Dunsandel Stormwater Scheme

 

​​​​​​​​​​​<< ​​5-Waters-Volume-4-Stormwater​​ ​​6   Dunsandel Stormwater Scheme

6.1       Scheme Summary

Description​	​	Quantity
Scheme Area​		137.79ha
Scheme Coverage (as at 1 Jan 2021)	Rating numbers	174
System components ​​ ​​ ​​ ​​ ​	Piped (m)	890
	Swales (m)	1,052
	Drains (m)	482
	Manholes/Inspection Chambers (No.)	6
	Treatment	N/A
	Other	7 soakholes
Value ($) ​	Replacement Cost	$241,268
	Depreciated Replacement Cost	$146,422
Financial	​Operator cost (scheduled and reactive maintenance) per connection	$10.29/connection
Planning ​	Stormwater Management Plan	Draft
	No. SDC stormwater consents	1
Demand ​	Mean Annual Rainfall (mm)	688
	10% AEP (10 year) 1hr rainfall depth (mm)	16.7
Sustainability	Sustainable drain management practices	Adopted and Encouraged

 

6.2       Key Issues

The following key issues are associated with the Dunsandel Stormwater Scheme.  A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.

​Table 6‑1 Dunsandel Scheme Issues

What's the Problem​	What we plan to do
Flooding during moderate rainfall events.	Develop a SWMP and undertake catchment management study

6.3       Overview & History

Dunsandel stormwater discharges either to ground or via surface water to a natural waterway on the eastside of town.​

​

Figure 6‑2 Scheme Schematic

6.4       Resource Consents

Council have applied for a global consent for the Dunsandel scheme (CRC186172). The application is in progress with ECan.

 

6.5       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 Dunsandel stormwater management plan was updated in April 2017 and submitted to ECan. ​

6.6       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.      Soakholes – Are used to dispose of stormwater to ground in areas where the ground water table is low and soil permeability is high.   

c.      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 6‑3 and Figure 6‑4.

​Figure 6‑3 Pipe Material - Dunsandel

​Figure 6‑4 Pipe Diameter – Dunsandel

 

6.7       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.

 

6.8       Photos of Main Assets

​ Photo 1: Typical Drain

​​
​6.9       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 6‑5 shows the predicted flooding for Dunsandel.

Figure 6‑5 Rapid Flood Modelling, Dunsandel

6.10       Risk Assessment

A risk assessment has been undertaken for the Dunsandel 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 6‑2 details the risk priority rating and Table 6‑3 outlines the risks for this scheme.

Table 6‑2 Risk Priority Rating

Risk Score​	Level of Risk	Risk Response
> 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 6‑3 Risks - Dunsandel

​Risk	Action/Project	Year Identified	2014 Risk Rating	2017 Risk Rating	Residual Risk Rating
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.

 

6.11       Asset Valuation Details

The total replacement value of assets within the Dunsandel Scheme is $241,268 as detailed in Table 6‑4 below. 

Table 6‑4 Replacement Value, Dunsandel

Asset Class 1​	Asset Class 2	Sum of Replacement Value
Stormwater Reticulation ​​ ​​	Channel	$57,271
	Manhole	$41,830
	Pipe	$142,168

6.12       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 6‑6 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2019/20.​

​Figure 6‑6 Dunsandel Stormwater Renewal Profile

​ 

6.13       Critical Assets

The criticality model for Dunsandel 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 6‑5 and Figure 6‑7 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.

​Table 6‑5 Length of Assets per Criticality Level

Criticality Bands​	​	Length (m)
5	Low	1,823
4	Medium-Low	36
3	Medium	30​
2	Medium-High	0
1	High	0

​

6.14       Asset Condition

The asset condition model was run for Dunsandel 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 6‑8 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.

​Table 6‑6 provides a description of the condition rating used within the condition model.

Table 6‑6 Asset Condition Grading

Condition Rating​	Grading
1.0	Excellent
2.0	Good
3.0	Moderate
4.0	Poor
5.0+	Fail

 

6.15       Funding Program

The 10 year budgets are shown by Table 6‑7. Bu​dgets 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 6‑7 Budget Summary

Years	Projects	Capital Projects
2021/2022	-	-
2022/2023	-	-
2023/2024	-	$50,000
2024/2025	-	-
2025/2026	-	-
2026/2027	-	-
2027/2028	-	-
2028/2029	-	-
2029/2030	-	-
2031/2032	-	-
Total	-	$50,000

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 6​‑8 Key Projects​

Account Label	GL	Description​	Year 1($)	Year 2($)	Year 3($)	Years 4 to 10	Funding Split *
​Capital Projects​	-	Add treatment	-	-	$50,000	-	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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