Mehdi Ahmadi

Bio: Mehdi Ahmadi is an academic researcher from SINTEF. The author has contributed to research in topics: Asset management & Erosion. The author has an hindex of 9, co-authored 14 publications receiving 336 citations. Previous affiliations of Mehdi Ahmadi include Institut national des sciences Appliquées de Lyon & Centre national de la recherche scientifique.

Combining suspended sediment monitoring and fingerprinting to determine the spatial origin of fine sediment in a mountainous river catchment

Abstract: An excess of fine sediment (grain size <2 mm) supply to rivers leads to reservoir siltation, water contamination and operational problems for hydroelectric power plants in many catchments of the world, such as in the French Alps. These problems are exacerbated in mountainous environments characterized by large sediment exports during very short periods. This study combined river flow records, sediment geochemistry and associated radionuclide concentrations as input properties to a Monte Carlo mixing model to quantify the contribution of different geologic sources to river sediment. Overall, between 2007 and 2009, erosion rates reached 249 ± 75 t km−2 yr−1 at the outlet of the Bleone catchment, but this mean value masked important spatial variations of erosion intensity within the catchment (85-5000 t km−2 yr−1). Quantifying the contribution of different potential sources to river sediment required the application of sediment fingerprinting using a Monte Carlo mixing model. This model allowed the specific contributions of different geological sub-types (i.e. black marls, marly limestones, conglomerates and Quaternary deposits) to be determined. Even though they generate locally very high erosion rates, black marls supplied only a minor fraction (5-20%) of the fine sediment collected on the riverbed in the vicinity of the 907 km2 catchment outlet. The bulk of sediment was provided by Quaternary deposits (21-66%), conglomerates (3-44%) and limestones (9-27%). Even though bioengineering works conducted currently to stabilize gullies in black marl terrains are undoubtedly useful to limit sediment supply to the Bleone river, erosion generated by other substrate sources dominated between 2007 and 2009 in this catchment.

Temporal variability of suspended sediment sources in an alpine catchment combining river/rainfall monitoring and sediment fingerprinting

Abstract: Influence of the rainfall regime on erosion and transfer of suspended sediment in a 905‐km$^2$ mountainous catchment of the southern French Alps was investigated by combining sediment monitoring, rainfall data, and sediment fingerprinting (based on geochemistry and radionuclide concentrations). Suspended sediment yields were monitored between October 2007 and December 2009 in four subcatchments (22–713 km$^2$. Automatic sediment sampling was triggered during floods to trace the sediment origin in the catchment. Sediment exports at the river catchment outlet (330 $\pm$ 100 t km$^{‐2}$ yr$^{‐1}$) were mainly driven (80%) by widespread rainfall events (long duration, low intensities). In contrast, heavy, local and short duration storms, generated high peak discharges and suspended sediment concentrations in small upstream torrents. However, these upstream floods had generally not the capacity to transfer the sediment down to the catchment outlet and the bulk of this fine sediment deposited along downstream sections of the river. This study also confirmed the important contribution of black marls (up to 70%) to sediment transported in rivers, although this substrate only occupies c . 10% of the total catchment surface. Sediment exports generated by local convective storms varied significantly at both intra‐ and inter‐flood scales, because of spatial heterogeneity of rainfall. However, black marls/marly limestones contribution remained systematically high. In contrast, widespread flood events that generate the bulk of annual sediment supply at the outlet were characterized by a more stable lithologic composition and by a larger contribution of limestones/marls, Quaternary deposits and conglomerates, which corroborates the results of a previous sediment fingerprinting study conducted on riverbed sediment.

Sewer asset management – state of the art and research needs

Abstract: Sewer asset management gained momentum and importance in recent years due to economic considerations, since infrastructure maintenance and rehabilitation directly represent major investments. Becau...

Fingerprinting sediment sources in the outlet reservoir of a hilly cultivated catchment in Tunisia

Abstract: Purpose Approximately 74 % of agricultural soils in Tunisia are affected by water erosion, leading to the siltation of numerous human-made reservoirs and therefore a loss of water storage capacity. The objective of this study was to propose a methodology for estimating the relative contributions of gully/channel bank erosion and surface topsoil erosion to the sediment accumulated in small reservoirs.

