Analysis of Soil Erosion Induced by Heavy Rainfall: a Case Study from the NE Abruzzo Hills Area in Central Italy

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Article Analysis of Soil Erosion Induced by Heavy Rainfall: A Case Study from the NE Abruzzo Hills Area in Central Italy

Tommaso Piacentini 1,2,* , Alberto Galli 3, Vincenzo Marsala 3 and Enrico Miccadei 1,2

1 Department of Engineering and Geology, Università degli Studi “G. d’Annunzio” Chieti-Pescara, Laboratory of Tectonic Geomorphology and GIS, Via dei Vestini 31-66100 Chieti Scalo (CH), Italy; [email protected] 2 Istituto Nazionale di Geoﬁsica e Vulcanologia, Sezione Roma 1, Via di Vigna Murata 605, 00143 Rome, Italy 3 SGI Studio Galli Ingeneria S.r.l., Via della Provvidenza, 15, 35030 Sarmeola di Rubano (PD), Italy; [email protected] (A.G.); [email protected] (V.M.) * Correspondence: [email protected]; Tel.: +39-0871-355-6422

Received: 21 July 2018; Accepted: 11 September 2018; Published: 22 September 2018

Abstract: Soil erosion induced by heavy rainfall deeply affects landscape changes and human activities. It depends on rainfall distribution (e.g., intensity, duration, cumulative per event) and is controlled by the interactions between lithology, orography, hydrography, land use, and vegetation. The Abruzzo piedmont coastal hilly area has been affected by several heavy rainfall events in the last decades. In this work, we investigated three ~1-day heavy rainfall (>35 mm/h and 100–220 mm/day) events in 2007, 2011, and 2012 that occurred in the clayey hilly coastal NE Abruzzo area, analyzing cumulative rainfall, intensity, and duration while mapping triggered geomorphological effects (soil erosion and accumulation) and evaluating average erosion. The analysis provides contributions to a soil erosion assessment of clayey landscapes that characterizes the Adriatic hilly area, with an estimation of rainfall-triggering thresholds for heavy soil erosion and a comparison of erosion in single events with rates known in the Mediterranean area. The triggering threshold for heavy soil erosion shows an expected value of ~100–110 mm. The estimated average soil erosion is from moderate to high (0.08–3.08 cm in ~1-day heavy rainfall events) and shows a good correlation with cumulative rainfall and a poor correlation with peak rainfall intensity. This work outlines the strong impact of soil erosion on the landscape changes in the Abruzzo and Adriatic hilly areas.

Keywords: heavy rainfall; soil erosion; geomorphology; Mediterranean environment; clay hills; Abruzzo; Central Italy

1. Introduction Rainfall and heavy rainfall events are among the most important factors inducing soil erosion and landslides, particularly in Mediterranean countries [1–4]. Their impact on slope instabilities is mainly related to the interaction between lithology, orography, hydrography, land use, and vegetation [5–10]. The relationship between rainfall intensity/duration and the geomorphological effects on the landscape, especially in terms of heavy soil erosion, is an open issue [1–3,5]. This is particularly true in the framework of changing climate/land use and increasing ﬂash ﬂooding induced by extremely heavy rainfall events [8,11–13]. These events have a strong impact on landscape changes and deeply affect human activities, inducing large losses and even casualties [14–17]. For these reasons, they are studied with different approaches and data in Mediterranean and arid environments (e.g., rain gauge data, satellite rainfall data, time series statistical analysis, remote sensing, climate modeling, empirical

Water 2018, 10, 1314; doi:10.3390/w10101314 www.mdpi.com/journal/water Water 2018, 10, 1314 2 of 29 connections between rainfall and geomorphological effects, morphometry, automated reconstructions, etc.) [18–20]. The Mediterranean areas, and particularly the hilly areas all around the Mediterranean coasts (e.g., Apennines hills, Sicily, Liguria, Greece, and also islands, Southern Spain, occasionally the Maghreb areas), are characterized by moderate to low annual precipitation, but occasionally are affected by heavy rainfall events (up to 400–500 mm/day) [21–27]. Due to these events, the landscape might suffer severe changes in terms of landslides, river modifications, coastal variations, and soil erosion, etc. Clayey hills, such as those characterizing the eastern piedmont of the Apennines and the Adriatic coastal hills of southern, central, and northern Italy [28] are largely used for high-quality plantations (e.g., olive groves, vineyards). This landscape suffers severe soil erosion from sheet-rill erosion, gullies, channel incisions, mud flows, and flooding (among many other issues [14,29–31]). Moreover, the coastal areas are devoted to touristic villages and facilities and have suffered strong land-use change and urbanization in recent years, inducing heavy soil consumption [32]. Over the last decades, the Abruzzo region has been affected by several heavy rainfall events (e.g., January 2003, October 2007, March 2011, September 2012, December 2013, February–March 2015, January 2017 [7,25,33,34]). These events have triggered different types of geomorphological instability (i.e., landslides, soil erosion, and flooding), with distribution and types varying from event to event, and in many cases led to an official natural disaster declaration at the regional government level [35]. Rainfall-induced soil erosion is investigated either by means of indirect estimations (e.g. Tu, RUSLE, USPED [36,37]), or with direct measurements (e.g., field plots [38], pins [39], detailed digital elevation models, DEM, morphometry and photogrammetry [40], field survey [39], reservoir sedimentation [36,37], and morphotectonic analysis [41]). The former are more suitable at large basin scales and for average soil erosion assessment [37,42], while the latter are more suitable at the local scale and in small catchment investigations as well as for single event investigations [42] (except for reservoir sedimentation analysis). In this study, we had the opportunity to perform a direct survey with a geomorphological approach (field mapping combined with detailed aerial photos analysis) of the effects of heavy rainfall events on the clayey and sandy hilly landscape of northern Abruzzo (Figure1). This area is largely dedicated to agricultural activities and high-quality cultivation (e.g., olive groves, vineyards), but is also affected by urbanization-related soil consumption [32]. We investigated three heavy rainfall events that occurred from 2007 to 2012 in terms of rainfall intensity-duration, surface geomorphological effects, and soil erosion, and we compared the very short-term soil erosion effects revealed by this work with wider and more long-term soil erosion in Mediterranean clayey landscapes [39–42]. The investigated events had rainfall from 100 mm to >200 mm over ~1 day and affected, to different extents, northern Abruzzo (Figure2) on several dates: 6–7 October 2007 (in a small part of the hilly-coastal Teramo area); 1–2 March 2011 (in the hilly-coastal Teramo and Pescara area); and 5–6 and 13–14 September 2012 (in the hilly coastal Teramo area). For each of them, the rainfall amount, intensity, and duration were investigated by means of rain gauge data (18 pluviometric stations; Figure2). The survey of geomorphological effects induced by the events was focused on some small coastal catchment with investigation in the field and by means of aerial photos and technical reports and geographic information system (GIS) mapping. The data processing enabled the analysis and comparison of geomorphological effect and soil erosion estimations with hourly rainfall intensity, event cumulative rainfall, daily rainfall, and rainfall before the event. The aims of the work are as follows: (1) to outline the correlation between rainfall parameters and geomorphological effects in terms of soil erosion in NE Abruzzo; (2) to assess the impact of soil erosion on the clayey landscape (which is largely characterized by high-quality plantations); and (3) to contribute to the empirical definition of rainfall threshold for the triggering of heavy soil erosion through local inventories (for the debate on this issue see also Segoni et al. [43] and references therein). These types of investigations may contribute to applied studies for the stabilization and management of slopes and minor or major drainage basins, and for general land management. Finally, this work is expected to contribute to quantitatively investigate and mapping land sensitivity to soil Water 2018, 10, 1314 3 of 29

Water 2018, 10, x FOR PEER REVIEW 3 of 28 erosion, and for deﬁning future scenarios of the impact of soil erosion on areas increasingly devoted to high-qualityquality agricultural agricultural deployment deployment (e.g., (e.g.,vineyards, vineyards, olive olive groves groves),), which which sustainable sustainable land landplanning planning and andmanagemen managementt should should be based be based on [14 on,29 [14,31,29]., 31 ].

2. Study Area and Rainfall Events 2. Study Area and Rainfall Events 2.1. Regional Setting 2.1. Regional Setting The NE Abruzzo region is located in the central eastern part of the Italian peninsula along the The NE Abruzzo region is located in the central eastern part of the Italian peninsula along the piedmont area of the Central Apennines and the hilly coastal area (Figure1). It includes the lower part piedmont area of the Central Apennines and the hilly coastal area (Figure 1). It includes the lower of the main SW–NE to W–E ﬂuvial valleys (the Tronto, Vibrata, Salinello, Vomano, and Saline rivers), part of the main SW–NE to W–E fluvial valleys (the Tronto, Vibrata, Salinello, Vomano, and Saline and the small tributary catchment of the main rivers and those incising the coastal slopes. rivers), and the small tributary catchment of the main rivers and those incising the coastal slopes. The elevation ranges from 0 to ~600 m along the main drainage divides between the main rivers, The elevation ranges from 0 to ~600 m along the main drainage divides between the main rivers, and the terrain gradient (derived from a 10-m-cell digital elevation model, DEM) ranges from 0◦ along and the terrain gradient (derived from a 10-m-cell digital elevation model, DEM) ranges from 0° along the river and coastal plains to >40◦, with local vertical scarps along the slopes. the river and coastal plains to >40°, with local vertical scarps along the slopes.

