**1. Introduction**

Scour, which represents an intriguing and complex engineering problem involving many physical mechanisms and interactions, has motivated a vast amount of research following different but complementary methodological approaches such as experimental, numerical, and field methods. Notwithstanding the many research findings that can be found in published literature, the subject is far from being completely understood. Scour continues to be one of the main hazards for many structures such as bridges, submarine pipelines, offshore wind turbines, etc., as is shown in the catastrophic failure of Houfeng Bridge in September 2008 (Figure 1) and the serious scour problems downstream of the grade-control structure leading to the demise of Zhongzheng Bridge in July 2013 (Figure 2). Consequently, this subject remains a very active area of research with important unanswered questions and practical challenges that need to be resolved.

Published evidence reinforce the need for more research in other scour types besides pier and abutment scour, particularly considering the importance of the use of alternative sources of energy other than hydrocarbon, with the aim to alleviate the threat of carbon dioxide emission and climate change. With the number of wind farms built in recent years or to be built in the near future by European and Asian countries (particularly China), obtaining a comprehensive understanding of the mechanism of scour around monopiles and finding the most cost-effective scour countermeasure are particularly needed. Despite the important works done in Europe on monopile scour, e.g., Whitehouse et al. [1], the authors did not incorporate the effect of vibrations on scour. Such limitations were highlighted in a recent review paper by Fredsøe [2], who lamented such a lack of consideration in past researches. However, more recent researchers, e.g., Guan et al. [3], have begun to include this concern in monopile scour studies. The effect of vibrations on scour is presently still in its infancy stage, and more effort is clearly needed in this area, whether experimentally or numerically. The effect of vibrations on scour is not limited to monopiles but also submarine pipelines. Scour around submarine pipelines started with researchers working in the 2-dimensional case, in which laboratory tests were conducted in narrow flumes. The early empirical 2-dimensional pipeline scour researches [4,5] were extended to numerical studies [6,7]; and later to 3-dimensional scour; with the latter better epitomizing the prototype condition. Some of the earliest 3-dimensional works were empirically conducted by [8–10], which were supplemented by numerical studies, although some of the earlier numerical studies have not been vigorously verified due to the lack of experimental data [11]. The effect of vibration on pipelines is equally an important consideration, which also was ignored by these researchers. This effect was only seriously examined in the work of Li et al. [12], who showed how vibrations significantly amplify the scour formation process. Later works include the experimental works of [13–16].

**Figure 1.** Failure of Houfeng Bridge in Dajia River, Taiwan due to pier-scour in September 2008 (courtesy of Yee-Meng Chiew).

(**a**) (**b**) 

**Figure 2.** Scour downstream of grade-control structure leading to failure of Zhongzheng Bridge in Touqian River, Taiwan (**a**) scour downstream of grade-control structure in 2009 (courtesy of Yee-Meng Chiew); (**b**) Scour leading to bridge failure in 2013 (courtesy of Dr. Hong JH).

In comparison with pier- or even pipeline-induced scour, propeller scour receives notably less attention. Most of the earlier works were carried out in Europe, especially those conducted in Queen's University Belfast, with the first doctoral dissertation by Hamill [17] and subsequent doctoral studies. A second group of propeller scour works was conducted in Singapore, with very meticulous flow field

measurements collected using the particle image velocimetry technique [18], giving comprehensive descriptions of the propeller-induced flow field and providing new insights into the scouring mechanism associated with the development of the scour hole. While weirs often are built along rivers, sometimes for flow regulation or as a water intake structure (as a side weir), their effects on scour have not been extensively studied. Interestingly, when the weir is very high, it essentially becomes a dam. On the other hand, if it is low, it acts similarly to a sill and eventually resembles a grade-control structure.

Reservoirs that are caused by dam construction act to impound water in rivers, mainly for water supply and flow regulation, invariably modifying the channel bed slope. Accordingly, their formation has a tendency to induce sediment deposition, which is a kind of negative scour [19,20], while a grade-control structure can have a profound influence on local scour [21]. Reservoirs also interrupt the course of sediment transport through river systems, causing decreased sediment loads because of trapping by upstream dams [22,23]. The interference of sediment trapping by reservoir exerts considerable morphological effects on downstream river channels, including riverbed incisions, riverbank instabilities, scour-induced damages to infrastructure (e.g., bridges, pipeline-crossings, embankments, and levees), channel width variations, and coastal erosion. Many reservoir managemen<sup>t</sup> strategies and effective countermeasures have been investigated to reduce sediment deposition in reservoirs and increase sediment supply for downstream river systems [24,25]. To remove trapped sediment deposits from reservoirs, mechanical dredging is one of the commonly used measures. However, the disposal of dredged sediment is costly.

From another viewpoint, dredged sediments may be considered as a resource that provides effective environmental benefits. By adding sediment to a river, an approach termed sediment replenishment or sediment augmentation can be applied to compensate for the lack of sediment loads downstream of dams. In field practice, it can be planned and practiced in specific hydrological and geographic areas. Okano et al. [26] investigated reservoir sedimentation managemen<sup>t</sup> by depositing coarse sediment (mainly sand and gravel sizes) for downstream replenishment. The conceptual idea of the replenishment method is to place sediments on the downstream floodplain before the arrival of floods. During the flood season, the replenished sediment will be scoured and distributed downstream. The applied replenishment method requires a sufficient amount of discharge in order to scour the replenished sediment. In practice, sediment deposited in a reservoir can be periodically dredged and strategically placed on the floodplain downstream of the dam. Most commonly, the sediments added are gravel and sand obtained from gravel quarries in the floodplain or other sources. Additionally, sediments also may be dredged from the reservoir delta deposits [27,28]. Ock et al. [28] reviewed methods in the context of sediment replenishment and compared implementation activities undertaken in Japan and the USA. According to sediment placement, sediment replenishment methods are implemented with mechanical rehabilitation for re-creating gravel or sand bar features through fluvial processes. This comparative study provides useful information for engineers to adopt proper methods corresponding to river-specific high-flow and sediment regimes. Laboratory experiments and field investigations have been conducted to improve the understanding of the transport processes of sediment replenishment on the river basin scale [29,30]. Notwithstanding these research, additional works clearly are needed to ensure the most cost-effective solution to address the dire problems of reservoir storage and downstream bed load reduction that are prevalent worldwide presently.

The editors of this Special Issue have carefully selected fourteen papers to provide a wide view of scour types using different research approaches. We hope that the contents of these papers, which do not originate from them, will provide guidelines for practicing engineers and researchers in their works.
