Tero Niemi

Role of spatial anisotropy in design storm generation: Experiment and interpretation

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

The spatial shape of a precipitation event has an important role in determining the catchments 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.

Development and application of an automated subcatchment generator for SWMM using open data

Abstract An open source subcatchment generator program was developed for the Stormwater Management Model (SWMM) to automate tedious stages in the model construction process. The generator divides the investigated area into subcatchments using a uniform computation grid and connects the grid cells together and to the underlying stormwater network. The system was tested by applying it to two small urban catchments with different fractions of impervious surfaces in Helsinki, Finland, using mostly openly available data. The simulated discharge results were compared to measured data and to results obtained from manually built models. The proposed system significantly accelerated the setup of a SWMM modelling project, as the routing between the subcatchments as well as the subcatchment slopes and flow widths were directly derived from the computation grid. Automatically generated and manually constructed SWMM models produced discharge results that differed only slightly from each other.

Spatio-temporal patterns of major ions in urban stormwater under cold climate

Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland Department of Built Environment, Aalto University, Espoo, Finland Department of Environmental Sciences, University of Helsinki, Helsinki, Finland Correspondence Maija Taka, Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, FI‐00014 Helsinki, Finland. Email: maija.taka@helsinki.fi Funding information European Union Life + programthe Academy of Finland, Grant/Award Number: 263335. 263320. 263308.

Effects of LID-Based Urban Designs on Water Balance

Low impact development (LID) has received significant attention in supplementing urban drainage designs. This study demonstrates the effects of LID-based urban designs on water balance components for an urban catchment. Two LID-based urban designs (A, B) are simulated for short term (E1) and long term (E2) periods and the effects of the various designs on different water balance components are assessed. The goal of the study is to demonstrate the performance of the designs to realize the sponge city concept. For the intensive short event, scenario A with 15% LID is not effective whereas scenario B with 49% LID is highly effective. For the longer period, both scenarios are highly effective in replicating sponge city concept. In addition, the A scenario possesses a potential for producing sponge city concept for rain events for return periods lower than 100 years.