Torrential Catchments Research Articles

Evaluation of sediment budgets in catchments prone to flash flood-related debris flows: A case study from the southern Apennines (Italy)

The evaluation of sediment budgets of flash-flood prone torrential catchments is important for the assessment of flood susceptibility and hazard in piedmont areas. Here, we developed and tested a predictive, quantitative geomorphological technique for estimating the magnitude of debris flows related to flash floods triggered by extreme rainfall events. We applied our method to steep torrential catchments that allowed testing of our technique through quantification of the effects of recently-occurred flash floods. The three study catchments are located in southern Italy and experienced flash flood-related debris flows triggered by a rainstorm with a recurrence interval > 200 yrs. The study basins are small size, steep, narrow and the stream network has a low order (maximum 3rd order). We estimated the volumes of loose sediment stored in the bottoms of the mainstem valleys of the investigated mountainous catchments prior to the flash flood and compared our results with the sediment volumes entrained by the debris flows. The latter volumes were estimated by post-event data collected by means of field surveys. The results were mutually consistent so supporting our approach. The approach is fit to the analysis of torrential catchments with geomorphological features comparable to those of our study basins, provided (i) the stream is confined by hard rock slopes not affected by channel lateral erosion, (ii) all stored sediment is eroded and transported downstream during the flood, and (iii) no landslide occurs along the hillslopes. For these types of ungauged catchments, our method can be applied extensively, even in the absence of field data and measurements, for flash flood hazard assessment and planning of mitigation strategies.

Assessing torrentiality in catchments of the tropical Andes: A morphometric approach

Torrential flows represent one of the most catastrophic natural hazards in the Andes, and Colombia is not an exception. We computed and analyzed ten morphometric indexes, classified by the catchment geometry and relief characteristics, for 149 torrential catchments across Colombian Andes three branches (Western, Central, and Eastern cordilleras). We divided the study area into six tectonic-stratigraphic regions, and employed statistical tests such as the Shapiro-Wilk and ANOVA One-Way Permutation tests to assess index distributions within each region. The ANOVA One-Way Permutation test results revealed significant differences in the distributions of form factor (p-value 0.048), mean gradient (p-value 0.009), relief ratio (p-value 0.003), Melton ratio (p-value 0.006), and hypsometric integral (p-value 0.000) indices among regions. Likewise, the pairwise comparison tests identified that the following regions are different between them for Melton ratio: Central Cordillera and Plateau Antioqueño (p-value 0.032), Garzón Massif and Plateau Antioqueño (p-value 0.015), Garzón Massif and Santander Massif (p-value 0.05), and Plateau Antioqueño and Western Cordillera (p-value 0.032); for relief ratio: Central Cordillera and Plateau Antioqueño (p-value 0.0219), Estern Cordillera and Plateau Antioqueño (p-value 0.0425), Garzón Massif and Plateau Antioqueño (p-value 0.0058), Plateau Antioqueño and Western Cordillera (p-value 0.053); for hypsometric integral: Central Cordillera and Western Cordillera (p-value 0.0004), Eastern Cordillera and Western Cordillera (p-value 0.0004), Garzón Massif and Western Cordillera (p-value 0.0004), Plateau Antioqueño and Western Cordillera (p-value 0.0161), and Santander Massif and Western Cordillera (p-value 0.002). Based on the findings we computed confidence intervals for the population median of the studied morphometric parameters according to the region employing a non-parametric bootstrapping method. The statistical analysis highlights the importance of the geological and geomorphological features in the morphometric indices analysis as part of the torrential flow susceptibility assessment.Applying our findings, we tested proposed thresholds in a case study of the Santander Massif (Eastern Cordillera of Colombia), focusing on the 2020 Piedecuesta Flow. In the study case, we demonstrated the relevance of identifying the feeder catchments where the morphometric indices would be calculated from, especially in areas where significant hydrologic changes occur. Volumetric concentration variations along the Piedecuesta Flow path highlight the need to prioritize the recognition of feeder catchments as an imperative part of the susceptibility assessment. Our approach enriches the knowledge base for identifying torrential-prone basins and has important implications for natural hazard management, aiming at mitigating the impact of torrential flows in the Andes.

