Eleni Tzanou

Extreme weather events have increasingly resulted in significant disasters, impacting human life and economic development. Recently, short-term floods have caused substantial damage worldwide. Such was the case of the catastrophic flood that struck the region of Thessaly, Greece, in September 2023. In this study, we investigate the application of Earth Observation satellite data – optical, SAR, and gravimetric – for mapping flood events, focusing on the catastrophic flood that occurred in Thessaly. The rationale for incorporating both EO imagery and gravimetric observations lies in their complementary strengths: while optical and SAR sensors provide detailed, high-resolution snapshots of surface hydrological changes, gravimetry captures changes in the total water mass, enabling an assessment of broader hydrological conditions that may not be visible in conventional imagery. Our analysis was divided into three main components. Firstly, we utilized post-event satellite images from the Sentinel-2 platform, filtered by area of interest (AOI) and date range, with a constraint of less than 20% cloud cover. The Normalized Difference Water Index (NDWI) was applied to identify water bodies in optical imagery, serving as a reference for subsequent flood mapping methodologies. Secondly, we employed Sentinel-1 Synthetic Aperture Radar (SAR) data from pre- and post-event acquisitions. SAR images underwent edge masking and were merged to generate two mosaics. Flooded areas were detected by identifying regions with low backscatter values, specifically where the vertical vertical polarisation (VV) band exhibited values below a threshold of −17 in post-event mosaics relative to pre-event counterparts. Visual inspection confirmed the accuracy of flood detection against Sentinel-2 imagery. Lastly, we integrated GRACE and GRACE-FO Level-2 spherical harmonic coefficients provided by the Jet Propulsion Laboratory (JPL) to derive Liquid Equivalent Water Thickness (EWT), representing water thickness per grid cell. Despite the spatial resolution limitation of monthly GRACE products, EWT effectively captured spatial characteristics during short-term flood events. Simulating water level rise involved calculating flow direction, accumulation, and depressions using SRTM DEM data. The water spread simulation incorporated EWT values, modeling water movement and accumulation across terrain. This approach considered filled depressions and flow dynamics, providing a realistic representation of flood scenarios by accounting for terrain elevation and water volume. Modeled flooded areas closely resembled the spatial extent of the actual event, although volumetric accuracy was constrained by the spatial resolution of GRACE mission data.

The determination of physical heights is of key importance for a wide spectrum of geoscientific applications and, in particular, for engineering projects. The main scope of the present work is focused on the determination of a high-accuracy and high-resolution gravimetric and hybrid geoid model, to determine orthometric heights without the need of conventional leveling. Both historical and newly acquired gravity data have been collected during dedicated gravity campaigns, around the location of a dedicated GNSS network as well as areas where the existing land gravity database presented voids. Geoid determination was based on the classical remove–compute–restore (RCR) technique and spectral and stochastic approaches. The low frequencies have been modeled based on the XGM2019e global geopotential model (GGM) and the topographic effects have been evaluated with the residual terrain model (RTM) reduction. The evaluation of the final geoid model was performed over 462 GNSS/leveling benchmarks (BMs), where the newly determined gravimetric geoid has shown an improvement of 3.1 cm, in the std of the differences to the GNSS/leveling BMs, compared to the latest national geoid model. A deterministic and stochastic fit to the GNSS/leveling data has been performed, investigating various choices for the parametric models and analytical covariance functions. The scope was to determine a hybrid geoid model, tailored to the area and GNSS/leveling data, which will be the one used for the direct estimation of high-accuracy orthometric heights from GNSS observations. After the deterministic fit, a std to the GNSS/leveling data of 10.1 cm has been achieved, with 54.8% and 83.1% of the absolute height differences being below the 1 cm and 2 cm per square root km of baseline length. The final hybrid geoid model, i.e., after the stochastic treatment of the adjusted residuals, gave a std of the difference to the GNSS/leveling data of 1.1 cm, with 99.8% and 99.9% of the height difference being smaller than the 1 cm and 2 cm standard errors, thus achieving a 1 cm accuracy regional geoid.

