Showing papers by "Tim D. Fletcher published in 2008"

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

Abstract: 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.

Hydraulic and pollutant removal performance of fine media stormwater filtration systems.

Abstract: Stormwater runoff from urban areas has multiple negative hydrologic and ecological impacts for receiving waters. Fine media stormwater filtration systems have the potential to mitigate these effects, through flow attenuation and pollutant removal. This work provides an overall assessment of the hydraulic and pollutant removal behavior of sand- and soil-based stormwater filters at the laboratory scale. The influence of time, cumulative inflow sediment, cumulative water volume, wetting and drying, and compaction on hydraulic capacity was investigated. The results suggested that the primary cause of hydraulic failure was formation of a clogging layer at the filter surface. Loads of sediment and heavy metals were effectively retained; however, the soil-based filters leached nitrogen and phosphorus for the duration of the experimental period. Media pollutant profiles revealed significant accumulation of all pollutants in the top 20% of the filter profile, suggesting that elevated discharges of nutrients was du...

Reuse of Urban Runoff in Australia: A Review of Recent Advances and Remaining Challenges

Abstract: The degradation of aquatic ecosystems due to hydrologic and water quality impacts of urbanization, combined with increasing water scarcity, has generated increasing interest in the harvesting of urban storm water. This paper reviews the rationale for integrated storm water treatment and harvesting and synthesizes recent advances and trends and knowledge gaps that limit its application. Storm water harvesting is shown to be a viable alternative water supply and to provide a potential solution to the increases in runoff frequency and peak flows that occur as a result of catchment urbanization. In general, treatment technologies for storm water harvesting have been adapted from existing "water-sensitive urban design" approaches, with limited use of traditional water supply and wastewater technologies. Risk management is often lacking, in part due to a lack of relevant guidance. Reported performance shows variable levels of potable water savings, with cases of up to 100% substitution recorded. Costs of storm water harvesting systems are shown to be inversely related to their scale. The limited cost data show the importance of context, with the harvested water costing more or less than alternative supplies, depending on the cost of the alternative. Limited data exist on environmental benefits, such as reductions in pollutant loads and flow peaks. Implementation of storm water harvesting systems is impeded by inadequate data on risk, lifecycle costs, externalities, and water-energy tradeoffs. Furthermore, retrofit of storm water harvesting into existing urban areas is proving to be a challenge, creating an urgent need for specific technologies for use in retrofit situations.

Urban stormwater harvesting - sensitivity of a storage behaviour model

Abstract: The harvesting of urban stormwater to supply non-potable water demands is emerging as a viable option, amongst others, as a means to augment increasingly stressed urban water supply systems. This paper investigates the sensitivity of an urban stormwater harvesting system's capacity-yield-reliability relationship to variations in the behaviour modelling method used, focusing on the storage and demand components of a single reservoir system. The aim is to enhance our understanding of the appropriate computational method for assessing such volumetric reliability/storage capacity relationships. Four reference scenarios were developed, based on two different climates and two different water demand patterns. A sensitivity analysis was conducted, which considered the following computational, storage and demand parameters: yield-spillage order, modelling time-step, length of rainfall record, initial storage volume, open/closed storage surface, dead storage volume, diurnal and weekly pattern of water demand, and inter-annual variability of seasonal water demand. It was found that several parameters had an insignificant impact on the estimation of volumetric reliability for the scenarios tested, whilst the three most significant parameters were: length of rainfall record, inter-annual variability of seasonal demand, and storage surface type. Recommendations about the minimum length of rainfall record used and the inclusion of both the inter-annual variability of seasonal demand and net evaporative losses in the case of an open store are made.

A new saturated/unsaturated model for stormwater infiltration systems

Abstract: Infiltration systems are widely used as an effective urban stormwater control measure. Most design methods and models roughly approximate the complex physical flow processes in these systems using empirical equations and fixed infiltration rates to calculate emptying times from full. Sophisticated variably saturated flow models are available, but rarely applied owing to their complexity. This paper describes the development and testing of an integrated one-dimensional model of flow through the porous storage of a typical infiltration system and surrounding soils. The model accounts for the depth in the storage, surrounding soil moisture conditions and the interaction between the storage and surrounding soil. It is a front-tracking model that innovatively combines a soil-moisture-based solution of Richard's equation for unsaturated flow with piston flow through a saturated zone as well as a reservoir equation for flow through a porous storage. This allows the use of a simple non-iterative numerical solution that can handle ponded infiltration into dry soils. The model is more rigorous than approximate stormwater infiltration system models and could therefore be valuable in everyday practice. A range of test cases commonly used to test soil water flow models for infiltration in unsaturated conditions, drainage from saturation and infiltration under ponded conditions were used to test the model along with an experiment with variable depth in a porous storage over saturated conditions. Results show that the model produces a good fit to the observed data, analytical solutions and Hydrus.

