspection of the shape and area of the components within and outside of this development were so similar, that half of the volume calculated by SMEC Urban was attributed to each, even though the southern portion is at this time not residentially zoned.
The 0.5ha developed runoff is equivalent to the rural runoff from the current undeveloped 1.86ha area as demonstrated on Appendix 3.
Appendix 3 also includes a breakdown of the pre and post development drainage calculations and the area set aside for the basin and WSUD purposes.
5.0 Basin Volumes -
5.1 Northern Catchment -
Area (sm) / 126615 / Coefficient of Runoff / 0.70 / Storm Frequency (Yr)Time of Concentration (mins) / Q (In) [l/sec] / Q (Out) [l/sec] / Intensity (mm/hr) / Poertner Method / South Barwon / 100 yr
2 / 5139 / 1100 / 208.72 / 485 / 2 / 208.72
4 / 4545 / 1100 / 184.63 / 893 / 4 / 184.63
6 / 3910 / 1100 / 158.82 / 1144 / 6 / 158.82
8 / 3437 / 1100 / 139.61 / 1320 / 8 / 139.61
10 / 3080 / 1100 / 125.12 / 1452 / 10 / 125.12
12 / 2802 / 1100 / 113.82 / 1556 / 12 / 113.82
14 / 2578 / 1100 / 104.72 / 1638 / 14 / 104.72
16 / 2394 / 1100 / 97.22 / 1704 / 16 / 97.22
18 / 2238 / 1100 / 90.92 / 1757 / 18 / 90.92
20 / 2105 / 1100 / 85.52 / 1801 / 20 / 85.52
22 / 1990 / 1100 / 80.84 / 1835 / 22 / 80.84
24 / 1889 / 1100 / 76.74 / 1862 / 24 / 76.74
26 / 1800 / 1100 / 73.10 / 1884 / 26 / 73.10
28 / 1720 / 1100 / 69.85 / 1899 / 28 / 69.85
30 / 1648 / 1100 / 66.93 / 1910 / 30 / 66.93
32 / 1583 / 1100 / 64.29 / 1917 / 32 / 64.29
34 / 1523 / 1100 / 61.88 / 1920 / 34 / 61.88
36 / 1469 / 1100 / 59.67 / 1919 / 36 / 59.67
38 / 1419 / 1100 / 57.64 / 1916 / 38 / 57.64
5.2 Southern Catchment –
Area (sm) / 122440 / Coefficient of Runoff / 0.70 / Storm Frequency (Yr)Time of Concentration (mins) / Q (In) [l/sec] / Q (Out) [l/sec] / Intensity (mm/hr) / Poertner Method / South Barwon / 100 yr
2 / 4969 / 585 / 208.72 / 526 / 2 / 208.72
4 / 4396 / 585 / 184.63 / 950 / 4 / 184.63
6 / 3781 / 585 / 158.82 / 1221 / 6 / 158.82
8 / 3324 / 585 / 139.61 / 1420 / 8 / 139.61
10 / 2979 / 585 / 125.12 / 1577 / 10 / 125.12
12 / 2710 / 585 / 113.82 / 1705 / 12 / 113.82
14 / 2493 / 585 / 104.72 / 1814 / 14 / 104.72
16 / 2315 / 585 / 97.22 / 1906 / 16 / 97.22
18 / 2164 / 585 / 90.92 / 1987 / 18 / 90.92
20 / 2036 / 585 / 85.52 / 2057 / 20 / 85.52
22 / 1925 / 585 / 80.84 / 2119 / 22 / 80.84
24 / 1827 / 585 / 76.74 / 2174 / 24 / 76.74
26 / 1740 / 585 / 73.10 / 2224 / 26 / 73.10
28 / 1663 / 585 / 69.85 / 2267 / 28 / 69.85
30 / 1593 / 585 / 66.93 / 2307 / 30 / 66.93
32 / 1530 / 585 / 64.29 / 2342 / 32 / 64.29
34 / 1473 / 585 / 61.88 / 2373 / 34 / 61.88
36 / 1421 / 585 / 59.67 / 2402 / 36 / 59.67
38 / 1372 / 585 / 57.64 / 2427 / 38 / 57.64
40 / 1328 / 585 / 55.77 / 2450 / 40 / 55.77
42 / 1287 / 585 / 54.04 / 2470 / 42 / 54.04
44 / 1248 / 585 / 52.43 / 2488 / 44 / 52.43
46 / 1212 / 585 / 50.93 / 2504 / 46 / 50.93
47 / 1179 / 585 / 49.52 / 2465 / 48 / 49.52
48 / 1148 / 585 / 48.20 / 2428 / 50 / 48.20
6.0 Water Quality Treatment Measure -
Water quality performance objectives as set out in “Urban Stormwater – Best Practice Environmental Management Guidelines” and relate to the percentage removal of several nominated pollutants.
