\r\n\tAs an analytical technique with realms of applications, EIS has seen major progress during the past few years. One reason could be the feasibility of implementing EIS in a system, and the other would be the usefulness of EIS data in determining properties such as reaction rate, as well as the diffusion coefficients. Applications of EIS varied between corrosion analysis and inhibition, food and drug analysis, monitoring the performance of batteries, and developing biosensors, etc. Impedance microbiology which is used to monitor bacterial growth in a sample, and tissue electrical impedance which is basically used for detecting abnormalities in morphology and health of tissues, are other examples for the applications of EIS.
Clostridium perfringens (C. perfringens) is a toxin-producing anaerobic Gram-positive bacterium, which is well known for its role in human tissue infections and food poisoning. It is readily isolated from soil and a component of normal human intestinal and vaginal flora in many individuals. Apart from the classic clostridial myonecrosis of gas gangrene, C. perfringens can be responsible for a range of other clinical scenarios including sepsis, aspiration pneumonia, brain abscess, and enteritis necroticans. The potent exotoxins produced by various strains of C. perfringens are central to their effectiveness as pathogens, and include four major toxins used in strain classification: a phospholipase C (alpha-toxin, PLC), two pore-forming toxins (beta and epsilon toxins); and an ADP-ribosylation toxin (iota toxin). C. perfringens gas gangrene is one of the most fulminant necrotizing infections affecting humans. The infection can become well established in traumatized tissues in as little as 6-8 h and the destruction of adjacent healthy muscle can progress several inches per hour despite appropriate antibiotic coverage. Shock and organ failure occur in 50% of patients, and 40% of these individuals die. Even with modern medical advances and intensive care regimens, the centuries-old practice of radical amputation on an emergent basis remains the single best treatment. Histologically, this infection is characterized by widespread destruction of muscle and the absence of polymorphonuclear leukocytes at the site of infection. Instead, leukocytes accumulate within adjacent vessels.
C. perfringens alpha-toxin is the major virulence factor in gas gangrene with inflammatory myopathies (Williamson and Titball 1993, Awad et al. 1995). The toxin, which exhibits phospholipase C (PLC) and sphingomyelinase activities, causes hemolysis, necrosis, and death, and the activation of neutrophils and release of cytokines (Sakurai, Nagahama and Oda 2004). Bryant reported that the intramuscular injection of alpha-toxin caused a rapid and irreversible decline in skeletal muscle blood flow due to toxin-induced intravascular aggregates of plates, leukocytes and fibrin (Bryant et al. 2000a, Bryant et al. 2000b). Neutrophils in these aggregates often bordered the endothelium but all remained intravascular (Bryant et al. 2000a). These findings suggested that the large heterotypic aggregates of platelets and leukocytes generated by alpha-toxin also contributed to impairment of the tissue inflammatory response. We have reported that alpha-toxin-induced activation of endogenous PLC and sphingomyelinase via a pertussis toxin (PT)-sensitive GTP-binding protein (Gi) plays an important role in the hemolysis of rabbit and sheep erythrocytes, respectively (Ochi et al. 1996, Ochi et al. 2004, Oda et al. 2008).
Recently, we revealed that the tyrosine kinase A (TrkA) receptor plays an important role in the release of superoxides and cytokines (Oda et al. 2006, Oda et al. 2008). This review will present findings about the signal transduction via TrkA receptor induced by alpha-toxin and summarize information about its likely role in inflammatory disease, especially septic shock.
The TrkA receptor is a 140-kDa transmembrane protein encoded by a proto-oncogene located on chromosome 1 (Martin-Zanca, Hughes and Barbacid 1986). The family of Trk receptor tyrosine kinases consists of TrkA, TrkB and TrkC. While these family members have highly conserved sequences, they are activated by different neurotrophins: TrkA by nerve growth factor (NGF), TrkB by Brain-derived neurotrophic factor (BDNF) or neurotrophin 4 (NT4), and TrkC by NT3. TrkA regulates proliferation and is important for development and maturation of the nervous system (Pierotti and Greco 2006). This receptor comprises a tyrosine-kinase domain in its intra-cytoplasmic region and five extracellular domains, including two immunoglobulin-like domains involved in NGF binding and responsible for the specific selectivity to bind NGF (Wiesmann et al. 1999). In humans, the TrkA receptor is expressed on cells throughout the nervous system (Muragaki et al. 1995) as well as on structural cells and other non-neuronal cells in the immune and neuroendocrine systems (Levi-Montalcini et al. 1995, Aloe et al. 1997, Bonini et al. 2002, Levi-Montalcini 1987). When NGF binds to the TrkA receptor, it induces receptor homodimerization, which initiates kinase activation and transphosphorylation (Kaplan et al. 1991). This kinase activation involves small G proteins (Ras, Rac, Rap-1), PLCγ, protein kinase C (PKC) and phosphatidylinositol-3 kinase (PI3K) in neural cells (Obermeier et al. 1993b, Obermeier et al. 1993a, Melamed et al. 1999, York et al. 2000, Wu, Lai and Mobley 2001). Phosphorylation at Tyr490 is required for association with Shc and activation of the Ras-MAP kinase cascade. Residues Tyr674/675 lie within the catalytic domain, and phosphorylation at this site reflects TrkA kinase activity (Segal and Greenberg 1996, Stephens et al. 1994, Obermeier et al. 1993a, Obermeier et al. 1993b, Yao and Cooper 1995). Point mutations, deletions and chromosomal rearrangements (chimeras) cause ligand-independent receptor dimerization and activation of TrkA.