. Figure 1. Location map of (a) the Abruzzo region and (b) of the study area in the main physiographic Figure 1. Location map of (a) the Abruzzo region and (b) of the study area in the main physiographic domains of the Abruzzo region. domains of the Abruzzo region. There are four lithologic bedrock complex, or groups of rock units, in the area, each comprising differentThere marine are four sedimentary lithologic bedrock rock types complex, varying or groups in strength of rock from units, hard in the to a weakrea, each and comprising soft rocks: (1)different sandstone marine and sedimentary claystone (in rock the innertypes hillyvarying areas); in strength (2) clay (infrom the hard lower to slopes weak ofand the soft coastal rocks hilly: (1) areas);sandstone (3) sand and andclaystone weaksandstone (in the inner (in the hilly upper areas); slopes (2) ofclay the (in hilly the coastal lower areas); slopes and of (4)the conglomerate coastal hilly andareas); consistent (3) sand graveland weak (at thesandston top ofe the(in the main upper coastal slopes hills). of the In thehilly recent coastal official areas); geological and (4) conglom- map of Italyerate [and44,45 consistent], these complexes gravel (at are the pertaining top of the to main Neogene coastal turbiditic hills). In arenaceous the recent and official pelitic geological rocks, and map to Plio-Pleistoceneof Italy [44,45], marinethese complexes clay–sand–sandstone–conglomerate are pertaining to Neogene rocks. turbiditic The bedrock arenaceous units and are coveredpelitic rocks, with Middleand to Plio Pleistocene–Holocene-Pleistocene marine clasticclay–sand continental–sandstone deposits–conglomerate (slope, landslides, rocks. The fluvial bedrock deposits) units are at cov- the basesered with of the Middle slopes Pleistocene and in the– mainHolocene valleys clastic (Figure continental2). The hillslopesdeposits (slope and coastal, landslides, slopes fluvial are mantled depos- byits)clay- at the and bases sand-rich of the slopes eluvial and and in colluvialthe main cover,valleys for (Figure which 2 thickness). The hillslopes ranges and from coastal < 1 m slopes to >10 are m dependingmantled by on clay morphology- and sand- andrich lithologyeluvial and (thicker colluvial in clay cover bedrock, for which and within thickness minor ranges valleys). from Finally, < 1 m theto >10 superficial m depending soil cover on morphology reflects the lithologicaland lithology types, (thicker with in thickness clay bedrock ranging and from within a few minor centimeters valleys). toFinally, >1 m. the The superficial piedmont soil and cover coastal reflect hillys the areas, lithological as well as types, theriver with andthickness coastal ranging plains, f arerom mostly a few characterizedcentimeters to by>1 luvisoils,m. The piedmont vertisoils, and and coastal cambisoils hilly area (Figures, as3 well[ 46 ])as developed the river and on clayeycoastal andplains sandy, are parentmostly materialscharacterized [47], by which luvisoils, are typical vertisoils of terrains, and cambisoils affected by (Figure high erosion3 [46]) developed rates [48]. Theon clayey structural and settingsandy parentis defined materials in the inner [47], part which by arethrust typical reliefs, of which terrains are affected affected by by highregional erosion NNW–SSE rates [ Pliocene48]. The thrustsstructural and setting minor is high-angle defined in normalthe inner faults part by and, thrust in the reliefs outer, which part, byare aaffected large slightly by regional NE-dipping NNW– homoclineSSE Pliocene [44 thrusts,45,49– 53and]. minor high-angle normal faults and, in the outer part, by a large slightly NE-dippingThe piedmont homocline coastal [44, area45,49 features–53]. a hilly landscape carved by cataclinal valleys (SW–NE) on arenaceous–peliticThe piedmont thrustedcoastal area and features faulted successionsa hilly landscape and on carved a gently by NE-dippingcataclinal valleys homocline (SW– onNE) clay, on arenaceous–pelitic thrusted and faulted successions and on a gently NE-dipping homocline on clay, sand, and conglomerate deposits. This landscape is incised by the main river valleys (featuring up to 4-km-wide fluvial plains) and by minor catchments flowing toward the main valleys and the coastal Water 2018, 10, 1314 4 of 29 sand, and conglomerate deposits. This landscape is incised by the main river valleys (featuring up to 4-km-wide fluvial plains) and by minor catchments flowing toward the main valleys and the coastal plainWater (featuring 2018, 10, radial,x FOR PEER trellis, REVIEW and angular drainage patterns [49]). The coastal area is characterized4 of 28 by wide coastal slopes and a coastal plain up to >1 km wide [49,54]. The geomorphological processes are mainlyplain fluvial, (featuring gravity-induced, radial, trellis, and and angular mass-wasting. drainage Thesepatterns processes [49]). The arecoastal frequently area is characterize activated byd the heavyby rainfall wide coastal events slopes that affectand a thecoastal region. plain Fluvial up to >1 processes km wide [ affect49,54]. the The main geomorphological rivers, alternating processes between channelare incisionsmainly fluvial, and flooding.gravity-induced The slope, and processesmass-wasting due. toThese running processes water are mostly frequently affect activated the clayey by and arenaceous–peliticthe heavy rainfall hills events of piedmont that affect and the theregion. coastal Fluvial areas, processes generating affect badlands the main rivers, and minor alternating landforms between channel incisions and flooding. The slope processes due to running water mostly affect the such as rills, gullies, and mudflows [41]. Mass-wasting processes have induced the formation of a clayey and arenaceous–pelitic hills of piedmont and the coastal areas, generating badlands and minor hugelandforms number ofsuch landslides as rills, gullies and mass, and movements,mudflows [41] mostly. Mass-wasting affecting processes the hilly have piedmont induced areathe for- and the chainmation area and, of a locally,huge number the coastal of landslides area [54 and–57 ].mass Moreover, movement thes, clayey mostly hill affecting areas the are hilly largely piedmont affected by both short-termarea and the and chain long-term area and heavy, locally soil, the erosion coastal processes area [54– [5758].]. Moreover The long-term, the clayey average hill erosion areas are rate is documentedlargely affected to be ~0.1–14 by both mm/year, short-termand and thelong short-term-term heavy erosion soil erosion rate isprocesses 1–90 mm/year [58]. The in long this-term area and in theaverage central erosion Italy clayey rate is hillsdocumented [29,39–42 to]. be ~0.1–14 mm/year, and the short-term erosion rate is 1–90 Themm/y hillyear in piedmont this area and coastal in the central area is Italy characterized clayey hills [ by29,39 a– maritime42]. Mediterranean climate [59]. The averageThe annual hilly piedmont precipitation coastal is area 600–800 is characterized mm/y, with by a occasional maritime Medite heavyrranean rainfall climate (>100 mm/day[59]. The and 30–40average mm/h) annual [33,34 precipitation,60–62]. In the is 600 last–800 decades, mm/y, thiswith area occasional was affected heavy rainfall by flash (>100 flood mm/d events and induced 30–40 by mm/h) [33,34,60–62]. In the last decades, this area was affected by flash flood events induced by heavy heavy rainfall ranging from 60 to 80 mm in a few hours to >200 mm in one day (e.g., January 2003, rainfall ranging from 60 to 80 mm in a few hours to >200 mm in one day (e.g., January 2003, October October2007, 2007, March March 2011, 2011,September September 2012, Dec 2012,ember December 2013, Feb 2013,ruary– February–MarchMarch 2015, January 2015, 2017 January). 2017).

FigureFigure 2. Geologic 2. Geologic scheme scheme of of the the Abruzzo Abruzzo regionregion (modiﬁed(modified from from [44 [44,45,45,63,63]). ]).The The red redboxes boxes indicate indicate locationslocation of thes of studythe study areas areas (grid (grid in in UTM UTM WGS84 WGS84 coordinates).coordinates). Water 2018, 10, 1314 5 of 29 Water 2018, 10, x FOR PEER REVIEW 5 of 28

In terms of of land land use, use, the the hilly hilly coastal coastal area area of ofNE NE Abruzzo Abruzzo is characterized is characterized by arable by arable lands lands,, and andthe number the number of high of- high-qualityquality plantations plantations (mainly (mainly olive groves olive groves and vineyards and vineyards)) has increas hased increased in the last in thedecades; last decades; minor urban minor areas urban and areas sparsely and sparsely urbanized urbanized rural areas rural are areas also are present. also present. The fluvial The ﬂuvialplains plainsshow alternating show alternating cropland cropland and industrial and industrial commercial commercial areas. Finally, areas. Finally, the coastal the coastal plain is plain mostly is mostly seam- seamlesslylessly urbanized urbanized with withvillage villages,s, tourist tourist facilities, facilities, roads, roads, railroad, railroad, ports, ports, etc. etc.

Figure 3.3. Soil scheme of the study area (modiﬁed(modified from Soil Map of Italy [4646]).]).

2.2. Rainfall Events and Affected Areas The investigated investigated heavy heavy rainfall rainfall events events occurred occurred on 6 on–7 October 6–7 October 2007, 2007,1–2 March 1–2 March2011, and 2011, 5– and6/13– 5–6/13–1414 September September 2012, and 2012, affected and affectedthe Tortoreto the Tortoreto area, the area, Pineto the and Pineto Salinello and Salinello River area, River and area, the andPineto the area, Pineto respectively area, respectively (Figure (Figure2). 2). 2.2.1. Rainfall Event of 6–7 October 2007 2.2.1. Rainfall Event of 6–7 October 2007 The 2007 heavy rainfall event affected, for a short time (14–16 h), a local area in the northern The 2007 heavy rainfall event affected, for a short time (14–16 h), a local area in the northern Abruzzo (hilly and coastal Tortoreto area; Figure2), including tributary catchments between the Abruzzo (hilly and coastal Tortoreto area; Figure 2), including tributary catchments between the Sa- Salinello and Vibrata rivers and coastal catchment (total area of 43.7 km2). River and coastal plains linello and Vibrata rivers and coastal catchment (total area of 43.7 km2). River and coastal plains (slope (slope gradient > 5◦) cover 35% of the area, while 65% is hills (slope gradient 5–30◦, with local gradient > 5°) cover 35% of the area, while 65% is hills (slope gradient 5–30°, with local scarps). The scarps). The event occurred after two dry months and affected a very poorly vegetated landscape. event occurred after two dry months and affected a very poorly vegetated landscape. The agricultural The agricultural areas (arable land, vineyards, and olive groves; Figure4a) were largely plowed on areas (arable land, vineyards, and olive groves; Figure 4a) were largely plowed on erodible clay bed- erodible clay bedrock with sands and conglomerates on the top of the hill, both largely covered with rock with sands and conglomerates on the top of the hill, both largely covered with clayey eluvial clayey eluvial and colluvial cover and landslides (Figure4b). and colluvial cover and landslides (Figure 4b). Water 2018, 10, 1314 6 of 29 Water 2018, 10, x FOR PEER REVIEW 6 of 28

FigureFigure 4. 4Tortoreto. Tortoreto area: area: (a ()a) land land use use map map (modiﬁed(modified fromfrom l landand u usese map map of of the the Abruzzo Abruzzo region region [64 [64]) ]) andand (b )(b lithologic) lithologic scheme scheme (grid (grid in in UTM UTM WGS84 WGS84 coordinates). coordinates). Water 2018, 10, 1314 7 of 29 Water 2018, 10, x FOR PEER REVIEW 7 of 28

