Drainage

Purpose of the plan

1. Provide an overview of the City of Whittlesea’s drainage assets;
2. Identify drainage management procedures
3. Identify forward drainage strategies

Summary

Definition

Drainage assets comprise pits and pipes, WSUD systems and legal points of discharge (LPD) that are located in and service road reserves, other reserves, easements and private property. Assets in this class will generally be individually identified and registered in the Asset Management System.
Ownership of the pipework connection to the LPD in easements is unclear as to whether it is the responsibility of the property owner it serves, or Council.
Gross Pollutant traps and litter baskets in standard pits are installed on the drainage networks to stop litter being discharged to the waterways.
As Water Sensitive Urban Design (WSUD) systems become installed, a new class of drains will need to be separately identified for maintenance and valuation. These swale drains will contain filter media and slotted agricultural pipes to remove nutrients from stormwater.

SCOPE OF THE PLAN

Assets covered

There are currently (April 2003)
a. 30717 pits;
b. 30606 pipe segments with a total length of 747.3 km;
c. ……… rural culverts with a total length of …….. km;
d. ……… rural culvert endwall structures;
e. ……… km of biofiltration trenches associated with WSUD swale drains;
f. ……… rain gardens having a total area of ……… sq.m;
g. ……… wetlands having a total area of ……… sq.m;
h. ……… km of other open overflow swales required for 100 year design flows;
i. Quantities of LPD are unknown but could be assumed to approximate one per property (47,000).
j. 60 Gross Pollutant Trap pits;
k. 115 litter baskets in other pits.

Useful Life

Drainage Useful Life Table

The table below details the estimated useful life of a range of drainage assets.

Type Useful Life
Stormwater Pipes 100 Years
Stromater Pits 100 Years
Box Culverts 100 Years
GPTs ?? Years
WSUD Assets ?? Years

Asset Condition

The structural condition of underground pipe drains is not well known. A preliminary CCTV inspection (in 2003) of 1.5 km of concrete pipe drains in the oldest part of the Thomastown drainage network indicated that the expected life of those pipes will exceed the assumed design life of 100 years. Further inspection and analysis is required to make a determination on the overall network condition and expected life.
A sample condition survey in a number of areas with varying conditions should be undertaken in order to assess whether condition information is an important component of managing drains.

Assets not covered

1. Drainage assets which are the responsibility of other responsible road authorities – VicRoads; Parks Victoria; Department of Sustainability and Environment.
2. Main drainage assets which are the responsibility of Melbourne Water.
3. Drainage assets on private property which may discharge into Council’s drainage systems – examples are drains on commercial premises including car parks, drains on body corporate roads.
4. Drains on subdivisions which have not yet been accepted for practical completion by Council

DRAINAGE ASSET SYSTEM MANAGEMENT FRAMEWORK

Key Assumptions to Managing Risk of Flooding

1. The existing maintenance practices on drains are appropriate to the level of service provided.
2. The drainage system is inspected for maintenance
a. On a locality basis, which is a practice developed from attending to public complaints;
b. Giving attention to known drainage problem locations, including:
• pits where there is a risk of overflow through private property (e.g. at court bowls);
• commercial areas where litter build up may block drainage systems;
• pipelines known to be below desirable design capacity;
• Water Sensitive Urban Design locations subject to blockage – swales and rain gardens;
• Rural culverts known to carry a high debris or silt load.
c. Following a significant rain event
3. The drainage system CCTV inspection program is to target:
a. The oldest pipe systems to build up knowledge of structural integrity and probability of structural pipe failure;
b. New subdivisions to audit the installation work of contractors for acceptance purposes.
4. The WSUD biofiltration monitoring and audit system is to target:
a. Visual assessment of filter performance following a significant rainfall event;
b. Periodic audit of filter performance as deemed necessary from the visual assessment in (a) or from property owner complaint;
c. Maintaining filter areas free of litter and impediment to flow of stormwater through or into the system as designed.

Risk Management

1. The risk of public trip and fall is minimised through the inspection and pit lid replacement program.
2. Risk of flooding property is minimised through the inspection and cleaning of drains and pits.
3. Confined Space Training is provided to those staff who require entry into pits and manual handling training is provided for the lifting of pit lids.
4. Restricted and secured entry points into large pits and outfall drains is in place to minimise public access to large drains.

