It must be noted that this type of stormwater disposal system should not be limited to Western Australia and this article is prepared for Hydraulic Services Consultants to have a greater awareness of On-Site Stormwater Disposal System, (OSSDS). There are two main types of on site disposal systems that are selected based upon the area of catchment and anticipated volumes that need to be catered for in the design. These systems are as follows:

• Compensation basins - generally required for large catchment or where high ground water tables limit the effective depth of soak wells. In Western Australia most Local Government Authorities require compensation basins to be designed by a civil engineer. Compensation basins utilize evaporation and percolation to dispose of the runoffs collected. Sizing and general design requirements for compensation basins should be referred to a practicing Civil Engineer.
• Soak wells - circular pre-cast louvered concrete soak well liners installed with precast concrete covers or perforated plastic pipe culvert sections (domestic installations only). The concrete soak well liners are manufactured to standard diameters being 600mm, 900, 1070, 1200, 1500 and 1800. Larger capacity liner can be utilized where additional storage is required. The larger diameter liners are not louvered. Soak wells dispose of runoff by percolation in to the surrounding substrata.
• Alternative methods- such as dispersal drains or leaching drains are very rarely used due to the low storage capacity of such systems and high installation and maintenance costs.

The Advantages of an OSSDS.
Local Authority road drainage systems can be reduced in size as the regional catchment areas can be significantly reduced. Extensive internal drainage systems can be reduced as point of disposal is usually in close proximity of the catchment area.

The Disadvantages of an OSSDS.
Maintenance is required periodically to remove silt and debris that will enter the system. Some Local Authorities require a written statement accepting a minimum period of maintenance as part of the Building Approval. Pollutants could enter the system and become an environmental issue especially in areas where artesian water supplies are used. (Refer to pollution control.)

Requirements for an OSSDS
The effectiveness of any soak well design is dependent on many factors. The conditions vary from region to region and even within a site and carefully consideration must be given to the following when designing or locating a OSSDS

Type of soil
Soak wells should only be considered where the natural ground conditions allow free draining into the surrounding soil. Factors such as a high silt or clay content of the surrounding soil reduces the infiltration rate and in such conditions exist additional storage capacity is required. In addition to the increased storage capacity some Local Authorities require an aggregate surround to be installed around the soak well (normally 300 thick) to assist in the dispersal of runoff. Such designs must be discussed with the Local Authority engineer to ensure that overloading of the system does not occur.

General site topography
It is imperative that the location of an soak well allows sufficient area for percolation into the ground and that below structures such as basements are not adversely affected. It is preferred to have soak wells installed externally and usually at the lowest end of the site. This will permit shallower soak wells to be installed and in the advent of extreme rainfall conditions any flooding would be limited to the lowest portion of the site.

Peak Rainfall / Storm Conditions
If the region is subject to frequent storms of high intensity the soak well design must cater for retention of any surplus inflow that exceeds the rate of percolation/ disposal rate. It is not recommended to install soak wells where monsoons could occur unless flood control measures, such as an overflow discharge to street drainage systems, are incorporated.