Session PL3

[Programme]

PL3 | Hydro-geological effects of extreme events (e.g., floods, landslides, erosions, coastal dynamics, storm surges etc.)

Hydro-geological effects of extreme events (e.g., floods, landslides, erosions, coastal dynamics, storm surges etc.)

Conveners: Marco Borga, Giorgio Boni, Michalis Diakakis

Orals

| Thu, 03 Oct, 12:00–13:30|Lecture room

Posters

| Attendance Thu, 03 Oct, 11:15–12:00 | Display Tue, 01 Oct, 09:00–Thu, 03 Oct, 16:30|Poster hall

The countries bordering the Mediterranean coast are affected each year by disasters that create economic damage and victims, with a serious impact on the economy and the development of the area. Among them hydro-geological disasters are among the worst. It is therefore of extreme interest to increase knowledge on the physical mechanisms that generate extreme hydro-geological effects, in order to improve the ability to model, evaluate and predict the risks associated with them. The session accepts contributions that present research on the observations, modeling and forecasting of the extreme hydro-geological processes typical of the Mediterranean and mediterranean-like climates, with special reference to the assessment, forecast and mapping of hazard scenarios, aimed at the assessment and forecast of the hydro-geological risks.

Orals: Thu, 3 Oct | Lecture room

12:00–12:15

Plinius18-60

Impacts of extreme storm and flood events on coastal areas: Data from recent disasters in the Eastern Mediterranean region.

Michalis Diakakis, Marilia Gogou, Christos Filis, Spyridon Mavroulis, Andromachi Sarantopoulou, Ioannis Kapris, Emmanuel Vassilakis, Katerina-Navsika Katsetsiadou, Evelina Kotsi, Nafsika Spyrou, Aliki Konsolaki, Eleftheria Stamati, and Efthymis Lekkas

The projected increase in frequency of flood extremes, attributed to climate change, poses a significant threat to coastal regions throughout the Mediterranean. Consequences encompass extensive geomorphological changes, infrastructural degradation, property damage, pollution of the aquatic environment, and other adverse socio-economic impacts, also threating the blue economy—a vital economic driver of the region. Acknowledging the pivotal role of coastal areas as critical nodes for both economic activities and ecologically valuable natural landscapes, it becomes imperative to deepen our understanding of the mechanisms and extents to which extreme flood events can impact these vulnerable coastal zones.

This work focuses on exploring the impacts on the coastline of recent extreme storm events in the Eastern Mediterranean. The study aims to explore and classify the typology of effects, the severity of impacts and examine their spatial distribution as means to contribute to an improved understanding of extreme storm and flooding consequences in the region.

How to cite: Diakakis, M., Gogou, M., Filis, C., Mavroulis, S., Sarantopoulou, A., Kapris, I., Vassilakis, E., Katsetsiadou, K.-N., Kotsi, E., Spyrou, N., Konsolaki, A., Stamati, E., and Lekkas, E.: Impacts of extreme storm and flood events on coastal areas: Data from recent disasters in the Eastern Mediterranean region., 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-60, https://doi.org/10.5194/egusphere-plinius18-60, 2024.

12:15–12:30

Plinius18-100

The future of the Portuguese most vulnerable coastal areas under climate change – shoreline evolution and future extreme coastal flooding from downscaled bias corrected ensembles

(withdrawn)

Gil Lemos, Ivana Bosnic, Carlos Antunes, Michalis Vousdoukas, Lorenzo Mentaschi, Miguel Santo, Vanessa Ferreira, and Pedro M. M. Soares

12:30–12:45

Plinius18-120

Analysing extreme flood events in the Mediterranean region

Ioanna Stamataki, Thomas R. Kjeldsen, Joan Rossello, and Ramtin Sabeti

12:45–13:00

Plinius18-6

Chronicle of a disaster foretold: The Storm Daniel dam-breaching flood at Derna, Libya

