In the runoff process, the particles covered the former topsoil with different amount and particle sizes, over and over again. Consequently, particle stratification was clearly existent in the DF profiles since the particles were deposited at different times. The horizontal distribution of particle sizes controlled the appearance of the soil layers.
The occurrence of preferential flow, which may transport chemicals or contaminant materials into the groundwater quickly, was mainly influenced by the layered soils of the DF Zhao et al. The finding of stratification in the DFs agreed with the results of Boll et al.
To identify the most erodible particle-size group in this region, the enrichment ratio ER of various particles in the two DFs was compared Figure 4. In this study, the ER value was calculated by the mean proportion of various particles in the DF, by dividing the proportions of particles collected from controlled sloping areas.
For a more specific range, a more detailed classification than the Chinese taxonomy was applied see x-axes of Figure 4. In D1, the range of 0. The same result was found in D2. This means that the 0. This result contradicted the study of Young , who found that 0.
This can be explained by the fact that soil erosion and soil loss were mainly associated with heavy rains on the CLP, and the eroded soil was sandy loam with high sa contents, as explained by Hu et al. Consequently, the various eroded environments and materials, and mainly rainfall intensity and soil texture, would determine the most erodible particle-size group.
This implies that these fractions were easily detached in the soil erosion process. Soils with very high sa are not well-aggregated and thus tend to erode before the other particles. The results illustrated that the fine particles i.
However, Ampontuah et al. From the results of this research, the characteristics of the high silt content were relatively stable with regard to the water erosion on the CLP. For the fraction 0. As a consequence, this range has a lower ER than the fractions of 0. Wang et al. As it is well-known, the content of soil organic C has a positive linear relationship with the value of fine particles e.
Despite the higher sand fraction, more carbon was captured in the DF than in sloping areas under no tillage. Consequently, the DF on the CLP contribute greatly to carbon sequestration, due to the large scope and long time of this soil conservation practice. The particle size has a great effect on soil water distribution and movement Zhao et al. The relationship between the particles and the volumetric soil water content was analyzed by SPSS The soil water contents were assessed on different dates by a neutron probe to represent wet and dry conditions Zhao et al.
Table 3 shows Pearson's correlation coefficients between soil water content and various particles of D1 for several random days. All particles size groups were significantly related with soil water content. Due to the large pore structure, the low soil water content is more significantly related with cas in the DF. Hu et al. This result indicates the trend of most particle size groups to show low correction with the soil water content, except for the csa fraction, which tends to be more prevalent with soil water content in the DF.
The range of 0. Generally, disregarding their significance level, cl and si were positively and sa negatively correlated with soil water content.
However, no relationship was found between particle-size group and soil water content for D2 in this study. This could be explained by deposition effects, leading to the retention of great amounts of shallow groundwater by the dam body, as well as to the presence of air and water in the pores.
This air affects the relationship between particles and soil water content negatively. This study reported a layered structure of various particle-size groups in the profiles of deposited farmland soil.
The distribution patterns of the particles were the basis of soil layers that strongly influence water distribution and movement. This study reported an increased erodibility of soils with high sand fractions and low clay and silt contents. In this research, the particle size range of 0. The range of erodible particles differed according to the different eroded material and environmental conditions.
Particles of 0. With increasing soil water content, a significant correlation between particles and increased soil water was found, except for the coarse sand fraction, with an opposite trend. We express our most sincere thanks to the reviewers and editors for their important contributions to this paper. We thank Dr. Timm sincerely for his help on portuguese abstract.
Abrir menu Brasil. Abrir menu. Assessment of soil particle redistribution on two contrasting cultivated hillslopes. Geoderma, , AMER, A. Moisture adsorption capacity and surface area as deduced from vapour pressure isotherms in relation to hygroscopic water of soils. Biologia, , The effect of plantation silviculture on soil organic matter and particle-size fractions in Amazonia. Solo, , BOLL, J. Using ground penetrating radar to detect layers in a sandy field soil.
GAO, Z. Study on the construction and layout of check-dams on the Loess Plateau. Effects of particle size distribution and water content at compaction on saturated hydraulic conductivity and strength of high sand content root zone materials.
Soil Sci. HU, W. Soil water content temporal-spatial variability of the surface layers of a Loess Plateau hillside in China. Influence of vegetal cover on sediment particle size distribution in natural rainfall conditions in a semiarid environment. Catena, , Soil organic matter particle size fractions in soils of the semiarid Argentinian Pampas. TANG, K.
Academia Sinica MWR, , Physical properties of dystrophic Red Latosol Oxisol under different agricultural uses. WANG, Y. Profile variability of soil properties in check dam on the loess plateau and its functions.
XU, M. To accelerate the construction of check-dams in the Loess Plateau. Yellow River, , ZHAO, P. Soil water spatial distribution in dam farmland on the Loess Plateau, China. Acta Agr. B - Soil Plant, , Soil water distribution and movement in layered soils of a dam farmland. Water Res. History Accepted 17 Aug Received 31 Mar Pei Zhao.
