Gert Verstraeten | KU Leuven (original) (raw)

Papers by Gert Verstraeten

Sediment dynamics during the Holocene are mainly controlled by either climate change or human imp... more Sediment dynamics during the Holocene are mainly controlled by either climate change or human impact, as is evidenced by numerous field studies. In order to identify the relative importance of climate or human impact, it is foremost vital to have a good idea about the intensity of these driving forces. Within the territory of the ancient city of Sagalassos, located

Boardman/Soil Erosion in Europe, 2006

… 2009, held 19-24 …, Apr 1, 2009

During the course of the Holocene, humans have impacted the sediment dynamics of hillslopes, rive... more During the course of the Holocene, humans have impacted the sediment dynamics of hillslopes, river channels and even global river systems, mostly with ever-increasing intensity. However, there is no clear one on one relation between the intensity of human presence (population density) and the magnitude of impact on sediment transfers. Several thresholds have to be crossed for coupling between the various landscape elements in a river catchment to occur, ensuring that anthropogenic landscape disturbances impact ...

Catena, 2009

A Holocene sediment budget was constructed for the 758 km 2 Dijle catchment in the Belgian loess ... more A Holocene sediment budget was constructed for the 758 km 2 Dijle catchment in the Belgian loess belt, in order to understand long-term sediment dynamics. Hillslope sediment redistribution was calculated using soil profile information from 809 soil augerings, which was extrapolated to the entire catchment using morphometric classes. As large parts of the forests within the catchment prove to have undergone little or no erosion since medieval times, a correction was applied for the presence of forests. Total Holocene erosion amounts 817 ± 66 Mt for the catchment, of which 327 ± 34 Mt was deposited as colluvium. This corresponds with a net Holocene soil erosion rate of 10.8 ± 0.8 × 10 3 Mg ha − 1 for the entire Dijle catchment. Alluvial deposits were studied through 187 augerings spread over 17 cross-valley transects. The total alluvial sediment deposition equals 352 ± 11 Mt or 42% of total eroded sediment mass. Results indicate that at the scale of a medium-sized catchment the colluvial sediment sink is as important as the alluvial sediment sink and should not be neglected. As a result the estimation of erosion through alluvial storage and sediment export would yield large errors. Dating of sediment units show an important increase in alluvial deposition from medieval times onwards, indicating the important influence of agricultural activities that developed from that period. Mean sediment export rates from the catchment for the last 1000-1200 years range between 0.8 and 1.3 Mg ha − 1 a − 1 and are consistent with present suspended sediment measurements in the Dijle. Erosion for agricultural land for this period is 9.2 ± 2.2 Mg ha − 1 a − 1 . Sediment budgets for the various tributary catchments provide an insight in the sources and sinks of sediment at different scales within the catchment.

Floodplain deposition rates have increased markedly under influence of anthropogenic land use thr... more Floodplain deposition rates have increased markedly under influence of anthropogenic land use throughout the Late Holocene in many Western and Central European catchments. These variations in sedimentation rates have changed the geomorphology and ecology of many floodplains as is shown here for the River Dijle (760 km 2) in the Belgian Loess Belt. Based on coring and pollen data, the geomorphology and ecology of the floodplain as well as the regional vegetation has been reconstructed for ...

Quaternary Science Reviews, 2014

Reconstructing and quantifying human impact is an important step to understand how, when and to w... more Reconstructing and quantifying human impact is an important step to understand how, when and to what extent humans have changed the landscape during the Holocene. In this study we present a reconstruction of vegetation changes throughout the Holocene based on palynological data of six study sites in the Dijle catchment, located in the Belgian loess belt. A reconstruction of human impact in the catchment is extracted from the palynological study based on statistical analyses (cluster analysis and non-metric multidimensional scaling (NMDS)). The NMDS analysis on the pollen data do not detect large-scale Mesolithic or Neolithic human activities on the Dijle catchment. In these periods, human impact in the catchment was probably limited to local disturbances and small-scale forest clearances. Only from the Bronze Age onwards (ca 3900 cal a BP) human impact was clearly detected in the pollen records and vegetation gradually changed. Human impact further increased from the Iron Age onwards, except for a temporary halt between ca 1900 and 1600 cal a BP, possibly coupled with the Migration Period in Europe. The general vegetation development and increasing human impact are rather similar at the catchment scale, beside some local variations in timing and intensity of the human impact in the different subcatchments. The applied methodology, cluster analysis and NMDS, proves to be a useful tool to provide semi-quantitative insights in the temporal and spatial vegetation changes related to increasing human impact.

