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5 Doyleston Stormwater Scheme

5.1 Scheme Summary

Description		Quantity
Scheme Area		44.68ha
Scheme Coverage (as at 1 Jan 2021)	Rating numbers	121
System components	Piped (m)	1,590
	Swales (m)	184
	Drains (m)	2,943
	Manholes/Inspection Chambers (No.)	19
	Treatment	N/A
	Other	N/A
Value ($)	Replacement Cost	$679,235
Value ($)	Depreciated Replacement Cost	$583,887
Financial	Operator cost (scheduled and reactive maintenance) per connection	$14.80/connection
Planning	Stormwater Management Plan	Draft
Planning	No. SDC stormwater consents	1 (application in progress)
Demand	Mean Annual Rainfall (mm)	629
Demand	10% AEP (10 year) 1hr rainfall depth (mm)	19.2
Sustainability	Sustainable drain management practices	Adopted and Encouraged

5.2 Key Issues

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

Table 5‑1 Doyleston Scheme Issues

What's the Problem	What we plan to do
Flooding occurs in the township during heavy rainfall events with classified drains overtopping.	Continue to provide early warning of flooding events. Provide funding for minor upgrades to the network and agree in conjunction with the Community.
Increasing expectations from the community regarding level of service received from stormwater and drainage assets	Increase maintenance budgets to allow for additional maintenance particularly of open drains

5.3 Overview & History

Doyleston is located 2.5km northeast of Leeston. Stormwater runoff is typically conveyed by open channel or piped system to the '82 Drain Road North' land drainage drain. Stormwater runoff from the remaining catchment discharges to Boggy Creek on the northern boundary of the township.

Flooding has been known to occur during heavy rain when drain levels are high e.g. June 2013 when flooding of Greenan Place, Petticoat Lane and Railway Terrace occurred. The local volunteer fire service undertook sand bagging. A level transducer has been installed in the Osbornes Park Drain to provide flood warning. The alarm is mentioned by Sicon. The Swale in Greenan Place is influenced by the classified drain level. The swale underdrain has been repaired and water blasted. The swale is planned to be planted in 2013/14.

Figure 5-1
DOYLESTON Stormwater - Scheme map
Open larger map: 'Stormwater Scheme'

Figure 5‑2 Scheme Schematic

5.4 Resource Consents

Council have applied for a global stormwater consent for the Doyleston scheme (CRC186171). The application is underway with ECan.

Consent CRC063693 is currently held by Horncastle Homes Limited (the developer) and expires 27-Jul-2041.

5.5 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 Doyleston stormwater management plan was updated in April 2017 and submitted to ECan.

5.6 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.

A summary of material and diameter for channels and pipes, where known, is shown below in Figure 5‑3 and Figure 5‑4.

Figure 5‑3 Pipe Material - Doyleston

Figure 5‑4 Pipe Diameter – Doyleston

5.7 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.

5.8 Photos of Main Assets

The photos below provide a summary of the types of assets found within this stormwater management area.

Photo 1: Typical swale after light rain

5.9 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 5‑5 shows the predicted flooding for Doyleston.

V4 Doyleston - Rapid Flood Modelling, Doyleston.png

Figure 5‑5 Rapid Flood Modelling, Doyleston

5.10 Risk Assessment

A risk assessment has been undertaken for the Doyleston 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 5‑2 details the risk priority rating and Table 5‑3 outlines the risks for this scheme.

Table 5‑2 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 5‑3 Risks – Doyleston

Risk	Action/Project	Year Identified	2014 Risk Rating	2017 Risk Rating	Residual Risk Rating
Major flooding	Review options to reduce flooding	2014	10	10	10
Non-consented activities	Renewal of consents	2014	27	27	6
Major flooding	Upgrades	2017		9	4
Stormwater management	Stormwater management plan	2017		6	2.1

The list of district wide risks can be found in 5Waters Activity Management Plan: Volume 1.

5.11 Asset Valuation Details

The total replacement value of assets within the Doyleston Scheme is $679,235 as detailed in Table 5‑4 below.

Table 5‑4 Replacement Value, Doyleston

Asset Class 1	Asset Class 2	Sum of Replacement Value
Stormwater Reticulation	Channel	$104,393
	Inlet-Outlet-Point	$28,514
	Lateral	$15,370
	Manhole	$55,339
	Pipe	$475,619

5.12 Renewals

The renewal profile has been taken from the 2019 5 Waters Valuation. A graph showing the renewals for this scheme is shown by Figure 5‑6 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2037/38.

Figure 5‑6 Doyleston Stormwater Renewal Profile

5.13 Critical Assets

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

Table 5‑5 Length of Assets per Criticality Level

Criticality Bands		Length (m)
5	Low	2,993
4	Medium-Low	180
3	Medium	334
2	Medium-High	13
1	High	0

Figure 5-7
DOYLESTON Stormwater - Criticality map
Open larger map: 'Stormwater Criticality'

5.14 Asset Condition

The asset condition model was run for Doyleston 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 5‑8 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.

Figure 5-8
DOYLESTON Stormwater - Condition map
Open larger map: 'Stormwater Condition'

Table 5‑6 provides a description of the condition rating used within the condition model.

Table 5‑6 Asset Condition Grading

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

5.15 Funding Program

The 10 year budgets are shown by Table 5‑7. 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 5‑7 Budget Summary

Years	Projects	Capital Projects
2021/2022	-	-
2022/2023	-	$35,000
2023/2024	-	$20,000
2024/2025	-	$600,000
2025/2026	-	-
2026/2027	-	-
2027/2028	-	-
2028/2029	-	-
2029/2030	-	-
2031/2032	-	-
Total	-	$655,000

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 5‑8 Key Projects

Account Label	GL	Description	Year 1($)	Year 2($)	Year 3($)	Years 4 to 10	Funding Split *
Capital Projects	-	Doyleston flood diversion/main drain upgrade	-	$30,000	-	$500,000	100% LoS
Capital Projects	-	Stormwater treatment upgrades	-	-	-	$100,000	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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