Anthony R. Ladson

Stream restoration in urban catchments through redesigning stormwater systems: looking to the catchment to save the stream

Abstract Restoration of streams degraded by urbanization has usually been attempted by enhancement of instream habitat or riparian zones. Such restoration approaches are unlikely to substantially improve instream ecological condition because they do not match the scale of the degrading process. Recent studies of urban impacts on streams in Melbourne, Australia, on water chemistry, algal biomass and assemblage composition of diatoms and invertebrates, suggested that the primary degrading process to streams in many urban areas is effective imperviousness (EI), the proportion of a catchment covered by impervious surfaces directly connected to the stream by stormwater drainage pipes. The direct connection of impervious surfaces to streams means that even small rainfall events can produce sufficient surface runoff to cause frequent disturbance through regular delivery of water and pollutants; where impervious surfaces are not directly connected to streams, small rainfall events are intercepted and infiltrated. We, therefore, identified use of alternative drainage methods, which maintain a near-natural frequency of surface runoff from the catchment, as the best approach to stream restoration in urban catchments and then used models of relationships between 14 ecological indicators and EI to determine restoration objectives. Ecological condition, as indicated by concentrations of water-quality variables, algal biomass, and several measures of diatom and macroinvertebrate assemblage composition, declined with increasing EI until a threshold was reached (EI = 0.01–0.14), beyond which no further degradation was observed. We showed, in a sample catchment, that it is possible to redesign the drainage system to reduce EI to a level at which the models predict detectable improvement in most ecological indicators. Distributed, low-impact design measures are required that intercept rainfall from small events and then facilitate its infiltration, evaporation, transpiration, or storage for later in-house use.

Impediments and Solutions to Sustainable, Watershed-Scale Urban Stormwater Management: Lessons from Australia and the United States

In urban and suburban areas, stormwater runoff is a primary stressor on surface waters. Conventional urban stormwater drainage systems often route runoff directly to streams and rivers, thus exacerbating pollutant inputs and hydrologic disturbance, and resulting in the degradation of ecosystem structure and function. Decentralized stormwater management tools, such as low impact development (LID) or water sensitive urban design (WSUD), may offer a more sustainable solution to stormwater management if implemented at a watershed scale. These tools are designed to pond, infiltrate, and harvest water at the source, encouraging evaporation, evapotranspiration, groundwater recharge, and re-use of stormwater. While there are numerous demonstrations of WSUD practices, there are few examples of widespread implementation at a watershed scale with the explicit objective of protecting or restoring a receiving stream. This article identifies seven major impediments to sustainable urban stormwater management: (1) uncertainties in performance and cost, (2) insufficient engineering standards and guidelines, (3) fragmented responsibilities, (4) lack of institutional capacity, (5) lack of legislative mandate, (6) lack of funding and effective market incentives, and (7) resistance to change. By comparing experiences from Australia and the United States, two developed countries with existing conventional stormwater infrastructure and escalating stream ecosystem degradation, we highlight challenges facing sustainable urban stormwater management and offer several examples of successful, regional WSUD implementation. We conclude by identifying solutions to each of the seven impediments that, when employed separately or in combination, should encourage widespread implementation of WSUD with watershed-based goals to protect human health and safety, and stream ecosystems.

Improving Stream Health in Urban Areas by Reducing Runoff Frequency from Impervious Surfaces

Abstract In urbanised catchments, the frequent direct delivery of water and pollutants from impervious surfaces to streams has a detrimental effect on stream health. Recent studies of a range of ecological indicators have shown that where there is opportunity for attenuation of these inputs, that is, where the link between impervious surfaces and streams is less direct, the damage to stream health may be mitigated. This suggests that improving stream health, in areas subject to urbanisation, involves finding ways to decrease the efficiency of water delivery from impervious surfaces. We have undertaken a feasibility study to examine options to reduce the area of impervious surfaces that are directly connected to waterways by efficient drainage systems. These options include: rainwater tanks where the captured water is used to replace a portion of the mains water supply; permeable pavements that reduce runoff from roads; and swale drains and bioretention systems along roads, rather than piped drainage direct to streams. Models of the relationship between effective imperviousness and a range of ecological indicators, from previous studies, are used to assess the likely effect of these options on stream health. A large-scale experiment is now proposed to test this new approach to urban stream restoration.

