Multiple Rainfall Events Research Articles

The Effects of Rainfall Events on the Composition and Diversity of Microplastics on Beaches in Xiamen City on a Short-Term Scale.

Coastal beaches are vulnerable to microplastic pollution originating primarily from terrestrial and marine sources or the in situ weathering of plastic waste. The present study investigates the effects of rainfall events on the composition and diversity of microplastics on beaches in Xiamen City on a short-term scale. In the results, the quantity of microplastics in beach sediments was 245.83 ± 11.61 items·kg-1 (mean ± standard error). The abundance of microplastics did not differ after each rainfall event but significantly decreased after multiple rainfall events. When the diversity of microplastics in the coastal area was evaluated, the Shannon-Wiener index and Pielou's index also decreased from 3.12 and 0.64 to 2.99 and 0.62, respectively, after multiple rainfall events. Rainfall had varying effects on microplastics depending on their size and shape, with particles smaller than 500 μm experiencing pronounced reductions. There was a significant negative correlation between the abundance of microplastics and the grain size of sand, but a positive correlation with sediment moisture content. We encourage the consideration of the potential impact of rainfall events during sample collection to ensure the reliability of the data. We also recommend using diversity indexes to help in understanding the influence of physical processes on microplastic distribution and their mechanisms.

The Influence of Rainfall and Evaporation Wetting–Drying Cycles on the Open-Pit Coal Mine Dumps in Cam Pha, Quang Ninh Region of Vietnam

Among the slope hazards caused by rainfall, not all of them occur directly during storm washout, and the wetting–drying cycles’ effect on the rainfall–evaporation process is an important cause of shallow slope instability. In this study, taking the slope of the open-pit coal mine dumps in Cam Pha, in the Quang Ninh region of Vietnam, as the research object, we carry out experiments on the physical properties of the rock body under different wetting–drying cycles, as well as numerical analyses. The results show that the wetting–drying cycles significantly affect the physical and mechanical parameters and permeability of the rock body. In the process of the wetting–drying cycle, a transient saturated zone occurs on the surface of the slope, and the range of the unsaturated zone inside the slope body decreases with the increase in the number of wetting–drying cycles. Moreover, the infiltration line keeps moving downward, but the rate of downward movement is slowed down by the decrease in the gradient of matrix suction affected by rainfall. Under the influence of the wetting–drying cycles, the slope displacement, plastic zone, and maximum shear strain increment range gradually approach the slope surface with the wetting–drying cycles, and the displacement peak gradually increases. A dump is a site for the centralized discharge of mining waste, formed by the crushing and stockpiling of the original rock formation. Bang Nau is the name of the dump considered in this study. After multiple rainfall events, the slope stability under five wetting–drying cycles decreases from 1.721 to 1.055, and the landslide mode changes from a whole landslide to a single-step shallow landslide, with a certain landslide risk. It is necessary to strengthen the slope stability as the landslide risk is very high, and it is necessary to strengthen the monitoring and inspection of the slope.

Analysis of Effects of Spatial Distributed Soil Properties and Soil Moisture Behavior on Hourly Streamflow Estimate through the Integration of SWAT and LSM

This study addresses the challenge of accurately estimating hourly flow and soil moisture by integrating the Soil and Water Assessment Tool (SWAT) with a Land Surface Model (LSM). Our approach enhances SWAT by incorporating spatially distributed soil properties and a physically-based soil moisture process, using the Noah LSM for hourly soil moisture estimation. This integration captures spatial variations in soil moisture and hydraulic properties from remote sensing across the watershed. The parameter sensitivity analysis and the calibration of hourly flow were significantly impacted by the physically-based hourly soil moisture routing and the incorporation of spatially distributed soil properties. Consequently, the modified SWAT model showed improved accuracy in hourly flow simulations for long-term and multiple rainfall events. Validation results showed significant improvements, with Coefficient of Determination (R2) and Nash and Sutcliffe Efficiency (NSE) increasing by 25.95% and 33.3%, respectively, and Percent Bias (PBIAS) decreasing by 85.8%. Notably, the average error for peak flows across eight events decreased by 49%. These findings highlight the importance of initializing soil parameters based on spatial soil moisture distribution and incorporating physical process-based moisture routing to enhance hourly flow simulation accuracy. Future research should focus on validating the physical feasibility of the soil parameter set in the study area with detailed hourly flow and soil moisture data and exploring its applicability in various regions. This study provides valuable insights for the scientific community, water resources, and agricultural decision-makers regarding integrated modeling of soil moisture and hourly flow, which can inform dam operation management, disaster planning, and crop yield improvement.

