L Beuselinck

Grain-size analysis by laser diffractometry: comparison with the sieve-pipette method

Abstract Results of a traditional sedimentation technique for grain-size analysis (the sieve-pipette method) were compared with those of a laser diffraction grain-size analyser, the Coulter LS-100. Milled quartz samples and silty soil samples were used to investigate the effect of particle shape. The reproducibility of the Coulter LS-100 was better than that of the sieve-pipette method, except for the sand fraction. The agreement between the two methods is strongest for the milled quartz samples. The Coulter LS-100 underestimated the clay content of the silty soil samples and generally overestimated the clay content of the milled quartz samples. This indicates the importance of particle morphology. The milled quartz grains are very angular and somewhat elongated whereas the clay minerals in the clay fraction of the silty soil samples usually have a platy shape. Reduced major axis relationships are calculated which may be used to convert Coulter LS-100 results to those of the sieve-pipette method. Correlation is strongest for percentiles size data. The Coulter LS-100 underestimates the clay, but correctly estimates the sand fraction. This results in a weak relationship for the total silt fraction as measured by the two techniques. Therefore, an alternative method is proposed for estimating this fraction. A comparison of our data with published data and relationships suggests that the calibration relationships are quite robust as long as the clay mineralogy of the samples is similar.

Sediment export by water from an agricultural catchment in the Loam Belt of central Belgium

Abstract An agricultural drainage basin of 250 ha was selected in the Belgian Loam Belt to evaluate sediment export by water. Water discharge was continuously measured at the outlet of the catchment and suspended sediment samples were taken proportional with discharge during rainfall events. From these samples supended sediment concentration and grain-size distribution were determined. Within the catchment, regular surveys were conducted in order to obtain detailed data on vegetation cover, soil surface parameters and erosion and sedimentation features. Important variations in sediment yield were detected at various time-scales, ranging from a year, over a season to one event. These variations could be linked with processes occurring within the catchment. Due to the development of a vegetation cover, the suspended sediment concentrations were lower during the summer period than during the winter period for the same discharge. The evolution in both suspended sediment concentration and grain-size distribution during the winter period is explained by variations in the soil surface state of the fields as well as the development of a rill and gully network. The positive hysteresis observed within a single event is explained by taking into account the distance between the sediment source and the catchment outlet and not by sediment flushing. Our analysis shows that the role of the main factors controlling the geomorphic response of such a catchment can be identified only if measurements at the outlet are complemented by detailed data on the conditions within the catchment. Catchment model calibration and validation are therefore impossible if this information is lacking.

The value of a physically based model versus an empirical approach in the prediction of ephemeral gully erosion for loess-derived soils

Abstract A data set on soil losses and controlling factors for 58 ephemeral gullies has been collected in the Belgian loess belt from March 1997 to March 1999. Of the observed ephemeral gullies, 32 developed at the end of winter or in early spring (winter gullies) and 26 ephemeral gullies developed during summer (summer gullies). The assessed data have been used to test the physically based Ephemeral Gully Erosion Model (EGEM) and to compare its performance with the value of simple topographical and morphological indices in the prediction of ephemeral gully erosion. Analysis shows that EGEM is not capable of predicting ephemeral gully cross-sections well. Although conditions for input parameter assessment were ideal, some parameters such as channel erodibility, critical flow shear stress and local rainfall depth showed great uncertainty. Rather than revealing EGEMs inability of predicting ephemeral gully erosion, this analysis stresses the problematic nature of physically based models, since they often require input parameters that are not available or can hardly be obtained. With respect to the value of simple topographical and morphological indices in predicting ephemeral gully erosion, this study shows that for winter gullies and summer gullies, respectively, over 80% and about 75% of the variation in ephemeral gully volume can be explained when ephemeral gully length is known. Moreover, when previously collected data for ephemeral gullies in two Mediterranean study areas and the data for summer gullies formed in the Belgian loess belt are pooled, it appears that one single length ( L )–volume ( V ) relation exists ( V =0.048 L 1.29 ; R 2 =0.91). These findings imply that predicting ephemeral gully length is a valuable alternative for the prediction of ephemeral gully volume. A simple procedure to predict ephemeral gully length based on topographical thresholds is presented here. Secondly, the empirical length–volume relation can also be used to convert ephemeral gully length data extracted from aerial photos into ephemeral gully volumes.

