Raindrop Impact Research Articles

Continuum damage mechanics based computational framework for prediction of the lifetime and degradation of wind turbine coatings with defects

The repeated raindrop impact significantly decreases the life of wind turbine blades (WTB) coating. Several computational frameworks have been developed in the literature to understand the damage behaviour and to predict the safe life of WTB's coating under such conditions. Experimental findings show that coatings may contain voids which adversely affect the coating's life. However, the computational models of coating often ignore this effect. This paper develops a new framework based on Continuum Damage Mechanics (CDM) to predict the life of WTB’s coating with voids due to rain. To determine the fatigue life of WTB’s coating, (i) a stochastic rain field simulation model has been employed, which provides data on the raindrop size distribution and impact locations. (ii) The stresses due to the impact of these raindrops are computed using smoothed particle hydrodynamics. (iii) Thereafter, CDM is implemented to predict fatigue damage life. Since damage can be visualized as nucleation and propagation of pits, voids, and cracks, hence, the complete life of the wind turbine blade's coating can be predicted by CDM. The computational result of the proposed CDM framework has been verified with fatigue damage results available in the literature. This study establishes that the proposed CDM-based computational framework is a reliable tool to predict the rain-induced fatigue life of the blade's coating in the presence of flaws.

Growth rates and nutrient accumulation in aerial biomass of sunn hemp juncea and spectabilis sown in spring

Crotalaria juncea and spectabilis have been widely used in green manuring, mainly due to the fact that they incorporate large amounts of N into the soil, through biological fixation of atmospheric N2, and also because they have a vigorous and branched root system, characteristics that help to recycle nutrients from the deeper layers of the soil. The present work was carried out in the municipality of Mercês, in the Mata Mineira area. Growth rates and nutrient accumulation were evaluated in the aerial part of sunn hemp and sunn hemp, grown in soil with good nutrient availability. The soil in the chosen area had good fertility, with no exchangeable aluminum in the 0 to 20 cm layer. Soon after the first spring rains, the soil was subsoiled, plowed, and harrowed, and the study was then installed, conducted in an experimental design in randomized blocks with five replications. The plots consisted of six furrows of five meters in length with 0.50 meters of spacing. Sowing was carried out in the first week of October. Evaluations of dry matter accumulation and plant height were carried out in the four central furrows of the plots, at 30; 45; 60; 75; 90 and 105 days after emergence (D.A.E.), with a sample area of 1.0 m of furrow (0.5 m2). In the biomass collected at 105 D.A.E, phenological stage of grain filling, the contents of nitrogen, phosphorus, potassium, calcium, magnesium and sulfur were determined, with the objective of quantifying the accumulation of these elements in the aerial part of sunn hemp. The results obtained in this edaphoclimatic environment led to the conclusion that sunn hemp juncea had faster vegetative development than sunn hemp spectabilis, therefore, a plant with greater soil protection potential against the impact of raindrops than sunn hemp spectabilis. Although they have very different growth rates, both for sunn hemp and sunn hemp, the highest rates of growth and dry matter accumulation in the shoots occurred in the periods from 45 to 60 D.A.E., with mean values, respectively, of 406, 7 and 140.0 kg per hectare per day. The accumulation of dry matter in the aerial part, as a function of the age of the plants, can be described by the quadratic equation, y = 0.0024 x2 + 0.5267x -11.8086 (R2= 0.9888), for sunn hemp, and y = 0.0008 x2 + 0.2003x -4.4657 (R2= 0.9928), for sunn hemp spectabilis. For the adopted production system, and in this climatic environment, sunn hemp was a better recycled/fixing plant than sunn hemp spectabilis, thus, green manuring with sunn hemp was more advantageous than with sunn hemp spectabilis.