Elad Dente, Moshe Armon, and Yuval Shmilovitz

Storm Daniel, the deadliest recorded Mediterranean tropical-like (medicane) storm, led to severe floods in large parts of the eastern-central Mediterranean, including Greece and northern Libya. Extreme rainfall, reaching more than 400 mm day^-1, triggered a flash flood in Wadi Derna (Libya)– an ephemeral river with a drainage area of 575 km² that crosses the city of Derna at its outlet to the Mediterranean Sea. In the 1970s, dams were built in Wadi Derna basin to mitigate flood risks. However, during Storm Daniel, at least two dams were breached by a flash flood that inundated much of the city of Derna and resulted in over 5,000 casualties, thousands of missing persons, and tens of thousands of displaced people. The devastating event was the focus of media coverage for a long time, but many questions with implications for other dammed Mediterranean regions are still open. Here, we focus on three main research questions: (a) How unique and extreme were the storm and meteorological conditions of Storm Daniel? (b) How extreme was the flood? And (c) What could have been the flood outcome if dams hadn't been built upstream in the first place?

To analyze the characteristics of Storm Daniel over Wadi Derna, the catchment’s hydrological response, and the impact of the flood on the city of Derna, we integrate various datasets and models. Satellite-based precipitation estimations (IMERG) were used to quantify spatiotemporal storm properties and the catchment-scale rainfall, which were fed into the KINEROS2-RHEM hydrological model to quantify surface runoff. The modeled flood hydrograph is then fed into a 2D hydraulic model (HEC-RAS) to test three end-member scenarios: (a) dam filling, overflow, and collapse, (b) dam overflow but no collapse, and (c) no dams exist in the wadi. This combination of methods reveals that the peak discharge during the flood was ~1,400 m³ s^-1, falling below the expected maximum extreme flood for this region. Based on the total discharge volume, we estimated the return period of the flood as 33-50 years. In the dam-collapse scenario, the populated flooded area is 40% larger and ten-fold more destructive than the no-dam scenario. Given the high variability of precipitation in the Mediterranean and the projected increase in extreme precipitation intensity under climate change, the Wadi Derna flood should serve as a warning sign for other populated areas downstream of dams in similar environments.

How to cite: Dente, E., Armon, M., and Shmilovitz, Y.: Chronicle of a disaster foretold: The Storm Daniel dam-breaching flood at Derna, Libya, 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-6, https://doi.org/10.5194/egusphere-plinius18-6, 2024.

13:00–13:15

Plinius18-113

Impact of temperature threshold selection on future changes of liquid precipitation return levels based on convection-permitting models

Matteo Pesce, Eleonora Dallan, Francesco Marra, Giorgia Fosser, Petr Vohnicky, Rashid Akbary, and Marco Borga

In mountainous regions, temperature conditions directly affect precipitation phase (liquid or solid) and, in turn, runoff formation and the probability of flood events. The projected temperature increase due to global warming therefore directly affects the fraction of liquid precipitation during heavy storms, leading to a potential intensification of the flood regime. In this study we assess the impact of temperature threshold selection for splitting precipitation into rainfall and snowfall, on the projected changes in the liquid fraction of precipitation during extreme events in the upper Adige River catchment (Eastern Italian Alps). To this aim, we use an ensemble of convection-permitting climate models (CPM), which are well suited to the task given their ability to explicitly represent deep convection and to resolve the mountainous topography. The CPM data provided by the CORDEX-FPS Convection project at 1 hour temporal resolution and remapped to 3 km spatial resolution cover historical and far future (2090-2099) time periods under the extreme climate change scenario (RCP8.5). Future changes of rainfall extremes are obtained using the Simplified Metastatistical Extreme Value approach, which is applied to the CPM simulations for frequency analysis. This approach provides estimates of extreme return levels with reduced uncertainty with respect to traditional methods. Three different temperature thresholds are used (i.e. 1, 1.5 and 2°C) and different elevation bands are considered within the catchment. Our preliminary results indicate an increase of liquid precipitation return levels that is dependent both on temperature threshold and elevation. In particular, larger increments are obtained for lower temperature thresholds and higher elevations. Moreover, the event duration seems to have an impact on the results, with a stronger signal for long duration storms. The results highlight the importance of addressing uncertainty in the quantification of future rainfall extremes in a mountainous area, with strong implications for water resources management and adaptation strategies.

