School of Ecosystem and Forest Sciences - Research Publications
Now showing items 1-12 of 218
A suburban sediment budget: Coarse-grained sediment flux through hillslopes, stormwater systems and streams
Sediment in urban stormwater systems creates a significant maintenance burden, while a lack of coarse‐grained bed sediment in streams limits their ecological value and geomorphic resilience. Gravel substrates, for example, provide benthic habitat yet are often scoured from the channel bed only to end up in a detention basin or treatment wetland. This dual problem of both ‘too much’ and ‘too little’ coarse‐grained sediment reflects a watershed sediment budget that is profoundly altered. We developed a conceptual urban coarse‐grained (>0.5 mm) sediment budget across three domains: hillslopes (urban land surfaces), the built stormwater network and stream channels. We then quantified key sources, sinks and storages for a suburban case study, using a combination of hillslope and in‐channel monitoring, and interrogation of local government records. Around 36% of the sediment supplied to the stormwater network reached the catchment outlet, a level of sediment delivery much higher than observed in similar‐sized natural catchments. The remainder was deposited in the sediment cascade and either stored, or extracted and removed from the catchment (e.g. material deposited in sediment ponds and gross pollutant traps). Conventional urban drainage networks are characterized by high hillslope sediment supply and low storage, resulting in efficient sediment delivery. Channel erosion, deposition in (and extraction from) pipes and channels, and floodplain deposition are small compared to sediment transport through the cascade. An understanding of the sediment budget of urban headwater catchments can provide stormwater and waterway managers with the information they need to address specific sediment problems such as sedimentation in stormwater assets and geomorphic recovery of urban streams.
Urban catchment runoff increases bedload sediment yield and particle size in stream channels
Physical degradation of urban rivers negatively impacts the environmental and social values they provide, and imposes significant financial costs on waterway management agencies. While the impact of urban stormwater runoff on streams is well recognised, the influence of altered bed sediment regimes on urban stream geomorphology is poorly understood. This study reports bedload sediment yields and bedload particle size distributions measured with sediment traps in nine small streams in eastern Melbourne, Australia, across a gradient of urbanization. We assessed relationships between the yield and size of bedload sediment and measures of catchment urbanization (including total imperviousness, effective imperviousness, road density and pipe density) and hydrology (measured through flow gauging at each site). Bedload yields were greater and bedload sediments were coarser-grained in more urbanized catchments. Bedload yields were strongly related to drainage connection of the urban land surface to the stream (captured by measures such as effective imperviousness and pipe density). The increase in bedload yield and calibre in urban catchments was driven mainly by the increase in sediment-transporting runoff from connected impervious surfaces. This study found no evidence of urban land cover severely limiting coarse-grained sediment supply. Most of the bedload material appeared to originate from imported sediment sources (e.g. construction and surfacing materials) in upland urban hillslope areas, which were connected to the channel by efficient transport reaches such as pipes and rock-lined channels, rather than from channel-derived erosion. Findings of this study suggest a rethink of the coarse-grained sediment-supplying potential of urban catchments, which has long been assumed to be low due to sealing of the land surface with hard materials.
How urban stormwater regimes drive geomorphic degradation of receiving streams
(SAGE PUBLICATIONS LTD, 2020-01-13)
<jats:p> For streams draining urban catchments, sediment transport capacity is the key driver of physical impacts including bed sediment removal and channel incision. The main unanswered question is the relative role of flow alteration compared to sediment supply in influencing sediment transport capacity. With this objective, we computed sand and gravel bed sediment transport capacity using the Wilcock and Kenworthy two-fraction bedload transport relation for nine streams in catchments covering a gradient of urbanisation. Computations were done for typical natural bed surface material, based on conditions in the least urban study streams. We compared transport capacity distributions and cumulative transport capacity over one-year between the streams. Transport capacity was up to three orders of magnitude higher in urban streams than in forested-catchment streams. This was driven overwhelmingly by the urbanisation-induced alterations to the flow regime, with only minor feedback from channel form changes. Transport capacity was two to three orders of magnitude greater than measured bedload transport in all but the least urban streams. This excess bedload transport capacity mobilises and removes bed sediment, produces channel incision and enlargement and reduces channel complexity. Rebalancing transport capacity with sediment supply therefore requires significant flow mitigation towards pre-urban conditions. Other responses, which may theoretically help to regain this balance – channel widening, grade control, increasing roughness, sediment augmentation – are either inappropriate or only feasible following flow mitigation measures. </jats:p>
Shared responsibility, community engagement and resilience: international perspectives
(AUSTRALIAN EMERGENCY MANAGEMENT INST, 2019-07-01)
Australia’s National Strategy for Disaster Resilience (Attorney-General’s Department 2011) has a central principle of shared responsibility that has influenced the policies and practices of Australia’s emergency management sector. However, the notion of ‘shared responsibility’ remains controversial. As part of a research project examining aspects of shared responsibility, seven international authorities in natural hazard mitigation policy were interviewed about their understandings of hazard threats, shared responsibility and community resilience in their own countries. The aims of this study were to analyse these international views to clarify what constitutes shared responsibility as a policy to develop resilience and to better understand how it might operate effectively. While there were differences in perspectives compared with the Australian policy, the centrality of the role of government agencies was acknowledged by all and the importance of community education was emphasised by some. Several aspects of shared responsibility were considered problematic, especially relationships between government agencies and community groups. Findings point to shared responsibility involving government and community organisations being viable if they are collaborative endeavours. A framework is suggested to assess the levels of collaboration in such endeavours.