Progression toward maximum leaf surface storage during the rainfall interception process

AbstractDuring rainfall events, leaves in the canopy absorb kinetic energy from falling raindrops causing the leaves to vibrate after each individual drop impact. Additionally, intercepted water gradually increases on foliar surfaces adding greater mass exerted on each leaf. This accumulated mass of water increases the leaf inclination angle and may enable the drainage of water from the leaf surface as the water drop retention angle is surpassed. This study explored the dynamic process of leaf vibration and changes in steady‐state leaf inclination angles after raindrop impact for three species (Acer saccharinum, Quercus gambelii, and Ulmus pumila) and with four different drop sizes. An incremental increase in steady‐state leaf inclination angle was measured from processing single frames of videos as each additional raindrop impacted leaf surfaces. In general, larger drops increased leaf inclination angles to a larger degree than the smaller drops. As maximum leaf surface storage or the water drop retention angle was approached, water drained from leaf surface causing an abrupt rebound of the leaf inclination angle. The maximum changes in steady‐state leaf inclination angle before rebound (LIAmax) and the number of drops needed for leaf inclination angle rebound were significantly correlated with the initial leaf inclination angle (LIAi) for experiments with A. saccharinum and U. pumila, but not for the experiments with Q. gambelii. No significant correlations were found between LIAmax and any leaf trait (i.e., leaf area, leaf mass, petiole length, petiole diameter, leaf hydrophobicity, and water drop retention) for A. saccharinum and U. pumila; however, a significant negative correlation was found for Q. gambelii between maximum changes in leaf inclination angle and petiole diameter. Of the leaf characteristics examined, leaf surface storage is most influenced by LIAi before drop impact.

Linear Macropore Installation to Reduce Red-Soil Erosion in Sugarcane Fields

This study determines the cause of soil erosion in red soils in sugarcane fields, especially even with the use of subsoiling fissures, and to compare the effectiveness of a novel artificial linear-macropore with the insertion of fibrous material into the fractures. Four column treatments (tillage, subsoiling, linear-macropore with plant residue fillings, and no-tillage-with-mulching) were established. A subsoiler was used to break up hard soil layers to enhance infiltration, whereas mulching reduced the impact of raindrops on the soil. Sugarcane residue was inserted in the empty fissure to reinforce the structure, making linear macropore. Simulated rainfall with 20 mmh−1 was applied to the soil surface for 6 h per day for two days. Surface runoff, soil erosion, and drainage were measured during each run. Erosion was minimal (1/7 reduction), and bottom drainage was observed in the linear-macropore and no-tillage-with-mulching plots. Conversely, due to the formation of an impermeable layer or surface crust, high erosion (0.282 t-C ha−1 yr−1) and decreased drainage levels were detected in the subsoiling and tillage plots. Moreover, the aboveground protrusion of fibrous material at the linear-macropore maintained infiltration, even following crust formation. Field application of these four management strategies revealed the effectiveness of linear-macropore and mulching in reducing surface flow. Linear-macropore application maintains appropriate levels of infiltration, and insertion of plant residue fillings reinforces the macropore structure while also avoiding clogging. Hence, the linear-macropore scheme may represent an effective strategy for reducing surface runoff and red soil erosion.

Understanding Multiple Stressors Which Degrade Antisoiling Coatings: Combined Effect of Rain, Abrasion, and UV Radiation

Antisoiling coatings (ASCs) are nano or microlayer coatings which reduce the settlement of dust on the surface of the PV module and are extensively investigated as a dust mitigation strategy. Since the coating is applied on the outer surface of the PV module, they are exposed to various environmental stressors, and the durability of these coatings is essential. Various studies have reported the effect of individual stressors that degrade ASCs. However, there are very few reports on the impact of the combination of stressors relevant to outdoor field conditions. In this article, we investigate the impact of the combination of the following stressors that degrade ASCs: 1) the combination of the impact of raindrops and acidic water, 2) the combination of UV radiation, the impact of raindrops, and acidic water, and 3) the combination of abrasion, UV radiation, the impact of raindrops and acidic water. The key takeaways from this study are 1) coating life reduced by 21× (average of all coatings) when exposed to the rainy season compared with those exposed to the nonrainy season. 2) All coated samples show lower coating life varying from 10× to 48× when exposed to the impact of raindrops with pH 7 water than those exposed to water immersion/water contact test with pH 7 water. This indicates that, during a rain event, the impact of raindrops hitting the coated surface causes more significant damage than water contact. This study shows that coatings A, B, C, and D are unreliable for long-term use in warm and humid climate zones, considering 25-year module lifetime. However, other chemistries of hydrophobic and hydrophilic solutions could be effective if they exhibit long-term reliability.

