Urban snowmelt processes – current research and modelling needs

Despite the water balances of cold region towns being dominated by low intensity, long duration snowmelt events, urban drainage systems continue to be designed according to standards developed for short, high intensity rain storms. During the 1980s and early 1990s, work in Scandinavia (Bengtsson, 1983, 1984, 1986; Westerström, 1984; Bengtsson and Westerström, 1992; Thorolfsson, 1990) and Canada (Xu and Buttle, 1987; Buttle and Xu, 1988, Todhunter et al., 1992) identified fundamental differences between rural and urban snowmelt processes. They found that snow properties such as density and albedo varied both between town and country and within the town depending on land-use. Moreover, both the longwave and shortwave radiation balances are heavily modified by buildings. Thus melt and runoff generation occurs at different times and rates. Town centres can have melt rates almost double that of residential areas. Despite snow removal policies, snowmelt in town centres is extremely important as these areas are the most likely to have combined sewer systems. These revelations will come as no surprise to practitioners working in cold regions, however, there is a lack of published material. This paper documents urban snow research from the last decade, it is both a summary and continuation of the state-of-the-art review found in a UNESCO special report on urban drainage in cold regions (Chapter 2, Semadeni-Davies and Bengtsson, 2000). Topics discussed include snow distribution, snow energy balance, frozen soil and runoff generation and modelling approaches - water quality issues are outside the scope. How to improve temporal and spatial resolution with limited budgets and limited data availability are ongoing problems, however, recent coupling between major urban drainage models such as SWMM and MOUSE and Geographic Information Systems offers a glimmer of hope. While full physically-based snow melt modelling is still out of the question, GIS could allow improved representations of snow distribution and energetics

Derivation of Economic and Social Indicators for a Spatial Decision Support System to Evaluate the Impacts of Urban Development on Water Bodies in New Zealand

There is mounting evidence that urban development in New Zealand has contributed to poor water quality and ecological degradation of coastal and fresh water receiving waters. As a consequence, local governments have identified the need for improved methods to guide decision making to achieve improved outcomes for those receiving waters. This paper reports progress on a research programme to develop a catchmentscale spatial decision-support system (SDSS) that will aid evaluation of the impacts of urban development on attributes such as water and sediment quality; ecosystem health; and economic, social and cultural values. The SDSS aims to express indicators of impacts on these values within a sustainability indexing system in order to allow local governments to consider them holistically over planning timeframes of several decades. The SDSS will use a combination of deterministic and probabilistic methods to, firstly, estimate changes to environmental stressors such as contaminant loads from different land use and stormwater management scenarios and, secondly, use these results and information from a range of other sources to generate indicator values. This paper describes the project’s approach to the derivation of indicators of economic and social well being associated with the effects of urban storm water run-off on freshwater and estuarine receiving waters.Environmental Economics and Policy,

SUDS, LID, BMPs, WSUD and more - The evolution and application of terminology surrounding urban drainage

Open Access articleThe management of urban stormwater has become increasingly complex over recent decades. Consequently, terminology describing the principles and practices of urban drainage has become increasingly diverse, increasing the potential for confusion and miscommunication. This paper documents the history, scope, application and underlying principles of terms used in urban drainage and provides recommendations for clear communication of these principles. Terminology evolves locally and thus has an important role in establishing awareness and credibility of new approaches and contains nuanced understandings of the principles that are applied locally to address specific problems. Despite the understandable desire to have a ‘uniform set of terminology’, such a concept is flawed, ignoring the fact that terms reflect locally shared understanding. The local development of terminology thus has an important role in advancing the profession, but authors should facilitate communication between disciplines and between regions of the world, by being explicit and accurate in their application

Urbanwater management vs. climate change: Impacts on cold region waste water inflows