The mitogen-activated protein kinase (MAPK) pathways are activated next: extracellular-regulated protein kinase (ERK) by the small G proteins; ERK, p38 and JUN-N-terminal kinase (JNK) MAPK by PKC; and p38 and JNK by PI3K (Kaplan and Miller 1997). PI3K in turn induces activation of protein kinase B (PKB or Akt) and PKCξ (York et al. 2000)(Fig. 1).
The generation of superoxide in neutrophils has been reported to be stimulated by zymosan, 12-O-tetradecanoylphorbol 13-acetate (TPA), Ca2+ ionophores, and bacterial chemotatic peptides (Babior 1999). The signal transduction process leading to the stimulation has been studied extensively using N-formyl-methionyl-leucyl-phenylalanine (fMLP) (Kusunoki et al. 1992), platelet-activating factor (Yasaka, Boxer and Baehner 1982), and TPA (Nick et al. 1997, Pongracz and Lord 1998). It has been reported that these stimuli activated MAPK or PI3K in neutrophils (Shenoy, Gleich and Thomas 2003, Yamamori et al. 2004). Furthermore, these studies have demonstrated that the interaction of the ligands with receptors on neutrophils activates endogenous PLC with the formation of diacylglycerol (DG), which activates PKC, and inositol 1, 4, 5-trisphosphate (IP3), inducing the release of Ca2+ from the endoreticulum, and that these products act synergistically to generate superoxide. Several studies also reported that phosphorylation of tyrosine kinases and activation of phospholipase D (PLD) were closely related to the generation of superoxide in neutrophils stimulated with agonists (Garland 1992, Mitsuyama, Takeshige and Minakami 1993) and that activation of PLD resulted in the formation of PA, which was linked to the activation of NADPH oxidase (Bellavite et al. 1988, Olson, Tyagi and Lambeth 1990). We revealed that alpha-toxin-induced generation of superoxide is closely related to the activation of endogenous PKCθ via a combination of two events: production of DG on activation of PLC through a PT-sensitive GTP-binding protein and activation of phosphatidylinositide kinase 1 (PDK1) through the TrkA receptor (Oda et al. 2006).
There are three classes of PKC isotypes: classical PKC isotypes (PKCα, -β, and -γ) which have a C1 and C2 domain, bind DG, 1-oleoyl-2-acetyl-3-phosphoglycerol (OAG) and TPA, and are regulated by DG and Ca2+; novel PKC isotypes (PKCδ, -ε, -η, and -θ), which have a C1 domain and novel C2 domain and are regulated by DG but not Ca2+; and atypical isotypes (ζ/λ), which do not bind DG and are not regulated by these classical ligands (Le Good et al. 1998). Alpha-toxin induced phosphorylation of PKCθ and PKCζ/λ, and the generation of superoxide induced by the toxin was inhibited by rottlerin and calphostin C, an inhibitor of PKCθ. We reported that the formation of DG induced by alpha-toxin in rabbit neutrophils plays an important role in the generation of superoxide (Ochi et al. 2002). It therefore appears that the toxin-induced generation of superoxide is dependent on the activation of PKCθ, through binding of PKCθ phosphorylated by PDK1 to DG (Parekh, Ziegler and Parker 2000, Toker and Newton 2000). PKCθ has been reported to play an important role in activation of the protein 1 and NF-κB signaling pathway in T cells, production of interleukin-2, and apoptosis (Altman, Isakov and Baier 2000, Fan et al. 2004, Villalba et al. 1999, Villunger et al. 1999). Our data may provide clues to the role of PKCθ in neutrophils.
We reported that the alpha-toxin-stimulated generation of superoxide was related to the formation of DG through activation of endogenous PLC by a PT-sensitive GTP-binding protein in rabbit neutrophils (Ochi et al. 2002). U73122, an inhibitor of endogenous PLC, blocked the toxin-induced generation of superoxide and formation of DG in the cells, supporting that the toxin-induced increase in superoxide is dependent on the formation of DG by endogenous PLC. However, when the level of OAG incorporated into the cells was the same as the level of DG in the cells treated with 25 nM of the toxin, the level of OAG did not induce superoxide generation in the absence of the toxin but did in the presence of a near threshold dose (2.5 nM) of the toxin which did not induce production of DG. The result shows that the toxin-induced production of superoxide requires not only the formation of DG, but also the activation of other events.
