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5-Waters-Volume-4-Stormwater
20 West Melton Stormwater Scheme
20.1 Scheme Summary
Scheme Area
| 163.42 ha |
| Scheme Coverage (as at 1 Jan 2021) | Rating numbers | 734
|
System components
| Piped (m) | 6,610
|
| Swales (m) | 7,306
|
| Drains (m) | 0 |
| Manholes/Inspection Chambers (No.) | 76
|
| Treatment | 6 Infiltration basins |
| Other | 57 soakholes
|
Value ($)
| Replacement Cost | $3,584,615
|
| Depreciated Replacement Cost | $3,346,407
|
Financial
| Operator cost (scheduled and reactive maintenance) per connection
| $60.14/connection
|
Planning
| Stormwater Management Plan | Updated 2021
|
| No. SDC stormwater consents | 1
|
Demand
| Mean Annual Rainfall (mm) | 642
|
| 10% AEP (10 year) 1hr rainfall depth (mm) | 19.6 |
| Sustainability | Sustainable drain management practices | Adopted and Encouraged |
20.2 Key Issues
The following key issues are associated with the West Melton Stormwater Scheme. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.
Table 20‑1 West Melton Scheme Issues
| New stormwater system | Monitor performance of new systems |
20.3 Overview & History
The majority of the West Melton stormwater assets are within the Gainsborough and Prestons Down subdivision. The system comprises a network of swales discharging to stormwater basins.
Figure 20‑2 Scheme Schematic
20.4 Resource Consents
There is a global consent held by Selwyn District Council for this stormwater management area.
Table 20‑2 Resource Consents
CRC167467
Issued - Active | to discharge contaminants into and onto land | West Melton, Selwyn | 12/04/2017 | 12/04/2052 |
20.5 Integrated Stormwater Management Plan
An ISMP has been approved for West Melton. The ISMP was last updated in 2021.
20.6 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. Swale (Grassed) – Is a longitudinal open channel which is lined with grass. The swale both conveys and treats stormwater.
b. Soakage trench – Carries out the same function as soakholes but is orientated in a horizontal direction rather than vertically. They are particularly useful in areas with reduced infiltration rates or higher ground water tables.
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 20‑3 and Figure 20‑4.
Figure 20‑3 Pipe Material – West Melton
Figure 20‑4 Pipe Diameter – West Melton
20.7 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.
20.8 Photos of Main Assets
The photos below provide a summary of the types of assets found within this stormwater management area.
Photo 1 – Basin 1 Gainsborough
| 
Photo 2 – Basin 2 Gainsborough
|
20.9 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 20‑5 shows the predicted flooding for West Melton
Figure 20‑5 Rapid Flood Modelling, West Melton
20.10 Risk Assessment
A risk assessment has been undertaken for the West Melton 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 20‑3 details the risk priority rating and Table 20‑4 outlines the risks.
Table 20‑3 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 20‑4 Risks – West Melton
| 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.
20.11 Asset Valuation Details
The total replacement value of assets within the West Melton Scheme is $3,584,615 as detailed in Table 20‑5 below.
Table 20‑5 Replacement Value, West Melton
Stormwater Reticulation
| Channel | $443,528
|
| Inlet-Outlet-Point | $248,269
|
| Management Device | $455,545
|
| Manhole | $543,490
|
| Pipe | $1,816,539
|
| Soakhole | $77,244
|
20.12 Renewals
The renewal profile has been taken from the 2019 5 Waters Valuation. A graph showing the renewals for this scheme are shown by Figure 20‑6 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2025/26, 2039 to 2042.
Figure 20‑6 West Melton Stormwater Renewal Profile
20.13 Critical Assets
The criticality model for West Melton has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 20‑6 and Figure 20‑7 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.
Table 20‑6 Length of Assets per Criticality Level
5
| Low | 9,672
|
4
| Medium-Low | 888
|
3
| Medium | 495
|
2
| Medium-High | 0 |
1
| High | 34
|
20.14 Asset Condition
The asset condition model was run for West Melton in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). Figure 20‑8 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.
Table 20‑7 provides a description of the condition rating used within the condition model.
Table 20‑7 Asset Condition Grading
| 1.0 | Excellent |
| 2.0 | Good |
| 3.0 | Moderate |
| 4.0 | Poor |
| 5.0+ | Fail |
20.15 Funding Program
The 10 year budgets are shown by Table 20‑8. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1.
All figures are ($) not adjusted for CPI “inflation". They are calculated on historical data, and population growth where relevant.
Table 20‑8 Budget Summary
| 2021/2022 | -
| -
|
2022/2023
| -
| $10,000
|
| 2023/2024 | -
| -
|
| 2024/2025 | -
| -
|
| 2025/2026 | -
| -
|
| 2026/2027 | -
| -
|
| 2027/2028 | -
| - |
2028/2029
| -
| -
|
2029/2030
| -
| -
|
2031/2032
| -
| -
|
| Total | -
| $10,000
|
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 20‑9 Key Projects
Capital Projects
| -
| Weedons Ross Road swale upgrades
| -
| $10,000
| -
| -
| 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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