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Paolo Billi

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Francesco Fidolini

Università degli Studi di Firenze (University of Florence)

Williams Themison

James Cook University

Filippo D'Oriano

Małgorzata Pisarska-jamroży

Adam Mickiewicz University in Poznań

Muséum National d'Histoire Naturelle

hasan sözbilir

Dokuz Eylül University

Jutta Winsemann

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Papers by Paolo Billi

PLEISTOCENE LACUSTRINE FAN DELTA DEPOSITS OF THE VALDARNO BASIN, ITALY l

JOURNAL OF SEDIMENTARY PETROLOGY, 1991

During the Early Pleistocene, mouth bar fan deltas formed on the northern margin of the shallow w... more During the Early Pleistocene, mouth bar fan deltas formed on the northern margin of the shallow water lacustrine Valdarno Basin, which developed during the last phases of the Northern Apennines evolution. One fan delta, studied in detail here, is a coarse-grained, fluvial-dominated system and shows characteristics different from the classic Gilbert-type model. The apex is morphologically and sedimentologically similar to those reported for alluvial fans of humid climates. Very course-grained, poorly organized deposits, dumped as lobes during intense floods or laid down in braided channels, predominate. Downfan, in the transition zone, braided channels change into stable straight channels lacking evidence of lateral accretion. These consist of lenticular pebbly sand and sandy bodies included within massive mud and fine sand. In the subaqueons zone the straight distributary channels, cut into muddy lacustrine deposits, are filled with sand. Distally, such channels give way to lenticular bodies composed of sands arranged into coarsening-and thickening-upward sequences. The occurrence of drowned channels is explained by scouting during periods of low water stand or due to hyperpycnal flows. The very shallow water, the small size, and the low energy of the lake favored the formation of mouth-bar fan deltas instead of Gilbert-type foresets.

HYDRO-MORPHOLOGY OF DISCONTINUOUS GULLIES: AN ETHIOPIAN EXAMPLEASTSN Gullies Ethiopia

Atti Soc. Tosc. Sci. Nat., Mem., Serie A,, 2017

Gully erosion is very common in Ehtiopia affecting large areas with different geomorphological, p... more Gully erosion is very common in Ehtiopia affecting large areas with
different geomorphological, pedological and climatic characteristics.
The amount of soil loss due to gullying has become a very serious
problem in the recent decades leading to remarkable depletion of cultivated
land. Field investigations on gully morphological development
were carried out in two study areas of Ethiopia, the Lakes Region in
the Main Ethiopian Rift Valley and the central highlands, representative
of different geo-morphological and environmental conditions.
Three main types of discontinuous gullies were identifies on the basis
of their morphological characteristics. In order to investigate the main
causes originating the different types of gullies, data on geomorphic
parameters were collected in the field. From the analysis of field data
hypotheses on the mechanisms responsible for gully development in
the study areas were derived. The short- and mid-term gully expansion
rate and the main processes involved are discussed as well.

A new early warning drought index for Ethiopia

Journal of Water and Climate Change |, 2018

This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A n... more This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A new index based on the rainfall delay (Rd) with respect to the expected onset (and traditional) seeding time and other indices, i.e., the aridity index and the Z-score, alternatives to the Standard Precipitation Index (SPI), are used to test the validity of the new Rd index in identifying severe droughts extending back to 1955. Although only data of rain gauges located in the district of Dire Dawa were used, they proved, albeit with different accuracies, able to identify nationwide droughts.