How to cite: Pesce, M., Dallan, E., Marra, F., Fosser, G., Vohnicky, P., Akbary, R., and Borga, M.: Impact of temperature threshold selection on future changes of liquid precipitation return levels based on convection-permitting models, 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-113, https://doi.org/10.5194/egusphere-plinius18-113, 2024.

13:15–13:30

Plinius18-122

Comparing extreme sub-daily rainfall projections from convection-permitting climate models and temperature-scaling across an Alpine gradient

Rashid Akbary, Marco Marani, Eleonora Dallan, and Marco Borga

Understanding projected changes in sub-daily extreme rainfall in mountainous basins can help increase our capability to adapt to and mitigate against flash floods and debris flows. Here we compare the changes in extreme rainfall projections from apparent Clausius-Clapeyron (CC) temperature scaling against those obtained from convection-permitting climate model simulations. Temperature and precipitation projections are obtained from an ensemble of convection-permitting climate models (CPM), which are suitable to the task given their ability to explicitly represent deep convection and to resolve the mountainous topography. The CPM data provided by the CORDEX-FPS Convection project at 1-hour temporal and remapped to 3 km spatial resolution, cover historical and far-future (2090-2099) time periods under the extreme climate change scenario (RCP8.5). Due to the computational demands however, CPM simulations are still too short (typically 10-20 years) for analyzing extremes using conventional methods. We use a non-asymptotic statistical approach (the Metastatistical Extreme Value, MEVD, Marani and Ignaccolo, 2015) for the analysis of extremes from short time periods, such as the ones of CPM simulations. We use hourly precipitation and temperature data from 174 stations in an orographically complex area in northeastern Italy as a benchmark.

In this study, we considered two temperatures approach for computing CC scaling: the mean annual temperature and the temperature during extreme events (top 20% of ordinary events). Our findings indicate that elevation significantly influences temperature changes during storms, with higher elevation areas experiencing more pronounced temperature increases in the future. This is further highlighted by seasonal shifts in storm occurrence, as we found storms moving from summer to fall in lowlands, suggesting a lower delta T for those storms. This same pattern was detected using the temperature during storms for CC scaling, showing that extremes are increasing more in higher elevations than in lowlands. We found this comparably captured by CC scaling approach, however, variations in return levels are also notable in CPMs when considering different return periods, as we find CPM changes depending on them, which contradicts the results from both CC scaling approaches. These findings identify that CC scaling agrees with CPM changes to some extend when the right temperature is selected, however, it emphasizes the need for a nuanced understanding of the scaling method's applicability under various conditions.

How to cite: Akbary, R., Marani, M., Dallan, E., and Borga, M.: Comparing extreme sub-daily rainfall projections from convection-permitting climate models and temperature-scaling across an Alpine gradient, 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-122, https://doi.org/10.5194/egusphere-plinius18-122, 2024.

Posters: Thu, 3 Oct, 11:15–12:00 | Poster hall

Display time: Tue, 1 Oct, 09:00–Thu, 3 Oct, 16:30

P15

Plinius18-31

Exploring historical gully dynamics in an agricultural setting in Mediterranean climate

George Olivier, Marco Van De Wiel, Bastien Dieppois, and Willem de Clercq

Gully erosion is a significant degradation process affecting land (on-site effects) and water resources (off-site effects) in regions exhibiting a Mediterranean climate. Climate change projections suggest lower mean annual rainfall and higher intensity rainfall events, which could amplify gully erosion processes during the 21^st century. Analysing gully erosion over extended periods is essential to understanding better and predicting future gully perturbations and degradation. However, few studies monitor gully evolution on such temporal scales. We analyse long-term (1938-2023) gully morpho-dynamics in a Mediterranean catchment to estimate the impact of climate and land use drivers thereon.

The Sandspruit catchment in the Western Cape of South Africa exhibits a distinct Mediterranean climate. Natural vegetation is sparse, interspersed only between extensive agricultural fields mainly used for dryland agriculture. Four gully networks were analysed using aerial imagery and climate data. Gully changes and land use activities were mapped from the sequential aerial imagery. Climatic variability during the study period was assessed in terms of mean annual rainfall, rainfall intensity derivatives, and the El Niño–Southern Oscillation cycles (ENSO) to identify significant trends and association with gully evolution.

