Ecohydraulics is an interdisciplinary science studying the hydrodynamic factors that affect the survival and reproduction of aquatic organisms and the activities of aquatic organisms that affect hydraulics and water quality. Considerations include habitat maintenance or development, habitat-flow interactions, and organism responses. Ecohydraulics assesses the magnitude and timing of flows necessary to maintain a river ecosystem and provides tools to characterize the relation between flow discharge, flow field, and the availability of habitat within a river ecosystem. Based on this relation and insights into the hydraulic conditions optimal for different species or communities, ecohydraulics-modeling predicts how hydraulic conditions in a river change, under different development scenarios, the aquatic habitat of species or ecological communities. Similar considerations also apply to coastal, lake, and marine eco-systems.
In the past century, hydraulic engineers have been challenged by habitat modeling, complicated by lack of knowledge regarding ecohydraulics. Since the 1990s, especially after the first International Symposium on Ecohydraulics in 1994, ecohydraulics has developed rapidly, mainly to assess the impacts of human-induced changes of water flow and sediment conditions in river ecosystems...
Ecohydraulics analyzes, models, and seeks to mitigate the adverse impacts of changes in hydraulic characteristics caused by dam construction and other human activities, on the suitability of habitat for organisms, such as fish and invertebrates, and to predict changes in biological communities and biodiversity. Many articles report research findings about fluvial ecohydraulics. For example, the International Association for Hydro-Environment Engineering and Research (IAHR) and Taylor & Francis have been publishing the Journal of Ecohydraulics since 2016. The journal spans all topics in natural and applied ecohydraulics in all environmental settings.
Key Concepts An aquatic ecosystem is defined as a community of aquatic organisms, with the species dependent on each other and on their physical-chemical environment and linked through flows of energy and materials. The distribution patterns of species are affected by the spatial and temporal characteristics of water flow.
Flow velocity affects the delivery of food and nutrients to organisms. It can also dislodge organisms and prevent them from remaining at a site. Some vertebrates and invertebrates, such as the shellfish Corbicula fluminea, filter their food through flowing water. Flow velocity and turbulence are critical to the life activities of many species. For example, some fish migrate and some fish spawn when they detect high flows. However, extremely high flow velocity, or high intensities of turbulence, created by hydraulic engineering infrastructure can exert pressure on most fish and invertebrates and even kill them. When the flow velocity is below 0.1 m/s, the biological community in a river is similar to that in a lake. Usually, in rivers, flow velocity between 0.1–1 m/s is most suitable for major-stream fish species.
High flow velocity and turbulence are cues for timing migration and spawning of some fish. Asian carp lay floating eggs when they sense increasing discharge resulting from a spring flood flow. The settling velocity of the eggs varies in the range of 0.7-1.5 cm/s. Once a carp egg settles on the riverbed, the egg cannot hatch. Only if flow velocity exceeds the settling velocity can an egg remain in suspension and complete incubation within 24–40 hours.
Golden mussels (Limnoperna fortune) are an invasive filter-feeding macro-invertebrate species. Dense attachment of the species to the boundaries of water-transfer tunnels and pipelines results in biofouling, causing high resistance to water flow and damage to pipeline walls. This consequence, along with the decay of dead mussels, harm water quality. Golden mussel larvae can be killed by high-frequency turbulence and increased flow velocity. Experiments show that the larvae can be killed in a flow field with velocities in excess of 0.08-0.15 m/s and a turbulence frequency higher than 30 Hz. Preliminary results have shown that the higher the turbulence intensity the higher the mortality of golden mussel larvae. On the other hand, low vertical mixing or turbulence is a key factor in favoring the development of harmful algal blooms.
Reservoirs are operated according to the requirements of power generation, water supply, navigation, and, in recent decades, environmental flows. Thus, the time and magnitude of peak discharge of floods may change, which thereby affect the life cycle and habitat of aquatic bio-communities. Most faunal species in a river cannot adapt to the non-natural change of flow and disappear from the reach downstream of the dam. Fish stranding caused by reservoir operation has occurred downstream of hydropower stations in many countries. A hydro-power dam, such as Fengshuba Dam on the East River, China, releases water suddenly during daytime and shuts off at night to meet an unsteady power demand. The instantaneous fluctuation in flow discharge and velocity kills most species except for those (e.g., the small shrimp, Palaemonidae) that can hide in crevices in riverbed sediment.
Water depth is crucial for large fauna. The habitats created by shallow rapids of small rivers in mountainous areas typically suit invertebrates and small vertebrates. Only mountain streams with many deep pools can have medium-sized creatures such as rainbow trout. White-flag dolphin, Chinese sturgeon, and finless porpoise require the water depths associated with the middle and lower reaches of the Yangtze River, where there is sufficient water depth for them to grow and hide. On the other hand, few animals can live in the lower layers of deep lakes and reservoirs, because of low dissolved-oxygen (DO) concentration.
