Introduction
Drop structures are widely used hydraulic constructions in water supply networks, irrigation and drainage channels, sewage treatment systems, and erosion control infrastructures. These structures dissipate the flow energy primarily via a hydraulic jump occurring downstream, with substantial energy dissipation also occurring when the flow plunges from the structure's edge before reaching the hydraulic jump. Inclined drops, specifically, are implemented in drainage and irrigation canals, as well as mountainous areas where the constructed channel slope is milder than the natural terrain slope. They are effectively employed to transfer flow from higher elevations to lower levels, simultaneously dissipating excess kinetic energy. Considering the engineering importance of energy dissipation in hydraulic infrastructures, this study aims to provide researchers with a comprehensive overview of inclined and vertical drop structures, highlighting their effectiveness in dissipating kinetic energy through supplementary structural arrangements. Pagliara and Chiavaccini (2006a, b) investigated the effects of base materials and reinforced block ramps on flow energy dissipation, while Pagliara et al. (2008) explored submerged flow conditions over reinforced slopes constructed with rigid blocks. Methodology This review systematically followed these stages:
1. Identification and review of literature on inclined drop structures.
2. Identification and review of literature on vertical drop structures.
3. Investigation of previous studies focused on energy dissipation mechanisms.
4. Analysis and synthesis of findings from steps (1), (2), and (3).
5. Examination of various stilling basin designs associated with drop structures.
6. Evaluation of auxiliary structures and their role in energy dissipation within drop structures.
Despite the limited number of studies explicitly targeting drop structures, substantial research exists on spillways. Although spillways and drops share similar functional objectives, their hydraulic and geometric characteristics are notably different. Generally, drop structures are restricted to heights that do not exceed roughly 4 meters, while spillways can greatly exceed this height limitation. Literature review revealed that comprehensive research on drop structures mainly emerged after 2012, with a notable concentration of studies conducted between 2018 and 2023. The literature reviewed in this research was sourced from recognized academic databases, including Google Scholar, Science Direct, CAS, and various esteemed publishers such as Elsevier, Springer, Taylor & Francis, and MDPI. In total, 49 articles spanning the years 1932 to 2023 were critically assessed, with the majority reflecting recent research advances. Results and Discussion Analysis of inclined, vertical, and stepped drops revealed critical insights into their hydraulic performance and energy dissipation capabilities. Results indicated that relative energy dissipation generally decreases with increasing relative critical depth and decreasing channel slope angles for flow rates ranging between 3.31 and 3.5 liters per second. Comparative analysis demonstrated that under identical geometric and hydraulic conditions, vertical drops exhibited the highest relative energy dissipation, followed by stepped and inclined drops. The relatively lower energy dissipation observed in inclined drops is primarily attributed to the absence of a substantial plunge pool and jet formation, resulting in diminished structural energy dissipation. An essential factor influencing energy dissipation efficiency is the relative height of drop structures. Lower heights in stepped drops inhibit the formation of fully developed two-phase (air-water) flows, reducing their energy dissipation capacity compared to vertical drops. In vertical drops, substantial energy dissipation occurs due to significant drop heights, the formation of plunge pools, and pronounced two-phase flows. The inclined drop configurations examined typically involved slopes of 26.56° and 33.7°, highlighting their sensitivity to structural angle variations in terms of energy dissipation. Conclusion Drop structures are vital components in systems designed to transfer water flow from higher to lower elevations, serving key roles in flood control and water supply management. Among the various drop configurations, vertical drops with adequately designed stilling basins demonstrate superior energy dissipation capabilities. As such, vertical drops remain highly desirable within hydraulic engineering practice. Conversely, inclined drops transfer flows downstream more rapidly due to their structural geometry, relying heavily on hydraulic jumps to dissipate energy. However, critical areas such as cavitation risks, pressure, and velocity distribution remain largely understudied for both inclined and vertical drop structures. Future research is required to address these knowledge gaps comprehensively, ultimately contributing to safer, more efficient hydraulic structure designs.
Keywords: Inclined drop, vertical drop, stepped drop, energy dissipation, stilling basins, hydraulic structures.
References
Pagliara, S., & Chiavaccini, P. (2006a). Energy dissipation on block ramps. Journal of Hydraulic Engineering, 132(1), 41-48.
Pagliara, S., & Chiavaccini, P. (2006b). Flow resistance of rock chutes with protruding boulders. Journal of Hydraulic Engineering, 132(6), 545-552.
Pagliara, S., Das, R., & Palermo, M. (2008). Energy dissipation on submerged block ramps. Journal of irrigation and drainage engineering, 134(4), 527-532.

Results and Discussion
By examining different types of drops with common geometries, including inclined, vertical and stepped drops has been conducted. The results obtained show a decrease in relative energy dissipation with an increase in relative critical depth and a decrease in the channel angle in the flow rate range of 3.5-3.31 liters per second. A comparison of the anodic dissipation in the range of variables of the present study shows that under equal geometric and hydraulic conditions, the maximum amount of relative energy dissipation is allocated to the vertical drop, with stepped and inclined drops in the second and third place, respectively. The reason for this is that in inclined drops, due to the lack of formation of a pool and a falling jet, the energy dissipation caused by the structure itself is much lower than that of other drops. One of the most important parameters that increases energy dissipation is the relative height of the structure. The low height of stepped drops prevents two-phase flow of water and air from occurring on the steps, which is a factor in the low depreciation compared to vertical drops. While in vertical drop, due to the flow falling from a high height and the formation of pools and two-phase flows, more energy will be taken from the flow. For the inclined drop used in the research, they are from two angles with values of 26.56 and 33.7 degrees.
Conclusion