Preliminary results show that human activity is the primary influence on gully evolution. Agricultural intensification increased erosion up to 1938, whereafter mitigation efforts reduced gully sizes up to five times in 29 years. In subsequent years, contraction and expansion were observed. However, contemporary gully erosion since 1999 shows active growth. The increase in activity could be due to a reduction in the efficiency of the ageing mitigation measures but may also be caused by a changing climate. Rainy Day Normal, a rainfall intensity proxy based on mean annual rainfall, shows no distinct trend. However, ENSO cycles coupled with the timing of high-intensity events may play a critical role.

How to cite: Olivier, G., Van De Wiel, M., Dieppois, B., and de Clercq, W.: Exploring historical gully dynamics in an agricultural setting in Mediterranean climate, 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-31, https://doi.org/10.5194/egusphere-plinius18-31, 2024.

P16

Plinius18-76

Ripple effects: analyzing Cascade effects triggered by extreme storms and floods in the Eastern Mediterranean

Michalis Diakakis, Andromachi Sarantopoulou, Christos Filis, Spyridon Mavroulis, Marilia Gogou, Ioannis Kapris, Emmanuel Vassilakis, Aliki Konsolaki, and Efthymis Lekkas

P17

Plinius18-105

Integrated analysis of sediment connectivity and geomorphic effects of Storm Ciaran in two mountain catchments in Tuscany (Italy).

Federica Fiorucci, Stefano Crema, Lorenzo Marchi, Marco Piantini, Jacopo Rocca, Alessandro Sarretta, and Marco Cavalli

Extreme rainfall events over mountain catchments often trigger mass failures on hillslopes and sediment transport along the channel network. Understanding the connection between sediment sources and downstream transfer pathways is crucial in areas where settlements are impacted by hydro-geomorphic processes, to plan effective risk mitigation measures.

From November 2 to 5, 2023, northern Tuscany (Italy) experienced a severe flood due to storm Ciarán, causing landslides and flooding. This disaster led to eight deaths, displaced 300 people, and caused around €1.9 billion in damages.

This study focuses on two adjacent catchments, covering a total area of approximately 35 km², within the area affected by the storm event. To analyse sediment dynamics at the catchment scale, an integrated approach was devised, encompassing the analysis of high-resolution (0.5 m) satellite imagery combined with the field-based mapping of hillslope instabilities. After the instabilities census, the analysis was integrated with an Index of Connectivity map (created using SedInConnect 2.3 software) to characterize pre-event structural connectivity in both catchments. The integration of these datasets enables the determination of the type, extent, and characteristics of mass movements providing sediments to the channel network, as well as understanding the interaction between sediment transport in the main channel and its morphological modifications. This finding highlights that inventory maps developed soon after extreme events, combined with sediment connectivity data, can provide vital information for future land management and risk mitigation. These insights can help prioritize structural interventions in specific areas to reduce connectivity or disconnect inhabited areas from hydro-geomorphic systems, thereby preventing future damage.

The research is currently funded by the APPARE project within the framework of the Extended Partnership of the main project RETURN of the Italy’s Recovery and Resilience Plan (Next Generation EU). The APPARE project seeks to improve the understanding of flash floods in mountainous and hilly regions. This initiative aims to increase community resilience through better risk planning and management in these areas.

How to cite: Fiorucci, F., Crema, S., Marchi, L., Piantini, M., Rocca, J., Sarretta, A., and Cavalli, M.: Integrated analysis of sediment connectivity and geomorphic effects of Storm Ciaran in two mountain catchments in Tuscany (Italy)., 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-105, https://doi.org/10.5194/egusphere-plinius18-105, 2024.