Temperature is an important factor for many species. Salmon can only survive in cold water rivers. The Mississippi and Yangtze rivers are not suitable for salmon due to high temperatures. However, aquatic insects grow and develop more rapidly in tropical and subtropical rivers than in temperate rivers. Some species may complete two or more generations per year at warmer sites yet only one or fewer at cooler sites. Some dragonfly species on the Tibetan Plateau live for more than ten years in cold water before attaining sexual maturity and eclosion.
Variability of hydraulic characteristics is essential for biodiversity. A wide variety of flow velocities, water depths, and temperatures, both spatial and temporal, are needed to maintain high levels of biodiversity in aquatic ecosystems.
Eutrophication refers to the enrichment of a water body by nutrients to a level that results in algal blooms, deterioration in water quality, and undesirable disruption to the balance of an aquatic ecosystem. Eutrophication and algal blooms occur in rivers, lakes, estuaries, coastal, and marine waters. Algal blooms in lakes and coastal waters may lead to massive fish kills. The onset and the risk of algal blooms are closely related to the hydraulic flow and vertical turbulent mixing processes. This relationship has been shown by a real-time forecasting and warning system established to monitor algal and DO dynamics. Monitoring shows that diurnal DO fluctuations mirror the algal biomass. Algae of high density can increase fluid viscosity by more than 100%. Real-time monitoring and early warning systems can help with adaptive management to mitigate the harmful effects of massive algal blooms.
Emergent vegetation (e.g., reeds and bulrushes) on floodplains and riparian wetlands imposes significant resistance to overbank flow. The resistance of emergent vegetation is so great that the resistance coefficient in the equations of hydraulics requires adjustment. For instance, the Manning's n increases tenfold as flow depth increases from 0.03 to 0.5 m, mainly due to emergent vegetation. Emergent and submerged vegetation change the turbulence structure and sediment transportation, and may cause these quantities to vary with flow velocity over a floodplain.
Aquatic animals may change flow and sediment transportation. Initiation of motion for sediment and transportation are affected by salmonid spawning. Clustering of bed gravel is important to embryo survival of the species. The spawning fish move the riverbed pebbles and bury their eggs underneath and the egg burial depth tends to be just deeper than the observed scour depth. The species has adapted its egg placement strategy to the process of flood scouring. Beavers may construct wood dams across small streams and the beaver dams alter the hydrological process and hydraulic characteristics of a stream. Invasion of Zebra mussels and Golden mussels into pipelines, such as the cooling water pipeline of a hydro-power plant, can block pipelines and hamper power generation.
Habitat is an area where plants or animals normally live, grow, feed, reproduce, and otherwise exist for any portion of their life cycle. Because each species responds differently to environmental and biotic conditions, the term habitat is specific to a species, and in more general terms, specific to guilds of species; for example, 'fish habitat' is specific to fish. Hydraulic attributes are considered to be the most important features of habitat for almost all organisms in rivers. The biological diversity and species abundance in streams depend on the diversity of available habitat. The slope, planform, confinement, and cross-sectional shape and dimensions of a stream, and the grain-size distribution of bed sediment affect aquatic habitat. Under less disturbed situations, a narrow, steep-walled cross section provides less physical area for habitat than does a wide cross section. A steep, confined stream is a high-energy environment that may limit the occurrence, diversity, and stability of habitat.
Substrate is a general term that refers to all material that constitutes a riverbed or stream bed, which in most cases mainly comprise sediment. Stream-bed and bank erosion, sediment transportation, and deposition are among the most important factors that affect aquatic habitat. Stable streams are streams with a stable channel bed, which normally features energy-dissipation structures and little bed-load motion (transport of particles from a bed). Such streams have the best habitat for fish and benthic invertebrates. Incised streams are streams experiencing channel bed erosion, which provide the second-best habitat. Streams with intensive bed-load motion and sedimentation provide bad habitat for organisms. The taxa richness or biodiversity of these different types of rivers varies extremely because of different magnitudes of erosion, sedimentation, and sediment transportation. A uniform sand bed in a stream provides less potential habitat diversity than a bed with a step-pool system, boulder cascades, rapids, pool-riffle sequences, or other types of "bed structures" because of the resting places such feature provide.
Hyporheic zone is a layer of substrate on the riverbed in which benthic animals normally live or exist for any portion of their life cycle. Animals in the hyporheic zone usually are protected from severe washouts and temperature extremes. Other species prefer the stream bed surface for its higher DO concentration, direct contact with flowing water, and high food availability. Macro-invertebrates inhabit a sediment bed layer with a thickness of about 40–55 cm in gravel beds, 60 cm in cobble beds, 10–30 cm in coarse sand beds, and 5–10 cm in fine sand beds. The thickness of the zone in clay and silt beds is about 30 cm because the bed is relatively soft; some macro-invertebrates can move within the fluid mud layer.