The impact of gully networks on the time-to-peak and size of flood hydrographs

European settlement of SE Australia has been associated with the extension and deepening of drainage lines by gully erosion. There has been considerable research into the hydrological impacts of clearing vegetation from catchments, but none into the hydrological implications of gully networks. Using surveyed data from a 5 km2 catchment in the Southern Tablelands of NSW we demonstrate, using the RAFTS hydrological model, that gully erosion alone (with all catchment parameters held constant) is likely to increase flood peaks from 12% to 20% and decrease time-to-peak by 20% to 24% over original conditions for the 100 year and the one year flood respectively. These increases are primarily due to reductions in channel roughness. Thus, the impact of gully erosion alone is likely to have had an impact on Australian stream ecology, and should be considered in flood design.