Debris-flow activity in the Japanese Alps is controlled by extreme precipitation and ENSO – Evidence from multi-centennial tree-ring records

Japanese mountains are recurrently affected by mass movements and sediment-related disasters. Large efforts have therefore been undertaken to enhance understanding of the impacts and triggering of recent disasters, but ancient and continuous records to put recent events into perspective are still lacking. Here, we present a multi-centennial tree-ring-based debris-flow reconstruction in two size-contrasting torrential catchments of the Japanese Alps. We analyzed 197 affected trees growing on debris-flow fans and along torrential channels, which allowed us to identify 62 past events (38 and 24 in each catchment) from the late 19th century to today. The new chronology shows a non-stationary behavior of debris flows in the long-term, and an upward trend in debris flow activity over the last decades. Analysis of the 3-day rainfall legacy explains the occurrence of debris flows best, whereas rainfall thresholds show a positive trend linked to an increase in the occurrence of extreme precipitation over the last decades. Our results clearly support a link between debris flows with the passage of typhoons as well as La Niña events. The unprecedented length of the debris-flow record provided here opens new avenues for climate change and sediment-disaster research in Japan. Thus, our findings have important implications for the understanding of past, recent, and near-future debris-flow trajectories under climate change scenarios.

Torrential Hazards’ Mitigation Measures in a Typical Alpine Catchment in Slovenia

Different sediment-related disasters due to torrential hazards, such as flash floods, debris flows, and landslides, can occur in an Alpine torrential catchment. When protecting infrastructure and human lives, different structural and non-structural protection measures can be used to mitigate permanent and future risks. An overview of the mitigation measures constructed near the Krvavec ski resort in northwest Slovenia (Central Europe) is presented. In May 2018, an extreme debris flood occurred in this area, causing significant economic damage. After the May 2018 event, different field investigations (i.e., geological and topographic surveys) and modeling applications (e.g., hydrological modeling, debris flow) have been conducted with the purpose of preparing the required input data for the design of protection measures against such disasters in future—due to climate change, more disasters are expected to happen in this torrential watershed. The mitigation includes the restoration of local streams, the construction of a large slit check dam for sediment retention, the construction of several smaller check dams and the construction of 16 flexible net barriers with an estimated ~8000 m3 retention volume for controlling in-channel erosion in steep torrential streams. Additionally, in order to observe and monitor potential future extreme events, an extensive monitoring system has been established in the investigated area. This monitoring system will cover measurements of flexible net corrosion, the estimation of concrete abrasion at check dams, periodical geodetic surveys using small drones (UAV), hydro-meteorological measurements using rainfall gauges and water level sensors. The recent extreme floods of August 2023 also hit this part of Slovenia, and this combination of technical countermeasures withstood the event and prevented large amounts of coarse debris from being transported to the downstream section and devastating infrastructure, as was the case in May 2018 during a less extreme event. Therefore, such mitigation measures can also be used in other torrential catchments in the Alpine environment.

Statistical modeling of sediment supply in torrent catchments of the northern French Alps

Abstract. The ability to understand and predict coarse-sediment transport in torrent catchments is a key element for the protection against and prevention of the associated hazards. In this study, we collected data describing sediment supply at 99 torrential catchments in the northern French Alps. The sample covers a wide range of geomorphic activity: from torrents experiencing debris flows every few years to fully forested catchments exporting small bed load volumes every decade. These catchments have long records of past events and sediment supply to debris basins. The mean annual, the 10-year return period and the reference volume (i.e., the 100-year return level or the largest observed volume) of sediment supply were derived for the studied torrents. We examined the relationships between specific sediment supply volumes and many explanatory variables using linear regression and random forest approaches. Results showed that the ratio of sediment-contributing area (bare soil or rock) to catchment area is the most important predictor of the specific sediment production volumes (m3 km−2). Other variables such as the Melton index or the indices of sediment connectivity also have an influence. Several predictive models were developed in order to estimate the sediment supply in torrents that are not equipped with debris basins.