Diamondback moth egg susceptibility to rainfall: effects of leaf surface properties on the direct and indirect impact of simulated rain

AbstractDiamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Plutellidae), eggs are vulnerable to rainfall, but the impact is dependent on host plant properties and the nature of oviposition sites. The effects of host plants, common cabbage (Brassica oleracea L. var. capitata) and Chinese cabbage (Brassica rapa L. var. pekinensis) (Brassicaceae), and oviposition sites on the fate of eggs subjected to simulated raindrops were investigated in laboratory experiments. In oviposition studies, DBM deposited more eggs on leaf veins than on leaf laminas on both host plants. In common cabbage plants, eggs laid on lower leaves (leaf 2) were less susceptible to dislodgment by rainfall than eggs laid on upper leaves (leaf 6). Eggs laid on the veins on the abaxial surfaces of leaves 2 and 6 of common cabbage plants were less susceptible to dislodgment by raindrops than eggs laid at other sites on this host plant. In Chinese cabbage plants, eggs laid on the veins were less susceptible to dislodgment by raindrops than eggs laid on leaf laminas. When DBM laid eggs in clusters, eggs were less susceptible to dislodgment by raindrops. The presence of waxes on leaf surfaces and concave veins affected egg placement and the retention of eggs at oviposition sites when subjected to raindrops. Eggs were more resistant to raindrops on the concave veined and less waxy leaves of Chinese cabbage plants than on waxy surfaces of smooth common cabbage leaves. When epicuticular wax crystals were removed from common cabbage leaves before oviposition, eggs were more resistant to dislodgment by raindrops. Leaf orientation or angle (horizontal plane vs. an angle of 45°) did not affect the susceptibility of eggs to raindrops. However, eggs were affected indirectly, and vibrations caused by the impact of raindrops resulted in egg removal from both adaxial and abaxial surfaces. Overall, eggs were less likely to be dislodged by raindrops when laid on concave veins and less waxy leaf surfaces. In oviposition tests, DBM laid fewer eggs on plant parts from which they were most easily removed by raindrops, and when they did lay eggs at these sites, they often deposited them in clusters which were more resistant than single eggs to removal by raindrops. The study reiterates the species‐specific nature of DBM‐host plant interactions and shows that female DBM can modify their behaviour to improve the probability of egg survival when substrates available for oviposition are sub‐optimal.

Resistance to sheet flow induced by raindrop impact on rough surfaces

Raindrops impact can affect the flow resistance to sheet flow by interfering with flow velocity profile. However, quantitative information regarding the flow resistance induced by raindrops impact to sheet flow (Δf) under the influence of different surface roughnesses is limited. In this study, multiple experimental simulations of rainfall and inflow were conducted on flumes with five unit width inflow discharges ranging from 0.02-0.1 m2 min−1, five rainfall intensities ranging from 40 to 120 mm h−1, and three levels of surface roughnesses ranging from 0.009 to 0.25 mm to identify flow resistance due to raindrop impact under different surfaces. Results showed that surface roughness was able to increase the total flow resistance without raindrop impact (fo) and with raindrop impact (fr), respectively. Under rainfall conditions, raindrop impact was able to increase fr on different surfaces. The surface roughness played a stronger role on contributing the fr than the raindrop impact. The major control of fr was the Reynolds number (Re), and fr decreased with the increasing Re. Δf were positively affected by surface roughness and rainfall intensity. Rough surface can increase the effects of raindrop impact on water flow by reducing the flow velocity of base flow or inflow, which leads to the increase in Δf. The major control of Δf was the Re, and Δf decreased with Re. Flow resistance induced by raindrop impact was larger for shallower and slower moving sheet flow with the low Re. In addition, a new equation has been developed to predict Δf under the conditions of Fr < 2.16 by using rainfall intensity, surface roughness, velocity, and water depth. This study helps to estimate the flow resistance induced by raindrops impact to sheet flow under surface roughness and provide the scientific insights for improving hydrologic and soil erosion models.

