Rainstorm Magnitude and Debris Flows in Pyroclastic Deposits Covering Steep Slopes of Karst Reliefs in San Martino Valle Caudina (Campania, Southern Italy)

Guido Leone,Libera Esposito,Pasquale Clemente,Francesco Fiorillo

doi:10.3390/w13162274

Abstract

Debris flows that have occurred in the area of San Martino Valle Caudina (Campania, Southern Italy) are described by geomorphological and hydrological analyses, focusing on the recent event of December 2019. This area can be considered a key example for studying debris-flow phenomena involving the pyroclastic mantle that covers the karstified bedrock along steep slopes. A hydrological analysis of the time series of the maximum annual rainfall, of durations of 1, 3, 6, 12 and 24 h, was carried out based on a new approach to assess rainstorm magnitude. It was quantified by measuring the deviation of the rainfall intensity from the normal conditions, within a specified time period. As the time series of annual maxima are typically skewed, a preliminary transformation is needed to normalize the distribution; to obtain the Z-value of the standard normal distribution, with mean µ = 0 and standard deviation σ = 1, different probability distribution functions were fitted to the actual data. A specific boxplot was used, with box width Z = ±1 and whiskers length Z = ±2. The deviations from these values provide the performance of the distribution fits. For the normalized time series, the rates shown by the trends and relative significance were investigated for the available time series of 11 rain gauges covering the Western–Central Campania region. The most critical condition for the debris-flow initiation appears to occur when a severe or extreme rainfall has a duration ≥ 12 h. The trend analysis did not detect statistically significant increases in the intensity of the rainfall of duration ≥ 6 h.

Highlights

In recent years, great attention has been dedicated to the study of changes in the frequency of intense rainstorms and the increase in the severity of rainfall events
The interest in these topics has increased, especially for those areas where rainfall represents the main driver of catastrophic events, such as floods and debris flows
This case study focused on a sector of the Partenio Mts., where rainfall represents a main source of risk for urban settlements, as it was the driver of the catastrophic floods and the debris flows which have hit the area many times in the past

Summary

Introduction

Great attention has been dedicated to the study of changes in the frequency of intense rainstorms and the increase in the severity of rainfall events The interest in these topics has increased, especially for those areas where rainfall represents the main driver of catastrophic events, such as floods and debris flows. The extraordinary storm of 26 October 1954 that occurred along the Amalfi Coast (Lattari Mts. in Figure 1), falls into the first category, with 459 mm in 6 h [11] and after a long dry period [4] It caused more than 300 deaths and severe damage to infrastructures, buildings and local agriculture.

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Results

Conclusion