Adaptive Management of Environmental Flows: Lessons for the Murray-Darling Basin from Three Large North American Rivers

Abstract Throughout the Murray-Darling Basin it is recognised that rivers are being degraded by changes to flow caused by development of water resources. Proposals to reverse this degradation include the provision of environmental flows, that is, water specifically managed to meet environmental needs. So far, implementation of environmental flows has proved difficult because of competition for water and uncertainties in environmental water requirements. Economic losses from allocating water to the environment are easy to quantify but the benefits can only be vaguely specified, which makes changes difficult to justify. A possible way forward is to use an adaptive management approach to the implementation of environmental flows. The promise of adaptive management is that it facilitates public policy in the face of uncertainty through a structured dialogue between scientists and managers and allows meaningful participation of stakeholders. Adaptive management has been used to address natural resources management issues in large river systems in the United States and three case studies are considered here: the Columbia, Colorado and Mississippi River projects. The case studies show that adaptive management can promote rapid learning and address complex environmental management issues but the process can be also be derailed. Barriers to adaptive management include: jurisdictional and institutional complexity, lack of credible science, and difficulties with developing system models. Adaptive management is more likely to be successful where there are few, well defined points of intervention, early successes for experimental management, shared goals and strong political support. Where experimental policies are seen as too risky or costly, the promise of adaptive management has not been realised. There is the potential to use adaptive management to address implementation of environmental flows in the Murray-Darling Basin but it will be necessary to select a project wisely.

A standard approach to baseflow separation using the Lyne and Hollick filter

Abstract The digital filtering approach to baseflow separation suggested by Lyne & Hollick (1979) has been widely used and is available in a number of computer packages. However, details of the approach used by different authors vary and so do the results. This means baseflow volumes and indices reported by different authors, and at different times, are difficult to compare. We propose a standard method for baseflow separation using the Lyne and Hollick digital filter. This includes reflecting the flow series at the start and end of the record to reduce “warm up” effects and the adoption of specific starting values for each filter pass.

Flow-regime management at the urban land-parcel scale: Test of feasibility

AbstractOvercoming the hydrologic shortcomings of conventional approaches to stormwater management requires the protection or restoration of flow regimes at small scales. A better understanding of how stormwater management strategies can achieve this aim is needed. This study modeled 28,800 design configurations of a typical stormwater management strategy at the scale of urban land parcels across a range of urban densities and climatic conditions. Realistic design configurations that achieved three hydrologic response targets were identified as part of this modeling. It was found that meeting the targets required a combination of stormwater harvesting (using tanks) and infiltration (using rain gardens). This was possible primarily because the amount of harvested impervious roof runoff made a large contribution to a hydrologic target, which measured the ability to restore volumetric losses. Management of flow regimes at small scales will require policy mechanisms that necessitate both stormwater harvesting...

Evaluating stream rehabilitation projects: reasons not to, and approaches if you have to

Abstract Over the last thirty years, the work of river engineers has shifted from the protection of physical and economic assets, to include the protection and enhancement of environmental assets. River engineers now work in multi-disciplinary teams that include biologists and social scientists. Environmental flow, and river rehabilitation projects are typical of this work. Unlike the well established engineering designs and practices of the past, there is now less confidence in the effectiveness of interventions, particularly where the outcomes involve animals, ecological functions, or imprecise ideas of ‘stream health’. Quite rightly, there is pressure to ‘evaluate’ the effectiveness of every public project. However, there is confusion about the type of evaluation that is appropriate for rehabilitation projects. In this paper we propose a hierarchy of evaluation types. We begin with measures of outputs, responsiveness, and appropriateness. We also discuss the dangers of moving to the next level of evaluation that is often proposed: measuring biological or water-quality variables in order to identify the outcomes of interventions. We demonstrate that it is rare that this type of evaluation is feasible, given the variability of fluvial systems. Apart from the cost (which usually exceed the physical cost of the project), we argue that such evaluation attempts can do more harm than good by appearing to ‘conclusively’ demonstrate that there has been no effect, when the problem lies with the experimental design. We also describe the levels of ‘confidence’ that each type of evaluation provides, as well as the risks associated with each type. The hierarchy of methods described allows managers to clearly specify the ‘level’ of evaluation that they are proposing to apply to any given project, and will help managers to avoid committing to infeasible evaluations.

