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5-Waters-Volume-4-Stormwater
10 Lake Coleridge Stormwater Scheme
10.1 Scheme Summary
Scheme Area
| 21.77ha |
| Scheme Coverage (as at 1 Jan 2021) | Rating numbers | 69 |
System components
| Piped (m) | 830
|
| Swales (m) | 81.3
|
| Drains (m) | 0
|
| Manholes/Inspection Chambers (No.) | 0
|
| Treatment | N/A
|
| Other | 7 soakholes
|
Value ($)
| Replacement Cost | $113,799
|
| Depreciated Replacement Cost | $67,270
|
Financial
| Operator cost (scheduled and reactive maintenance) per connection
| $13.79/connection
|
Planning
| Stormwater Management Plan | Draft
|
| No. SDC stormwater consents | 1
|
Demand
| Mean Annual Rainfall (mm) | 820 |
| 10% AEP (10 year) 1hr rainfall depth (mm) | 14.8
|
| Sustainability |
Sustainable drain management practices | Adopted and Encouraged |
10.2 Key Issues
The following key issues are associated with the Lake Coleridge Stormwater Scheme. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.
Table 10‑1 Lake Coleridge Scheme Issues
| Increased expectation from the community regarding level of service received from the stormwater network | Identify capacity restrictions in the system, design upgrades and budget for physical. |
| Soakholes in Ryton Place do not have the capacity to take moderate rainfall events resulting in road flooding. Ground in this area is not free draining. | Investigate other options for SW detention. |
| Surface ponding outside some properties in Harper Place. | Discuss options with the community to prioritise areas of need. Budget for works to resolve. |
10.3 Overview & History
The stormwater scheme comprises a number of relatively short sections of pipework discharging either to ground or surface water (ultimately the Rakaia River).
Soakholes in Ryton Place do not cope with significant rainfall. New soakholes were attempted to be installed in 2013 but at 3.5m no free draining material was found, only clay. Bore logs show sediments between 16 - 75m. Localised flooding occurs on Harper Place and piping is proposed to main behind affected properties.
Figure 10‑2 Scheme Schematic
10.4 Resource Consents
Council have applied for a global consent for the Lake Colerdige stormwater scheme (CRC186174). The application is underway with ECan.
10.5 Integrated Stormwater Management Plan
Environment Canterbury's Land and Water Regional Plan became operative on the 13th December 2018. Under policy 4.16, any reticulated stormwater system for an urban area requires a stormwater management plan. Additionally, from 1 January 2025, operators of reticulated stormwater systems will become responsible for the quality and quantity of all stormwater discharged from the reticulated stormwater system. The Lake Coleridge stormwater management plan was updated in April 2017 and submitted to ECan.
10.6 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. 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.
b. Soakholes – Are used to dispose of stormwater to ground in areas where the ground water table is low and soil permeability is high.
Figure 10‑3 Pipe Material – Lake Coleridge
Figure 10‑4 Pipe Diameter – Lake Coleridge
10.7 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.
10.8 Photos of Main Assets
No photos are available for Lake Coleridge.
10.9 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, no flooding is expected in Lake Coleridge.
10.10 Risk Assessment
A risk assessment has been undertaken for the Lake Coleridge 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 10‑2 details the risk rating priority, Table 10‑3 outlines the risks and the list of key projects is found in Table 10‑8.
Table 10‑2 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 10‑3 Risks – Lake Coleridge
| Poor drainage | Investigate stormwater disposal options | 2014 | 12 | 12 | 12 |
| 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.
10.11 Asset Valuation Details
The total replacement value of assets within the Lake Coleridge Scheme is $113,979 as detailed in Table 10‑4 below.
Table 10‑4 Replacement Value, Lake Coleridge
Stormwater Reticulation
| Channel | $2,902
|
| Inlet-Outlet-Point | $16,935
|
| Lateral | $36,524
|
| Pipe | $53,576
|
| Soakhole | $3,862
|
10.12 Renewals
The renewal profile has been taken from the 2019 5Waters Valuation. A graph showing the renewals for this scheme are shown by Figure 10‑5 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2019/20.
Figure 10‑5 Lake Coleridge Stormwater Renewal Profile
10.13 Critical Assets
The criticality model for Lake Coleridge has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 10‑5 and Figure 10‑6 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.
Table 10‑5 Length of Assets per Criticality Level
5
| Low | 387 |
4
| Medium-Low | 0 |
3
| Medium | 0 |
2
| Medium-High | 0 |
1
| High | 0 |
10.14 Asset Condition
The asset condition model was run for Lake Coleridge in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). Figure 10‑7 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.
Table 10‑6 provides a description of the condition rating used within the condition model.
Table 10‑6 Asset Condition Grading
| 1.0 | Excellent |
| 2.0 | Good |
| 3.0 | Moderate |
| 4.0 | Poor |
| 5.0+ | Fail |
10.15 Funding Program
The 10 year budgets are shown by Table 10‑7. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1.
All figures are ($) not adjusted for CPI “inflation". They are calculated on historical data, and population growth where relevant.
Table 10‑7 Budget Summary
| 2021/2022 | -
| -
|
2022/2023
| -
| -
|
| 2023/2024 | -
| -
|
| 2024/2025 | -
| $95,000
|
| 2025/2026 | -
| -
|
| 2026/2027 | -
| -
|
| 2027/2028 | -
| - |
2028/2029
| -
| -
|
2029/2030
| -
| -
|
2031/2032
| -
| -
|
| Total | -
| $95,000
|
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 10‑8 Key Projects
Capital Projects
| -
| Ryton Place stormwater overland flow to creek, including design
| -
| -
| -
| $95,000
| 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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