Shaping and Sharing Responsibility: Social Memory and Social Learning in the Australian Rural Bushfire Landscape
(TAYLOR & FRANCIS INC, 2018-01-01)
Responding to increased frequency and severity of bushfires, Australian governments called for “shared responsibility” for bushfire preparation and mitigation. This requires engagement between all sectors of community—government agencies, businesses, not-for-profit, and residents. Fire management agencies remain concerned about whether all communities in fire-prone landscapes are equally equipped to participate in sharing responsibility. A related question is how experience of bushfire influences subsequent community fire management practices. This paper addresses social learning and social memory in a landscape that has experienced repeat bushfires between 2006 and 2013. It examines the relationships between memory, learning and practice among a farming community in western Victoria and government agencies with bushfire management responsibility. Findings suggest that social learning and social memory interact and new practices emerge as the participants embrace “shared responsibility.” However, ambiguities remain about “what” is being shared and what being “responsible” means at different points in preparation and response.
Communities and responsibility: Narratives of place-identity in Australian bushfire landscapes
(PERGAMON-ELSEVIER SCIENCE LTD, 2020-02-01)
This research explores the complex interplay among (a) landscape and social memory, (b) the construction of place-identity, and (c) notions of responsibility sharing in hazard preparedness and response. Our aims are to (a) establish greater clarity about the gap between social responses to technologically driven risk management strategies favoured by governments and agencies within the context of Australian bushfire management policies and practices, and (b) provide insight into the practical implications of policies espousing greater community involvement in readiness for bushfire, and shared responsibility between communities and governments. Case study based ethnographic research involves communities in rural, peri-urban and remote Australia. We find that maintaining a sense of place-identity in the face of economic rationalist government-at-a-distance may be of greater urgency to communities than planning for possible bushfire threat. Community imperatives to retain control over local social and landscape memory and narratives may result in push-back against well-intentioned outsider agency interventions aimed at promoting greater community awareness and preparedness. Within the entanglement of landscape, remembering and everyday social practices – the taskscape – there is fluidity between individual place-identity and the ways social memory is (re)constructed to make sense of emergent events and support imagined community futures. We conclude that a consequence for land and fire management agencies is to recognise the importance of framing community engagement around the ways each community functions within and as part of its landscape, rather than a singular focus on potential bushfire impacts.
People’s concepts of place and the implications for successful implementation of prescribed burning in Australian landscapes
(Australasian Fire and Emergency Service Authorities Council Limited, 2021-03-01)
Catchment-scale urbanization diminishes effects of habitat complexity on instream macroinvertebrate assemblages
While provision of in-stream habitat complexity remains common practice in efforts to restore streams, the evidence of positive effects on in-stream communities is inconsistent. In streams of urban catchments, where both reach-scale habitat manipulation and catchment-scale actions to ameliorate the disturbance regime of urban stormwater runoff are common management responses, clearer understanding of the effects of habitat complexity under different degrees of urban impact are needed. We experimentally assessed the effect of increased surface complexity in wood, the dominant hard substrate in our 18 study reaches on 14 small streams, on in-stream macroinvertebrate assemblages across a range of urban impact. Increased surface complexity increased abundance of most taxa, but this effect was less pronounced in urban streams, partly because of the reduced species pool tolerant of urban stormwater impacts, and partly because of a lesser response of some species to increased complexity in more urban streams. Collectively these taxon-specific effects resulted in small, uncertain increases in taxon richness with increased complexity in rural streams, and no change in richness of the less diverse assemblages of urban streams. Increased abundances suggest increased availability of refugia or resources with increased surface complexity, while the reduced effect of complexity in urban streams suggests that any refuge or resource provided by greater surface complexity is less effective in more disturbed environments receiving urban stormwater runoff. The reduced abundance of sensitive taxa in more urban streams, and the resultant reduced richness, confirms that urban stormwater runoff acts as a strong environmental filter, limiting the species pool available for community assembly. Restoration of habitat complexity in streams without catchment-scale drivers of degradation is likely to have positive benefits to in-stream biotic assemblages, but the efficacy of such approaches in catchments subject to urban stormwater runoff will be greatly diminished. In such cases, restoration activities should first be aimed at controlling the larger-scale problem.
