Environmental Hydraulics (EH) is the scientific study of environmental water flows and their related transport and transformation processes affecting the environmental quality of natural water systems, such as rivers, lakes, and aquifers, on our planet Earth [1]. Hence EH constitutes a subset of the Environmental Fluid Mechanics (EFM), which includes both water and airflow in natural systems [1]. These water flows and processes can be tackled by using theoretical analyses, field studies, laboratory measurements on physical models, and numerical simulations. In this broad sense, EH studies the motion of water at several different scales, from millimeters to kilometers, and from seconds to years the fate and transport of species, dissolved and suspended, carried along by this fluid, and the interactions among those flows and geological, biological, and eventually engineered systems. Interestingly, as EH flows are ultimately investigated to achieve an acceptable quality of the aquatic ecosystem, the above definition of EH is not in contrast with that of environmental flow as the hydrological regime required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on them [2]. While classical Hydraulics deals with the design and operation of water supply and urban and rural drainage networks, EH is mostly aimed at predicting and decision about water quality in natural channels. Hence, EH integrates the traditional analyses of classical Hydraulics in terms of discharge, velocity, water level, and pressure with that carried out in terms of mass loading rate, flux, and concentration. With the exception of groundwater flows, EH flows are ubiquitously turbulent, because of the large scales that they typically occupy in natural water systems [1]. Turbulence and stratification, that is related to density differences due to heat, salinity, or suspended matter, are the main ingredients of EH [1].

EH involves several substances (Table 1). They may be gases, solutes, or solids, and they can be naturally present or be produced by human activities. They can be reactive and non-reactive substances. These substances may be discharged into natural waters through point sources, which have a well-defined point of discharge, such as a pipeline, and are typically continuous, and non-point or diffuse sources, which are not confined to a specific location but can enter a given river or lake via overland runoff or through the land and water surface.

Several processes are analyzed in the natural aquatic systems considered in EH. They can be categorized into two broad groups:

• Transport processes are those moving a substance from a point to another within the bulk water and across its surrounding boundaries, such as advection, molecular and turbulent diffusion, gas-transfer, sediment transport, hyporheic exchange.
• Transformation processes are those changing a substance into another substance or the same substance in another form. There are physical, chemical, and biological transformations.

Since its foundation, the EH field has experienced a deep evolution, which was subjected to a rapid shift in the last two decades. This large shift in terms of problems and methods was the main reason for the present study. The question is: which are the most widely investigated EH topics and approaches emerging from a bibliometric analysis of papers published in the last 15 years at the International Symposium on Environmental Hydraulics (ISEH), which is a conferences series specialized in EH, and since its launch (i.e., 2001) in the journal Environmental Fluid Mechanics (EFMC)?

The study first presents the historical evolution of EH during three different paradigms or ages: Public Health Age, when the unique focus was on microbiological pollution and its impact on human communities, Water Quality Age, when the focus was on water quality of aquatic systems, Integrated Environmental Hydraulics Age when the traditional problems are integrated with advanced treatment of turbulence and full consideration of the exchanges processes involving any natural water system. Second, the applied methodology of the bibliometric analysis, the tools Citespace and Leximancer, is introduced. Third, the results in terms of nationality and affiliations are presented to identify patterns of collaboration among scientific institutions (universities and research institutes) and authors, which is followed by an analysis of the temporal evolution of the EFMC impact index as well as its highly-cited articles. Finally, the major EH topics are presented with a comparison between the topics extracted from the two different sources.

The close relationship between humans and water goes back to the origin of mankind. Hence, the concerns about the quality of water in aquatic systems, and particularly in those used for drinking water and irrigation, date back to the earliest human communities. In this sense, the first interest in EH flows and processes originate from those communities. However, until people lived by hunting, fishing, and gathering in small groups constantly on the move, waterborne health risks were very limited, except for parasites.

Since approximately 10,000 years ago, most people on Earth became sedentary farmers starting to live concentrated around wells, lakes, rivers, and other sources of water in growing urban centers acting as sources of wastewater discharges [3]. The management of urban wastewater discharge to minimize the risk of microbiological pollution with the associated impacts on human communities represents the original focus of EH, which was unique or largely predominant until the 20th century. Hence, we can propose calling such a long period of time as the Public Health Age of EH. The oldest sewerage systems are documented in Mesopotamia and Persia (ca. 4000–2500 BC), Minoan Civilization (ca. 3200–1100 BC), Indus or Harappan Civilization (ca. 3200–1900 BC), and Egypt and China (ca. 2000–500 BC) [4,5,6] (Figure 1). Alcmaeon of Croton (ca. 470 B.C.) is known to be the first Greek doctor to state that the quality of water may influence the health of people, while later Hippocrates of Kos, (ca.460–ca.370 BC), who is considered the Father of Medicine, presented in detail in the treatise “Air, waters, places” (Figure 1) the different sources, qualities, and health effects of water [7]. Greeks and Romans used different methods to improve the quality of the water if it did not satisfy their quality requirements, such as settling tanks (even in series), sieves, filters through sands to remove sediments and the boiling of water [7]. Furthermore, it is believed that Romans used dropshafts for a vertical drop in invert elevation, kinetic energy dissipation, and also flow aeration, to improve water quality [8,9]. If the Romans are commonly celebrated for their aqueducts, which brought vast amounts of water to towns, their sewer systems are quite poorly known. In large towns sewerage systems were vast, but in many other cases they were partially or totally lacking, and streets and rivers worked as sewers. Hence, streets were constantly flushed for cleaning by the waters of aqueducts [4,7]. On the other hand, another EH process, i.e., sediment transport, was present in Antiquity and relevant to irrigation and drainage channels in Mesopotamia and Egypt [5], and the Dujiangyan city system (ca. 300 BC) for controlling silt deposition and flooding in Sichuan province of China (Figure 1) [10].