Experimental Study on Runoff and Sediment Production of the Fully Weathered Granite Backfill Slope under Heavy Rain in Longling, Yunnan Province

Heavy or intermittent rainfall can cause slopes to become unstable and erode, resulting in significant damage, loss of life, and destruction of property. Targeted management solutions are based on an analysis of slopes’ flow generation and sediment production patterns during periods of rainfall. This study used a fully granite backfill slope as its research subject and examined the features of slope erosion during intermittent rainfall. We examined the processes of slope flow generation and soil erosion during intermittent rain through indoor artificially simulated rainfall experiments. Three intermittent rainfall events with a 220 mm/h intensity were designed during the experiment. Each rainfall event lasted for 60 min, with an interval of 60 min between the events. By analyzing multiple rainfall events, this study reveals the patterns of runoff and sediment yield on different slopes in response to variations in rainfall intensity and slope gradient. The runoff volume on other slope surfaces exhibits a similar pattern in reaction to changes in rainfall events. As the frequency of rainfall events increases, the surface runoff tends to be higher. Additionally, with variations in slope steepness, the runoff volume generally follows an increasing trend. Notably, the slope with a 20° incline shows the smallest runoff volume. The sediment yield on different slope surfaces gradually increases as the slope increases. In particular, on a 20° slope, the sediment yield experiences a substantial increase, indicating that the impact of the slope on the sediment yield becomes more pronounced. In different rainfall events, the morphology of the slope changes due to the influence of gravity and hydraulics, resulting in oscillations in both the average runoff rate and sediment yield. Furthermore, as the slope steepens, the amplitude of these oscillations increases. The process of slope erosion involves three stages: raindrop splash erosion, runoff erosion, and collapse damage. The sequence of slope damage locations is as follows: footslope, mid-slope, and hilltop. For the backfilled slope of completely weathered granite, the artificial slope can be controlled to around 20°. Erosion on the slope mainly occurs after the formation of gullies, and slope management should focus on preventing gully formation before it happens.

Robust optimal design of urban drainage systems: A data-driven approach

The design of urban drainage systems often relies on the coupling of optimization algorithms and process-based models, with the latter informed by design storms. However, such practice may not yield systems that are robust with respect to a broad array of rainfall conditions. To tackle this problem, we contribute a novel computational framework characterized by two distinguishing features. First, we use multiple rainfall events throughout the design process; and, second, we rely on faster emulators (instead of process-based models) to manage the inevitable increase in the computational requirements. In particular, our framework moves through five steps accounting for the generation of stochastic rainfall events, the selection of decision variables (via sensitivity analysis), emulator identification, emulation-based optimization, and ex-post robustness analysis. To test our framework, we implement it on a 33-km2 basin in Ho Chi Minh City (Vietnam) and benchmark it against a more traditional approach based on design storms. Results show that there are untapped opportunities for improving drainage performance without altering investment costs. For example, we found that a budget of $30 million can yield an overflow reduction of about 85% or 50% depending on whether we rely on stochastic rainfall events or design storms. Importantly, such improvements do not affect the computational requirements, owing to the use of emulators. The framework is applicable to other basins and could be readily extended to other sources of uncertainty, such as land use change, or climate change scenarios, which are expected to pose additional strains on urban drainage systems.