The objectives are –
80% reduction in Total Suspended Solids (TSS)
45% reduction in Total Phosphorus (TP), and
45% reduction in Total Nitrogen (TN.
A MUSIC (Model for Urban Stormwater Improvement Conceptualisation) assessment has been carried out on these two simple post development catchments. The tabulated outcomes have been based on the City of Greater Geelong MUSIC guidelines and the areas for the Bio-Retention System are shown in Appendix 3.
It is unknown as to whether any end of line allowance has been made in the Horizon@Leopold development, but the basins shown have sufficient areas within their bases to accommodate the respective catchments.
A schematic section through a basin below, indicates the general arrangement for storage, extended detention and drainage within each basin.
7.0 Services Availability & Development Affect -
An enquiry was made to all other servicing authorities to ascertain their ability to provide assets to the Ash Road west area.
Each was confirmed, although in the case of electricity, drainage, water and sewer, the following comments were made as they impact on cost, possible easement rights and timing.
7.1 Drainage & Sewer -
In general the provision for drainage and sewerage iscontingent upon the completion of the Horizon Estate to the west and the extension oftheseinfrastructure outlets to the boundary.
The timeframe for this to occur based on the current development rate we are advised,should see these at our common boundary in approximately two (2) years.
If this schedule is not achieved, then downstream drainage and sewerage easements would be necessary, however development would by this time be a lot nearer than at present.
7.2 Electricity -
While electricity can be provided, at some stage approximately 900 metres of single phase supply will need to be upgraded to three phase in Ash Road.
The associated costs will be determined, together with the relevant contribution by the customer, as the application for each stage of residential development progresses.
Similarly the retirement of any existing overhead supply would be funded by the developer.
7.3 Water Supply -
This asset can also be provided, however the north-west corner of this development requires a connection to a high level supply in Greenbank Court which will require works external to the site at the developers expense.
7.4 Road Infrastructure -
While these are provided by the developer of an independent stage, certain parcels can only be developed once an adjoining parcel has preceded it.
As well certain parcels have lengths of “one sided” roads around future basins, public open space and indeed the southern boundary that have an impact on cost and may effect the development timing for “landlocked” stages.
8.0 Infrastructure Constraints, Triggers & Costs -
The post development intention is for the areas north and south of Walkers Road to discharge into two basins which will treat the water before discharging it, hopefully by that stage,to downstream piped drainage and overflow routes.
The estimated cost associated with each basin is shown in Appendix 3.
These estimates include the construction of the basin, bio-wetland and drainage components to satisfy the 1 in 10 and 1 in 100 ARI outlet requirements.
The progress of development to the west presently suggests that the northern portion of its development is likely to reach our joint boundary before the southern portion.
That said, the Horizon Estate’s north-east corner is affected by the earlier referred to water supply issue and it will be looking for a connection through this development to Greenbank Court in order to complete this stage of its construction.
We have for the purposes of this section, numbered the landholdings as 1-8 on Appendix 1.
This has not been done to indicate a development staging,but simply as a method by which details can be referenced to the discussion below.
In each of the two catchment areas, the drainage and sewerage outfalls will ultimately located on the western boundary, while the remaining services are currently to be found in the east in Ash Road.
A lot will depend on the Horizon Estate’s completion date, as the construction of Walkers Road through to Ash Road will establish a priority, especially from a City of Greater Geelong perspective and from a logical extension of services and outfalls from the west towards Ash Road.
There is an area abutting Ash Road in Area 1, which along with lots abutting the existing Hazelwood Crescent development, could from a sewerage perspectivedevelop now, although this would be at the expense of a future more economical sewer layout.
Drainage from these lots could discharge to the existing farm dams once emptied as an interim measure, before further development necessitated the construction of the basin and outlets.
Everything considered and presuming this rezoning coincides with the completion of the Horizon Estate, only Areas 2 and 7 could not proceed immediately as they require access to be established viaadjoining areas proceeding first.