Quaternary Science Reviews, 1999

Since 1990 archaeological research by the Catholic University of Leuven (Belgium) has been carrie... more Since 1990 archaeological research by the Catholic University of Leuven (Belgium) has been carried out at the ancient site of Sagalassos (Ag\ lasun, Burdur province, Southwestern Turkey). At first, research focused on the excavation of the city and the study of the immediate vicinity which provided it with raw materials. The main objective was to obtain a clear picture of the history and development of the city. Since 1993 research has also incorporated a study of the territory of the Roman city, from prehistoric to modern times, in order to understand why the site was selected for settlement, why it developed into a middle-sized town, its economy and subsistence, how it affected and exploited the environment, its decline, and what changes have taken place in the district subsequently. The focus has now shifted towards obtaining a better understanding of the linkages between human and environment systems so that inter-relations between the two can be more readily understood. As a result, a number of environmental topics concerning the territory of the Roman city are presently being studied. This territory extended from Lake Burdur in the West to the Aksu canyon in the East, from the Ag\ lasun Dag\ lari in the North to Mt. Kestel in the South. Interdisciplinary research revealed that for the early Neolithic and the Roman period there was a slightly warmer climate, a richer vegetation and more fertile soils for agricultural practice. 0277-3791/99/$ -see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 -3 7 9 1 ( 9 8 ) 0 0 1 0 5 -X

Journal of Hydrology, 2007

Sediment yield data derived from long-term sedimentation rates in 26 small farm dams in SE Austra... more Sediment yield data derived from long-term sedimentation rates in 26 small farm dams in SE Australia were used to calibrate a spatially distributed soil erosion and sediment delivery model (WATEM/SEDEM) that takes into account contribution from gully erosion in areas of concentrated flow. For three different land use categories (poor, moderate and good vegetative cover), a sediment transport capacity coefficient was calibrated. All other parameters being equal, it was found that sediment transport capacity for cropland is 2 times higher than for degraded pasture and 20 times higher than for native forest and good pasture. Model efficiencies for the prediction of specific and total sediment yield are 0.56 and 0.89, respectively. These model efficiencies are much higher compared to those obtained when intense erosion in concentrated flow areas is not considered explicitly. Several of the catchments that were used for the calibration have very high sediment yield rates, which are attributed to the presence of gullies. The good performance of WATEM/SEDEM to all catchments suggests that the model accounts well for gully erosion. Next, the calibrated WATEM/SEDEM was applied to the Murrumbidgee River basin (30,000 km 2 ). A mean annual sediment input into the river channels from the hillslopes of 478,000 t was predicted. The spatial pattern of hillslope-derived suspended sediment delivery in the Murrumbidgee indicates that most of the sediment originates from a few tributaries downstream of Burrinjuck Reservoir. Given the fact that high-resolution datasets (including digital elevation models) are becoming available at reasonable cost, WATEM/SEDEM provides a powerful tool to predict hillslope sediment delivery under 0022-1694/$ -see front matter ª

International Journal of Geographical Information Science, 2005

The RUSLE (Revised Universal Soil Loss Equation) is integrated within a GIS framework to calculat... more The RUSLE (Revised Universal Soil Loss Equation) is integrated within a GIS framework to calculate soil loss spatially. For this module, algorithms and procedures were developed to derive the slope length factor (L) and steepness factor (S) from a DEM, then integrated with the R, K, C, and P factors to develop homogeneous patches (sub‐units) within each field or river