Characteristics of sediment deposits formed by intense rainfall events in small catchments in the Belgian Loam Belt

Erosion and deposition patterns within two agricultural catchments in the Belgian Loam Belt were mapped and their volumes measured after an intense rainfall event. From these data, the total erosion and deposition budget was calculated. The surveys clearly indicated that deposits could be differentiated according to the type of process that caused deposition. For most deposits topography was the controlling factor. However, important deposits were also found at field borders where a vegetation barrier caused deposition. Vegetation-controlled deposition occurs at significantly higher slope gradients than slope-controlled sediment deposition. This implies that vegetation-controlled deposition has an important effect on the spatial distribution of deposited sediment and on the sediment delivery ratio at the catchment outlet. The undispersed aggregate-size distribution of sediment deposits in front of vegetation barriers is finer than the sediment deposited under topographically-controlled conditions. However, the dispersed particle-size distributions of both types of sediment are very similar and only slightly coarser than the dispersed particle-size distribution of the source material. During these extreme rainfall events, sediment is eroded, transported and deposited in aggregated form. The aggregates themselves have a particle size distribution, closely resembling the source material. Consequently, considerable quantities of fine material and associated pollutants, which are expected to be exported to the river system, are trapped within the catchment.

Characteristics and controlling factors of bank gullies in two semi-arid mediterranean environments

Abstract Bank gullies are gullies that are formed due to a height drop caused by a terrace or a river bank, which develop by headward retreat in erodible hillslopes. This study aims (i) to investigate the morphology of actively eroding bank gullies, i.e., geometrical characteristics resulting from past erosion and active erosion processes shaping the gully, and, (ii), to find relationships with environmental site characteristics, such as topographical parameters, material properties and climate. The ultimate goal is to identify the most important controlling factors of past and present bank gully erosion. Fifty-five active bank gullies formed in different lithologies by various erosion processes have been selected in the Guadalentin basin and the surroundings of Guadix (Southeast Spain). For each bank gully site, geometrical and topographical parameters of both the channel and the drainage basin were measured. Erosion features indicating activity at the gully head, such as tension cracks, plunge pools, undercutting, fluting, piping and rill or sheet erosion on sloping side walls were mapped, and samples were taken from distinct lithological layers that were considered to influence the type and intensity of erosion processes. A relationship could be shown between the presence of piping and fluting and a number of material characteristics, including particle size distribution, dispersion behaviour and electrical conductivity. On the other hand, lithology appeared not to be a differentiating factor on gully development in the long run, as expressed by the total eroded volume ( V ). This parameter was most strongly related to the drainage basin area in which the entire bank gully had been formed ( A o ), explaining 66% of the variance. The relationship is V =1.75* A o 0.59 . No significant difference was found between regression lines through sub-datasets of different soil textural classes. Finally, multiple regression was used to include both topographical parameters and material characteristics in an explanatory and/or predictive equation for the total eroded bank gully volume. The results of the analyses using the entire dataset, including the sites in the Guadalentin as well as in the Guadix area, have been compared with the results for the separate study areas. Differences are not only related to topographical and lithological characteristics, but may also be the consequence of a different climate in the two areas.

Sediment transport by overland flow over an area of net deposition

Flume studies were conducted in order to evaluate the influence of slope, sediment size, discharge and inflow sediment concentration on sediment deposition by overland flow. Additionally, experiments were carried out to measure transport capacity of overland flow at low slopes, using a wide range of discharges. The experimental data show that the hydraulic conditions where net deposition occurs can be divided into two domains. The first domain is characterized by hydraulic conditions where transport capacity is not significant. In the second domain net deposition still occurs but transport capacity is significant. The size of the latter domain is dependent on the sediment size distribution, on the hydraulic roughness and on the inflow sediment concentration. The experiments clearly indicate the necessity of incorporating a threshold value in any deposition equation. These experiments demonstrate that shear stress is a valuable threshold for deposition modelling. Copyright