Failure to account for non-climatic changes to water systems, such as design and operation, within climate change impact assessments leads to misconceptions because these activities buffer the human built enviroment from bio-physical impacts. Urban drainage in cold regions, which is dominated by snowmelt, is especially vulnerable to climate change and is investigated in this paper within the context of future rehabilitation of the sewer network. The objectives are to illustrate the relative response of urban drainage to changes in both the pipe network and climate and demonstrate the use of response surfaces for climate change studies. An incremental climate scenario approach is used to generate two sensitivity analyses for waste water inflows to the Lycksele waste water treatment plant in north-central Sweden. Air temperature and precipitation data ( 1984 - 1993) are altered incrementally between - 5 and + 15degreesC and - 10 and + 40% respectively. These data are then used to drive a hydrological transformation model to obtain estimates of sewer infiltration from groundwater. The results are presented as winter and spring response surfaces - these are graphical representations of a response matrix where each point relates to a single model run. Climate scenario envelopes which summarise a series of GCM runs ( ACACIA; Carter, 2002, pers. comm.) are overlaid to indicate the range of plausible waste water inflows. Estimates of natural multi-decadal variability are also included. The first sensitivity analysis assumes no change to the drainage system while the second simulates sewer renovation in which the system is fully separated and sewer infiltration is reduced. The main conclusions are that innovations in drainage network technology have a greater potential to alter waste water inflows than climate change and that, while the direction of climate change is fairly certain, there is great uncertainty surrounding magnitude of those changes and their impacts

Winter performance of an urban stormwater pond in southern Sweden

Evidence from cold regions in North America has shown that the performance of stormwater ponds differs between winter and summer. The pond hydraulics change seasonally, and winters have lowered removal efficiency due to a combination of an ice cover, cold water and de-icing salts. This study examines the function of the Backaslov stormwater pond under the more mild conditions of southern Sweden, where there are several snow and melt cycles per year. Event sampling in the summer of 1997 showed good removal efficiencies for nutrients, total suspended solids (TSS) and a selection of metals (Cd, Cu, Pb, Zn), but winter grab-tests taken in 1995-96 and 1997-98 suggest that the pond acts as a pollutant source under cold conditions. To better assess winter and spring pond performance, water at the inflow and outflow was sampled from January to April 2003. The low intensity of runoff delivery and slow inflow velocities meant that time- rather than flow-weighted sampling was used. Five consecutive events were sampled and analysed for TSS, chloride and the metals As, Cd, Cr, Cu, Hg, Ni, Pb and Zn. YSI probes were in place at both the inlet (pH, temperature) and outlet (pH, temperature, conductivity, dissolved oxygen) to determine the timing of pollution flows. In addition, profiles of the same quality indicators allowed snapshots of pond processes. De-icing salt has a major effect on pond hydraulics. Strong stratification occurred after each snowmelt-generated flow event and up to 80% of chloride could be retained by the pond. However, continuous conductivity measurements show that chloride is flushed between events. Ice changes retention times and causes oxygen depletion, but bed scour was not observed. Pond performance decreased during the winter and spring, albeit not as badly as the grab tests suggest. A seasonal comparison of the removal efficiencies showed that removal of Cd (75%) and Cu (49%) was about the same for summer and winter-spring, but removal of Pb, Zn and TSS dropped from 79%, 81% and 80% to 42%, 48% and 49% respectively. The removal efficiencies for the other metals sampled in 2003 were: As, 50%; Cr, 39%; Hg, 56%; Ni, 41%