The clogging behaviour and treatment efficiency of a range of porous pavements

Abstract: This paper presents the findings of a laboratory investigation into the clogging behaviour of three different porous pavements that were most representative of the available range and their pollutant removal efficiency over time. These pavements were monolithic Porous Asphalt (PA), Permapave (PP), and modular Hydrapave (HP). The pavements were dosed with a semi-synthetic stormwater mixture over a continuous period of 20 weeks, at a flow rate of 3.9 mm/hr, the intensity of which corresponds to the 90 th percentile of an average recurrence interval (ARI) storm in Melbourne or the mean of an ARI storm in Brisbane, Australia. Inflow and outflow samples were collected and analysed for key pollutants such as total suspended solids (TSS), total phosphorus (TP), total nitrogen (TN), heavy metals and dissolved nutrients. Flow rates, pH and temperature readings were also measured. A 1 in 5 year Brisbane storm, which is equivalent to more than a 1 in 100 year Melbourne storm was simulated in the 6 th , 10 th and 17 th weeks to evaluate the rate of clogging; this corresponded to 5.5, 10.4 and 17.5 years of operation in Melbourne, or about half these periods in Brisbane. Under normal dosing, all three systems were capable of removing approximately 100% of TSS, 30% of TP and 20% of TN after 17 years of operation, with no large differences between the systems. For the ‘average’ conditions, none of the pavements showed signs of clogging even after 17 years of operation in Melbourne or 8.5 years in Brisbane. However, for the flooding conditions, HP started to pond after 5.5 years in Melbourne (or less than 3 years in Brisbane), while PA showed signs of clogging after 10.4 years in Melbourne (5.2 years in Brisbane). However, Permapave has still not showed signs of clogging after 17 years of operation in Melbourne, even for a 1 in 100 year event.

Removal of nutrients, heavy metals and pathogens by stormwater biofilters

Abstract: Biofilters (bioretention systems) are a potentially effective treatment option for the treatment of pollutants in urban stormwater, such as nutrients (nitrogen and phosphorus in particular), heavy metals, and even pathogens. A large-scale column study was conducted in Melbourne, Australia, to quantify the treatment performance of biofilters for the above mentioned pollutants, and to assess the effect of a range of different factors on the removal efficiency: presence and type of vegetation, depth and type of filter media, the magnitude of storms and their pollutant inflow concentrations, the presence of an anoxic zone in the bottom of the biofilter and the presence of drought periods. The results demonstrated that vegetation selection is critical for nitrogen removal (e.g. columns planted with Carex showed 70% removal after 8 months of exposure to stormwater). Phosphorus was efficiently removed (>80%) by most biofilter designs, providing that no organic matter was added to the filter media. All biofilter configurations performed well for heavy metals (80% for lead and >98% for copper and zinc), if their depth was >300mm. Although antecedent dry weather periods had a negative influence on pathogen treatment, mean removal was found to be >80% for indicators of viruses, bacteria and protozoa. Questions remain as to whether this is adequate to allow safe non-potable use of water discharged from stormwater biofilters.

The impact of vegetation on the hydraulic conductivity of stormwater biofiltration systems

