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5-Waters-Volume-4-Stormwater
15 Rolleston Stormwater Scheme
15.1 Scheme Summary
Scheme Area
| 975.46 ha |
Scheme Coverage (as at 1 Jan 2021) | Rating numbers | 6,438
|
System components
| Piped (m) | 27,340
|
Swales (m) | 8,086
|
Drains (m) | 27.8
|
Manholes/Inspection Chambers (No.) | 197
|
Treatment | 31 Infiltration basins, 74 Proprietary devices, 11 silt traps, 4 rain gardens
|
Other | Versitanks 1,685 soakholes
|
Value ($)
| Replacement Cost | $5,890,885
|
Depreciated Replacement Cost | $5,315,761
|
Financial | Operator cost (scheduled and reactive maintenance) per connection
| $31.20/connection
|
Planning
| Stormwater Management Plan | Yes
|
No. SDC stormwater consents | 15
|
Demand
| Mean Annual Rainfall (mm) | 643 |
10% AEP (10 year) 1hr rainfall depth (mm) | 19.6
|
Sustainability | Sustainable drain management practices | Adopted and Encouraged |
15.2 Key Issues
The following key issues are associated with the Rolleston Stormwater Scheme. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.
Table 15‑1 Rolleston Scheme Issues
Capacity upgrades which are a result from increasing expectations from ratepayers that a higher level of service LOS is provided. | Identify capacity restrictions in the system, design upgrades and budget for physical works in LTP. |
15.3 Overview & History
Due to the relatively free draining soils which underlie the township of Rolleston, the stormwater system generally consists of roadside sumps discharging directly to soak holes.
A number of the newer subdivisions have incorporated soakage basins as part of the overall stormwater management. The Izone industrial area also incorporates a number of proprietary treatment devices.
Figure 15‑2 Scheme Schematic
15.4 Resource Consents
The Rolleston stormwater scheme has a number of resource consents. Table 15‑2 shows the stormwater discharge permitted by the resource consents for this scheme.
Table 15‑2 Resource Consents
CRC054637
Issued - Active | To discharge stormwater on to land | Burnham School Road, ROLLESTON | 5/10/2005 | 30/09/2040 |
CRC091220.2
Issued - Active | To discharge contaminants into land. | Hoskyns Road, ROLLESTON | 9/11/2010 | 28/06/2041 |
CRC091824.1
Issued - Active | To discharge contaminants into land. | Hoskyns Road, ROLLESTON | 15/04/2009 | 28/06/2041 |
CRC100025
Issued - Active | To discharge contaminants into land. | Izone Drive, ROLLESTON | 15/05/2009 | 28/06/2041 |
CRC100027
Issued - Active | To discharge contaminants into land. | Izone Drive, ROLLESTON | 15/05/2009 | 28/06/2041 |
CRC100057
Issued - Active | To discharge contaminants into land. | Izone Drive, ROLLESTON | 15/05/2009 | 28/06/2041 |
CRC100069
Issued - Active | To discharge contaminants into land. | Izone Drive, ROLLESTON | 15/05/2009 | 28/06/2041 |
CRC110923
Issued - Active | To discharge contaminants into land. | Izone Drive, ROLLESTON | 24/06/2011 | 28/06/2041 |
CRC133608
Issued - Active | To discharge contaminants into land. | 10 George Holmes Road, ROLLESTON | 1/02/2013 | 1/02/2043 |
CRC100819.1
Issued - Active | To discharge contaminants into land. | Railway Road, ROLLESTON | 18/02/2010 | 19/11/2044 |
CRC100886.1
Issued - Active | To discharge contaminants into land. | Railway Road, ROLLESTON | 18/02/2010 | 19/11/2044 |
CRC132527
Issued - Active | To discharge stormwater to land | Rolleston Township, Selwyn District | 16/01/2014 | 16/01/1949 |
CRC145959
Issued - Active | To discharge contaminants into land.
| Izone Industrial Park, ROLLESTON | 1/07/2013 | 24/06/2046
|
CRC173495 Issued - Active
| To discharge contaminants into land
| Lot 1 DP 488477 (Izone Industrial Park)
| 24/06/2016
| 24/06/2046
|
CRC142372 Issued - Active
| To discharge stormwater
| East Maddisons Road, Rolleston
| 28/06/2013
| 02/11/2040
|
Council obtained a global stormwater consent for the existing township stormwater disposal in January 2014 (CRC132527). Eleven residential consents were surrendered. The consent requires treatment in specific areas and excludes all industrial and commercial developments.