2.2.2.2.2.2. Rainfall EventEvent of 1–21–2 MarchMarch 20112011 TheThe 20112011 heavyheavy rainfall event affected the entire northeastern Abruzzo piedmont hillyhilly area (hilly andand coastalcoastal Teramo areaarea betweenbetween thethe Vomano, Tordino,Tordino, Salinello,Salinello, andand VibrataVibrata rivers;rivers; FigureFigure2 2)) for for a a moderatelymoderately short timetime (22–26(22–26 h),h), afterafter ~10~10 days of moderate rainfall (total 50 mm). After this event, twotwo areasareas werewere investigatedinvestigated inin detaildetail becausebecause they suffered thethe heaviest rainfall and induced effects: thethe SalinelloSalinello RiverRiver valleyvalley andand thethe Pineto-AtriPineto-Atri hillyhilly areaarea (Figure(Figure2 ).2). The The Salinello Salinello area area includes includes the the lowerlower partpart ofof thethe Salinello River valley and the surrounding hillslopes (including part of the Tortoreto hillyhilly areaarea affectedaffected byby thethe 20072007 event).event). Over 50% of the area pertains to the river and the coastal plains; ◦ thethe remainingremaining areaarea incorporatesincorporates thethe hillyhilly reliefrelief up up to to ~300 ~300 mm a.s.l.a.s.l. withwith aa slopeslope gradientgradient ofof 5–305–30° (total(total 2 areaarea 39.739.7 kmkm2; Figure5 5).). WithWith respectrespect toto thethe 20072007 event,event, whichwhichpartially partially affected affected the the same same area area (lower (lower SalinelloSalinello hillslopes),hillslopes), thethe 20112011 eventevent occurredoccurred onon aa moderatelymoderately vegetatedvegetated landscapelandscape withwith agriculturalagricultural areasareas (arable(arable land,land, vineyards,vineyards, andand oliveolive groves;groves; FigureFigure5 5a)a) at at an an initial initial crop crop growth growth stage stage and and during during grassgrass development.development. This event occurredoccurred againagain onon clay–sand–conglomerateclay–sand–conglomerate bedrock blanketedblanketed byby aa clayeyclayey eluvialeluvial andand colluvialcolluvial covercover andand landslideslandslides (Figure(Figure5 b).5b). TheThe Pineto-AtriPineto-Atri areaarea consistsconsists ofof threethree coastalcoastal catchments,catchments, thethe interveningintervening coastalcoastal slopes,slopes, andand anan 2 upup toto <1<1 kmkm widewide coastalcoastal plainplain (total(total areaarea 60.660.6 kmkm2;; FigureFigure6 6).). TheTheelevation elevation rangesrangesfrom from sea sea level level to to ~450~450 mm a.s.l.a.s.l. TheThe coastalcoastal andand riverriver plainplain areasareas makemake upup ~15%~15% of the area, while the hillyhilly areaarea (slope(slope ◦ gradientgradient 5–355–35° withwith locallocal scarps)scarps) makes makes up up the the remaining remaining 85%. 85%. Similar Similar to to the the others, others, the the Pineto Pineto area area is characterizedis characterized by by arable arable land land and and high-quality high-quality plantations plantations in in the the hilly hilly areas, areas, butbut alsoalso largelarge barebare clayclay areasareas withinwithin widespreadwidespread badlandsbadlands affectingaffecting thethe south-facingsouth-facing clayclay slopes.slopes.

FigureFigure 5.5. LowerLower Salinello Salinello area: area: (a) (landa) land use use map map (modified (modiﬁed from from land landuse map use of map the ofAbruzzo the Abruzzo region region[64]) and [64 (])b) and lithologic (b) lithologic scheme scheme (legend (legend is in Figure is in Figure4, grid4 ,in grid UTM in UTMWGS84 WGS84 coordinates). coordinates).

TheThe coastalcoastal areaarea is,is, again,again, seamlesslyseamlessly urbanized (Figure(Figure6 6a).a). TheThe hillslopeshillslopes areare onon clayclay bedrock;bedrock; inin thethe hilltops,hilltops, sand,sand, sandstonesandstone andand conglomerateconglomerate are present.present. As for the other areas,areas, thethe slopesslopes areare coveredcovered withwith eluvial–colluvialeluvial–colluvial coverscovers andand landslideslandslides (Figure(Figure6 b).6b). Water 2018, 10, 1314 8 of 29 Water 2018, 10, x FOR PEER REVIEW 8 of 28

FigureFigure 6. 6Pineto. Pineto area: area: ( a(a))land-use land-use mapmap (modiﬁed(modified from from land land use use map map of of the the Abruzzo Abruzzo region region [64]) [64 ])and and (b()b lithological) lithological scheme scheme (modiﬁed (modified from from [[60],60], gridgrid inin UTM WGS84 coordinates). coordinates).

Water 2018, 10, 1314 9 of 29

2.2.3. Rainfall Event of 5–6 and 13–14 September 2012 During September 2012, after a >1-month dry time, the hilly coastal area of the northeastern Apennines (between the Pescara and Vibrata rivers; Figure2), was affected by two rainfall events a short time from one another: a moderate one on 5–6 September and a heavy one on 13–14 September. After both of these events, the Pineto-Atri area was investigated in detail (the same three minor coastal catchments analyzed in 2011; Figure6). The 2012 event occurred on poorly vegetated landscape with agricultural areas (arable land, vineyards, and olive groves) that were mainly plowed.

3. Methods This work deals with the rainfall-induced soil erosion issue with a geomorphological approach based on direct field observation and remote air-photo analysis. It is based on the comparison of rain gauge data and geomorphological effects on the piedmont and coastal area of the NE Abruzzo region during three heavy rainfall events. We used a rainfall dataset from a network of 18 meteorological pluviometric stations (blue dot in Figure2) provided by the Functional Center and Hydrographic Office of the Abruzzo Region. In the study area, 5–15 min sampled rain gauges (for at least 6 days around the main events, accuracy 0.2–1 mm) were analyzed, as well as daily data (1–2 months before the event). For each event, the data (from single rain gauges or from networks of rain gauges) enabled the analysis and comparison of: (1) hourly rainfall intensity; (2) event cumulative rainfall; (3) event duration; (4) daily rainfall; and (5) pre-event rainfall [2,43,65]. In this work, we do not consider the uncertainty in the rainfall duration triggering the geomorphological instabilities [65] because all the events are well defined in terms of time, and, focusing on the overall soil erosion, we consider all the effects induced by the entire events as an assumption. The geomorphological effects of the heavy rainfall events were investigated through different types of available data: (1) 1–2-day post-event field surveys (1:5000; 2011 and 2012 events); (2) analysis of 2-day post-event aerial photos (2007 event; aerial photo scale 1:5000 taken on 9 October 2007 provided by Abruzzo Region Cartographic office) combined with ground truthing through direct landform measurements; and (3) effects inventories and technical reports (all events). This allowed us to be confident that the mapped effects were triggered by the events [66]. The field survey investigated the lithological features of bedrock and superficial deposits cover and the field data were compared to the results of borehole investigations and technical reports. The field geomorphological investigations focused on the type and distribution of the geomorphological effects (i.e., sheet-rill areas, gully areas, major gullies, channel incision, mud flows, flooding areas, crevasse splays, bank failures). For all of the landforms, the length, width, and surface area were mapped in the field (~5 m plan resolution by means of GPS measurements and mapping on 1:5000 topographic maps) and digitized in GIS software (ArcMap® 10.1, ESRI, Redlands, CA, USA). The incision of the erosional landform and the thickness of the depositional landforms were directly measured in the field (~1–5 cm precision) on~55% of the landforms and inferred from remote observation on the remaining ~45%. For the 2007 event, the remote observation from aerial photos (0.5-m resolution) allowed us to map, with GIS software, the surface distribution of the landforms and partly interpret their incision and thickness. Some landforms (from four to ten) for each type were observed and measured in the field for ground-truthing. Through these combined field and remote investigations, a dataset including (1) 1192 features for 2007 (~40 points of direct measurements), (2) 299 features for 2011 (~180 points of direct measurements), and (3) 614 features for 2012 (~330 points of direct measurements), was defined including soil erosion and flooding features induced by the three events. This allowed us to define or estimate the average erosion depth or accumulation thickness for all the landforms. Considering the surface area (for areal landforms), length and width (for linear features), and the average erosion depth or accumulation thickness of the landforms, the eroded volume, as well as the sedimented volume, were estimated for each landform for the overall investigated areas and for each event as follows:

(1) areal landform erosion = ∑i=1−n avg.erosion depthi × landform areai Water 2018, 10, 1314 10 of 29

