From past to present: hydrological and morphological characteristics of Wangi Creek, Northern Territory

Flooding in the tropical Northern Australia is under researched relative to other regions of the continent yet the entire northern portion of the continent is affected by Australia’s most significant climatic phenomena, the Australian Indo Monsoon. This study of Wangi Creek (13° 09\u27 49 S, 130° 41\u27 07 E) in the Litchfield National Park investigates flood dynamics during the 2013-14 wet season. Wangi Falls cascades off an ungauged catchment on the Tabletop Plateau in the Litchfield National Park. The study reach extends from the plunge pool 3.1 km downstream, a key site investigated in previous paleohydrological studies due to the interpretation of extreme discharges in the Last Glacial Maximum. Using existing high resolution topographic data (LiDAR), plunge pool bathymetry, flood flow stage levels, catchment rainfall and soil moisture, an analysis of wet season rainfall-runoff relationships was undertaken. This was then used for a detailed analysis of inundation patterns and hydraulic characteristics in the study reach. Various scenarios of flow stage conditions were modelled using a 1D steady-state hydraulic model (HEC-RAS), resulting in the quantification of inundation patterns during differing flood stages. Results show that maximum daily rainfall was 201 mm and hourly rainfall 72.6 mm with peak hydrographic responses occurring within 30 minutes of maximum rainfall, Modelled discharge estimates of the ~ 4 m flood flows in 2014 range from 350-500 m 3/s, depending on selected roughness parameters. Previous estimates of extreme discharge of 8 m flow depth (3600 m3/s) are examined and the hydraulic model supports such estimates in the upper part of the study reach. However, the presence of a migrating knickpoint (lowering modern levels by up to 2 m) is shown to have a large impact on predicted discharge. Precipitation intensities and unit discharge required to produce such predicted runoff are investigated within the thesis. There have been no observed required precipitation intensities in the Northern Territory to produce the past paleodischarge estimates nor has there been any observed unit discharge values recorded high enough to produce such discharge estimates for a catchment area the size of Wangi Creek. There are however estimates nationally (based on Probable Maximum Precipitation – PMP) that suggest that these rainfall intensities are possible and this is globally verified. Such issues are discussed in the context of previous research on this river which has suggested maximum floods during the Last Glacial Maximum, pointing to a much wetter and extreme climatic period. Beyond this local case study, such an approach provided an improved understanding of flooding dynamics and rainfall-runoff patterns in the seasonally flooded ‘Top End’ and may prove useful in linking past and present flood hydrology