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Bridge failures due to scour occur more frequently than expected, although bridge design considers conservative scenarios, evidencing a stochastic scour behaviour and the need for improvements in current design procedures. In this paper, a new method for determination of design local scour depth at bridge piers is proposed. The results show that peaks-over-threshold series are better suited than annual-maximum series of maximum scour depths for frequency analysis. Extreme scour depths follow the heavy-tailed Cauchy distribution. Scour depths with a 100-year return period resulted in 22 to 202% of the equilibrium scour depth currently used in design, evidencing the need for a different criterion to select the design scour depth. The proposed method computes the design local scour depth based on its probability of occurrence, introducing a novel probabilistic approach in bridge design for enhanced infrastructure risk management compatible with multi-hazard analysis.

Description and characterization of bed forms in different environmental settings requires an assessment of three-dimensionality. This is related to the irregularity of the bed form shape and the degree of dissimilarity from strictly 2D bed forms, those bed forms that are straight, continuous, lack of defects and are arranged in groups with an aligned pattern. Three-dimensionality assessments are commonly vague and subjective, and even when some numeric relations for three-dimensionality quantification exist, they are based mainly on the characteristics of the bed form crest and cannot be easily implemented or automated to be applied to bed form fields. In this work an objective and general method is presented for the assessment of bed form three-dimensionality. The method introduces a three-dimensionality index Tb, which considers the correlation between bed profiles orthogonal to a target direction and a reference mean longitudinal bed profile. It is shown that the suggested method can serve as an objective characterization tool for ranking the three-dimensionality of groups of bed forms at different scales, with a continuous scale between 0 and 1. For archetypical bed forms, which were synthetically generated from sinusoidal functions, Tb successfully reproduced the intuitive hierarchy of the degree of three-dimensionality. Moreover, when applied to real subaqueous dunes in one-directional decelerating flows, Tb was able to reveal a gradual decrease of the bed configuration three-dimensionality with flow strength increase. The method also proved to be useful to automatically infer the orientation angle of bed forms by finding the angle of minimum three-dimensionality. This is exemplified with superimposed ripples on a Martian barchan-dune, using the grey scale of a high-resolution image instead of bed elevations. The new approach may be also useful to validate morphodynamic models, and to develop three-dimensionality maps for identifying heterogeneous regions or zones with a bad quality of survey data.

Scientists in Egypt are particularly interested in the sustainable management of water and land resources. Global climate change will have a dramatic impact on the Egyptian water and land resources as well as its coastline and agriculture. Egypt is likely to become one of the most vulnerable countries in the world in the next several decades. Many climate scenarios predict that climate change will severely affect rainfall in the Nile basin and the flow of the Nile River in general and the High Aswan Dam Reservoir (HADR) in particular. Global warming and the higher temperatures will lead to higher evaporation rates, which, in turn, will result in less water availability at the HADR. Egypt’s Ministry of Water Resources and Irrigation predicts that the evaporation losses will, compared to the mean annual evaporation rates for the last 30 years, be approximately 3% to 10% higher by the year 2100. Since the construction of the High Aswan Dam fifty years ago, high sediment loads are a tremendous problem. 6.6 Billion Cubic Meter (km3) of sediments were deposited in the HADR during this period. The sediment has raised the lakebed level as well as the water level and caused a larger surface area. These developments have decreased the storage capacity of HADR and have increased the evaporation rate. The presented paper investigates the impact of lowering the lakebed by removing sediments from the HADR with a distinct emphasis on evaporation losses. A digital elevation model for the HADR was developed to describe the hydrological characteristics and to assess the consequences of removing sediment deposits. The results show that the removal of sediments will reduce evaporation losses by about 1.1 km3 projected for 2100, which represents 6.5% of the total projected evaporation losses.

The pier scour caused by flood waves is analyzed, introducing the dimensionless effective work by the flow on the sediment bed around the pier, W*. The three-parameter exponential function is adopted to relate the normalized scour depth Z* with W*. A nov

The time-dependent bridge pier scour during flood waves is analysed. Scour experiments were conducted in a novel installation able to produce complex hydrographs with high precision. Experimental data were used to test scour formulas including a new mathematical model. Results confirm the reliability and superior performance of the proposed dimensionless, effective flow work based model under steady and unsteady hydraulic conditions. Analyses highlight the impact of different hydrographs on scour, demonstrating the strong control by the hydrograph shape of the temporal evolution of scour depth and scour rate, although final scour after a flood only depends on the total effective flow work exerted by the hydrograph on the sediment bed. Hysteresis between flow discharge and scour rate is reported and explained. Flow acceleration is shown to play a minor role in scouring. The proposed model is a promising alternative for computation of local scour under highly unsteady hydraulic conditions.

The local scour process during flood waves is modeled introducing the idea of an effective work by the flow on the sediment bed around the pier. Dimensional considerations show different possible formulations for the dimensionless effective flow work W* that, in each case, is shown to be a generalization of the flow intensity concept, commonly used in existing scour formulas. A novel experimental installation able to reproduce any hydrograph with high precision in the laboratory flume is used to carry out steady and unsteady flow experiments in order to calibrate and validate the mathematical model. Results confirm the uniqueness of the relationship between the dimensionless, effective flow work W* and the relative scour depth Z*, highlighting the high predictive capacity of scour depth caused by any hydrograph. The proposed model provides good performances allowing a straightforward prediction of maximum scour depth and its evolution.

Scales involved in bridge pier scour include the Reynolds number, the Froude number, and the ratio of the pier to the sediment diameter, among others. Consequently, it is not possible to achieve perfect similitude, and thus scale effects between model and prototype arise. In fact, these scale effects distort the scour estimations typically used in hydraulic design of bridges, as scour formulas are based mainly on results from scour experiments at the laboratory scale. In this paper, proper scales of the flow, sediment, and scour are derived theoretically. The suitability of these scales to achieve perfect similitude of scour is tested through carefully designed laboratory experiments conducted in three different flumes having different sizes and piers of different diameters, with five bed materials of different sizes and densities. Results show that the dimensionless effective flow work, W* is a suitable criteria for scaling the effects of river flows on the local scour process around a pier in a bed composed by grains represented by the dimensionless grain diameter D*. In this context, scale effects of the ratio of the pier to the sediment diameter is quantified and shown to be negligible.

[1] Bed form geometry and dynamics in a straight section of the Elbe River in Germany is analyzed considering the measured bed surfaces as two-dimensional random fields of bed elevations. Statistically derived roughness parameters are evaluated from high-resolution digital elevation models, which were available for a range of flow rates from low flows to floods. The key results relate to the identification of characteristic scaling regions in the bed surface spectra, and to observed relationships between water discharge and both the standard deviation and a factor of the “−3” spectral law of bed elevations. Two-dimensional second-order structure functions of bed elevations are also analyzed to gain further insight into the spatial structure of sand wave beds. In addition, the interrelations between flow rate hysteresis and the statistical structure of bed forms, as well as effects of channel modification by groynes, are highlighted and discussed. The reported results demonstrate that statistical parameters of bed forms may be used for characterization and prediction of flow-dependent sand bed roughness.