Figures 9 Tables 3. Google Google Scholar. Water Sci Technol 25 October ; 76 8 : — The presence of a first flush FF of suspended solids SS in stormwater runoff has important implications for the design of treatment facilities, as does the particle size of solids. In this study, the FF behaviour of SS was investigated in five size ranges, sampled from an urban stormwater drainage system located in Dublin, Ireland.
In the majority of rain events, the FF strength increased with decreasing particle size, probably related to the lower intensities required to dislodge solids at the onset of rainfall.
Although FF strength was correlated with rain event characteristics, prediction intervals were too broad to confirm FF presence based on rainfall data alone. Therefore, the design of smaller treatment volumes based on an assumption of FF must be justified by local monitoring data. The FF is an important phenomenon due to its potential short-term toxicity for receiving water biota Kang et al.
Furthermore, the design of stormwater treatment systems is often based on the assumption of a FF effect. However, the possibility exists that a flush of pollutants may be observed at the drainage outlet at any time in the storm event, in response to peaks in rainfall intensity or runoff Aryal et al. Many researchers have examined the links between rainfall characteristics and the FF strength of SS. FF strength may be considered as the mass of pollutants transported in the initial storm event volume, although precise definitions vary.
While some researchers have identified relationships for specific catchments Deletic , there has been a greater difficulty in finding common explanatory variables for FF strength across different catchments and surface types Bertrand-Krajewski et al. This difficulty arises from attempting to represent complex physical transformations which occur during the transport of solids through urban drainage systems by relatively simple regression relationships.
One of the confounding factors is the selective transport of SS with respect to particle size in the build-up and washoff of solids from urban surfaces Zafra et al. Other factors influencing FF strength may include re-suspension of previously deposited sediments Aryal et al.
Suspended solids FF is often analysed in terms of a single size fraction, typically 1. This aggregation of particle sizes may mask significant factors influencing FF occurrence. There have been a number of studies of size-fractionated FF. In this study, limited data three storm events were available for the analysis of size-fractionated FF, but it was noted that larger particles were washed out earlier in the storm events than fine particles.
From grab sampling of highway runoff, Li et al. Whilst the preceding studies have focussed on the pattern and timing of FF, there has been limited study of the relationship between rain event characteristics and the FF strength of SS in different size fractions, which may allow the prediction of a significant FF effect for a given catchment.
This paper addresses three key questions, supported by detailed field data, relating to the FF of SS transported to the outlet of a stormwater drainage system: Does the FF strength of SS vary by particle size? FF characteristics were investigated by sampling the outlet of a stormwater drainage system over an month period.
Discrete samples were analysed for SS to ascertain FF behaviour. Additionally, analysis of particle size distribution PSD in a number of storm events allowed FF behaviour in different SS size fractions to be assessed. The catchment was monitored between March and August , during which 19 rain events were analysed for suspended solids concentration SSC , of which eight were also analysed for PSD.
For each event, flow was measured at 2-minute intervals using an area-velocity probe ISCO Rainfall data were provided by a tipping-bucket rain gauge 0. Samples were retrieved at the outlet of the drainage system using an automatic sampler ISCO , with the sampler inlet secured at the base of the pipe.
Whilst stratification of stormwater solids may present difficulties for representative sampling by fixed point sampling inlets Selbig et al. For future studies, the use of depth-integrated sampling should be examined.
This approach was designed to collect more samples on the rising limb of the hydrograph, where changes in SSC and PSD were more rapid compared to the falling limb. The discrete samples were first passed through a 1-mm nylon mesh to remove gross solids. PSDs of stormwater solids were measured by a laser diffraction instrument Malvern Mastersizer in the size range of 0.
The PSD was measured as the percentage of total volume within each size range, assuming a spherical particle shape. Distributions were calculated in five size fractions: 0. The fractions were combined with the corresponding discrete sample measurement of SSC to generate SS loads in each size fraction, e.
The methodology assumed a constant density of particles with respect to particle size. Since the specific gravity of stormwater solids is strongly influenced by the fraction of organic material present, the assumption of constant density was examined by testing the organic content of stormwater solids see Supplementary Information.
A value of b equal to 1 represents a uniform distribution of pollutant mass over the rain event, with values less than 1 indicating a FF effect, and lower values of b equating to a stronger FF effect.
Saget et al. The FF coefficient was also calculated to enable comparison of the findings with previous studies. Regression models which predict FF strength based on rainfall characteristics are attractive since they can potentially identify sites which are susceptible to FF. However, this approach is predicated on there being significant relationships between FF strength and the explanatory variables, and the regression model must be suitable for the geographic region and land use in question.