The size distribution metrics and kinetic energy of raindrops above and below an isolated tree canopy in urban environment

Raindrop impact on bare soils is the initial phase of rainfall-induced soil erosion which is altered under any type of vegetation due to the interactions of rainfall with the canopies. This study examines the drop size distribution (DSD) and kinetic energy (KE) of raindrops above and below the birch tree (Betula pendula Roth.) canopy in a research plot in the city of Ljubljana, Slovenia using a one-year observation of 63 rainfall events and the effect of meteorological variables under moderate continental climate. Simultaneous measurement of the microstructures of open rainfall and throughfall was carried out using an optical disdrometer. The result of our analysis revealed that throughfall DSD showed two distinct major peaks (bimodal) occurring primarily on smaller drop sizes while open rainfall has only one. The cumulative drop number, median drop-volume diameter (D50), and drop fall velocity of throughfall were 16.4%, 26.6%, and 5.0% lower than those of open rainfall, respectively. Also, the relative volume percentage of raindrops > 1.5 mm is 1.5 times higher than those observed in throughfall drops which indicates that the presence of the canopy caused the fractionation of larger drops into smaller droplets. These reductions significantly differ depending on the phenoseasons of the canopy with the leafed state being higher than the leafless state. Similarly, the Kruskal-Wallis H test result revealed that birch tree elicits a statistically significant change in the kinetic energy of open rainfall, thus weakening the mean rainfall KE by 33.7%. On the other hand, KE is positively affected by the phenological condition of the canopy with higher attenuation being observed during its leafed state. Also, the correlation analysis demonstrated that vapor pressure deficit, air temperature, and relative humidity have stronger associations with throughfall kinetic energy among meteorological variables considered. These findings underscore the necessity of an optimized selection of tree species for afforestation programs.

Estimating Manning’s coefficient n for sheet flow during rainstorms

Many hydrological and soil erosion models employ Manning’s coefficient (n) to calculate the velocity of sheet flow (SF). However, Manning’s coefficient of SF cannot be estimated accurately due to the strong influences of rainfall, slope gradient, ground surface roughness, sediment size, etc. The effects of rainfall, especially rainstorms, on Manning’s n are usually ignored. In this study, flume tests were conducted with rainfall intensities ranging from 40 to 120 mm h−1, unit width inflow discharges ranging from 0.02 to 0.06 m2 min−1, bed surface roughness values ranging from 0.009 to 0.25 mm, and slope gradients ranging from 3° to 7°. Manning’s n and the correction (Δnr) induced by raindrops were calculated. The results indicated that Manning’s n without rainfall increased with increasing surface roughness and decreasing unit width inflow discharge. In rainstorms, Manning’s n increased with increasing rainfall intensity and surface roughness and decreasing unit width inflow discharge. Δnr increased with increasing rainfall intensity and surface roughness and decreased slightly with increasing unit width inflow discharge. Manning’s n significantly decreased with increasing Froude number (Fr) according to a power function, while Δnr significantly decreased logarithmically with increasing Fr. The Δnr values of this study were greater than 0 when Fr was less than approximately 2.15. For slope gradients of 3°, 5° and 7°, the rainfall increased n by 1.47–49.33 %, 0.44–21.81 % and 0.43–23.94 % for the cases of Fr < 2.15, respectively. This result indicates that the effects of rainfall on Manning’s n of SF cannot be ignored for laminar flows and low Reynolds numbers (<900). Additionally, two empirical equations for predicting Manning’s n and Δnr under rainfall conditions were devised. The correction Δnr due to raindrop impact was affected by the inflow or base flow regimes. This study experimentally provides basic insights into the effects of rainfall on Manning’s coefficient.