Analysis of Geotechnical-Assisted 2-D Electrical Resistivity Tomography Monitoring of Slope Instability in Residual Soil of Weathered Granitic Basement

Prolong heavy rainfall is increasingly inducing slope instabilities on the high-risk hills of weathered granitic basement in Penang. These slope instabilities are spatially controlled with changes in geotechnical properties of the slope soils. A reliable method to include density as part of geotechnical properties to calibrate Electrical Resistivity Tomography (ERT) resistivity distribution in slope instability monitoring is still rare. Hence, we present six ERT data that were acquired with survey length of 60 m and 1.5 m electrode spacing using Wenner-Schlumberger array from 2019-2020. The results were calibrated with the laboratory determined geotechnical properties- Moisture Content (MC), Particle Size Distribution (PSD), density and Hydraulic Conductivity (HC). The result of the analysis of ERT models classified resistivity distribution into saturated zones of 20% silt and clay, weak zones of 600-3000 Ωm and basement rocks of >5000 Ωm. The presence of floaters and boulders of resistivity >4000 Ωm overlie saturated zones coupled with multiple rainfall events that act as triggering factors for slope instability and failure. Geotechnical results show strong correlation of R≈0.94 between density and resistivity values which are crucial for the calibration of the ERT models because low-resistivity <600 Ωm areas have high MC, 30.1% with low density, 1176 kg/m3 and HC, 2.02x10-5 m/s whereas high resistivity <3000 Ωm areas have lower MC, 11.4% with relatively high density 1458 kg/m3 and HC 1.34x10-2 m/s. Therefore, we conclude that low-resistivity areas are composed of earth materials that are less dense low-permeable unstable zones of displacement which constitute subsurface drainage paths that are precursors to slope instability.

Soil moisture assimilation in urban watersheds: A method to identify the limiting imperviousness threshold based on watershed characteristics

Data assimilation is well suited for merging observed and simulated values to improve hydrologic forecasts. Typically, observation data that is assimilated into hydrologic models can be obtained from in situ gauges, radar, and remote sensing. In an urban environment, the benefits of soil moisture assimilation are not clearly defined. This work aims to identify the feasible imperviousness range at which it is advantageous to assimilate remotely sensed soil moisture to improve hydrologic forecasting in an urban watershed. A synthetic experiment was set up to simulate the retrieval of soil moisture onto a spatial grid and assimilate it into a hydrologic model. Sub-catchments were set up such that they would represent areas within that retrieval grid with varying levels of development. Multiple rainfall events were simulated with forecasts of up to 12 hours for each sub-catchment. The results of these simulations indicate that when areal average soil moisture is assimilated into an urban sub-catchment model which has a level of impervious that exceeds a threshold value, there is a decrease in model forecast performance, indicating that soil moisture assimilation is no longer beneficial. A quick way to determine the imperviousness threshold was then derived using a modified NRCS-CN method which matches the results of the synthetic experiments. This methodology was then further tested using real-world urban watersheds and shown to be a valid approach that can be used to quickly determine whether soil moisture assimilation would be beneficial for the watershed of interest. In general, this work has shown that hydrologic forecasting in an urban watershed can benefit from soil moisture assimilation even when imperviousness reaches 65–75%. Additionally, this work found a quick way to validate further if soil moisture assimilation is beneficial using some simple watershed characteristics.

Estimation of Field‐Scale Variability in Soil Saturated Hydraulic Conductivity From Rainfall‐Runoff experiments

AbstractSaturated hydraulic conductivity (Ks) is among the important soil properties that influence the partitioning of rainfall into surface and subsurface waters. Point estimates of Ks are difficult to determine and exhibit large spatial variability in fields. Often, data from field‐scale rainfall‐runoff experiments are utilized to assess the properties of the Ks random field that are required in the use of field‐scale infiltration models. Standard methods of calibration are confounded by nonuniqueness and identifiability problems associated with experimental data. In this study, a new method that employed a field‐averaged infiltration model and Monte Carlo simulations was used to obtain the possible range of distributions of Ks that would describe experimental observations over a field for a rainfall event. A Shannon information‐theoretic approach was subsequently adopted to consolidate the ranges of Ks distributions over multiple rainfall events to yield the best range of Ks distributions. The method was applied to data from several rainfall‐runoff events observed under natural conditions over an experimental field characterized by a silty loam soil and a small surface slope. Results suggest the existence of numerous parameter combinations that could satisfy the experimental observations over a single rainfall event, and high variability of these combinations among different events, thereby providing insights regarding the identifiable space of Ks distributions from individual rainfall experiments. Validation results showed that the method provides a realistic estimate of our ability to quantify the spatial variability of Ks in natural fields from rainfall‐runoff experiments.

