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ECMWF COPERNICUS REPORT

Copernicus Climate Change Service

Urban SIS: Use case urban flooding

Stockholm

D441.6.2.1

Status report

Issued by: Swedish Meteorological and Hydrological Institute

Date: 30/04/2017

Ref: C3S_441_Lot3_Urban_SIS_D441.6.2.1

Official reference number service contract: 2017/C3S_441_Lot3_SMHI/SC2

This document has been produced in the context of the Copernicus Climate Change Service (C3S).

The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts,

operator of C3S on behalf of the European Union (Delegation Agreement signed on 11/11/2014). All information in this

document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.

The user thereof uses the information at its sole risk and liability. For the avoidance of all doubts, the European Commission

and the European Centre for Medium-Range Weather Forecasts has no liability in respect of this document, which is merely

representing the authors view.


C3S_441_Lot3_UrbanSIS_SC2 - D441.6.2.1 Use case urban flooding Stockholm of 12

Contributors

EDITOR

- Anna Åkesson, WSP - Sofie Thurin, WSP



Table of Contents

1. Purpose of the study 5

1.1 Description of the general problem 5

1.2 Decision support – benefits for the end user 5

1.3 Temporal and spatial scales of interest 6

1.3.1 Spatial resolution 6

1.3.2 Temporal resolution 6

2. Status of the Stockholm use case 6

3. What has been accomplished so far? 7

4. What is envisioned for the near future? 9

References 10



Urban SIS: Use case urban flooding Stockholm The Urban SIS case study for Stockholm city is a modelling demonstration of using spatially and temporally distributed rainfall inputs to urban flooding modeling. The outcomes of this first report describes modelling of urban flooding scenarios using downscaled, high-resolution precipitation data for historical time series, whereas future work will also include runoff modelling for future climate conditions.

1. Purpose of the study

1.1 Description of the general problem

Presently, urban hydrological climate change impact assessment is highly limited by the comparatively coarse resolution in available climate projection data (>10×10 km²), in which small-scale rainfall extremes is difficult to adequately describe. In the project Urban SIS, climate projections are downscaled to 1×1 km² grids over European cities which makes it possible to describe the expected changes in short-duration rainfall extremes, and in turn assess urban flood risk, with higher accuracy. The presented study demonstrates the use of the urban SIS as input to local Hydrological and Hydraulic models and local assessments. New methodology of downscaled precipitation data is together with high resolution land use data used as inputs to hydraulic modelling of urban flooding using the software MIKE21 developed by DHI. The project comprises quantifications and evaluation of the consequences of a dynamic rainstorm moving over Stockholm, for present conditions as well as for a scenario in a future climate (a 5-year period around year 2050). The urban flooding will be driven by intensive rainfall through both direct surface run-off over the city infrastructure and through the influence of heavy rainfall upstream the city, the latter manifested as flooding of the watercourses inside the city borders. The novelty lies in that the precipitation input is variable in space as well as time, which mimics the real dynamics of the intense rainfall events that are expected to become more common in the near future. The demonstrated rainfall events comprise short duration, high intensity storms over limited areas. At these instances, urban drainage systems are generally overfilled, implying that a large proportion of the rainfall will be transported as surface flow. The questions related to the future climate are similar to the present-day problems, with the notion that the occurrence of high-density precipitation will become more frequent in the future. Apart from changes in rainfall pattern, future climate scenarios are believed to display other temperature conditions than currently. Changes in temperature and/or precipitation will influence the soil moisture, which in turn will induce changes in infiltration capacity compared to the present conditions. If modelling goes smoothly, a possible extension of the analyses could also be included to address effects of seasonally varying soil moisture conditions. These questions will be further analysed by Urban SIS final data and the results will be reported in detail in the final report D441.6.2.2.

1.2 Decision support – benefits for the end user

The application of the proof of concept demonstrated by the case studies has both methodological and practical benefits. For the end user Stockholm City, the identification of vulnerable areas prone to flooding after intense rainfall is of course a major result. Once the model is set up, flood



mitigation measures could also be evaluated – i.e. to optimize flood protection solutions, by e.g. infrastructure and/or by allowing certain areas to be flooded during extreme conditions. On a more theoretical level, the high spatial and temporal resolution of short-duration rainfall extremes, and in turn the augmented possibility to assess urban flood risk, has significant consequences on the runoff modelling in urban areas in general. The results provides a means of utilizing newly developed scientific methods in urban planning, where the foremost novelty presented within the Urban SIS project is that rainfall events which are non-uniform in space and time are modelled.

