There are a number of physical factors that will affect the design of the drain which should be considered.
In a conventional network, the minor drainage system is usually a pipeline with sufficient capacity to contain the nuisance flows. These pipelines prevent stormwater damage to properties and also limit the frequency and quantity of surface water to a level acceptable to the community. They're usually designed to cater for flows with a five year average recurrence interval (ARI). The pipelines don't always follow the natural drainage path and are usually aligned along property boundaries and the roadway kerb and channels.
The major drainage system caters for runoff from storms producing higher peak flows than the minor drainage system has been designed. The major drainage system is designed to handle overland flows resulting from storms with a 100 year ARI. A roadway, a drainage channel in a reserve, a pipeline or a combination of the above may act as a drainage line. The finished surface level of the allotments adjacent to the overland flow path must be at least 300mm above the 100 year ARI flood level.
Special details such as reverse fall nature strips and depressed footpaths may need to be provided at road low points to allow transverse flow of flood waters where adjacent to drainage reserves. Footpaths shouldn't be depressed if the flow is traveling longitudinally along the roadway. Designers should refer to the "Floodway Safety Criteria" Appendix A, for guidelines on channel flow design.
The design of the drainage system begins with the subdivisional layout. Roads and reserves must be located along the valley floor to serve as the major drainage system. Courts should not run downhill to the court bowl unless provision is made for the 100 year ARI flow downstream of the low point in the court bowl. This can be achieved by either a suitable drainage reserve at the court bowl, or a design pipeline. Easements for the minor drainage system must be determined when preparing the subdivisional layout.
The design of the waterways, drainage and water quality works within any project area must be consistent with the optimum design for the entire catchment area and any future extension of the system must be taken into account. This may affect the drain location, capacity, type, diameter, depth and manhole location.
Ground topography and geomorphology
Natural water courses should always be retained and drainage works should be designed to follow the natural surface depressions as closely as possible. The natural waterway morphology should also be retained or mimicked where possible in the design. This will minimise the amount of excavation required during construction.
In some cases, the depth of the drain can be reduced by offsetting the drain from the natural watercourse. This will be possible only if there's enough space available in the reserve and the inlet connections can still be made.
When designing drainage pipelines, the design must be extended to cross any road or proposed road widening that's adjacent to the development. This ensures any existing or proposed services within the road reserves are known and the proposed drainage works can cross these services.
As well as generally following the natural watercourse alignment, drains should, as far as practicable, be designed to follow easements or reserves intended for drainage purposes. Where a drainage reserve or easement forms part of the major drainage system it must be wide enough to cater for flood flows of a 100 year ARI.
Easements and reserves must be sized to cover the width of the drain as well as provide for access, construction and maintenance.
Construction and maintenance requirements are particularly important considerations when designing channel or waterway works. Relevant considerations include:
the maximum slope and height of the grassed batters
whether trucks require access to the channel invert
the location of inlets and roads
whether there are limitations on the use of certain construction techniques (when adding to, or duplicating existing drains in built-up areas for example)
the distance from the proposed alignment to any existing services (particularly if there are any issues with side loading effects during excavation)
the location of overhead power lines and the width of streets
Location of an underground drain within the pavement area of a road is acceptable if it's not practical to locate the pipeline outside the pavement area.
Possible limitations on the use of certain construction techniques need to be considered when selecting an underground drain alignment. This is particularly the case when adding to, or duplicating existing drains in built-up areas.
Surface obstructions such as buildings, electricity supplier poles, native vegetation or large trees etc. may affect the proposed alignment of the drain. These should be located during the initial survey and inspection of the site. If the proposed drain location can't avoid or closely approaches the obstruction, it'll be necessary to contact the owner, local council or relevant authority. The designer should compare the construction costs to either avoid, or where feasible, relocate the obstruction before deciding on the final route of the drain.
Existing bridges or culverts can also influence the design. These must be checked to ensure they're capable of coping with the additional discharge from the proposed drain. The alignment and capacity of the existing structure may need to be improved during construction if necessary.
Underground services that have the potential to affect drainage design include:
underground electricity cables
traffic signal cables
A search should be undertaken to check for major services at least 100m both upstream and downstream of the proposed works. The depth of a future drain may be affected by some or all of these services, and also by any building foundations or similar below-ground structures. Changing the level or location of one section of a proposed drain could avoid future costly changes to major services.
Ourselves, water companies or relevant service agencies should be contacted for information on their assets, such as size, depth and location. Site inspection and proving excavations should also be utilised to identify and confirm potential underground obstructions.