Physically based numerical model for the landscape evolution of soil-mantled watersheds driven by rainfall and overland flow

Soil and gully erosion is a critical threat to sustainable agricultural land management and productivity. Soil erosion by rain splash, sheet runoff, and concentrated flow are complex problems conditioned by the combined interactions of soil physical properties, hydrology, human activity, landscape topography, and climate. Effective management of erosion processes driven by rainfall and surface runoff requires a combined effort of field observation, physical experimentation, and numerical simulation. A physically-based hydrodynamic numerical model, CCHE2D with updated raindrop erosion effects, is applied to simulate landscape evolution processes as a result of raindrop impact and overland flow. The numerical model simulates a thin layer of runoff and concentrated flow over complex terrains, and predicts sediment transport due to raindrop splash detachment, transport and soil surface erosions. Soil erosion and landform evolution simulations due to rainfall are presented for an experimental landscape and a tilled large scale agricultural field. Photogrammetric field data were collected using an unmanned aerial system (UAS) following planting and subsequent rainstorms. Numerical simulation results agreed well with the observed experimental soil surface topography, sediment yield, and drainage network developments. The simulated soil and gully erosion in the tilled field also agreed well with observations. This research successfully demonstrates how high resolution spatial and temporal topographic measurements in experimental and field landscapes can be simulated and interrogated by 2D physically-based hydrodynamic and morphodynamic models. The accurate predictions of tillage field and gully erosion process in agriculture watersheds with complex tillage conditions also indicated the usefulness of the model for studying soil and landscape degradation.

Effects of surface topography and specimen thickness on high-speed raindrop impact damage of CFRP laminates

Raindrop impact erosion has been observed since early days of aviation, and can be catastrophic for exposed materials during supersonic flight. A single impact waterjet apparatus is established for mimicking drop impacts at the velocities between 350 m/s and 620 m/s. Carbon Fiber Reinforced Polymer (CFRP) laminates with three different surface morphologies and specimen thicknesses are tested here. A central region with no visible damage has been noticed, surrounded by a “failure ring” with common damage patterns including resin removal, matrix cracking, fiber breakage and mass fiber loss. Asymmetric features are presented in the “failure ring” whose whole scope extends larger along the longitudinal direction than the transverse direction of the top layer. The mechanism of the resin removal is related to fiber–matrix debonding, and its onset and propagation can be facilitated by initial surface asperities with the shear action of the lateral jetting. In cases of multiple impact, good surface quality can slow down the evolution of resin removal and fiber exposure on the CFRP surface, reducing the erosion speed and delaying the occurrence of structural damages in the subsequent impacts. Rayleigh wave dominates the occurrence of matrix cracking on the CFRP surface, and subsequently, results in material loss and peeling of the top-layer because of lateral flow. With the increase of the specimen thickness, both the interlaminar and intralaminar failures decrease as the impact damage mechanism changes from plate bending stress to the reflection of stress waves.

Modeling rainfall-infiltration-runoff processes on sloping surfaces subject to rapidly changing soil properties during seal formation

The effect of rapidly changing soil properties during surface sealing is usually neglected in hillslope infiltration-runoff modeling practices despite its substantial impact. Based on the modified seal formation model and a two-layer infiltration-runoff model, a physics-based, computationally efficient modeling framework is developed to simulate the rainfall-infiltration-runoff process on slopes during seal formation. The framework considers the rapid variations of soil properties of the surface layer over time to reflect the soil sealing process, while accounting for the steepness of the slope. On sloping surfaces, the raindrop impact can both facilitate soil sealing via the normal component to the slope (dominant role) and reverse the process because of the tangential component (secondary role). The proposed solution approach needs no numerical grid adjustment to address the progressive sealing layer compaction. The model simulations accord well with the runoff experiments of different rainfall intensities and slope gradients. During seal formation, rainfall intensity and slope gradient can significantly affect the variation of soil hydraulic properties. The new approach is compared to the case where the seal layer formation is not accounted for and to the case where a completely formed seal is assumed at the rainfall onset. It is found that accounting for the sealing process affects the hydrological processes. Neglecting seal formation or assuming a completely formed seal layer would lead to large errors in both infiltration and runoff predictions. The proposed model can improve rainfall-infiltration-runoff models applicable for hillslopes subject to fast changing soil properties during surface sealing or rapid soil compaction.