(2) linear landform erosion = avg.erosion depth × landform length × landform width Water 2018, 10, x FOR PEER REVIEW ∑i=1−n i i 10 iof 28 (3) areal landform accumulation = ∑i=1−n avg.accumulationi × landform areai (3) areal landform accumulation = ∑ 푎푣푔. 푎푐푐푢푚푢푙푎푡𝑖표푛 × 푙푎푛푑푓표푟푚 푎푟푒푎 The total eroded volume was calculated푖=1−푛 from (1) and (2),푖 and the total accumulated푖 sediment from (3).The total eroded volume was calculated from (1) and (2), and the total accumulated sediment fromThe (3). percentage and areal distribution of these effects were also analyzed for the different events and comparedThe percentage to the land and useareal and distribution crop conditions of these in effects the agricultural were also analyzed areas [42 for]. Averaging the different the events eroded volumesand compared in the investigated to the land use areas and (average crop conditions erosion in = totalthe agricultural eroded volume/hilly areas [42]. Averaging surface area) the providederoded thevolumes average in erosion the investigated depth for areas each ( event.average In erosion the calculation, = total eroded only volume/hi the hilly areaslly surface (with area) slope provided > 5◦) were considered,the average excluding erosion depth valley for floors each andevent. fluvial In the and calculation, coastal plains.only the hilly areas (with slope > 5°) were consideredWe exploited, excluding soil erosion valley valuesfloors and fluvial rainfall and data coast in eachal plains. event (heuristic expert method according to BrunettiWe exploited et al. [66 soil]) in erosion order tovalues outline and arainfall correlation data in and each the event expected (heuristic threshold expert method for the triggeraccord- of ing to Brunetti et al. [66]) in order to outline a correlation and the expected threshold for the trigger heavy soil erosion features in the investigated area. Finally, the erosion rates/rainfall ratio were of heavy soil erosion features in the investigated area. Finally, the erosion rates/rainfall ratio were compared to short-term and long-term erosion rates known for the Mediterranean area in order to compared to short-term and long-term erosion rates known for the Mediterranean area in order to outline the impact of heavy soil erosion on the clayey and sandy Apennine piedmont and coastal outline the impact of heavy soil erosion on the clayey and sandy Apennine piedmont and coastal hilly landscape. hilly landscape. 4. Results 4. Results The results of the rainfall distribution analysis and post-event investigations are presented for the The results of the rainfall distribution analysis and post-event investigations are presented for 2007, 2011, and 2012 events. For each event, only the most significant rainfall graphs are presented as the 2007, 2011, and 2012 events. For each event, only the most significant rainfall graphs are presented well as the mapping, features, and distribution of the geomorphological effects. as well as the mapping, features, and distribution of the geomorphological effects. 4.1. Rainfall Event of 6–7 October 2007 4.1. Rainfall Event of 6–7 October 2007 4.1.1. Rainfall Amount and Duration 4.1.1. Rainfall Amount and Duration The rainfall event occurred from about 20:00 to 21:00 on October 6 to about 11:00 to 12:00 on The rainfall event occurred from about 20:00 to 21:00 on October 6 to about 11:00 to 12:00 on 7 7 October 2007, for a total duration of some 14–16 h. The cumulative rainfall was from 60–80 mm October 2007, for a total duration of some 14–16 h. The cumulative rainfall was from 60–80 mm in the in the coastal area to >200 mm in the hilly area (205 mm, Nereto station; Figure7a). The intensity coastal area to >200 mm in the hilly area (205 mm, Nereto station; Figure 7a). The intensity was from was from moderate to high, with values ranging from 10 mm/h in the coastal area to 40 mm/h in moderate to high, with values ranging from 10 mm/h in the coastal area to 40 mm/h in the hilly area the hilly area (Nereto station; Figure7a). The daily rainfall was comparable to the cumulative for (Nereto station; Figure 7a). The daily rainfall was comparable to the cumulative for the event (~100 themm event up to (~100 a maximum mm up toof 2 a10 maximum mm, Tortoreto of 210 and mm, Nereto Tortoreto station ands, Figures Nereto 7 stations,b and 8). FiguresAlong the7b coast, and8). Alongthe recorded the coast, daily the recordedprecipitation daily was precipitation around 60–80 was mm around (Figure 60–80 7b). mmMoreover, (Figure this7b). event Moreover, occurred this eventafter occurred a fairly dry after period a fairly, with dry very period, low September with very lowrainfa Septemberll (<20 mm; rainfall Figure (<20 8). mm; Figure8). InIn summary, summary the, the Tortoreto Tortoreto 2007 2007 event event had had high high intensity intensity (10–40 (10–40 mm/h,mm/h, up up to to >200 >200 mm/day) mm/day) and and highhigh cumulative cumulative rainfall rainfall (up (up to to 210 210 mm) mm) andand occurringoccurring after > >11 month month of of very very low low rain rainfall.fall.

FigureFigure 7. 7Heavy. Heavy rainfall rainfall event event of of 77 OctoberOctober 2007:2007: ( a)) hourly hourly and and cumulative cumulative rainfall rainfall at at the the Nereto Nereto (TE) (TE) stationstation and and (b ()b maximum) maximum dailydaily rainfallrainfall at the the stations stations surrounding surrounding the the study study area area (location (location in Figure in Figure 2). 2). Water 2018, 10, 1314 11 of 29 Water 2018, 10, x FOR PEER REVIEW 11 of 28

FigureFigure 8 8.. SeptemberSeptember–October–October 2007 daily rainfall at the ( a) Tortoreto station andand ((b)) NeretoNereto station.station.

4.1.2. Geomorphological Effects This event induced heavy soil erosion processes on the slopes (sheet, rill, and gully erosion), rapid mud mud flows flows at at the the bases bases of ofslopes slopes and and minor minor drainage drainage basins, basins, and andflooding flooding within within the main the mainriver riverand coastal and coastal plains, plains, mostly mostly at the atoutlets the outletsof minor of tr minoributary tributary catchments catchments (Figures (Figures 9 and 109 )and. On 10 the). Ondownstream the downstream side, along side, the along urbanized the urbanized area and area the and coastal the coastal plain, plain,large volumes large volumes of sediment of sediment were weredeposited deposited on roads, on roads, at the atstream the stream outlet outlet,, and along and alongpaleovalleys paleovalleys (covered (covered with urban with urban areas). areas). This Thisinduced induced serious serious problems problems at the at circulation the circulation and also and affected also affected the safety the safety of the of population. the population. 2 Hilly catchments and slopes were affected by gully erosion (4.36 km 2, 10.0%10.0% of the investigated 2 area; FiguresFigures9 9 and and 10 10) and) and sheet-rill sheet–rill erosion erosion (4.84 (4.84 km km, 11.1%2, 11.1% of of the the investigated investigated area; area; Figures Figures9 and 9 and10). The10). lowThe gradientlow gradient slopes slopes and ridges and ridges with vineyardswith vineyards and olive and groves, olive groves, or, in general, or, in general, those not those plowed, not wereplowed, mostly were affected mostly byaffected moderate by moderate sheet-rill sheet erosion.‑rill Rillerosion. features Rill were features observed were observed as 1–5 cm as deep 1–5 and cm withdeep 1–10and with m spacing 1–10 m in spacing these areas. in these The areas. average The erosion average in erosion the sheet-rill in the erosionsheet–rill areas erosion was calculatedareas was 3 tocalculated be as much to be as ~1as cm.much The as total ~1 cm. calculated The total eroded calculated volume eroded was ~48,400 volume m was. The ~48,400 high gradient m3. The slopes, high particularlygradient slopes, where parti plowedcularly in where a downslope plowed direction,in a downslope were incised direction, by gullieswere incised (Figure by9). gullies Gullies (Figure were measured9). Gullies were mostly measured from 20 mostly cm up tofrom ~100 20 cm deepup to and~100 40–200cm deep cm and wide, 40–200 usually cm wide, spaced usually 4–8 m spaced apart. The4–8 m average apart. erosionThe average in the erosion gully erosion in the areagully was erosion calculated area was to reach calculated ~14 cm. to The reach total ~14 eroded cm. The volume total 2 waseroded calculated volume as was ~610,900 calculated m . Alongas ~610,900 the main m2. Along channels the of main the tributarychannels catchments,of the tributary channel catchments, incision 2 occurredchannel incision (channels occurred up to 2 (channels m deep and up 3–4to 2 m m wide). deep and The 3 area–4 m affected wide). wasThe area 0.23 affected km and was the 0.23 average km2 3 depthand the was average ~100 depth cm. The was total ~100 eroded cm. Th volumee total eroded was calculated volume was as ~228,400calculated m as. ~228,400 m3. Mud flows flows were were the the result result of of heavy heavy soil soil erosion erosion on the on theslopes, slopes, inducing inducing the mobilization the mobilization of huge of hugesediment sediment volumes volumes of clay of-silt clay-silt and minor and minor volumes volumes of sand of sandfrom fromthe eluvial the eluvial and colluvial and colluvial cover. cover. Ex- Extensivetensive mud mud and and water water flooding flooding affected affected both both coastal coastal plains, plains, coming coming more more from from the the small small catchments catchments than fromfrom thethe main main rivers rivers (Figure (Figure 10 10). In). In the the lower lower part part of theof the slopes slopes and and at the at outletsthe outlets of the of catchments, the catch- 2 mudments, flows mud wereflows very were common very common (total (total area ofarea ~0.37 of ~0.37 km ,km ~0.9%2, ~0.9 of% the of the investigated investigated area), area), showing show- muding mud accumulated accumulated thickness thickness ranging ranging from from 50 50 to to 200 200 cm. cm. The The total total estimated estimated sedimentsediment volumevolume was 3 250,900 mm3. Mud Mud-rich-rich floodingflooding areasareas occurredoccurred at at the the bases bases of of the the slopes, slopes, along along the the channels channels of tributaryof tribu- 2 catchments,tary catchments, and atand the atoutlets the outlets of catchments of catchments to the to fluvialthe fluvial or coastal or coastal plains plains (~3.97 (~3.97 km ,km ~9.1%2, ~9.1% of the of 3 investigatedthe investigated area); area); accumulated accumulated mud mud was was 0–10 0 cm–10 thick,cm thick, for a for total a total estimated estimated volume volume of 153,000 of 153,000 m . m3. Water 2018, 10, 1314 12 of 29 Water 2018, 10, x FOR PEER REVIEW 12 of 28

Water 2018, 10, x FOR PEER REVIEW 12 of 28

FigureFigureFigure 9. Geomorphological9 .9 Geomorphological. Geomorphological effects effectseffects induced induced induced by by the by the2007 the 2007 2007 heavy heavy heavy rainfall rainfall rainfallevent event event inin thethe inTortoretoTortoreto the Tortoreto hilly area hilly (ortophotoareaarea (ortophoto (ortophoto taken 2 taken daystaken after 2 days the afterafter event): the the event): (event):a) gully (a )( ga erosionully) gully erosion erosion and and (b) and( rill-gullyb) rill (b‑gully) rill erosion‑ gullyerosion erosion features. features features. .