Geomorphology, 2001

. Data on sediment yield from 26 small cultivated catchments 10-10 000 ha in central Belgium were... more . Data on sediment yield from 26 small cultivated catchments 10-10 000 ha in central Belgium were collected, based on sedimentation rates in small flood retention ponds. Area specific sediment yield varies from 0.4 to 20.6 t ha y1 year y1 for catchments ranging from 7 to 4873 ha. Various catchment properties were analysed in order to understand this large variation in sediment yield. Catchment area alone explains 64% of the observed variance in area specific sediment yield. Although no other parameter could explain more, it is concluded that catchment area is not a valid parameter to link with sediment yield. Overall catchment geomorphology, e.g. presented by the hypsometric integral, is much more important in controlling sediment yield. The lack of data, however, made this not very clear. A multiple regression model including three Ž y1 . morphological catchment properties is a valuable tool to predict total sediment yield t year from catchments in central Belgium. This model has an efficiency of 0.81. Furthermore, it was concluded that the within-catchment spatial distribution of parameters, for instance slope steepness, is very important but difficult to obtain one single representative value. The use of spatially distributed erosion and sediment delivery models is therefore necessary. Accurate sediment yield data, like the one presented in this study, however, are needed to validate these models. q

Geomorphology, 2006

In this article a statistical multivariate method, i.e., rare events logistic regression, is eval... more In this article a statistical multivariate method, i.e., rare events logistic regression, is evaluated for the creation of a landslide susceptibility map in a 200 km 2 study area of the Flemish Ardennes (Belgium). The methodology is based on the hypothesis that future landslides will have the same causal factors as the landslides initiated in the past. The information on the past landslides comes from a landslide inventory map obtained by detailed field surveys and by the analysis of LIDAR (Light Detection and Ranging)-derived hillshade maps. Information on the causal factors (e.g., slope gradient, aspect, lithology, and soil drainage) was extracted from digital elevation models derived from LIDAR and from topographical, lithological and soil maps. In landslideaffected areas, however, we did not use the present-day hillslope gradient. In order to reflect the hillslope condition prior to landsliding, the pre-landslide hillslope was reconstructed and its gradient was used in the analysis. Because of their limited spatial occurrence, the landslides in the study area can be regarded as "rare events". Rare events logistic regression differs from ordinary logistic regression because it takes into account the low proportion of 1s (landslides) to 0s (no landslides) in the study area by incorporating three correction measures: the endogenous stratified sampling of the dataset, the prior correction of the intercept and the correction of the probabilities to include the estimation uncertainty. For the study area, significant model results were obtained, with pre-landslide hillslope gradient and three different clayey lithologies being important predictor variables. Receiver Operating Characteristic (ROC) curves and the Kappa index were used to validate the model. Both show a good agreement between the observed and predicted values of the validation dataset. Based on a qualified judgement, the created landslide susceptibility map was classified into four classes, i.e., very high, high, moderate and low susceptibility. If interpreted correctly, this classified susceptibility map is an important tool for the delineation of zones where prevention measures are needed and human interference should be limited in order to avoid property damage due to landslides.

Catena, 2003

Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion b... more Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion by water is the objective of many national and international research projects. However, over the last decades, most research dealing with soil erosion by water has concentrated on sheet (interrill) and rill erosion processes operating at the (runoff) plot scale. Relatively few studies have been conducted on gully erosion operating at larger spatial scales.

