**2. Contributions**

This Special Issue comprises fourteen papers, the majority of which examine the scour topic from the empirical or experimental approaches (64%); the others are founded on the numerical and field approaches, and a review paper. The types of structure involved are piers, pipelines, propellers, turbines, weirs, reservoirs, and grade-control structures (GCS), with piers leading the group with five

papers or 36%. The review paper summarizes research works to-date relating to bridge foundation, namely piers and abutments, in supercritical flows. The number of papers published in terms of the research methodology or structure type, namely experimental approach and pier scour, respectively, is representative of the number of papers most commonly found within scour research in archival journals and conference proceedings. Notwithstanding the number of published research works in a particular area, this does not negate the importance of scour research in the other areas and research approaches. The limited number of scour research using field data in this Issue highlights the need for more e fforts in the field approach. The papers of this Special Issue, which are listed in alphabetical order of the lead author, are summarized as follows:

#### **Chavan et al. Turbulent Flow Structures and Scour Hole Characteristics around Circular Bridge Piers over Non-Uniform Sand Bed Channels with Downward Seepage**

This paper [31] presents the experimental results of the scour hole characteristics forming around single vertical pier sets on a non-uniform sand bed with and without downward seepage. Empirical equations for the evaluation of the scour hole characteristics such as the length, width, area, and volume, including the downward seepage parameter, are proposed and experimentally tested. Predictions give reasonably good agreemen<sup>t</sup> with the experimental data.

#### **Cui et al. Scour Induced by Single and Twin Propeller Jets**

The authors conduct laboratory experiments to investigate scour induced by single and twin propeller jets [32]. Propellers with three and six blades typically used in British ports and harbors are scaled according to the similitude theory and reduced propeller models were created with a 3D printer using biodegradable the polylactic acid filament, PLA, material. A laser rangefinder is used to measure the scour depth during the experiments. Empirical relations for the computation of the scour depth along the propeller axis are provided.

#### **Guan et al. Scour Evolution Downstream of SubmergedWeirs in ClearWater Scour Conditions**

The authors propose exponential equations to estimate the temporal variation in the scour depth and scour length downstream of a submerged weir [33]. The study is useful in the development of models capable of estimating scour depth downstream of weirs in rivers or coastal areas, for which the overtopping conditions are very common. The proposed equations for scour hole dimensions, profiles, and sizes will be good tools for hydraulic engineers in the design of scour countermeasures.

#### **Lee et al. Scouring of Replenished Sediment through Reservoir Flood Discharge A** ff**ects Suspended Sediment Concentrations at Downstream River Water Intake**

This paper describes a comprehensive study on the transport of replenished sediment following two methodological approaches, namely numerical modelling and physical scale modelling [34]. Valuable field data are used for the models' calibrations and validations. The study makes an important contribution to the use of dredged reservoir sediments focusing on fine (cohesive) sediments for sediment replenishment, which in turn contributes to the morphological stability of the river downstream a dam. In this way, an alternative for the disposal of dredged fine sediments taking advantage of scour is provided.

#### **Lin et al. Visible Light Communication System for O** ff**shore Wind Turbine Foundation Scour Early Warning Monitoring**

This paper proposes a scour monitoring system for o ffshore wind turbine installations using visible light communication (VLC) modules [35]. The feasibility of the proposed system is investigated through laboratory scour experiments. The novel technique is a promising low-cost alternative for field scour monitoring.

#### **Link et al. Scour at Bridge Foundations in Supercritical Flows: An Analysis of Knowledge Gaps**

Scour at bridge foundations, viz., piers and abutments caused by supercritical flows, is reviewed and knowledge gaps are identified and discussed [36]. Based on published data, the review paper finds that the scour depth caused by supercritical flows is unexpectedly much lower than that found in subcritical flows, and argues that the reasons of this behavior remain uncertain. It surmises that interactions of the horseshoe vortex with the detached hydraulic jump and the wall jet flow observed in supercritical flows could be a reason. Finally, it provides recommendations for future research in this topic.