Urban Snowmelt Processes: Modelling and Observation

In northern Scandinavia and other cold regions, urban drainage systems are often unable to cope with the high volumes of melt water which can be sustained for several weeks. High waste water treatment plant inflows, combined system overflows and low water quality are a few problems associated with snowmelt. However, reports detailing urban snowmelt processes are lacking in the hydrological literature. This thesis reports research into the snow hydrology of the City of Luleå, Sweden. There are four objectives: to investigate (a) the relationship between snow and landuse; (b) the effects of urbanisation on snowmelt processes; (c) the effect of melt water on urban hydrology; and (d) the feasibility of modelling urban snowmelt processes. These are met in a series of five appended papers. Papers I and II investigate snowmelt sensitivity to the urban environment. Paper III concerns seasonal patterns of the town water balance. Paper IV presents a snow survey that reaffirms the assumptions about snow cover and characteristics made in Papers II and III. Paper V is a literature review that questions the validity of urban runoff routing models on the basis of findings from the earlier papers. Urbanisation is associated with extreme heterogeneity and diminished surface permeability. The urban hydrograph shows high peak flows and rapid responses after short, low intensity events. Snowmelt processes are the same in both rural and urban areas, but climate and snowpack properties differ greatly. Urban snow, for instance, is subject to ploughing. Snowmelt induced runoff generation is further complicated by a more pronounced thermal component of the water balance compared to rain events. Snowpack energy fluxes are greatly influenced by the urban environment. Melt is earlier and more intense due to enhanced radiation to the south of buildings in comparison to snowpacks in open ground or to the shaded north. Urban snow has low albedo and high density than that of rural snow leading to speeded melt. The role of surface type and snow cover characteristics upon storm- and waste water inflow is demonstrated. Predictions of waste water inflow are improved by spatially weighting snow types according to a priori assumptions of snow properties and coverage. For instance, the proportion of undisturbed snow coverage decreases as landuse intensity increases. Snow piles in suburbs tend to be on permeable ground, but they are more likely to be on sealed surfaces in the central city. There are seasonal differences in runoff volumes and flow pathways to the waste water treatment plant and receiving waters. Increased volumes of runoff, reduced concentration times and long event duration lead to high waste water loads at the treatment plant. The surface water component of sewage originates from direct flow into pipe inlets and infiltration into sewer pipes. Current methods of estimating urban snowmelt, such as the degree-day method, and runoff are found to be problematic. However, a lack of data for developing, testing or operating more sophisticated models restricts urban engineers

Response surfaces for climate change impact assessments in urban areas

Assessment of the impacts of climate change in real-world water systems, such as urban drainage networks, is a research priority for IPCC (intergovernmental Panel of Climate Change). The usual approach is to force a hydrological transformation model with a changed climate scenario. To tackle uncertainty, the model should be run with at least high, middle and low change scenarios. This paper shows the value of response surfaces for displaying multiple simulated responses to incremental changes in air temperature and precipitation. The example given is inflow, related to sewer infiltration, at the Lycksele waste water treatment plant. The range of plausible changes in inflow is displayed for a series of runs for eight GCMs (Global Circulation Model; ACACIA; Carter, 2002, pers. comm.). These runs are summarised by climate envelopes, one for each prediction time-slice (2020, 2050, 2080). Together, the climate envelopes and response surfaces allow uncertainty to be easily seen. Winter inflows are currently sensitive to temperature, but if average temperature rises to above zero, inflow will be most sensitive to precipitation. Spring inflows are sensitive to changes in winter snow accumulation and melt. Inflow responses are highly dependent on the greenhouse gas emission scenario and GCM chosen

Radiation balance of urban snow: a water management perspective

The radiation balance of urbanised catchments differs from their rural counterparts, with snowpacks experiencing either enhanced or decreased irradiance depending on snowpack location and condition. As snowmelt is largely driven by radiation inputs, changes to localised irradiance (and melt rates) have implications for urban runoff generation. Storm- and wastewater drainage systems in cold regions are currently dimensioned for rain according to practices developed for temperate climates. They are not designed to cope with wintry conditions, which can lead to surface flooding, hydraulic overloads and poor water quality at receiving waters. Net allwave radiation measurements over snow made at the Swedish city of Luleå during April 1997 and 1998 are presented. The 1997 measurements were made in the vicinity of a matt-black-painted metal building at Luleå University of Technology, whereas the 1998 measurements are from a specially constructed 3×6-m black plastic-clad wall built on an open field just outside the town. Black minimises multiple reflections between the snow and walls, while maximising absorption of shortwave radiation by walls. The data were compared to the outputs of an urban radiation model. The results show that urban structures significantly alter radiation over snow. The temperature of the south-facing walls translates to longwave enhancements in the order of 150 W m−2 for several metres from the walls on sunny days. Shaded snow near the north-facing wall showed a net allwave radiation loss of the same order of magnitude. Radiation inputs to snow are similar both to the north and south of walls when the sky is overcast. The need to include snowmelt energetics within design and management techniques is discussed in light of the results