Fleer, D. 1992. Wanilla Seepage and Waterlogging Control Project. Department of Agriculture, South Australia, Technical Report No. 198.

This project was undertaken to assess the feasibility of interceptor drains (open and reverse) on two farms, 5 kilometres west of Wanilla, in the Lower Eyre Peninsula, South Australia, which have been impacted by waterlogging and soil salinity. The project area of 330 hectares is located on two adjoining properties owned by D. Oats and G. Giddings.

Waterlogging is known to be a major problem in the Lower Eyre Peninsula, South Australia. Based on farmer reports, the Department of Agriculture estimates that approximately 30% of productive land within the region is affected by surface flooding and waterlogging, on average, one year of two. Waterlogging is believed increase the potential for soil salinity, together with a major retarding effect on plant growth. Soil salinisation occurs on the lower slopes and drainage lines.

The site is gently undulating with slopes generally not exceeding 5%. The duplex soils are dominantly loamy sands to sandy loams, with a layer of ironstone gravel overlying a hard setting impervious yellow clay. It has been thought that when the soil becomes water logged, the ironstone gravel layer acts as a transmissive unit transporting excess water downslope. The subsoil is a light clay to light medium clay with 35-45% clay content and occasional medium grained sand.

The aim of the interceptor drains is intercept and remove substantial quantities of surface and subsurface water inot a suitable water way, to minimise waterlogging. The reverse bank interceptors and open interceptors were constructed using a grader or bulldozer and excavated through the sandy topsoil into the clay subsoil. A total length of 16.2 km of interceptor drains, surveyed at 0.6 and 0.7% slope were installed, together with the establishment of a 0.5 km length of man-made waterways. Additionally, 5.5 km of refencing was carried out with a total of 20 hectares being sown to salt tolerant grasses.

Project results have demonstrated that where waterlogging was apparent, interceptor drains provide wheat yield increases, both above and below the drain. However the effect is quite local Downslope effects appeared to extend for between 26 and 36 metres and uplsope effects 20 to 36 metres. Combining these figures to ensure overlap of downslope and upslope effects indicates that the optimum drain spacing is between 46 and 72 metres.

The area affected by salinity in the case study was not affected by the interceptor drains. Hopwever as they were effective in reducing water logging, where this waterlogging occurs near a salinised area, interceptor drains could be installed upslope to remove through-flowing fresh water which could further be salinised and spread the salinity effect

Cost - benefit analyses completed for the 1989 and 1990 monitoring years imply the drains provide increased gross margins and are payed for within or less than four years of continuous cropping. It is emphasised by the author that the construction of interceptor drains in the Lower Eyre Peninsula should only be considered for paddocks that experience waterlogging frequently (1 season in 2 when cropping), to ensure that the drains pay for themselves in a reasonable time period.

This study has demonstrated the technical and economic viability of seepage interceptor drains in dryland areas, given appropriate soil conditions (ie. duplex soils) and installation specifications. Detailed knowledge of site conditions are also required as it is emphasised by the authors that the engineering option is only likely to be economically feasible on paddocks that experience frequent waterlogging (1 season in 2 when cropping).

The following are key determining factors for the successful implementation of shallow surface interceptor drains to control waterlogging and saline soils in dryland areas:

• drain spacing adequate to maintain the water table at a particular depth below the ground surface;
• a level of economic return from the land subject to the drains;
• a level of economic value of the re-used water;
• ability to re-use the drained water; and
• a suitable drainage disposal or storage strategy.