Browsing by Author "Niemi, Tero J."

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Applicability of open rainfall data to event-scale urban rainfall-runoff modelling
(2017-04) Niemi, Tero J.; Warsta, Lassi; Taka, Maija; Hickman, Brandon; Pulkkinen, Seppo; Krebs, Gerald; Moisseev, Dmitri N.; Koivusalo, Harri; Kokkonen, Teemu
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Rainfall-runoff simulations in urban environments require meteorological input data with high temporal and spatial resolutions. The availability of precipitation data is constantly increasing due to the shift towards more open data sharing. However, the applicability of such data for urban runoff assessments is often unknown. Here, the feasibility of Finnish Meteorological Institute’s open rain gauge and open weather radar data as input sources was studied by conducting Storm Water Management Model simulations at a very small (33.5 ha) urban catchment in Helsinki, Finland. In addition to the open data sources, data were also available from two research gauges, one of them located on-site, and from a research radar. The results confirmed the importance of local precipitation measurements for urban rainfall-runoff simulations, implying the suitability of open gauge data to be largely dictated by the gauge’s distance from the catchment. Performance of open radar data with 5 min and 1 km² resolution was acceptable in terms of runoff reproduction, albeit peak flows were constantly and flow volumes often underestimated. Gauge adjustment and advection interpolation were found to improve the quality of the radar data, and at least gauge adjustment should be performed when open radar data are used. Finally, utilizing dual-polarization capabilities of radars has a potential to improve rainfall estimates for high intensity storms although more research is still needed.
Impact of alternative land cover descriptions on urban hydrological model simulations
(2019-02-07) Kokkonen, Teemu; Warsta, Lassi; Niemi, Tero J.; Taka, Maija; Sillanpää, Nora; Pusa, Mikko; Kesäniemi, Outi; Salo, Heidi; Koivusalo, Harri
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Availability of remotely sensed and openly available land cover datasets is rapidly improving. This opens promising possibilities for utilizing such data in urban hydrological assessments. However, it remains unknown how the performance of readily available land cover data compares with manually collated information when used to construct detailed model parameterizations required in urban hydrological models. In this study, model runs with alternative land cover data are conducted for three small study areas with varying urban densities in Helsinki, Finland. The study demonstrates how different spatial data sources with varying resolutions produce different urban runoff simulation results. The results suggest that an openly available detailed land cover description can perform equally well with a laboriously collated manual land cover description. However, mixed land cover types of the pan-European Urban Atlas dataset are problematic in describing directly connected impervious areas, which leads to poor modelling results in low density urban areas.
Role of spatial anisotropy in design storm generation: Experiment and interpretation
(2016-01-08) Niemi, Tero J.; Guillaume, Joseph H A; Kokkonen, Teemu; Hoang, Tam M T; Seed, Alan W.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Rainfall accumulation depths over a given area are strongly dependent on the shape of the storm together with its direction of advection. A method to produce design storms exhibiting anisotropic spatial scaling is presented by combining a state-of-the-art stochastic rainfall generator STEPS with the linear generalized scale invariance (GSI) notation. The enhanced model is used to create ensembles of design storms based on an extreme storm with a distinct rainband shape observed in Melbourne, Australia. Design storms are generated both with and without accounting for anisotropy. Effect of anisotropy on precipitation characteristics is studied using the entire region covered by the radar (radar scale) and at a significantly smaller catchment scale. A rainfall-runoff model is applied to route the rainfall through the catchment into streamflow. Accounting for anisotropy allows for a more realistic description of precipitation features at the radar scale. At the catchment scale, anisotropy increases the probability of high rainfall accumulations, which translates into greater flood volumes. No discernible difference was observed in streamflow characteristics after controlling for the accumulation over the catchment. This could be explained by a lower importance of anisotropy relative to other factors affecting streamflow generation, and by the difficulties in creating representative rainfall temporal properties at the catchment scale when the radar scale is used for model calibration. The proposed method provides a tool to create ensembles of design storms when the anisotropic shape of the fields is of importance.
Role of spatial anisotropy in design storm generation: Experiment and interpretation
(2015) Niemi, Tero J.; Guillaume, Joseph H. A.; Kokkonen, Teemu; Hoang, Tam M. T.; Seed, Alan W.