Quantification of potential flood inundation areas in the marsh wetland of Honghe National Natural Reserve, Northeast China

Wetland protection and recovery needs scientific foundations for a better understanding of wetland floods, since the flooding existence and its dynamics are the key aspects in the formation and evolution of wetland ecosystems. The Honghe National Natural Reserve (HNNR) in the Northeast China has been listed in the Ramsar Convention on Wetlands since 2002. The specific dates for determining of an average inundation boundary caused by a regular flooding were firstly fixed from multiple rainfall events by a hydro-meteorological analysis based on the long-term rainfall data. The flood inundation areas in the HNNR marshes were mapped according to the correlation between hydro-geomorphology and regular flood progress. The mapping of the flood inundation areas were supported technically from the application of remote sensing images, combining the hydro-geomorphologic analysis with meteorological statistical data. The final map with the spatial-temporal content of an inundation created by hydro-geomorphological analysis and remote sensing data was validated by the long-term rainfall, soil moisture and surface water level data in the HNNR. The results indicated that the flood inundation areas were mainly distributed in large patches along the two main river courses of Nongjiang River and its branch of Woyalan River in the northern and central HNNR. The flood inundation patches were scattered as seasonal marshes and permanent ponds in the southern and eastern HNNR. The extracted flood inundation area accounted for more than a quarter of the total area of HNNR. The flood inundation area of Nongjiang was much larger than that of Wuyalan.

Effects of rock fragment content, size and cover on soil erosion dynamics of spoil heaps through multiple rainfall events

Spoil heaps are the main sources of soil erosion on disturbed land surfaces as an artificial accelerated erosion form, and rock fragments are an important component of spoil heaps. This study examined different fragment contents (0, 20, 40, 60 mass percentage) and sizes (1–4, 4–7, 7–10 cm) on hydrological processes, sediment yielding processes and rock fragment cover evolution through three sequential simulated rainfalls with a constant intensity of 1.5 mm min−1. The rock fragments in spoil heaps were found to reduce the soil loss amount by 35.23–76.84% through effects on runoff production and rock fragment cover. Runoff rates decreased with increasing rock fragment content, and size class 4–7 cm had the strongest reduction effect, but for a definite treatment runoff rates have little change in three rainfall events. The expanding rock fragment cover with cumulative rainfall in the multiple rainfall events, which increased fastest during the first rainfall period led to a significant decreasing soil detachment rate. Multiple regression analysis shows that the average detachment rate under each rainfall event could be estimated using a power function of average runoff rate and median rock fragment cover. These findings indicated that the presence of rock fragments in spoil heaps has an obvious mitigating effect on soil erosion, with the rock fragment content making a larger percentage contribution than rock fragment size.

Use of the Minimax Regret Approach for Robust Selection of Rainfall-Runoff Model Parameter Values Considering Multiple Events and Multiple Performance Indices

When hydrological simulation models are calibrated, the optimized parameter set based on a single event often does not show good accuracy for other events, even using the same performance measure. Therefore, this study improved the robust parameter set (ROPS) selection method for hydrological simulation models to consider multiple rainfall events by combining the Minimax Regret Approach (MRA) with Pareto optimums. A multi-event objective function, which is the linear combination of three weighting values and three Nash-Sutcliffe coefficients, was used and individually solved using a genetic algorithm. All available 63 multi-objective functions with weighting values based on three events were determined and then, the Pareto optimum was derived. These optimized parameter sets were considered as the ROPS candidates for the final selection. This study used two approaches. First, Nash-Sutcliffe efficiencies (NSEs) for the additional three rainfall events were used to identify the ROPS. Second, four performance indices, the NSE, the Peak Flow Error (PFE), the Root Mean Square Error (RMSE) and the Percent Bias (PBIAS), were used for the other three events. From the results, it can be concluded that the optimized parameter set from the best weighted multi-objective function using multiple events can simulate most rainfall events well with relatively high precision for both the NSE only and the four performance indices. Furthermore, it can be extended to combine multiple events from multiple sites.