Investigating the interplay of the hydrometeorological and seasonal forest vegetation role in regulating the nitrate flushing in a small torrential catchment

Forest vegetation is a very topical research subject as part of nature-based solutions for flood protection, soil erosion, water quality improvement, etc. However, limited capabilities of such measures are rarely investigated. Therefore, in this paper, study on the interplay of the hydrometeorological and seasonal forest vegetation role in regulating the nitrate‑nitrogen (NO3-N) flushing from a forested, torrential catchment is presented. For the 43 identified rainfall events it was found that there are no statistically significant seasonal differences in NO3-N concentrations; however, during baseflow conditions such differences were noted. The rainfall events were described by 17 hydro-meteorological and vegetation variables to investigate similarities between the events from the NO3-N export point of view using clustering methods. Additionally, the relationship between explanatory and dependent variables, i.e. NO3-N concentration and export variables, was modelled. In the models, the first four principal components were used as explanatory variables after the reduction of the initial number of variables. It was found that phenological phases as indicators of the forest vegetation seasonal activity are generally not able to considerably influence the NO3-N concentration and flushing dynamics. The results indicate that during baseflow conditions, the influence of the forest vegetation in the torrential catchment becomes predominant. During rainfall events, the role of vegetation is blurred and might be generally considered as insignificant. The characteristics of rainfall events apparently exceed the limits of a forest's ability to control NO3-N flushing. Given the pronounced impact of rainfall intensity on NO3-N flushing, the challenge for the future will be to mitigate the potential negative effects of climate change related impacts, especially through measures able to reduce the intensity of rainfall-runoff formation. The seasonal role of vegetation in diminishing the intensity of nutrient flushing in natural torrential catchments might be relatively constrained and probably also overwhelmed by the hydrometeorological conditions.

Statistical Modelling of Sediment Supply in Torrent Catchments of the Northern French Alps

The ability to understand and predict sediment transport in torrent catchments is a key element for the protection and prevention against the associated hazards. In this study, we collected data describing sediment supply at 100 torrential catchments in the Northern French Alps. These catchments have long records of past events and sediment supply due to debris deposition basin management enabling estimation of sediment supply frequency. The mean annual, the 10-year return period and the reference volume (i.e. the 100-year return level or the largest observed volume) of sediment supply were derived for studied torrents. We examined the relationships between sediment supply volumes and several explanatory variables using multivariate statistical analyses. Several predictive models were developed in order to estimate the sediment supply in torrents that are not equipped with sedimentation structures.

Mapping and classification of heavy rainfall-induced hazards in the Petrich town area (SW Bulgaria)

The torrential catchments located on the northern slopes of Belasitsa Mountain (SW Bulgaria), in the area of Petrich town were affected by an intense rainfall event in December 2021 that triggered a cascade of hazard processes like floods, landslides, debris flows/floods that caused major damage to the road network, properties, and buildings, showing the urgent need to take hazard and risk assessment and mitigation measures. For assessment purposes, we perform field and UAV-based analysis, and mapping of the affected areas, the geomorphological changes after the flooding event, and the different types of identified mass-wasting processes. We also investigated the effects of early December 2021 heavy rain on a regional scale.

Monitoring the role of soil hydrologic conditions and rainfall for the triggering of torrential flows in the Rebaixader catchment (Central Pyrenees, Spain)

Torrential flows (debris flows and debris floods) are mainly triggered by precipitation and soil hydrological processes. Most early warning systems in torrential catchments are rainfall-based. However, this approach can result in frequent false positives, due to its pure black-box nature, in which soil water conditions are neglected. We aim to contribute to the understanding of the conditions required for triggering torrential flows by considering also in situ measurements of soil water content. Herein, monitoring data of 12 years of rainfall and torrential flow occurrence (2009–2020) and 8 years of soil hydrologic conditions (2013–2020) in the Rebaixader catchment (Central Pyrenees, Spain) are analyzed. The dataset includes more than 1000 rainfall events and 37 torrential flows. First, rainfall thresholds using maximum rainfall intensity (Imax) and mean intensity (Imean) are defined. For the 2013–2020 dataset, which includes 15 torrential events, the Imean threshold predicted 2 false negatives and 73 false positives (positive predictive value, PPV, of 15.1%) and the best Imax threshold predicted also 2 false negatives but only 11 false positives (PPV of 54.2%). However, our observations confirmed quantitatively that the lower is the soil moisture the higher is the rainfall intensity to trigger torrential flows. Then, we combined Imax and volumetric water content at 15 and 30 cm depth to define a hydro-meteorological threshold. This latter threshold reduced false negatives to 1 and false positives to 8 and increased the PPV to 63.6%. These results confirm that soil hydrological conditions are key factors for torrential flow triggering and may improve early-warning predictions.

