Abstract
The aim of this study was to assess the applicability of indirect techniques for streamflow and suspended sediment concentration estimation and their use in the calculation of suspended sediment transport rate in the small mountain watersheds of Madeira Island, Portugal. Emphasis was given to the application of salt dilution gauging to the indirect determination of the flow rate and the use of water turbidity data to estimate the concentration of suspended sediments. The field and laboratory work carried out are briefly described, and the main experimental results and the field data from the short measurement campaign performed in the Ribeira Seca stream in Faial, on the north side of the island, are presented and discussed. Whilst the measurement campaign carried out was temporally and spatially limited, it was pioneering for Madeira and allowed to verify the applicability of the indirect hydrometric and sedimentometric techniques used in this exploratory study.
Highlights
IntroductionThe occurrence of extreme hydrometeorological events such as intense rainfall and flash floods can have a significant impact on the environment and the landscape [1], with potentially severe and even catastrophic consequences for the af fected communities and populations [2, 3].The erosion processes caused by rainfall in these regions, often intensified by the inappropriate use of the soil as well as the soil disintegration and the pos sible occurrence of landslides caused by the action of heavy rainfall on the mountain slopes and steep hill sides, are directly responsible for the increase in the solid load in major and secondary mountain water courses.In addition to the direct action of rainfall erosiv ity, the effect of rainfall is manifested indirectly by fluvial erosion, since rainfall leads to an increase in streamflow, being the mechanical action of the wa ter on the stream bed and the banks—which is much more accentuated in flood situations— responsi ble for increasing the solid load carried by mountain rivers and streams.As underlined by Lenzi [4], the temporal and spa tial variations in the concentration of suspended sed iments in mountain watercourses are typically very pronounced, with changes in concentration over time strongly dependent on flow variations.On the other hand, the scarcity or even lack of di rect flow measurements in many regions leads to the use of rainfall data for its estimation [5, 6]
Are presented the experimental results obtained in laboratory from synthetic samples pre pared with stream water and stream sediments at con centrations similar to those observed, together with the field measurements made in Ribeira Seca stream
The temporal and spatial limitations of the measurement campaign performed in the Ribeira Seca stream did not allow to obtain a more represen tative conclusion on the suspended sediment trans port in the watershed under study, the work carried out allowed assess and master the field and laboratory methods and techniques used in this type of study, as well as to verify the applicability of such tech niques to the chosen watercourse, and by extension to other similar mountain torrential streams on Madeira Island, where other traditional hydrometric and sed imentometric techniques, and more recent tech niques like acoustic Doppler profiling (ADP) may not be the most suitable or are even impracticable under severe hydrological conditions
Summary
The occurrence of extreme hydrometeorological events such as intense rainfall and flash floods can have a significant impact on the environment and the landscape [1], with potentially severe and even catastrophic consequences for the af fected communities and populations [2, 3].The erosion processes caused by rainfall in these regions, often intensified by the inappropriate use of the soil as well as the soil disintegration and the pos sible occurrence of landslides caused by the action of heavy rainfall on the mountain slopes and steep hill sides, are directly responsible for the increase in the solid load in major and secondary mountain water courses.In addition to the direct action of rainfall erosiv ity, the effect of rainfall is manifested indirectly by fluvial erosion, since rainfall leads to an increase in streamflow, being the mechanical action of the wa ter on the stream bed and the banks—which is much more accentuated in flood situations— responsi ble for increasing the solid load carried by mountain rivers and streams.As underlined by Lenzi [4], the temporal and spa tial variations in the concentration of suspended sed iments in mountain watercourses are typically very pronounced, with changes in concentration over time strongly dependent on flow variations.On the other hand, the scarcity or even lack of di rect flow measurements in many regions leads to the use of rainfall data for its estimation [5, 6]. In addition to the direct action of rainfall erosiv ity, the effect of rainfall is manifested indirectly by fluvial erosion, since rainfall leads to an increase in streamflow, being the mechanical action of the wa ter on the stream bed and the banks—which is much more accentuated in flood situations— responsi ble for increasing the solid load carried by mountain rivers and streams. In mountain areas, the distribution of rainfall is highly influenced by the wind direction and intensity and the very rugged topography, which has strong implica tions for the estimation of rainfall over the study area watersheds [7], even more so considering the usual lack of weather radars and the low spatial density of rain gauge and weather stations frequently observed in these areas [8]
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