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Project

voorspelling en beheer van extreme regenval dat risico's veroorzaakt in een stedelijke omgeving.

A. Context

Extreme local rain storms may induce severe floods and related socio-economic impacts on the urban environment (Belgian cities). While floods along rivers have been studied already extensively, quantification, forecasting, control and management of inundations along sewer systems and urban rivers have to face particular difficulties. They need fine-scale (local, short duration) rainfall estimation and nowcasting (= short-term forecasting in real time). They also require involvement of local authorities, which typically have low capacity in setting up risk quantification, forecasting, control and management systems.

B. Objectives

This PLURISK project supported the local authorities in the quantification, forecasting, warning, control and management of pluvial floods. Methodologies and software tools are being developed for:

(1) Nowcasting of fine-scale extreme rainfall, using advanced techniques for storm cell tracking and integrating national (C-band) and local (X-band) radar technology, numerical weather prediction (NWP); quantification of the uncertainty in this nowcasting.

(2) Two-dimensional fine-scale modelling, mapping and nowcasting of inundations in urban areas, and quantification of uncertainties on these inundation quantifications.

(3) Socio-economic risk quantification, incl. material and immaterial (social, ecological) damage assessment, quantification of risk perception (awareness), coping capacity and recovery capacity, impacts on built heritage, and uncertainty estimation on these impacts.

(4) Risk communication and flood risk warning based on the nowcasting results; also extreme rainfall and lightning warnings will be addressed.

(5) Risk reduction by both prevention/management, with particular focus on new management strategies, i.e. better interfacing between spatial planning, eco-management and urban water management (e.g. green – blue water; role of landscape architecture; restoration of biodiversity in urban areas incl. ecotechnologies on buildings and very dense urbanized areas, and considering the services of biodiversity for human population)

While the nowcasting system for extreme weather conditions developed in (1) has been developed nation-wide (covering the whole Belgian territory), the methodologies and software tools developed in (2) - (5) have been tested and demonstrated for selected Belgian cities as case-studies. More specifically, the cities of Leuven and Gent (area of Oostakker and Sint-Amandsberg) were considered as case studies. For the Leuven case, the PLURISK project was linked to the RainGain Interreg IVB NEW project, which completed in 2015. The methodologies and tools developed within PLURISK are, however, applicable to any city or urban area in Belgium.

C. Conclusions

The main conclusions on the research activities and results are:

For WP1 - Nowcasting of fine-scale extreme rainfall:

A detailed international literature review was conducted on methods and experiences in radar based fine scale rainfall estimation. Methods were selected, advanced and applied to integrate the X-band with the C-band radar observations and with rain gauge data, in order to generate hybrid spatial rainfall composites with high resolution. The results show that for both the X-band and C-band radar data, the radar estimates could be greatly improved by all the adjustment procedures. The gauge-radar residuals however remain quite large, even after the adjustments. We moreover concluded that the adjusted X-band radar measurements are not always better estimates than the corresponding C-band measurements. Further investigation showed that the rain gauges and radars could simulate the spatially more uniform winter storms with almost the same accuracy. The results are different for summer events, where the added value of the radar data becomes more evident.

Moreover, important progress was achieved with the implementation of the Short-Term Ensemble Prediction System (STEPS). The STEPS framework has shown flexibility to meet the scientific objectives of WP1 such as the non-conservative extrapolation of rain fields, uncertainty quantification of both radar observations and nowcasts, etc. STEPS-BE was set up for ensemble quantitative precipitation estimation and forecasting as well as the deterministic, probabilistic and ensemble verification of STEPS nowcasts. Additionally, the STEPS-BE software suite is able to run both in an operational environment on real-time data, as well as on archived data for case studies. Nowcasts were produced for selected storms and applied for testing the integration with the urban flood modelling in WP2 in the PLURISK case studies.

The real-time mode of STEPS-BE is being used side-by-side with INCA-BE, the current operational deterministic nowcasting system at the RMI. A new lightning module was added to this deterministic system, and the performance of this module is continuously monitored in a real-time verification utility.

Weather radar data were also prepared and delivered to WP3 in order to extend the multivariate depth-damage models with rainfall-damage models. They consist of monthly maximum hourly rainfall accumulations derived from the quantitative precipitation estimation product of the Wideumont radar.

For WP2 - Two-dimensional urban flood modelling, mapping and nowcasting:

For the Leuven case, a detailed sewer system model, implemented in InfoWorks-CS, was calibrated and validated based on in-sewer measurements of water levels and flow velocities. After that, the model was applied to simulate more than 50 rainfall events, including 10 extreme rain storms. For the Gent case, the sewer model is up and running as well, and 10 extreme rain storms were simulated. The rainfall inputs were based on the different radar adjustment and fine-scale rainfall estimation methods developed in WP1, to study the effect on the sewer flow and water level results.

Based on these simulation results and applying the dual drainage approach, sewer surface inundations were modelled in a 2D way. Detailed and simplified full 2D versus 1D approaches were implemented and tested for that task. For the full 2D approach, different mesh resolutions were tested and the optimal resolution selected. To enhance the application of such 2D surface inundation modelling and mapping, simplified approaches – but identified and calibrated to the detailed approaches – were tested and implemented. This sewer inundation model has also been applied for simulation of the STEPS nowcasts provided by WP1 for selected storms, to obtain urban flood hazard maps to be combined with the urban damage functions obtained by WP3 to obtain urban flood risk maps.