Monitoring of Rainfall and Soil Moisture at the Rebaixader Catchment (Central Pyrenees)

ABSTRACT The instrumental monitoring of torrential catchments is a fundamental research task that provides necessary information to improve our understanding of the mechanisms of debris flows. While most monitoring sites include meteorological sensors and analyze the critical rainfall conditions, very few contain soil moisture measurements. In our monitoring site, the Rebaixader catchment, 11 debris flows and 24 debris floods were detected during the last 9 years. Herein, the initiation mechanisms of these torrential flows were analyzed, focusing on the critical rainfall conditions and the soil water dynamics. Comparing the temporal distribution of both rainfall episodes and torrential flows, the Kernel density plots showed maximum values for rainfalls at the beginning of June, while the peak for torrential flows is on July 20. Thus, the antecedent rainfall, and especially the soil moisture conditions, may influence the triggering of torrential flows. In a second step, a new updated rainfall threshold was proposed that included total rainfall duration and mean intensity. The analysis of soil moisture data was more complicated, and no clear trends were observed in the data set. Therefore, additional data have to be recorded in order to quantitatively analyze the role of soil moisture on the triggering of torrential flows and for the definition of thresholds. Some preliminary results show that the soil moisture at the beginning of a rainfall event affects the maximum increase of soil moisture, while a slight trend was visible comparing the initial soil moisture with the necessary rainfall amount to trigger a torrential flow.

Morphometrical analysis of torrential flows-prone catchments in tropical and mountainous terrain of the Colombian Andes by machine learning techniques

Assessment of susceptibility to torrential flows at the catchment scale is a necessary step for prioritising zones that can be identified by hazard and risk maps. In the Andean region of Colombia, where these phenomena are common and highly destructive, few studies have been carried out on the subject. Considering that the effects of torrential flows will likely increase in the future due to extreme weather events and urbanisation of low prone areas, effective land management plans should include the assessment of susceptibility to this type of event. In this study, machine learning techniques were applied with morphometric parameters of mountainous catchments in the northern Colombian Andes to analyse torrential susceptibility in tropical environments. Several feature selection methods were implemented using statistics to evaluate the relationship between morphometric parameters and torrential susceptibility. The parameters that showed the strongest relationship with torrential catchments were selected for application of machine learning techniques, along with evaluation of performance and prediction ability. The analysis was carried out on morphometric parameters regarding the catchment’s drainage network, basin geometry, drainage texture, and relief characteristics. According to the results, the catchments characteristics related to relief and drainage texture are the most important for the study of torrential susceptibility. Within these groups of predictor parameters, relief ratio and constant of channel maintenance are the highest ranked variables. Although the Melton index shows a good prediction capacity, thresholds proposed in the literature are not valid in the Colombian Andes.

Regional-scale inventory of periglacial moving landforms connected to the torrential network system

Abstract. When connected to torrential channels, periglacial moving landforms (including rock glaciers, push moraines and high-altitude landsliding masses) may constitute important active sediment sources for gravitational and torrential transfer processes such as debris flows. However, still very little is known about the location and the number of such types of sedimentary connection in given regions, as well as about the typical sediment transfer rates that can be expected. Therefore, this contribution aims at (i) describing a new methodology developed to identify and characterize moving landforms connected to the torrential network system at a regional scale and (ii) presenting the results yielded from the application of this method in a 2000 km2 region in the southwestern Swiss Alps. The developed approach is based on the analysis of simple data such as a high-resolution digital elevation model (DEM), time series of aerial images and a slope movement inventory. The approach allowed both the fast identification of moving landforms connected to torrential channels and the estimation of annual sediment transfer rates for these inventoried landforms. In the study region, results showed that such types of sedimentary connection appeared to be rather rare. Results also showed that most connected moving landforms were characterized by relatively low sediment transfer rates (<500 m3 yr−1) but several sites were identified as transferring large amounts of sediment into the torrents (>1000 m3 yr−1). As sediment transfer rates depend on the kinematical behavior of the landforms, values calculated may change in regard to the evolution of the surface velocities, which are currently generally increasing in the European Alps. When connected to torrents, periglacial moving landforms may thus represent substantial active sources of sediments for the development of debris flows and should be considered in the management of torrential catchments.