15.5 Integrated Stormwater Management Plan
An Integrated Stormwater Management Plan (ISMP) was prepared for Rolleston as part of Council's global discharge consent application in November 2012. The consent was granted January 2014. The ISMP is currently being updated.
15.6 Scheme Assets
Council has a wide variety of stormwater assets within the district. A brief description of the assets within this scheme is provided below:
a. Humeceptor - Is a hydrodynamic separator which helps to reduce mass sediment load from the discharge, some removal of hydrocarbons is also achieved.
b. Swale (Grassed) – Is a longitudinal open channel which is lined with grass. The swale both conveys and treats stormwater.
c. Reticulated network – Includes pipes, manholes, sumps. The primary purpose of the reticulated network is to collect and convey stormwater. Historically these systems were designed for the 2 year storm event. Today's engineering standards require the piped network to be designed for a 10 year event with overland flow provision for up to the 50 year event.
d. Infiltration basin – is a stormwater management device which is used to store, treat and dispose of stormwater to the ground via soakage.
e. Soakholes – Are used to dispose of stormwater to ground in areas where the ground water table is low and soil permeability is high.
A summary of material and diameter for channels and pipes, where known, is shown below in Figure 15‑3 and Figure 15‑4.
Figure 15‑3 Pipe Material – Rolleston
Figure 15‑4 Pipe Diameter – Rolleston
15.7 Operational Management
The stormwater network is operated and maintained under two maintenance contracts as follows:
- Contract 1241: Water Services Contract. Contract is with SICON who undertakes investigations, conditions inspections, proactive and reactive maintenance and minor asset renewals.
- Contract 1202: Parks and Reserves Contract. Contract is with SICON who undertakes the maintenance of land scape features related to water services e.g. mowing, gardens etc.
Water quality sampling is completed under an agreement with Food and Health Ltd as required.
15.8 Photos of Main Assets
The photos below provide a summary of the types of assets found within this stormwater management area.
Photo 1: Stormwater Basin
|
Photo 2: Stormwater Basin |
15.9 Rapid Flood Modelling
The Council has undertaken 'Rapid Flood Hazard Assessment' modelling for its main townships. The modelling uses DHI MIKE 21 to simulate rainfall on grid with the outputs processed through ArcGIS producing maps illustrating a range of flood deeps during different rainfall intensities and durations.
The rapid flood assessment has been generated to provide a high level summary of potential flood and ponding areas across the district during extreme rainfall events. The results from this study are not to be used to set floor levels. The results have not been ground tested and therefore are indicative only.
For a 50 year event, Figure 15‑5 shows the predicted flooding for Rolleston.
Figure 15‑5 Rapid Flood Modelling, Rolleston
15.10 Risk Assessment
A risk assessment has been undertaken for the Rolleston scheme. The key output from the risk assessment is the identification of any extreme and high risks which need to be mitigated. In order to mitigate these risks they have been included and budgeted for in the projects within this LTP. Table 15‑3 details the risk priority rating, Table 15‑4 outlines the risks and the list of key projects is found in Table 15‑9.