P18

Plinius18-111

Understanding the impact of climate change on areal reduction factors using convection-permitting models

Petr Vohnicky, Eleonora Dallan, Francesco Marra, Giorgia Fosser, Matteo Pesce, and Marco Borga

Areal reduction factors (ARFs) are essential in hydrological modeling as they enable the conversion of point design precipitation into the average design precipitation for a catchment area. This study aims to assess the projected variations in ARFs during precipitation events for different return levels, focusing on the upper Adige River, Italy (Eastern Italian Alps). The study utilizes simulations from the ensemble of convection-permitting climate models (CPMs), which are well-suited for this purpose due to their ability to explicitly represent deep convection and to resolve the mountainous topography. The CPM data, provided by the CORDEX-FPS Convection project at 1-hour temporal resolution and remapped to a common 3 km spatial resolution, cover historical and far-future (2090-2099) time periods under the extreme climate change scenario (RCP8.5). The method applied for estimating the ARFs utilizes gridded precipitation data from CPMs and is based on precipitation quantiles derived from frequency curves. The extreme quantiles are estimated by the Simplified Metastatistical Extreme Value approach, known for its reduced uncertainty compared to conventional approaches. The ARFs are then calculated as the ratio of extreme quantiles for a selection of events with different duration and return period, which allows the ARF to vary where necessary. Our preliminary results indicate that ARFs for short-duration events are likely to increase in the future, suggesting a potentially larger spatial structure for storms. Additionally, for longer duration storms, ARFs are expected to remain relatively same, although a slight negative trend is observed for higher return period events. These results highlight the importance of considering advanced statistical methods and high-resolution climate models to address emerging challenges in hydrology and climate science.

How to cite: Vohnicky, P., Dallan, E., Marra, F., Fosser, G., Pesce, M., and Borga, M.: Understanding the impact of climate change on areal reduction factors using convection-permitting models, 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-111, https://doi.org/10.5194/egusphere-plinius18-111, 2024.

P19

Plinius18-137

The Importance of the observation network density in describing spatiotemporal characteristics of events producing urban flooding: a case study in the city of Genoa, Italy.

Giorgio Boni, Arianna Cauteruccio, Francesco Faccini, Nicola Loglisci, Massimo Milelli, Guido Paliaga, and Antonio Parodi

The rainfall climatology of the Mediterranean region is characterized by high probability of very intense rainfall events with a duration of a few hours or less and a very limited spatial scale. There is also evidence that this probability is increasing due to climate change.

In urban areas, these extreme rainfall events produce floods and pluvial floods, often in very small areas, depending on the physical-geographical layout of the area. It is therefore of paramount importance, in order to adequately model the ground effects and evaluate flash and pluvial flood risk in small urban catchments, to have an adequate monitoring network for rain events that are highly concentrated in space and time.

This research analyses the event of the 27th and 28th August 2023 that occurred in the city of Genoa, Italy, producing local floods in the historical center and surroundings due to the overflow of the sewer system and very small urban catchments. During the event, a maximum of 400 mm of rainfall in 6 hours was recorded in the eastern sector of the historic centre of Genoa.

We combined rainfall observations and estimates from official or “authoritative” networks (rain gauges and meteorological radar) and rain gauge networks inspired by citizen science principles. The analysis of combined observations reveals a spatial variability of the precipitation field at hourly and sub-hourly timescale that cannot be captured by the current spatial density of the authoritative measurement stations even in an area, like that of the Genoa Region, that shows one of the highest raingauge network density of Italy.

Furthermore, the analysis of short-duration annual maximum time series recorded by the authoritative rain gauge network of the Genoa region shows significant differences even at distances of less than 2 km in the average rainfall depth accumulated over a sub-hourly duration.

In conclusion, the results show that, to have a reliable description of urban flooding-producing rainfall events adequate hydrometeorological monitoring is one of the most important aspects. This may help both in correctly evaluate ground effects of occurred events as well as to improve the methods to draw design hyetographs for flood hazard assessment, especially for urban areas. In this perspective, the integration between authoritative and citizen science networks can provide a very interesting solution.

How to cite: Boni, G., Cauteruccio, A., Faccini, F., Loglisci, N., Milelli, M., Paliaga, G., and Parodi, A.: The Importance of the observation network density in describing spatiotemporal characteristics of events producing urban flooding: a case study in the city of Genoa, Italy., 18th Plinius Conference on Mediterranean Risks, Chania, Greece, 30 Sep–3 Oct 2024, Plinius18-137, https://doi.org/10.5194/egusphere-plinius18-137, 2024.