Conceptual Framework for Sediment Management in Torrents

Torrential catchments are complex systems showing a huge diversity of sediment and flow regime reactions. Therefore, it seems reasonable to decrease the complexity by introducing a conceptual framework to obtain a general idea of the torrential system’s behavior. The aim of the manuscript is to provide a conceptual framework and a stepwise procedure to develop a reproducible sediment management concept. This involves starting with a definition of torrential process types. These processes are then routed through the drainage system, which is divided into homogeneous reaches. Each reach transfers a flow process from the upstream node to the adjacent downstream reach, based on driving geomorphic factors of sediment mobilization and deposition. Additionally, a sediment budget, derived from cumulative erosion and deposition volumes, is calculated. The estimated flow process together with the sediment budget enables the selection of a sediment management strategy. The implementation of the strategy is based upon the selection of functions, which define the desired modification with regard to the torrential system. Ultimately, the sum of each function’s performance must lead to the fulfilment of the overall protection objective.

Predicting Flood Hazard Indices in Torrential or Flashy River Basins and Catchments

Flood hazard maps are one of the main components of any flood risk management strategy. It is predicted that the degree of flood risk is going to significantly increase in the future due to climatic and environmental changes, and hence it is increasingly important that state-of-the-art methods are implemented for assessing human stability in floodwaters. Therefore, this paper focuses on proposing more accurate and detailed guidelines for predicting flood hazard indices in small and steep river basins or catchments, prone to the occurrence of flash flooding. The results obtained in this study indicate that for river basins with an average bed gradient greater than 1% (i.e. torrential or flashy river basins or catchments), then the flood hazard indices should be predicted using criteria which are based on the physical interpretation of the processes that affect the human stability in floodwaters, i.e. mechanics based and experimentally calibrated flood hazard assessment methods.

An historical database on alluvial events in torrential catchments of Campania region (southern Italy)

Alluvial events in Italy are a constant and continuous problem due to the peculiar orographic and climatic setting of the territory. Even though less frequent than in the Alps, debris-flows and floods greatly contributed also along the Southern Apennines to build alluvial fans, a widespread geomorphic unit located at the catchment mouth. These areas have been frequently chosen as sites for the urban development, as they were considered safer than the valleys, for the lower probability of being affected by floods. Due to the orographic position, the heavy rainfall events characterizing the study area can lead to flash floods with large debris loads. Such phenomena may be extremely risky for the population, and cause heavy damages. Starting from the consideration that past events can provide information on future events, we compiled a database on alluvial events in torrential catchments of Campania region, a highly prone area to such phenomena. By consulting different types of information sources we collected about 500 events. The database is being constantly updated, and contains temporal and spatial information for each event, since availability of these data is considered a mandatory step in order to perform a correct hazard and risk analysis.

Geomorphic response detection and quantification in a steep forested torrent

Extreme events such as flash floods and debris flows are frequent phenomena that occur in steep torrential catchments; these kinds of events can cause notable geomorphic changes. Repeated terrestrial laser scanning (TLS) surveys were performed in a steep forested catchment of the Kuzlovec torrent (drainage area ~0.7km2) in central Slovenia, where a ~200-m long section of the torrent was scanned in 2013, 2014, and 2015. The main aim of this study was to perform the geomorphic response detection in the torrent due to hydro-meteorological events of different magnitudes. After applying several pre-processing steps, digital terrain models (DTMs) with a cell resolution of 5cm were produced. The geomorphic change detection was performed using the DTM of Difference approach (DoD). Several above-average flow events occurred in the period from 2013 to 2015 (some of them can be regarded as floods). The 2014 August extreme flash flood that was initiated by the rainfall event with a return period exceeding 100years, where maximum 1-minute rainfall intensities were up to 288mm/h, led to erosion rates of an order of magnitude higher than average annual erosion rates. Moreover, the analysis of the geomorphic changes shows that the August 2014 flash flood caused intense sediment transport processes that resulted in the changes at the location of the main stream channel thalweg and reduced channel roughness. The unit stream power for the scanned section of the torrent was assessed to be approximately 500W/m2 during this extreme event. This is above the thresholds that were suggested to differentiate between the situations where significant geomorphic changes can occur and the situations where geomorphic changes are not notable.

