Effect of an Extreme Storm Event on Catchment Hydrology and Sediment Transport in the Magela Creek Catchment, Northern Territory

The Magela Creek catchment lies within the Alligator Rivers Region and is approximately 220 km east of Darwin. Located in the wet-dry tropics, the region experiences a distinct wet season from October to April and stream flow, as a consequence, is highly seasonal. During the 2006-07 wet season an extreme rainfall event occurred across the Magela Creek catchment over a three-day period between 27 February and 2 March 2007. Total rainfall on the lower part of the catchment area was 785 mm, the largest three-day rainfall recorded in the Top End. This extraordinary rainfall event resulted in the highest flood levels measured within the Magela Creek catchment since recording began in 1971. Several gauging stations within the catchment were completely submerged by flood waters. Peak discharge along the main channel was approximately eight times the mean annual flood discharge and more than double the 1:100 y flood event discharge. Fine suspended sediment concentration was monitored upstream and downstream of the Energy Resources of Australia Ranger Mine, which is located within the lower part of the Magela Creek catchment. Fine suspended sediment load measured at the downstream station during the flood was significantly elevated compared to both the upstream load and corresponding event discharge characteristics. This result has important implications for future impact assessment of mining-related activities at the Ranger Mine, particularly during rehabilitation when the site could be especially vulnerable to high rainfall events.