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15 Rolleston Stormwater Scheme

15.1 Scheme Summary

Description		Quantity
Scheme Area		975.46 ha
Scheme Coverage (as at 1 Jan 2021)	Rating numbers	6,438
System components	Piped (m)	27,340
	Swales (m)	8,086
	Drains (m)	27.8
	Manholes/Inspection Chambers (No.)	197
	Treatment	31 Infiltration basins, 74 Proprietary devices, 11 silt traps, 4 rain gardens
	Other	Versitanks 1,685 soakholes
Value ($)	Replacement Cost	$5,890,885
Value ($)	Depreciated Replacement Cost	$5,315,761
Financial	Operator cost (scheduled and reactive maintenance) per connection	$31.20/connection
Planning	Stormwater Management Plan	Yes
Planning	No. SDC stormwater consents	15
Demand	Mean Annual Rainfall (mm)	643
Demand	10% AEP (10 year) 1hr rainfall depth (mm)	19.6
Sustainability	Sustainable drain management practices	Adopted and Encouraged

15.2 Key Issues

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

Table 15‑1 Rolleston Scheme Issues

What's the Problem	What we plan to do
Capacity upgrades which are a result from increasing expectations from ratepayers that a higher level of service LOS is provided.	Identify capacity restrictions in the system, design upgrades and budget for physical works in LTP.

15.3 Overview & History

Due to the relatively free draining soils which underlie the township of Rolleston, the stormwater system generally consists of roadside sumps discharging directly to soak holes.

A number of the newer subdivisions have incorporated soakage basins as part of the overall stormwater management. The Izone industrial area also incorporates a number of proprietary treatment devices.

Figure 15-1
ROLLESTON Stormwater - Scheme map
Open larger map: 'Stormwater Scheme'

Figure 15‑2 Scheme Schematic

15.4 Resource Consents

The Rolleston stormwater scheme has a number of resource consents. Table 15‑2 shows the stormwater discharge permitted by the resource consents for this scheme.

Table 15‑2 Resource Consents

Consent	Description	Location	Date Issued	Expiry Date
CRC054637 Issued - Active	To discharge stormwater on to land	Burnham School Road, ROLLESTON	5/10/2005	30/09/2040
CRC091220.2 Issued - Active	To discharge contaminants into land.	Hoskyns Road, ROLLESTON	9/11/2010	28/06/2041
CRC091824.1 Issued - Active	To discharge contaminants into land.	Hoskyns Road, ROLLESTON	15/04/2009	28/06/2041
CRC100025 Issued - Active	To discharge contaminants into land.	Izone Drive, ROLLESTON	15/05/2009	28/06/2041
CRC100027 Issued - Active	To discharge contaminants into land.	Izone Drive, ROLLESTON	15/05/2009	28/06/2041
CRC100057 Issued - Active	To discharge contaminants into land.	Izone Drive, ROLLESTON	15/05/2009	28/06/2041
CRC100069 Issued - Active	To discharge contaminants into land.	Izone Drive, ROLLESTON	15/05/2009	28/06/2041
CRC110923 Issued - Active	To discharge contaminants into land.	Izone Drive, ROLLESTON	24/06/2011	28/06/2041
CRC133608 Issued - Active	To discharge contaminants into land.	10 George Holmes Road, ROLLESTON	1/02/2013	1/02/2043
CRC100819.1 Issued - Active	To discharge contaminants into land.	Railway Road, ROLLESTON	18/02/2010	19/11/2044
CRC100886.1 Issued - Active	To discharge contaminants into land.	Railway Road, ROLLESTON	18/02/2010	19/11/2044
CRC132527 Issued - Active	To discharge stormwater to land	Rolleston Township, Selwyn District	16/01/2014	16/01/1949
CRC145959 Issued - Active	To discharge contaminants into land.	Izone Industrial Park, ROLLESTON	1/07/2013	24/06/2046
CRC173495 Issued - Active	To discharge contaminants into land	Lot 1 DP 488477 (Izone Industrial Park)	24/06/2016	24/06/2046
CRC142372 Issued - Active	To discharge stormwater	East Maddisons Road, Rolleston	28/06/2013	02/11/2040

Council obtained a global stormwater consent for the existing township stormwater disposal in January 2014 (CRC132527). Eleven residential consents were surrendered. The consent requires treatment in specific areas and excludes all industrial and commercial developments.