Assessment of Soil Erosion Hazards in Kambiti Subcatchment, Murang’a County, Kenya

Erosion is a global problem that destroys soil and adversely affects ecosystem productivity. Soil erosion generally involves many processes but the major activities involve particles being transported and deposited to another location. With an increasing population, soil erosion, water availability, energy production, and biodiversity loss are some of the most pressing environmental problems around the world. Erosion is a hazard associated with agriculture in tropical and semi-arid areas. Kambiti sub catchment is part of the upper Tana catchment. The upper Tana catchment includes 25% of Kenya gazette forest. A large area of land has been degraded, resulting in a drastic reduction in surface water availability during the dry season and poor-quality water during the wet season caused by high silt levels. The main objective of the study is to assess erosion hazards using RUSLE model in Kambiti sub catchment area, Murang’a County. The specific objectives of the study were to determine the effect of rainfall erosivity and soil erodibility factor in Kambiti Sub catchment area in Murang’a County, to determine effect of slope factor to soil loss in Kambiti Sub-catchment area in Murang’a County, to determine the effect of crop protection and management factor in Kambiti Sub catchment area in Murang’a County to soil loss and to determine the strategies for management of soil and water in Kambiti Sub catchment area in Murang’a County. Data was collected from the catchment and analyzed using arc Geographical Information System to obtain the specific parameters in the revised universal soil loss equation model. Interpolation method was used to determine the mean annual precipitation. The k factor is a function of soil texture. Shape file for geological structure for Kenya was obtained from Kenya Agriculture and livestock Research Organisation and analyzed by use of arc GIS to obtain soil erodibility factor. The slope factor was analyzed using digital elevation model from arc view. Digital elevation model was gotten from STRM download. The C factor was derived from Landsat imagery from sentinel of 30metres by 30 metres. It was further analysed by unsupervised classification from Arc GIS. The sentinel clip of Kambiti sub-catchment was joined with ground trothing observations. The results were useful in estimation of soil loss therefore profiling the areas prone to soil loss. Study findings indicated rain drop impact and runoff were primarily responsible for causing erosion in Kambiti sub catchment. Anthropogenic factors played an important role in amplifying the severity of the damage, such as persistent vegetative degradation and destruction of soil structure due to organic matter depletion and routine shallow tillage. In recent years, erosion control has been hampered by the occurrence of gaps in knowledge regarding the integrated nature of erosion processes, leading to land damage caused by rill and inter-rill erosion going unaddressed. Poor people and those lacking capital to invest in reclaiming land are the main causes of abandoning degraded land. Through strategic awareness campaigns and education, soil erosion will be assessed and the knowledge gap will be closed. Participation of farmers in land use decisions is inevitable as it ensures that people who utilize land resources are recognized as equal partners in identifying problems and designing solutions. It was also recommended that Identification and operationalization of alternative off-farm income.&#x0D;

Modelling the dynamics of seal formation and pore clogging in the soil and its effect on infiltration using membrane fouling models

In the paper, we evaluated some of the prominent mathematical models derived from Darcy’s law used to describe filtration membrane fouling occurring during the clarification of turbid fluids. The fouling can occur by basic mechanisms like surface pore blocking, pore constriction/obstruction, caking, or their combination. The paper presents a novel application of these models to the infiltration of overland flow into the soil. The soil, as a porous medium, is assumed to be the filter. The soil infilling that occurs during structural seal formation is assimilated to the membrane fouling process, and the turbid fluid is the flow enriched in fine particles due to the illuviation of the soil particles detached by raindrop impact and overland flow. Explicit mathematical equations were derived for the dynamics of the free surface area for infiltration, the resistance of the infiltration porous medium, the infiltration rate, the cumulative infiltration volume and the saturated hydraulic conductivity. The fluid characteristics and the validation dataset were provided by the SERLAB (Soil Erosion LABoratory, 42°59′34′’N 12°17′27′’E, Italy) of the University of Perugia and derives from a sequence of rainfall-simulation experiments in a silty clay loam. The better fits were obtained by combining the models that simulate the basic mechanisms of the free surface pore area blocking and the increase in the medium resistance due to pore obstruction by infilling fine particles. Both processes occur during structural crust formation. As a model reproducing the fine particles infilling pores both at the surface and at the underneath layer could describe the behaviour of the infiltration data, the contribution of this process should be added in modelling the dynamics of structural seal formation and its effect on infiltration.