It has been reported that the PI3K signaling pathway has an important role in several effector functions including the generation of superoxide (Yamamori et al. 2004). PI3K is known to generate phosphatidylinositol 3, 4, 5-trisphosphate (PIP3), which is recognized by a pleckstrin homology domain identified as a specialized lipid-binding module (Le Good et al. 1998). Several papers have reported that PDK1 requires PIP3 as its activator for effective catalytic activity (Le Good et al. 1998). Le Good et al. reported that there is a cascade involving PI3K, PDK1, and various members of the PKC superfamily in signal transduction (Le Good et al. 1998). Furthermore, the function of PKC family members is reported to depend on the phosphorylation of an activation loop by PDK1 (Le Good et al. 1998). LY294002 and wortmannin, both PI3K inhibitors, inhibited alpha-toxin-induced generation of superoxide and phosphorylation of PDK1 but did not affect the toxin-induced formation of DG. The result shows that the toxin-induced activation of PI3K occurs upstream of the phosphorylation of PDK1, which is an important step in the toxin-induced generation of superoxide. It is likely that the toxin-induced phosphorylation of PDK1 is a process independent of the toxin-induced formation of DG.
Tyrosine phosphorylation is thought to be crucial to the regulation of effector functions in neutrophils (Rollet et al. 1994). It is known that stimuli that induce tyrosine kinase activity in cells evoke the generation of PIP1, PIP2, and PIP3. This tyrosine kinase activity is linked to the NGF receptors with intrinsic tyrosine kinase activity. Kannan et al. reported that NGF enhances the generation of superoxide induced by TPA in murine neutrophils (Kannan et al. 1991). Ehrhard et al. reported that human monocytes express the trk proto-oncogene, encoding the signal-transducing receptor unit for NGF, and that the interaction of NGF with monocytes triggers respiratory burst activity (Ehrhard et al. 1993). NGF, which did not induce the generation of superoxide in rabbit neutrophils, potentiated the events triggered by the toxin and caused superoxide to form in the presence of OAG, suggesting that a combination of the production of DG and stimulation of the NGF receptor induces severe activity in the generation of superoxide. The TrkA receptor was detected in rabbit neutrophils and found to be phosphorylated when the cells were treated with the toxin. Furthermore, immunoprecipitation using the anti-TrkA receptor antibody revealed direct binding of the toxin to the TrkA receptor. In addition, the antibody inhibited the toxin-induced generation of superoxide. These observations indicate that the interaction of alpha-toxin with TrkA receptors is important to the production of superoxide. In rabbit neutrophils, K252a, a TrkA inhibitor, and LY294002 inhibited the toxin-induced generation of superoxide and phosphorylation of PDK1 within specific concentration ranges, but PP2, a Src inhibitor, and AG1478, a epidermal growth factor receptor inhibitor, did not, supporting the finding that the TrkA receptor is involved in the toxin-induced increase in superoxide. The results obtained with the anti-TrkA antibody, LY294002, and K252a show that the activation of PI3K through direct binding of the toxin to the TrkA receptor results in production of PIP3, which activates PDK1. In addition, PT inhibited the alpha-toxin-induced generation of superoxide and formation of DG, but not phosphorylation of PDK1, suggesting that a PT-sensitive GTP-binding protein plays a crucial role in the coupling to endogenous PLC, but not phosphorylation of PDK1. These observations indicate that the toxin independently induces activation of both endogenous PLC via a PT-sensitive GTP-binding protein and PDK1 via the TrkA receptor.
NGF, which binds to the TrkA receptor, is reported to be required for the differentiation and survival of sympathetic and some sensory and cholinergic neuronal populations (Howe et al. 2001). Furthermore, it has been reported that NGF is involved in inflammatory responses, an increase in mast cells in neonatal rats (Woolf et al. 1996), the degranulation of rat peritoneal mast cells (Woolf et al. 1996), and the differentiation of specific granulocytes (Kannan et al. 1991). The injection of C. perfringens cells or alpha-toxin into tissues is known to cause inflammation. Therefore, it is possible that the activation of the TrkA receptor by alpha-toxin is related to inflammation caused by C. perfringens in humans and animals.
H148G induced phosphorylation of PKCθ, but not production of DG, suggesting that the enzymatic activity of the toxin is essential for activation of endogenous PLC, but not activation of the TrkA receptor. It has been reported that binding of the C-domain, which does not contain the enzymatic site, to erythrocytes is important for the hemolysis induced by the toxin (Nagahama et al. 2002). It therefore is possible that the C-domain, the binding domain of alpha-toxin, plays a role in the binding of the toxin to the TrkA receptor and in the activation of signal transduction via the TrkA receptor.
Several studies have reported that the activation of PKC by various stimuli results in the generation of superoxide via the activation of MAPK systems (Coxon et al. 2003, Dewas et al. 2000, McLeish et al. 1998, Zu et al. 1998). K252a and U73122 inhibited the toxin-induced phosphorylation of PKCθ and ERK1/2 and generation of superoxide, suggesting that the toxin-induced production of superoxide is linked to the stimulation of the MAPK system via the activation of PKCθ. The toxin causes phosphorylation of ERK1/2, but not p38 and SAPK/JNK, implying that the process is dependent on a MAPK system containing MEK1/2 and MAPK/ERK1/2, but not systems containing p38 and SAPK/JNK.
It has been reported that PA directly or indirectly activated NADPH oxidase in a cell-free system of neutrophils (Erickson et al. 1999) and that PKCδ regulates phosphorylation of p67phox in human monocytes (Zhao et al. 2005). PKC also has been reported to activate directly NADPH oxidase (Johnson et al. 1998). However, PD98059 almost completely inhibited the toxin-induced production of superoxide near the inhibitory threshold dose of the inhibitor. Thus, it is unlikely that PA and PKC directly activate NADPH oxidase under the conditions used here.