Table 15‑3 Risk Priority Rating
> 50 | Extreme | Awareness of the event to be reported to Council. Urgent action to eliminate / mitigate / manage the risk. Document risk and action in the AMP. |
35-50 | Very High | Risk to be eliminated / mitigated / managed through normal business planning processes with responsibility assigned. |
14-35 | High | Manage risk using routine procedures. |
3.5-14 | Moderate | Monitor the risk. |
< 3.5 | Low | Awareness of the event to be reported to Council. Immediate action required to eliminate / mitigate / manage the risk. Document risk and action in the AMP. |
Table 15‑4 Risks - Rolleston
Complaints about appearance of stormwater basins | Beautify stormwater basins | 2014 | 4 | 4 | 2 |
Soil contamination | Basin sediment sampling | 2014 | 12 | 12 | 6 |
Non-consented activities | Renewal of consents | 2014 | 27 | 27 | 6 |
The list of district wide risks can be found in 5Waters Activity Management Plan: Volume 1.
15.11 Asset Valuation Details
The total replacement value of assets within the Rolleston Scheme is $5,890,885 as detailed in Table 15‑5 below.
Table 15‑5 Replacement Value, Rolleston
Stormwater Reticulation
| Chamber | $10,440
|
Channel | $413,775
|
Inlet-Outlet-Point | $433,082
|
Lateral | $134,043
|
Management Device | $1,156,512
|
Manhole | $1,100,933
|
Pipe | $2,028,009
|
Soakhole | $614,090
|
15.12 Renewals
The renewal profile has been taken from the 2019 5 Waters Valuation. A graph showing the renewals for this scheme are shown by Figure 15‑6 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2025/26.
Figure 15‑6 Rolleston Stormwater Renewal Profile
15.13 Critical Assets
The criticality model for Rolleston has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 15‑6 and Figure 15‑7 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.
Table 15‑6 Length of Assets per Criticality Level
5
| Low | 13,093
|
4
| Medium-Low | 1,231
|
3
| Medium | 194
|
2
| Medium-High | 0
|
1
| High | 0 |
15.14 Asset Condition
The asset condition model was run for Rolleston in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). Figure 15‑8 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.
Table 15‑7 provides a description of the condition rating used within the condition model.
Table 15‑7 Asset Condition Grading
1.0 | Excellent |
2.0 | Good |
3.0 | Moderate |
4.0 | Poor |
5.0+ | Fail |
15.15 Funding Program
The 10 year budgets are shown by Table 15‑8. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1.
All figures are ($) not adjusted for CPI “inflation". They are calculated on historical data, and population growth where relevant.
Table 15‑8 Budget Summary
2021/2022 | -
| $10,000
|
2022/2023
| -
| $60,000
|
2023/2024 | -
| $20,000
|
2024/2025 | -
| $60,000
|
2025/2026 | -
| $10,000
|
2026/2027 | -
| $10,000
|
2027/2028 | -
| $10,000
|
2028/2029
| -
| $10,000
|
2029/2030
| -
| $10,000
|
2031/2032
| -
| $10,000
|
Total | -
| $210,000
|
An explanation of the categories within the budgets are as follows below:
-
Expenditure consists of operation and maintenance costs;
-
Renewals are replacement of assets which are nearing or exceeded their useful life;
-
Projects are investigations, decisions and planning activities which exclude capital works; and
-
Capital projects are activities involving physical works.
Table 15‑9 Key Projects
Capital Projects
| -
| Retrofit stormwater treatment
| $10,000
| $50,000
| $10,000
| $50,000
| 100% LoS
|
Capital Projects
| -
| Rain garden enhancement
| -
| $10,000
| $10,000
| $70,000
| 100% LoS
|
* Where LoS refers to Level of Service and G refers to Growth
The list of district wide projects can be found in 5Waters Activity Management Plan: Volume 1.
Discussion on Projects
Projects have been determined based on their:
- Relevance to the scheme
- Requirement to be completed under legislation
- Ability to bring the scheme up to or maintain the Level of Service required under council's Asset Management Policy.
Many projects are jointly funded by more than one scheme and activity. Each scheme pays a pro-rata share only, equivalent to the number of connections.
Discussion on Capital and Projects
Where relevant, Capital (Levels of Service) and Capital (Growth) projects have been included in the scheme financial details.
Levels of Service Projects and growth splits have been provided to ensure the costs of population driven works are clear.
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