In this study, the regression analysis consisted of two steps. In step 1, correlations between single rain event characteristics and FF strength were examined. In step 2, multiple linear regression MLR was used to identify significant combinations of FF explanatory variables. This pattern is consistent with the findings of Zafra et al. It follows that during the initial part of rain events, the finest particles will be eroded first, and so are more likely to exhibit a FF effect than are larger particles.
Examining size effects within events, FF strength generally increased with decreasing particle size for events 5, 6, 7 and 8. However, the FF strength increased with increasing particle size for events 2 and 3 and to a marginal extent for event 9 , which was unexpected.
The researchers separated events into flow-limited and mass-limited groups, based on the mean event runoff rate, observing no clear relationships between FF strength and particle size for the flow-limited events, but a larger FF effect for larger particle sizes in the mass-limited events.
This would indicate that the FF strength is related to the particle size and antecedent mass of solids available. This implied that for events 2 and 3, the FF for fine solids was limited by the available mass. Correlations between the full SS gradation in the complete dataset 14 events and reduced dataset 7 events are shown in Table 2. These negative correlations could be explained by the fact that large rain events in terms of rainfall depth tended to be longer in duration and lower in rainfall intensity.
These trends were repeated in the reduced dataset, and in some cases the correlation coefficients were greater than the complete dataset. For example, the R 2 value for MF increased from 0. This may have occurred because the reduced dataset consisted of mostly single flow peak events 5 of 7 events , whereas the complete dataset contained a lower proportion of single peak events 7 of 14 events. Prediction of FF strength using aggregate rain event characteristics is less effective for events with multiple flow peaks since the explanatory variables cannot account for effects such as the order in which the PF occurs, or variations in available washoff mass for successive flow peaks.
This phenomenon was explored further using an MLR approach. Interestingly, the ADP appeared in all of the top performing models, which indicated that the available mass of solids was an important factor for FF strength.
The storm duration also appeared in four of the five models. For the fine solids, it is likely that the available SS mass on the pavement surface was influential. The catchment sampled in this study was also subject to measurement of solids build-up rates during dry weather Morgan et al. This supports the theory that FF effects for fine solids may have been influenced more by the limited mass available for washoff than rain event characteristics. For example, if low quantities of fine solids were available at the start of the event, they may have been eroded in a sporadic fashion as they may have been adhered to larger particles or the road surface.
Conversely, where a significant mass of fine particles was available, erosion and FF patterns may have been more consistent with rain event characteristics. Examining the limited correlations for coarser particles, the influence of the drainage system must be considered. Consequently, a FF effect may be observed for solids in source-area runoff but not for solids transported to the outlet of the drainage system. This may have been partly due to the additional PSD analysis step, which would have introduced additional experimental error.
In some cases, such as models 3 and 5, the correlation coefficients were significantly lower or zero in the size-fractionated models. Therefore, the application of the MLR has limited benefit in identifying the likelihood of a strong FF effect on the catchment, using rainfall characteristics alone. Furthermore, if the model was to be applied to a different catchment, re-calibration of the model coefficients would be required. The design of stormwater treatment systems is commonly based on the assumption of a FF.
For example, the UK Highways Agency recommends treating at least 10 mm of runoff on the basis that this is often the most seriously polluted UK Highways Agency Similarly, a minimum water quality treatment volume of However, the findings of this research, which are consistent with some previous studies, demonstrate that a significant mass FF is uncommon for SS transported to the outlet of stormwater drainage systems.
Therefore treatment structures, such as detention ponds, should be effective in treating SS throughout the storm event, not just the initial runoff. If treatment structures are designed to target the FF, they should be capable of removing this particle size range. Where treatment volumes are limited, a FF filtration component could be incorporated to ensure retention of fine solids Pitt et al. There were reasonable correlations between multiple rain event characteristics and FF strength R 2 of 0.
The purpose of such models would be to identify catchments with a potential for a significant FF effect. In these cases, smaller treatment volumes could be justified. However, the high uncertainty associated with the MLR model predictions imply that reduced treatment volumes should only be applied where a FF effect has been confirmed with field data.
The FF of SS from the outlet of an urban residential drainage system was investigated, in both the complete gradation and in different size fractions. Of the 14 events where the complete SS gradation was analysed, 11 of the 14 events exhibited a moderate FF effect, according to the definition of Saget et al. This was expected, since the erosive power required to dislodge and transport finer particles would have been lower; thus finer particles would have been transported to the outlet earlier in the rain event.
In two of the remaining events and to a limited extent in one event , the FF strength increased with increasing particle size. It is hypothesised that these events were mass limited for finer particles. The relationship between FF strength and rain event characteristics was investigated.
The lower correlations for the finer fractions were attributed to the limited mass of solids available for washoff. For future research, continuous models of build-up and washoff of SS should be used to investigate the role of antecedent mass in determining FF strength.
The study findings have important implications for stormwater management. Therefore, stormwater best management practices BMPs should be designed to treat the entire rain event volume.
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