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5-Waters-Volume-4-Stormwater
18 Springston Stormwater Scheme
18.1 Scheme Summary
Scheme Area
| 37.16ha |
| Scheme Coverage (as at 1 Jan 2021) | Rating numbers | 195
|
System components
| Piped (m)
| 2,350
|
| Swales (m) | 570
|
| Drains (m) | 3,660
|
| Manholes/Inspection Chambers (No.) | 11
|
| Treatment | 1 Basin |
| Other | 3 soakholes
|
Value ($)
| Replacement Cost | $935,688
|
| Depreciated Replacement Cost | $734,860
|
| Financial | Operator cost (scheduled and reactive maintenance) per connection
| $26.72/connection
|
Planning
| Stormwater Management Plan | Required
|
| No. SDC stormwater consents | 2
|
Demand
| Mean Annual Rainfall (mm) | 614 |
| 10% AEP (10 year) 1hr rainfall depth (mm) | 18.8 |
| Sustainability | Sustainable drain management practices | Adopted and Encouraged |
18.2 Key Issues
The following key issues are associated with the Springston Stormwater Scheme. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.
Table 18‑1 Springston Stormwater 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 works. |
18.3 Overview & History
Stormwater runoff is discharged either to the Leeston Road Drain or Sargents New Drain with the ultimate discharge to Te Waihora/Lake Ellesmere.
The Everest Way subdivision manages the stormwater (water quality and quantity) via a dry attenuation basin. The basin has been retrofitted with a spring flow outlet control structure which operates via a float. And can be locked open manually in the event of sustained presence of spring water.
Figure 18‑2 Scheme Schematic
18.4 Resource Consents
There is one stormwater discharge consent held by Selwyn District Council for this stormwater management area. Council have applied for a global consent for the Springston stormwater network. The application is underway with ECan.
The discharge of stormwater from Everest Way was transferred to Council in January 2014. The spring water outlet was not constructed as per consent plans however a compliance monitoring report was issued by Environment Canterbury on 29 June 2010 stating that revised plans were reviewed by Environment Canterbury and consent criteria have been achieved.
Table 18‑2 Resource Consents
CRC155154
Issued - Active | To discharge contaminants to water | Junction of Leeston Road and Ellesmere Junction Road, SPRINGSTON | 22/06/2013
| 20/06/2040
|
CRC186177
Application on Hold
| To discharge stormwater to land and to surface water from the existing stormwater network of Springston.
| Springston
| TBC
| TBC
|
18.5 Integrated Stormwater Management Plan
An integrated stormwater management plan was prepared and submitted to ECan in April 2017, as part of the application for the global discharge consent (CRC186177).
18.6 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. Swale (Wetland) – Is a longitudinal open channel which is lined with wetland plant species. The swale both conveys and treats stormwater and is particular useful in areas with high groundwater 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.
f. Open drains – are channels used to convey stormwater. They are cost effective means to convey large volumes of water.
A summary of material and diameter for channels and pipes, where known, is shown below in Figure 18‑3 and Figure 18‑4.
Figure 18‑3 Pipe Material - Springston
Figure 18‑4 Pipe Diameter – Springston
18.7 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.
18.8 Photos of Main Assets
The photos below provide a summary of the types of assets found within this stormwater management area.
Photo 1 – Everest Way Basin
|
Photo 2 – Everest Way Basin in flood
|
18.9 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 18‑5 shows the predicted flooding for Springston.
Figure 18‑5 Rapid Flood Modelling, Springston
18.10 Risk Assessment
A risk assessment has been undertaken for the Springston 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 18‑3 details the risk priority rating and Table 18‑4 outlines the risks for this scheme.
Table 18‑3 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 18‑4 Risks - Springston
| Groundwater issues (water bubbling up, basin was full all 2013 winter) | Review basin upgrade options | 2014 | 12 | 4 | 4 |
| Stormwater management | education on where to pump flood water | 2017 | | 6 | 2.1 |
| 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.
18.11 Asset Valuation Details
The total replacement value of assets within the Springston Scheme is $935,688 as detailed in Table 18‑5 below.
Table 18‑5 Replacement Value, Springston
Stormwater Reticulation
| Chamber | $11,663
|
| Channel | $187,961
|
| Inlet-Outlet-Point | $34,578
|
| Lateral | $145,422
|
| Management Device | $13,491
|
| Manhole | $70,477
|
| Pipe | $472,096
|
18.12 Renewals
The renewal profile has been taken from the 2019 5 Waters Valuation. A graph showing the renewals for this scheme are shown by Figure 18‑6 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2038/39.
Figure 18‑6 Springston Stormwater Renewal Profile
18.13 Critical Assets
The criticality model for Springston has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 18‑6 and Figure 18‑7 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.
Table 18‑6 Length of Assets per Criticality Level
5
| Low | 4,424
|
4
| Medium-Low | 339
|
3
| Medium | 212
|
2
| Medium-High | 15 |
1
| High | 0 |
18.14 Asset Condition
The asset condition model was run for Springston in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). Figure 18‑8 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.
Table 18‑7 provides a description of the condition rating used within the condition model.
Table 18‑7 Asset Condition Grading
| 1.0 | Excellent |
| 2.0 | Good |
| 3.0 | Moderate |
| 4.0 | Poor |
| 5.0+ | Fail |
18.15 Funding Program
The 10 year budgets are shown by Table 18‑8. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1.
All figures are ($) not adjusted for CPI “inflation". They are calculated on historical data, and population growth where relevant.
Table 18‑8 Budget Summary
| 2021/2022 | -
| -
|
2022/2023
| -
| $100,000
|
| 2023/2024 | -
| -
|
| 2024/2025 | -
| $40,000
|
| 2025/2026 | -
| -
|
| 2026/2027 | -
| -
|
| 2027/2028 | -
| - |
2028/2029
| -
| -
|
2029/2030
| -
| -
|
2031/2032
| -
| -
|
| Total | -
| $140,000
|
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 18‑9 Key Projects
Capital Projects
| -
| Culvert upgrades in L2; sizes DN600 - DN750
| -
| $100,000
| -
| -
| 100% LoS
|
Capital Projects
| -
| Basin low-flow system
| -
| -
| -
| $40,000
| 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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5-Waters-Volume-4-Stormwater