Sediment transport of the Blue Nile at Khartoum

Quaternary International, 2010

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

Rhythmic roughness elements and channel morphology of gravel bed rivers

with 17 figures and 3 tables Summary. A first aim of this paper is to investigate the existence, ... more with 17 figures and 3 tables Summary. A first aim of this paper is to investigate the existence, if any, and the parameters defining the occurrence conditions of a continuum of rhythmic, transverse roughness elements of gravel-bed rivers such as transverse ribs, step-pools and riffle and pool sequences. The investigation is carried out by the analysis of a large data set consisting of almost all the data available in the literature and unpublished field data measured by the authors on a few rivers in the eastern Alps and the northern Apennines. Open questions about the origin of these bedforms are considered and tested against the data set available. A few equations relating the main geomorphic parameters are obtained. They can be used for river restoration and paleohydrological inference. The jamming factor (ratio between the size of the coarser particle and channel width) is found as a relevant parameter in the development of the bedforms considered. The conditions and the interplays between the jamming factors and others parameters such as bedform wavelength and streambed gradient are considered in order to discriminate the fields of existence of the larger roughness transverse, cyclic elements of gravel bed rivers and to show that transverse ribs, step-pools and riffle and pool sequences are part of a coarse grained bedform continuum.

Channel processes and sedimentology of a boulder-bed ephemeral stream in the western Afar margin

Zeitschrift für Geomorphologie, 2016

Few papers report and very little is known about the geomorphology and sedimentology of present d... more Few papers report and very little is known about the geomorphology and sedimentology of present day very coarse-grained, ephemeral streams. This paper aims to shed some light on fluvial processes acting in this kind of drylands rivers and to provide some insight into their sedimentological characteristics as a diagnostic tool for the interpretation of ancient deposits. Such topics are investigated in a field study on the Golina River, a sandy boulderbed ephemeral stream of the Kobo basin in northern Ethiopia, subjected to intermittent flow generated by episodic, high intensity rainfall. Though the main gemorphological characteristics of the braid bars and channels are apparently similar to those of perennial rivers, field investigations show that the channel morphology and sedimentology of the study reach result from different depositional/erosion processes. A model based on the superimposition of coarse-grained bedload sheets, with the characteristics described by Whiting et al. (1988), and subsequent dissection during the receding flood flow is considered. This model was found to explain well the morphological and sedimentological features observed in the investigated boulder-bed, ephemeral stream.

Geomorphology of the Archaeological Area of Aksum

World Geomorphological Landscapes, 2015

Landforms and landscapes vary enormously across the Earth, from high mountains to endless plains.... more Landforms and landscapes vary enormously across the Earth, from high mountains to endless plains. At a smaller scale, nature often surprises us by creating shapes which look improbable. Many physical landscapes are so immensely beautiful that they received the highest possible recognition-they hold the status of World Heritage Sites. Apart from often being immensely scenic, landscapes tell stories which not uncommonly can be traced back in time for tens of millions of years and include unique geological events such as meteorite impacts. In addition, many landscapes owe their appearance and harmony not solely to the natural forces. For centuries, and even millennia, they have been shaped by humans who have modified hillslopes, river courses and coastlines, and erected structures which often blend with the natural landforms to form inseparable entities. These landscapes are studied by geomorphology-'the science of scenery'-a part of Earth Sciences that focuses on landforms, their assemblages, surface and subsurface processes that moulded them in the past and that change them today. To show the importance of geomorphology in understanding the landscape, and to present the beauty and diversity of the geomorphological sceneries across the world, we have launched a book series World Geomorphological Landscapes. It aims to be a scientific library of monographs that present and explain physical landscapes, focusing on both representative and uniquely spectacular examples. Each book will contain details on geomorphology of a particular country or a geographically coherent region. This volume presents the impressive geomorphic legacy of Ethiopia which hosts many landscapes and landforms of global significance. Examples include the otherworldly Afar Depression with its sun-baked volcanoes, high-elevation basalt plateaus, the East African Rift valley with its splendid lakes, to name just a few. Ethiopia is also a country where geomorphology and people have remained in particularly close association since time immemorial. These relationships can be examined by referring to the pastas demonstrated by the chapter on geoarcheology of Aksum, but perhaps more importantly, with the reference to the present-day environmental problems arising from land use, soil erosion, water resources depletion and settlement growth. A number of chapters in this book remind us that geomorphological landscapes are not only beautiful; they are also very fragile if used improperly. The World Geomorphological Landscapes series is produced under the scientific patronage of the International Association of Geomorphologists (IAG)-a society that brings together geomorphologists from all around the world. The IAG was established in 1989 and is an independent scientific association affiliated with the International Geographical Union (IGU) and the International Union of Geological Sciences (IUGS). Among its main aims are to promote geomorphology and to foster dissemination of geomorphological knowledge. I believe that this lavishly illustrated series, which keeps to the scientific rigour, is the most appropriate means to fulfil these aims and to serve the geoscientific community. To this end, my great thanks go to Prof. Paolo Billi, a person long involved in geomorphological research in Ethiopia, for agreeing to coordinate this timely volume in the series. I am also very grateful to all individual authors who accepted invitations to contribute and, often, delivered stories which contained original, not yet published research.