Sheet–Rill Gully Channel Mud Flooding (b) TOTAL ErosionSheet-Rill ErosionGully IChannelncision MudFlows FloodingAreas (b) TOTAL Number of features 389Erosion 506Erosion Incision45 Flows128 Areas124 1192 Depth/thickness (cm) 1–5 20–100 100–200 50–150 0–10 Number of features 389 506 45 128 124 1192 Spacing (m) Sheet1–10– Rill 4Gully–8 Channel Mud Flooding Depth/thickness(b) (cm) 1–5 20–100 100–200 50–150 0–10 TOTAL Area (km2) Erosion4.36 4E.84rosion 0.23Incision 0.37 Flows 3.97 Areas 13.77 % ofSpacing total area (m) 10.0 1–1011.14–8 0.9 9.1 31.00 Number of features2 389 506 45 128 124 1192 Area (km 3) 4.36 4.84 0.23 0.37 3.97 13.77 Depth/thicknEroded volessume (cm) (m ) 48,4001–5 610,90020–100 228,400100 –200 50–150 0–10887,700 Sediment%ed of vol totalume area (m3) 10.0 11.1 250,900 0.9 9.1153,000 31.00 403,900 SpacingEroded (m) volume (m3) 1–1048,400 610,9004–8 228,400 887,700 2 SedimentedFigureArea (km 10. ) volumeGeomorphological (m3) 4.36 effects triggered4. 84by the 2007 heavy0.23 rainfall 250,900 event0 in.37 153,000the Tortoreto3 403,900.97 hilly 13.77 % of total area 10.0 11.1 0.9 9.1 31.00 and coastal area (Tortoreto 2007): (a) map (grid in UTM WGS84 coordinates) and (b) table of the effects FigureEroded 10. volGeomorphologicalume (m3) 48,400 effects triggered610,900 by the 2007 heavy228,400 rainfall event in the Tortoreto hilly887,700 distribution. 3 Sedimand coastalented vol areaume (Tortoreto(m ) 2007): (a) map (grid in UTM WGS84 coordinates)250,900 and (b)153,000 table of the403,900 effectsFigure distribution. 10. Geomorphological effects triggered by the 2007 heavy rainfall event in the Tortoreto hilly and coastal area (Tortoreto 2007): (a) map (grid in UTM WGS84 coordinates) and (b) table of the effects distribution. Water 2018, 10, 1314 13 of 29 Water 2018, 10, x FOR PEER REVIEW 13 of 28 Water 2018, 10, x FOR PEER REVIEW 13 of 28 4.2.4.2. RainfallRainfall EventEvent ofof 1–21–2 MarchMarch 20112011 4.2. Rainfall Event of 1–2 March 2011 4.2.1.4.2.1. RainfallRainfall AmountAmount andand DurationDuration 4.2.1. Rainfall Amount and Duration TheThe rainfallrainfall eventevent occurredoccurred fromfrom aboutabout 02:0002:00 onon 11 MarchMarch toto 02:0002:00 onon 22 MarchMarch 2011,2011, forfor aa totaltotal durationdurationThe ofof rainfall somesome 22–2622 event–26 h.h occurred. TheThe cumulativecumulative from about rainfallrainfall 02:00 waswas on100–130 1001 March–130 mmmmto 0 at2:00at mostmost on of2of March thethe stationsstations 2011, (e.g.,for(e.g., a PinetototalPineto duration of some 22–26 h. The cumulative rainfall was 100–130 mm at most of the stations (e.g., Pineto station;station; Figure 11a), up up to to a amaximum maximum of of 211 211 mm mm in inthe the hilly hilly area area (Nereto (Nereto station; station; Figure Figure 11b). 11 Theb). station; Figure 11a), up to a maximum of 211 mm in the hilly area (Nereto station; Figure 11b). The Therainfall rainfall intensity intensity was wasaround around 15–20 15–20 mm/h mm/h and up and to up35 mm/h to 35 mm/h (e.g., Pineto (e.g., Pinetoand Nereto and Nereto stations; stations; Figure rainfall intensity was around 15–20 mm/h and up to 35 mm/h (e.g., Pineto and Nereto stations; Figure Figure11). The 11 daily). The rainfall daily was rainfall ~80– was120 mm/d ~80–120 in the mm/day entire hilly in the area, entire up to hilly a maximum area, up of to 180 a maximum mm (Nereto of 11). The daily rainfall was ~80–120 mm/d in the entire hilly area, up to a maximum of 180 mm (Nereto 180station; mm (NeretoFigure 12 station;a). Moreover, Figure 12thisa). event Moreover, occurred this after event a occurredmoderately after humid a moderately winter period humid (~50 winter mm station; Figure 12a). Moreover, this event occurred after a moderately humid winter period (~50 mm periodin a 10- (~50day mmtime inspan a 10-day before time the event, span before Figure the 12 event,b). Figure 12b). in a 10-day time span before the event, Figure 12b). InIn summary,summary, the the 2011 2011 event event affected affected a regional a regional area area (Pineto (Pineto 2011 2011 and andSalinello Salinello 2011) 2011) for a formod- a In summary, the 2011 event affected a regional area (Pineto 2011 and Salinello 2011) for a moderately short duration (22–26 h) with high intensity (15–35 mm/h, up to >180 mm/d) and high cumu- moderatelyerately short short duration duration (22 (22–26–26 h) with h) with high high intensity intensity (15– (15–3535 mm/h, mm/h, up to up >180 to >180mm/d) mm/day) and high and cumu- high lative rainfall (up to 211 mm) and after ~10 days of moderate antecedent rainfall. cumulativelative rainfall rainfall (up to (up 211 to mm) 211mm) and after and after~10 days ~10 of days moderate of moderate antecedent antecedent rainfall. rainfall.

FigureFigureFigure 11.11 11.Hourly H. Hourlyourly and and and cumulative cumulative cumulative rainfall rainfallrainfall during during the thethe heavy heavyheavy rainfall rainfall rainfall event event event on on 1–2on 1 –1 March2– 2March March 2011, 2011, 2011, at at (a at)(a the )( a) Pinetothethe Pineto Pineto (TE) (TE)station (TE) station station and and( andb) the ((bb)) Nereto thethe NeretoNereto (TE) (TE) station. stationstation..

Figure 12. Rainfall event on 2 March 2011: (a) daily rainfall at stations in the NE Abruzzo hilly area, FigureFigureand ( 12.b12) daily.Rainfall Rainfall and eventcumulativeevent onon 22 MarchrainfallMarch 2011: 2011:in the ( (aFebruarya)) dailydaily rainfallrainfall–March atat2011 stationsstations time interval inin thethe NENE (Pineto AbruzzoAbruzzo station) hillyhilly. area,area, andand ((bb)) dailydaily andand cumulativecumulative rainfallrainfall inin thethe February–MarchFebruary–March 2011 2011 time time interval interval (Pineto (Pineto station) station). . 4.2.3. Geomorphological Effects 4.2.2.4.2.3. GeomorphologicalGeomorphological EffectsEffects The geomorphological analysis performed after the event outlined soil erosion due to sheet and rillThe Theerosion, geomorphologicalgeomorphological gully erosion, and analysisanalysis sedimentation performedperformed as afterafter mud thethe flows eventevent and outlinedoutlined flooding soilsoil (Figure erosionerosion 13), duedue similar toto sheetsheet to the andand rillrill2007 erosion,erosion, event gully gullybut with erosion,erosion, a different andand sedimentation sedimentationdistribution and asas minor mudmud total flowsflows extent. andand flooding floodingIn the hilly (Figure(Figure catchment13 13),), similar ssimilar and slopes to to the the 20072007of the eventevent Pineto butbut- withAtriwith area aa differentdifferent (Figure distributionsdistribution 13a,b, and and14anda), minorminorsoil erosion totaltotal extent.extent.occurred InIn as thethe gully hillyhilly areas, catchmentscatchment with gulliess andand slopesslopes 20– ofof50 thethe cm Pineto-AtriPineto deep -andAtri spaced area area (Figure ( Figure3–20 m 13s (over13a,ba,b, and an and area Figure 14 ofa), 1.1314 soila), km erosion soil2, 1. erosion9% occurred of the occurred investigated as gully as gully areas, area areas,, Figurewith with gullies 14 gullies); the 20 – 2 20–5050total cm cm deeperoded deep and volume and spaced spaced was 3– 20133,000 3–20 m (over m (overm3 .an Major area an area gulliesof 1.13 of 1.13 upkm to km2, 1.1 9m,% 1.9% deep of the of and theinvestigated 2 investigated m wide (totalarea area,, Figuregully Figure length 14); 14 the ); total eroded volume was 133,000 m3. Major gullies up to 1 m deep and 2 m wide (total gully length Water 2018, 10, 1314 14 of 29 the total eroded volume was 133,000 m3. Major gullies up to 1 m deep and 2 m wide (total gully lengthWater 2018 10.2, 10 km;, x FOR Table PEER1a, REVIEW Figure 14a) occurred along the channels of tributary catchments and14 of slope 28 undulations, in some cases enlarging natural or manmade notches or plowing incisions (Figure 13b); 10.2 km; Table 1a, Figure 14a) occurred along the channels of tributary catchments and slope undu- the total estimated volume was 8900 m3. At the bases of the slopes within the main valleys and along lations, in some cases enlarging natural or manmade notches or plowing incisions (Figure 13b); the the coastal slope rapid mud flows occurred from 20 cm to 150 cm thick (total area of 0.26 km2, 0.4% total estimated volume was 8900 m3. At the bases of the slopes within the main valleys and along the 3 ofcoastal the investigated slope rapid area;mud flows Table occurred1a, Figure from 14a); 20 thecm to estimated 150 cm thick accumulated (total area volumeof 0.26 km was2, 0.4 60,400% of the m . Alonginvestigated the main area; rivers Table and 1a, at Figure the outlet 14a); to the the estimated coastalplain, accumulated crevasse volume splays was occurred, 60,400 withm3. Along a sediment the 2 3 thicknessmain rivers of 10–50and at cmthe (totaloutlet areato the 0.16 coastal km plain,, 0.3% crevasse of the area;splays estimated occurred, volumewith a sediment 20,000 m thickness), as well 2 asof flooding 10–50 cm areas, (total with area mud0.16 km 0–52, cm0.3% thick of the (total area; area estimated 2.01 km volume, 3.3% 20,000 of the m area;3), as estimatedwell as flooding volume 3 26,100areas, m with). mud 0–5 cm thick (total area 2.01 km2, 3.3% of the area; estimated volume 26,100 m3).

FigureFigure 13. 13Geomorphological. Geomorphological instabilitiesinstabilities triggered by by the the 2011 2011 heavy heavy rainfall rainfall event: event: (a ()a Pineto,) Pineto, gully gully erosionerosion area area on on nonvegetated nonvegetated cropland;cropland; (b) Pineto, major major gully gully on on a a vegetated vegetated cropland; cropland; (c) ( cSalinello) Salinello River,River crevasse, crevasse splay splayand and fluvialfluvial erosionerosion scarps; ( dd)) Salinello Salinello R River,iver, fluvial fluvial erosion erosion scarp scarp affecting affecting a a valleyvalley road; road; (e ()e Salinello) Salinello River, River, flooding flooding areaarea andand crevassecrevasse splays on the the main main fluvial fluvial plain. plain.