A potential problem also arises when the development intentions of some landholders do not accord with those of others.
The areas for the proposed basins, the timing of their ultimate construction, the reimbursement for land lost and construction costs will presumably be written into a Developer Contribution Plan (DCP).
However access to even the existing dams, were they considered by Council as acceptable interim measures, requires the agreement of predominately Landowners 2, 4 and 8.
As a short term measure each landholder if he wished to develop, could employ with Council’s permission, a temporary retention basin in the low corner of their holding, on the understanding that drainage rights to and through the Horizon Estate would need to exist and that these temporary basins would ultimately be replaced by the two nominated on the Indicative Plan of Subdivision.
8.1 Basin Construction -
8.1.1 Northern Basin -
The catchment size allowed for by SMEC in its original stormwater assessment was 8.775 hectares.
This proposal intends to capture the area north of Walkers Road and direct it to the SMEC outlet location; however the post development area is now 12.662 ha in total.
With no obvious staging for this northern section and given that the area being commanded back to the SMEC system exceeds theiroriginal catchment area, the construction of the northern basin would be necessary with the development of any first stage.
The other prevailing factor is that the downstream development is entirely residential housing, where protection from upstream drainage overflow is paramount.
8.1.2Southern Basin -
The southern catchment fares better in that the SMEC catchment approximates that of the proposed post-development and has the added benefit of the existing dam being substantial.
As well a downstream floodway reserve “exists” clear of residential housing.
The existing dam would need to be emptied and with some pipework could be used as an interim measure if permitted by Council.
Water quality issues could also be dealt with via the floodway reserve.
That said, Council is concerned that it not be seen to allow too much development to occur wherein any downstream problems are sheeted home to it for failure to implement satisfactory stormwater safeguards.
It would seem however and having regard to the above, that interim measures around the existing dam might see the development of fifty (50) percent of this catchment prior to the need for the construction of the ultimate basin.
Finally under the current City of Greater Geelong Infrastructure Development Manual parameter, an ANCOLD assessment of both basins will be required as part of the design submission and prior to their construction.
While the outcome of this assessment is unknown, an allowance of fifteen (15) percent should be made on top of the construction costs shown in Appendix 3 for the purpose of the assessment and any additional construction requirements.
8.2 Extraordinary Road Costs -
As remarked upon earlier, there are lengths of roadway adjacent to the basins and public open space in particular, that represent additional costs to the developer of that stage.
These are in place at Council’s insistence with resultant development potential on one side only.
As well, in the case of Landholding 8, there is a one-sided road adjacent to the landscape buffer; also condition of Council.
These situations are requirements of the rezoning and can be regarded as a “benefit” to the other landholders in the development area.
Accordingly the DCP should make allowance for the half road cost in these locations. Allowing for servicing assets,it is estimated that $900 per linear metrerepresents the half road cost.
8.2.1 Northern Open Space -
The length of road around this area of public open space is approximately 370 linear metres, resulting in a cost of $333,000.
8.2.2 Northern Basin -
The length of road around this basin including Walkers Road, is approximately 165 linear metres, resulting in a cost of $148,500.
8.2.3Southern Basin/Open Space/Landscaping Buffer -
The length of road associated with the basin/open space is approximately 190 metres with an additional 255 metres along the southern boundary.
The cost as a result for these construction works is $400,500.
These costs exclude GST and are estimated at today’s rates.
Although CPI does not necessarily equate to construction industry changes, some means of dealing with inflation will be necessary and so the current rate for Melbourne (June 2011)of 175.6 is noted.
9.0 Conclusion -
The outcomes of this report can be summarised as follows –
- The majority of the area can be commanded back to the west, retarded and water quality requirements met within basins on site,
- Downstream outlet parameters have been provided in an earlier approved drainage strategy,
- Minor and major flows can be directed towards these basins and hence the nominated outlet points, either by pipe or overland flow,
- All services can be made available to the development area, and
- Aspects relating to joint development costs will be dealt with by a Developer Contribution Plan.
In essence this strategy indicates that the development area stormwater can be retarded to accord with downstream infrastructure, treated to best practice and that it can be provided with all services.
Appendices
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Peter Berry & Associates Pty. Ltd.
Civil Engineers
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Peter Berry & Associates Pty. Ltd.
Civil Engineers
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