Dynamics of gravel-bed rivers / edited by P. Billi ... [et al. ]

ABSTRACT

Morphology and sediment dynamics of ephemeral stream terminal distributary systems in the Kobo Basin (northern Welo, Ethiopia)

Geomorphology, 2007

... Although our knowledge of the distributary systems of ephemeral rivers has greatly increased ... more

Geomorphological investigation on gully erosion in the Rift Valley and the northern highlands of Ethiopia

CATENA, 2003

Gully erosion phenomena are very common in Ethiopia. They affect large areas with different morph... more Gully erosion phenomena are very common in Ethiopia. They affect large areas with different morphological, pedological and climatic characteristics. The amount of soil loss due to gullying has become a very serious problem in the recent decades as it was associated to remarkable depletion of cultivated land. Field investigations on gully morphology and its genetic processes were carried out in two study areas of Ethiopia, representative of different geo-environmental conditions: the Lakes Region in the Rift portion north of Shashamene and the area surrounding the town of Mekele in Tigray. Two main types of gullies were identified on the basis of their morphological and hydraulic geometry characteristics: (1) discontinuous gullies which generally develop on low gradient slopes (1-5j on average) and the hydraulic radius of which increases from an upstream minimum to a maximum, at approximately their mid length, and decreases again to a relative minimum at their downstream end; and (2) stream gullies, formed by deep erosion processes typically migrating upslope. In order to investigate the main causes originating the different types of gullies, shear stress data were collected in the field from their hydraulic geometry. Hypotheses on the mechanisms responsible for both discontinuous and stream gullies development and for their different characteristics are discussed considering the pattern of shear stress variation in the downstream direction.

Bedload transport in the instrumented catchment of the Rio Cordon

CATENA, 1999

The paper illustrates the results of 10 years of research conducted on the Rio Cordon catchment (... more The paper illustrates the results of 10 years of research conducted on the Rio Cordon catchment (5 km 2), where an experimental station has been operating for the integral and continuous automatic recording of water discharge and coarse sediment transport. The device operates by separating bedload from water discharge and fine sediment and subsequently measuring the two solid components. The recorded floods are grouped in two distinct categories according to the bedload transport rates: 'ordinary' and 'exceptional' floods (with return times exceeding 30-50 years). Based on data collected and field studies, the aspects characterising threshold conditions of bedload transport at flood initiation and cessation are illustrated. Then, a description is given of the conditions which are necessary for two distinct mechanisms of incipient motion of bed particles to occur: selective transport and equal mobility of stream bed material.

An investigation of the effects of breakwaters on beach sediment characteristics

CATENA, 1984

The south-east reach of Castiglione della Pescaia beach, Italy, has been studied to demonstrate t... more The south-east reach of Castiglione della Pescaia beach, Italy, has been studied to demonstrate the effect of the breakwaters on sediment characteristics as regards as the cuspate shape of the shoreline. Several samples have been collected to investigate the longshore distribution of sediment parameters such as mean size, sorting and skewness. In addition~ comparison with data from natural cuspate beaches has been made. It shows an opposite sediment size distribution while sorting and skewness are almost the same for cusps and bays. That is a consequence of different hydrodynamic processes evenly resulting in a similar shoreline morphology. The grain size analysis has also allowed to state that the sediment of the protected beach is substantially of the same quality of that where natural conditions prevail and that the influence of breakwaters can be neglected in terms of skewness sign. Statistical tests, using mean size, confirmed the main longshore direction of sediment transport, calculated theoretically. Core samples have been taken in order to examine the subsurface sediment characteristics in the breakwater protection area. The core sediments show a predominant positive skewness while the grab and the berm samples are more frequently negatively skewed. The analysis of core stratigraphy taken in different periods shows a sea-bed scour of at least 1 m in the aggradational protected area during ordinary sea-storms.