InIn the the Salinello Salinello River River valleyvalley (Figure(Figures 13 13c,d,ec,d,e and and Figure14b), limi 14b),ted limited soil erosion soil erosionoccurred occurred on the mod- on the moderatelyerately vegetated vegetated slopes, slopes, while while most most of the of the effects effects were were related related to floodingto flooding along along the the main main river. river. HeavyHeavy mud mud and and water water flooding flooding were were the the prevailing prevailing effects effects of of this this event, event, affectingaffecting thethe coastalcoastal plainsplains as as well as the river plain almost seamlessly. Extensive overbank flooding along the rivers induced the formation of wide and long crevasse splays on the floodplain (Figure 13c–e). Soil erosion occurred Water 2018, 10, 1314 15 of 29 well as the river plain almost seamlessly. Extensive overbank flooding along the rivers induced the Water 2018, 10, x FOR PEER REVIEW 15 of 28 formation of wide and long crevasse splays on the floodplain (Figure 13c–e). Soil erosion occurred inin terms terms of of areas areas affected affected byby sheet-rillsheet–rill erosionerosion areas (Table (Table 11b,b, Figure 1144b),b), with with an an average average erosion erosion 2 3 ofof 5–10 5–10 cm cm (0.10(0.10 kmkm2,, 0.2% 0.2% of of the the area; area; estimated estimated volume volume 5700 5700 m3), m with), withgullies gullies 20–100 20–100 cm deep cm and deep 3 and0.5– 0.5–2.02.0 m wide m wide (for (fora total a total length length of ~4.5 of ~4.5km; estimated km; estimated eroded eroded volume volume 4400 m 44003). Along m ). the Along main the mainchannel channel of the of Salinello the Salinello River River bank, bank, failure failure occurred occurred (for a (for total a totallength length of ~2.0 of km; ~2.0 Figure km; Figure 13c,d), 13 in-c,d), inducingducing severe severe damage damage to to valley valley roads. roads. At At the the bases bases of of the the slopes, slopes, mud mud flows flows were were formed formed (Table (Table 1b,1 b, 2 3 FigureFigure 14 14b)b) over over a a 0.15 0.15 km km2area, area, asas thickthick as 50–15050–150 cm (total estimated estimated volume volume 65,500 65,500 m m3). ).Flooding Flooding 2 areasareas occurred occurred (Table(Table1 1b,b, FigureFigure 14b) over over 3.62 3.62 km km2 (8.3%(8.3% of the of theinvestigated investigated area), area), accumulating accumulating 0–5 0–5cm cm of mud of mud (estimated (estimated volume volume 46,700 46,700 m3). All m3 along). All the along flooding the flooding areas, crevasse areas, crevassesplays formed splays (Figure formed (Figure13e) over 13e) 0.32 over km 0.322 (0.7% km2 of(0.7% the area), of the from area), 20 from cm to 20 40 cm cm to thick 40 cm (estimated thick (estimated volume 83,700 volume m 83,7003). m3).

FigureFigure 14. 14.Map Map of of thethe geomorphologicalgeomorphological effects triggered triggered by by the the 2011 2011 heavy heavy rainfall rainfall event event (grid (grid in in UTMUTM WGS84 WGS84 coordinates): coordinates): ((aa)) Atri-PinetoAtri-Pineto hilly and and coastal coastal area area (Pineto (Pineto 2011) 2011) and and (b ()b Salinello) Salinello River River valleyvalley (Salinello (Salinello 2011). 2011).

Water 2018, 10, 1314 16 of 29

Table 1. Table of the geomorphological effects distribution of the 2011 heavy rainfall event: (a) Pineto area and (b) Salinello area.

Gully Major Mud Crevasse (a) Flooding Areas TOTAL Erosion Gullies Flows Splays Number of features 8 26 38 13 5 90 Depth/thickness (cm) 20–50 50–100 20–150 0–5 10–50 Spacing (m) 3–20 Area (km2) 1.13 10.20 0.26 2.01 0.16 13.76 % of total area 1.9 0.4 3.3 0.3 5.9 Eroded volume (m3) 133,000 8900 141,900 Sedimented volume (m3) 60,400 26,100 20,000 86,500 Sheet-rill Major Bank Mud Flooding Crevasse (b) TOTAL Erosion Gullies Failure Flows Areas Splays Number of features 27 84 15 31 32 20 209 Depth/thickness (cm) 5–10 20–100 100–200 50–200 0–5 20–40 Spacing (m) 1–2 Area (km2)/Length (km) 0.10 4.50 2.00 0.50 3.62 0.32 % of total area 0.2 0.3 8.3 0.7 9.5 Eroded volume (m3) 5700 4400 2000 12,100 Sedimented volume (m3) 65,500 46,700 83,700 195,900

4.3. Rainfall Event of 5–6 and 13–14 September 2012

4.3.1. Rainfall Amount and Duration This event is the result of two rainfall periods separated by a 7-day dry period and has to be considered as two separated events (according to Peruccacci et al. and Brunetti et al. [7,66]). The first rainfall event occurred from about 17:00 on 5 September to 11:00 on 6 September 2012, for a total duration of some ~18 h, with peaks in the evening and the morning; the second event occurred from about 18:00 on 13 September to 18:00 on 14 September 2012, for a total duration of ~24 h. The cumulative rainfall was ~30–110 mm for the first event and ~80–190 mm for the second event (Figure 15a,b). The recorded rainfall intensity was 10 to >60 mm/h for the first event and ~15–45 mm/h for the second event (Figure 15a,b). The daily rainfall was ~60–110 mm/day for the first event and up to a maximum of 190 mm for the second event (Figure 15c,d). Moreover, these events occurred after more than a month of completely dry conditions (Figure 15c). In summary, the Pineto 2012 event was a double event, 18 and 24 h long, and affected and coastal hilly area within a 1-week interval, with high intensity (15–65 mm/h, up to >190 mm/day) and combined cumulative rainfall up to 280 mm, after a >1-month completely dry period. Water 2018, 10, 1314 17 of 29 Water 2018, 10, x FOR PEER REVIEW 17 of 28

FigureFigure 15. 15Double. Double September September 2012 2012 eventevent inin thethe Atri-PinetoAtri-Pineto area: ( (aa)) hourly hourly and and cumulative cumulative rainfall rainfall (5–6(5–6 September September 2012;2012; Atri station); station); (a ()a hourly) hourly and and cumulative cumulative rainfall rainfall (13–14 (13–14 September September 2012; Atri 2012; Atristation); station ();c) (dailyc) daily rainfall rainfall in August in August and September and September 2012 at 2012the Atri at thestation; Atri ( station;d) daily rainfall (d) daily over rainfall the overperiods the periods 5–6 and 5–6 13– and14 September 13–14 September 2012, events 2012, at events significant at signiﬁcant stations in stations NE Abruzzo. in NE Abruzzo.

4.3.2.4.3.2. Geomorphological Geomorphological Effects Effects TheThe geomorphological geomorphological analysis analysis performedperformed afterafter both the first first and and second second even eventt outlined outlined a alarge large distributiondistribution of of soil soil erosion erosion features features combinedcombined withwith mud flows, flows, flooding flooding areas, areas, and and minor minor crevasse crevasse splayssplay (Figures (Figure 16 16). ). OnOn 5–6 5– September6 September (Figure (Figure 17 17a,a, TableTable2 2aa),), minor minor effectseffects occurredoccurred in terms terms of of soil soil erosion erosion features, features, suchsuch as as gully gully areas areas with with 10–30 10–30 cm cm average average incisionincision (0.23(0.23 km2,, 0. 0.4%4% of of the the area; area; estimated estimated volume volume 33,90033,900 m m3)3 and) and gullies gullies 20–40 20–40 cmcm deepdeep andand 50–15050–150 cm wide (total length length 8 8.3.3 km; km; estimated estimated volume volume 27002700 m 3m;3 Figure; Figure 16 16a,b);a,b); sedimentation sedimentation featuresfeatures includedincluded mud flows flows 25 25–50–50 cm cm thick thick (o (overver 0.1 0.1 km km2, 20.,2 0.2%% ofof the the area; area; estimated estimated volume volume 30,900 30,900 mm33),), floodingflooding areas ( (FigureFigure 1616c)c) with with 0 0–3–3 cm cm thick thick mud mud (over (over 0.300.30 km km2,2 0.5%, 0.5% of of the the area; area; estimated estimated volumevolume 63006300 m3)),, and and minor minor crevasse crevasse splay splays.s. OnOn 13–14 13–14 September September (Figure (Figure 17 17b,b, TableTable2 2bb),), further further heavyheavy soilsoil erosion occurred, mostly mostly due due to to gullygully areas areas with with an an average average 20–40 20–40cm cm incisionincision (over(over 1.181.18 km2,, 2 2.0%.0% of of the the area; area; estimated estimated volume volume 236,500236,500 m m3)3 and) and major major gullies gullies 20–70 20–70 cmcm deepdeep and 1 1–2–2 m wide wide (total (total length length 48.8 48.8 km; km; estimated estimated volume volume 23,30023,300 m 3m; Figure3; Figure 16 16 d–f). d–f). Sheet-rill Sheet–rill erosion erosion areas areas were were observed observed with with a 2–5 a 2 cm–5 cm incision incision (over (over 0.92 0.92 km 2, 2 3 1.5%km of, 1.5% the area;of the estimated area; estimated volume volume 27,600 27,600 m3). m In). the In the lower lower part part of theof the slopes slopes and and at at the the junctions junctionsof 2 minorof minor tributaries tributaries to main to main streams, streams, mud mud flows flows occurred occurred up up to to 150 150 cmcm thickthick (over 0.18 0.18 km km,2 0., 0.3%3% of of 3 thethe area; area; estimated estimated volume volume 88,000 88,000 m m3).). AlongAlong the main fluvial fluvial plain plain and and the the coastal coastal plain, plain, flooding flooding areas occurred with 0–3 cm of accumulated mud (over 0.36 km2, 0.59% of the area; estimated volume areas occurred with 0–3 cm of accumulated mud (over 0.36 km2, 0.59% of the area; estimated volume ~5000 m3), as well as some local crevasse splay 10–30 cm thick (over 0.03 km2, 0.1% of the area; esti- ~5000 m3), as well as some local crevasse splay 10–30 cm thick (over 0.03 km2, 0.1% of the area; mated volume 5900 m3). estimated volume 5900 m3). Water 2018, 10, 1314 18 of 29 Water 2018, 10, x FOR PEER REVIEW 18 of 28

Figure 16. Geomorphological instabilities triggered by the 2012 double heavy rainfall event: (a) Pineto Figure 16. Geomorphological instabilities triggered by the 2012 double heavy rainfall event: (a) Pineto valleyvalley slope, slope, gully gully erosion erosion area area on on aa not-vegetatednot-vegetated cropland; ( (bb)) Pineto Pineto coastal coastal slope, slope, major major gully gully on on plowedplowed cropland; cropland; (c ()c Pineto) Pineto coastal coastal plain, plain, ﬂoodingflooding area with mud mud accumulation; accumulation; (d (d) )Atri Atri-Pineto‑Pineto hilly hilly slopes,slopes, widespread widespread sheet-rill sheet‑rill and andgully gully erosionerosion areas; ( e)) Atri Atri hilly hilly slopes, slopes, sheet sheet-rill–rill an andd gully gully erosion erosion areas;areas (f; )(f Atri) Atri hilly hilly slopes, slopes, major major gullies gullies alongalong thethe minorminor catchments. Water 2018, 10, 1314 19 of 29 Water 2018, 10, x FOR PEER REVIEW 19 of 28

FigureFigure 17. 17. Map of of geomorphological geomorphological effects effects triggered triggered by the by 2012 the 2012heavy heavy rainfall rainfall event in event the Atri in the- Atri-PinetoPineto hilly hilly-coastal-coastal area area (grid (grid in UTM in UTM WGS84 WGS84 coordinates) coordinates):: (a) 5– (a6) September 5–6 September (Pineto (Pineto 2012- 2012-1)1) and (b and) (b)13 13–14–14 September September (Pineto (Pineto 2012 2012-2).-2). Water 2018, 10, 1314 20 of 29

Table 2. Table of the geomorphological effects distribution of the 2012 heavy rainfall event: (a) 5–6 September and (b) 13–14 September.