Modeling Multi-Event Non-Point Source Pollution in a Data-Scarce Catchment Using ANN and Entropy Analysis

Event-based runoff–pollutant relationships have been the key for water quality management, but the scarcity of measured data results in poor model performance, especially for multiple rainfall events. In this study, a new framework was proposed for event-based non-point source (NPS) prediction and evaluation. The artificial neural network (ANN) was used to extend the runoff–pollutant relationship from complete data events to other data-scarce events. The interpolation method was then used to solve the problem of tail deviation in the simulated pollutographs. In addition, the entropy method was utilized to train the ANN for comprehensive evaluations. A case study was performed in the Three Gorges Reservoir Region, China. Results showed that the ANN performed well in the NPS simulation, especially for light rainfall events, and the phosphorus predictions were always more accurate than the nitrogen predictions under scarce data conditions. In addition, peak pollutant data scarcity had a significant impact on the model performance. Furthermore, these traditional indicators would lead to certain information loss during the model evaluation, but the entropy weighting method could provide a more accurate model evaluation. These results would be valuable for monitoring schemes and the quantitation of event-based NPS pollution, especially in data-poor catchments.

Detachment of fullerene nC60 nanoparticles in saturated porous media under flow/stop-flow conditions: Column experiments and mechanistic explanations

This study was aimed at investigating the detachment of fullerene nC60 nanoparticles (NPs) in saturated sand porous media under transient and static conditions. The nC60 NPs were first attached at primary minima of Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energy profiles in electrolyte solutions with different ionic strengths (ISs). The columns were then eluted with deionized water to initiate nC60 NP detachment by decreasing solution IS. Finally, the flow of the columns was periodically interrupted to investigate nC60 NP detachment under static condition. Our results show that the detachment of nC60 NPs occurred under both transient and static conditions. The detachment under transient conditions was attributed to the fact that the attractions acting on the nC60 NPs at primary minima were weakened by nanoscale physical heterogeneities and overcome by hydrodynamic drags at lower ISs. However, a fraction of nC60 NPs remained at shallow primary minima in low flow regions, and detached via Brownian diffusion during flow interruptions. Greater detachment of nC60 NPs occurred under both transient and static conditions if the NPs were initially retained in electrolyte solutions with lower valent cations due to lower attractions between the NPs and collectors. Decrease in collector surface chemical heterogeneities and addition of dissolved organic matter also increased the extent of detachment by increasing electrostatic and steric repulsions, respectively. While particle attachment in and subsequent detachment from secondary minima occur in the same electrolyte solution, our results indicate that perturbation in solution chemistry is necessary to lower the primary minimum depths to initiate spontaneous detachment from the primary minima. These findings have important implications for predicting the fate and transport of nC60 NPs in subsurface environments during multiple rainfall events and accordingly for accurately assessing their environmental risks.

Estimating Flood Quantiles on the Basis of Multi-Event Rainfall Simulation — Case Study

This paper presents an approach to estimating the probability distribution of annual discharges Q based on rainfall-runoff modelling using multiple rainfall events. The approach is based on the prior knowledge about the probability distribution of annual maximum daily totals of rainfall P in a natural catchment, random disaggregation of the totals into hourly values, and rainfall-runoff modelling. The presented Multi-Event Simulation of Extreme Flood method (MESEF) combines design event method based on single-rainfall event modelling, and continuous simulation method used for estimating the maximum discharges of a given exceedance probability using rainfall-runoff models. In the paper, the flood quantiles were estimated using the MESEF method, and then compared to the flood quantiles estimated using classical statistical method based on observed data.