We have shown that alpha-toxin induces formation of DG through the activation of endogenous PLC by a PT-sensitive GTP-binding protein and phosphorylation of PDK1 via stimulation of the TrkA receptor, so that DG and PDK1 synergistically activate PKCθ, and that the activation of PKCθ stimulates generation of superoxide through MAPK-associated signaling events in rabbit neutrophils (Fig. 2).
Cytokines are immunoregulatory peptides with a potent inflammatory action, mediating the immune/metabolic response to an external noxious stimulus and fueling the transition from sepsis to septic shock, multiple organ dysfunction syndromes, and/or multiple organ failure (Tracey et al. 1987, Dinarello 2004, Riedemann, Guo and Ward 2003). It is thought that synergistic interactions between cytokines can cause or attenuate tissue injury (Calandra, Bochud and Heumann 2002). TNF-α, which is released early from neutrophils and macrophages, is one of the important cytokines involved in the pathophysiology of sepsis (Tracey et al. 1987, Lum et al. 1999). TNF-α-induced tissue injury is largely mediated through neutrophils, that respond by producing elastase, superoxide ion, hydrogen
peroxide, sPLA2, PAF, leukotriene B1, and thromboxane A2 (Aldridge 2002). IL-1 stimulates the synthesis and release of prostagrandins, elastases, and collagenases and transendothelial microvascular cells, which respond by releasing the powerful neutrophil-stimulating agents, PAF and IL-8 (Leirisalo-Repo 1994). IL-1 and TNF-α are synergistic and share many biological effects in sepsis (Herbertson et al. 1995).
Anti-TNF-α antibody inhibited the death of mice induced by alpha-toxin. Furthermore, TNF-α-deficient mice were resistant to alpha-toxin. These observations suggest that the lethal effect of alpha-toxin is closely related to the release of TNF-α into the bloodstream. Stevens et al. and Bunting et al. suggested that alpha-toxin contributes indirectly to shock by stimulating production of endogenous mediators such as TNF-α and platelet-activating factor (Bunting et al. 1997, Stevens and Bryant 1997). It therefore appears that TNF-α released by alpha-toxin is important in enhancing the toxic actions of alpha-toxin in vivo. Consequently, inhibitors for release and expression of TNF-α may be worth pursuing as a novel therapeutic approach to the treatment of gas gangrene and sepsis caused by C. perfringens.
Cytokines such as the pro-inflammatory TNF-α, interleukin-1β (IL-1β) or transforming growth factor-β (TGF-β), increase the synthesis of NGF in airway structural cells. This stimulation has been evidenced in vitro in human pulmonary fibroblasts (Olgart and Frossard 2001, Micera et al. 2001), A549 epithelial cells (Pons et al. 2001) and bronchial smooth muscle cells (Freund et al. 2002). Studies also show that pro-inflammatory cytokines can act in concert to stimulate additional NGF secretion: TNF-α, for example, increases the secretion of NGF induced by IL-1β and interferon γ (IFN-γ) in fibroblasts (Hattori et al. 1994) and by interleukin-4 (IL-4) in astrocytes (Brodie et al. 1998). NGF synthesis in inflammatory conditions has also been demonstrated in vivo: elevated NGF concentrations are observed in cutaneous inflammation (Safieh-Garabedian et al. 1995) and in asthmatic airways (Olgart and Frossard 2001, Kassel, da Silva and Frossard 2001, Virchow et al. 1998). Taken together, these results suggest that pro-inflammatory cytokines, which are present at high levels in the airways of patients with asthma (Tillie-Leblond et al. 1999), might contribute to the elevated levels of NGF synthesis.
Corticosteroids are well known for their anti-inflammatory properties, particularly in asthmatic airways. Numerous studies report that the glucocorticoids dexamethasone and budesonide affect NGF expression. They cause a significant reduction in the increased NGF expression induced by pro-inflammatory cytokines; in one study, this action was shown to result from the repression of NGF gene transcription in endoneural fibroblasts from the rat sciatic nerve (Lindholm et al. 1990). Olgart and Frossard have reported that glucocorticoid treatment decreases the NGF secretion that the pro-inflammatory cytokines IL-1β and TNF-α stimulate in cultures of human pulmonary fibroblasts (Olgart and Frossard 2001) and in A549 epithelial cells (Pons et al. 2001).
These results suggested that the initial release of pro-inflammatory cytokines induced by alpha-toxin in vivo leads to the production of NGF, and the NGF released synergistically causes systemic inflammation such as sepsis and shock via activation of the TrkA receptor (Fig. 3).
C. perfringens alpha-toxin, the main agent involved in the development of gas gangrene and septicemia, induces death, hemolysis, and the activation of macrophages and neutrophils. The toxin activated the MAPK-associated signal transduction from phospholipid metabolism and phosphorylation of TrkA. Penicillin is known to be highly effective in preventing the growth of microorganisms. In conclusion, treatment with TrkA inhibitors (tyrosine kinase inhibitors) and high doses of penicillin would be effective against diseases caused by C. perfringens.