Gully Mud Crevasse (a) Major Gullies Flooding Areas TOTAL Erosion Flows Splays Number of features 23 66 4 13 1 106 Depth/thickness (cm) 10–30 20–40 50–200 0–3 30 Spacing (m) 4–6 Area (km2) 0.23 8.3 0.10 0.30 > 0.001 8.93 % of total area 0.4 0.2 0.5 1.1 Eroded volume (m3) 33,900 2700 36,600 Sedimented volume (m3) 30,900 6300 300 37,500 Sheet-rill Gully Major Channel Mud Flooding Crevasse (b) TOTAL Erosion Erosion Gullies Incision Flows Areas Splays Number of features 52 76 321 4 23 24 8 508 Depth/thickness (cm) 2–5 20–40 20–70 30–60 50–200 0–3 10–30 Spacing (m) 1–4 4–8 Area (km2) 0.92 1.18 48.80 0.90 0.18 0.36 0.03 52.37 % of total area 1.5 2.0 0.3 0.6 0.1 4.5 Eroded volume (m3) 27,600 236,500 23,300 2500 289,900 Sedimented volume (m3) 88,000 5000 5900 98,900

5. Discussion A direct investigation (field mapping and air-photo) analysis of the geomorphological effects induced by three heavy rainfall events in the NE Abruzzo hilly coastal area provided the mapping of >2000 soil erosion and accumulation features. The amount of soil erosion was estimated along the slopes and in the hilly areas for the different types of landforms in all of the investigated events (Table3). The distribution of mud flows, flooding areas, and crevasse splay outlined the estimated volume of sediment accumulated at the bases of slopes, at the outlets of minor catchments, and on the river and coastal plains (Table4). For the 2007 event, the estimated soil erosion volume on the hillslopes and catchments was about 887,700 m3 (mostly due to gully erosion; Table3). The average overall erosion calculated in the hilly area (65% of the investigated area) was ~3.08 cm in this single event. At the base of the slopes, in the valley floors, and in the coastal and river plains, mud flows and floods accumulated an estimated 403,800 m3 volume of sediment. This means that more than half of the eroded soil (55%) was removed from the hilly system to the sea. For the 2011 event in the Pineto-Atri area, the estimated amount of soil erosion was about 141,900 m3 (mostly due to gully erosion; Table3). The average overall erosion calculated in the slopes of the hilly area (~85% of the investigated area) was ~0.29 cm. At the bases of slopes, in the valley floors, and in the coastal and river plains, mud flows and floods accumulated an estimated 106,500 m3 volume of sediment. Again, 25% of the eroded soil was removed from the hilly system to the sea. In the Salinello River area, the estimated eroded volume was very low at 12,100 m3 (overall erosion estimated in the hilly area <0.01 cm), while the sedimented volume was as great as 196,000 m3 (mostly due to mud flows and crevasse splay; Table3). This confirms that this case is in a different geomorphological framework (main river valley instead of coastal hilly area) and the main geomorphological effects were related to flooding in the main river (mostly coming from upstream and the main valley) and not to soil erosion in the hilly slopes. For the 2012 double event, the estimated amount of soil erosion (Table3) was about 36,500 m 3 on 5–6 September (Pineto 2012-1) and 289,900 m3 on 13–14 September (Pineto 2012-2). The average overall erosion in the slopes of the hilly Pineto-Atri area (~85% of the investigated area) was ~0.08 (5–6 September) and ~0.60 cm (13–14 September). At the bases of slopes, in the valley floors, and in the coastal and river plains, mud flows and floods accumulated an estimated amount of about 37,500 m3 of sediment (5–6 September, roughly corresponding to the eroded volume) and 98,900 m3 of sediment (13–14 September, about one-third of the eroded volume). Again, this means that 66% of the eroded soil was removed from the hilly system to the sea. Moreover, for all the events, considering that large part of the sediment Water 2018, 10, 1314 21 of 29 was accumulated in urban/industrial areas and removed after the events, the actual soil loss was even higher.

Table 3. Geomorphological effects and landform distributions for the investigated events, soil erosion-sedimentation, and soil loss estimation.

Tortoreto 2007 Salinello 2011 Pineto 2011 Pineto 2012-1 Pineto 2012-2 Erosion m3 % m3 % m3 % m3 % m3 % Sheet-rill erosion 48,400 5 5700 56 0 0 0 27,600 10 Gully erosion 610,900 69 0 133,000 94 33,900 93 236,500 82 Major gullies 0 0 4400 44 8900 6 2700 7 23,300 8 Bank failure 0 0 2000 20 0 0 0 0 100 0 Channel incision 228,400 26 0 0 0 0 2500 1 Total eroded 887,800 12,100 141,900 36,600 289,900 Sedimentation m3 % m3 % m3 % m3 % m3 % Mud ﬂows 250,900 62 65,500 33 60,400 57 30,900 82 88,000 89 Flooding areas 153,000 38 46,700 24 26,100 25 6300 17 5000 5 Crevasse splays 0 0 83,700 43 20,000 19 300 1 5900 6 Total sedimented 403,800 195,900 106,500 37,500 98,900 Soil loss −484,000 −55 183,800 +1519 −35,400 −25 +900 +2 −191,000 −66

Table 4. Summary of rainfall and soil erosion–sedimentation in the investigated rainfall events. Negative values indicate sediment loss; positive values indicate sedimented volumes higher than the eroded (see text for explanation).

Hourly Cumulative Hourly Rainfall Rainfall Eroded Average Sedimented Rainfall Soil Loss Event Rainfall Intensity Max Duration Volume Erosion Volume Intensity (m3) (mm) (mm/h) (h) (m3) (cm) (m3) Mean (mm/h) 1 Tortoreto 2007 210 40 14.7 14–16 887,700 3.08 403,800 −483,900 2a Salinello 2011 211 35 8.8 22–26 10,100 < 0.01 196,000 185,900 2b Pineto 2011 120 35 5.0 22–26 141,900 0.29 106,500 −35,400 3a Pineto 2012-1 120 60 6.7 18 36,600 0.08 37,500 900 3b Pineto 2012-2 ~160 45 7.0 22–24 289,900 0.60 98,900 −191,000

The events were characterized by variable heaviness (i.e., cumulate rain and intensity) and different areas, but in comparable lithological-geomorphological settings (coastal clay–sand–conglomerate hills covered with eluvial–colluvial and landslide deposits), except for the Salinello River valley (major river valley and hillslopes). The main rainfall parameters of the events in the different areas (i.e., cumulative rainfall, which in this case roughly corresponds to daily rainfall, rainfall peak intensity, and rainfall duration) were compared to the soil erosion estimations (eroded volume, erosion averaged over the affected hilly slope areas, sediment volume, sediment loss) (Table4). Despite encompassing a limited number of investigated events, rainfall data and soil erosion estimations were analyzed (Figure 18) in order to outline the controlling factors and possibly a triggering threshold for the heavy soil erosion features in this area. Considering the distribution of rainfall values, the strong influence on soil erosion of cumulative rainfall and the poor influence of maximum rainfall intensity are outlined in this case. More specifically, eroded volume vs., cumulative rainfall (Figure 18a), which roughly corresponds to daily rainfall for these events as well as average erosion vs. cumulative rainfall (Figure 18b), shows good correlation (the latter not depending on the size of the areas). Otherwise, average erosion vs. rainfall intensity (Figure 18c) shows a scattered distribution and poor control of the peak intensity in soil erosion for these events. Only the 2011 Salinello event is completely incomparable (2a in Figure 18), as expected, being in a different geomorphological framework (main river valley vs., coastal hilly area) and possibly due to the condition of slopes in terms of vegetation cover. The event occurred at the beginning of March, with the cultivated areas already covered by incipient vegetation, which prevented the slopes from undergoing heavy soil erosion. This is partially true also for the Pineto 2011 area (in terms of Water 2018, 10, x FOR PEER REVIEW 22 of 28

Water 2018, 10, 1314 22 of 29 etation cover. The event occurred at the beginning of March, with the cultivated areas already covered by incipient vegetation, which prevented the slopes from undergoing heavy soil erosion. This is landformpartially distribution),true also for the in whichPineto sheet-rill2011 area features (in terms did of notlandform form due distribution), to the vegetation in which cover sheet of‑rill the slope,features and did only not gully form areas due to and the major vegetation gullies cover incised of the the slope, slopes. and only gully areas and major gullies incisedSeveral the slopes. studies outlined that the main parameters controlling the soil erosion are related to the interactionSeveral studies among outlined rainfall that parameters the main parameters and lithology, controlling orography, the soil hydrography, erosion are related land-use to the and vegetationinteraction [ 5among–10]. In rainfall this case, parameters considering and thelithology, relatively orography, homogeneous hydrography, lithologic–geomorphological land-use and vegeta- settings,tion [5–10 we]. mainlyIn this outlinedcase, considering the roleof the land-use relatively and homogeneous vegetation cover lithologic (as mentioned–geomorphological above) and set- the controltings, we of rainfallmainly parametersoutlined the (i.e., role cumulative of land-use rainfall) and vegetation on the soil cover erosion. (as mentioned above) and the control of rainfall parameters (i.e., cumulative rainfall) on the soil erosion.