School of Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Rainfall accumulation depths over a given area are strongly dependent on the shape of the storm together with its direction of advection. A method to produce design storms exhibiting anisotropic spatial scaling is presented by combining a state-of-the-art stochastic rainfall generator STEPS with the linear generalized scale invariance (GSI) notation. The enhanced model is used to create ensembles of design storms based on an extreme storm with a distinct rainband shape observed in Melbourne, Australia. Design storms are generated both with and without accounting for anisotropy. Effect of anisotropy on precipitation characteristics is studied using the entire region covered by the radar (radar scale) and at a significantly smaller catchment scale. A rainfall-runoff model is applied to route the rainfall through the catchment into streamflow. Accounting for anisotropy allows for a more realistic description of precipitation features at the radar scale. At the catchment scale, anisotropy increases the probability of high rainfall accumulations, which translates into greater flood volumes. No discernible difference was observed in streamflow characteristics after controlling for the accumulation over the catchment. This could be explained by a lower importance of anisotropy relative to other factors affecting streamflow generation, and by the difficulties in creating representative rainfall temporal properties at the catchment scale when the radar scale is used for model calibration. The proposed method provides a tool to create ensembles of design storms when the anisotropic shape of the fields is of importance.
A simple and effective method for quantifying spatial anisotropy of time series of precipitation fields
(2014) Niemi, Tero J.; Kokkonen, Teemu; Seed, Alan W.
School of Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The spatial shape of a precipitation event has an important role in determining the catchment's hydrological response to a storm. To be able to generate stochastic design storms with a realistic spatial structure, the anisotropy of the storm has to be quantified. In this paper, a method is proposed to estimate the anisotropy of precipitation fields, using the concept of linear Generalized Scale Invariance (GSI). The proposed method is based on identifying the values of GSI parameters that best describe isolines of constant power on the two-dimensional power spectrum of the fields. The method is evaluated using two sets of simulated fields with known anisotropy and a measured precipitation event with an unknown anisotropy from Brisbane, Australia. It is capable of accurately estimating the anisotropy parameters of simulated nonzero fields, whereas introducing the rain-no rain intermittency alters the power spectra of the fields and slightly reduces the accuracy of the parameter estimates. The parameters estimated for the measured event correspond well with the visual observations on the spatial structure of the fields. The method requires minimum amount of decision making and user interaction, making it suitable for analyzing anisotropy of storm events consisting of long time series of fields with a changing spatial structure.
Stormflow against streamflow – Can LID-provided storage capacity ensure performance efficiency and maintenance of pre-development flow regime?
(2021-11) Khadka, Ambika; Kokkonen, Teemu; Koivusalo, Harri; Niemi, Tero J.; Leskinen, Piia; Körber, Jan Hendrik
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The goal of Low Impact Development (LID) is to restore and maintain the pre-development flow regime. The static storage capacity, which is often used as a parameter in LID designs, provides the maximum capacity of an LID type and is easily quantifiable already at the design phase. However, the static storage approach does not consider the inter-event recovery of storage capacity by infiltration and evapotranspiration. This study investigated dynamic storage capacities of three stormwater management designs with increasing proportions of LID units on a 1.2 ha urban residential block in Southern Finland, to compare their cost-efficiency, as well as their potential in restoring the pre-development flow regime. The cost-efficiency of LID designs was assessed based on their ability to contribute to water losses, and on the additional construction costs required when comparing them to conventional solutions (e.g. asphalt replaced with permeable pavement). The design with a small storage capacity and a large capture ratio, i.e., the ratio of contributing area to LID area, was the least efficient albeit its small construction cost. The design with an appropriate balance between the capture ratio and the LID provided storage capacity was the most efficient option. In assessing the potential of stormwater designs in restoring the pre-development flow regime, the sum of infiltration and flow in storm sewer networks was more representative of the catchment total runoff than flow alone. Finally, an extensive simulation of a large set of differently placed LID units proved useful in a priori identification of the most influential units in the treatment train.
Towards natural water cycle in urban areas : Modelling stormwater management designs
(2020-08-08) Khadka, Ambika; Kokkonen, Teemu; Niemi, Tero J.; Lähde, Elisa; Sillanpää, Nora; Koivusalo, Harri
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Urbanization modifies the natural water cycle particularly by reducing the water storage capacity. We analysed the storage capacity of three stormwater management designs in south-western Finland to demonstrate how an urban catchment releases stormwater and how storage contributes to flood resilience. The analysis relies on EPA SWMM5.1 simulations of water balance for a seven-month period including two extreme rain events during the summer and autumn. The enhanced storage capacity provided by the designs increased resilience against flooding and released stormwater with slower rates leading to reduced peak flows. Even the design with the least storage (10% LID coverage) was efficient at regulating floods due to controlled flow in a vegetated swale, whereas the design with the highest storage capacity (60% LID coverage) demonstrated the possibility of restoring nearly natural water cycle in urban catchments. The study suggests storage capacity can act as a flood resilience indicator directly linked with the physical catchment characteristics.