Hydraulic Conditions of Incipient Rill by Raindrop-Induced Overland Flow on Steep Slopes of Sandy Soil

Hydraulic characteristics of rill incipience on steep slopes with sandy soils are different from those of gentle slopes in the mountainous watersheds. In other to better understand the processes of the incision and expansion of rill, rainfall simulations were conducted in a laboratory soil box with an inflow device installed at the top of the steep slope. The location and shape of the rill incision were tested with respect to different slopes (20, 24, and 28°), rainfall intensity (80 and 100 mm/h), and segment distances with various inflow rates (0.0, 8.8, 17.5, and 26.3 ml/s). The result showed that rills with heatcut were mostly incised on the 2.4 m segments and progressively moved upslope on the 3.2 m segments. The steps of rill evolution with increase of inflow are identified as four stages; (1) sheet flow with impact of raindrops, (2) overland flow with flowpaths, (3) microrill with headcut incision, and (4) rill enhanced by headcut expansion. It appears that incision and headcut of rill is prevalent when the Froude number (Fr) changes from subcritical flow to supercritical flow and the maximum velocity of rill reaches over 0.065 m/s. Unit stream power using the maximum velocity of rill was the best parameter for estimating rill erosion as rill evolution greatly depends on maximum velocity of the local flow enhanced by rill incision and expansion As a result, it was found that critical conditions for rill incipient of sandy soils on steep slope are observed greater than those of loamy soils or gentle slopes.

Modified inderbitzen equipment for simulation of water erosion in undeformed soil samples

Several devices have been developed to assess soil losses by water erosion. However, they rarely assess the combined effects of raindrop impact and surface runoff together with vegetation cover on varying slopes. This study aimed to design and validate an equipment, on laboratory scale, to evaluate the effect of water erosion on undisturbed soil samples with and without plant cover, as well as to assess the kinetic energy of simulated rain and the resultant shear stress at varying runoff intensities. The equipment is composed of a rainfall simulator, an adjustable stand for different slopes and falling heights, and a runoff ramp for testing undisturbed soil samples measuring 15 x 20 x 40 cm (height, width, and length). In this study, the equipment simulated and evaluated the effect of precipitation and runoff on soil losses, allowing to obtain different values of the kinetic energy of precipitation and runoff. For a f low rate of 12 L min-1 and slope of 35%, the shear stress could reach up to 8 Pa. Furthermore, the equipment showed the effect of vegetation cover and slope on soil losses in different granulometric fractions (&lt; 0.106 mm, 0.106 to 0.25 mm, 0.25 to 0.5 mm, 0.5 to 1.0 mm, 1.0 to 2.0 mm, and &gt; 2.0 mm), revealing the potential of its use in several erosion studies on a laboratory scale.

Superhydrophobic Strategy for Nature-Inspired Rotating Microfliers: Enhancing Spreading, Reducing Contact Time, and Weakening Impact Force of Raindrops

Wind-dispersal of seeds is a remarkable strategy in nature, enlightening the construction of microfliers for environmental monitoring. However, the flight of these microfliers is greatly affected by climatic conditions, especially in rainy days, they suffer serious raindrop impact. Here, a hierarchical superhydrophobic surface is fabricated and a novel strategy is demonstrated that the superhydrophobic coating can enhance spreading while reduce contact time and impact force of raindrops, all of which are beneficial for the rotating microfliers. When the surface rotating speed exceeds a critical value, the effect of centrifugal force becomes considerable so that the droplet spreading is enhanced. The rotating superhydrophobic surface can rotate an impacting droplet by the tangential drag force from the air boundary layer, and the rotation of the droplet generates a negative pressure zone inside it, reducing the contact time by more than 30%. The impact force by the droplet on the rotating superhydrophobic surface also has a remarkable reduction of 53% compared to that on unprocessed hydrophilic surfaces, which helps maintain the flight stability of the microfliers. This work pioneers in revealing the droplet impact effect on rotating microflier surfaces and demonstrates the effectiveness of protecting microfliers with superhydrophobic coatings, which shall guide the manufacture and flight of microfliers in rainy conditions.