Effect of thinning and burning fuel reduction treatments on forest carbon and bushfire fuel hazard in Eucalyptus sieberi forests of South-Eastern Australia
More frequent hot and windy weather in fire prone forested landscapes requires that a full suite of fuel reduction measures be investigated for effectiveness in fuel hazard reduction, environmental impact and carbon (C) outcomes. Although prescribed fire and thinning are routinely applied in forests of North America to reduce fuel loads, there are few detailed studies from Australia. We report the impacts of fuel reduction treatments including burning, mechanical thinning and the combination of both on forest C and fuel hazard in open forests dominated by Eucalyptus sieberi in south-eastern Australia. Carbon losses to the atmosphere and redistribution within the forest were calculated from stocks within each fuel category before and after treatment. Mechanical thinning + burning was the most effective treatment for reducing aboveground C and fuel hazard, with major reductions in dead trees, stumps and understorey, as well as stems removed for sale as pulpwood. However forest floor fuel loads increased in thinned treatments relative to control forests. The overall fuel hazard rating in the burn only treatment was significantly reduced from extreme to low immediately following burning. In thinned only stands, the overall fuel hazard rating did not change from the pre-treatment rating of extreme, due to high surface and forest floor fuel loads and loose and flammable bark on the retained overstorey trees. This result suggests the current fuel hazard guide in use in Australia should be revised to enable it to better describe the benefits of thinning for fuel reduction - in this case the removal of about 50% of aboveground C mostly as overstorey trees, and a significant reduction in understorey, dead trees and stumps.
Impact of mechanical thinning on forest carbon, fuel hazard and simulated fire behaviour in Eucalyptus delegatensis forest of south-eastern Australia
(ELSEVIER SCIENCE BV, 2017-12-01)
Forest mega-fires have become a global phenomenon in recent decades including in south-eastern Australia where large areas of forest have been fire-killed with loss of human lives and property and impacting carbon sequestration and greenhouse gas emissions. The vast extent and impact of mega-fires has induced a re-evaluation of fuel reduction methods asa key management strategy in wildfire risk mitigation in many countries. This study investigated the impact of a commercial thinning in Eucalyptus delegatensis forest on fuel hazard, fuel loads and wildfire behaviour, eight years after completion of a bay and outrow thinning operation. At the stand level, thinning reduced overstorey tree stocking by more than 50%, increased canopy openness and stimulated the growth of retained trees. Thinning also encouraged the profuse regeneration of over 1000 saplings ha−1 of E. delegatensis, mostly in the outrows, compared with no sapling regeneration in unthinned forest. A system of additive biomass equations was developed to estimate total biomass and component biomass (stem wood, bark, branches and foliage) of individual trees. The aboveground tree carbon was 433±49MgCha−1 in unthinned forest and 322±47MgCha−1 in thinned forest. Thinning decreased surface fuel hazard ratings and fuel loads but had no significant effect on the mass of coarse woody fuels. Fire simulation under severe to extreme weather conditions, as occurred in the 2006/7 Great Divide Fires, indicated an almost 30% reduction in fireline intensity and about 20% reduction in the rate of spread and spotting distance in thinned forest compared with unthinned forest. This study indicates the potential of thinning to reduce wildfire severity and to increase the fire-survival of E. delegatensis.
Importance of disturbance history on net primary productivity in the world's most productive forests and implications for the global carbon cycle
Analysis of growth and biomass turnover in natural forests of Eucalyptus regnans, the world's tallest angiosperm, reveals it is also the world's most productive forest type, with fire disturbance an important mediator of net primary productivity (NPP). A comprehensive empirical database was used to calculate the averaged temporal pattern of NPP from regeneration to 250 years age. NPP peaks at 23.1 ± 3.8 (95% interquantile range) Mg C ha-1 year-1 at age 14 years, and declines gradually to about 9.2 ± 0.8 Mg C ha-1 year-1 at 130 years, with an average NPP over 250 years of 11.4 ± 1.1 Mg C ha-1 year-1 , a value similar to the most productive temperate and tropical forests around the world. We then applied the age-class distribution of E. regnans resulting from relatively recent historical fires to estimate current NPP for the forest estate. Values of NPP were 40% higher (13 Mg C ha-1 year-1 ) than if forests were assumed to be at maturity (9.2 Mg C ha-1 year-1 ). The empirically derived NPP time series for the E. regnans estate was then compared against predictions from 21 global circulation models, showing that none of them had the capacity to simulate a post-disturbance peak in NPP, as found in E. regnans. The potential importance of disturbance impacts on NPP was further tested by applying a similar approach to the temperate forests of conterminous United States and of China. Allowing for the effects of disturbance, NPP summed across both regions was on average 11% (or 194 Tg C/year) greater than if all forests were assumed to be in a mature state. The results illustrate the importance of accounting for past disturbance history and growth stage when estimating forest primary productivity, with implications for carbon balance modelling at local to global scales.