A note on cluster bedform behaviour in a gravel-bed river

CATENA, 1988

The behaviour of cluster bedforms was investigated in the Farma R., Central Italy. A study reach ... more The behaviour of cluster bedforms was investigated in the Farma R., Central Italy. A study reach was surveyed and the three components of the pebble dusters occuring on the surface of a gravel bar were painted using different colours. Before and after each flood all the clusters were photographed and a comparison was made. This simple methodology allowed the evolution of the painted dusters to be followed and also allowed a survey of the dislodgment of the clusters particles and the addition of new ones. The experiment shows that the mobility of the wake grains is higher than that of the stoss particles. A pebble cluster generally loses the wake grain first, followed by the stoss .pebbles. The obstacel clast is commonly the last to be entrained and acts as a single particle after the wake and stoss particles have been dislodged. Different explanations of this unexpected behaviour are postulated and the interaction of pebble with bed armouring and bedload size distribution is discussed.

A new early warning drought index for Ethiopia

Journal of Water and Climate Change, 2018

This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A n... more This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A new index based on the rainfall delay (Rd) with respect to the expected onset (and traditional) seeding time and other indices, i.e., the aridity index and the Z-score, alternatives to the Standard Precipitation Index (SPI), are used to test the validity of the new Rd index in identifying severe droughts extending back to 1955. Although only data of rain gauges located in the district of Dire Dawa were used, they proved, albeit with different accuracies, able to identify nation-wide droughts.

Long-term soil erosion modelling of a cultivated terraced system in the area of Aksum (Ethiopia), with the support of archaeological data

by Rossano Ciampalini, Lorenzo Borselli, and Paolo Billi

The evolution of agricultural landscapes is the result of soil redistribution processes caused by... more The evolution of agricultural landscapes is the result of soil redistribution processes caused by water erosion and human activity. Some agricultural areas, well-documented by historical and archaeological data, are useful to understand the evolution of the landscape structures and soil erosion dynamics. This research has been developed in one of the most relevant archaeological areas of East Africa, the site of Aksum (Ethiopia), where the flourishing and decay of the Aksumite Kingdom (400BC 800AD) took place. This site provided favorable conditions for a long-term approach to the study of soil conservation techniques, since such techniques have been maintained from ancient times to the present. In the current study, the surfaces of three terrace systems have been surveyed and analyzed in terms of soil loss, with the plough mark methodology (Ciampalini et al., 2008(Ciampalini et al., , 2012, providing the amount of soil loss within the last two millennia. In this context, we performed long-term modelling for tillage and water erosion with the expert model, LandSoil (Ciampalini et al., in press), which is based on a raster distributed approach and accounts for the principal components in soil erosion-deposition processes.

Increased frequency of flash floods in Dire Dawa, Ethiopia: Change in rainfall intensity or human impact?

by Rossano Ciampalini and Paolo Billi

Natural Hazards, SPRINGER, Apr 1, 2015

In the first decade of the twenty-first century, Ethiopia has been subjected to an increased fre... more In the first decade of the twenty-first century, Ethiopia has been subjected to an
increased frequency of flash floods, especially in the town of Dire Dawa. The results of
international organizations studies point to no evidence of a climate-driven change in the
magnitude/frequency of floods, though increases in runoff and risk of floods in East Africa are expected. Flash floods are posing constraints to the economic growth and the development process of a low-income country such as Ethiopia, and, in order to mitigate such hazard, it is crucial to understand the relative roles of two main factors: rainfall intensity and land use change. This study analyses the recent trends of rainfall intensity across
Ethiopia and investigates the relative role of rainfall intensity and land use change in
augmenting the frequency of flash flooding of the town of Dire Dawa by the Dechatu
River. Results indicate that the increase in rainfall intensity is a more important factor than land use change in controlling the increased frequency of flash flood in Dire Dawa.

