Paolo Billi

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

Abstract 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 (1–5° 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.

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

Abstract 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 clusters 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 clusters 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.

Environmental conditions and human drivers for changes to north Ethiopian mountain landscapes over 145 years

As quantitative or spatially distributed studies of environmental change over truly long-term periods of more than 100 years are extremely rare, we re-photographed 361 landscapes that appear on historical photographs (1868-1994) within a 40,000 km(2) study area in northern Ethiopia. Visible evidence of environmental changes apparent from the paired photographs was analyzed using an expert rating system. The conditions of the woody vegetation, soil and water conservation structures and land management were worse in the earlier periods compared to their present conditions. The cover by indigenous trees is a notable exception: it peaked in the 1930s, declined afterwards and then achieved a second peak in the early 21st century. Particularly in areas with greater population densities, there has been a significant increase in woody vegetation and soil and water conservation structures over the course of the study period. We conclude that except for an apparent upward movement of the upper tree limit, the direct human impacts on the environment are overriding the effects of climate change in the north Ethiopian highlands and that the northern Ethiopian highlands are currently greener than at any other time in the last 145 years.

Morphodynamics of a pseudomeandering gravel bar reach

Abstract A large number of rivers in Tuscany have channel planforms, which are neither straight nor what is usually understood as meandering. In the typical case, they consist of an almost straight, slightly incised main channel fringed with large lateral bars and lunate-shaped embayments eroded into the former flood plain. In the past, these rivers have not been recognised as an individual category and have often been considered to be either braided or meandering. It is suggested here that this type of river planform be termed pseudomeandering . A typical pseudomeandering river (the Cecina River) is described and analysed to investigate the main factors responsible for producing this channel pattern. A study reach (100×300 m) was surveyed in detail and related to data on discharge, channel changes after floods and grain-size distribution of bed sediments. During 18 months of topographic monitoring, the inner lateral bar in the study reach expanded and migrated towards the concave outer bank which, concurrently, retreated by as much as 25 m. A sediment balance was constructed to analyse bar growth and bank retreat in relation to sediment supply and channel morphology. The conditions necessary to maintain the pseudomeandering morphology of these rivers by preventing them from developing a meandering planform, are discussed and interpreted as a combination of a few main factors such as the flashy character of floods, sediment supply (influenced by both natural processes and human impact), the morphological effects of discharges with contrasting return intervals and the short duration of flood events. Finally, the channel response to floods with variable sediment transport capacity (represented by bed shear stress) is analysed using a simple model. It is demonstrated that bend migration is associated with moderate floods while major floods are responsible for the development of chute channels, which act to suppress bend growth and maintain the low sinuosity configuration of the river.

Flash flood sediment transport in a steep sand-bed ephemeral stream

Abstract This paper reports about flow and sediment transport measurements undertaken during the rainy season (July-August) 2008 on Gereb Oda, a steep, sand-bed ephemeral stream, draining the western margin of the Kobo structural basin in northern Ethiopia. Gereb Oda streambed is dry for most of the year since it is subjected only to sporadic flash floods in response to individual, high intensity and spatially confined rainstorms. Information about hydraulic and sediment transport processes in steep sand-bed, ephemeral stream is very poor and, though only two flash floods occurred during the field campaign, the data gathered may contribute to improve our knowledge on these kind of rivers. Froude numbers were calculated in order to verify the occurrence of supercritical flow conditions as postulated by a few authors from the analysis of sedimentary structures characteristics. Flow data are also compared to simple models to predict flow velocity and discharge since flow recording systems are seldom installed on ephemeral streams in remote areas and developing countries. Dryland rivers are known for their very high suspended sediment transport, but very little bedload data exist for sand-bed, ephemeral streams. The variation of suspended sediment concentration with discharge is analysed and simple rating curves for both suspended and bed load transport are derived. A few equations to predict bedload are tested against Gereb Oda data and the relationship between en masse bedload transport processes as thin sand sheets and the development of horizontal lamination through the migration of leaf-shaped, sheet-like bedforms is investigated. Individual, large boulders were observed to move at flow depths of the same order of magnitude of their size. A few functions to predict the threshold conditions for large particles entrainment are used to verify if and to what extent they match Gereb Oda field conditions. The increased density of the water-sediment mixture, for the very high suspended sediment concentrations ranging from 100,000 to 200,000 ppm, is considered as well.

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

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.

Digital Photographic Archives for Environmental and Historical Studies: An Example from Ethiopia

Abstract The illustrative power of a set of repeat photographs is often stronger than that of other scientific output, but historical photographs are also a very useful research tool and object. Here we demonstrate how historical photographs in Ethiopia are used in studies on landscape and land use changes, soil erosion and geomorphology, history and historical context, livelihood and archaeology. Further, using the case study of Ethiopia, the location, accessibility and quality of digital archives of historical photographs is discussed as well as the way in which they can be used in research. Historical landscape photographs that are available in major archives may now largely be traced and often obtained through the Internet, as libraries increasingly are digitising collections and making them available online. Institutions that do not make at least part of their collection available online see decreasing numbers of end users, as opposed to a majority of archives which revaluate their collection by making it available online.

Estimating land cover effects on evapotranspiration with remote sensing : a case study in Ethiopian Rift Valley

Abstract A methodology for the analysis of land surface control on evapotranspiration and heat fluxes is presented, based on the assimilation of remotely sensed data. The method is suitable for application in areas with limited data availability since only standard micro-meteorological measurements and land surface temperature maps are required, while no a priori information about land cover is required. The land cover effect on the heat exchange is inferred from the assimilation in terms of heat transfer coef-ficients. The methodology has been applied in the Ethiopian Rift Valley where desertification processes are progressing at a high rate in association with remarkable temperatures recorded in the last decades. Measurements of radiation, temperature and wind profiles were taken in a field campaign. The land surface temperature maps were obtained from NOAA-AVHRR. The results show reasonable estimates of the retrieved surface fluxes and spatial patterns of heat transfer coefficients consistent with those of land cover and vegetation.

Skiability conditions in several skiing complexes on Piedmontese and Dolomitic Alps

Nivo-meteorological data from six daily recording stations located in the Italian Alps (three in the Piedmont Region, to the West, and three in the Dolomites, to the East), operating since 1991 and representative for quite large areas, have been analysed in order to point out nivometric regimes. The data gathered were also used to infer time and space distribution of parameters (thickness of snow cover, wind intensity, air humidity, amount of liquid precipitation and daily minimum, average and maximum air temperature) affecting skiability, and therefore influencing the touristic potential of alpine area.

The Climate of Ethiopia

Several papers have been published on different issues regarding the climate of Ethiopia or of some specific region. This presentation attempts to revise the knowledge of the climate of Ethiopia by means of updated, longer time series and including a larger number of meteo-stations than previous studies. Basic climatic parameters such as temperature, rainfall, relative humidity, wind, evapotranspiration, and aridity are considered and their spatial distribution is analyzed. The main results of such elaborations have been regionalized to obtain climatic maps by means of geostatistical interpolation, also taking into account topogeographic variables. These parameters were also used to update the Koppen classification of the Ethiopian territory. Climate change is a very important issue with worrying repercussions on agriculture and hence the social and economic development of the country. Trends of temperature and annual, spring, and summer rains were interpolated for the last 3–5 decades. Temperature shows a markedly increasing trend especially as regards the minimum values, whereas annual rainfalls tend to decrease with the spring, ‘small rains’ decreasing at faster rate.