Influence of available data on sewer inspection program efficiency

Abstract: One key aspect of sewer inspection programs is the prediction of sewer condition. Despite the development of deterioration models, the influence of available data on models' predictive power has not been studied in depth. In this article, numerical experiments on a semi-virtual asset stock have been conducted to answer two main questions: how to establish a list of the most informative factors and whether it is better to have data imprecision instead of data incompleteness in a utility database. Two approaches for establishing a list of the most informative factors are compared. The results show a statistical analysis (a priori analysis) can predict the impact of available data on inspection program efficiency (a posteriori analysis). This can be used to plan data acquisition programs. Finally, we show that using the notion of “district” (data imprecision) can provide efficient results when the most informative factor “age” is not available (data incompleteness).

Sampling: Design and Analysis

Abstract: In his seminal book, Shewhart (1931) makes no demand on the distribution of the characteristic to be plotted on a control chart. How then can we explain the idea that normality is, if not required, at least highly desirable? I believe that it has come about through the many statistical studies of control-chart behavior. If one is to study how a control chart behaves, it is necessary to relate it to some distribution. The obvious choice is the normal distribution because of its ubiquity as a satisfactory model. This is bolstered by the existence of the Central Limit Theorem.

The evolution of sediment source fingerprinting investigations in fluvial systems

Abstract: This contribution reviews the evolution of sediment source fingerprinting investigations since the beginning of such studies in the mid-1970s. Attention is directed to key advances and developments during this period, to the present status of source fingerprinting techniques and to the scope for future development. An analysis of the number of papers reporting sediment source fingerprinting investigations or associated methodologies published annually since the mid-1970s to date indicates that the number of such papers has increased near exponentially. The key drivers behind the expansion of such work are examined and linked to both the progress of academic enquiry and the need to support the development of sediment management strategies aimed at countering environmental problems associated with fine sediment. Instead of providing a chronological review of the various advances and developments evidenced by the expanding literature, attention focuses on seven key areas of development which are seen as having together contributed to the current state of the art. These include the expanding range of fingerprint properties employed; the use of statistical tests to confirm the ability of particular fingerprint properties to discriminate between potential sources and to assist in the selection of the ‘best’ properties for inclusion in the final composite fingerprint; the use of numerical mixing models to obtain quantitative estimates of the relative contribution of different sources; recognition of the need to confirm the conservative behaviour of the sediment properties employed as fingerprints and to take account of contrasts in grain size composition and organic matter content between source material and target samples; extension of the approach to include a greater range of targets and potential sources; addition of a temporal dimension, in order to consider changes in sediment source through time; and recognition of the need to direct increased attention to the uncertainty associated with the results of such studies. At the present time, sediment source fingerprinting techniques can be seen as being in a transition from a scientific tool to an operational or management tool, but further development will be required before successful transition to the latter can be fully achieved.

Connectivity as an emergent property of geomorphic systems

Abstract: Connectivity describes the efficiency of material transfer between geomorphic system components such as hillslopes and rivers or longitudinal segments within a river network. Representations of geomorphic systems as networks should recognize that the compartments, links, and nodes exhibit connectivity at differing scales. The historical underpinnings of connectivity in geomorphology involve management of geomorphic systems and observations linking surface processes to landform dynamics. Current work in geomorphic connectivity emphasizes hydrological, sediment, or landscape connectivity. Signatures of connectivity can be detected using diverse indicators that vary from contemporary processes to stratigraphic records or a spatial metric such as sediment yield that encompasses geomorphic processes operate over time and space. One approach to measuring connectivity is to determine the fundamental temporal and spatial scales for the phenomenon of interest and to make measurements at a sufficiently large multiple of the fundamental scales to capture reliably a representative sample. Another approach seeks to characterize how connectivity varies with scale, by applying the same metric over a wide range of scales or using statistical measures that characterize the frequency distributions of connectivity across scales. Identifying and measuring connectivity is useful in basic and applied geomorphic research and we explore the implications of connectivity for river management. Common themes and ideas that merit further research include; increased understanding of the importance of capturing landscape heterogeneity and connectivity patterns; the potential to use graph and network theory metrics in analyzing connectivity; the need to understand which metrics best represent the physical system and its connectivity pathways, and to apply these metrics to the validation of numerical models; and the need to recognize the importance of low levels of connectivity in some situations. We emphasize the value in evaluating boundaries between components of geomorphic systems as transition zones and examining the fluxes across them to understand landscape functioning.