PLEISTOCENE LACUSTRINE FAN DELTA DEPOSITS OF THE VALDARNO BASIN, ITALY l

JOURNAL OF SEDIMENTARY PETROLOGY, 1991

During the Early Pleistocene, mouth bar fan deltas formed on the northern margin of the shallow w... more During the Early Pleistocene, mouth bar fan deltas formed on the northern margin of the shallow water lacustrine Valdarno Basin, which developed during the last phases of the Northern Apennines evolution. One fan delta, studied in detail here, is a coarse-grained, fluvial-dominated system and shows characteristics different from the classic Gilbert-type model. The apex is morphologically and sedimentologically similar to those reported for alluvial fans of humid climates. Very course-grained, poorly organized deposits, dumped as lobes during intense floods or laid down in braided channels, predominate. Downfan, in the transition zone, braided channels change into stable straight channels lacking evidence of lateral accretion. These consist of lenticular pebbly sand and sandy bodies included within massive mud and fine sand. In the subaqueons zone the straight distributary channels, cut into muddy lacustrine deposits, are filled with sand. Distally, such channels give way to lenticular bodies composed of sands arranged into coarsening-and thickening-upward sequences. The occurrence of drowned channels is explained by scouting during periods of low water stand or due to hyperpycnal flows. The very shallow water, the small size, and the low energy of the lake favored the formation of mouth-bar fan deltas instead of Gilbert-type foresets.

HYDRO-MORPHOLOGY OF DISCONTINUOUS GULLIES: AN ETHIOPIAN EXAMPLEASTSN Gullies Ethiopia

Atti Soc. Tosc. Sci. Nat., Mem., Serie A,, 2017

Gully erosion is very common in Ehtiopia affecting large areas with different geomorphological, p... more Gully erosion is very common in Ehtiopia affecting large areas with
different geomorphological, pedological and climatic characteristics.
The amount of soil loss due to gullying has become a very serious
problem in the recent decades leading to remarkable depletion of cultivated
land. Field investigations on gully morphological development
were carried out in two study areas of Ethiopia, the Lakes Region in
the Main Ethiopian Rift Valley and the central highlands, representative
of different geo-morphological and environmental conditions.
Three main types of discontinuous gullies were identifies on the basis
of their morphological characteristics. In order to investigate the main
causes originating the different types of gullies, data on geomorphic
parameters were collected in the field. From the analysis of field data
hypotheses on the mechanisms responsible for gully development in
the study areas were derived. The short- and mid-term gully expansion
rate and the main processes involved are discussed as well.

A new early warning drought index for Ethiopia

Journal of Water and Climate Change |, 2018

This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A n... more This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A new index based on the rainfall delay (Rd) with respect to the expected onset (and traditional) seeding time and other indices, i.e., the aridity index and the Z-score, alternatives to the Standard Precipitation Index (SPI), are used to test the validity of the new Rd index in identifying severe droughts extending back to 1955. Although only data of rain gauges located in the district of Dire Dawa were used, they proved, albeit with different accuracies, able to identify nationwide droughts.

Sediment transport of the Blue Nile at Khartoum

Quaternary International, 2010

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

Rhythmic roughness elements and channel morphology of gravel bed rivers

with 17 figures and 3 tables Summary. A first aim of this paper is to investigate the existence, ... more with 17 figures and 3 tables Summary. A first aim of this paper is to investigate the existence, if any, and the parameters defining the occurrence conditions of a continuum of rhythmic, transverse roughness elements of gravel-bed rivers such as transverse ribs, step-pools and riffle and pool sequences. The investigation is carried out by the analysis of a large data set consisting of almost all the data available in the literature and unpublished field data measured by the authors on a few rivers in the eastern Alps and the northern Apennines. Open questions about the origin of these bedforms are considered and tested against the data set available. A few equations relating the main geomorphic parameters are obtained. They can be used for river restoration and paleohydrological inference. The jamming factor (ratio between the size of the coarser particle and channel width) is found as a relevant parameter in the development of the bedforms considered. The conditions and the interplays between the jamming factors and others parameters such as bedform wavelength and streambed gradient are considered in order to discriminate the fields of existence of the larger roughness transverse, cyclic elements of gravel bed rivers and to show that transverse ribs, step-pools and riffle and pool sequences are part of a coarse grained bedform continuum.