Hydraulic structures play an important role in drainage, irrigation, and hydraulic projects. If hydraulic structures fail, it may cause serious damages of wealth, properties, and environment as well as losses of life and injury to economy. The water related infrastructures are constructed at the aims to facilitate human needs/desires and enhance the quality of life such as drainage channel, river/channel, irrigation canal, bank/foot protection work, embankment, dam, spur dike/groyne, bridge/culvert, regulator, barrage/large regulator, aqueduct, pump station, siphon, and sluice. The details of some of the hydraulic structures are presented below.
Hydraulic structures are structures that are fully or partially submerged in water. The essence of building hydraulic structures is to either divert, disrupt, store, or completely stop the natural flow of water bodies. Based on the work they are designed to perform on streamflow, hydraulic structures are categorized as water-retaining structures (dams and barrages), water-conveying structures (artificial channels), and special-purpose structures (structures for hydropower generation or inland waterways) .
The dam is an essential hydraulic structure that all other structures directly or indirectly relied upon. Dams and barrages are typical water-retaining structures that are built purposely to impound water. The retained water behind dams and barrages could be used for other purposes such as irrigation, recreational activities, navigation, and a lot more. As of September 2019, there are 57,985 registered dams in the world . Regardless of their size and type, dams demonstrate high complexity in their load response and interactive relationship with site hydrology and geology. Dams are of different sizes and shapes and made of various materials such as soil or rockfill embankment, mass concrete, reinforced concrete, masonry, and wood. However, based on the construction materials used, dams are broadly classified into concrete dams and embankment dams.
Concrete dams comprised of gravity (PG), arch (VA), buttress (CB), barrage (BM), and multiple-arch dams (MV) as shown in Figure 1a–e. All these dams are constructed of mass concrete and sometimes of masonry with appropriate structural quality [1, 2]. Recent statistics show that concrete dams occupied only 20–22%, while embankment dams accounted for 78–80%.
Embankment dams are of two types, earthfill (TE) and rockfill (ER), both of which are constructed by mass filling of naturally existing ground materials (soil and rocks). The construction materials are graded and well compacted to resist seepage and sliding. Embankment dams are characterized by having similar moderate face slopes at both upstream and downstream. This feature gives rise to a broad trapezoidal cross section and a high construction volume, which is relative to the dams’ height that can cover >300 m .
Any artificial facility cut in the ground with the sole purpose of transporting water diverted from main sources (river and dams) is termed as the water-conveying structure. These types of structures are comprised of canals (Figure 2a) and tunnels (Figure 2b) (usually made from soil and rocks) or siphons, aqueducts (Figure 2c), flumes (Figure 2d), and pipelines (usually made from concrete and metals) . Before the construction of any water-conveying structure, a detailed geotechnical soil test and analysis is recommended to avail the surface and subsurface properties of the soil on which the structure is upon rest. The same soil test and analysis also applies to other types and classes of hydraulic structures to ensure safety and to save resources.
As the name implies, special-purpose hydraulic structures are built as an integral part of hydraulic project to meet a special purpose such as hydropower generation (e.g., surge towers and shafts, forebays, and head ponds), navigation (e.g., landings, berths, substations for ship repair, etc.), fishing (e.g., fish nursery ponds, fish lifts and locks, fishways, etc.), water supply for domestic and industrial uses (e.g., water intakes to treatment plant, pumping stations, etc.), waste disposal/sewerage (e.g., sewage headers, pumping stations, channels after treatment plant to water bodies, etc.), and land reclamation (e.g., irrigation canals, drainage systems, silt tanks, etc.) [1, 7].
Hydraulic structures are purposely for managing and controlling the flow of water in natural and built environment systems. Moreover, the primary purposes may include the following flood control, water conveyance, irrigation, navigation, power generation, domestic and industrial purposes, environment protection, and recreation, among others.
Flooding is a geophysical hazard that nonuniformly dispersed in both space and time. Over a decade, several watershed areas are frequently suffering from flood disaster, which causes massive destruction and loss of lives, farmlands, crops, access roads, and houses . The effective way of flood control and reducing its negative impacts is by the construction of dams, water conveyance structures, culverts, canals, and reservoirs . Many control structures are not solely constructed mainly for dealing with flood control only. However, sometimes, hydraulic structures are purposely built for flood control only. In the designing and building of flood control structures, some vital point of views must be taken into consideration in such that the cost of construction of such a project structure should be of benefit, concerning the damage reduction and the public interest when comparing to similar benefits to be derived by the alternative means. Also, the flood control structures should be reliable and effective as predicted. Even in some instance, the methods of controlling floods should rather be automatic, not manual.
Hydropower generation is the production of electrical energy from running water through turbines without reducing its quantity. The flexibility; long-lasting, storing capability; less environmental pollution; and the cost-effectiveness of hydropower plants make it attract more investment as a renewable energy source and role as a way of drought mitigation . It has been demonstrated that hydropower generated about 16.4% of the global total electricity supply equivalent to the installed capacity of about 1064 GW . The hydropower system is the leading global source of an estimated 71% of total renewable energy. Furthermore, hydropower plant reservoirs can also be used as a tool in minimizing the adverse impacts of climate change and in achieving sustainable development goals .