Data Protocol

1. The GIS is the key register for drainage data. Drainage data has been transferred from the previously used Paramap system and a process of validating the quantity and quality of this data was undertaken in 2003.
2. As new drainage assets are created, the data is captured in the GIS.
3. Council is currently involved with a project to obtain digital data from the design consultants to directly up-load to the GIS and to avoid translation from hard copy plans to the GIS.

Use of Asset Data

1. The prime use of the data stored in the GIS is for visual display of location and network component information.
2. The data in the GIS is to be also used for
a. asset valuation;
b. recording of condition
3. Gross Pollutant Traps (GPT’s) are separately identified due to the need to specifically manage these components which have an environmental impact. They are recorded in Authority Assets for Work Order management

Work Orders

Work on drainage assets is substantially routine cleaning with a lesser proportion of reactive maintenance. As the identification of the work is principally by location and involves a network of assets, planned activities and history of work on individual pipes and pits is not warranted.
To plan for future work and to capture work history on these assets, the Work Order will be linked to the nearest property to the asset and a Work Order Category representing drainage activities will be assigned to each Work Order.
In the case of GPT’s, a record of each GPT is in the Asset Module as a Subtype 2 of 6410. Asset values are not recorded in Authority as the Asset valuation of the GPT’s is derived from the GIS data.

LEVELS OF SERVICE

Design

Urban drainage pipes and WSUD swale drains or rain gardens in new areas are designed to have a hydraulic capacity to cope with the following rainfall events:

  • Residential drains and backbone WSUD pipe systems – 1 in 5 years
  • Industrial drains – 1 in 20 years
  • Closed system outfalls and overflow systems – 1 in 100 years
  • WSUD swales and rain gardens – 1 in 2 years
  • Biofiltration trenches – 1 in 3 months with filtration flow of ……………

Rural culverts are designed to have a hydraulic capacity to cope with the following rainfall events:

  • Vehicle speed 80 – 100kph: 1 in 100 years
  • Vehicle speed 60 – 80 kph: ……………….
  • Vehicle speed less than 60kph: ………………

Maintenance by the Operations Group

1. Urban drainage pits are serviced based on the municipality being divided into areas …………. The program within each area is based on:
a. cleaning pits (and associated pipes as necessary) with a frequency that addresses the sensitivity of the area as follows:
• Shopping Centres - three times per year
• Main Roads- twice per year
• Industrial Areas- bi-annually
• Residential Areas- bi-annually
• Gross Pollutant Traps - annually
b. pits (and associated pipes as necessary) in Hot Spot Areas- twice per year
2. Rural culverts are serviced on a needs basis based on visual assessment of rainfall event siltation or blockage from debris.
3. From the inspection process a program of pit and pipe cleaning is identified and scheduled.
4. Pit lids are replaced through the same inspection program and on a reactive basis.
5. Root cutting and major clearing works includes the programmed maintenance of Gross Pollutant Traps.
6. The drainage maintenance budget includes allocations for maintenance of Open Drains.

Links to Strategies & Plans

  • Urban Stormwater Management Plan

Practices, Policies & Standards

  • Design Standard: Australian Standard 4058, 1992
  • Standard drawings Council uses are SD 201 and SD 202

The practice at Whittlesea is to only use steel reinforced concrete pipes.

FINANCE

Current Replacement Cost

a. Pits and endwalls - $27.5m
b. Pipes and culverts – $102.9m
c. Biofiltration trenches in swales - $……………
d. Rain gardens - $ ……..
e. Wetlands - $ ………
f. Overflow swales - $ ………..
g. LPD - $ ………..
h. Gross Pollutant traps - $ ………
i. Litter baskets - $ ………

Funding sources

Sources for expenditure on municipal drainage assets includes the following:
a. Council rate revenue;
b. Roads to Recovery Program from the Federal government;
c. Special Charges;
d. Developer contributions.

Maintenance

Asset maintenance activities include
• pit, pipe, culvert and WSUD facility inspections, clearing and cleaning;
• root cutting;
• inspection, clearing and cleaning of Gross Pollutant Traps;
• open drain and culvert maintenance.