This research aims to evaluate the operational effectiveness of current flood protection infrastructure and measures in a flood-prone area using geo-referenced information systems and SWOT analysis. To achieve this, all existing flood protection measures and works in the case study basin, namely Strymonas River basin in Greece, were mapped and recorded. These data, along with water-related spatial information, were stored in a geo-referenced database created within an open-source GIS environment. Additionally, the system was populated with the basin’s recorded historic floods, derived from the European Union’s Floods Directive implementation process. The outputs of the research, which include a spatial comparison of flood protection measures and works with flood event occurrences as well as analyses of the figures, density, and locations of flood protection works, were evaluated as an integrated system and further processed using SWOT analysis. The latter was informed by questionnaire results, and the identified strengths and weaknesses of the flood protection infrastructure were used to explore potential opportunities and threats, which could respectively reinforce or jeopardize the basin’s capacity to effectively respond to future floods. The research framework can be applied to any river basin and could provide important assets in flood protection planning at a basin scale.

The increased rate of floods occurrence during the last few decades, which is mainly attributed to climate change and mankind pressures on the hydrosystems, results on large scale horizontal flood control and protection policies. At European Union (EU) scale, the Directive on the Assessment and Management of Flood Risks of the year 2007 aims, after implementing sequential processes which amongst other include remote sensing and hydraulic modeling coupling, at proposing specific measures for mitigating the flood risks and the derived socioeconomic devastating impacts. The current research demonstrates the usefulness of geo-spatial technologies for assessing the operationality of the current anti-flood infrastructures together with the historic flood events and the necessity of maintaining the infrastructures. For doing so, all the flood control structures in the case study area were mapped in a geographic information system (GIS). Additionally, information regarding the floods? spatial and temporal placement were used to populate the GIS database, while the repeatability of the works regarding the maintenance and/or restoration and/or failure recovery of the flood control structures was attributed in monetary terms to evaluate the feasibility of the projects. The case study area is the Greek part of the Struma/Strymonas transboundary river basin, which is shared between Bulgaria and North Macedonia and Greece. The outputs of the research demonstrated the usefulness of the current flood protection projects, however, there were particular cases where the annual maintenance cost necessitates the promotion of new and more financial independent solutions.

Urban flooding occurs to populated and build environments when there is excess water due to intense rainfall, extreme river flows, or storm occurance. Flood protection procedures and processes are important and critical tools in flood-vulnerable and flood-prone areas. The consequences of the occurrence of such phenomena can have a seriously negative impact on a social, economic and environmental level. The first two categories are particularly affected in urban environments, where flooding might lead to severe casualties. The assessemnt for optimal use of mobile systems of mobile flood protection dams/barrires as short-term flood prevention and non-permanent/ nonstructural measures in combination with the permanently existing protection works and infrastructures in the urban environment is the subject of this study.As a field of application (case study) of this research and the evaluation of different flooding and intervention scenarios, a stream section of an important transboundary watercourse that flows through the city of Serres, Greece was chosen. For this stream, the river bed and the surrounding areas as well as the built environment and all the technical works along the stream were measured by land observation methods (topographic and remote sensing data).In order to draw into conclusions, the assessment of the hydrological characteristics and the water flow characteristics of the stream and the catchment area was carried out. Then, the simulation of the hydraulic characteristics for the current state of the stream and for various different flooding scenarios through the use of mobile flood barriers/small dams of different types and geometrical characteristics was applied.The result of the study has led to a “roadmap” of how, when and where non-permanent protection measures and can be implemented in urban environments, useful to local authorities and civil protection in charge.The evaluation of the capacity and performance of mobile barrier systems (based on their characteristics) was carried out, in order to be effectively used in varying flooding events, with different characteristics and in site-specific locations in various scenarios, through hydraulic simulations. The results of the hydraulic simulations resulted in the barrier systems’ evaluation and the formation of a methodology, which concerns their application efficiency and their inter-operability in the pilot area, while determining the optimal management and the overall cost at the same time.

Flood occurrence consequences are inextricably linked to the loss of human life and material damage. The latter has a direct economic impact and requires financial resources for repair or reconstruction in order to continue to provide protection. In the study area of the Strymon River in northern Greece, the implemented flood protection and hydraulic structural works combined with failure repeatability, their initial construction costs, and damage/repair costs were thoroughly assessed and correlated. This methodology provides a roadmap to support decision-making procedures to formulate flood protection action plans based on the valuation of current flooding results in established infrastructure.