Inland water transportation plays a significant role in the national and global markets. Building dams and draining of river streams will considerably raise the capacity of inland water transportation, thereby allowing the smooth movement of a shipping vessel. An important point to note is that a chain of storage reservoirs would advance navigation depth, straightening out navigation channels, and support the passage of both small, medium, and even large ships. However, it is recommended to provide pathways or locks for vessels when dam structures are built on a large river stream for easy navigation from upstream to the downstream. Also, the topography of the surrounding environment should be taken into consideration. Hence, the pathways might be an integral part of the dam or a completely different structure.
Recently, it was reported that about 20% of the global total arable land is under different forms of irrigation schemes. More than 70% of freshwater withdrawn from rivers is utilized for irrigating crops, and 75% of the total water hardly returns to the rivers . In many regions of the world, with water scarcity, farming without irrigation would not be possible. The quantity of water kept in the storage reservoirs and the power required for water pumping are provided by hydropower plants, which are integral parts of the multipurpose hydraulic structure. In the present world, irrigation projects depend on the supply from multipurpose hydraulic dams, reservoirs, and rivers. For irrigation schemes to be successful, the water supply from sources must be adequately available whenever needed and at a reasonable cost of investment. Also, the operation and maintenance of such a structure should be smooth and cost-effective.
A large quantity of freshwater is being consumed daily by food processing; mineral mining and processing; textile, paper, and pulps; nuclear and thermal power plants; and drugs and pharmaceutical, petrochemical, and metallurgical industries, among others. However, some of the major industries that use a large volume of water are nuclear and thermal power plants. To meet both domestic and industrial needs, due to the higher demand for water by many industries, especially in industrially developed nations, large capacity storage structures are always built to store local rainfall runoff and water diverted from other river basins. Multipurpose hydraulic structures are the primary storage and sources of most water supply for domestic and industrial purposes. Although public water consumption constitutes nearly only 10% of the water consumed by the industries, still the immediate needs of public water supply must be taken seriously . The water supply from hydraulic projects should always meet the standards of quality required for domestic and industrial uses in terms of its color, test, hardness, odor, and bacterial purity. Also, the treatment methods for the water should be cost-effective and daily available all year round. Necessary control and protection measures should be provided in the river basin areas where the hydraulic project is sited which are mainly for the municipal water supply. The need for hydraulic projects is also in a region with the seasonal variation of rainfall distribution of the year.
Another vital reason for hydraulic projects is for environmental protection and water management, which may include farmland improvement by controlling soil erosion; environmentally friendly hydropower supply; improved quality water supply for human, animal, and industrial consumption; aquatic food supply; and recreational development . Nevertheless, the negative impacts posed by the massive hydraulic structures on the environment and public safety should always be considered in the course of design and construction processes . The essential environmental issues are for the well-being of people living around the hydraulic projects and to the other plants and animals for the social needs of humankind.
Many hydraulic projects also serve as a place for tourism, recreational, and sports activities. In fact, in some countries, sometimes hydraulic projects are specially constructed for recreation purposes. Some recreational activities carried out at the hydraulic project sites might include swimming, fishing, boating, canoeing, scuba diving, and lakeside walking. Recreational activities provide job opportunities to the teeming population and generate incomes to the government and, at the same time, conserve the natural environment.
Strategies for sustainable operation and maintenance of hydraulic structures are initiated before design and are optimized during its service life for the safety of lives and properties, which stabilizes the environment and the national economy. Consequently, improper hydraulic structures’ operation and maintenance may lead to loss of life, properties, economy, and the environment. The responsibilities for the operation and maintenance of hydraulic systems vary in different countries, depending on the ownership and purposes. In Nigeria, the responsibilities rest on the central government, coordinated by the department of water resources. This section has highlighted the necessary strategies for safe operation, maintenance, and consequences due to failure. The strategies can be long term, seasonal, frequent, and daily. The primary tasks to exemplary operation and maintenance of hydraulic structures according to Chen  are as follows: hydrologic monitoring and forecasting, detection and mitigation of aging of structures, safety surveillance and instrumentations, and remedial actions.
Safe operation and management of hydraulic structure primarily depend on the efficiency of metrological stations to provide independent data of water regime and observation. The data obtained can be used during the analysis and prediction of future hydrologic events. Nowadays, automated facilities are installed at various locations in the catchment area to provide hydrologic data. After the analysis of the data, the forecasted value and period must be provided with some reliable accuracy. The short-term forecasting, developed on runoff and other fundamental theories, provides the basis of flood controls in the catchment. Mid- and long-term forecasting give essential information to the hydropower sector .
The continuous, systematic assessments of the physical condition of hydraulic structures without compromise are encouraged. The large capacity hydraulic structures constitute a more significant threat to downstream life and properties. Mostly, failure arises from extreme flood events and inter- or obvious structural distress, which necessitates safety surveillance and instrumentation programs to detect the possible symptom and specific problem at an early stage in hydraulic structures and create strategies for the solution to the possible abnormalities [1, 13]. The selection and installation of equipment or instrumentation at appropriate locations in the surveillance area, adequate interpretation of the surveillance data, and immediate actions are more important than the number of instruments installed.