Sub-seasonal behaviour of Asian summer monsoon under a changing climate: assessments using CMIP5 models

Numerous global warming studies show the anticipated increase in mean precipitation with the rising levels of carbon dioxide concentration. However, apart from the changes in mean precipitation, the finer details of daily precipitation distribution, such as its intensity and frequency (so called daily rainfall extremes), need to be accounted for while determining the impacts of climate changes in future precipitation regimes. Here we examine the climate model projections from a large set of Coupled Model Inter-comparison Project 5 models, to assess these future aspects of rainfall distribution over Asian summer monsoon (ASM) region. Our assessment unravels a north–south rainfall dipole pattern, with increased rainfall over Indian subcontinent extending into the western Pacific region (north ASM region, NASM) and decreased rainfall over equatorial oceanic convergence zone over eastern Indian Ocean region (south ASM region, SASM). This robust future pattern is well conspicuous at both seasonal and sub-seasonal time scales. Subsequent analysis, using daily rainfall events defined using percentile thresholds, demonstrates that mean rainfall changes over NASM region are mainly associated with more intense and more frequent extreme rainfall events (i.e. above 95th percentile). The inference is that there are significant future changes in rainfall probability distributions and not only a uniform shift in the mean rainfall over the NASM region. Rainfall suppression over SASM seems to be associated with changes involving multiple rainfall events and shows a larger model spread, thus making its interpretation more complex compared to NASM. Moisture budget diagnostics generally show that the low-level moisture convergence, due to stronger increase of water vapour in the atmosphere, acts positively to future rainfall changes, especially for heaviest rainfall events. However, it seems that the dynamic component of moisture convergence, associated with vertical motion, shows a strong spatial and rainfall category dependency, sometimes offsetting the effect of the water vapour increase. Additionally, we found that the moisture convergence is mainly dominated by the climatological vertical motion acting on the humidity changes and the interplay between all these processes proves to play a pivotal role for regulating the intensities of various rainfall events in the two domains.

Estimating short-term soil erosion rates after single and multiple rainfall events by modelling the vertical distribution of cosmogenic 7Be in soils

The 7Be method is a promising tool for estimating soil erosion rates at time scales of days to months. However, so far its applicability is limited to steady state concentrations of the tracer. In this paper a diffusion-sorption model is presented, which takes into account atmospheric input rates, transport down the soil column and radioactive decay of 7Be. It allows simulation of the non-steady-state 7Be inventories and depth distributions in soil and their modification by discrete erosion events as well as by tillage. This model is used to quantify erosion rates for 12 individual heavy rainfall events over a period of two years at an agricultural study site in Germany. Comparisons of soil collected at the bottom of our study site with predictions made using our proposed radioberyllium technique generally show good to excellent agreement even for multiple erosion events.

Antecedent controls on rainfall runoff response and sediment transport in a high arctic catchment

In the High Arctic, late summer rainfall events have the potential to increase discharge and transport substantial amounts of sediment. Past studies have not considered the role of antecedent soil moisture with respect to rainfall‐runoff response, particularly in the context of changing climate. We investigate these processes through multiple major rainfall events and the associated discharge and suspended sediment response, measured in a small watershed at ape ounty, elville sland, unavut. Fluvial and sedimentological responses to two major rainfall events on 9 ul. and 23 ul., 2012, totaling 35.4 and 10.6 mm, respectively, resulted in negligible responses in discharge and suspended sediment mobilization. The magnitude and intensity of these events were not unusual in this region, but the lack of response was substantial when compared with past events of similar magnitudes. The two‐week antecedent period leading up to the 9 ul. rainfall was characterized by exceptional temperatures and high solar radiation, decreasing soil moisture and increasing infiltration potential, all of which substantially diminished the runoff and resultant suspended sediment transport. These results demonstrate that antecedent catchment conditions may substantially attenuate suspended sediment transport responses to multiple major rainfall events and decrease runoff ratios and characterization of these conditions is necessary to accurately model catchment response.