In Europe, after the fall of the Roman Empire, water supply and sewage systems experienced a decline in maintenance and extension being limited to castles, monasteries, and large towns. The lack of proper sanitation increased the effects of epidemics during the Middle Age and the Renaissance Age in Europe (Black Death, Great Plague of London, etc.). Interestingly, it was just during the Renaissance Age that the Italian word turbolenza (turbulence) appears, both as a noun and in its adjectival forms, for the first time just throughout the writings of Leonardo [11,12]. He used that term both in the context of a furious battle, to depict the rapid movement of the soldiers, and of water motion, where he extensively referenced a range of scales of eddies and their random nature at small scales). He wrote: “The small eddies are almost innumerable, and large things are only turned round by large eddies and not by small ones, small things revolve both in small eddies and large.” [13] (Figure 1). Such and other descriptions have led Gad-el-Hak et al. [14] and others to conclude that they presage the concept of coherent structures and the Richardson–Kolmogorov cascade [12]. It was argued that he brought to his scientific work the background of a formal artistic apprenticeship and a profound consideration of what was beautiful at the time [12]. In the end, this close relationship between his mathematical and mechanical studies with his work as an artist suggests that Leonardo’s vision of Nature lies at the border between the Hermetic-Cabalist Philosophy of the Renaissance and the Mechanical Philosophy of Nature of the Modern Age [15].

Initiation of sediment motion was first described by Albert Brahams (1692–1758), who suggested that initiation of sediment motion takes place if the near-bed velocity is proportional to the submerged bed material weight to the one-sixth power, using an empirically based proportionality coefficient [16].

Since the 18th Century, the industrialization and urbanization of the Western World made more urgent the need to properly manage public health, and the governments started to develop water supply and sanitation. In the meantime, in the 19th, the role of water in the transmission of several important diseases was identified, and filtering of the entire water supply of a town was introduced, while the systematic chlorination of drinking water started in the early 20th century [7]. However, new challenges related to the impact of wastewater discharges on dissolved oxygen and nutrient levels emerged in the early 20th century. This created the first shift in EH focus from human health only to the water quality of surface and groundwaters. Hence, we may suggest that EH moved from Public Health Age to the Water Quality Age, starting from the 1920s (Figure 2).

The historical development of the Water Quality Age of EH was well outlined by Chapra [17] with emphasis on theoretical analysis and numerical modeling. The first, seminal EH study in the modern sense was that about river water quality from Streeter and Phelps (1925) [18], who developed a model to address the dissolved oxygen cycle in the Ohio River that was impaired by wastewater discharges. Subsequent investigations provided a means to evaluate dissolved oxygen levels in streams and estuaries [19,20,21,22]. In addition, bacteria models were also developed [23]. Because of the non-availability of computers, model solutions were closed-form and limited to simple geometries and linear kinetics in steady-state conditions. Since the 1960s, the number of studies related to the EH processes increased rapidly, even because in the 1970s, the rising ecological consciousness drew attention within society on environmental problems, and eutrophication and, later, in the 1980s toxic pollution came under investigation. The first textbooks related to EH were published in those years, i.e., Fischer et al. [24], Thomann and Mueller [25], and later, Chapra [17].

Since the new millennium, the scientific focus of EH was oriented to the integration of classical water quality-based approaches with (1) the large body of new knowledge about fluid turbulence, (2) the impressive advancements in river morphodynamics, and (3) the full consideration of the connections between physical, chemical and biotic components of a natural water system around the concept of environmental interface [28]. This originates the Integrated Environmental Hydraulics Age (Figure 3).

Despite the recent progress, the treatment of turbulence is still a difficult task. Turbulent flows are complex, three-dimensional, intrinsically irregular, and chaotic and are characterized by intense mixing and dissipation at a large range of spatio-temporal scales, and by the coexistence of coherent structures and random fluctuations [29]. Turbulence strongly controls several EH transport processes and their equations [29].

An environmental interface can be defined as a surface between two either abiotic or biotic systems that are in relative motion and exchange mass, heat, and momentum through biophysical and/or chemical processes. These processes are fluctuating temporally and spatially. These interfaces in the EH realm are: air-water [30], water-sediment [10], water-aquatic organisms [31], water-vegetation [32], and water-porous medium [33] interfaces (Figure 3). The concept of environmental interface is also consistent with Professor Nikora’s remark on the need for integration among Aquatic Ecology, Biomechanics and Environmental Fluid Mechanics into the Hydrodynamics of Aquatic Ecosystems that deals with two key interconnected issues, i.e., physical interactions between flow and organism and ecologically relevant mass-transfer-uptake processes [34]. Further studies are also needed to improve our current knowledge about exchange processes at the environmental interfaces.

In the end, during the last two decades in the Integrated Environmental Hydraulics Age, the analysis of EH processes found great benefit from the continuous advancement in the power of computational resources and accuracy/resolution of field/laboratory instrumentation as well as from the rapid advancements of both experimental and numerical studies about turbulence in environmental flows, sediment transport and morphodynamics [10,29,35,36,37]. EH is currently a vast and broad subject dealing with a large number of complex and interdisciplinary problems in natural water systems using a fully 3D approach and cutting-edge methodologies and technologies.