Environmental hydraulic engineering: a perspective

Hydraulic engineering is the science of water in motion, and mainly dealing with the interactions between the flowing fluid and its surrounding environment. The technical challenges are huge [2]. The levels of complexity are closely linked with the variety of water systems, the broad range of relevant time and length scales, the variability of water systems from droughts to gigantic floods, the complexity of basic fluid mechanics with non-linear governing equations, natural fluid instabilities, interactions between water, solid, air and biological life, and the total dependence of humanity on water [3]. The challenges are not only scientific but also geo-political, as discussed below. This special issue intends to cover some of recent progresses in understanding environmental flows in hydraulic engineering. In our history, water resources and water-related issues have been and continue to be present in numerous armed conflicts [2, 11]. A number of conflicts were directly impacted by natural events (typhoons, tsunamis, wind setup) [7], sometimes considered as natural catastrophes or ‘divine’ salvation depending upon the side. Historical man-made use of water resources included artificial droughts and floods. Droughts were created by diverting river courses and drying up surface and ground-water reserves: e.g., during the siege of Khara Khoto [12]. Artificial flooding was caused by the construction of a dam and its destruction, and the breaching of dykes [3]. Some related case was the bombing of the Sorpe and Möhne dams during the dam buster campaign in 1943 [13], and the anticipation of German dam destruction at the German-Swiss border to stop the crossing of the Rhine river by the Allied Forces during WWII [9]. Since WWII, international armed conflicts have been numerous, with increasing risks of conflict from competition for water resources and food security with prevailing climate change tendencies [6]. The scope of the relevant