What is a graded or sheeted catchment? How do they work?

Stock Journal Feature, November 2022

A graded or sheeted catchment is an area of land that is specifically managed to maximise water catchment and run-off into a dam. When considering a sheeted catchment there are a number of factors to consider including approvals, the type of catchment, planning and siting, size and water storage options, soils for foundation and bank materials, and location and proximity to where water will be used. These factors will impact the total cost of construction as well as the ongoing maintenance and  costs.

When thinking about installing a catchment, it is recommended that you undertake research on the topic, visit other catchments and talk to the owners. Some of the important considerations are discussed below, however this discussion is not exhaustive.

Contact your local Landscape Board and Department for Environment and Water for information and advice on permits and approvals for any proposed catchment. Development approval from your local council may also be required depending on the capacity of the ‘dam’, contact your local council for further details.

There are a number of different methods for creating a catchment, including the use of existing roads, smooth rock outcrops, unlined graded catchments and plastic lined catchments.   For unlined catchments run off is dependent on exposing and rolling less permeable clay soils.   The amount of run off from unlined soil is estimated to vary between 5% and 40%, depending on rainfall.   Where existing rock outcrops act as the catchment, run off estimates are 45-50% and with the use of plastic sheeted catchments  constructed of welded plastic sheets or where the catchment area is bituminised, runoff estimates are 75-90%.

Considerations must include the amount and seasonality of rainfall at the site as this will effect the size of catchment needed to collect the required supply of water.   The volume of water required will depend on livestock needs, volumes required for spraying, any domestic use and capacity required for dry periods/drought and evaporation from the dam.

Other considerations include; siting the catchment in an elevated area to help reduce pumping costs; ensuring that there is sufficient room to expand the catchment in the future if required; and avoiding steep ground. A level area with a gentle slope is best.

When constructing a catchment, it is important to ensure the catchment and storage dam surfaces are smooth and level to prevent water pooling and evaporation, and where the surface is to be lined, it must be free of rocks and sticks or other objects which may damage the plastic liner.

Fencing of the catchment and storage dam is important to prevent livestock and wildlife damaging the lining. Considerations for catchment and dam liners include the length of the warranty, safety of people (as lined dams are very slippery install a ladder or safety rope) and how the liner will be held down (tyres are generally used but do not underestimate how many are required!). If the dam is to be used for water storage, consider the amount of evaporation from the dam, how this may be minimised and management of algae growth.

Catchments are not necessarily a cheap water option, but costs can be reduced with some careful planning.

Further info:

The publication Farm Dams – A Guide to Siting, design, construction and management on Eyre Peninsula is a good source of information.