15.5 Integrated Stormwater Management Plan

An Integrated Stormwater Management Plan (ISMP) was prepared for Rolleston as part of Council's global discharge consent application in November 2012. The consent was granted January 2014. The ISMP is currently being updated.

15.6 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. Humeceptor - Is a hydrodynamic separator which helps to reduce mass sediment load from the discharge, some removal of hydrocarbons is also achieved.

b. Swale (Grassed) – Is a longitudinal open channel which is lined with grass. The swale both conveys and treats stormwater.

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 15‑3 and Figure 15‑4.

Figure 15‑3 Pipe Material – Rolleston

Figure 15‑4 Pipe Diameter – Rolleston

15.7 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.

15.8 Photos of Main Assets

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

Photo 1: Stormwater Basin

Photo 2: Stormwater Basin

15.9 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 15‑5 shows the predicted flooding for Rolleston.

Figure 15‑5 Rapid Flood Modelling, Rolleston

15.10 Risk Assessment

A risk assessment has been undertaken for the Rolleston 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 15‑3 details the risk priority rating, Table 15‑4 outlines the risks and the list of key projects is found in Table 15‑9.

Table 15‑3 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 15‑4 Risks - Rolleston

Risk	Action/Project	Year Identified	2014 Risk Rating	2017 Risk Rating	Residual Risk Rating
Complaints about appearance of stormwater basins	Beautify stormwater basins	2014	4	4	2
Soil contamination	Basin sediment sampling	2014	12	12	6
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.

15.11 Asset Valuation Details

The total replacement value of assets within the Rolleston Scheme is $5,890,885 as detailed in Table 15‑5 below.

Table 15‑5 Replacement Value, Rolleston

Asset Class 1	Asset Class 2	Sum of Replacement Value
Stormwater Reticulation	Chamber	$10,440
	Channel	$413,775
	Inlet-Outlet-Point	$433,082
	Lateral	$134,043
	Management Device	$1,156,512
	Manhole	$1,100,933
	Pipe	$2,028,009
	Soakhole	$614,090

15.12 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 15‑6 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2025/26.

Figure 15‑6 Rolleston Stormwater Renewal Profile

15.13 Critical Assets

The criticality model for Rolleston 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 15‑6 and Figure 15‑7 below show the calculated criticality for all of the assets within this scheme that have a recorded known length.

Table 15‑6 Length of Assets per Criticality Level

Criticality Bands		Length (m)
5	Low	13,093
4	Medium-Low	1,231
3	Medium	194
2	Medium-High	0
1	High	0

Figure 15-7
ROLLESTON Stormwater - Criticality map
Open larger map: 'Stormwater Criticality'

15.14 Asset Condition

The asset condition model was run for Rolleston 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 15‑8 below shows the level of asset condition for all of the assets within this scheme that have a recorded known condition.

Figure 15-8
ROLLESTON Stormwater - Condition map
Open larger map: 'Stormwater Condition'

Table 15‑7 provides a description of the condition rating used within the condition model.

Table 15‑7 Asset Condition Grading

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

15.15 Funding Program

The 10 year budgets are shown by Table 15‑8. 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 15‑8 Budget Summary

Years	Projects	Capital Projects
2021/2022	-	$10,000
2022/2023	-	$60,000
2023/2024	-	$20,000
2024/2025	-	$60,000
2025/2026	-	$10,000
2026/2027	-	$10,000
2027/2028	-	$10,000
2028/2029	-	$10,000
2029/2030	-	$10,000
2031/2032	-	$10,000
Total	-	$210,000

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 15‑9 Key Projects

Account Label	GL	Description	Year 1($)	Year 2($)	Year 3($)	Years 4 to 10	Funding Split *
Capital Projects	-	Retrofit stormwater treatment	$10,000	$50,000	$10,000	$50,000	100% LoS
Capital Projects	-	Rain garden enhancement	-	$10,000	$10,000	$70,000	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.