Channel processes and sedimentology of a boulder-bed ephemeral stream in the western Afar margin

Zeitschrift für Geomorphologie, 2016

Few papers report and very little is known about the geomorphology and sedimentology of present d... more Few papers report and very little is known about the geomorphology and sedimentology of present day very coarse-grained, ephemeral streams. This paper aims to shed some light on fluvial processes acting in this kind of drylands rivers and to provide some insight into their sedimentological characteristics as a diagnostic tool for the interpretation of ancient deposits. Such topics are investigated in a field study on the Golina River, a sandy boulderbed ephemeral stream of the Kobo basin in northern Ethiopia, subjected to intermittent flow generated by episodic, high intensity rainfall. Though the main gemorphological characteristics of the braid bars and channels are apparently similar to those of perennial rivers, field investigations show that the channel morphology and sedimentology of the study reach result from different depositional/erosion processes. A model based on the superimposition of coarse-grained bedload sheets, with the characteristics described by Whiting et al. (1988), and subsequent dissection during the receding flood flow is considered. This model was found to explain well the morphological and sedimentological features observed in the investigated boulder-bed, ephemeral stream.

Geomorphology of the Archaeological Area of Aksum

World Geomorphological Landscapes, 2015

Landforms and landscapes vary enormously across the Earth, from high mountains to endless plains.... more Landforms and landscapes vary enormously across the Earth, from high mountains to endless plains. At a smaller scale, nature often surprises us by creating shapes which look improbable. Many physical landscapes are so immensely beautiful that they received the highest possible recognition-they hold the status of World Heritage Sites. Apart from often being immensely scenic, landscapes tell stories which not uncommonly can be traced back in time for tens of millions of years and include unique geological events such as meteorite impacts. In addition, many landscapes owe their appearance and harmony not solely to the natural forces. For centuries, and even millennia, they have been shaped by humans who have modified hillslopes, river courses and coastlines, and erected structures which often blend with the natural landforms to form inseparable entities. These landscapes are studied by geomorphology-'the science of scenery'-a part of Earth Sciences that focuses on landforms, their assemblages, surface and subsurface processes that moulded them in the past and that change them today. To show the importance of geomorphology in understanding the landscape, and to present the beauty and diversity of the geomorphological sceneries across the world, we have launched a book series World Geomorphological Landscapes. It aims to be a scientific library of monographs that present and explain physical landscapes, focusing on both representative and uniquely spectacular examples. Each book will contain details on geomorphology of a particular country or a geographically coherent region. This volume presents the impressive geomorphic legacy of Ethiopia which hosts many landscapes and landforms of global significance. Examples include the otherworldly Afar Depression with its sun-baked volcanoes, high-elevation basalt plateaus, the East African Rift valley with its splendid lakes, to name just a few. Ethiopia is also a country where geomorphology and people have remained in particularly close association since time immemorial. These relationships can be examined by referring to the pastas demonstrated by the chapter on geoarcheology of Aksum, but perhaps more importantly, with the reference to the present-day environmental problems arising from land use, soil erosion, water resources depletion and settlement growth. A number of chapters in this book remind us that geomorphological landscapes are not only beautiful; they are also very fragile if used improperly. The World Geomorphological Landscapes series is produced under the scientific patronage of the International Association of Geomorphologists (IAG)-a society that brings together geomorphologists from all around the world. The IAG was established in 1989 and is an independent scientific association affiliated with the International Geographical Union (IGU) and the International Union of Geological Sciences (IUGS). Among its main aims are to promote geomorphology and to foster dissemination of geomorphological knowledge. I believe that this lavishly illustrated series, which keeps to the scientific rigour, is the most appropriate means to fulfil these aims and to serve the geoscientific community. To this end, my great thanks go to Prof. Paolo Billi, a person long involved in geomorphological research in Ethiopia, for agreeing to coordinate this timely volume in the series. I am also very grateful to all individual authors who accepted invitations to contribute and, often, delivered stories which contained original, not yet published research.