Abstract: Stormwater biofiltration systems are an increasingly popular treatment technology and are being installed in all major cities in Australia. Their concept is very simple and easily implemented in all urban forms. Unfortunately, we know very little about the performance of biofiltration systems, particularly with respect to their long-term hydraulic performance. There is some concern regarding the ability of biofiltration systems to treat the design volume of stormwater for the duration of their intended lifespan, or whether factors such as compaction of the filter media and surface clogging will impede their infiltration performance. To address this knowledge gap, the Facility for Advancing Water Biofiltration has been monitoring the hydraulic capacity of field-scale biofiltration systems. Vegetation was shown to be critical in maintaining the infiltration capacity of biofiltration systems, helping them to recover from the inevitable reduction in hydraulic conductivity due to initial compaction of the filter media under hydraulic loading. The creation of macropores due to root growth and senescence is thought to contribute to this behaviour. Biofiltration systems were shown to attenuate mean peak flows by 80% (range 45 - 96%). A performance analysis of a lined biofiltration system demonstrated that, on average, 42% of an event volume (range: 15 - 83%) was retained by the filter media and subsequently lost via evapotranspiration. This high level of losses is mainly due to the fact that the monitored events were largely small to medium in their size (monitoring of large events was not conducted).

Influence of time and design on the hydraulic performance of biofiltration systems for stormwater management

Abstract: Hydraulic conductivity of soil used in biofiltration systems is one of the key parameters to assess when designing and building such systems. However, changes in hydraulic conductivity over time may be a real issue and may lead to a loss of treatment capacity, more frequent overflow and problems with stagnant water. This article aims to study clogging phenomena using a laboratory approach, in order to assess the influence of design parameters (vegetation, size, soil type and inflow concentration) on hydraulic conductivity and its evolution. Hydraulic conductivity was measured on 125 biofiltration columns over a 72 week period. Initial values were within Australian guidelines (50 – 200 mm/h), with a median value of 186 mm/h. After almost 1.5 year of operation, hydraulic conductivity dropped to 27% of the initial value (median of 51 mm/h). Vegetation did not have an influence on clogging, except when a species with course roots, such as Melaleuca, is used. In this case, hydraulic conductivity increases over time. The size of the system relative to its catchment is of prime importance. Small systems (in our case, systems designed at 0.7% of their catchment) are more prone to rapid clogging than are larger systems (designed at 2% or 4% of their catchment). Clogging rate is also influenced by the inflow concentration, increasing with higher concentrations of sediment. These results will inform improved design guidelines for biofilters.

Heavy metal removal by stormwater biofilters: can it withstand alternative wetting and drying conditions?

Abstract: Urban stormwater contains substantial loads of Cu, Pb and Zn, which are considered as key stormwater contaminants. Stormwater biofiltration is a promising option to treat these contaminants. Biofilters are exposed to an alternate cycle of drying and wetting, and the influence of this on pollutant removal performance is as-yet unknown. To investigate the effect of drying and subsequent rewetting on the retention of heavy metals by stormwater biofilters, a laboratory study has been conducted using three groups of biofilter columns, which were dosed with semi-synthetic stormwater according to three different drying and wetting regimes. Some biofilters were fitted with a submerged zone combined with a carbon source, at the bottom of the filter. Overall, the biofilters were very effective in heavy metal removal, provided that they received regular stormwater input. However, after drying extending to three or four weeks, removal of heavy metals decreased significantly. A statistically significant correlation between antecedent dry days and metal removal was shown. Furthermore, a clear effect of the submerged zone was found: after extended dry periods, biofilters with this feature performed significantly better than those without it. In particular, the removal of Cu was clearly increased both during wet and dry periods; for Pb the negative effect of drying was completely eliminated by introducing a submerged zone.

Stream Restoration through Stormwater Runoff Management and Retrofit: New Objectives, New Approaches

Abstract: The Little Stringybark Creek restoration project is the first of its kind, focusing on reducing stormwater runoff using LID strategies across an entire sub-watershed. Urban streams around the globe demonstrate common characteristics associated with the increased imperviousness of their watersheds, including a flashy hydrograph, elevated concentrations of pollutants, altered channel morphology, and increased dominance of pollution tolerant species. Urban streams cannot be restored to pre-disturbance stream health conditions without addressing the combined water quality and hydrologic disturbance (increased volume and frequency of polluted stormwater runoff) from impervious areas delivered by drainage infrastructure in developed watersheds. This poses a great challenge for stream restoration, since it is much easier to implement local or reach scale in-stream or riparian projects than to reduce the stormwater impacts of impervious areas in a catchment. One of the key needs for the protection or restoration of streams in urban or urbanizing catchments is, therefore, a better understanding of specific and practical stormwater management objectives at the catchment and site scale aimed at addressing hydrologic characteristics that affect streams. © 2009 American Society of Civil Engineers.