Green Courtyard System to Remove Fluoride from Stormwater: Modeling and Field Measurements

In this work, performance of an innovative in-situ passive engineered natural system to remove fluoride from stormwater and to reduce the peak stormwater flow rate was investigated for multiple rainfall events (from August 13 to October 20, 2010). Water flow was measured on site and samples were taken for fluoride analysis throughout several rain events. A mathematical model was developed and used to predict stormwater flow and fluoride concentration in the effluent of an in-situ passive treatment system consisting of roof, green courtyard basin, and an activated alumina (AA) treatment cell. This model is a combination of a mass balance and one-dimensional advection-dispersion model, which involves three components: (1) flow of water, (2) fluoride build up and wash-off on a roof, and (3) fluoride transport. For rainfall events investigated, the fluoride that had accumulated on the roof by deposition during dry weather quickly washed off during the beginning of the storm event. The green courtyard system reduced concentrations of fluoride contaminated runoff. Model and field results indicate that under conditions encountered, the AA effectively removed fluoride from stormwater to undetectable levels (<1 mg/L). Data from this study can be used to aid in determining engineering design parameters needed for scale up and economic analysis. This technology may be appropriate for a range of stormwater contaminants if the appropriate reactive media is used within the system to reduce runoff and the environmental impact of runoff associated contaminants.

Effect of antecedent conditions and fixed rock fragment coverage on soil erosion dynamics through multiple rainfall events

The effect of antecedent conditions and specific rock fragment coverage on precipitation-driven soil erosion dynamics through multiple rainfall events was investigated using a pair of 6-m × 1-m flumes with 2.2% slope. Four sequential experiments – denoted E1, E2, E3 and E4, involved 2-h precipitation (rates of 28, 74, 74 and 28 mm h-1, respectively) and 22 h without rainfall – were conducted. In each experiment, one flume was bare while the other had 40% rock fragment coverage. The soil was hand-cultivated and smoothed before the first event (E1) only, and left untouched subsequently. Sediment yields at the flume exit reached steady-state conditions over time scales that increased with sediment size. Experiments were designed such that both steady and non-steady effluent sediment yields were reached at the conclusion of E1. Results from subsequent experiments showed that short-time soil erosion was dependent on whether steady-state erosion was achieved during the preceding event, although consistent steady-state effluent sediment yields were reached for each sediment size class. Steady-state erosion rates were, however, dependent on the rainfall intensity and its duration. If steady-state sediment yields were reached for a particular size class, that class’s effluent sediment yield peaked rapidly in the next rainfall event. The early peak was followed by a gradual decline to the steady-state condition. On the other hand, for size classes in which steady state was not reached at the end of the rainfall event (i.e., E1), in the following event (E2), the sediment yields for those classes increased gradually to steady state, i.e., the sharp peak was not observed. The effect of rock fragment cover (40%) on the soil surface was also found to be significant in terms of the time to reach steady state, i.e., their presence reduced the time for steady conditions to be attained. Effluent sediment yields for the bare and rock fragment-covered flumes (E1) showed steady conditions were reached for the latter, in contrast to the former. We used the Hairsine-Rose (H-R) model to simulate the experimental data as it explicitly models soil particle size classes. Experiments E1 and E2 involved soil compaction by raindrops, and in this case the model predictions were found to be unsatisfactory. However, compaction was effectively completed by the end of experiment E2, and the model provided reasonable predictions for experiments E3 and E4.