Influence of roofing materials and lead flashing on rainwater tank contamination by metals

Abstract The quality of rainwater collected in polyvinyl chloride (PVC) tanks from six trial roofs (glazed tile, pre-painted steel and aluminium-zinc coated steel, each with and without uncoated lead flashing) was monitored for nine months. Samples of water and sediment were collected at three monthly intervals and analysed for concentrations of metals (Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn), in order to determine the influence of roof materials and uncoated lead (Pb) flashing upon metal contamination within the tanks. Lead concentration in tank water exceeded Australian Drinking Water Guidelines for all roof types where there was lead flashing. Lead flashing also contributed to contamination of tank sediments. In all cases, pH was low which contributed to a large proportion of lead being in the dissolved form.

Addressing Climatic Non-Stationarity in the Assessment of Flood Risk

Abstract Present-day flood estimation practise is underpinned by the assumption that flood risk in a future climate will reflect historical flood risk as represented by the instrumental record. This assumption, which is commonly referred to as the assumption of stationarity, recently has been questioned as a result of both an increased appreciation of the natural variability in our hydroclimate at temporal scales beyond that of the instrumental record, as well as the projected intensification of the hydrologic cycle due to anthropogenic climate change. These developments have led some authors to suggest that the stationarity assumption should henceforth be considered invalid, thereby calling into question all the methods that are underpinned by it, including flood frequency analysis using observed streamflow records, and rainfall-runoff modelling informed by instrumental precipitation and streamflow records. In this paper we review a wide range of possible sources of non-stationarity in the Australian climate record, and highlight that the primary sources of non-stationarity relevant for flood risk assessments include natural climate modes that vary at timescales similar to the length of the instrumental record, as well as long-term trends and step changes that are attributable to anthropogenic climate change. Although prescriptive guidelines that describe how to address this non-stationarity are currently unavailable in Australia, this review nonetheless highlights the importance of using long records for flood analysis, possibly by extending records using nearby stations. Furthermore, it will become increasingly necessary to develop plausible estimates of how the climate will evolve, and we describe some climate modelling tools that allow for the development of future climate scenarios. Finally, we emphasise that removing the assumption of stationarity will inevitably result in an increase in the uncertainty associated with future flood estimates, and suggest that this may require new methods to conceptualise and manage future flood risk.

Estimating extractable soil moisture content for Australian soils from field measurements

The amount of water that can be stored in soil and evaporated or actively used by plants is a key parameter in hydrologic models and is important for crop and pasture production. Often, the active soil moisture store is estimated from laboratory measurements of soil properties. An alternative approach, described in this paper, is to estimate the extractable soil moisture capacity from direct measurements of soil moisture content in the field. A time series of soil moisture values, over the depth of the soil, shows the actual changes in water content. The difference between the wettest and driest profiles is an estimate of the extractable soil moisture storage. We have gathered data on extractable soil water capacity for 180 locations over Australia and have compared our values with published results from the Atlas of Australian Soils (AAS), derived from profile descriptions and pedo-transfer functions. Our results show that data from the AAS provide a useful lower bound for measured extractable soil moisture storage, but of the sites examined, 42% had values >2 times those in the AAS. In part, this was because total soil depths were underestimated in the AAS results compared with the active depths from the measured data. Active depths are strongly related to vegetation type.