When designing a drain to run parallel to a sewer, it's important to consider how this will affect house connections. This aspect should be checked early in the design stage to enable the most economic solution to be adopted.
Where a proposed drain passes over an existing or proposed sewer, the designer must consider the effects of the drain on the sewer and vice versa. For drains of 825 millimetre diameter or more with less than 500 millimetre clearance, either a special base will be required or the sewer will need to be built into a concrete make-up section.
Where the drain passes under an existing sewer, the design must provide support for the sewer both during construction and following backfilling of the trench. The designer should refer to the Water Services Association of Australia Sewerage Code for guidance.
If feasible, at least one metre horizontal clearance should be provided between a drain and another service. In special circumstances less horizontal clearance may be adopted but the relevant authority must be consulted for approval.
It's usually more economical to raise or lower a minor underground obstruction than to change the design depth of the drain. Major services such as oil pipelines and HP gas transmission lines however, are costly and difficult to change. It may be necessary to design the drain as a twin conduit where it crosses the service. Alternatively, if the service is located near the top of the drain and the drain invert cannot be lowered, a 'duckunder' structure may be used subject to approval.
Note: If spigot and socket pipes are to be used then clearances should be checked from pipe collar to pipe collar unless specified otherwise by the relevant service authority.
Some factors that influence the drain depth include:
the hydraulic considerations
the provision of space above a drain for other services
the size and depth of existing culverts and bridges
the ability to adequately service upstream catchment properties likely to be developed in the future
the provision of sufficient grade for the future downstream extension of the pipeline
sufficient cover for future road grading and pavement depth
Where possible, the drain should be designed as high as possible to minimise construction costs. The minimum cover over underground pipe drains is 850 millimetres. However less clearance is permissible for short lengths where justified by local circumstances.
The cover over the pipe is to be considered to ensure it does not compromise pavement design or location of other services.
Ground conditions affect the design depth of both underground pipes and channels. If clay or soft material overlies rock, it may be necessary to install a larger drain with a flatter grade at a higher level or change the cross sectional shape to reduce the amount of rock excavation.
Where surface drains are to be located in areas where a high water table exists, the channel should be designed at a depth to minimise uplift forces on any lining and reduce de-watering problems during construction. The use of a "natural" open waterway rather than an artificially lined channel will always be preferred.
The presence of friable, dispersive or soluble clay can also be challenging when designing open drains or structures with weepholes. Two potential issues are soil erosion and the leaching of clay material through weepholes. Channel depths may have to be minimised in these areas to overcome possible problems.
Designers should consult an experienced geo-technical consultant to determine suitability of proposed works to the geo-technical conditions of the site. Sufficient test pits and/or bore samples should be taken to determine ground water levels and rock depths etc. prior to commencement of design. Geo-technical consultants may provide advice for construction techniques or application of materials standards to lower construction costs.
It's often necessary to excavate a temporary outfall at the downstream end of the drainage pipeline. The design of the temporary outfall must be undertaken following discussions with the affected landowner/s.
Consideration must be given to factors including:
access and stock crossings
stockpile or removal of excavated material
stabilisation of bed and banks of temporary channel
Wherever possible the design and alignment of underground and surface drainage should avoid or minimise the impact on existing environmental values. This includes direct impacts such as tree removal and habitat disturbance as well as ecological process issues, such as changes in water regimes for wetlands or grasslands. The following issues should be considered:
impact on remnant indigenous trees and shrubs
impact on remnant native grasslands or wetlands
impact on native fauna, including local and migratory species
impact on listed threatened species
potential to encourage weed invasion
Culture and heritage assets
A heritage place is a specific area or site, perhaps a large area such as a whole region or landscape, or a small are such as a feature or a building, which is valued by people for its natural and/or cultural heritage significance.
The community places a high value on the significance of indigenous and non-indigenous places and sites. In turn, communities place expectations on all levels of government to protect our natural and cultural heritage. In planning for the future, it's important to ensure all elements of significance are protected.
Drainage design should always consider the impact on cultural and heritage assets. The developer must undertake all necessary investigations to determine the presence of any assets and ensure the drain design protects, preserves and promotes cultural and heritage assets.
Government policy is set up to protect the environment and support development. The requirement of the policy is to demonstrate the environment isn't adversely affected by the development. Therefore, the designer must follow the major/minor approach to drainage systems as outlined in Elements 12a and 12b of the Victorian Code for Residential Development (ResCode) or Australian Rainfall and Runoff (1998).