Roles of raindrop impact in detachment and transport processes of interrill soil erosion

To date the roles of raindrop impact in sediment entrainment and transport processes of interrill soil erosion are not yet fully quantified. The objectives are to 1) evaluate the effects of raindrop impact on sediment entrainment, 2) systematically quantify the relative importance of raindrop-driven and flow-driven transport, and 3) characterize sediment size distributions in different sediment transport regimes. A loam soil with 48.4% sand and 23% clay was packed into flumes (L × W × H: 1.8 × 0.5 × 0.1 m) and subjected to intensities of 60, 90, 120 mm h−1 at gradients of 5, 10, and 16°. Air filter and tarp covers were used to vary impact energy and flow length. Results show that sediment is largely entrained by raindrop impact and transported by raindrop-impacted flow. Interrill erosion consists of two composite processes: raindrop-induced detachment/entrainment and raindrop-impacted flow transport. The former includes direct detachment by raindrop impact and ‘flow detachment’. The latter includes raindrop-driven and flow-driven transports. The proportions between the two transports vary with slope steepness, slope length, flow depth (or flow discharge), rainfall intensity, and sediment characteristics. Raindrop-driven transport is competent but inefficient, while the opposite is true for flow-driven transport. Because raindrop impact plays dual roles in detaching soil (and/or entraining sediment) and enhancing sediment transport, a drop energy factor must be included in interrill erosion models to adequately simulate the dual roles. More studies are required to derive a drop energy function that takes into consideration the rainfall energy dissipation by canopies of various plants under natural conditions.

Theoretical and Experimental Studies of the Impact of High-Speed Raindrops on the Structural Elements of Modern Technology.

Objects of modern technology located in the rain zone experience additional loads and can be destroyed due to water droplet erosion. With a significant number of successive impacts in a certain period of time, rain causes damage to the surface of materials or structures. It should be noted that supersonic water droplet impact has a low probability of occurrence; however, the peak pressure impulse of the water hammer (up to GPa level) far exceeds the strength of many materials, and a small number of impacts are enough to damage the material or structure. Therefore, it is very interesting to determine the external load caused by a water droplet’s impact and its response to various obstacles. In this work, the external load is determined on the basis of experimental studies. To carry out such tests, a single-jet generator is most widely used, which, with a certain ease of operation, makes it possible to investigate the mechanisms of damage to materials and the effect of water droplet impact erosion on structural elements. Based on the obtained research results, mathematical modeling of the droplet impact with an obstacle is provided. The examples are considered.

Development and Calibration of a Low-Cost, Piezoelectric Rainfall Sensor through Machine Learning.

In situ measurements of precipitation are typically obtained by tipping bucket or weighing rain gauges or by disdrometers using different measurement principles. One of the most critical aspects of their operational use is the calibration, which requires the characterization of instrument responses both in laboratory and in real conditions. Another important issue with in situ measurements is the coverage. Dense networks are desirable, but the installation and maintenance costs can be unaffordable with most of the commercial conventional devices. This work presents the development of a prototype of an impact rain gauge based on a very low-cost piezoelectric sensor. The sensor was developed by assembling off-the-shelf and reused components following an easy prototyping approach; the calibration of the relationship between the different properties of the voltage signal, as sampled by the rain drop impact, and rainfall intensity was established using machine-learning methods. The comparison with 1-minute rainfall obtained by a co-located commercial disdrometer highlights the fairly good performance of the low-cost sensor in monitoring and characterizing rainfall events.