Process type identification in torrential catchments in the eastern Alps

Torrential hazards are omnipresent in the alpine regions, as it frequently causes damage to infrastructures. In some cases, even people's lives are endangered. The classification of these processes takes place according to factors like sediment concentration and flow behaviour and ranges from fluvial process types, including water floods and fluvial sediment transport processes, to fluvial mass movements such as debris flows. Following the hypothesis of this study, a context exists between basic geomorphological disposition parameters and potential dominant flow process types in a steep headwater catchment.Thus, examined catchments were selected based on a historical event documentation of torrential events in the Austrian Alps. In total, 84 catchments could be analysed, and 11 different morphometric parameters were considered. To predict the dominant torrential process type within a catchment, a naive Bayes classifier, a decision tree model, and a multinomial regression model was trained against the compiled geomorphological disposition parameters. All models as well as their combination were compared. Based on bootstrapping and complexity, we present the classification model with the lowest prediction error for our data that might help to identify the most likely torrential process within a considered catchment.

Time of concentration in torrential catchments in Serbia

Maximal discharge Qmax,p is the basic input data for the design of longitudinal and transversal objects in torrential beds. Calculation of maximal discharge Qmax, on unstudied catchments is performed by the following methods: combined procedure (the theory of synthetic unit hydrograph and SCS methodology for the separation of effective rainfall Pe from total rainfall Pb), rational theory and theory of maximal runoff intensity. Rational theory is a suitable method with representative time of concentration Tc. The time of concentration was determined on the basis of investigation on 93 control profiles, south from the Sava and the Danube. Time of concentration and dominant physical-geography characteristics of the catchments were the basis for deriving the equations with regional significance [Tc=f(A), Tc=f(L), Tc=f(L, Lc, Iu), Tc=f(L, Iu), Tc=f(L, Isr), Tc=f(L, Iu, Isr)]. The relation between time of concentration Tc and lag time tp was determined [Tc=f(tp)].

Efficacité des barrières végétales pour la lutte contre l'envasement des retenues de barrages

Erosion in marly torrential catchments in the French Southern Alps causes silting of hydroelectric dam impoundments established along the Durance river. One solution to assist in solving this problem consists of retaining the sediments within the catchments. To do this, vegetation and bioengineering works can be used as efficient and sustainable means to avoid sediment erosion and transport to dam impoundments. Effectively, the cover provided by vegetation and bioengineering works acts as a barrier to soil erosion. The studies showed that vegetation barriers can stop sediment yield at the exit of gullies only partially covered with vegetation, if the vegetation is located downstream of the gully. This paper presents the current and improved knowledge about the influence of vegetation barriers on erosion, and explains how to use this knowledge to propose optimal strategies for reduction of the sediment load in rivers and hydroelectric dam impoundments, by the use of ecological engineering techniques.

Lag time on torrential catchments in Serbia

Maximal discharge Qmax(p) is the basic input data for designing longitudinal and transversal objects in torrential beds. Calculation of maximal discharge Qmax, on unstudied catchments, is carrying out by usage the theory of synthetic unit hidrograph and SCS methodology for separation effective rain Pe by total rainfall Pb. One of the basic time characteristics of torrential floods was analyzed: lag time tp. Lag time tp was determined on the basis of processing of hydrographs and deriving representative unit hydrographs, on 93 control profiles. Dominant physical-geography characteristics were determined: magnitude of the catchment area A, length of the catchment area along the main stream L, the distance from the centroid of the catchment area to the outlet profile Lc, average slope of the river bed Iu, average slope of terrain on the catchment area Isr. Lag time and dominant physical-geography characteristics were the basis for deriving some equations with regional significance (tp=f(L, Lc, Iu); tp=f(L, Lc, Iu, Isr)). The relation between lag time tp and time of concentration Tc was determined (tp=f(Tc)).