Funding in 2004-2005 is $ …………….. administered by:
a. Operations ($ ………..); and
b. Parks and gardens’ rural maintenance section ($ ………)

These activities aim to maintain public safety and keep assets in reasonable working condition to pass design flows.
Maintenance programs are developed in support of the budget, with notice taken of:
a. Customer requests
b. Inspection results
c. Relative risk to the travelling public and to property
d. Emergency response requirements as detailed in the Whittlesea Road Management Plan and the Whittlesea Road Maintenance Management Plan
e. Available finance

Asset Renewal

Asset renewal refers to the refurbishment or replacement of existing drainage assets due to structural failure of the asset component.
With pipe systems of a relatively young age the funding requirements of asset renewal are relatively minor at this stage.
Finance in the 2004-2005 budget of $……….. has been made available to cove minor asset modifications which arise from time to time and is also available for any unplanned asset renewal issues.
Predictive modelling of pipe asset renewal funding requirements is currently based on an age profile, assuming that concrete pipes have a life of 100 years. Predictive modelling will be expanded in the future to cover the following needs:
a. Pits, endwalls, LPD’s, GPT’s and Litter Baskets from continued visual assessment and estimation of expected life;
b. Pipelines, following collection of additional CCTV data and prediction of expected life;
c. Biofiltration trenches in swales and rain gardens from monitoring and audit programs;
d. Wetlands from visual assessment of siltation.

New Assets

Asset creation may be:
• From new road works;
• From establishing of new drainage networks or pipelines or structures;
• From upgrading the caacity of existing drainage networks to cope with an expansion of the urban catchment or redevelopment of the catchment into higher densities that produce more runoff in design storms.

Funding of asset creation can come from
a. Developer contributions - a drainage contribution overlay (strategy) is proposed to charge a contribution on medium density residential and extensions to industrial development;
b. Rate revenue; and
c. Special Charge Schemes targeted at specific improvements for property owners who gain secial benefit from those improvements

Council’s 2004-2005 budget provides for $ ……….. for Capital upgrade/renewal
Catchment studies are proposed to identify areas of capacity risk in Council’s urban drainage network. Identified catchments having capacity risk problems are:
a. Lalor-Thomastown catchment – study underway in 2004-2005
b. Darebin Creek catchment

??? Estimated costs of $3,178,000 maintenance and capital renewal in next 1-5 years, $2,688,000 in next 6-10 years and $3,187,000 in next 11-15 years (based on 2% increase per annum allowance for replacement of drains).
The estimate for Capital Upgrade and expansion of drains is $1,405,000 in the next 1-5 years.
Asset acquisition Whilst drainage asset acquisition from subdivision construction (gifting from developer to Council) does not in itself createa capital cost it does create an on-going maintenance cost which must be factored into Council’s long-term finance allocation.
In addition, there can be future outlays where capacity upgrades are required and as the road network or developed areas are expanded in accordance with growth pressures.
Future Demand In Green field developments, the drain design (approved through the planning process) minimises the impact on existing systems through the use of flow to waterways or wetlands. Infill development will place some demand on existing network hydraulic capacity.
Some older areas of Lalor and Thomastown as well as the area in Epping, north of Cooper Street, have been identified as having under-capacity drainage. This requires completion of current investigations to quantify remedial works. Each drainage catchment should be analysed for capacity on a progressive basis.
Water quality monitoring at drainage outfalls is required in conformity with the Urban Stormwater Management Plan. Future wetlands to assist in cleaning stormwater are a likely result.

Suggested Plan improvements

1. Conduct a sample survey of drain pipes by internal CCTV inspection.
2. Based on the results of item 1 determine a future inspection regime to ensure that the condition of drains is reasonably quantified.
3. Use the financial model for the pavement management system to determine network deterioration and expected replacement forecasts.
4. Provide operational data in GIS on gross pollutant traps such as pit lid type and accessibility.
5. Define location in GIS of litter baskets in pits.
6. Graphical representation of pit cleaning history to assist in planning optimum frequency for programmed cleaning.
7. Progressively determine whether pipelines in drainage catchments are undersized, with the following priorities:
a. Lalor – Thomastown Drainage Catchment to Edgars Creek (incl Westall St)
b. Cooper – Miller Street catchments to Darebin Creek

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