Figure 18. Comparison of soil erosion values (eroded volume, average erosion, sediment loss) and Figure 18. Comparison of soil erosion values (eroded volume, average erosion, sediment loss) and rainfall distribution (cumulative rainfall, max and mean rainfall intensity). The event numbers refer to rainfall distribution (cumulative rainfall, max and mean rainfall intensity). The event numbers refer Table3. to Table 3. We compared the average single-event erosion estimated in the investigated events (0.8–30.8 mm) We compared the average single-event erosion estimated in the investigated events (0.8–30.8 with the short- and long-term erosion rates known in the literature (0.05–90 mm/year [67–75]; mm) with the short- and long-term erosion rates known in the literature (0.05–90 mm/year [67–75]; Table5). The values resulting from this work are comparable to the highest short-term values Table 5). The values resulting from this work are comparable to the highest short-term values known known for badlands (10–30 mm/year) and landslides (30–90 mm/year) from direct measurements for badlands (10–30 mm/year) and landslides (30–90 mm/year) from direct measurements at local at local scale and in small catchments [29,39,41] and about one order of magnitude higher than the scale and in small catchments [29,39,41] and about one order of magnitude higher than the values valuesknown known at basin at scale basin (0.05 scale–8 (0.05–8mm/year mm/year)) obtained obtainedfrom indirect from estimations indirect estimations [37,69,75] and [37, 69the,75 long] and- theterm long-term erosion rates erosion (0.06 rates–15 mm/y (0.06–15ear) mm/year)derived from derived trapped from sediment trapped calculations sediment and calculations morphotec- and morphotectonictonic reconstructions reconstructions [41,67–70] (Table [41,67 –5).70 ]Even (Table if 5these). Even values if theseare related values to are different related types to different of pro- typescesses of (episodic processes vs. (episodic, continue vs., processes), continue processes),taking into takingaccount into that account they affect that comparable they affect comparable lithologic, lithologic,geomorphological geomorphological,, and climatic and environments climatic environments (or in some (or incases, some the cases, same the area, same 10 area, in Table 10 in Table5), this5), thiscomparison comparison highlights highlights that that the the impact impact of ofsoil soil erosion erosion on on the the landscape landscape is islargely largely underestimated, underestimated, particularlyparticularly in in the the long-term long-termanalysis, analysis, inin respectrespect toto what is outlined by the the recent recent heavy heavy rainfall rainfall events. events. Water 2018, 10, 1314 23 of 29

Table 5. Erosion rates in Mediterranean and surrounding areas (modiﬁed after Buccolini et al. [41]).

ID Locality Environment Lithology Method Erosion Rate Period Reference Sediment volume trapped in valleys and 1 Alps Mountain chain Various 1.77 mm/year Late Glacial [67] lake basin Mountain chains, hills and 2 Black Sea source area Various Sediment volume trapped in Black Sea 0.063 mm/year Holocene [68] plains 3 Adriatic Central Italy Main ﬂuvial basin Alluvial deposits Thermochronometry 0.7–1.5 mm/year Last 20,000 year [69] Lac Chambon (Massif 4 Mountain basin Lacustrine deposits Qualitative estimation 0.12 mm/year Last deglaciation [72] Central, France) Direct measures on erosion plots in Present 5 Ebro Basin (NE Spain) Badlands on hilly area Clayey bedrock 5.6–11.2 mm/year [73] badland a (1991–1993) 15–30 mm/year 6 Southern Tuscany Hilly area Clayey bedrock Direct measures Present [18] (badlands) 60–90 mm/year (landslides) Direct measures compared with indirect 10–25 mm/year (badlands) 7 Central Italy Hilly areas Clayey bedrock estimation from geomorphometry of Present [39,74] 30–40 mm/year (landslides) drainage network 8 Europe Various Soil Various 10–20 t ha−1 year−1 (overall) Present [75] 455 t ha−1 year−1 (gully erosion max. value) Slope deposits, clayey 7.8 mm/year Last 20,000 year 9 Mt. Ascensione (Area 1) High hills Radiometric dating and GIS analysis [41] bedrock 15.6 mm/year Holocene Colluvial deposits, clayey 2.4–3 mm/year Last 20,000 year 10 Atri (Area 2) Coastal hills Radiometric dating and GIS analysis [41] bedrock 4.8–6 mm/year Holocene Camastra Clayey–marly–calcareous 1392 T km−2 year−1 11 reservoir Mountain basin Indirect assessment (Tu, RUSLE, USPED) Present [37] bedrock ~1 mm/year Basilicata) Clayey- marly–calcareous 12 Verde Basin Mountain-piedmont basin Indirect assessment (Tu) ~0.05–8.0 mm/year Present [69] bedrock Water 2018, 10, 1314 24 of 29

Concerning the issue of the rainfall threshold for geomorphological effects of heavy rainfall events, which is widely debated in the literature ([43,70,71] and references therein), this work provides a local contribution that could be representative of a larger area. The rainfall thresholds for soil erosion are usually defined for short-term intensities (e.g., from 5.6 to 20 mm/h [70,71]). In this work, even after a few investigated cases, the average erosion vs. cumulative rainfall graph (Figure 18b) suggests a threshold for the triggering of heavy soil erosion features around 100–110 mm of cumulative rainfall in ~1-day events, according to the intersection of the correlation line with the x-axis (excluding the 2011 Salinello event, 2a; Figure 18). The hourly intensity threshold is less defined in this case (scattered distribution in Figure 18b) at roughly ~30–40 mm/h. These values are more meaningful for agricultural areas (e.g., arable lands, vineyards, olive groves) during the plowed stage (in autumn and winter) than during the unplowed vegetative stage (spring and summer), when soil erosion distribution is less intense. These results can also be representative of larger areas, considering that the geologic and geomorphological features of the investigated areas are typical of large parts of the Adriatic clay hills. When compared with other direct and indirect methods applied to soil erosion [36–42], the direct field survey approach appears to be suitable for intermediate-scale (several km2) investigations and is useful for assessing the impact of soil erosion to hilly and agricultural landscapes. Other direct observations provide greater advantages at the local scale (~1 km2 and less) and in small basin investigations, while indirect estimations are more suitable for large-scale (hundreds of km2) soil erosion assessment of large drainage basins and entire physiographic domains. Moreover, the used approach is more suitable for single or multiple event investigations as in this work, and for applied issues, while other direct and indirect methods provide average long-term and short-term assessments that, however, might underestimate the assessment of soil erosion and its impact on the landscape.

6. Conclusions In this work, we investigated three ~1-day heavy rainfall events (combined or not with antecedent rainfall) in 2007, 2011, and 2012 in different areas along the clayey hilly-coastal NE Abruzzo area, largely characterized by agricultural areas and urban tourist areas (affected by strong urbanization along coast and hilly area). The events were investigated in terms of cumulative rainfall, intensity and duration, triggered geomorphological effects (soil erosion and accumulation), and average erosion. The investigation offers several contributions to widely debated issues: (1) soil erosion assessment and impact on clayey landscapes typical of the entire Adriatic coastal hilly area; (2) comparison of estimated erosion with short- and long-term erosion rates in the Mediterranean area; and (3) rainfall triggering threshold for heavy soil erosion. The soil erosion assessment shows average erosion from 0.08 cm to 3.08 cm in a single ~1-day heavy rainfall event (Figure 18). The erosion values show a good comparison (even if over a few events) with cumulative rainfall and not with peak rainfall intensity. The correlation is significant for the hilly coastal areas (Tortoreto, Pineto) but not for the Salinello area, incorporating the lower part of a main river valley and processes that are more connected to the main river than to the hillslopes. More specifically, the geomorphological features of the 2011 event, which occurred in March with partly vegetated cropland, outline the positive contribution (reduced erosion values) of vegetated landscape for preventing slopes from heavy soil erosion with respect to other events occurring in a heavily plowed landscape. The average erosion values obtained for single events are comparable to the annual erosion rates obtained from previous studies for badlands (10–30 mm/y) and up to about one order of magnitude greater than the basin scale values (0.05–8 mm/y) and the long-term erosion rates (0.06–15 mm/y). Since these events occurred several times in the last decades and heavy rainfall is one of the main issues in future scenarios in terms of climate change, it is possible to outline the underestimation of the impact of soil erosion on the Adriatic hilly landscapes. This is fundamental in areas largely dedicated to agriculture, including high-quality plantations (e.g., olive groves, vineyards), and urban tourist Water 2018, 10, 1314 25 of 29 areas, which are also suffering a strong loss of free soil due to land-use change and urbanization in recent years. In terms of a heavy soil erosion triggering thresholds for clayey hilly landscapes, the correlation of cumulative rainfall and average erosion shows an expected threshold value as high as ~100–110 mm/day (Figure 18). This value, even if it results from a few local cases, could be representative of larger areas of the Adriatic clay hills with a similar morphoclimatic setting and it is comparable with the results of previous works, although mostly defining hourly rainfall intensity thresholds. Finally, comparing the field mapping approach used in this work with other direct and indirect approaches for soil erosion assessment, it appears to be suitable mostly for intermediate-scale investigations and for single-multiple events analysis and can be used to verify and calibrate the evaluation of erosion rates with other methods. In further development of this research, we expect to analyze new events, which occasionally affect the Adriatic clayey and sandy hills in order to verify the results. Moreover, we expect to compare the direct investigations with erosion modeling, and to use this approach to outline future scenarios of soil erosion impact in the framework of climate changes in terms of the debated issue of increasing heavy rainfall events.

Author Contributions: Conceptualization and supervision, E.M. and V.M.; rainfall and geomorphological analysis, GIS mapping, T.P.; rainfall analysis, A.G. and V.M.; discussion and conclusion, T.P., E.M., A.G., and V.M.; writing—original draft preparation, review and editing, T.P. Funding: This research and the APC were funded by Università degli Studi “G. d’Annunzio” Chieti-Pescara, grant Miccadei and grant Piacentini. Acknowledgments: The authors wish to thank F. Fonzi, O. Ranalli, and V. Mancinelli, who contributed the geomorphological survey and rainfall analysis. They are also very grateful to the three anonymous reviewers, whose precious comments greatly improved this paper. The rainfall data were provided by the Functional Center and Hydrographic Office of the Abruzzo Region (Centro Funzionale e Ufficio Idrografico Regione Abruzzo). The topographic data and aerial photos were provided by the Cartographic Office of the Abruzzo Region by means of the OpenGeoData Portal (http://opendata.regione.abruzzo.it/). Conflicts of Interest: The authors declare no conflict of interest.

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