불포화토 칼럼시험을 통한 연속강우와 반복강우의 강우침투속도 분석

본 연구는 불포화 풍화토별 강우지속시간 및 비강우시간에 따른 강우침투속도 관계를 파악하기 위하여 국내에서 산사태 발생빈도가 높은 선캠브리아기 편마암 풍화토와 백악기 화강암 풍화토를 대상으로 불포화 풍화토 칼럼시험을 하였다. 본 연구에서는 일정시간 간격으로 체적함수비를 측정하기 위하여 함수비 측정 TDR센서를 이용하였다. 강우강도 조건은 20 mm/h로 선정하여 연속강우와 반복강우를 재현하였으며, 반복강우의 경우 강우시간과 비강우시간을 조절하였다. 그리고 흙의 단위중량 조건은 편마암 풍화토의 경우 현장 건조단위중량보다 낮고 칼럼상부유출이 일어나지 않는 1.35 <TEX>$g/cm^3$</TEX>, 화강암 풍화토의 경우 현장 건조단위중량인 1.21 <TEX>$g/cm^3$</TEX>로 선정하였다. 편마암 풍화토와 화강암 풍화토 총 강우량 200 mm인 조건에서 <TEX>$2.090{\times}10^{-3}{\sim}2.854{\times}10^{-3}$</TEX> cm/s와 <TEX>$1.692{\times}10^{-3}{\sim}2.012{\times}10^{-3}$</TEX> cm/s로 총 강우량 100 mm에서의 <TEX>$1.309{\times}10^{-3}{\sim}1.871{\times}10^{-3}$</TEX> cm/s와 <TEX>$1.175{\times}10^{-3}{\sim}1.581{\times}10^{-3}$</TEX> cm/s보다 강우침투속도가 빠르게 나타났다. 이는 동일 시간당 토층 내 주입되는 물의 양이 200 mm조건에서 100 mm조건보다 많기 때문이다. 완전 건조 상태의 강우침투속도와 강우가 반복되어 물을 함유하고 있는 상태의 강우 재침투속도를 비교해 보면, 편마암 풍화토와 화강암 풍화토의 최초 강우침투속도인 <TEX>$1.309{\times}10^{-3}{\sim}2.854{\times}10^{-3}$</TEX> cm/s와 <TEX>$1.175{\times}10^{-3}{\sim}2.012{\times}10^{-3}$</TEX> cm/s보다 강우 재침투속도가 <TEX>$1.307{\times}10^{-2}{\sim}1.718{\times}10^{-2}$</TEX> cm/s와 <TEX>$1.789{\times}10^{-2}{\sim}2.070{\times}10^{-2}$</TEX> cm/s로 높게 나타났다. 이는 토층 내 공기의 함입량이 줄어들어서 불포화 투수계수에 영향을 미치는 흡입력(matric suction)이 감소한 것이 원인으로 생각된다. Unsaturated soil column tests were performed for weathered gneiss soil and weathered granite soil to assess the relationship between infiltration velocity and rainfall condition for different rainfall durations and for multiple rainfall events separated by dry periods of various lengths (herein, 'rainfall break duration'). The volumetric water content was measured using TDR (Time Domain Reflectometry) sensors at regular time intervals. For the column tests, rainfall intensity was 20 mm/h and we varied the rainfall duration and rainfall break duration. The unit weight of weathered gneiss soil was designed 1.21 <TEX>$g/cm^3$</TEX>, which is lower than the in situ unit weight without overflow in the column. The in situ unit weight for weathered granite soil was designed 1.35 <TEX>$g/cm^3$</TEX>. The initial infiltration velocity of precipitation for the two weathered soils under total amount of rainfall as much as 200 mm conditions was <TEX>$2.090{\times}10^{-3}$</TEX> to <TEX>$2.854{\times}10^{-3}$</TEX> cm/s and <TEX>$1.692{\times}10^{-3}$</TEX> to <TEX>$2.012{\times}10^{-3}$</TEX> cm/s, respectively. These rates are higher than the repeated-infiltration velocities of precipitation under total amount of rainfall as much as 100 mm conditions (<TEX>$1.309{\times}10^{-3}$</TEX> to <TEX>$1.871{\times}10^{-3}$</TEX> cm/s and <TEX>$1.175{\times}10^{-3}$</TEX> to <TEX>$1.581{\times}10^{-3}$</TEX> cm/s, respectively), because the amount of precipitation under 200 mm conditions is more than that under 100 mm conditions. The repeated-infiltration velocities of weathered gneiss soil and weathered granite soil were <TEX>$1.309{\times}10^{-3}$</TEX> to <TEX>$2.854{\times}10^{-3}$</TEX> cm/s and <TEX>$1.175{\times}10^{-3}$</TEX> to <TEX>$2.012{\times}10^{-3}$</TEX> cm/s, respectively, being higher than the first-infiltration velocities (<TEX>$1.307{\times}10^{-2}$</TEX> to <TEX>$1.718{\times}10^{-2}$</TEX> cm/s and <TEX>$1.789{\times}10^{-2}$</TEX> to <TEX>$2.070{\times}10^{-2}$</TEX> cm/s, respectively). The results reflect the effect of reduced matric suction due to a reduction in the amount of air in the soil.