The safety inspection is a regular inspection of some deteriorations to determine the current state of hydraulic structures based on purposes related to the operation. Safety inspections are categorized into routine, specialized, and periodic inspections. Specifically, the embankments of large capacity structures should be closely and routinely examined against any physical defect . This inspection is categorized into routine, specialized, and periodic inspections , and thus, their cumulative records determine whether a defect is new, gradual, and/or rapidly changing in the structures . The routine inspection aims to identify the physical deficiencies of the hydraulic structures during day-to-day operations. Periodic inspections are carried out by experienced technical crews at an interval of 2–3 years and are meant to detect physical defects on the structures by visual examination so that strategic remedial actions can be taken. Specialized inspections include earthquake and check-flood inspections. Earthquake and check-flood are identified as a potential threat to hydraulic structures. Their inspection is carried out by experienced and well-trained dam engineers. Thus, the documented reports for mitigations are then put into the remedial action plan.
Surveillance is the continuous monitoring of physical conditions through medium to large instruments. It is being done to check the deterioration concerning the actual performance of the hydraulic structure and its trends for compliance with the design expectations. In this operation, the collection, presentation, and evaluation of data from the equipment installed in the system are paramount. The equipment must cover critical components and should be installed at positions where normal behavior is anticipated. It is a good practice to draft an ideal instrumentational plan at an early stage to eliminate the less essential provisions until an adequate, balanced, and affordable plan is determined. In large-scale structures such as a dam, surveillance through high-level technology should be enhanced. Monitoring of change in temperature and cracks occurring in the embankments are used to reveal seepage and sediments during operations.
Remedial actions are meant to prevent failures of hydraulic structures, especially the large capacity structures that pose a significant threat to lives and properties. The deficiencies are classified as minor, moderate, and major accidents . Their remedial actions are necessary before the failure of the entire structure. The defects may earlier be detected through surveillance, and the defects may probably be design-related, such as improper design capacity, or construction-related such as inappropriate choice of materials. The common and challenging operation- and maintenance-related incidents are the rapid rises in seepage, overtopping of earth embankment, excessive beaching, erosion of spillway and embankments, cracking in the concrete dam and spillway, and fractured gates. The remedial actions to be considered depend on the condition of structures and hydrologic events. The remedial measures included:
Preventive control to reduce the condition from escalation
Short-term actions to modify the nearby catchment conditions, such as increasing surveillance, emergency evacuations, and lowering the overtopping
Long-term remedies in the structures, such as reinforcements, gates, dredging, and abdication
Erosion control: During floods, the use of polyethylene sheeting and sandbag controls the erosion of the slope embankment .
Overtopping control: Overtopping must be avoided, and the provision of temporary barrier above the predicted altitude is applied.
Seepage control: The seepage must not be allowed to saturate the downstream slope, and if saturated, the provision of permeable material to reduce pressure buildup on the embankment is needed.
Aging of a hydraulic structure is referring to the time-related deformations in the properties of the material and its foundation used during construction of the hydraulic structures, which developed within at least 5 years of working period. Also, it is the entire lifespan of hydraulic structure before abdication or decommissions. The deterioration of the structures may be due to the defects developed through unusual events such as an earthquake or a result of environmental factors during service life.
Detection of aging should start during the operation and maintenance of hydraulic structures. Factors that influence the degradation of the structural properties of hydraulic systems should be identified and must immediately be managed. Alternatively, nondestructive examinations could be essential to detect the aging of hydraulic structures. The nondestructive examinations are the direct and indirect evaluation of information regarding the state of the hydraulic structure. This is to allow for immediate interventions in the situation and avoid severe consequences. Indirect assessment of aging should be accomplished by monitoring the effects and consequences of aging.
On the other hand, the direct assessment is performed by inspecting and testing the data of the structural properties of the hydraulic structures. The laboratory experiments and the in situ assessments, where the physical and mechanical properties of the sediment, including concrete, are extracted and analyzed, are examples of destructive examination. According to Chen , the destructive examination with in situ tests may or may not be destructive. The destructive examinations may include (i) hydraulic pumping tests for porosity and (ii) permeability and leak detection through a physical and chemical test of catchment and leakage, among others.
Similarly, a nondestructive examination is designed to ascertain the flows of materials while it protects the object’s usability, successfully nondestructive tests, and requires an understanding of its limitations and data manipulation. Various methods, such as electromagnetic, resistivity, acoustic, induce polarization, and visual assessment, are employed.
Adequate mitigations of aging of hydraulic structures start during the designs, effected during construction, which continues through monitoring and surveillance in operation and maintenance stages. The prior understanding of the factors that influence the degradation of the structural properties of the materials used in the constructions of the hydraulic structures must be scrutinized. Also, the provision of extra quality to meet the designed lifespan of the system must be put into consideration during the constructions. Alternatively, the following mitigations steps are commendable:
Analysis: The analysis of the aging process is carried out to ascertain its severity to the safety of life, properties, national economy, and environment.
