Sediment Deposit Thickness Research Articles

Holocene alluvial sediment storage in a small river catchment in the loess area of central Belgium

Soil erosion and sediment deposition widely affect landscape development, particularly in erosion-prone areas with loess-derived soils. Nevertheless, until now, few attempts were made to quantify soil losses and sediment storage over long (centennial or millennial) timescales. In this study, the Holocene alluvial sediment storage in a small river catchment (52 km 2) of the Belgian loess belt is estimated, and a preliminary sediment budget for the catchment is presented. In the valley of the Nethen River (c. 13 km long), a detailed survey of the alluvial sediment archive was conducted. Hand augerings and percussion drillings were made along cross-valley transects at 12 locations in the catchment. AMS 14C dating of peat samples provided a temporal framework for the interpretation of the cores. Results show that the thickness of Holocene sediment deposits in the Nethen valley is 4 to 6 m, which corresponds to a total clastic sediment mass of ∼14 × 10 6 t stored in the valley bottom. Three alluvial units could be distinguished and associated with deposition phases from 9600 to 2900 B.C., 2900 B.C. to A.D. 1000 and A.D. 1000 to present. In contrast to the older sediments (units 1 and 2), deposits from the last 1000 year (unit 3) contain little organic matter. They are seldom intercalated with peat layers, and devoid of tufa. Unit 3 reaches a thickness of c. 2 m, thereby representing 50% of the Holocene sediment mass stored in the alluvial plain. The mean sedimentation rate in the alluvial plain for this last phase is ∼26 t ha − 1 a − 1 , which is about tenfold larger than the sedimentation rates calculated for the older Holocene sediment units. Sediment supply towards the alluvial plain has therefore increased tremendously since Medieval times. These results are in contrast to dating results obtained for colluvial sediments in a nearby dry valley within the catchment of the Nethen, where soil erosion and sediment deposition started in the early Iron Age and was already substantial during the Roman Age. This means that there is a time lag of about one millennium between the onset of high sedimentation rates in the upstream area and high deposition rates in the alluvial plain. This is probably caused by a change in coupling (sediment connectivity) between the plateau, slopes, and rivers. As soil erosion proceeds, first the dry zero-order valleys in the catchment act as sediment traps, and only after these are filled sediment reaches the floodplains. The preliminary sediment budget for the Nethen catchment illustrates that 50% of the sediment that was eroded during the Holocene was stored in colluvial deposits, which are mainly located on footslopes and in dry valley bottoms. Another 29% of the sediment mass is stored in the alluvial plain.

Spatial evaluation of a physically-based distributed erosion model (LISEM)

Generally, spatially distributed erosion models are `validated' using only data on runoff and sediment load collected at the catchment outlet. This implies that one of the most important aspects of such models, i.e. the prediction of the spatial variation of erosion and deposition within the catchment is not at all tested. After an extreme event in a small agricultural catchment in the Belgium loam belt, erosion and deposition patterns were mapped by measuring rill and gully volumes and the thickness of sediment deposits. Volumes of interrill erosion were estimated. From these data, the erosion and deposition budget was calculated. The physically based soil erosion model LISEM was then used to simulate the event. Model parameters which could not be derived from the field data were estimated using guidelines for LISEM proposed for South Limburg (Netherlands), an area very near and similar to the Belgian catchment. The model was calibrated on total erosion within the catchment by varying the infiltration parameters. The results of the simulations show that the model predicts reasonably well the overall sediment delivery ratio of the event (ca. 60%). However, the observed variation in erosion rates for different crop types is not well-predicted. In general, LISEM strongly overpredicts erosion rates on fields with an important vegetation cover. Furthermore, the correlation between predicted and measured erosion rates per crop type at locations of the measured rill transects is weak. The predicted pattern of deposition is generally similar to the mapped deposition pattern. Discrepancies are mainly due to the fact that, in some cases, important (unpredicted) deposition was observed upstream of vegetation barriers or alongside roads. The study clearly shows that it is not possible to evaluate the performance of spatially distributed erosion models by using catchment outlet data alone. Such a `validation' can mask important spatial variations within the catchment which are not accurately predicted. It is therefore possible to obtain a good agreement between observed and simulated outlet data, while the spatial pattern erosion and deposition within the catchment is not well-described.

Deposition of Sediment in Transient Flow

An analytical method of predicting the change in riverbed elevation at transient phases is developed. A computer solution is presented along with the method in order that a sample computation can be made to check the stability and convergency of the method. For channels in which the sediment-laden water can be considered completely homogeneous and in which the velocity can be considered uniform over the cross-section, equations of continuity and the equation of motion of sediment-laden water are presented. Introducing the assumption that the difference between the original bed slope and the new bed slope within a short period is very small, these three basic partial differential equations are successfully reduced to two partial differential equations. The method of characteristics is then employed to solve for the mean velocity, v , and the mean depth of flow, h . In order to estimate the sediment deposit thickness, z , the equation of continuity for sediment is transformed directly into finite-difference form.