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​​​1​   Arthurs Pass Stormwater Scheme

1.1       Scheme Summary

Description​	​	Quantity
Scheme Area		23.18ha
Scheme Coverage (as at 1 Jan 2021)	Rating numbers	136​
System components ​​ ​​ ​​ ​​ ​	Piped (m)	920
	Swales (m)	42
	Drains (m)	938
	Manholes/Inspection Chambers (No.)	7
	Treatment	2 (Humeceptor + Oil Interceptor)
	Other	Infiltration trench
Value ($) ​	Replacement Cost	$332,902
	Depreciated Replacement Cost	$270,933
Financial	Operator cost (scheduled and reactive maintenance) per connection​	$57.11/connection
Planning ​	Stormwater Management Plan	Draft
	No. SDC stormwater consents	2
Demand ​	Mean Annual Rainfall (mm)	4330
	10% AEP (10 year) 1hr rainfall depth (mm)	37.4
Sustainability	Sustainable drain management practices	Adopted and Encouraged

 

1.2       Key Issues

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

​Table 1‑1 Arthurs Pass Scheme Issues

What's the Problem​	What we plan to do
Area outside the Outdoor Centre floods from the surrounding hillsides during heavy rainfall.  Includes septic tank flooding.	Monitor effectiveness of minor improvements made, determine responsibility.
Soakage trench by the Railway underpass has limited capacity and other floods during large rainfall events.	Options analysis undertaken by Opus consultants in 2013 and Community Committee opted to improve maintenance and undertake no capital works.
Costs of undertaking maintenance works in the township are high including proprietary device cleaning due to travel distances.	Continue to work alongside roading contractor and the Community to achieve best value for money maintenance.

 

1.3       Overview & History

The Arthurs Pass stormwater system services the car and bus parking and public toilet facility at Arthur​​​​​​s Pass.  These facilities sit outside the NZTA system.  Stormwater from the "bus and car parking area" is directed via kerb and channel to a rock-lined swale, and then piped to a proprietary device before discharging to a gravel soakage trench.  Stormwater from the "DoC carpark" is directed via kerb and channel to a petrol and oil interceptor and then discharges via open drain to the same soakage trench.

Arthurs Pass has a high annual rainfall of 4330mm, which is the highest in the district.

The community committee raised concerns about the flooding of the underpass when the soakage trench capacity was exceeded (Refer to Photo 3 in Section 1.8). An investigation of options to improve drainage of the soakage trench was undertaken by Opus Consultants in 2013. The community committee opted to remain with the status quo with additional maintenance.

Flooding is known to occur outside the outdoor centre from the drain on private property behind the store, minor improvements to the culvert was made in 2013 by the roading team and will be monitored.

Due to the isolated nature of the community in Arthurs Pass, a high level of communication with the community committee is required to help identify issues and opportunities for improvement.

There are a number of stormwater issues in Arthurs Pass on private property which are outside Council control.​

​ 

Figure 1‑2 Scheme Schematic

1.4       Resource Consents​

The Arthurs Pass stormwater scheme has a number of resource consents. Table 1‑2 shows the stormwater discharge permitted by the resource consents for this scheme.

Table 1‑2 Resource Consents

Consent	Description	Location	Date Issued	Expiry Date
CRC080003	To discharge stormwater to land from roofing and hardstand	State Highway 73, ARTHURS PASS	30/10/2007	25/10/2042
CRC084392	To discharge hardstand stormwater to ground.	Cloudesley Road, BEALEY SPUR	N/A	N/A
​CRC186179	​To discharge stormwater to land and to surface water from the existing stormwater network of Arthur's Pass	​Arthur's Pass	​Application on hold	​-

Consent CRC084392 is located outside the stormwater management area but is included for completeness.

 

1.5       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 Arthur​​s Pass stormwater management plan has been lodged as part of the application for consent CRC186179​. ​

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1.6       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.      Oil Interceptor – Is a proprietary device which uses baffles to trap and contain hydrocarbons (oils and fuels).

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

d.      Swale (Rock lined) – Is a longitudinal open channel which is lined with rocks.  The swale both conveys and treats stormwater.

e.      Soakage trench – Carries out the same function as soakholes but is orientated in a horizontal direction rather than vertically.  They are particularly useful in areas with reduced infiltration rates or higher ground water tables.

A summary of material and diameter for channels and pipes, where known, is shown below in Figure 1‑3 and Figure 1‑4.

​Figure 1‑3 Pipe Material – Arthurs Pass

​Figure 1‑4 Pipe Diameter – Arthurs Pass

 

1.7       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.

 

1.8       Photos of Main Assets

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

​ Photo 1: Soakage Trench	​ Photo 2: Rock lined swale
​​ Photo 3: Arthurs Pass in Flood​

The soakage trench in Photo 1 requires an annual clean of accumulated sediment.

 

1.9       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 Arthurs Pass.

1.10       Risk Assessment

A risk assessment has been undertaken for the Arthurs Pass 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 1‑3 details the risk priority matrix and Table 1‑4 outlines the risks for this scheme.

Table 1‑3 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 1‑4 Risks – Arthurs Pass

Risk​​	Action/Project	Year Identified	2014 Risk Rating	2017 Risk Rating	Resid​ual Risk Rating
Non-consented activities	Renewal of consents	2014	27	27	6
Network maintenance	review public / private drains	2017		12	2
Scheme capacity	infiltration basin overflow design + consent	2017		27	12
Scheme capacity	infiltration basin overflow construction	2017		27	12

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

 

1.11       Asset Valuation Details

The total replacement value of assets within the Arthurs Pass Scheme is $332,902 as detailed in Table 1‑5 below. 

Table 1‑5 Replacement Value, Arthurs Pass

Asset Class 1​	Asset Class 2	Sum of Replacement Value
​Stormwater Reticulation​	Channel	$59,488
	Inlet-Outlet-Point	$17,280
	Lateral	$23,181
	Management Device	$37,648
	Manhole	$48,801
	Pipe	$146,503

 

​1.12       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 1‑5 below. The majority of assets requiring renewal are culverts/pipes which occur in the year 2040/41.​

​Figure 1‑5 Arthurs Pass Stormwater Renewal Profile

 

1.13       Critical Assets

The criticality model for Arthurs Pass 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 1‑6 and Figure 1‑6 below show​ the calculated criticality for all of the assets within this scheme that have a recorded known length.

​Table 1‑6 Length of Assets per Criticality Level

Criticality Bands​	​	Length (m)
5	Low	711
4	Medium-Low	216
3	Medium	148
2	Medium-High	0
1	High	52​