1.3 Temporal and spatial scales of interest

1.3.1 Spatial resolution The geographical/spatial domain included in the Stockholm case study is delimited by the borders of Stockholm City (Stockholm Stad), see red delineation in Figure 1. The modelled extent of Stockholm is subdivided into four different geographical domains, as to make the data-heavy modelling exercise more convenient. The geographical resolution of the modelled raster cells is 4 x 4 m2. The precipitation input data is however supplied in grids of 1 x 1 km2, which implies that the spatial resolution of the land cover and the urban hydraulic flooding model has an even denser spatial resolution. The total modelled area for the Stockholm use case is about 220 km2, and comprises about 13 million raster cells.

Figure 1. The location and delineation (in red) of the modelled area in Stockholm City.

1.3.2 Temporal resolution The supplied precipitation data is presented in 15 minute intervals, which is a higher temporal resolution than what generally is available for rainfall modelling. The historical rainfall event chosen for the modelling demonstration has a duration of little more than two hours, and to simulate the runoff event in its entirety, the model is being run for several hours after the time when precipitation ceases.

2. Status of the Stockholm use case Despite some initial problems with the portal and the storage/downloading of the data, the data for the historical period has successfully been downloaded from the web portal and is now in pre-processing for later insertion in the MIKE21 model. The presented data included the years 2006-2007 and 2012-2014, and the available data series was scrutinized to identify a rainfall event with large magnitude and short duration, which influenced a



large proportion of the modelled domain of Stockholm City. The best identified rainfall event was found on August 23rd 2006, with a peak at 09:00 (07:00 GMT + 2 hours) as presented in Figure 2

Figure 2. An illustration of the procedure to identify a short-duration, large rainfall event which influenced a large proportion of Stockholm City. After identifying this, appropriate rainstorm, the gridded data was downloaded and prepared for reprocessing before being used as input to the hydraulic model (MIKE 21).

3. What has been accomplished so far? The first modelling task of an intense rainfall in based on historical data, in one of the four geographical subdivisions of Stockholm case study, is now under way. The precipitation data as identified in Chapter 2 has been downloaded as a grid and imported into a GIS system where the major part of the data processing is made (Figure 3).



Figure 3. An example of the gridded precipitation data for the rainfall event over Stockholm on August 23rd 2013 for one timestep. The geographical non-uniformity of the rainfall is here evident. The resolution of the grids is 1 x 1 km.

The precipitation data is combined with infiltration capacity, so that only excess water contributing to surface flow is entered into the hydraulic model MIKE21. Additional losses due to the filling of the stormwater network are also subtracted from the precipitation in quantities corresponding to a rain event with a 5-year return period. To determine the infiltration capacity of each of the 4 x 4 m2 grids, a land use map supplied by Stockholm City (Stockholm Stads stadskarta) is used, Figure 4. Each land use class is assigned an infiltration capacity (as a ratio), and by multiplying the precipitation in each grid with the infiltration coefficient, the losses as infiltration are quantified in each grid, and the remaining excess precipitation is entered into the hydraulic model. The used hydraulic model has previously been setup in a project performed with Stockholm City, and the model is used with the permission of Stockholm City.



Figure 4. Land use categories within central Stockholm City as used as inputs to determine infiltration capacity and roughness for the respective types of land use.

4. What is envisioned for the near future? For the near future, we will first make sure to compile all the input data into the required format. When assured that the first of the four modelling domains in the Stockholm area is successfully performed, the remaining three areas will be modelled. Thereafter, the flooding maps over Stockholm City can be compiled presented for the rain event of Aug 23rd 2013. Outcomes from the first modelling period (based in historical data) are planned to be presented in a poster at the ECCA2017 conference in Glasgow, co-authored with representatives from SMHI and ARPAE. When available, similar modelling will be made for future climate conditions. If time permits, further modelling, investigating the effects of temporally changes in soil moisture will also be included in the analyses of the future conditions.



References Skyfallsmodellering för Stockholms stad – Simulering av ett 100-årsregn i ett framtida klimat (år 2100), J. Pramsten, Stockholm Vatten AB, 2015.


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