Dynamics of gravel-bed rivers / edited by P. Billi ... [et al. ]

ABSTRACT

Morphology and sediment dynamics of ephemeral stream terminal distributary systems in the Kobo Basin (northern Welo, Ethiopia)

Geomorphology, 2007

... Although our knowledge of the distributary systems of ephemeral rivers has greatly increased ... more

Geomorphological investigation on gully erosion in the Rift Valley and the northern highlands of Ethiopia

CATENA, 2003

Gully erosion phenomena are very common in Ethiopia. They affect large areas with different morph... more Gully erosion phenomena are very common in Ethiopia. They affect large areas with different morphological, pedological and climatic characteristics. The amount of soil loss due to gullying has become a very serious problem in the recent decades as it was associated to remarkable depletion of cultivated land. Field investigations on gully morphology and its genetic processes were carried out in two study areas of Ethiopia, representative of different geo-environmental conditions: the Lakes Region in the Rift portion north of Shashamene and the area surrounding the town of Mekele in Tigray. Two main types of gullies were identified on the basis of their morphological and hydraulic geometry characteristics: (1) discontinuous gullies which generally develop on low gradient slopes (1-5j on average) and the hydraulic radius of which increases from an upstream minimum to a maximum, at approximately their mid length, and decreases again to a relative minimum at their downstream end; and (2) stream gullies, formed by deep erosion processes typically migrating upslope. In order to investigate the main causes originating the different types of gullies, shear stress data were collected in the field from their hydraulic geometry. Hypotheses on the mechanisms responsible for both discontinuous and stream gullies development and for their different characteristics are discussed considering the pattern of shear stress variation in the downstream direction.

Bedload transport in the instrumented catchment of the Rio Cordon

CATENA, 1999

The paper illustrates the results of 10 years of research conducted on the Rio Cordon catchment (... more The paper illustrates the results of 10 years of research conducted on the Rio Cordon catchment (5 km 2), where an experimental station has been operating for the integral and continuous automatic recording of water discharge and coarse sediment transport. The device operates by separating bedload from water discharge and fine sediment and subsequently measuring the two solid components. The recorded floods are grouped in two distinct categories according to the bedload transport rates: 'ordinary' and 'exceptional' floods (with return times exceeding 30-50 years). Based on data collected and field studies, the aspects characterising threshold conditions of bedload transport at flood initiation and cessation are illustrated. Then, a description is given of the conditions which are necessary for two distinct mechanisms of incipient motion of bed particles to occur: selective transport and equal mobility of stream bed material.

An investigation of the effects of breakwaters on beach sediment characteristics

CATENA, 1984

The south-east reach of Castiglione della Pescaia beach, Italy, has been studied to demonstrate t... more The south-east reach of Castiglione della Pescaia beach, Italy, has been studied to demonstrate the effect of the breakwaters on sediment characteristics as regards as the cuspate shape of the shoreline. Several samples have been collected to investigate the longshore distribution of sediment parameters such as mean size, sorting and skewness. In addition~ comparison with data from natural cuspate beaches has been made. It shows an opposite sediment size distribution while sorting and skewness are almost the same for cusps and bays. That is a consequence of different hydrodynamic processes evenly resulting in a similar shoreline morphology. The grain size analysis has also allowed to state that the sediment of the protected beach is substantially of the same quality of that where natural conditions prevail and that the influence of breakwaters can be neglected in terms of skewness sign. Statistical tests, using mean size, confirmed the main longshore direction of sediment transport, calculated theoretically. Core samples have been taken in order to examine the subsurface sediment characteristics in the breakwater protection area. The core sediments show a predominant positive skewness while the grab and the berm samples are more frequently negatively skewed. The analysis of core stratigraphy taken in different periods shows a sea-bed scour of at least 1 m in the aggradational protected area during ordinary sea-storms.

