ABSTRACT
Roma town in Queensland, Australia, has a long history of significant flooding. Of particular interest are the 3 consecutive years of flooding of 2010, 2011, and 2012. Some facts of the unprecedented 2012 flood which was estimated to be over 1% annual exceedance probability were one fatality, and about 580 submerged properties above floor level. A Stage 1 flood mitigation project consisting of 5.2 km of earthen levee (average height of 2.8 m) was constructed in 2014 to protect areas of Roma against a repeat of the 2012 flood event with a freeboard of 300 mm. This paper presents strategies used to establish grasses to control erosion on the flood levee batters. Establishing grasses also provided an astatic pleasing environments.
Five levee sections were selected for treatment on both sides (total area 5.5 ha). There was no topsoil available to cover the batters, therefore, organic wastes from the council were processed with conditioners to generate soil that was conducive for establishing grasses.
Roma town water supply from boreholes had to be sourced for irrigation, but chemical analysis of the water indicated it is alkaline, saline and sodic and, thus, not conducive for establishing grasses unless treated (Table 1). The water pH of nearly 9 was adjusted by mixing 85% phosphoric acid with the water in poly tanks that supplied water to the irrigation system, the right amount added determined by titration tests carried out in a laboratory. A mixing ratio of 200 mL of acid per 5000 L of water reduced the pH to 7.1 which is conducive for grass establishment. The sodicity of the water was improved by mixing 50 L of a liquid gypsum in 15,000 L truck and sprayed periodically on the batters.
Buffel and Rhodes grass were seeded as they are popular for permanent pasture and, therefore, present no risk of introducing obnoxious weed. Buffel grass is considered the most drought tolerant grass in Australia and has adapted to a range of soil texture, surviving in regions with annual rainfall between 300 mm and 750 mm. Soil pH between 7 and 8 is ideal for buffel grass establishment.
As a result of water scarcity in the arid environment, precision irrigation involving the use of drip laterals has been an integral part of erosion control within the region (Gyasi-Agyei, 2007, 2011, 2013). The dripline networks, designed using hydraulic principles and simulation, consisted of several bays which had different lengths and numbers of lateral rows as dictated by fence lines of property boundaries and access road ramps. To ensure adequate mixing of the acid, the water was recirculated in the tanks using a pump for about 30 minutes before pumping through the irrigation networks.
The treatments imposed on the batters have worked very well with over 80% grass cover established at all sections on the levee batters. However, being the only green pasture within the vicinity during the initial winter period, kangaroos and cattle fed on the treated sites which prevented the grass runners from developing as expected. This factor was not considered as previous study did not encounter this problem. With the grass coverage achieved, erosion is expected to be minimal and by the end of the wet season, the coverage is expected to reach 100% at all sections.
