Although river discharge is essential hydrologic information, it is often absent, especially for small rivers and remote catchment areas. Practical difficulties frequently impede the installation and operation of gauging stations, while satellite-sensed data have proved to be relatively useful only for discharge estimation of large-scale rivers. In this study, we propose a new methodology based on UAV-sensed data and photogrammetry techniques combined with empirical hydraulic equations for discharge estimation. In addition, two different riverbed particle size distributions were incorporated, to study the effect of fine sediment inclusion (or exclusion) in the estimation process. Accordingly, 17 study sites were selected and six different approaches were applied in each. Results show that at 75% of sites at least one approach produced an accurate discharge estimation, while in 10 out the 17 sites (58.8%) all six approaches produced accurate estimations. A strong correlation between a threshold value for the hydraulic radius (Rh = 0.3 m) of cross-sections and high estimation errors for sites exceeding it was also observed. The fine sediment inclusion improved only the performance of certain approaches and did not have a consistently positive effect. Overall, the relatively high percentage of sites with satisfactory discharge estimates indicates that using UAV-derived data and simple hydraulic equations can be used for this purpose, with an acceptable level of accuracy.

Recreational use of urban rivers is becoming popular since rivers may act as amenity corridors with the potential for exhibition, recreation, leisure, relaxation and retreat. However, several point and non-point pollution sources contribute to the degradation of urban rivers’ water quality, thereby impeding their beneficial uses and amenities. The physicochemical and microbiological quality of a Greek urban river (Kifisos–Athens) was analyzed over a period of 12 months. A sampling campaign was implemented, collecting monthly data from five sites. Spearman’s analysis showed significant correlation of the Hellenic Water Quality Index with specific nutrients. The total physicochemical status of all sampling stations was characterized as poor or bad. The annual average concentration of Escherichia coli (E. coli) was extremely high in four sites out of five, ranging from 16,822 to 26,780 cfu/100 mL. Bacteriological quality was unacceptable, as the study demonstrated the widespread occurrence of E. coli and low-quality physiochemical conditions. The spatiotemporal distribution of pollution levels revealed hotspots to be monitored further via automatic monitoring stations. A series of management and restoration measures, including tracing the exact pollution routes, should be initiated to minimize pollution pressures and establish the good ecological status of an important Mediterranean river.

In this research work, the implementation of Flow Duration Curves (FDC) was assessed, as a mean of estimating the environmental flow (e-flow) in rivers segments in Greece. The FDC of fifteen river basins varying in size and geomorphological characteristics and covering a wide area of mainland Greece, were derived by using high quality monthly discharge measurements of two years period. The e-flows were first calculated by applying the current Greek regulation method, which makes use of measurements of only the summer and September months, and by relating the e-flow as a percentage of the Mean Annual Flow (MAF). The e-flows were then plotted upon the FDC and compared with specific quartiles. The results show that the e-flows calculated by both the Greek regulation methodology and the 10% of MAF are a very good approximation of the 90% quantile of the FDC (Q90) while the 30% MAF falls very close to the 70% quantile (Q70). Therefore this study suggests that the Q90 could be used insted of the Greek regulation method, as it produces quite similar but safer (higher) discharge values. It was also indicated, that a strong correlation of 10% MAF to Q90 and 30%MAF to Q70 exists.

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