A note on cluster bedform behaviour in a gravel-bed river

CATENA, 1988

The behaviour of cluster bedforms was investigated in the Farma R., Central Italy. A study reach ... more The behaviour of cluster bedforms was investigated in the Farma R., Central Italy. A study reach was surveyed and the three components of the pebble dusters occuring on the surface of a gravel bar were painted using different colours. Before and after each flood all the clusters were photographed and a comparison was made. This simple methodology allowed the evolution of the painted dusters to be followed and also allowed a survey of the dislodgment of the clusters particles and the addition of new ones. The experiment shows that the mobility of the wake grains is higher than that of the stoss particles. A pebble cluster generally loses the wake grain first, followed by the stoss .pebbles. The obstacel clast is commonly the last to be entrained and acts as a single particle after the wake and stoss particles have been dislodged. Different explanations of this unexpected behaviour are postulated and the interaction of pebble with bed armouring and bedload size distribution is discussed.

A new early warning drought index for Ethiopia

Journal of Water and Climate Change, 2018

This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A n... more This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A new index based on the rainfall delay (Rd) with respect to the expected onset (and traditional) seeding time and other indices, i.e., the aridity index and the Z-score, alternatives to the Standard Precipitation Index (SPI), are used to test the validity of the new Rd index in identifying severe droughts extending back to 1955. Although only data of rain gauges located in the district of Dire Dawa were used, they proved, albeit with different accuracies, able to identify nation-wide droughts.

Long-term soil erosion modelling of a cultivated terraced system in the area of Aksum (Ethiopia), with the support of archaeological data

by Rossano Ciampalini, Lorenzo Borselli, and Paolo Billi

The evolution of agricultural landscapes is the result of soil redistribution processes caused by... more The evolution of agricultural landscapes is the result of soil redistribution processes caused by water erosion and human activity. Some agricultural areas, well-documented by historical and archaeological data, are useful to understand the evolution of the landscape structures and soil erosion dynamics. This research has been developed in one of the most relevant archaeological areas of East Africa, the site of Aksum (Ethiopia), where the flourishing and decay of the Aksumite Kingdom (400BC 800AD) took place. This site provided favorable conditions for a long-term approach to the study of soil conservation techniques, since such techniques have been maintained from ancient times to the present. In the current study, the surfaces of three terrace systems have been surveyed and analyzed in terms of soil loss, with the plough mark methodology (Ciampalini et al., 2008(Ciampalini et al., , 2012, providing the amount of soil loss within the last two millennia. In this context, we performed long-term modelling for tillage and water erosion with the expert model, LandSoil (Ciampalini et al., in press), which is based on a raster distributed approach and accounts for the principal components in soil erosion-deposition processes.

Increased frequency of flash floods in Dire Dawa, Ethiopia: Change in rainfall intensity or human impact?

by Rossano Ciampalini and Paolo Billi

Natural Hazards, SPRINGER, Apr 1, 2015

In the first decade of the twenty-first century, Ethiopia has been subjected to an increased fre... more In the first decade of the twenty-first century, Ethiopia has been subjected to an
increased frequency of flash floods, especially in the town of Dire Dawa. The results of
international organizations studies point to no evidence of a climate-driven change in the
magnitude/frequency of floods, though increases in runoff and risk of floods in East Africa are expected. Flash floods are posing constraints to the economic growth and the development process of a low-income country such as Ethiopia, and, in order to mitigate such hazard, it is crucial to understand the relative roles of two main factors: rainfall intensity and land use change. This study analyses the recent trends of rainfall intensity across
Ethiopia and investigates the relative role of rainfall intensity and land use change in
augmenting the frequency of flash flooding of the town of Dire Dawa by the Dechatu
River. Results indicate that the increase in rainfall intensity is a more important factor than land use change in controlling the increased frequency of flash flood in Dire Dawa.