For WP3 - Socio-economic risk quantification:

Based on a theoretical and analytical literature on flood damages and risks (functions), a specifically targeted questionnaire was distributed among flood victims. We managed to get the permission from the Belgian Privacy Commission to allow us to use the data from the Belgian Disaster Fund so that we could execute a large-scale written questionnaire. This created possibilities for innovative statistical analysis, which were completed. Two distinct types of multivariate flood damage models, “depth-damage” models and “rainfall-damage” models, were developed. Moreover, an intangible damage analysis was conducted with interesting findings. Also the potential damage to historic buildings and sites was explored. The “depth-damage” model was implemented for integration with the 2D sewer inundation depth results of WP2. The same was done for the “rainfall-damage” model, in view of WP4.

For WP4 - Risk communication and flood risk warning:

All PLURISK components (rainfall nowcasts, radar – rain gauge merged products, urban inundation model, socio-economic impact assessment approach) were brought together for the Gent case study to come up with a prototype integrated and multi-disciplinary urban inundation risk nowcasting system. By running the different STEPS-BE ensemble members, the rainfall nowcast uncertainty is propagated to the inundation hazard and risk results. The other uncertainties on the inundation modelling and socio-economic impact assessment are being added and propagated in that modelling chain. Different visualization approaches of the uncertainties were tested, which formed the basis for the urban inundation risk warning.

For WP5 - Risk reduction by eco-solutions:

A state of the art of concepts for eco-solutions and evaluation of such solutions was reviewed, which identified three main "gaps" in the literature. These include gaps reg. the definition of green spaces, the typology of green spaces, as well as the role of different types of green spaces in water regulation. The analysis of infiltration and runoff data collected from the literature highlighted a lack of consensus, showing the difficulty to evaluate the (function of) ecosystem services.

Assuming that ecological processes / ecosystem services depend on the spatial structure of green spaces, this structure was analysed by calculating different indices of the landscape. Five cities in the same region were studied to identify possible similarities between spatial patterns. The total amount of green spaces for the five cities was found comparable to other European cities. The size of green spaces was found increasing along an urban-rural gradient. At the same time, their density decreases. We also find that the connectivity of green spaces increases and that the shape becomes more complex along the urban-rural gradient.

Four categories of green infrastructures were identified as being useful in rainwater management, actively preventing flooding by reducing runoff. They are green roofs, bio-swales, infiltration trenches and rain gardens. Each one was developed in a specific technical leaflet. In parallel, study cases were analysed in Gent. The studied neighbourhood regularly flooded under heavy rains. A holistic analysis has been made from a spatial planning perspective to integrate the most appropriate green infrastructures in order to prevent flooding and to enhance the urban biodiversity.

D. Contribution of the project in a context of scientific support to a sustainable development policy

The PLURISK project focused on:

  • “Natural risks” related to the “extreme meteorological phenomena”: extreme rain storms, urban floods;
  • “Areas at risk”, being the Belgian society in the urban environment, and the “material cultural heritage”;

and considered the following chain to describe the risks:

  • Hazards: probability that fine-scale extreme high rainfall (that might induce urban flooding) will occur (variability) at a certain intensity, time and given place; and highlighting precursory signs (based on NWP), conditions that will induce urban floods (sewer hydraulics), as well as the potential aggravation of risks caused by a combination of other hazards and site effects (high downstream water levels). Uncertainties as regards to the fine-scale extreme rainfall hazards were reduced through different types of rainfall-based data sources, and explicitly quantified.
  • Vulnerability: identification and evaluation of the impacts on and potential damage to, as well as potential resilience of, at-risk areas. The research in particular took into account the multiple socio-economic and environmental factors which determine or influence such vulnerability.
  • Scientific support for managing risk: this involved i) evaluating risk based on the integration of scientific knowledge of the risk and the vulnerability in question; ii) analysing measurements of risk management while looking for a balance between measures to ensure early detection, prevention, impact limitation and restoration to support reduction or adaptation of the risk; and iii) analysing perception of the risk, concerns and values of society in order to suggest how to manage risk in a way that is acceptable to society and how to communicate this in a suitable manner.

The proposed research thus was multidisciplinary and systemic, and covered all three elements of the considered risk chain.

The PLURISK project more concretely aimed to support the local authorities in the quantification, forecasting, warning, control and management of pluvial floods. This aim was reached through the development and testing of methodologies and software tools for:

  1. Nowcasting of fine-scale extreme rainfall, using advanced techniques for storm cell tracking and integrating national (C-band) and local (X-band) radar technology, and numerical weather prediction; quantification of the uncertainty in this nowcasting (WP1);
  2. Two-dimensional fine-scale modelling, mapping and nowcasting of inundations in urban areas, and quantification of uncertainties on these inundation quantifications (WP2);
  3. Socio-economic risk quantification, incl. material and immaterial (social, ecological) damage assessment, quantification of risk perception (awareness), coping capacity and recovery capacity, impacts on built heritage, and uncertainty estimation on these impacts (WP3);
  4. Risk communication and flood risk warning based on the extreme rainfall and urban flood nowcasting results (WP4);

Risk reduction by both prevention/management and real-time control actions. New management strategies were developed by better interfacing between spatial planning, eco-management and urban water management (e.g. green – blue water; role of landscape architecture; restoration of biodiversity in urban areas incl. ecotechnologies on buildings and very dense urbanized areas, and considering the services of biodiversity for human population) (WP5).

Datum:1 mei 2012 →  31 okt 2017
Trefwoorden:urban environment, extreme rainfall
Disciplines:Milieubeleid, Stedelijk en regionaal ontwerp, ontwikkeling en planning