#### **Ma et al. Case Study: Model Test on the E** ff**ects of Grade Control Datum Drop on the Upstream Bed Morphology in Shiting River**

This case study describes the construction of a 1:80 undistorted physical model of a 1.3 km-long river reach in Shiting River, China, to investigate the impact of the grade-control datum (GCD, which is defined as the crest elevation of the grade-control structure) on the upstream bed morphology [37]. It provides a plausible designed consideration for the solution of scour countermeasure against excessive erosion around an existing bridge on the river.

#### **Mathieu et al. Two-Phase Flow Simulation of Tunnel and Lee-Wake Erosion of Scour below a Submarine Pipeline**

This paper presents a numerical investigation of the scour phenomenon around a submarine pipeline [38]. Numerical simulations are performed using a two-phase flow model for sediment transport implemented in an open-source computational fluid dynamics (CFD) toolbox. The paper focuses on the sensitivity of the granular stress model and the turbulence model with respect to the predictive capability of the two-phase flow model. The numerical results show no sensitivity to the granular stress model. However, the results are strongly dependent on the choice of turbulence model.

#### **Quezada et al. Numerical Study of the Hydrodynamics of Waves and Currents and Their E**ff**ects in Pier Scouring**

The authors use the Reynolds averaged Navier–Stokes (RANS) equations via REEF3D numerical modeling to simulate the hydrodynamic field and estimate the scour around cylindrical piles due to co-directional and opposite waves and currents [39]. The relative velocity of a current is defined as an indicator that adequately measures the interactions between currents and waves under the condition of co-directional flows. It concludes that the dimensionless scour depth will be less when waves and currents come from opposite directions.

#### **Sun et al. Temporal Evolution of Seabed Scour Induced by Darrieus-Type Tidal Current Turbine**

This paper presents an experimental study on the temporal evolution of seabed scour around the monopile foundation of a Darrieus-type tidal current turbine [40]. Their results show that the scour depth is inversely correlated with the tip–bed clearance between the turbine and seabed. Empirical equations are also proposed for the prediction of the temporal scour depth around the turbine, although this has not been extensively verified.

#### **Wang et al. Experimental Investigation of Local Scour Protection for Cylindrical Bridge Piers Using Anti-Scour Collars**

The authors conduct live-bed pier scour experiments to investigate the e ffects of collars as a scour countermeasure [41]. Three variables, namely the collar installation height, collar external diameter, and collar protection range, are tested. Important design guidelines such as the recommended collar shape, installation height, and collar diameter are provided.

#### **Wei et al. Vortex Evolution within Propeller Induced Scour Hole around a Vertical Quay Wall**

This study advances the technique of oblique particle image velocimetry (OPIV) to conduct a detailed experimental study on the flow field in a scour hole induced by a propeller, including the effect of a vertical quay wall [42]. Concurrent measurements of scour and flow during the temporal development of the scour hole are performed successfully. The evolution of the vortex system and its interaction with the developing scour hole are examined in detail, elucidating the distinct influence of di fferent quay types on the formation of flow patterns and the resulting scour profiles. The near-bed flow characteristics that are indicative of the erosive flow mechanisms are discussed; this provides new insights into the scour mechanism associated with the development of the scour hole.

**Williams et al. Examination of Blockage e** ff**ects on the Progression of Local Scour around a Circular Cylinder**

This paper describes investigations of the e ffect of the blockage ratio, which is defined as the ratio of pier diameter and flume width, on scour around a circular cylinder [43]. Unlike past researchers who stated how such e ffects are negligible when the blockage ratio exceeds 0.1 and 0.125 for clear-water and live-bed conditions, respectively, they concluded that the e ffect also is related to the relative coarseness, which is defined as the ratio of pier diameter and median grain size of the bed sediment. It suggests that further experiments are needed for the range of relative coarseness ratios exceeding 100 in order to establish the role of blockage ratio e ffects to refine existing scour estimation methods.

**Yang et al. Local Scour at Complex Bridge Piers in Close Proximity under Clear-Water and Live-Bed Flow Regime**

This experimental study investigates the characteristics of scour at complex bridge piers in close proximity under both clear-water and live-bed flow regimes [44]. The results are compared with those for a single complex pier. A di fferent scour pattern is observed when more than one complex pier with variable arrangements is present. Moreover, the resulting scour pattern also is di fferent for clear-water and live-bed conditions, with an important influence of migrating bed forms on scour under the latter condition.