EVALUATION OF THE SIMPLE SETTLING THEORY FOR PREDICTING SEDIMENT DEPOSITION BY OVERLAND FLOW

Experiments were conducted in the laboratory to evaluate the influence of sediment concentration, sediment grain-size distribution, bed slope and flow discharge on sediment deposition rates and patterns associated with a reduction in bed slope. The experimental data clearly indicate that sediment deposition by overland flow is a very selective process: fine particles remain almost entirely in suspension and coarse particles are deposited quickly. Analysis of the data shows that up to a critical unit discharge a simple settling equation without a transport term, assuming continuous mixing of the sediment and water, gives a good prediction of the overall sediment delivery ratio and the grain-size distribution of the deposited and the exported sediment. However, there are some discrepancies for the clay, the coarse silt and the sand fractions. The assumption of continuous mixing is tested by investigating the sedimentation patterns of very narrow size classes. The observed decrease of sediment concentration versus distance from the inflow point for these individual sediment size classes closely agrees with the prediction assuming continuous mixing. When the critical unit discharge is exceeded, hydraulic properties of the overland flow do influence the sediment delivery outcomes. At discharges exceeding the threshold value the simple settling theory underpredicts the sediment delivery ratio. In these hydraulic conditions, a transport term needs to be incorporated into the simple settling theory. It is shown that the transport capacity and the re-entrainment model yield similar expressions for the description of sediment transport by overland flow over an area of net deposition. The experimental data indicate that the re-entrainment of previously deposited sediment is non-selective. Copyright

The influence of rainfall on sediment transport by overland flow over areas of net deposition

Overland flow is often only a few millimetres deep. Consequently, the potential for raindrop impact to affect flow hydraulics and sediment transport is high. Furthermore, the relative importance of rainfall impact is highest for shallow low-energy flows on low slopes. In such flows net sediment deposition may occur. Therefore, laboratory experiments were conducted to study sediment deposition in the presence of rainfall over a range of hydraulic conditions. In order to investigate the impact of raindrops on sediment deposition by overland flow, these experimental data were compared to the experimental data collected in the absence of raindrop impact. Comparison of the experimental data shows that raindrop impact retards the flow velocity and has a clear positive effect on sediment delivery. Under rainfall significantly more coarse sediment is transported over areas of net sediment deposition. Subsequently, the experimental results are used to evaluate a multi-class net deposition theory, describing sediment transport and sediment sorting over areas of net deposition in the presence of both raindrop impact and flow-driven processes. The multi-class theory is calibrated using part of the experimental data. Evaluation of the model predictions using the other part of the data shows that the optimised model is able to accurately predict sediment delivery and sediment sorting over areas of net deposition.

Assessment of micro‐aggregation using laser diffractometry

In order to evaluate the influence of the measuring technique on the determination of (micro-)aggregation in soil and sediment samples, results of grain size distributions of undispersed silty soil samples obtained by the sieve-pipette method are compared with those obtained using a laser diffraction grain size analyser, the Coulter LS-100. Reduced major axis relationships are calculated which may be used to convert Coulter LS-100 results to those obtained by the sieve-pipette method. The relationships obtained are very similar to the reduced major axis relationships established for dispersed silty soil samples. The results also show that the Coulter LS measurements have a systematic bias compared to the sieve-pipette data. This implies that, if the percentage of (micro-)aggregation is determined, the (interpretation of the) results will be strongly dependent on the measurement technique used. Using the calibration relationships that were established, nomographs can be developed to predict the level of sieve-pipette (micro-) aggregation from Coulter LS-100 data. Copyright

An Alternative Approach to Modelling Sediment Deposition and Related Sorting

When sediment-laden overland flow passes across an area of reducing surface slope or increased hydraulic roughness, it often results in a reduction in the sediment load carried by that flow. We present a new model for describing the formation of such a deposit as the outcome of deposition by settling and resuspension occurring simultaneously. Consideration of size selectivity of the individual processes leads to predictions of moderate reduction of the proportion of coarse sediment passing an area of deposition while most fine sediment remains in suspension. We evaluate the model performance against results from a series of experiments for slope reductions using two sediment mixtures and a range of water flux, surface gradient and incoming sediment concentration. The model is found to perform well in predicting the gross sediment delivery ratio and the size composition of the sediment remaining in suspension. It is concluded that the model is a viable alternative to algorithms of Foster currently used in a wide range of sediment delivery models.