Prevention: It is well known that all structural materials have a finite lifespan and can be affected by the environment. The prevention stage to mitigate aging of a hydraulic structure is proceeded by detailed analysis to know the structure’s safety and its economic condition. If the effect is infinite, immediate remedial action such as an emergency action plan is necessary. However, if the effect is finite, and the structure has an economic lifespan, then, provision of concrete structures from uniquely selected materials is encouraged.
Rehabilitations: Many physical and chemical methods like geomembrane are employed to enhance waterproof. Additionally, the repair and replacement of corroded steels and the use of excellent impermeable materials are also administered for overlay operations.
The importance of hydraulic structures cannot be overemphasized, and therefore their maintenance and safe utilization are critical. The structures should neither leak nor erode; channels and structures should be clean and free from siltation with noncorrosive or rotten moving parts. The breakdown or failure of these hydraulic structures can lead to a disastrous situation within the surrounding areas. For instance, a catastrophic dam collapse could lead to flooding and erosion.
The challenges of maintaining hydraulic structures at the initial stage can be achieved by managing the characteristic of the flow to meet the desired goal of the project needs. According to Chen , this can be realized by considering the public safety and ecological, environmental, and the design objectives of each structure. Some of the challenges facing hydraulic structures and the way they can be addressed are further discussed in the subsequent section.
Soil is a nonrenewable resource that supports human and animal life. Soil provides living beings with food, fiber, and protection from harsh environmental conditions such as high temperatures and heavy rainfall. Soil is lost due to erosion as a result of continuous cultivation of land, drastic reduction in vegetation, and collapsing of hydraulic structures such as dams. Erosion is the washing away of the topmost soil layer by the agents of erosion, including water, wind, and human activities . Erosion by water is caused by overland flow and transport of sediments due to the interactive action of water flow and heavy rain droplets. In hydraulic structures, erosion can occur in canals, for example, in an unlined canal at downstream or lined canal section that receives water jet flow from a gate or pipe or water that spills over a weir. This type of erosion can be remediated by dissipating the energy of the incoming water through the construction of a stilling basin as part of the hydraulic structure immediately downstream of the pipe or weir . Another critical point of canals that is prone to erosion is the intersection of a lined and unlined canal, that is, the transition point from a lined canal to the unlined canal, as shown in Figure 3. This type of problem is called undermining and, if not taken care of, can cause a collapse of the lining and destruction of the structure . So, periodic maintenance should be observed to solve this problem. Undermining can be avoided or controlled by the provision of cutoff that will protect the foundation of the structure.
Leakage in hydraulic structures refers to the ability of confined or upstream water bodies to exploit the least available exit, space, or crack underneath or along the structure to escape to the downstream or unconfined surrounding area. The moment the water found these small spaces, then there is a leakage problem, which is the beginning of erosion in the area. These small openings and cracks are widened with time and intensity of leakage. Thus, the soil is washed away as time goes on and the structure will collapse. At this point, preventing the collapse of such a structure will be very difficult. Take a dam, for instance, the water level is very high at the upstream. Water can flow along the dam embankment; if no measure is taken to save the structure, it can be undermined and collapse due to erosion .
It has been recommended by van den Bosch and Snellen  to observe and identify leakages at their initial stage and correct them. Leakages in the crack can be repaired by cleaning the wall or the floor where the crack is located. Then remove any sand, clay, plant growth, or debris. Open up the crack to become broader and more in-depth. Prepare cement-sand mortar to fill the hole and smoothen it with a trowel. Provide adequate curing to the repaired crack.
On the other hand, vertical cutoffs can be constructed on the structures to obstruct the flow of water underneath and along with the structure. An example of a cutoff wall in a dam is showcased in Figure 4a. Similarly, drop structures can also be equipped with cutoffs to block the water flow along and underneath the structure (Figure 4b). The cutoffs are part of the structure, driven into the embankments of a canal by digging deep into the banks of the canal and canal bed. During the installation, the earth around the canal banks and the cutoffs must be well compacted.
Siltation is the process of deposition of debris and sand particles and their buildup in hydraulic structures that obstruct the full functioning of the structures. The problems caused by siltation are usually the changes in water flow, changes in velocities and water levels, decreased energy dissipation, and so on. Examples of these problems include deposition of large volumes of sand in the intake chamber of pumps, which usually causes damage to the pumps and subsequent silting of the canals by sand particles. Another instance is siltation at the stilling basin. This type of sand deposits reduces the energy dissipation of the structure. Similarly, the changes in flow and velocities of water inflow division box are affected by sand particles deposited in the structure . Because of these problems, large sand traps are usually constructed at the end of the upper main canal to collect the sand deposits and remove them by periodic cleaning.
Hydraulic structures are made from different materials, including concrete, wood, or steel. These structures are liable to deterioration with time and with alternating wet and dry conditions subjected. The wooden parts in the structure, for instance, rot and decompose, whereas the steel parts corrode, as a rule, causing their expansion, and get jammed in the sliding slots. Such a condition affects the smooth operation of the structures. Routine maintenance is necessary to curtail the problems and reduce their effects. Painting of the affected parts can preserve them against corrosion. Lubrication of moving parts (steel) such as sluice gates and valves can prevent jamming.