Earth and Planetary Science Letters, 2007

Despite the availability of studies on the frequency density of landslide areas in mountainous re... more Despite the availability of studies on the frequency density of landslide areas in mountainous regions, frequency-area distributions of historical landslide inventories in populated hilly regions are absent. This study revealed that the frequency-area distribution derived from a detailed landslide inventory of the Flemish Ardennes (Belgium) is significantly different from distributions usually obtained in mountainous areas where landslides are triggered by large-scale natural causal factors such as rainfall, earthquakes or rapid snowmelt. Instead, the landslide inventory consists of the superposition of two populations, i.e. (i) small (b 1-2 · 10 − 2 km 2 ), shallow complex earth slides that are at most 30 yr old, and (ii) large (N 1-2 · 10 − 2 km 2 ), deep-seated landslides that are older than 100 yr. Both subpopulations are best represented by a negative power-law relation with exponents of −0.58 and −2.31 respectively. This study focused on the negative power-law relation obtained for recent, small landslides, and contributes to the understanding of frequency distributions of landslide areas by presenting a conceptual model explaining this negative power-law relation for small landslides in populated hilly regions. According to the model hilly regions can be relatively stable under the present-day environmental conditions, and landslides are mainly triggered by human activities that have only a local impact on slope stability. Therefore, landslides caused by anthropogenic triggers are limited in size, and the number of landslides decreases with landslide area.

Geomorphology, 2005

Sediment deposition in reservoirs is a serious off-site consequence of soil erosion in Tigray (No... more Sediment deposition in reservoirs is a serious off-site consequence of soil erosion in Tigray (Northern Ethiopia). So far insufficient and less reliable sediment yield data have been collected for Northern Ethiopia. Nor are there any adaptable methodologies for sediment yield assessment in the country as a whole, which could be used when designing new reservoirs. This study addresses those problems by (1) undertaking reservoir sediment deposition measurements and (2) by calibrating and adapting the Pacific Southwest Inter Agency Committee (PSIAC) and the Factorial Scoring Model (FSM) sediment yield assessment models to Ethiopian conditions. Field rating of catchment characteristics and the sediment yield data from the reservoir survey were used for calibration and validation of the models. Our reservoir survey indicates that specific sediment yield (SSY) varies significantly between catchments: i.e. 487 t km À2 year À1 to 1817 t km À2 year À1 with an average of 1054 (F 446) t km À2 year À1 . Since the variability of SSY is high between the studied reservoirs, care should be taken in the study area to adopt representative SSY values during reservoir and soil water conservation planning. The PSIAC SSY prediction is found to fit well with observed SSY without adjustment. While the FSM was found to have, after modifying the description of factors and incorporating new controlling factors, a good fit between the predicted and observed SSY. Studies of the relationship between the known sediment yield rates and the catchment conditions using semi-quantitative approaches such as PSIAC and FSM can be of substantial benefit in extrapolating data for areas where no detailed information is available in a cheap and quick way. However, calibration and modification of such models may be necessary if they are to be used beyond the region where they were developed. D

Catena, 2001

Field observations in central Belgium indicate a significant soil degradation process which has h... more Field observations in central Belgium indicate a significant soil degradation process which has hitherto not been considered in assessments of soil erosion rates: i.e., soil losses due to root crop Ž .

Geomorphology, 2003

An existing dataset of area-specific sediment yield (SSY) for 60 catchments in Spain that was ret... more An existing dataset of area-specific sediment yield (SSY) for 60 catchments in Spain that was retrieved from sediment deposition rates in reservoirs . Sediment yield at Spanish reservoirs and its relationship with the drainage basin area. ] reveals that catchment area alone explains only 17% of the variability in SSY. In this study, an attempt to explain the remaining variability in SSY was made using a quantitative and a semiqualitative approach for 22 catchments. During a field survey, the 22 selected catchments were characterised by topography, vegetation cover, lithology, shape and the presence of gullies in the broad vicinity of the reservoir. This information was used to develop a factorial scoring index model that provides a fairly accurate and reliable prediction of SSY. A classical multiple regression model using climatic, topographic and land use properties derived from regional datasets could not explain as much variance as the qualitative index model, nor did it appear to be as reliable. The same conclusion could be drawn when using the CORINE soil erosion risk map of southern Europe. The low prediction capability of the multiple regression models and the CORINE soil erosion risk map could be attributed mainly to the fact that these methods do not incorporate gully erosion and that the land cover data are not a good representation of soil cover. Both variables have been shown to be of great importance during the field surveys. Future assessments of SSY could be quickly and efficiently made using the proposed factorial scoring index model. In comparison with other models, which demand more data, the index model offers an alternative prediction tool. D