Toward an understanding of debris flow hazard in Tasmania

Colin Mazengarb

During a major rainfall event on the night of June 4, 1872, a large “landslide” was triggered high up on the flanks of kunanyi-Mt Wellington, adjacent to Hobart. The landslide entered the Humphrey Rivulet reaming out the rainforest in the upper reaches and transporting rocks, logs and mud through orchards and businesses in the lower part of the stream, a total distance of 8km. Surprisingly, no lives were lost. 

Several accounts exist of the noise that was heard during the event, lasting many minutes. In addition, a naturalist and a photographer walked up the stream and recorded their observations in following days.

The slope failure was different to anything the colonial settlers had ever experienced and the records of the day speculated the landslide created a large dam that subsequently breached, flooding the lowlands. Over the decades that followed, the event was largely forgotten, and without planning restrictions, the Greater Hobart urban region expanded into the affected area, including the construction of a large shopping centre.

Over a century later, myself and colleagues at Mineral Resources Tasmania, researched the 1872 event and concluded that it initiated as a debris-slide but shortly after transformed into a true debris flow. Over the last few kilometres of the flow, as the rock component was deposited on the lower stream gradient, it may be better described as a hyper-concentrated flow or debris flood. The awareness of debris-flows amongst society now is certainly much greater than in the 1870s especially as they have been captured on media many times world-wide in recent decades. 

Our conclusions raised important questions. Is this an isolated event in Tasmania? Could another event occur on the mountain? How likely is this to occur and what would be the consequences if this was to happen again?

To answer these questions, we undertook a regional investigation to determine the spatial and temporal distribution of other events in similar physiographic and geological situations. To date we have mapped many hundreds of debris-flows and entered these into a landslide database including volume and timing-constraint estimates. The mapping involved searching through state aerial photographs dating back to the 1940s, recent satellite imagery, historical records such as listed on the Trove website, as well as investigating recent events in the field. The most recent of these occurred in 2022, damaging roads, a pylon and buildings demonstrating that it is an important contemporary geomorphic process occurring in the Tasmanian landscape.

Based on the acquired information, a magnitude-frequency relationship was established over an eighty-year period as well as determining what parts of the landscape were susceptible to failure. This allowed us to devise probabilistic volume estimates for future events on kunanyi-Mt Wellington, including the Hobart Rivulet that runs through the centre of Hobart CBD. The remaining component of this study modelled future debris flows using the RiverFlow-2D software programme to predict the runout extents, depths and velocities of flows for each of the catchments using notional probabilities. These results in turn were used as inputs by colleagues at the University of Tasmania to design possible restraining structures on the Hobart Rivulet.

My talk will outline the methodology of this investigation and show the key results. Furthermore, it is fair to say that modelling debris flows contains uncertainty, and this will also be open for discussion, especially as it has relevance to other areas in Australia.

About the speaker

Colin Mazengarb

Colin Mazengarb has worked as a regional mapping geologist and an engineering geologist in New Zealand, Australia and the USA. During his 20 years employed by the Tasmania state government he led a landslide zoning project, utilising his GIS and mapping skills in particular. Colin has recently ended full-time employment to focus on semi-retirement activities as well as developing his GIS courses and occasional consulting jobs. Colin and his family are based in Hobart.

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